An Experimental and Modeling 

Study of the Flammability of 

Fuel Tank Headspace Vapors 

from High Ethanol Content Fuels 

D. Gardiner, M. Bardon, and G. Pucher 

Nexum Research Corporation 

Mallorytown, K0E 1R0, Canada 

Subcontract Report 

NREL/SR-540-44040 

October 2008 
An Experimental and Modeling 

Study of the Flammability of 

Fuel Tank Headspace Vapors 

from High Ethanol Content Fuels 

D. Gardiner, M. Bardon, and G. Pucher 

Nexum Research Corporation 

Mallorytown, K0E 1R0, Canada 

NREL Technical Monitor: M. Melendez 

Prepared under Subcontract No. XCI-5-55505-01 

Period of Performance: August 2005 – December 2008 

Subcontract Report 

NREL/SR-540-44040 

October 2008 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

NREL is a national laboratory of the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

Operated by the Alliance for Sustainable Energy, LLC 

Contract No. DE-AC36-08-GO28308
NOTICE

This report was prepared as an account of work sponsored by an agency of the United States government. 

Neither the United States government nor any agency thereof, nor any of their employees, makes any 

warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or 

usefulness of any information, apparatus, product, or process disclosed, or represents that its use would not 

infringe privately owned rights. Reference herein to any specific commercial product, process, or service by 

trade name, trademark, manufacturer, or otherwise does not necessarily constitute or imply its endorsement, 

recommendation, or favoring by the United States government or any agency thereof. The views and 

opinions of authors expressed herein do not necessarily state or reflect those of the United States 

government or any agency thereof. 

Available electronically at http://www.osti.gov/bridge

Available for a processing fee to U.S. Department of Energy 

and its contractors, in paper, from: 

U.S. Department of Energy 

Office of Scientific and Technical Information 

P.O. Box 62 

Oak Ridge, TN 37831-0062 

phone: 865.576.8401 

fax: 865.576.5728 

email: mailto:reports@adonis.osti.gov

Available for sale to the public, in paper, from: 

U.S. Department of Commerce 

National Technical Information Service 

5285 Port Royal Road 

Springfield, VA 22161 

phone: 800.553.6847 

fax: 703.605.6900 

email: orders@ntis.fedworld.gov

online ordering: http://www.ntis.gov/ordering.htm

Printed on paper containing at least 50% wastepaper, including 20% postconsumer waste
i 

Acknowledgments 

This work was supported by the U.S. Department of Energy's (DOE) Vehicle Technologies 

Program and managed by DOE's National Renewable Energy Laboratory and the Clean Vehicle 

Education Foundation. 
ii 

Executive Summary 

Background 

Gasolines and alcohol/gasoline blends are mixtures of hundreds of individual pure compounds. 

An important result is that the composition of the headspace vapors in a fuel tank depends on 

both ambient temperature and how much liquid fuel is left in the tank. Light volatile fractions 

evaporate first, followed progressively by the heavier molecular weight compounds in the fuel. 

The compositions of liquid and vapor phases vary continuously as the fuel evaporates. Air is also 

present in a vented or pressure-equilibrated tank, mixed with the fuel vapor in the headspace. 

Gasoline is so volatile at most ambient temperatures that the headspace vapors in the tank are 

actually too rich to burn, as long as some liquid fuel remains. However, as temperature drops, or 

as the liquid fuel level goes down, the volatility of the fuel decreases. As liquid level drops, there 

is less fuel vapor mixed with the air in the tank. 

If the ambient temperature is cold enough and the tank is nearly empty, then the fuel-air mixture 

in the tank becomes flammable and can pose an explosion hazard if ignited. Similarly, when 

refueling a nearly empty tank at very cold temperatures, the headspace vapors are expelled as a 

plume of fuel vapor and air as the liquid enters the tank, producing a flammable plume. An 

ignition source within that plume could lead to a flame travelling back down into the tank and 

causing an explosion. This has always been the case with gasoline, but the temperatures and fill 

levels needed to produce a hazard are rarely encountered, so fires and tank explosions are very 

unlikely, although not impossible. 

Ethanol by itself in a fuel tank produces headspace vapors that are flammable at room 

temperature and over a broad range of commonly encountered ambient temperatures. Suitable 

precautions to deal with flammable vapors would therefore be necessary to avoid fires and 

explosions if pure ethanol were used as a fuel. 

Ethanol/gasoline blends generally have volatility characteristics between those of the two major 

constituents. Any given high-alcohol blend tends to produce flammable fuel tank vapors at 

higher (i.e., less cold) temperatures than pure gasoline. The extent of the difference, and hence of 

any increased risk, depends on composition of the gasoline part of the blend and how much 

gasoline is present in the fuel mixture. It is therefore prudent to assess the extent of any 

differences in the fire hazards of fuel tank headspace vapors between gasoline and ethanol fuel 

blends to ensure that consumers and fuel handlers can be protected from significant unexpected 

risks or appropriately advised if special precautions are needed. Depending on the level of 

increased risk, if any, it might also be appropriate for auto manufacturers and fuel suppliers to 

implement modifications to mitigate hazards in vehicles or fuel-handling equipment. 

Objectives of the Work Reported 

The tests and mathematical modeling carried out in this project were aimed at helping quantify 

differences between candidate E85 (approximately 85% ethanol, 15% gasoline) blends and 

gasolines with respect to flammability hazards related to fuel tanks and refueling processes. The 

following were specific tasks: 
iii 

1. Develop the experimental apparatus and testing protocols needed to evaluate the 

flammability of fuel tank headspace vapors under representative, small-scale laboratory 

conditions. 

2. Test a series of fuel blends supplied for the work and selected as representative candidate 

commercial E85 fuel blends or as gasolines and denatured ethanol to be used for comparison. 

3. Analyze the test results and draw conclusions relevant to the flammability observed. 

4. Develop mathematical models for the fuel tank tests based on existing simulation techniques 

previously shown to have been successful for alcohol/gasoline blends. Use the models to 

predict flammability of the fuels tested in the experimental work, and evaluate the utility of 

such modeling to assess fuel tank combustion hazard scenarios. 

5. Carry out preliminary analysis of the flammability risks associated with vapor/air plumes 

emitted from fuel tanks during refueling. 

Principal Results 

1. A test process using multiple small, closed test chambers fitted with spark plugs and pressure 

transducers was developed. It was found to be an effective means of comparing the 

flammability of numerous fuel samples in a relatively short period. All tests reported 

corresponded to a tank having 5% of its volume filled with liquid fuel and the remainder with 

fuel vapor and air at ambient conditions. By attempting to ignite the mixture at different test 

temperatures varying from room temperature to as low as -30oC (-22oF), the range of 

temperatures for which the headspace vapor was flammable was determined for all 10 test 

fuels. 

2. Seven E85 fuel blends, two types of gasoline, and denatured ethanol were compared. 

Headspace vapors for the two gasolines became flammable when the temperature dropped to 

approximately -19oC (-2oF) and -25oC (-13oF) or lower. The E85 blends, on the other hand, 

produced flammable vapors at temperatures below values ranging from -2oC (28oF) to -22oC 

(-8oF). Denatured ethanol was found to be flammable at room temperature and all 

temperatures down to approximately -6oC (22oF). 

3. Three E85 fuels that were blends of denatured ethanol and natural gasoline had flammability 

behavior similar to the summer gasoline tested. 

4. The limit temperature for flammability of the E85 and gasoline test fuels often corresponded 

to the ranking to be expected from their dry vapor pressure equivalent (DVPE, i.e., vapor 

pressure at 100oF in the ASTM standard apparatus used for that test). However, there were 

exceptions. DVPE alone did not predict the ranking of the fuels correctly in all cases. A more 

volatile fuel at 38oC (100oF) is not necessarily more volatile at -29oC (-20oF). 
iv 

5. In general, E85 is flammable at low temperatures, whereas denatured ethanol is flammable at 

warmer temperatures. If both types of fuels are stored in separate tanks at the same location, 

there is a wide range of ambient temperatures for which one or both of the tanks' headspace 

vapors will be flammable. This is relevant to the issue of splash blending of ethanol and 

gasoline at refueling stations and the mixing of ethanol with gasoline by consumers 

themselves. 

6. The mathematical modeling confirmed that DVPE by itself does not reliably rank the lowtemperature flammability hazards of fuel tank headspace vapors when comparing 

conventional gasolines with alcohol blends or even comparing among alcohol blends. 

Differences in gasoline distillation characteristics, particularly as they represent the more 

volatile light ends, must be accounted for to obtain reliable comparisons. 

7. Existing mathematical models for gasoline primers can be used for some alcohol blend 

comparisons, provided that both DVPE and distillation data of the model used are reasonable 

approximations of the actual primer's characteristics. Matching DVPE alone is insufficient. 

8. At any given temperature, fuels that are more volatile produce longer plumes during 

refueling and represent greater hazards. 

9. For the more dangerous situation of a flammable plume adjacent to flammable tank 

headspace vapors, the size and location of the plume is important. Some cases might have 

plume lengths of such a size as to present a serious hazard of ignition and subsequent tank 

explosion, whereas others might have plumes short enough to preclude ignition by typical 

ignition sources found in the immediate vicinity of the refueling equipment. This aspect was 

not assessed in detail in this project. 
v 

Table of Contents 

I. Experimental Study ........................................................................................................ 1 

1. Flammability versus Ignitability ............................................................................. 1 

2. Apparatus and Test Procedures ............................................................................... 1 

2.1 Flammability Test Chambers .......................................................................1 

2.2 Ignition Apparatus .......................................................................................3 

2.3 Test Procedure .............................................................................................4 

3. Results and Discussion ........................................................................................... 4 

3.1 Delivered Ignition Energy............................................................................4 

3.2 Pressure and Temperature Measurements with Denatured Ethanol at Room 

Temperature .................................................................................................6 

3.3 Pressure Rise and Rate of Pressure Rise Results for the Test Fuels at 

Different Ambient Temperatures .................................................................8 

3.4 Lower Flammability Limit of Denatured Ethanol .....................................13 

4. Conclusions and Recommendations from the Experimental Study ...................... 16 

II. Mathematical Modeling Study ..................................................................................... 17 

5. Background ........................................................................................................... 17 

5.1 Volatility Characteristics of Gasolines ......................................................17 

5.2 Volatility Characteristics of Gasoline/Alcohol Blends ..............................18 

5.3 Flammability Characteristics of Gasoline/Alcohol Blends........................19 

6. Scope of the Present Modeling Study ................................................................... 20 

7. Models Used to Represent the Fuels Tested Experimentally ............................... 20 

7.1 Gasoline Primer Simulations .....................................................................20 

7.2 Ethanol Vapor Pressure..............................................................................21 

7.3 Selection of Existing Primer Models for Each Test Blend ........................21 

8. Results of the Modeling ........................................................................................ 21 

8.1 Initial Predictions without Tuning the Model ............................................22 

8.2 Conclusions from this Initial Modeling .....................................................23 

8.3 Tuning the Off-the-Shelf Models...............................................................24 

8.4 Conclusions from the Tuned Modeling .....................................................25 

9. Conclusions from the Fuel Tank Flammability Modeling .................................... 26 

10. Plumes of Flammable Vapor Emitted from Vehicle Fuel Tanks during 

 Refueling ............................................................................................................... 26 
vi 

10.1 Background ................................................................................................26 

10.2 Analysis......................................................................................................28 

10.3 Conclusions from the Plume Modeling Study ...........................................32 

III. Overall Conclusions and Recommendations .............................................................. 33 

11. Experimental Conclusions .................................................................................... 33 

11.1 Conclusions on the Mathematical Modeling of Fuel Tank Flammability .34 

11.2 Conclusions on the Mathematical Modeling of Plumes Emitted During 

Refueling ....................................................................................................34 

IV. Recommendations....................................................................................................... 34 

References ......................................................................................................................... 36 

Appendix A. Yellow Fuel Blend ...................................................................................... 38 

Appendix B. Green Fuel Blend ......................................................................................... 39 

Appendix C. Red Fuel Blend ............................................................................................ 40 

Appendix D. White Fuel Blend ........................................................................................ 41 

Appendix E. Blue Fuel Blend ........................................................................................... 42 

Appendix F. "Natural Gasoline" Fuel Blend .................................................................... 44 

Appendix G. Type A Fuel Blend ...................................................................................... 46 

Appendix H. Type B Fuel Blend ...................................................................................... 47 

Appendix I. Type C Fuel Blend ........................................................................................ 48 

Appendix J. "Denatured Alcohol" Fuel Blend .................................................................. 49 

Appendix K. Fuel Properties for Off-the-Shelf Mathematical Gasoline Models ............. 50 
vii 

List of Figures 

Figure 1. Flammability Test Chamber ............................................................................................ 2 

Figure 2. Programmable Ignition System ....................................................................................... 3 

Figure 3. Example of the Current Waveform of the Spark ............................................................. 5 

Figure 4. Effect of Ambient Temperature on Delivered Spark Energy .......................................... 6 

Figure 5. Pressure Rise during Combustion of Denatured Ethanol at 18°C ................................... 7 

Figure 6: Rate of Pressure Rise during Combustion of Denatured Ethanol at 18°C ...................... 7 

Figure 7. Temperature Rise during Combustion of Denatured Ethanol at 18°C ............................ 8 

Figure 8. Pressure Rise Comparison for Gasoline E85 Blends ...................................................... 9 

Figure 9. Rate of Pressure Rise Comparison (Gasoline E85 Blends) ........................................... 10 

Figure 10. Pressure Rise Comparison for Natural Gasoline E85 Blends ..................................... 10 

Figure 11. Rate of Pressure Rise Comparison for Natural Gasoline E85 Blends ......................... 11 

Figure 12. Digital Curve Fit to Establish 50% Flammability Limit Temperature ........................ 12 

Figure 13. Upper Flammability Limit as a Function of Vapor Pressure for E85 Fuels 

and Reference Gasoline Blends ................................................................................. 12 

Figure 14. Pressure Rise Results for Denatured Ethanol .............................................................. 13 

Figure 15. Rate of Pressure Rise Results for Denatured Ethanol ................................................. 14 

Figure 16. Pressure Rise Comparison for Denatured Ethanol and E85Y ..................................... 15 

Figure 17. Flammability Limits of Denatured Ethanol and E85Y................................................ 15 

Figure 18. Distillation Data for LIC and Natural Gasoline .......................................................... 25 

Figure 19. Plume of Fuel-Air Vapor Expelled during Refueling ................................................. 27 

Figure 20. Typical Fuel Concentration Profiles in the Plume [21] ............................................... 27 

Figure 21. Decrease in Equivalence Ratio in the Plume Downstream of the Fuel Tank ............. 30 

Figure 22. Flammable Region in the Plume for Case 1: Headspace Vapors above the 

Rich Limit of Flammability ....................................................................................... 31 

Figure 23. Flammable Region in the Plume for Case 2: Headspace Vapors Flammable ............ 31 

Figure 24. Relative Flammable Plume Lengths ............................................................................ 32 
viii 

List of Tables 

Table 1. Measured Temperature Flammability Limits and Initial Predictions ............................. 22 

Table 2. Results for the Models Tuned to Match DVPE .............................................................. 24 

Table A-1. Characteristics of the Primer Used in this Blend Formulation ................................... 38 

Table B-1. Characteristics of the Primer Used in this Blend Formulation ................................... 39 

Table C-1. Characteristics of the Primer Used in this Blend Formulation ................................... 40 

Table D-1. Characteristics of the Primer Used in this Blend Formulation ................................... 41 

Table E-1. Characteristics of the Primer Used in this Blend Formulation ................................... 42 

Table F-1. Characteristics of the Primer Used in this Blend Formulation.................................... 44 

Table G-1. Characteristics of the Primer Used in this Blend Formulation ................................... 46 

Table H-1. Characteristics of the Primer Used in this Blend Formulation ................................... 47 

Table I-1. Characteristics of the Primer Used in this Blend Formulation .................................... 48 

Table J-1. Characteristics of the Denaturant Used in Modeling this Blend Formulation ............. 49 

Table K-1. Reid Vapor Pressures [21] .......................................................................................... 50 
1 

I. Experimental Study 

1. Flammability versus Ignitability 

A fuel-air mixture is considered to be flammable if, when ignited, it produces a flame that can 

propagate throughout the available mixture. A flammable mixture might fail to ignite if the 

ignition source is too weak. However, successful ignition does not guarantee successful flame 

propagation. In some cases, the ignition source is strong enough to initiate a small flame, but the 

flame is extinguished as it moves away from the ignition source. In such cases, only a portion of 

the available mixture is burned, and the mixture is not, therefore, considered flammable. 

In the present study, the objective was to determine the flammability of the headspace vapors 

from different fuels. As such, the strategy was to employ an ignition source strong enough to 

ensure that any flammable mixture could be ignited reliably. With this achieved, flammability 

experiments should show evidence of partial combustion when the limits are reached. 

The use of closed combustion chambers and pressure measurements in the experiments provided 

a means of differentiating between ignition failures, successful ignition followed by partial flame 

propagation, and substantially complete flame propagation. Tests in which the mixture fails to 

ignite show no increase in chamber pressure. Partial flame propagation produces a small but 

detectable pressure rise. 

Tests in which most of the mixture in the chamber is burned should exhibit similar peak pressure 

levels. Assuming nearly complete flame propagation, differences in peak pressure levels would 

mainly be due to differing amounts of heat lost to the chamber walls. As the flammability limits 

are approached, greater heat losses (therefore, lower peak pressure levels) are expected due to 

slower combustion. However, even these reduced pressure levels are well above those that occur 

when only a small portion of the mixture is burned (i.e., the partial propagation case). 

Another diagnostic tool for examining flame propagation in the chamber is the rate of pressure 

rise. The rate of pressure rise (obtained through differentiation of the pressure signal) reflects the 

overall heat release rate within the chamber. This rate is influenced by the laminar burning 

velocity of the mixture and the surface area of the flame front. For example, a weak flame that 

extinguishes near the ignition source will reach a relatively small maximum surface area and will 

have a relatively low burning velocity prior to extinction. These factors will result in a very low 

rate of pressure rise compared with a case in which combustion is nearly complete. 

The rate of pressure rise also provides a sensitive means of comparing cases with similar peak 

pressure levels but different combustion rates. The peak rate of pressure rise falls as the 

flammability limit is approached. 

2. Apparatus and Test Procedures 

2.1 Flammability Test Chambers 

Nine identical test chambers were constructed so that a number of fuel samples could be chilled 

simultaneously. Photographs of one of the test chambers are shown in Figure 1. The chambers 

were based on modified cast-aluminum electrical junction boxes. A lid was fabricated from 
aluminum plate to mount a spark plug and two ball valves. The interior volume of the chambers 

was 296 mL. 

Pressure Sensor 

Compressed 

Air Valve Fuel Valve 

Gas Thermocouples Fuel 

Gas Thermocouples Fuel 

Spark Plug 

Figure 1. Flammability Test Chamber 

The existing pipe thread fittings in each box were used to install the pressure sensor and 

thermocouples. The pressure sensor was a Honeywell model 19CP300PA4K absolute pressure 

sensor with a full-scale range of 0–2,070 kPa (0–300 psia). The sensor had a response time of 0.1 

ms, which was found to be fast enough to capture the transient features of the pressure rise 

events during combustion. 

The gas temperature inside the chamber was measured using a Nanmac “right angle” K-type 

thermocouple. This thermocouple was a fast-response design with a low-inertia ribbon junction. 

The thermocouple was not fast enough to detect the actual transient gas temperature during 

combustion, but it could track the dynamic temperature behavior well enough to provide relative 

comparisons between tests. 

The temperature of the liquid fuel at the bottom of the chamber was measured using a 

conventional 1.6-mm (1/16 in) sheathed K-type thermocouple with a grounded junction. This 

thermocouple was positioned so that it would be immersed in the liquid layer (about 5 mm deep). 

2 
The signals from the pressure sensor, the thermocouples, and the trigger signal from the ignition 

system (which showed the timing of the spark) were recorded with a Daqbook 100 16-bit data 

acquisition system. Signal conditioning modules that were part of this system were used for the 

thermocouple voltages (providing ice point compensation, amplification, and linearization) and 

to provide differential amplification for the pressure sensor output voltage. 

Each chamber was equipped with ball valves, one of which was connected to a compressed-air 

fitting. The valve without the fitting was used when fuel was loaded into the chamber with a 

syringe. The chamber was purged between tests by blowing compressed air through the other 

valve while the fuel valve was open to vent the purged fuel and air. It was then leak tested by 

closing the fuel valve and pressurizing the chamber to 550 kPa (80 psi). 

2.2 Ignition Apparatus 

The ignition source for the experiments was composed of an automotive spark plug and a 

laboratory programmable ignition system. The spark plug was a Champion model 7034, which 

had platinum pins in the center and ground electrodes. This type of spark plug was selected 

because the pins ensure that the spark location is consistent from spark to spark. The electrode 

gap was set at 2 mm (0.079 in). This particular spark plug model also had a projected tip, which 

placed the spark gap closer to the vertical center of the chamber. The spark plug had an internal 

resistor to reduce electromagnetic interference (like most modern spark plugs) and was used in 

combination with a resistive interference suppression ignition cable. These measures made it 

possible to record noise-free pressure and thermocouple signals. 

The programmable ignition system (Figure 2) was a proprietary design produced by Nexum 

Research Corp. The system uses current sensing and closed-loop control to produce repeatable 

spark-current waveforms at current levels and durations programmed by the user. The details of 

the spark characteristics used for the experiments are discussed in the results section of this 

report. The ignition system was located outside the cold chamber, and the ignition cables from 

the test chambers were extended through the chamber wall to the outside of the chamber near the 

ignition coil. The appropriate cable for the chamber being tested was connected to the coil prior 

to the ignition attempt. 

Figure 2. Programmable Ignition System 

3 
4 

2.3 Test Procedure 

The experiments were carried out in a refrigerated engine testing cell. During most of the 

experiments, all nine test chambers were chilled together. During some tests, the same type of 

fuel was used in each chamber. In other cases, a different fuel was used in each chamber, or three 

fuels were tested (three chambers for each fuel). The choice of fuel for the chambers depended 

on the test temperature and the results of previous tests. 

All the experiments were conducted using a 1/20th (5%) fill level. As shown by Vaivads et al.

[1], the fuel tank headspace vapors of gasoline and alcohol/gasoline blends are flammable at 

higher temperatures if there is less liquid fuel in the tanks. The 5% fill level was selected, in 

consultation with the project sponsor, to represent a worst-case scenario in terms of how low an 

automobile operator might allow the fuel level to become. 

Before each test, fuel at room temperature was extracted with a syringe from one of the 20-L fuel 

storage containers and transferred to the appropriate test chamber. The storage container was 

opened just long enough to extract the fuel (to limit vapor losses from the samples), and the test 

chamber was sealed immediately after the fuel was injected through the fuel valve. 

The sealed chambers were placed in the cold chamber, connected to the instrumentation cables, 

and chilled until the gas temperature inside the chamber reached the desired test temperature. 

This initially caused the gas pressure in the chambers to fall below atmospheric pressure. The 

chamber pressure levels were then equalized to atmospheric pressure by quickly opening and 

closing each fuel valve. Following equalization, the chambers were cold soaked until the gas and 

fuel temperatures were within 0.5°C of each other and then maintained at the test temperature 

(+/- 0.5°C) for at least 1 hour before ignition was attempted. 

During each ignition attempt, the data acquisition system was activated and the ignition system 

was triggered. The ignition system sparked at a frequency of 2 Hz once triggered. Flammable 

mixtures usually ignited with the first spark. 

3. Results and Discussion 

3.1 Delivered Ignition Energy 

An example of the current waveform of the spark is shown in Figure 3. This spark required 

almost 1 J of stored energy from the capacitor that was discharged through the primary winding 

of the ignition coil. This is an order of magnitude higher than a typical automotive ignition 

system. However, only a fraction of this energy was delivered across the spark plug gap to ignite 

the mixture owing to various losses in the ignition circuit. 
0

20

40

60

80

100

120

0 0.2 0.4 0.6 0.8 1 1.2

Time (ms)

Current (mA)

Figure 3. Example of the Current Waveform of the Spark 

Resistive losses in the coil windings, ignition cable, and spark plug resistor are proportional to 

the square of the spark current (I2R losses), thus sparks in which the current is maintained at a 

relatively high level throughout the discharge period are inherently inefficient. Furthermore, 

spark discharges at atmospheric pressure deliver much less energy across the spark plug gap than 

do discharges at elevated pressures (such as in a running engine [2]) because the effective 

resistance of the gas between the electrodes is low compared with the external resistance of the 

ignition circuit. 

In the present study, the ignition energy delivered to the spark gap was calculated based on 

measurements of the spark current and voltage. The spark current measurement was provided by 

the programmable ignition system, which uses a current-sensing resistor in the current loop from 

the secondary winding of the ignition coil. The spark voltage was measured using a Tektronix 

6015 1000:1 high-voltage probe attached to the spark plug terminal. The voltage and current 

values were recorded with a Tektronix TDS 2014 oscilloscope and transferred to a PC for 

analysis. 

The delivered energy was calculated as the integral of the spark power. Because the measured 

voltage included the voltage drop across the spark plug resistor, the power dissipated in the 

resistor (I2

R) was subtracted from the total power so that only the power dissipated between the 

electrodes was included in the calculation. 

Spark voltage and current measurements were made with a spark plug installed in a test chamber 

at atmospheric pressure and without fuel. The apparatus was installed in the cold chamber, and 

experiments were carried out at temperatures ranging from +18°C to -20°C. The results of the 

energy calculations are shown in Figure 4. 

5 
0

5

10

15

20

25

30

-25 -20 -15 -10 -5 0 5 10 15 20 25

Temperature (o

C)

Energy (mJ)

Figure 4. Effect of Ambient Temperature on Delivered Spark Energy 

The delivered ignition energy was 15–20 mJ at room temperature but showed a greater variation 

between sparks as the temperature became lower. In some cases, energy levels greater than 25 

mJ were obtained. This is consistent with the higher gas density at lower temperatures. 

The ignition energy provided in this study was greater than would be delivered by a standard 

automotive system at atmospheric pressure [2], and such systems are commonly used for 

determining the explosive limits of gases and vapors [3]. Thus, it was concluded that the ignition 

system used in the present study was more than adequate to ignite any fuel-air mixtures that were 

actually flammable (i.e., capable of flame propagation after ignition). 

3.2 Pressure and Temperature Measurements with Denatured Ethanol at Room 

Temperature 

Figures 5–7 show results from tests in which all nine chambers were loaded (5% fill level) with 

denatured ethanol and ignited at room temperature. As discussed further in this report, denatured 

ethanol was flammable at this temperature and could be ignited with 100% reliability. These 

figures and the following discussion are presented to illustrate the raw measurements used to 

produce the upcoming flammability plots for the test fuels. 

6 
0

100

200

300

400

500

600

0 0.2 0.4 0.6 0.8 1

Time (s)

Pressure (kPa)

Figure 5. Pressure Rise during Combustion of Denatured Ethanol at 18°C 

-2

-1

0

1

2

3

4

0 0.2 0.4 0.6 0.8

Time (s)

dP/dt (kPa/ms)

1

Figure 6. Rate of Pressure Rise during Combustion of Denatured Ethanol at 18°C 

7 
0

20

40

60

80

100

120

0 0.511.5

Time (s)

Temperature (C)

2

Figure 7. Temperature Rise during Combustion of Denatured Ethanol at 18°C 

Figure 5 shows overplots of the absolute pressure in the chambers following ignition. Such 

pressure records were used to generate the maximum pressure rise data (pmax-pmin) for each fuel. 

Those data were later used to determine whether the fuel was flammable at a given temperature. 

The peak pressure levels were similar for these tests under identical conditions. However, there 

were greater differences in the details of the pressure-time history, as some examples took longer 

than others to reach a given pressure level. These differences are accentuated in plots showing 

the rate of pressure rise in Figure 6. These curves were obtained by differentiation of the pressure 

plots. As discussed earlier, the rate of pressure rise provides a relative indication of the burning 

rate of the fuel-air mixture in the chamber. 

Figure 7 shows overplots of the temperature signal from the fast-response gas temperature 

thermocouple in the chamber. Unlike the pressure signal, the temperature signal is a point 

measurement that is sensitive to the location of the thermocouple junction. Consequently, the 

thermocouple signal is affected by the direction of the flame movement so that similar burning 

rates might produce different temperature/time profiles. Because of this, the gas temperature 

signals were less consistent than the pressure signals; therefore, the pressure signals were used 

for all the remaining analyses. 

3.3 Pressure Rise and Rate of Pressure Rise Results for the Test Fuels at 

Different Ambient Temperatures 

In the following analysis, the test fuels have been divided into two groups to improve the clarity 

of the graphs. The first group consists of four fuels (designated E85G, E85R, E85W, and E85Y) 

that were originally blended for a vehicle study [4]. These fuels all contained volume 

percentages of ethanol approaching 85% (80.7%–82.3%) and contained different grades of 

CARBOB base gasoline or pump gasoline as the main blending component. These fuels are 

8 
referred to as “gasoline E85 blends” in the remainder of the report. In addition, isopentane and 

toluene were added to some fuels. In essence, these were “near” E85 (approximately 85% 

ethanol, 15% gasoline) fuels in which custom tailoring of the composition of blended 

hydrocarbons was used to obtain the desired volatility characteristics. 

The second group of three fuels (E85A, E85B, and E85C) were all blends containing only 

denatured ethanol and natural gasoline (natural gas condensate). Thus, these fuels are referred to 

as “natural gasoline E85 blends” in the remainder of the report. The ethanol volume percentage 

of these fuels ranged from 69.4% to 79.1%. The desired volatility was achieved by varying the 

ratio of ethanol to natural gasoline such that the most volatile blend (E85C at 69.4% ethanol) had 

substantially lower ethanol content than the gasoline blends in the first group. 

Each graph also shows the results for a reference gasoline (used in the Chevron study) and pure 

natural gasoline. The reference gasoline had summer-grade volatility characteristics, whereas the 

natural gasoline (which is not sold commercially as a vehicle fuel) had greater volatility 

comparable to commercial winter-grade gasoline blends. Thus, these fuels represented the two 

extremes (in terms of volatility) that could exist for gasoline. 

The pressure rise results for the gasoline E85 blends are shown in Figure 8. Values of zero 

indicate cases in which the vapor failed to ignite. The results were clearly binomial in nature, as 

all the experimental results were either above 680 kPa or below 100 kPa. It is proposed that the 

upper values represent cases of complete or substantially complete flame propagation. The vapor 

was, therefore, flammable in these cases. The non-zero lower values are believed to represent 

cases of partial flame propagation. In these cases, the vapor could be ignited by the high-energy 

ignition system but was not, by this definition, flammable. 

0

100

200

300

400

500

600

-30 -20 -10 0 10

Temperature (°C)

Pressure Rise (kPa)

E85 Y

E85 R

E85 G

E85 W

Gas

Nat Gas

High Pressure

Low Pressure

Figure 8. Pressure Rise Comparison for Gasoline E85 Blends 

Further evidence that there were cases of partial flame propagation can be seen in the rate of 

pressure rise results of Figure 9. For the cases in the high-pressure group, the rate of pressure rise 

fell with increasing temperature, and this effect was greater than the effect of temperature on the 

9 
peak pressure rise. The results corresponding to those giving low peak pressures are identified on 

the graph. These cases resulted in rates of pressure rise less than 1 kPa/ms. 

0

2

4

6

8

10

12

14

16

18

-30 -20 -10 0 10

Temperature (°C)

Rate of pressure Rise (kPa/ms)

Gas

E85 Y

E85 R

E85 G

E85 W

Nat Gas

Low Pressure

Figure 9. Rate of Pressure Rise Comparison (Gasoline E85 Blends) 

Similar behavior can be seen in the results for the natural gasoline blends, shown in Figures 10 

and 11. The binomial nature of the results was more pronounced for these fuels. 

0

100

200

300

400

500

600

-30 -25 -20 -15 -10 -5 0 5

Temperature (°C)

Pressure Rise (kPa)

Gas

E85 A

E85 B

E85 C

Nat Gas

High Pressure

Low Pressure

Figure 10. Pressure Rise Comparison for Natural Gasoline E85 Blends 

10 
0

2

4

6

8

10

12

14

16

18

-30 -25 -20 -15 -10 -5 0 5

Temperature (°C)

Rate of Pressure Rise (kPa/ms)

Gas

E85 A

E85 B

E85 C

Nat Gas

Low Pressure

Figure 11. Rate of Pressure Rise Comparison for Natural Gasoline E85 Blends 

Returning to Figure 8, three of the four gasoline E85 blends (E85G, E85R, and E85Y) were 

flammable at substantially higher temperatures than the reference summer-volatility gasoline. 

The exception was the E85W fuel, which was blended with winter-volatility gasoline and 

isopentane (see Appendix A). This fuel gave results midway between those of the summervolatility gasoline and the high-volatility, natural gasoline. The vapor pressure of this fuel (58.8 

kPa dry vapor pressure equivalent [DVPE]) was slightly higher than that of the summer gasoline 

(53 kPa DVPE) but much lower than that of the natural gasoline (81.7 kPa DVPE). 

The rate of pressure rise results in Figure 9 show that, for cases in which all the fuels were 

flammable (circa -20°C), the peak rates of pressure rise were much higher with the E85G, E85R, 

and E85Y gasoline E85 fuels. In practice, this would represent a risk of a more destructive 

incident (i.e., a more violent explosion) if the vapors were ignited. On the other hand, the peak 

pressure rise rates for E85W were no higher than those of the test gasoline blends. 

Referring to Figure 10, the peak pressure rise results at a given temperature for the natural 

gasoline E85 fuels were similar to those for the summer-volatility gasoline. The rate of pressure 

rise results in Figure 11 were similar as well. Thus, it is likely that these E85 fuels would not 

pose fire and explosion risks substantially greater than those of a summer-volatility gasoline in 

use below its intended ambient temperature range. 

The results in Figures 8–11 give a qualitative indication of the relative flammability of the 

different fuel blends. For quantitative comparisons, it was necessary to derive a single value 

representing the upper flammability limit temperature for each fuel. This was accomplished via 

statistical analysis of the peak pressure rise data using a PROBIT technique [5]. The use of this 

digital technique was justified by the binomial nature of the data. 

11 

In the analysis, pressure rise values in the high-pressure range (representing flammable mixtures) 

were assigned a high logic value (1), while pressure rise values in the low-pressure range 
(ignitable but not flammable) and zero pressure rise values (ignition failure) were assigned a low 

logic value (0). The technique generated a most likely curve fit for the temperature region where 

both high-pressure and low-pressure results were recorded in the experiments. From this curve 

fit, the temperature at which there was a 50% probability (at 95% confidence) of a high-pressure 

event was determined. This temperature was used to define the upper flammability limit of the 

fuel. An example of this process for one of the fuels is shown in Figure 12. The analysis also 

provided the standard deviation for the limit temperature. This value was used, along with the 

errors inherent in the experimental temperature measurements, to calculate error bars for the 

limit values. 

0

0.2

0.4

0.6

0.8

1

1.2

‐25 ‐20 ‐15 ‐10

50% P robability of

E x plos ion (95%

C onfidence Level)

Figure 12. Digital Curve Fit to Establish 50% Flammability Limit Temperature 

The results of the upper flammability limit analysis are plotted versus the vapor pressure of the 

fuels in Figure 13. The limit temperature was strongly related to the DVPE, but vapor pressure 

alone would not correctly predict the ranking of the fuels in all cases. Natural gasoline blends 

E85A and E85B had limit temperatures only slightly higher than the summer-volatility gasoline, 

but their DVPE values were higher than that of the gasoline. 

-30

-25

-20

-15

-10

-5

0

5

30 40 50 60 70 80 90

DVPE (kPa)

Limit Temperature (deg C)

E85Y

E85G

E85R

E85A E85B

E85W

E85C

Nat Gas

Gas

Flammable region

Figure 13. Upper Flammability Limit as a Function of Vapor Pressure for E85 Fuels and Reference 

Gasoline Blends 

12 
Only two of the E85 fuels (E85C and E85W) had limit temperatures lower than the summervolatility gasoline. It is interesting to compare how this was achieved in each case. The natural 

gasoline blend E85C contained only 69.4% ethanol and had a DVPE value more than 16 kPa 

higher than the gasoline. This gave a limit temperature about 1.5°C lower than the gasoline. 

E85W had an ethanol content of 81.4%. The remainder of this fuel was high-volatility 

commercial gasoline (94.8 kPa DVPE) and crude isopentane. E85W had a DVPE value only 5.9 

kPa greater than the summer-volatility gasoline, but its limit temperature was lower by almost 

4°C. Thus, this fuel was noteworthy in its potential to provide good safety margins for 

flammability while maintaining high ethanol content and not requiring excessively high vapor 

pressure. 

The DVPE values for E85W that were measured after the conclusion of the experiments (and 

used on Figure 13) were virtually identical to those originally provided by the source of the fuel. 

This might suggest that this fuel was not highly sensitive to volatility loss (“weathering”) during 

storage or transfers between containers. 

3.4 Lower Flammability Limit of Denatured Ethanol 

As discussed earlier, no upper flammability limit was established for the denatured ethanol 

evaluated in the study. This fuel was flammable at room temperature, and no tests were 

conducted at elevated temperatures. Instead, the denatured ethanol approached its lower (lean) 

flammability limit as the ambient temperature was lowered and eventually would not ignite if it 

was too cold. 

Figures 14 and 15 show the results for the peak pressure rise and rate of pressure rise for the 

flammability experiments with denatured ethanol. In Figure 15, the rate of pressure rise was 

substantially below the maximum values when the ambient temperature approached 20°C. This 

indicates that this fuel was nearing the upper flammability limit at this temperature. 

0

100

200

300

400

500

-15 -10 -5 0 5 10 15 20 25

Temperature (o

C)

Pressure Rise (kPa)

13 

Figure 14. Pressure Rise Results for Denatured Ethanol 
0

2

4

6

8

10

12

14

-20 -10 0 10 20 30

Temperature (o

C)

Rate of Pressure Rise (kPa/ms)

Figure 15. Rate of Pressure Rise Results for Denatured Ethanol 

If E85 is blended at the refinery or fuel supply facility, the public would not be exposed to 

denatured ethanol alone at service stations. However, one possible scenario for the use of E85 

fuels involves blending ethanol and gasoline at the service station pump, rather than delivering 

pre-blended E85 to the service station. In a pump-blending approach, denatured ethanol would 

be stored onsite in its own storage tank. It would be blended at the pump with commercial 

gasoline (from one of the other storage tanks) to make E85 for flexible-fuel vehicles. 

This approach would result in E85 fuels that do not comply with the minimum vapor pressure 

specifications of ASTM D5798. The volatility of the gasoline available at the service station 

during a given season would be too low to produce specification-complying fuel if the gasoline 

was blended with denatured ethanol at 15% by volume. Nevertheless, dispenser blending is 

being promoted by some organizations for economic reasons. Furthermore, blending of ethanol 

and gasoline by “amateurs” and small organizations is known to exist, so there are isolated cases 

in which the onsite blending scenario would be applicable. 

The fuel resulting from such an approach might, in some cases, resemble the E85Y of the present 

study. This fuel was a blend of denatured ethanol and summer-grade CARBOB base gasoline. In 

Figure 16, the pressure rise results from both of these fuels are plotted together. Throughout the 

temperature range shown, the headspace vapors of at least one of the two fuels would always be 

flammable. 

14 
0

10

20

30

40

50

60

70

80

-25 -15 -5 5 15 25

Temperature (o

C)

Peak pressure (psi)

Denatured Ethanol

E85Y

Figure 16. Pressure Rise Comparison for Denatured Ethanol and E85Y 

In Figure 17, the temperature limits from the PROBIT analysis are depicted to show the regions 

of flammability (in terms of ambient temperature) if both fuels were present. There is a 

temperature region where both the denatured ethanol vapor (in the storage tank) and the E85 

vapor (in the vehicle tank) could be flammable simultaneously. 

-10 -8 -6 -4 -2 0 2 4

Temperature (deg C)

Both fuels 

flammable

Denatured ethanol 

flammable

E85Y flammable

Figure 17. Flammability Limits of Denatured Ethanol and E85Y 

15 
16 

4. Conclusions and Recommendations from the Experimental Study 

1. Experiments were carried out to determine the flammability of fuel tank headspace 

vapors as a function of temperature. Seven E85 fuel blends, two types of gasoline, and 

denatured ethanol were compared. For gasoline and E85, the fuels were flammable below 

a given critical temperature, as the vapor was too rich to be flammable at temperatures 

exceeding this value. Denatured ethanol was found to be flammable at room temperature 

and was not flammable below a critical temperature because the vapor was too lean. 

2. Three E85 fuels that were blends of denatured ethanol and natural gasoline (containing 

69.4%–79.1% ethanol) had flammability behavior similar to the test gasoline with 

summer-volatility characteristics. Three fuels that were blends of denatured ethanol and 

different commercial gasoline components (containing 80.7%–82.3% ethanol) were 

flammable up to substantially higher temperatures than the summer-volatility test 

gasoline. One fuel (81.4% ethanol) containing high-volatility commercial gasoline and 

isopentane had a limit temperature for flammability significantly lower than that of the 

summer-volatility test gasoline but higher than that of the high-volatility natural gasoline. 

3. The limit temperature for flammability of the E85 and gasoline test fuels was strongly 

related to their vapor pressure inspections (DVPE), but DVPE alone did not predict the 

ranking of the fuels correctly in all cases. 

4. In general, E85 is flammable at low temperatures, whereas denatured ethanol is 

flammable at warmer temperatures. If both types of fuels are stored at the same location, 

there is a wide range of ambient temperatures for which one or both of the fuels will be 

flammable. 

5. The E85 fuels used in the study represented a wide range of blending options but did not 

provide a blend matrix in which critical parameters (such as ethanol content, vapor 

pressure, and hydrocarbon composition) were varied in a systematic manner. It is 

recommended that such a fuel blend matrix be designed for future experimental work. 

6. The study addressed the flammability of tank headspace vapors using a strong, reliable 

ignition source. The relative hazards posed by weaker sparks that represent actual 

potential ignition sources should also be investigated. 

7. All the experiments were carried out using a relatively low (5%) tank fill level, as this 

represented a worst-case scenario for potential flammability. The effect of fill level on 

flammability should also be investigated. 

8. The experimental approach used in the present study (involving multiple small, closed 

test chambers and pressure rise measurements) was found to be an effective means of 

comparing the flammability of numerous fuel samples in a relatively short period. 

However, further refinements to the apparatus and experimental technique are needed 

before it can be recommended as standard test practice. 

 
II. Mathematical Modeling Study 

5. Background 

5.1 Volatility Characteristics of Gasolines 

For a pure compound such as a single-component hydrocarbon, the composition, and hence 

molecular weight, of the vapor and liquid phases are constant and identical regardless of how 

much has evaporated, and the vapor pressure and enthalpy of evaporation are functions only of 

temperature. The vapor pressure of pure compounds can be adequately described for most 

practical purposes by a simple equation such as the classical Clausius-Clapeyron equation, as 

follows: 

)/exp( sat 1 −= 2 TCCP (1) 

where 

Psat = equilibrium saturation pressure 

T = absolute temperature 

C1 and C2 = constants for any given pure substance 

On the other hand, commercial gasolines and other refinery products are mixtures of hundreds of 

individual pure hydrocarbon compounds. As a result, the compositions of the liquid and vapor 

phases vary continuously as the fuel evaporates. Light volatile fractions evaporate first, followed 

progressively by the heavier molecular weight compounds in the fuel. 

Modeling real fuels can in principle be done by expressing the vapor pressure of each of the 

hundreds of components using Equation 1 with the constants applicable for each component and 

then combining all components using Raoult’s Law for ideal mixtures: 

i sat

Ni

i

P satfuel ∑ iPX

=

=

= 1

 (2) 

where 

Pfuel sat = total vapor pressure of the hydrocarbon blend 

Xi = mole fraction of component i in the liquid phase of the blend at equilibrium 

Pi sat = equilibrium saturation pressure of component i alone 

In practice, this requires the detailed composition of the fuel blend to be known and the constants 

C1 and C2 for each component to be available. Equation 2 can then be used to find the vapor 

pressure of the blend at some given mass fraction evaporated (e.g., 20%). However, iteration is 

required because the mole fractions of each compound in the two phases are different from each 

other and are no longer the same as that of the initial mixture before evaporation occurred. A 

computer model can be developed on this basis, but the code for such a model tends to be large 

and relatively slow, and the exact composition of each blend to be evaluated must be known. 

17 
For many practical analyses, a simple and fast model is needed to allow blend volatility to be 

calculated. It has been shown [6,7] that hydrocarbon blends such as gasoline can be modeled 

satisfactorily for many purposes by describing their vapor pressure and other properties using the 

form of the Clausius-Clapeyron equation but using appropriate polynomial functions instead of 

the two constants C1 and C2 as follows: 

)/exp( sat 1 −= 2 TffP (3) 

where 

Psat = equilibrium saturation pressure 

T = absolute temperature 

f1 and f2 = functions of the extent of evaporation defined by VF where 

VF = mass fraction of the mixture in the vapor phase 

Equation 3 essentially separates the effects of temperature, expressed directly in the exponential 

term of this Clausius-Clapeyron format, from the extent of evaporation, contained exclusively 

within the functions f1 and f2. These latter two functions can be expressed as polynomials in the 

vapor fraction, VF. References 6 and 7 showed how the functions can de derived using only the 

ASTM distillation data [8] for the mixture and its specific gravity. 

Despite the drastic simplification of using Equation 3 to represent the complex evaporative 

behavior of a hydrocarbon mixture, experimental measurements [9] showed that this method 

gave satisfactory predictions of vapor pressure over a range of temperatures from 0oC to 

40oC. Subsequent measurements to temperatures as low as -40oC showed that the method 

worked well over the entire range of interest (-40oC to +40oC) for ambient conditions in 

North America. 

5.2 Volatility Characteristics of Gasoline/Alcohol Blends 

Based on the success of this simplified approach to modeling hydrocarbon blends, the method 

was extended to encompass blends of methanol or ethanol with gasoline. The approach [10] was 

to treat any blend of an alcohol with a hydrocarbon as if it were a pseudo-binary mixture, that is, 

a mixture of a single hydrocarbon component, represented by the model described above 

(Equation 3), and the alcohol, represented by the Clausius-Clapeyron equation (Equation 1) as 

usual for pure compounds. 

Such blends are somewhat more complicated to model than those involving only hydrocarbons, 

because mixtures of strongly polar compounds such as alcohols do not form ideal mixtures with 

non-polar compounds such as hydrocarbons. Thus, Raoult’s Law (Equation 2) does not apply as 

written, and a modification to account for the non-ideality of the resulting alcohol/hydrocarbon 

mixture was used. For a blend such as 85% methanol and gasoline (M85), this has the following 

form: 

M gsatgg PXPXP msatmm = γ + γ 85

where 

18 
PM85 = equilibrium saturation pressure of the M85 mixture 

 = activity coefficient for gasoline in the blend g γ

m γ = activity coefficient for methanol in the blend 

Xg = mole fraction of gasoline in the liquid phase 

Xm = mole fraction of methanol in the liquid phase 

Pgsat = saturated vapor pressure of the gasoline 

Pmsat = saturated vapor pressure of the methanol 

Note that Xg and Xm as well as Pgsat vary continuously as the fuel evaporates. Furthermore, at any 

point in the evaporation process, when the overall vapor fraction VFM85 is some fixed value 

(between 0 and 1), say 0.4, the vapor fraction of the gasoline component VFg is different from 

that of the methanol VFm and both are different than VFM85. Representative values for the two 

activity coefficients were determined for use with gasoline alcohol blends as part of that and 

subsequent work. Since the mole fractions of each of the two components in the liquid phase are 

not known a priori for some overall value of vapor fraction for the blend, iteration is required to 

determine the vapor pressure and other volatility characteristics needed, such as the molecular 

weight of the vapor phase. This makes the model more complex to use but again vastly simpler 

than the more rigorous methods. Experimental measurements over the temperature range −40oC 

to +40oC again showed that the model works satisfactorily for predicting the volatility properties 

of such pseudo-binary alcohol/gasoline blends [10]. 

The original work summarized above was motivated at the time by the need to predict the 

comparative cold-starting behavior of fuels, particularly alcohol/gasoline blends. To evaluate the 

performance of light refinery streams as cold-starting primers for alcohol blends, vapor pressure 

measurements were also carried out on high-methanol mixtures primed with either gasoline or a 

representative light isocrackate (LIC). The model was shown to give good predictions for this 

type of hydrocarbon primer as well [11]. 

5.3 Flammability Characteristics of Gasoline/Alcohol Blends 

Based on the success of the volatility predictions for alcohol/hydrocarbon blends, the model was 

used to predict the flammability of fuel tank headspace vapors. Under most conditions at 

moderate ambient temperatures, the vapor/air mixture in the headspace of a gasoline fuel tank is 

too rich to burn, despite what Hollywood action movies would lead the public to believe. As the 

fuel level decreases in a fuel tank during use, the vapor pressure decreases, and the vapor/air 

mixture in the tank becomes progressively less rich. At sufficiently low temperatures and a 

nearly empty tank, the vapor-air mixture eventually falls into the flammable range and then 

presents a potential explosion hazard. For winter-grade gasolines, this temperature occurs is quite 

low and historically has not been considered a serious risk. Alcohol/gasoline blends tend to be 

more sensitive to this effect of low fill level because there is a smaller total amount of the most 

volatile hydrocarbon components in the original fuel compared with 100% gasoline. 

19 

However, alcohol/gasoline blends are less volatile than gasolines and have different 

stoichiometries. As a result, the content of the headspace above the liquid fuel in the tank can 

present a hazard at ambient temperatures much higher than required for their gasoline 
20 

counterparts. The fuel volatility model described above was therefore evaluated to determine if it 

could be used to reliably assess fuel tank hazards with various alcohol/gasoline blends [1,12–15]. 

In those studies, the volatility model was used to assess composition at any given temperature 

and fuel fill level in a tank. Published, well-recognized flammability data [16] and the 

LeChatelier mixing rule [17] were used to evaluate the resulting flammability of the vapor phase. 

Results were satisfactory, and it was shown that this method can be used to predict the hazards 

associated with fuel tank vapor/air mixtures [1,12–15]. 

The above techniques for predicting vapor-phase properties and their flammability were used in 

the present study, focusing on comparisons between specific primed ethanol blends. 

6. Scope of the Present Modeling Study 

Because of the limited nature of this study, and the data available on primers used in the various 

blends, it was not possible to create models of the hydrocarbon components in each blend in 

accordance with the full modeling technique described in the references cited above. Instead, a 

number of existing models for representative gasoline primers were used. These existing models 

covered representative low-, medium-, and high-volatility gasolines, Indolene, and LIC, all 

resulting from the previous work. The following were the goals: 

1. Determine to what extent off-the-shelf gasoline or LIC models could be used to predict, a 

priori, the flammability of the blends in the study (which had various commercial gasolines 

and gasoline components as primers) without modeling the specific primers in detail. 

2. Determine what level of improvement over the a priori predictions could be obtained using 

the off-the-shelf gasoline models “tuned” by using the measured values of vapor pressure 

(DVPE [18]) at the standard Reid vapor pressure (RVP) conditions (100oF and vapor/liquid 

ratio of 4:1) [19] to adjust the calculated vapor pressures as a means of improving predicted 

flammability. 

3. Make recommendations about the potential for improving predictions by using the measured 

distillation data [8] and specific gravity of the actual hydrocarbon primer used in a given 

blend instead of the existing off-the-shelf gasoline models. 

7. Models Used to Represent the Fuels Tested Experimentally 

7.1 Gasoline Primer Simulations 

The off-the-shelf primers used for this study are listed below. The references cited in each case 

provide the polynomial expressions used in the model and the coefficients used in the 

polynomials for that particular primer. 

• Low-volatility gasoline [6] (LowVol) RVP = 4.24 psi (29.3 kPa) 

• Indolene [6] (Ind) RVP = 8.21 psi (56.6 kPa) 

• Medium-volatility gasoline [11] (MedVol) RVP = 11.45 psi (79.0 kPa) 

• Light isocrackate [11] (LIC) RVP = 11.5 psi (79.5kPa) 
• High-volatility gasoline [6] (HighVol) RVP = 15.1 psi (104 kPa) 

7.2 Ethanol Vapor Pressure 

The ethanol vapor pressure was modeled using the Antoine Equation: 

log )/( P 10 ethanolsat +−= TCBA

 The values of the coefficients used for ethanol were taken from Wilhoit and 

Zwolinski [20]. 

 

7.3 Selection of Existing Primer Models for Each Test Blend 

Appendices A–J show the 10 test fuels, some of their pertinent property data, the off-the-shelf 

primer models selected for use with that fuel blend, and the reasons for those particular 

selections. 

8. Results of the Modeling 

Mathematical modeling of the flammability hazard associated with fuel tank headspace vapors 

was carried out to help answer several questions: 

1. To what extent can DVPE [18] be used to rank fuels in order of flammability? 

• DVPE reflects the volatility of a fuel at 100oF and is usually readily available. 

Therefore, it would be the easiest property to use in comparing, qualitatively, the 

volatility and hence the flammability of the fuel vapor/air mixture in a tank at low 

ambient temperatures. Gasolines and alcohols cannot be compared to each other on 

this basis owing to their differences in volatility behavior with temperature. However, 

blends having a similar alcohol content, such as E85, might be expected to behave 

more similarly and therefore be more amenable to comparisons simply based on 

DVPE. 

2. To what extent can the existing gasoline models be used to rank fuels in order of 

flammability? 

• The full computational procedure described in the references [6,7] is complex and 

requires that the D86 distillation data and specific gravity of the hydrocarbon primer be 

known before it is added to the alcohol. Because the previous work examined five 

gasolines of varying volatility, it would be simple and convenient if the models already 

known for those samples could be used to represent other similar candidate primers in 

E85 blends. 

• There were two aspects to this part of the study. The first was simply an a priori

prediction in which the primer of the candidate blend was modeled using the off-the-shelf 

model that seemed to best reflect its volatility characteristics. The second stage was to see 

if some slight tuning of the off-the-shelf model to fit the measured DVPE of the alcohol 

blend could give significantly better results for predicting low-temperature flammability. 

21 
22 

3. To what extent can the existing gasoline models be used to predict the actual temperature 

flammability limits for new candidate blends? 

• Both a priori and tuned versions of the models were to be examined in this regard. 

8.1 Initial Predictions without Tuning the Model 

Modeling runs were carried out in two steps. The first step simply used the measured ethanol 

content of each blend along with whichever off-the-shelf hydrocarbon model seemed to best 

reflect the nature of the primer in the blend. Appendices A–K describe the reasons for the 

selection of the particular off-the-shelf gasoline primer used to model each fuel blend. 

The following table compares these initial predictions to the flammability data measured in the 

experimental part of this study. The fuels are listed in rank order of measured flammability by 

temperature. The DVPE of each fuel is also shown to illustrate how DVPE fares in ranking the 

flammability of these fuels. 

Table 1. Measured Temperature Flammability Limits and Initial Predictions 

Fuel Blend Measured 

Flammability 

Limit1 (oC) 

Initial Flammability 

Limit Prediction 

(oC) 

DVPE3

[18]

(psi) 

Predicted 

DVPE [with 

Primer shown] 

(psi) 

Yellow -1.8 +/- 0.47 +15.1 4.87 4.22 [LowVol] 

Green -6.49 +/- 2.05 +0.5 5.24 5.71 [Ind] 

Red -7.6 +/- 0.01 +1 5.60 6.05 [Ind] 

Type B -16.1 +/- 0.78 -13.5 8.22 8.66 [LIC] 

Type A -16.33, +/- 0.32 -12 7.98 8.29 [LIC] 

Blue -17.78 +/- 0.28 -11.5 7.69 8.21 [Ind] 

Type C -19.25 +/- 0.69 -16.5 9.37 9.57 [LIC] 

White -21.56 +/- 0.98 -10.5 8.54 7.92 [LIC] 

Natural 

Gasoline 

-25.3 +/- 1.02 -20.5 11.85 11.5 psi [LIC] 

Denatured 

Alcohol 

-5.73 +/- 4.23 

(lean limit)2

-15.5 2.97 [LIC] 

1

These are the rich temperature limits, i.e., the mixture in the tank is too rich to support combustion until the ambient 

temperature falls to this value. At that point, the mixture becomes flammable and remains so until continued cooling 

takes it to the lean limit of flammability at some lower temperature that was not measured in this study. 

2

Ethanol is flammable at room temperature, so this was the temperature at which it ceased to be flammable, i.e., the 

lean limit. The rich limit lies above room temperature and was not measured in this study. 

3

Values measured by the Alberta Research Council. 
23 

8.2 Conclusions from this Initial Modeling 

Several observations and conclusions can be drawn from the results shown in Table 1: 

1. The ranking of the Blue fuel (a commercial gasoline) compared with the E85 blend of similar 

DVPE (Type A) illustrates the well-known observation that gasolines and alcohols behave 

differently at low temperatures. Despite its slightly lower DVPE, the Blue fuel becomes 

flammable at a lower temperature than the Type A E85. The effect of temperature on 

volatility is different for the two blends, and the more volatile fuel at 100°F is less volatile at 

cold temperatures and becomes flammable under warmer ambient conditions in a fuel tank. 

A simple comparison of DVPE as an indicator of low-temperature flammability would be 

misleading. 

2. As might be expected intuitively, alcohol blends that are more volatile at DVPE conditions 

generally translate into safer fuels, i.e., higher DVPE fuels do not become flammable until 

lower ambient temperatures are reached. However, there are notable exceptions in which the 

DVPE fails to order flammability correctly. Although the order of Types A and B are 

reversed compared with their DVPE ranking, the experimental uncertainty gives them 

approximately the same measured flammability limit, so too much significance should not be 

placed on that comparison. However, the White fuel has a significantly lower DVPE than 

Type A but does not become flammable until a lower temperature, i.e., it is safer in that sense 

despite its lower DVPE. Thus, even when comparing alcohol blends to each other rather than 

to gasolines, the use of DVPE to assess low-temperature flammability can lead to significant 

errors. 

3. The a priori models predict measured DVPE reasonably well. However, like the DVPE, they 

also do not reliably rank the flammability of the various blends in the measured order. In fact, 

they are arguably worse overall in correlating flammability than DVPE, because the White 

fuel is even further out of the measured rank order of flammability. Simply using off-theshelf models of similar DVPE is not sufficient to assess low-temperature flammability of a 

particular fuel blend reliably. 

4. As noted above, the rich temperature flammability limits predicted a priori by the off-theshelf models are close in some cases and far from measured values in others, i.e., results are 

not generally encouraging viewed as a whole. However, a closer examination shows that the 

only cases in which the prediction can be said to be satisfactory involve Types A, B, and C 

and the “Natural Gasoline” (which was itself the primer for blends A, B, and C). This is also 

the case in which the D86 data for the primer used in those fuels was known, so the 

predictions could use a primer that was demonstrably similar, albeit not identical, to the 

primer actually used in the tested blends. For Types A, B, and C and Natural Gasoline, the 

predicted flammability limits are reasonably close to measured values, but the predicted rich 

flammability limits are all slightly warmer than measured values. Although the D86 for the 

Blue gasoline was also known, there was no off-the-shelf primer model that was a good fit to 

those data. 
24 

8.3 Tuning the Off-the-Shelf Models 

The second step was to determine whether more useful modeling results for low-temperature 

flammability could be obtained using the off-the-shelf mathematical models if they were tuned to 

predict exactly the measured DVPE of either the actual primer, if known, or the DVPE of the 

alcohol blend if the primer DVPE itself was unknown. This technique was evaluated for two 

classes of results: 

 

1. Yellow was studied as a sample of a blend in which the DVPE of the primer alone was 

known (4.8 psi [33.1 kPa]) and could be matched fairly well by one of the off-the-shelf 

models—in this case “Low Volatility Gasoline” (4.24 psi [29.3 kPa]). 

2. Types A,B, and C and their known primer, Natural Gasoline, were studied as examples of 

the case in which the actual distillation data of the primer were reasonably well matched by 

one of the off-the-shelf primer models—in this case LIC (11.5 psi [79.5kPa]). 

For each model, these tuned versions were run to determine whether there was any significant 

improvement over the a priori versions. 

Table 2. Results for the Models Tuned to Match DVPE 

Fuel Blend Measured 

Flammability 

Limit (oC) 

A Priori

Flammability 

Limit Prediction 

(oC) 

Tuned 

Flammability 

Limit Prediction 

(oC) 

Primer 

Model used 

Yellow -1.8 +/- 0.47 +15.1 +11.5 LowVol 

Type B -16.1 +/- 0.78 -13.5 -14.5 LIC 

Type A -16.33 +/- 0.32 -12 -13.5 LIC 

Type C -19.25 +/- 0.69 -16.5 -17.5 LIC 

Natural 

Gasoline 

-25.3 +/- 1.02 -20.5 -20.9 LIC 

 

As seen in Table 2, tuning the off-the-shelf model for the Yellow fuel blend improved the 

predicted rich flammability limit somewhat and moved it in the correct direction. However, the 

predicted limit was still substantially higher than the measured value. It is apparent that simply 

matching the DVPE of the gasoline primer is insufficient to give an adequate representation of a 

primer having different distillation characteristics. The difference in volatility behavior at 

temperatures other than the 100°F used in the DVPE test is significant and cannot be simulated 

merely by adjusting DVPE of an off-the-shelf model that has full distillation data that might not 

match the actual primer. 

The results for the case in which the off-the-shelf model for LIC was tuned so that its DVPE 

matched that of the Natural Gasoline were slightly better. However, the tuning required was 

small because the DVPE of the LIC was already close to that of the Natural Gasoline. Again, the 
changes were in the correct direction, but the improvement was small and still left discrepancies. 

Figure 18 compares the distillation data of the Natural Gasoline with those of the LIC used to 

simulate it. 

0

20

40

60

80

100

120

140

160

180

0 10 20 30 40 50 60 70 80 90 10

% Evaporated

T (C)

Natural Gasoline 

LIC 

0

Figure 18. Distillation Data for LIC and Natural Gasoline 

At low temperatures, there is little ethanol in the vapor phase. As a result, the flammability of the 

blend depends primarily on the mass of hydrocarbon components there. This is turn is a direct 

function of the hydrocarbon vapor pressure. The lightest, most volatile components in the fuel 

have the greatest influence, with higher boiling fractions having progressively less impact 

because their contributions to total vapor pressure are smaller. This graph shows that the lightest 

(i.e., most volatile) components in the Natural Gasoline have similar volatility to those of the 

LIC and would have a similar effect on flammability. However, the next most volatile fractions 

(seen here as the range from about 15%–70% evaporated on the distillation curve) are more 

volatile in the Natural Gasoline than in the LIC. The slight discrepancies in the predicted 

flammability limits when LIC was used to simulate the Natural Gasoline primer can be attributed 

to these more volatile fractions in the real primer. 

8.4 Conclusions from the Tuned Modeling 

1. Tuning an off-the-shelf model selected based on DVPE alone but not necessarily reflecting 

the actual hydrocarbon components of the real primer does not give reliable results. 

2. With a model that has both DVPE and distillation data that are reasonable representations of 

the actual primer, a priori results are already satisfactory. Simply tuning the model to match 

primer DVPE creates little improvement. 

25 
26 

9. Conclusions from the Fuel Tank Flammability Modeling 

1. DVPE by itself does not reliably rank the low-temperature flammability hazards of fuel tank 

headspace vapors when comparing conventional gasolines with alcohol blends or comparing 

among alcohol blends. Differences in distillation characteristics, particularly as they 

represent the more volatile light ends, must be accounted for to obtain reliable comparisons. 

2. Existing mathematical models for gasoline primers can be used for some alcohol blend 

comparisons, provided that both DVPE and distillation data of the model used are reasonable 

approximations of the actual primer. 

3. For confident use of these modeling techniques, the actual primer distillation data should be 

used to create the models [6,7]. The resulting alcohol blend models [1,9–15] will then be 

based on a faithful representation of their hydrocarbon components. 

4. The present study has shown that the modeling techniques used appear to give good 

predictions provided the primer model is a good match for the actual gasoline primer used in 

the alcohol blend. It would be desirable to carry out further studies of varied candidate E85 

blends for which the full primer information is known. Such a study could allow conclusions 

to be drawn about the accuracy and reliability of this modeling technique for making 

flammability hazard predictions when assessing future candidate alcohol blends. 

5. The mathematical model, in principle, could be used to predict tank safety more reliably than 

DVPE alone, but it is too complex for routine use in the field. However, further study using 

the model might permit a simpler correlation to be developed for field use that is more 

reliable than DVPE alone. 

10. Plumes of Flammable Vapor Emitted from Vehicle Fuel Tanks during 

Refueling 

10.1 Background 

When a fuel tank is being refilled, the headspace vapors present in the tank before fueling begins 

are progressively expelled out the filler neck. They flow out of the tank at approximately the 

same volumetric flow rate as that of the liquid fuel entering the tank and have the same 

properties at the exit as the headspace vapors. In many areas of the United States, service stations 

are equipped with vapor recovery systems at the fill nozzle that prevent the headspace vapors 

from being released to the atmosphere. All new passenger cars and trucks sold in the United 

States are equipped with onboard refueling vapor recovery systems. These emission control 

systems should also prevent the formation of flammable mixtures outside the tank during 

refueling. 

However, this technology might not be present or effective in some cases, such as the filling of 

portable fuel containers, older vehicles (amateur E85 conversions are known to exist), and 

trailered marine craft. If the vapor expelled from the fuel tank is not captured in some way, it will 

produce a plume of fuel-air mixture that flows outward and entrains air from the surroundings as 

it flows downstream. The fresh air being entrained progressively into the plume as it moves 

downstream dilutes the mixture and steadily reduces the fuel-air ratio inside the plume itself. 
Figure 19 shows the case of such a plume produced while refueling a vehicle in the presence of a 

crosswind. 

Figure 19. Plume of Fuel-Air Vapor Expelled during Refueling 

Depending on the presence or absence of an ambient wind, wind velocity, and rate of refueling, 

the plume might be vertical or blown downwind as shown in Figure 19. It is subject to shear 

forces at its boundary that entrain ambient air and buoyancy forces that might change the shape 

and direction of the plume. Within the plume, there is diffusion of the entrained air toward the 

core, dependant upon the turbulence intensity, mixture properties, and rate of air entrainment. 

Figure 20 shows the typical concentration profile of a component such as fuel vapor within the 

plume as it is progressively diluted. 

Transverse 

Distance 

Downstream Distance 

Figure 20. Typical Fuel Concentration Profiles in the Plume [21] 

As the plume moves downstream, the fuel concentration decreases. The mixture within the 

plume might be more or less uniform depending on turbulence intensity and rate of air 

entrainment. 

27 
28 

The details of the fuel concentration and plume dynamics can vary considerably, depending on 

the flow field fluid dynamics. However, some useful general conclusions can be deduced without 

analyzing the details of the flow. Overall, there are two different scenarios possible with respect 

to potential fire and explosion hazards. 

Case 1. Rich Tank Headspace Vapors. The first scenario occurs when the tank headspace 

vapors are above the rich limit of flammability. This is the normal case at warm ambient 

temperatures when the fuel is gasoline or an alcohol/gasoline blend. In this case, the plume is too 

rich to burn at the tank exit, but falls into the flammable region as entrained air dilutes it. Further 

air entrainment downstream eventually dilutes the mixture enough so that it passes the lean limit 

of flammability and thus becomes too lean to support combustion beyond that point. Therefore, 

there is a segment of plume inside which the mixture is flammable. A spark or other ignition 

source within this area would lead to a flame propagating throughout the flammable portion of 

the plume, including backward toward the tank. This is clearly a hazard because it could ignite 

other materials in the area or cause injuries to anyone within that region. However, the vapors 

inside the filler neck and the headspace vapors cannot be ignited because they are above the rich 

flammability limit, so the tank will not explode under this scenario. In general, fuels that are 

more volatile produce longer flammable plumes, and gasoline is more hazardous in this regard 

than alcohol blends [21]. 

Case 2. Flammable Tank Headspace Vapors. This scenario is more hazardous than the 

previous scenario. The vapors in the plume expelled from the tank are flammable immediately at 

the exit and remain so until the point downstream at which the mixture is sufficiently diluted by 

entrained air so that it falls below the lean flammability limit. As in Case 1, an ignition source 

within the flammable portion of the plume would lead to a flame propagating throughout the 

flammable portion of the plume, including backward toward the tank. In this case, however, the 

flame would propagate all the way back to the tank filler neck. Because the headspace vapors are 

themselves flammable, the flame would not stop at the tank entrance but would continue to 

propagate into the tank itself, producing an explosion, rupture of the tank walls, and violent 

dispersion of the liquid fuel to create a substantial fire. 

10.2 Analysis 

As mentioned above, the actual fluid mechanics of the plume are complex and depend up a 

number of factors. However, some general conclusions can be drawn based on the common 

aspects found in any of the possible plume types. 

The rate of air entrainment depends on the details of the flow field. However, for any of the 

candidate fuels of interest, the fluid properties responsible for the fluid dynamics of the plume 

are very similar because they are close to those of air at ambient temperature. As a result, 

regardless of what factors affect the shape and evolution of the plume, they are essentially the 

same for any of the candidate fuels. The rate of entrainment of ambient air into the plume 

depends on the flow field and at any point is given by 
flowfieldfn )( d

md air =

ξ ξ

&

 (4) 

whereξ is the curvilinear coordinate along the plume length with its origin at the tank filler neck 

and is the mass flow rate of the air within the plume at any cross section normal to the axis 

of the plume. The mass flow rate increases steadily owing to entrainment of ambient air into the 

plume, whereas the mass flow rate of fuel vapor at any cross section is constant and equal to that 

at the filler neck for a steady rate of flow of liquid fuel into the tank. 

mair &

The fact that the fuel flow rate is constant means that all air entrained along the plume length 

reduces the fuel-air ratio by a characteristic amount regardless of the fuel concerned. The rate of 

change of the flammability of the plume depends strongly on the flow field but is the same for all 

the candidate fuels. 

Equation 1 can be recast to express the overall fuel-air equivalence ratio,φ , of the mixture at any 

point along the plume location, where 

stoich

actual

f

f φ = (5) 

and 

air

fuel

m

m

f &

& = (6) 

And is the value of f for a stoichiometric mixture for the fuel in question. stoich f

With these definitions, a value of φ greater than 1 signifies a rich mixture and less than 1 

signifies a lean mixture. This is a particularly convenient convention because, for all the fuels in 

question, the rich and lean flammability limits occur at virtually the same values of φ . 

Expressing in terms of mair & φ : 

stoich

fuel

fuel

fuel

air air f

m

m

m

mm ⋅ = = φ

&

&

& && (7) 

Differentiating Equation (4) allows us to recast Equation (1) as follows: 

)( 2 flowfieldfn d

f

m

stoich

fuel − = φ

& φ (8) 

29 
 

The function on the right in Equation (5) depends on the details of the flow field. However, it is a 

positive function and is nearly constant for many cases of interest. For example, the rate of 

entrainment in a turbulent jet in still air is constant and that in a turbulent jet in a cross flow (such 

as shown in Figure 19) is only a very weak function of distance along the plume [17]. In a case 

in which the function is a constant, Equation (5) can be integrated, giving the following: 

1 )(1

1

ξξ

φ φξ −×+ = const

 (9) 

where state 1 is at the start of the plume (i.e., at the exit of the filler neck) when the plume 

properties are still those of the headspace vapors. is the distance downstream along the 

curved shape of the plume. 

)( −ξξ 1

Equation (9) shows that the equivalence ratio and hence the flammability of the mixture at any 

location along the plume has the shape shown in Figure 21. 

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50 60

Downstream Distance along Plume

Equivalence ratio

Figure 21. Decrease in Equivalence Ratio in the Plume 

Downstream of the Fuel Tank 

The two flammable plume scenarios described above now can be visualized as shown in Figures 

22 and 23. The distance Lf shows the flammable portion of the plume. In Figure 22, there is a 

rich section just downstream of the filler neck before the plume becomes flammable. 

30 
31 

0

1.5

2

2.5

3

3.5

10 20 30 40 50 60

Downstream Distance along Plume

Equivalence ratio

0.5

1

0

Rich limit of flammability 

Lean limit of flammability 

L f

Figure 22. Flammable Region in the Plume for Case 1: Headspace Vapors above the Rich Limit of 

Flammability 

 

0

0.5

1

1.5

2

2.5

3

3.5

0 10 20 30 40 50 60

Downstream Distance along Plume

Equivalence ratio

Lf

Lean limit of flammability 

Rich limit of flammability 

Figure 23. Flammable Region in the Plume for Case 2: 

Headspace Vapors Flammable 

 

In Figure 23, the distance Lf again shows the flammable portion of the plume. However, in this 

case, the flammable part of the plume is adjacent to the tank and the headspace vapors are 

themselves also flammable. 

The size and shape of the plume, as well as the rate of decrease of equivalence ratio φ

downstream inside it, depend on the particular flow field conditions. However, the general 

relationship in Equation (9) and the models for fuel blend volatility allow a limited quantitative 

comparison to be made between the relative flammability hazards of the candidate fuel blends. 
In the following illustrative comparison, an arbitrary value was used for the constant in Equation 

(9) so this equation could be used to compare the plume lengths of four fuels at both low and 

high ambient temperatures and draw general conclusions. The particular values given are of no 

significance except to define the relative sizes of the plumes with respect to each other. A 

detailed flow field model (still air, cross winds of various magnitudes, buoyancy effects, laminar 

or turbulent conditions, etc.) would need to be created to determine air entrainment rates and 

hence actual plume lengths in absolute terms. Different quantitative values would be found for 

any particular flow conditions, but the general trend is expected to be consistent with Figure 24. 

The results shown in Figure 24 were derived using the mathematical models used in the 

simulation of the Natural Gasoline and each of the three blends produced by mixing it with 

ethanol in various proportions, i.e., Type A, Type B, and Type C. The measured DVPE and the 

simulations both ranked these fuels, based on increasing volatility, as Type A, B, and C then 

Natural Gasoline (see Table 1). The relative plume lengths were evaluated at +20oC 

(representing Case 1: rich headspace vapor) and -20oC (representing Case 2: flammable 

headspace vapor). 

 

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

L/LType A

+20°C (Case 1) 

A B −20°C (Case 2) C NatGas 

Figure 24. Relative Flammable Plume Lengths 

10.3 Conclusions from the Plume Modeling Study 

The following qualitative conclusions are drawn from this study: 

1. Fuels that are more volatile produce longer plumes in both Case 1 and Case 2 scenarios. 

Usually, fuels that are more volatile at DVPE conditions are also more volatile at low 

temperatures, but this is not always the case, as previous studies have shown [11]. 

Nevertheless, at any given temperature, the more volatile fuel at that temperature will 

produce the longest flammable plume and represent the greatest hazard. 

 

32 
33 

2. For the more dangerous situation of Case 2 (a flammable plume adjacent to flammable tank 

headspace vapors), the size and location of the plume is important. It was beyond the scope 

of this study to examine the range of fluid mechanics scenarios applicable to the various 

practical cases of interest for refueling. However, some preliminary work carried out here, 

coupled with the results of one previous study of a refueling plume in a crosswind, suggest 

that some cases might have plume lengths of such a size as to present a serious hazard of 

ignition and subsequent tank explosion [21]. Other cases might have plumes short enough to 

preclude ignition by typical ignition sources found in the immediate vicinity of refueling 

equipment. A thorough quantitative assessment is needed of the various flow regimes 

possible during refueling to establish the actual sizes of the flammable plumes possible under 

different conditions. This assessment should consider buoyancy effects, still air, crosswinds 

of various magnitudes, the variability of refueling rates and equipment geometries, etc. 

Mathematical models already exist in the fluid mechanics literature for many of these 

possibilities, but some experimental confirmation of their applicability and reliability for this 

purpose will be needed. 

III. Overall Conclusions and Recommendations 

11. Experimental Conclusions 

1. Experiments were carried out to determine the flammability of fuel tank headspace vapors as 

a function of temperature. Seven E85 fuel blends, two types of gasoline, and denatured 

ethanol were compared. For gasoline and E85, the fuels were flammable below a given 

critical temperature, as the vapor was too rich to be flammable at temperatures exceeding this 

value. Denatured ethanol was found to be flammable at room temperature and was not 

flammable below a critical temperature when the vapor became too lean to be flammable. 

2. Three E85 fuels that were blends of denatured ethanol and natural gasoline (containing 

69.4%–79.1% ethanol) had flammability behavior similar to the test gasoline with summervolatility characteristics. Three fuels that were blends of denatured ethanol and different 

commercial gasoline components (containing 80.7%–82.3% ethanol) were flammable up to 

substantially higher temperatures than the summer-volatility test gasoline. One fuel (81.4% 

ethanol) containing high-volatility commercial gasoline and isopentane had a limit 

temperature for flammability that was significantly lower than that of the summer-volatility 

test gasoline but higher that that of the high-volatility natural gasoline. 

3. The limit temperature for flammability of the E85 and gasoline test fuels was strongly related 

to their vapor pressure specifications (DVPE), but DVPE alone did not predict the ranking of 

the fuels correctly in all cases. 

4. In general, E85 is flammable at low temperatures, whereas denatured ethanol is flammable at 

warmer temperatures. If both types of fuels are stored in separate tanks at the same location, 

there is a wide range of ambient temperatures for which one or both of the tanks' headspace 

vapors will be flammable. 
34 

5. The experimental approach used in the present study (involving multiple small, closed 

test chambers and pressure rise measurements) was found to be an effective means of 

comparing the flammability of numerous fuel samples in a relatively short period. 

11.1 Conclusions on the Mathematical Modeling of Fuel Tank Flammability 

1. DVPE by itself does not reliably rank the low-temperature flammability hazards of fuel tank 

headspace vapors when comparing conventional gasolines with alcohol blends or comparing 

among alcohol blends. Differences in distillation characteristics, particularly as they 

represent the more volatile light ends, must be accounted for to obtain reliable comparisons. 

2. Existing mathematical models for gasoline primers can be used for some alcohol blend 

comparisons, provided that both DVPE and distillation data of the model used are reasonable 

approximations to those of the actual primer. Matching DVPE alone is insufficient. 

3. Within the constraints of the present study, the modeling techniques used gave reasonable 

predictions of headspace vapor flammability, provided the primer model was a good match 

for the actual gasoline primer used in the alcohol blend. 

11.2 Conclusions on the Mathematical Modeling of Plumes Emitted During 

Refueling 

1. Fuels that are more volatile at the temperature of interest produce longer plumes during 

refueling and represent greater hazards. Usually, fuels that are more volatile at DVPE 

conditions are also more volatile at low temperatures, but this is not always the case, as 

previous studies have shown. 

2. For the more dangerous situation of a flammable plume adjacent to flammable tank 

headspace vapors, the size and location of the plume is important. It was beyond the scope of 

this study to examine the range of fluid mechanics scenarios applicable to the various 

practical cases of interest for refueling. Some cases might have plume lengths of such a size 

as to present a serous hazard of ignition and subsequent tank explosion, whereas others might 

have plumes short enough to preclude ignition by typical ignition sources found in the 

immediate vicinity of refueling equipment. 

IV. Recommendations 

1. The E85 fuels used in the current study represented a wide range of blending options but did 

not provide a blend matrix in which critical parameters (such as ethanol content, vapor 

pressure, and hydrocarbon composition) were varied in a systematic manner. It is 

recommended that such a fuel blend matrix be designed for future experimental work. 

2. The current study addressed the flammability of tank headspace vapors using a strong, 

reliable ignition source. The relative hazards posed by weaker sparks that represent actual 

potential ignition sources should also be investigated. 
35 

3. In the current study, all the experiments were carried out using a relatively low (5%) tank fill 

level, as this represented a worst-case scenario for potential flammability. The effect of fill 

level on flammability should also be investigated. 

4. Although the experimental approach used in the present study was found to be effective, 

further refinements to the apparatus and experimental technique are needed before it can be 

recommended as standard test practice. 

5. For confident use of these modeling techniques for the flammability of fuel tank headspace 

vapors, the actual primer distillation data should be used to create the models. The resulting 

alcohol blend models will then be based on a faithful representation of their hydrocarbon 

components. 

6. Further studies are needed of varied candidate E85 blends for which the full primer 

information is known. Such a study could then allow well-founded conclusions to be drawn 

about the accuracy and reliability of this modeling technique for making predictions of fuel 

tank flammability hazards of candidate alcohol blends. 

7. The mathematical model, in principle, could be used to predict tank safety more reliably than 

DVPE alone, but it is too complex for routine use in the field. Further study should be 

undertaken using the model to determine if a correlation could be developed that would be 

simple enough for field use but more reliable than DVPE alone. 

8. A thorough quantitative assessment is needed of the various flow regimes possible during 

refueling to establish the actual sizes of the flammable plumes emitted from the tank under 

different conditions. This assessment should consider buoyancy effects, still air, crosswinds 

of various magnitudes, the variability of refueling rates and equipment geometries, etc. 

Mathematical models already exist in the fluid mechanics literature for many of these 

possibilities, but some experimental confirmation of their applicability and reliability for this 

purpose will be needed. 
36 

References 

1. Vaivads, R.H., Bardon, M.F., Rao, V.K., and Battista, V., “Flammability Tests of 

Alcohol/Gasoline Vapors,” SAE Paper #950401, Society of Automotive Engineers, 

Warrendale, PA, 1995. 

2. Teets, R.E. and Sell, J.A., “Calorimetry of Ignition Sparks,” SAE paper #880204, Society 

of Automotive Engineers, Warrendale, PA, 1988. 

3. Persson, H., Bremer, P., Rosell, L., Arrhenius, K., and Lindstrom, K., “Fuel Vapor 

Composition and Flammability properties of E85,” SP Report 2007:39, Brandteknik, 

Boras, 2007. 

4. Fuel data provided by Lew Gibbs, Chevron Products Company. 

5. Lattin, J., Carroll, J.D., and Green, P.E., Analyzing Multivariate Data, Thomson, Brooks 

and Cole, 2003. 

6. Bardon, M.F. and Rao,V.K., “Calculation of Gasoline Volatility,” J. Inst. Energy, v57, 

343-348, Sept 1984. 

7. Rao, V.K. and Bardon, M.F., “Estimating the Molecular Weight of Petroleum Fractions,” 

Ind. Eng. Chem., Process Des. Dev., v27, n2, April 1985. 

8. ASTM D86 “Standard Test Method for Distillation of Petroleum Products at Atmospheric 

Pressure,” ASTM International, West Conshohocken, PA, 2008. 

9. Bardon, M.F., Rao, V.K., Vaivads, R., and Evans, M.J.B., “Measured and Predicted Effect 

of the Extent of Evaporation on Gasoline Vapor Pressure,” J. Inst. Energy, v59, n441, Dec 

1986. 

10. Bardon, M.F., Nicks, G.W., Rao,V.K., and Vaivads, R., “A Vapor Pressure Model for 

Methanol/Gasoline M85 Blends,” Paper # 870366, Trans SAE, J Fuels and Lubricants, 

v96, Section 5, pp38-45, 1987. 

11. Bardon, M.F., Rao,V.K., Clark, W., and Battista, V., “Gasoline and Light Isocrackate as 

Primers for High Methanol Blends,” ASME paper # 90-ICE-24, Energy Sources 

Technology Conference, New Orleans, Jan 14-18, 1990. 
37 

12. Bardon, M.F., Rao, V.K., Battista, V., “Modelling the Flammability Hazards of Methanol 

Gasoline Blends,” presented at the VIII International Symposium on Alcohol Fuels, Tokyo, 

Nov 13-16, 1988. 

13. Bardon, M.F., Rao,V.K., and Battista, V., “Volatility and Flammability of 

Methanol/Gasoline Blends,” Paper # 892062, Trans SAE, J Fuels and Lubricants, Section 

4, v98, pp803-809, 1989. 

14. Vaivads, R., Rao,V.K., Bardon, M.F., and Battista V., “Volatility and Flammability of 

Variable Fuel Vehicle Tank Contents,” SAE paper # 932776, presented at the Fuels & 

Lubricants Meeting and Exposition, Philadelphia, Oct 18-21 1993. 

15. Vaivads, R., Bardon, M.F., Rao,V.K., and Battista, V., “Flammability of Alcohol-Gasoline 

Blends in Fuel Tanks,” presented at the 4th International Symposium on Fire Safety 

Science, Ottawa, Jun 13-17, 1994 

16. Zabetakis, M.G., “Flammability Characteristics of Combustible Gases and Vapors,” U.S. 

Bureau of Mines, Bulletin 621, Washington, 1965. 

17. LeChatelier, H. and Bourdouard, O., “Limits of Flammability of Gaseous Mixtures” Bull. 

Soc. Chim. (Paris), v19, pp483-488, 1898. 

18. ASTM D5191, “Standard Test Method for Vapor Pressure of Petroleum Products (Mini 

Method),” ASTM International, West Conshohocken, PA, 2008. 

19. ASTM D323, “Standard Test Method for Vapor Pressure of Petroleum Products (Reid 

Method)," ASTM International, West Conshohocken, PA, 2008. 

20. Wilhoit, R.C. and Zwolinski, B.J., “Physical and Thermodynamic Properties of Aliphatic 

Alcohols ,” J. Phys. Chem. Ref. Data, v2 Supp. 1, 1973, pp.1.40-1.5.4. 

21. Rao, V.K. and Brzustowski, T.A., “Tracer Studies of Jets and Diffusion Flames in CrossFlow,” Combustion Science and Technology, 1982, v27, pp.229-239. 
Appendix A. Yellow Fuel Blend 

Properties and Supplementary Information 

Table A-1. Characteristics of the Primer Used in this Blend Formulation 

(blending data supplied by Lew Gibbs, Chevron Products Company, Richmond, CA) 

HC Category Mass% by GC Mass% in HC

Paraffins 2.16 13.0 

Iso-paraffins 8.31 50.2 

Olefins 0.6 3.6 

Naphthenes 1.56 9.4 

Aromatics 3.93 23.7 

Unknowns 0.13 

Ethanol 82.31 

Methanol 0.04 

Total 99.0 100 

Water 0.992 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%) NREL (mass%)

33.6 4.87 4.92 80.96 82.29 83.86 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

FR44986 (Yellow blend) was 85 vol% EtOH and 15 vol% 4.80 psi (33.1kPa) CARBOB 

(summer-grade CARBOB). 

CARBOB is the base gasoline used in California for ethanol blending to make California Phase 2 

reformulated gasoline when ethanol is added at 5.7 vol%. 

Existing Hydrocarbon Model used in the initial simulations: Low Volatility Gasoline [6] RVP = 

4.24 psi (29.3 kPa). This primer has a similar DVPE to the summer CARBOB. 

38 
Appendix B. Green Fuel Blend 

Properties and Supplementary Information 

Table B-1. Characteristics of the Primer Used in this Blend Formulation 

(blending data supplied by Mr. Lew Gibbs, Chevron Products Company, Richmond, CA) 

HC Category Mass% by 

GC 

Mass% in 

HC 

Paraffins 2.01 14.1 

Iso-paraffins 6.9 48.4 

Olefins 0.88 6.2 

Naphthenes 1.69 11.9 

Aromatics 2.78 19.5 

Unknowns 0.1 

Ethanol 84.95 

Methanol 0.04 

Total 99.4 100 

Water 0.693 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%) NREL (mass%)

36.1 5.24 7.31 83.58 80.67 82.9 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

FR45039 (Green) was a mixture of several blends of E85 to make a 7 psi (48.3 kPa) E85, which 

makes the composition difficult to determine. It contains all the hydrocarbon components except 

for the 4.80 psi (33.1 kPa) CARBOB (i.e., it presumably contains 5.92 psi [40.8 kPa] CARBOB, 

10.66 psi [73.5kPa] CARBOB, toluene, and crude isopentane). 

CARBOB is the base gasoline used in California for ethanol blending to make California Phase 2 

reformulated gasoline when ethanol is added at 5.7 vol%. 

Existing hydrocarbon model used in the initial simulations: Indolene [6] RVP = 8.21 psi (56.6 

kPa).

39 
Appendix C. Red Fuel Blend 

Properties and Supplementary Information 

Table C-1. Characteristics of the Primer Used in this Blend Formulation 

(blending data supplied by Mr. Lew Gibbs, Chevron Products Company, Richmond, CA) 

HC Category Mass % by 

GC 

Mass % in 

HC 

Paraffins 2.36 13.9 

Iso-paraffins 8.39 49.5 

Olefins 0.71 4.2 

Naphthenes 1.61 9.5 

Aromatics 3.89 22.9 

Unknowns 0.12 

Ethanol 82.27 

Methanol 0.04 

Total 99.4 100 

Water 0.65 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%) NREL (mass %)

38.6 5.60 5.6 80.78 80.74 82.42 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

FR44987 (Red) was 85 vol% EtOH, 11.9 vol% 5.92 psi (40.8 kPa) CARBOB, 2.5 vol% 10.66 

psi (73.5 kPa) CARBOB, and 0.5 vol% toluene. 

CARBOB is the base gasoline used in California for ethanol blending to make California Phase 2 

reformulated gasoline when ethanol is added at 5.7 vol%. 

Existing hydrocarbon model used in the initial simulations: Indolene [6] RVP = 8.21 psi (56.6 

kPa).

40 
Appendix D. White Fuel Blend 

Properties and Supplementary Information 

Table D-1. Characteristics of the Primer Used in this Blend Formulation 

(blending data supplied by Mr. Lew Gibbs, Chevron Products Company, Richmond, CA) 

HC Category Mass% by 

GC 

Mass% in 

HC 

Paraffins 3.78 23.8 

Iso-paraffins 6.64 41.9 

Olefins 1.25 7.9 

Naphthenes 1.42 9.0 

Aromatics 2.77 17.5 

Unknowns 0.08 

Ethanol 83.42 

Methanol 0.04 

Total 99.4 100 

Water 0.65 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%) NREL (mass%)

58.9 8.54 8.57 81.4 82.12 84.13 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

FR44927 (White) was 85 vol% EtOH, 11.4 vol% 13.75 psi (94.8 kPa) commercial gasoline, and 

3.6 vol% crude isopentane. 

The commercial gasoline was from a Rocky Mountain refinery chosen so as to get a winter grade 

with a high vapor pressure (13.75 psi [94.8 kPa]) that did not contain ethanol. 

The crude isopentane was obtained from a refinery sphere and contains butane and other 

hydrocarbons. The batch used was not inspected, but an earlier batch had an estimated vapor 

pressure of 22 psi (151.7 kPa). 

Existing hydrocarbon model used in the initial simulations: High Volatility Gasoline [6] RVP = 

15.1 psi (104 kPa). 

41 
Appendix E. Blue Fuel Blend 

Properties and Supplementary Information 

Table E-1. Characteristics of the Primer Used in this Blend Formulation 

(blending data supplied by Mr. Lew Gibbs, Chevron Products Company, Richmond, CA) 

HC Category Mass% by 

GC 

Mass% in 

HC 

Paraffins 8.29 8.4 

Iso-paraffins 53.85 54.6 

Olefins 6.52 6.6 

Naphthenes 8.71 8.8 

Aromatics 21.33 21.6 

Unknowns 1.3 

Ethanol 0 

Methanol 0 

Total 100.0 100 

Water 0 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%) NREL (mass%)

53 7.69 7.98 0 0 0 

D86 (ARC) °C D86 (ARC)

°F

D86(NREL)

°F

IBP 36.2 97.16 95.3 

5 52.6 126.68 126.3 

10 59 138.2 137.8 

20 68.2 154.76 155.4 

30 77.3 171.14 172.4 

40 86.5 187.7 190 

50 95.5 203.9 206.4 

60 103.8 218.84 222 

70 112.2 233.96 237.2 

80 123.4 254.12 257.5 

90 145.7 294.26 298 

95 331.3 

EP 186 366.8 342.3 

Residue 1.3 3.3 

loss 0.7 0.7 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

42 
43 

FR44928 (Blue) was 59.1 vol% 5.92 psi (40.8 kPa) CARBOB and 40.9 vol% 10.66 psi (73.5 

kPa) CARBOB. 

The Blue fuel was a gasoline reference with which to compare the driveability performance of 

the various E85s. The summertime federal vapor pressure limits are 7.8 psi and 9.0 psi, 

depending on location and emissions compliance. 

CARBOB is the base gasoline used in California for ethanol blending to make California Phase 2 

reformulated gasoline when ethanol is added at 5.7 vol%. 

Existing hydrocarbon model used in the initial simulations: Indolene [6] RVP = 8.21 psi (56.6 

kPa) chosen as a representative commercial gasoline of similar DVPE. 
Appendix F. "Natural Gasoline" Fuel Blend 

Properties and Supplementary Information 

Table F-1. Characteristics of the Primer Used in this Blend Formulation 

See above D86 data 

HC Category 

Mass % by 

GC 

Mass % in 

HC 

Paraffins 28.41 28.9 

Iso-paraffins 51.38 52.3 

Olefins 3.41 3.5 

Naphthenes 10.63 10.8 

Aromatics 4.39 4.5 

Unknowns 1.44 

Ethanol 0.34 

Methanol 0 

Total 100.0 100 

Water 0 

DVPE (kPa) DVPE (psi) NREL (psi)

81.7 11.85 

D86 (ARC) °C

D86 (ARC)

°F

IBP 31.1 87.98 

5 39.8 103.64 

10 41.9 107.42 

20 44.6 112.28 

30 47.1 116.78 

40 50.3 122.54 

50 54.9 130.82 

60 61.8 143.24 

70 72.9 163.22 

80 90.9 195.62 

90 119.8 247.64 

95 

EP 166 330.8 

Residue 1.1 

loss 0.7 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

44 
45 

Existing hydrocarbon model used in the initial simulations: Light Isocrackate [11] RVP = 11.5 

psi (79.5kPa). Although not exactly the same, “Natural Gasoline” is more similar in composition 

and D86 characteristics to LIC (Appendix K) than to a commercial gasoline. 
Appendix G. Type A Fuel Blend 

Properties and Supplementary Information 

Table G-1. Characteristics of the Primer Used in this Blend Formulation 

HC Category Mass% by GC Mass% in 

HC 

Paraffins 5.12 29.9 

Iso-paraffins 9.02 52.7 

Olefins 0.38 2.2 

Naphthenes 1.86 10.9 

Aromatics 0.72 4.2 

Unknowns 0.39 

Ethanol 81.64 

Methanol 0.04 

Total 99.2 100 

Water 0.867 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%)

55 7.98 7.75/7.87 79.12 79.25 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

Existing hydrocarbon model used in the initial simulations: Light Isocrackate [11] RVP = 11.5 

psi (79.5 kPa). This was selected because the Type A blend was understood to have been blended 

using the “Natural Gasoline” (Appendix F). Although not exactly the same, “Natural Gasoline” 

is more similar in composition and D86 characteristics to LIC [Appendix K] than to a 

commercial gasoline. 

46 
Appendix H. Type B Fuel Blend 

Properties and Supplementary Information 

Table H-1. Characteristics of the Primer Used in this Blend Formulation 

HC Category Mass% by GC Mass% in 

HC 

Paraffins 5.71 29.8 

Iso-paraffins 9.96 52.0 

Olefins 0.43 2.2 

Naphthenes 2.17 11.3 

Aromatics 0.87 4.5 

Unknowns 0.58 

Ethanol 79.45 

Methanol 0.04 

Total 99.2 100 

Water 0.844 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%)

56.7 8.22 9.38 76.73 76.08/76.93 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

Existing hydrocarbon model used in the initial simulations: Light Isocrackate [11] RVP = 11.5 

psi (79.5 kPa). This was selected because the Type A blend was understood to have been blended 

using the “Natural Gasoline” (Appendix F). Although not exactly the same, “Natural Gasoline” 

is more similar in composition and D86 characteristics to LIC (Appendix K) than to a 

commercial gasoline. 

47 
Appendix I. Type C Fuel Blend 

Properties and Supplementary Information 

Table I-1. Characteristics of the Primer Used in this Blend Formulation 

HC Category Mass% by GC Mass% in 

HC 

Paraffins 7.6 29.4 

Iso-paraffins 13.49 52.1 

Olefins 0.74 2.9 

Naphthenes 2.89 11.2 

Aromatics 1.15 4.4 

Unknowns 0.61 

Ethanol 72.76 

Methanol 0.04 

Total 99.3 100 

Water 0.763 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%) NREL (vol%)

64.6 9.37 10.03 69.44 72.58/70.72 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

Existing hydrocarbon model used in the initial simulations: Light Isocrackate [11] RVP = 11.5 

psi (79.5kPa). This was selected because the Type A blend was understood to have been blended 

using the “Natural Gasoline” [Appendix F]. Although not exactly the same, “Natural Gasoline” 

is more similar in composition and D86 characteristics to LIC [Appendix K] than to a 

commercial gasoline. 

48 
Appendix J. "Denatured Alcohol" Fuel Blend 

Properties and Supplementary Information 

Table J-1. Characteristics of the Denaturant Used in Modeling this Blend Formulation 

HC Category Mass% by GC 

Mass% in 

HC 

Paraffins 0.51 30.9 

Iso-paraffins 0.88 53.3 

Olefins 0.02 1.2 

Naphthenes 0.2 12.1 

Aromatics 0.04 2.4 

Unknowns 0.15 

Ethanol 97.18 

Methanol 0.04 

Total 99.0 100 

Water 1.025 

DVPE (kPa) DVPE (psi) NREL (psi) EtOH (vol%)

20.5 2.97 96.86 

Note: Unless otherwise noted, properties shown are values measured by the Alberta Research 

Council subsequent to the flammability testing at the Nexum Laboratory. 

Existing hydrocarbon model used in the initial simulations: Light Isocrackate [11] RVP = 11.5 

psi (79.5kPa). This was selected because the ethanol denaturant was understood to have been the 

“Natural Gasoline” (Appendix F). Although not exactly the same, “Natural Gasoline” is more 

similar in composition and D86 characteristics to LIC [Appendix K] than to a commercial 

gasoline. 

49 
Appendix K. Fuel Properties for Off-the-Shelf Mathematical Gasoline 

Models 

Table K-1. Reid Vapor Pressures [21] 

D863 Data

Volume % Recovered Low-Volatility 

Gasoline Indolene 

MediumVolatility 

Gasoline 

Light 

Isocrackate 

High-Volatility 

Gasoline 

 °C °F °C °F °C °F °C °F °C °F 

0 41 106 30 86 30 86 30 86 22 72 

5 39 102 39 101 

10 72 162 57 135 46 114 43 110 36 97 

20 83 181 76 169 57 135 49 121 47 117 

30 93 199 90 194 71 160 55 130 61 142 

40 103 217 99 210 87 188 60 139 76 169 

50 114 237 107 225 101 214 65 149 91 196 

60 126 259 116 241 119 246 71 159 105 221 

70 141 286 127 261 136 276 76 169 116 241 

80 158 316 142 288 150 302 82 180 125 257 

90 186 367 164 327 184 364 91 196 141 286 

95 209 408 100 212 

EP 244 471 198 388 220 428 138 280 184 363 

Residue/Loss unknown unknown 3% 1% unknown 

Low-Volatility Gasoline [6] RVP = 4.24 psi (29.3 kPa) 

Indolene [6] RVP = 8.21 psi (56.6 kPa) 

Medium-Volatility Gasoline [11] RVP = 11.45 psi (79.0 kPa) 

Light Isocrackate [11] RVP = 11.5 psi (79.5kPa) 

High-Volatility Gasoline [6] RVP = 15.1 psi (104 kPa) 

 

50 
F1146-E(10/2008) 

REPORT DOCUMENTATION PAGE Form Approved 

OMB No. 0704-0188

The public reporting burden for this collection of information is estimated to average 1 hour per response, including the time for reviewing instructions, searching existing data sources, 

gathering and maintaining the data needed, and completing and reviewing the collection of information. Send comments regarding this burden estimate or any other aspect of this 

collection of information, including suggestions for reducing the burden, to Department of Defense, Executive Services and Communications Directorate (0704-0188). Respondents 

should be aware that notwithstanding any other provision of law, no person shall be subject to any penalty for failing to comply with a collection of information if it does not display a 

currently valid OMB control number. 

PLEASE DO NOT RETURN YOUR FORM TO THE ABOVE ORGANIZATION.

1. REPORT DATE (DD-MM-YYYY)

October 2008 

2. REPORT TYPE

NREL Subcontract Report 

3. DATES COVERED (From - To)

August 2005-December 2008 

4. TITLE AND SUBTITLE

Experimental and Modeling Study of the Flammability of Fuel Tank 

Headspace Vapors from High Ethanol Content Fuels 

5a. CONTRACT NUMBER

DE-AC36-08-GO28308 

5b. GRANT NUMBER

5c. PROGRAM ELEMENT NUMBER

6. AUTHOR(S)

D. Gardiner, M. Bardon, and G. Pucher 

5d. PROJECT NUMBER

NREL/SR-540-44040 

5e. TASK NUMBER

FC080071 

5f. WORK UNIT NUMBER

7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)

Nexum Research Corporation 

1718 Blue Mountain Road 

Mallorytown, K0E 1R0, Canada 

8. PERFORMING ORGANIZATION

REPORT NUMBER

XCI-5-55505-01 

9. SPONSORING/MONITORING AGENCY NAME(S) AND ADDRESS(ES)

National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

10. SPONSOR/MONITOR'S ACRONYM(S)

NREL 

11. SPONSORING/MONITORING

AGENCY REPORT NUMBER

NREL/SR-540-44040 

12. DISTRIBUTION AVAILABILITY STATEMENT

National Technical Information Service 

U.S. Department of Commerce 

5285 Port Royal Road 

Springfield, VA 22161 

13. SUPPLEMENTARY NOTES

NREL Technical Monitor: M. Melendez 

14. ABSTRACT (Maximum 200 Words)

An experimental and modeling study was done to determine the flammability of fuel tank headspace vapors as a 

function of ambient temperature for seven E85 (85% ethanol, 15% gasoline) fuel blends, two types of gasoline, and 

denatured ethanol at a low tank fill level. Samples in small, closed chambers were tested simultaneously in a cold 

chamber to determine flammability and pressure rise when ignited by a strong electric spark. Gasoline and E85 fuels 

were flammable below a critical temperature; above it, the vapor was too rich to burn. Denatured ethanol was 

flammable at room temperature and colder, down to a critical temperature; below it, the vapor was too lean to burn. 

The flammability limit temperature of the E85 and gasoline fuels related to the dry vapor pressure equivalent (DVPE), 

but DVPE did not reliably rank the low-temperature flammability hazards of fuel tank headspace vapors when 

conventional gasolines were compared with alcohol blends. 

15. SUBJECT TERMS

E85; ethanol blends; fuel flammability studies; ethanol emissions; fuel tank vapors; gasoline vapors; denatured 

alcohol vapors 

16. SECURITY CLASSIFICATION OF: 17. LIMITATION 

OF ABSTRACT

UL 

18. NUMBER 

OF PAGES 

19a. NAME OF RESPONSIBLE PERSON 

a. REPORT 

Unclassified 

b. ABSTRACT 

Unclassified

c. THIS PAGE 

Unclassified 19b. TELEPHONE NUMBER (Include area code)

Standard Form 298 (Rev. 8/98) 

Prescribed by ANSI Std. Z39.18