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NBS TECHNICAL NOTE 666

U. S. DEPARTMENT OF COMMERCE/ National Bureau of Standards

Efflux of Gaseous Hydrogen

or Methane Fuels from the

Interior of an Automobile
NATIONAL BUREAU OF STANDARDS

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3 Part of the Center for Radiation Research.

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* Part of the Center for Building Technology.
onal Bureau of Standards

\ 2 1 1975 x rf CULL

Efflux of Gaseous Hydrogen or Methane

F Fuels from the Interior of an Automobile

J. M. Arvidson

J. Hord

D. B. Mann

Cryogenics Division

Institute for Basic Standards

\A,$> National Bureau of Standards

Boulder, Colorado 80302

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U.S. DEPARTMENT OF COMMERCE. Frederick B. Dent. Secretary

NATIONAL BUREAU OF STANDARDS Richard W Roberts Director

Issued March 1975
Library of Congress Catalog No. 75-600005

National Bureau of Standards Technical Note 666

Nat. Bur. Stand. (U.S.), Tech Note 666, 56 pages (Mar. 1975)

CODEN: NBTNAE

For sale by the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402

(Order by SD Catalog No. 013.46:666) $1.10
CONTENTS

Page

1.0 SUMMARY 1

2.0 INTRODUCTION 2

3.0 EXPERIMENTAL PLAN 4

4.0 EXPERIMENTAL APPARATUS 8

4.1 EXPERIMENTAL FACILITY 8

4.2 EXPERIMENTAL VEHICLE 10

4.3 INSTRUMENTATION DETAILS 10

5.0 TEST PROCEDURE 18

6.0 TEST RESULTS 40

7.0 DISCUSSION OF TEST RESULTS 41

8.0 CONCLUSIONS 44

9.0 RECOMMENDATIONS 45

10.0 REFERENCES 47

Appendix A. Physical Properties of H„ and CH, .... 49

Appendix B. Use of Figure 23 50

LIST OF FIGURES

Figure 1. Building in which experiment was conducted. Plastic

2 sheathing (approx. 13.4 m ) installed as a pressurerelief for building. Vehicle is situated behind

plastic 9

Figure 2. Schematic of sensor (1-8) and thermocouple (5-8)

locations 11

Figure 3. Sensor location 7 (ahead of front seat near floor).

Also shown is the duct, originating at the firewall,

2 with an area of 38.7 cm 12

Figure 4. System schematic for metering gas flow into vehicle

and typical thermocouple measuring circuit 13

Figure 5. Combustible detector locations 2, 3 and 5. Nitrogen

purge line shown in center of vehicle interior ... 14

XII
Figure 6. Sensor location 5 (near roof in center of vehicle).

The thermocouple (5) at this location is also shown . 16

Figure 7. Sensor location 8, thermocouple reference junction

(ice bath), constant temperature junction box (for

thermocouple leads) and electrical leads 17

Figure 8. Dispersion characteristics of H„ gas in the passenger

compartment of a 1970 sedan 24

Figure 9. Dispersion characteristics of H~ gas in the passenger

compartment of a 1970 sedan 25

Figure 10. Dispersion characteristics of H gas in the passenger

compartment of a 1970 sedan 26

Figure 11. Dispersion characteristics of H gas in the passenger

compartment of a 1970 sedan 27

Figure 12. Dispersion characteristics of H_ gas in the passenger

compartment of a 1970 sedan 28

Figure 13. Dispersion characteristics of H„ gas in the passenger

compartment of a 1970 sedan 29

Figure 14. Dispersion characteristics of CH, gas in the passenger

compartment of a 1970 sedan 30

Figure 15. Dispersion characteristics of CH, gas in the passenger

compartment of a 1970 sedan 31

Figure 16. Dispersion characteristics of CH4 gas in the passenger

compartment of a 1970 sedan 32

Figure 17. Dispersion characteristics of CH, gas in the passenger

compartment of a 1970 sedan 33

Figure 18. Dispersion characteristics of CH, gas in the passenger

compartment of a 1970 sedan 34

Figure 19. Dispersion characteristics of CH, gas in the passenger

compartment of a 1970 sedan 35

Figure 20. Threshold concentration (100% LEL) as a function of

time and flow rate for H

? and CH, gases in the

passenger compartment of a 1970 sedan 36

Figure 21. Effect of vent area and leakage flow rate on equilibrium concentrations of H_ gas in the passenger

compartment of a 1970 sedan 37

iv
Figure 22. Effect of vent area and leakage flow rate on equilibrium concentrations of CH, gas' in the passenger

compartment of a 1970 sedan 38

Figure 23. Potential fire and explosion hazards in the passenger

compartment of a 1970 sedan as a function of vent area and leakage flow rate 39

LIST OF TABLES

Table 1. Summary of hydrogen dispersion data for a passenger

vehicle 20-21

Table 2. Summary of methane dispersion data for a passenger

vehicle 22-23

EFFLUX OF HYDROGEN OR METHANE GASES FROM THE INTERIOR OF

A GASEOUS-FUELED AUTOMOBILE

J. M. Arvidson, J. Hord and D. B. Mann

ABSTRACT

Gasoline-powered automobiles are being converted to operate

on gaseous fuels such as H~ or CH, . These fuels are commonly

stored in containers located in the trunk of the car. Potential

leakage of these gaseous fuels into the passenger compartment

of the vehicle constitutes a safety threat. Definitive experiments were performed to identify the explosion hazards, establish

venting criteria and obviate general safeguards for H~ or CH,

fueled passenger vehicles. Appropriately designed ventilation

systems significantly reduce the safety hazards associated with

accumulated combustible gases. Vents are recommended for all

autos converted to burn H~ or CH, and may possibly be eliminated

in new cars that are designed for gaseous fuel operation. Combustible gas warning systems are recommended, at least in the

interim, for all (converted and new-design) gaseous fueled vehicles.

H_ and CH, gases appear equally safe as vehicular fuels if used in

properly designed vehicles.

Key words: Automobile; detection; dispersion; explosion; fire;

hydrogen; leakage; methane; safety; vents.

1 . SUMMARY

Conversion of gasoline-powered vehicles to operate on gaseous fuels

is becoming increasingly popular. One safety hazard with such conversions

is the potential for combustible gas to leak into the passenger compartment from its storage container in the trunk of the car. Controlled

experiments were performed to define fire and explosion hazards that

may result from this leakage. The rate of mixing within and efflux

of combustible gases (H„ or CH, ) from the passenger compartment of an American passenger vehicle were determined.

* This study was performed at the National Bureau of Standards and was sponsored, in part, by the General Services Administration.
These gas dispersion characteristics were found to depend heavily

on gas in-leakage flow rates and ventilation of the passenger compartment. Tests were conducted at injection leakage flow rates of 19 to

180 sees (standard cubic centimeters per second) and with roof (and

2 firewall) vent areas of 0.0 to 129 cm . All tests were performed with

the car doors and windows closed in both the vented and non-vented con- figurations. Combustible gas sensors were judiciously mounted within

the car interior to provide the desired dispersion data.

The test data clearly show the safety advantages of well-ventilated

passenger compartments. Roof (and firewall) vents appear highly desirable

in autos converted to operate on gaseous fuels, even if a membrane is installed to isolate the fuel system and passenger compartments. Appropriate

vents delay, and for low flows prevent, flammable concentrations of H„ or CH. in the passenger compartment. These vents also accelerate dilution

of flammable gas mixtures. Venting criteria and accompanying personnel

risks are developed herein.

Hydrogen disperses more rapidly than methane but for the same volumetric leakage flow rate identical vent areas are required to avoid

an incipient fire hazard in the car interior. Both gases appear equally

safe if used in a properly designed vehicle. Gaseous dispersion appeared

to occur mainly by buoyancy-induced flow.

It is believed that ventilation and combustible gas warning systems are highly desirable equipment for all passenger vehicles converted

to burn H„ or CH, . Autos designed for H„ or CH, fuels could conceivably

omit the ventilation system by incorporating appropriate integral design

features as commonly employed in gasoline-powered vehicles. The combustible gas detector system is recommended for all gaseous fueled cars until more operating experience is acquired.

2 . INTRODUCTION

With increasing emphasis on air quality, clean-burning fuels such

as hydrogen and methane make prime candidates for use in internal com- bustion engines [1-10]. In addition to reduced emissions a cost savings
may be realized in using gaseous fuels [9,10]. This economic advantage

is primarily due to the soaring price of gasoline and the absence of

a tax schedule on natural gas as a motor fuel. Taxation, scarcity of

gaseous fuels and concomitant increasing gas prices could eliminate

the economic incentive [10] to convert gasoline-powered vehicles to

gaseous fuel operation. Exhaust emissions are greatly reduced with

H or CH fuels; H -fueled autos [11] already satisfy 1975-1976 Federal

emission standards while CH, -fueled vehicles [10,18] apparently do not

quite meet these rigid specifications.

Due to increasing interest in gaseous fueled vehicles and numerous

fleet conversions [8,10] the safety of gaseous fueled vehicles has come under closer scrutiny. The Department of Transportation [12,13],

Massachusetts Turnpike Authority [10,14], state of California [15,16],

Compressed Gas Association [15], and General Services Administration

(this study and reference [9]) have initiated research and/or proposed

standards and regulations for the safe operation of such vehicles.

Definitive vehicle impact test data are available [12] and safety criteria

for gaseous vs gasoline-fueled vehicles have been developed [10]. To

perform a thorough analysis of fire and explosion hazards associated

with alternate fuels it is necessary to acquire mixing and dispersion

characteristics of candidate gaseous fuels. Such data are necessary

to evaluate the hazards associated with vehicle smash-ups and with

inadvertant accumulation of fuel gases inside of the vehicle. The

latter is far more subtle and is believed to be the cause of a nonfatal

incident involving a natural-gas fueled auto. Fuel apparently leaked

into the car interior and was ignited while starting the engine.

It was this event that provided impetus for the study reported

herein. The objective of this experiment is to determine the mixing

and efflux characteristics of hydrogen and methane when injected (leaked)

into the passenger compartment of a vehicle. The vehicle used for this

investigation was an American manufactured dual-fuel (natural gas/gasoline)

,

1970 sedan. This car, furnished by the General Services Administration,

is similar to the one that accidentally exploded as described above. The
dual-fuel car was modified to operate on either gasoline or natural gas.

The natural gas is stored as a liquid (at ~ 112 K) inside a 64 liter (17

gallon) dewar located in the trunk — with given weight or volume con- straints, hydrogen or methane in their liquid states provide a much

greater range for the vehicle than if stored as a gas.

3.0 EXPERIMENTAL PLAN

In this section we discuss the rationale used to design our experiment. We wish to simulate the leakage of gaseous fuel from the

trunk to the passenger compartment of the auto. Most gaseous-fuel

conversion kits for gasoline-powered autos use the trunk space for

storage, refueling, and venting operations. In some cases efforts

are made to seal-off the trunk from the passenger compartment and both

trunk and passenger spaces may be vented. Vent gas is normally ducted

outside of the trunk to the rear of the vehicle. Gas can conceivably

leak into the passenger compartment in a variety of ways: a defective,

damaged, or nonexistent trunk sealing membrane could permit entry of

gas from a defective, neglected or damaged fuel storage system, e.g.,

the result of a collision, loose connections, malfunctioning valves or regulators, improper design/construction/inspection, abuse, etc.

To cover a variety of these potential hazards we attempted to

answer the following questions: (a) What are reasonable inflow (leakage)

rates, cumulative leakage quantities and leakage injection locations?

(b) What are experimentally reasonable gas residence times in the

passenger compartment? (c) What is the effect of increased roof vent

area? (d) What is the effect of leakage gas temperature and environmental

temperature on test results? (e) What is the effect of wind direction

and velocity on the rate of efflux of gaseous fuels from the car interior?

and (f) What instrumentation is required?

(a) Inflow (leakage) : It was arbitrarily decided that the gas (H

2 or CH^)

would be injected into the passenger compartment just ahead of the rear window at the rear parcel shelf. The choice of this entry location is
slightly defensible as both H and CH, are lighter than air and tend

to rise to the top of the trunk compartment where the parcel shelf is

located. Leakage flow rates were also arbitrarily selected to range

from 19 to 180 sees (0.04 to 0.38 scfm) . These leakage rates may be

slightly high for most gaseous fuel installations but are considered

reasonable. In practice, the leakage rates will be governed by the

sealing quality of the trunk membrane. Higher or lower leakage rates

within the trunk space are certainly possible but it is assumed that

trunk vents would not permit membrane leakage rates in excess of 180

3 sees. Accordingly, only a trunkful (0.216 m ) of combustible gas was leaked into the passenger compartment during a single test. This quantity

of gas, while arbitrarily chosen, is also conducive to experimenter

safety because it limits a homogeneous mixture of H -air in the car interior to concentrations below the lower detonable limit (LDL)

(b) Gas residence times : The leakage flow rates and roof vent area,

as discussed in the next few paragraphs, are integrally related to the

residence time of the combustible gas within the passenger compartment. From both practical and experimental viewpoints it was felt

that residence times should not exceed several hours. Preliminary

experimental data indicated that this criterion could be met with the

chosen leakage flows and roof vent capacities

(c) Roof vent area : Injection of gaseous fuel causes expulsion of air,

and subsequently fuel, from the passenger compartment of the car. Fuel

escapes from the car through the roof vent and a multitude of smaller

openings by buoyant and diffusion flow mechanisms. For identical leakage

paths, flow modes, etc., H or CH, will expel air with an exit velocity,

V oc VAP ; AP = buoyant pressure (density) gradient. Thus,

e B B

V oc C Vp -p_, where C is approximately constant if a fixed volume of

e o

r Ar f o fuel gas is admitted to the car interior — C is numerically equivalent

to the square root of the fuel volume-to-area ratio, p = air density
and p = fuel density. Using the foregoing expression, buoyant forces

will favor the rapid efflux of H 9 gas by the ratio

(V eV

cc\/\^\2_ ^ 1>45

( VcH, V P A

" P CH/

4 4

Diffusion velocities also favor H~ dispersion by the ratio of « 0.63/0.20

3:1; however, diffusion flow rates are nearly negligible in comparison

with density (buoyancy) displacement rates.

While H should escape more readily than CH. from a specific auto

interior it must also inflow (leakage through trunk membrane) more

rapidly. The leakage of H_ vs CH, into the car through a fixed geometry

leak path favors CH ; volumetric leakage flow [17] of H gas will be

1.25 (viscous flow) to 3 times as large as CH, leakage (at identical

temperatures and pressures) . Thus, it appears that H_ could leak into

the car 1.25 to 3 times as fast as CH. and could leak out again 1.45

to 3 times as fast as could CH, . 4

Private consultants to the DoT (no source or reference is traceable)

have reportedly suggested that natural gas fueled cars be equipped with

2 3 1 in of roof vent area per 36 ft of car volume. Excluding trunk

3 volume the experimental car volume ~ 140 ft ; therefore, a vent area of

2 w 4 in would be prescribed. By performing buoyant flow calculations

2 we verified that a 4 in roof vent area is reasonable; a volume of air

equivalent to a trunkful of air could be expelled through the vent in less

than 5 minutes. These calculations must be considered rough estimates as the fuel concentration (and consequently fuel efflux) in the passenger

compartment is constantly changing during a test. Also, these estimates

are valid only if air and fuel expelled from the car are replaced with

make-up air at the same flow rate, i.e., inleakage of air around doors,

windows, floor boards, etc., must be sufficient to replace air and fuel

expelled through the roof vent. Otherwise, the air inleakage capacity

(dependent upon vapor-tightness of the car) will govern, or at least
influence, the rate at which the air-fuel mixture can escape through

the roof vent. This consideration prompted us to provide a fresh-air

vent through the car firewall near the bottom of the passenger compartment. In all vented tests the firewall vent area was identical to the

roof vent area. Tests were also conducted with the auto totally closed — no vents — to determine "worst case" results. Many autos already

feature fresh-air ducts and they could be easily installed at low cost

for use in favorable climates.

Our confidence in the efflux computations was rather low because

of 1) the uncertainty of existing flow paths into and out of the car

and 2) the lack of information on buoyant mixing and purging characteristics. Thus, it was decided that firewall and roof vent areas should be

varied to determine their true effect on gas residence times and efflux

rates.

(d) & (e) Environmental Conditions : In preliminary experiments with

natural gas the temperature of the injected gas was varied from 112 to

300 K with nearly identical mixing and dispersion results. On the basis

of these tests we decided to inject the gaseous fuels at room temperature and simply measure the temperature of the air-fuel mixture in the

passenger compartment. Environmental temperature was also controlled

near room temperature by placing the auto and experimental equipment

inside of a closed building, designed for hazardous experiments. By

locating the auto indoors the random influence of changing winds (on

efflux rates) was also eliminated. Our preliminary tests indicated

that changing winds were by far the most important environmental consideration — particularly in those tests that required several hours for

completion and especially in Boulder, Colorado.

(f) Essential Instrumentation : As previously explained, measurement of

gas injection temperature was not necessary and environmental temperature was held nearly constant. The barometric pressure (y 0.83 bar)
does not vary enough to affect efflux rates or justify concern. Airfuel temperature inside of the passenger compartment was continuously

measured using four strategically located thermocouples. Pressure

difference between the passenger compartment and auto environment was measured with a water-filled manometer. Gaseous fuel concentration was continually sensed at eight specified locations within the passenger

compartment using combustible gas detectors. Gas injection flow rates

and integrated (total) flow were also measured.

4.0 EXPERIMENTAL APPARATUS

The apparatus consisted of the experimental auto, appropriately

instrumented and situated within a properly equipped laboratory building.

4.1 EXPERIMENTAL FACILITY

The test vehicle was located in the test bay of a metal building

equipped with explosion-proof electrical fixtures and a roof-mounted

explosion-proof exhaust fan. Large access doors to the test bay were

sealed off with 0.015 cm thick plastic sheets to provide pressure relief

for the building in the event of an explosion in the auto, see figure

1. The roof fan was operated and all doors and windows in the building

were kept closed during a test; therefore, hydrogen (or methane)

exiting from the test vehicle escaped from the building through the

roof fan. A continuous and uniform flow of air into the building and

out through the roof fan is assured by the 'leaky' construction of this

type of prefabricated building.

Experimental control was performed in a partitioned room adjacent

to the test bay. Two steel plates (1/4-inch x 4 ft x 8 ft) were

strapped together to form a protective barrier 8 ft x 8 ft. This

barrier was secured to the partition separating the control room and

the test bay.

4.2 EXPERIMENTAL VEHICLE

The test vehicle was a standard 1970 sedan manufactured in the United

States. All doors and windows were in their normal closed positions during

all tests. The engine cowling and trunk lids were both kept in the 'up'

2 (open) positions during all tests. An unobstructed vent of 129 cm

2 (20 in ) area was cut in the center of the roof and an identical opening

was cut into the firewall. The firewall vent was ducted to the bottom

center of the vehicle just ahead of the front seat, see figures 2 and

3. Smaller roof and firewall vent areas were obtained by covering a

2 portion of the 129 cm areas. The rear seat and parcel shelf were

removed and aluminum panels (0.076 cm thick) were installed to isolate

the trunk area from the passenger compartment. The aluminum sheets were

held in place with sheet metal screws and the edges were sealed with

heat ing-and-ventilating duct tape. The rear seat was then replaced.

The system used for introducing gas (H„ or CH.) into the car and

for maintaining a precise flow rate is shown in figure 4. Flow modulation was accomplished by throttling with valve 4 or valve 5 and monitoring flowmeter 1 (FM1) or flowmeter 2 (FM2) . The vehicle was equipped

with three equally-spaced (0.317 cm dia.) copper tubes near the base of

the rear window; these gas inlet tubes were of equal length and were

manifolded to a supply line (0.635 cm dia.) connected to the control

panel.

A nitrogen purge line was used to inert the passenger compartment

upon conclusion of a test, see figures 2 and 5. The check valve located

near the tee-outlet was installed to prevent the possible back-flow of

combustible gas into the purge line.

Finally, the vehicle was instrumented as described in the following

section.

4.3 INSTRUMENTATION DETAILS

During an entire test the pressure inside of the passenger compartment, relative to pressure on the exterior of the vehicle, was monitored

10
Roof Vent*

'Vent Area:

Roof: 0.0, 32.3, 38.7, or 129cm^ Firewall: Same as roof vent area in all tests

Seal (0.076cm Al )to isolate

Trunk Area

H 2 & CH 4 Supply

Gas Inlet Ports (0.317cm dia . ) Copper

Tube

N

2 Purge

V.

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29" 30"

-22'

:30 "-^ 8,n

32"

M"r*—38"

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with a water-filled U-tube manometer. No observable pressure differences

could be detected at any time during the experiments with either FL or

CH, gases. This result was anticipated as the chosen gas leakage rates

were relatively low.

As indicated in figure 2, thermocouples were installed at sensor locations 5 to 8 to monitor the air-fuel temperature during a test. A

permanent continuous record of temperatures was produced by connecting

a strip chart recorder to the thermocouple outputs. The circuitry

schematic for a typical sensor is shown in figure 4. These copper-constantan thermocouples were placed approximately one inch from the combustible

gas detector heads, see figure 6. Reference junctions were maintained

at 0°C and all splices in electrical leads, connecting thermocouples in

the reference bath to instruments in the control room, were made in an insulated container designed for this purpose, see figure 7.

Leakage flow rates were measured using floating-ball flowmeters

3 (see FM1 and FM2 on figure 2). Total volumetric flow (0.216 m for all

tests) was measured by using a calibrated commercial dry-gas meter, see figure 2.

Solid-state combustible gas sensors were used to detect gaseous

fuel concentrations at various locations within the vehicle, see figure

2. Eight individual sensors were used, each consisting of a totally

independent system. This type of gas analyzer measures the gas concen- tration at the sensing head and does not depend upon withdrawal of a sample to a remote analyzing instrument. Consequently, the gas analysis

does not influence the mixture concentration in any way (other than the

presence of the sensing head itself) . Electrical outputs from all eight sensor leads were amplified and coupled to a multi-channel strip chart

recorder.

The overall uncertainties of the instrumentation, including calibration and readout errors, etc., are as follows: Pressure, ± 1.65 mm H„0;

15
16
17
temperature, ± 1 K; flow rate, ± 3 percent at all flow rates; total flow,

± 3 percent; gaseous fuel concentration in air, ± 3 percent of Lower

Explosive Limit (LEL) for each fuel (H„ or CH, )

# The combustible gas detectors were calibrated in-place many times

during the test program. In the beginning of the program calibration

checks were made before and after each test. This procedure was followed

to assure that no change in calibration occurred during a test. As

each detector (e.g., sensor No. 1) is a totally independent unit, all

eight sensors were individually calibrated with identical gas mixtures.

A known mixture of gas (H~ or CH. in air) was used to pressurize an evacuated bladder through a seal-off clamp. Next, a special threaded

fixture was attached to the end of the detector head located inside of

the passenger compartment. The bladder was then attached to the fixture

and the sensing head was ready for calibration. The combustible gas

analyzer, located in the control room, was zeroed and the calibration

mixture was released from the bladder. The analyzer unit then responded

to the stagnant calibration mixture by indicating a concentration level.

After detector stabilization was achieved, the analyzer instrument gain was adjusted so that the output indicator agreed with the calibration

gas mixture exposed to the detector head.

For hydrogen detection 99.2 volume percent air and 0.8 percent H~

(20% of the LEL) was used as the calibration gas mixture. Following the

initial calibration, all hydrogen tests were conducted with periodic

verifications of the initial calibration. For the series of runs involving

methane, a mixture of 97.5 volume percent air and 2.5 percent CH, (47% of

the LEL) was used to calibrate the sensing heads. Identical calibration

procedures r were used in both the H„ and CH. tests.

2 4

5.0 TEST PROCEDURE

Tests were conducted with the auto non-vented and vented (three

different roof and firewall vent areas were used) . In both vented and

non-vented tests all doors, windows and fresh-air vents were securely

closed. Identical test procedures were used for both H„ and CH, gases.
Prior to a test all electronic equipment, e.g., recorders, detectors,

thermocouple amplifiers, etc., were warmed-up enough to assure reliable

use. The test bay roof exhaust fan was turned on and the large access

doors to the test bay were opened to expose the frangible plastic curtain.

All system valves were secured in the closed position (see figure 4) and

the tests performed as follows:

1. Zero combustible gas analyzer and record pre-test thermocouple

readings (after commencement of test the thermocouple readings

are taken at 5 to 10 minute intervals throughout the run.).

2. Start strip chart recorders.

3. Open V-6 (valve 6).

4. Open V-2 or V-3 (depending on test gas desired).

5. Regulate gas pressure to approximately 20 psig (^2.2 bar abs.)

6. Select flow rate and use appropriate flowmeter with its calibration

curve. Throttle with V-4 or V-5.

Test FMl FM2

gas Range (sees) Range (sees)

H 0.0 to 657 0.0 to 103

CH. 0.0 to 251 0.0 to 42

4

7. After flow has stabilized initiate the run by simultaneously closing

V-6 and opening V-7

.

8. Monitor combustible gas sensor location No. 6 (located at gas inlet

to car)

.

9. When a measurable concentration is detected at sensor No. 6 begin

timing the run (response times ranged from ^ 2 to 10 seconds for

36, 90, and 180 sees flow rates).

10. Continuously monitor and record combustible levels at sensor locations 1 to 8 as a function of time.

11. When the required amount of combustible gas has been injected

3 (0.216 m for all tests) clos

at bottle (close V-2 or V-3).

3

(0.216 m for all tests) close V-7, open V-6, and turn off gas

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39
12. Continue to monitor and record the decay of concentration levels

for all detectors down to approximately 20 to 30 percent of LEL.

13. Terminate the run by purging the vehicle interior with nitrogen.

Open V-l, close V-6, and open V-8 — the N bottle pressure

regulator is set at approximately 25 psig (y 2.5 bar abs.).

14. Allow sufficient flow of N into the vehicle for adequate dilution

of the combustible gas.

15. Open the car doors to achieve thorough N purge and ventilation.

16. Close V-l and V-8.

With the building-controlled environment and the test procedure

outlined above test data were highly reproducible (not demonstrated

herein but verified by several repeat tests at identical flow rates and

vented-car conditions)

.

6.0 TEST RESULTS

The test data are summarized in tables 1 and 2 and are shown graphically in figures 8 to 23. The combustible sensor data designated on figures 8 to 19 correspond to those locations schematically illustrated

in figure 2. Temperature vs time plots for each test are shown on each

of figures 8 to 19 and the thermocouple locations (5 to 8) are coincident

with the mixture sensor locations.

Figures 8 to 13 illustrate the dispersion characteristics of hydrogen

at three flow rates and with two venting configurations (non-vented and

2 with 38.7 cm vents). The methane test results shown in figures 14 to 19 are at the same flow rates and venting conditions stated above for

hydrogen. This allows a direct comparison of hydrogen and methane dispersion data. Figures 9, 11, and 13 have an abscissa incremented in

20 minute intervals while all other figures have 10 minute increments.

The equilibrium concentration (percent of LEL) is given as a function

of leakage flow rate and vent area in figures 21 and 22 for H_ and CH,

.

From these data relationships between vent area, leakage flowrate and

equilibrium gas mixtures for H and CH, may be derived. One such

40
relationship is illustrated on figure 23. The equilibrium concentration

as designated on figures 21 to 23 is the steady-state gas mixture achieved

under sustained leakage flow.

Figure 20a shows the time required for any one of the eight sensors

to reach 100 percent of LEL after initiation of the test (vented or nonvented) ; figure 20b gives the time required (after leakage flow cutoff)

for all sensors to fall below 100 percent of LEL.

7.0 DISCUSSION OF TEST RESULTS

The data shown on figures 8 to 19 are considered representative

of the dispersion behavior of hydrogen and methane when released inside

of a passenger vehicle. These figures illustrate how quickly the LEL

is reached and in some cases how long combustible gas concentrations

can remain inside the vehicle. Several repetitive tests with each gas

exhibited remarkable reproducibility and strengthened our confidence in

the test results. While it is recognized that these data are precisely

applicable to but one auto, the test vehicle, it is felt that the results

are generally applicable to all passenger vehicles.

Some of the more important trends obviated by careful study of

2 figures 8 to 19 are: (1) vents in the roof and firewall (38.7 cm each)

effectively delay explosive accumulations of H„ or CH, ; (2) vents also

promote more rapid dilution of explosive gas mixtures (H~-air or CH.-air)

after leakage flow is stopped; (3) the lowest leakage flows (probably

most typical of loose fittings, cracked welds, tubing splits, trunk

membrane leaks, etc.) do not produce combustible gas mixtures in tests

2 with 38.7 cm vents, see figures 8 and 14; (4) the best location for an early-warning combustible gas detector is at the top-center of the

vehicle near the roof vent (sensor 5 in these tests)

.

These tests show that non-vented vehicles may retain hazardous gas

mixtures for an hour or more — about an order of magnitude longer than

for vented vehicles. This result suggests that a ventilation system

41
—forced or natural (as in these tests) — is very desirable for vehicles

converted to operate on gaseous fuels. Vehicles designed to operate on gaseous fuels could perhaps expunge this ventilation criterion by

proper placement of a well-vented fuel system exterior to a well-sealed

vehicle. Even so early-warning explosive gas detectors may be well advised,

To compare H~ and CH, fuels we must compare the test results from

identical tests, e.g., pair figures 8 and 14, 9 and 15, etc. At the

lowest flow rate we observe similar results for the two gases whether

the vehicle is vented or closed (figures 8 and 14, 9 and 15). H

disperses more rapidly than CH. with intermediate leakage into a vented

vehicle (figures 10 and 16); however, the two gases behave similarly with

intermediate leakage flow into a closed vehicle (figures 11 and 17).

At the highest flow rate H~ disperses more rapidly than CH, in both the

vented and closed vehicles (figures 12 and 18, 13 and 19). These observations support the contention that buoyant flow mechanisms are predominant and dispersion by diffusion is nearly negligible — refer to

rationale in section 3.0.

A generalized summary of figures 8 to 19 is shown on figure 20.

A threshold gas concentration of 100 percent of LEL at any location

within the car is used to illustrate mixing and efflux rates for the

two gases. Higher leakage flow rates create combustible concentrations

very rapidly in both the vented and non-vented vehicle configurations,

see figure 20a. Figure 20b clearly illustrates the advantages of

vehicle vents. Again, it is not surprising that H„ reaches, and

decays below, LEL concentrations more rapidly than CH, . Buoyant forces

favor the rapid dispersion of hydrogen over methane by the ratio of

1.45:1 and diffusion velocities favor H_ by the ratio of 3:1.

Tests were also performed with vent areas (roof and firewall) of

2 32.3 and 129 cm . These tests were designed to evaluate the effect

of vehicle vent area on mixing and efflux rates at various leakage flow

rates. The time parameter was eliminated from these tests by maintaining

42
leakage flow steady until all eight gas sensor outputs were constant,

i.e., an equilibrium (quasi steady-state) gas concentration was established

at each sensor location in the passenger compartment. Thus it is possible

to determine the vent area required, at each leakage flow rate, to

avoid combustible gas mixtures at any location inside of the vehicle.

Such data, for H and CH, , are shown in figures 21 to 23. Larger

vent areas accomodate larger leakage flow rates without reaching 100

percent of the LEL as illustrated in figures 21 and 22. Sensor 5, at

the roof vent, exhibited the highest concentration of combustible gas

in all of our tests. Consequently, data from sensor 5 as shown in

figures 21 and 22 .were used to derive vent area requirements for vehicles

subjected to potential H9 or CH, leaks. These vent requirements are plotted for two separate safety criteria on figure 23. The 20 percent

of LEL criterion was selected because it is the industrially accepted

limit for warning personnel of explosive hazards. The 100 percent of

LEL criterion provides a true indication of incipient fire and explosion

hazard.

Figure 23 shows that identical H? and CH, leakage flows require

the same vent areas to maintain combustible gas concentrations below

20 percent of the LEL. Also, the incipient fire hazard (100 percent

of LEL at sensor 5 near the roof vent) is almost identical for H? and

CH, at the same leakage flows and vent areas. These leftmost boundaries

of the shaded (high risk) regions on figure 23 are considered mainly as a fire hazard because only a small pocket of combustible gas exists near the roof vent. With the same vent area and higher flow rates,

additional gas sensors reach the LEL and the fire and explosion hazards are greatly amplified. The width of the shaded regions on this figure

indicates the tolerance of a vented vehicle to increased leakage flow

of H^ or CH, . The narrow bandwidth for CH, means that slightly increased

2 4 h

leakage flows in a specific vehicle result in explosive gas mixtures

throughout the upper region of the passenger compartment. By comparison,

43
H_ leakage must be significantly increased to fill the entire upper

region of the car interior with a flammable H -air mixture. An example,

explaining the use of figure 23 is given in Appendix B.

Figure 23 was composed from test data acquired with two vent areas

2 (32.3 and 129 cm ); therefore, the straight-line boundaries shown may

exhibit some slight curvature with the benefit of additional test data.

It was felt that additional tests, on this specific vehicle under the

prescribed test conditions, were unnecessary.

8.0 CONCLUSIONS

H„ and CH, gases were deliberately leaked into the passenger

compartment of a 1970 sedan. Tests were conducted with the car doors

and windows closed and in vented and non-vented configurations. Leakage

flow rates ranged from 19 to 180 sees and vehicle vent areas varied

2 from 0.0 to 129 cm . Combustible gas sensors, placed inside of the

vehicle, were used to detect mixing and efflux characteristics of the

combustible gases. The test results reported here are specifically

applicable to but one vehicle; however, the results are believed to be

characteristic and generally applicable to all passenger vehicles.

1. Vents in the roof and firewall effectively delay, and for low flows

prevent, explosive accumulations of H_ or CH, in the passenger

compartment. Low leakage flows considered typical of loose fittings,

cracked welds, membrane leaks from well sealed and vented trunks,

etc., do not produce flammable gas mixtures in tests where the

2

vent area > 38.7 cm .

2. Vents promote more rapid dilution of combustible gas mixtures after

leakage flow is stopped. Non-vented vehicles may hold flammable

mixtures for over an hour — nearly 10X the holding times for

vented vehicles.

3. High leakage flows of H or CH, produce combustible concentrations

very rapidly in both vented and non-vented vehicles. H„ reaches

and decays below flammable concentrations more rapidly than CH,

.

44
4. For identical leakage flow rates of H~ or CH, , identical vent

areas are required to avoid an incipient fire .hazard in the car interior (even though H_ disperses more rapidly) . Criteria for

acceptable vent areas — with varying levels of risk — are defined.

5. Buoyant flow mechanisms (rather than diffusion) were determined

to be the dominant mode of gaseous dispersion.

6. The prime location for an early-warning combustible gas detector

is at the topmost point of the vehicle roof, near the roof vent.

7. On the basis of these tests we conclude that neither H_ nor CH.

I 4

gas is safer than the other as a vehicular fuel and that the two

gases appear equally safe if used in properly designed vehicles.

These experimental results indicate that progressive development

of H„ and CH, fueled autos can be actively pursued with the full

expectation that such autos (properly engineered conversions and

new designs) can ultimately gain the acceptance of regulatory

organizations and the general public.

9 . RECOMMENDATIONS

This study indicates that gasoline-powered vehicles converted to

burn gaseous fuels should be equipped as follows: (1) The fuel system

should be adequately designed for fuel containment in the event of

collision; (2) the car trunk should be adequately vented if the fuel

tank is located in the trunk; (3) the fuel tank vent should be ducted

to the far rear exterior of the vehicle; (4) and a trunk membrane should

be provided to isolate the trunk (or fuel) and passenger compartments.

Optional, but highly desirable equipment for such vehicles includes a positive ventilation system and an early-warning combustible gas detector. Guidelines for the design of appropriate ventilation systems

are given herein. If used, a combustible gas sensor should be located

in the passenger compartment at the highest point of the roof (vented

or non-vented)

.

45
It is recognized that auto heating and air conditioning systems are not capable of handling the increased heating and cooling loads

imposed by the large vents prescribed in this paper. A more practical

solution is to locate the fuel supply system, as with conventional

gasoline-powered vehicles, on the exterior of the vehicle. The fuel

system (£L or CH, ) should be an integral part of the vehicle design

and located so that it can be well vented, readily serviced, protected

from foul weather and collision, and isolated from the passenger com- partment. With this integral design feature it seems plausible to omit

the roof and firewall vents; however, until we gain more experience

with H_ and CH. fueled cars it also seems advisable to install a com2 4

bustible gas warning system.

46
10 . REFERENCES

1. Billings, R., Hydrogens potential as an automotive fuel, Cryogenics

and Industrial Gases, Vol. 9, No. 1, 23-5 (1974).

2. Billings, R. E., and F. E. Lynch, History of hydrogen-fueled internal

combustion engines, Billings Energy Research Corp., Publication No.

73001 (1972).

3. Weil, K. H., The hydrogen I.C. engine- its origins and future in

the emerging energy-transportation-environment system, Proceed,

of the 7th IECEC, 1355-1363 (1972).

4. Murray, R. G., R. J. Schoeppel, and C. L. Gray, The hydrogen

engine in perspective, Proceed, of the 7th IECEC, 1375-81 (1972).

5. Swain, M. R., and R. R. Adt Jr., The hydrogen-air fueled automobile,

Proceed, of the 7th IECEC, 1382-87 (1972).

6. Corbet t, P. F., Natural gas and the environment - after combustion,

Outlook for Natural Gas - A Quality Fuel , from the Proceedings of

the Institute of Petroleum Summer Meeting, held at the Palace Court

Hotel, Bournemouth, England, p. 221-235 (6-9 June 1972).

7. Anon., Emission reduction using gaseous fuels for vehicular

propulsion, Final report on contract no. 70-69, Institute of

Gas Technology, IIT Center, Chicago, 111. 60616, (June 1971).:

Also PB201 410, Available from NTIS.

8. Johnson, E. F., Fire protection developments in CNG-fueled vehicle

operations, Fire Journal Vol. 6^6, No. 6 (Nov. 1972).

9. Anon., Pollution reduction with cost savings (report on the General

Services Administration's dual-fuel experiment), GSA DC71-10828

(1971)

.

10. Shooter, D. and A. Kalelkar, The benefits and risks associated

with gaseous fueled vehicles, report to the Massachusetts Turnpike

Authority, Arthur D. Little Case 74400-2 (May 1972).

11. Van Vorst, W. D., (Univ. of Calif., Los Angeles, Calif.) private

communication to the authors; and Bush, A. F., and Van Vorst, W. D.,

The UCLA hydrogen car, Book, Advances in Cryogenic Engineering J.9,

Ed. K. D. Timmerhaus, pp. 23-27 (Plenum Press, Inc., New York, N.Y.

,

1974).

12. Enserink, E., Dual-fuel motor vehicle safety impact testing,

DoT/HS-800 622, Available from NTIS, (Nov. 1971).

47
13. Docket No. HM-115; Notice No. 74-3, Transportation 49 CFR Parts

172, 173, 177, 178, 179, Cryogenic Liquids, U.S. Department of

Transportation, Office of Hazardous Materials, 2100 - 2nd St., S.W.,

Washington, D.C. 20590, provided by A. J. Mallen in private communication to the authors.

14. Ryan, L. E., Massachusetts Turnpike Authority, Suite 3000, Prudential

Center, Boston, Mass. 02199, private communication to the authors.

15. Proposed revisions to California regulations for LNG motor vehicle

fuel containers and regulations for compressed and liquefied gas

fuel systems, California Administrative Code, Title 13, Chapter 2,

Article 2, Sections 930-936, Department of California Highway

Patrol, P.O. Box 898, Sacramento, Calif. 95804: provided by W. M.

Heath in private communication to the authors. Also the Compressed

Gas Association, 500 Fifth Ave., New York, N.Y. 10036, is cooperating

with the State of California and DoT to develop specifications for

the design and construction of LNG motor fuel containers.

16. An editorial staff summary of California regulations for LNG auto

installations, Cryo. Tech., 132-134 (July-Aug. 1972).

17. Hord, J., Correlations for predicting leakage through closed

valves, Nat. Bur. Stand. (U.S.), Tech. Note 355 (1967).

18. Hill, J. E. , and Didion, D. A., Comparative performance of two

postal service vehicles operated on gasoline, compressed natural

gas, and propane, ASME paper 74-WA/HT-26, presented at the ASME

winter annual meeting, New York, N.Y. (Nov. 17-22, 1974); see also,

Hill, J. E., and Didion, D. A., Comparison of power output and

exhaust pollutants of two postal service vehicles operated on three hydrocarbon fuels, Nat. Bur. Stand. (U.S.), NBS-IR 74-460

(Sept. 1973).

19. Sparks, L. L., R. L. Powell, and W. J. Hall, Reference tables for

low-temperature thermocouples, Nat. Bur. Std's. (U.S.), NBS

Monograph 124 (June 1972)

48
Appendix A. Physical Properties

Physical Property

Molecular weight

Triple point pressure, atm

Triple point temperature, K

Normal boiling point (NBP) temperature, K

Critical pressure, atm

Critical temperature, K

3 Critical density, g/cm

3 Liquid density @ NBP, g/cm

3 Vapor density @ NBP, g/cm

3 Solid density @ triple point, g/cm

3 Gas density @ NTP, g/m

Density ratio: NBP liquid-to-NTP gas

Heat of fusion, J/g

Heat of vaporization @ NBP, J/g

Heat of combustion (low) , kj/g

Limits of flammability in air, vol. %

Limits of detonability in air, vol. %

Stoichiometric composition in air, vol. %

Minimum energy for ignition in air, J

Ignition temperature, K

Flame temperature in air, K

Percentage of thermal energy radiated to

surroundings from burning liquid pool, %

Flame velocity in NTP air, cm/s

Quenching gap in NTP air, cm

f H and CH.

1 4

P-H

2

CH.

4

2.016 16.043

0.0695 0.1159

13.803 90.680

20.268 111.632

12.759 45.355

32.976 190.53

0.0314 0.1628

0.0708 0.4226

0.00134 0.00182

0.0865 0.4872

83.764 651.19

845 649

58.23 58.47

445.59 509.88

119.96 50.02

4 to 75 5 to 15

18 to 59 —

29.53 9.48

0.02 0.29

858 810

2318 2148

25 23

265 39

0.06 0.22

NBP = normal boiling point

NTP = 1 atm and 293.15 K

49
Appendix B. Use of Figure 23

For this example a passenger vehicle similar to the one tested in

2 this report will be chosen; a vent area of 50 cm is assumed. As one progresses from zero to increasing leakage flow rates (on figure 23)

the first curve encountered is labeled "Preferred Vent Areas (low risk)".

The corresponding leakage flow rate (with H or CH.) is approximately

2 9 sees at 50 cm vent area. This means that a sustained leakage flow

rate of 9 sees will produce combustible gas concentrations in the

passenger compartment which are < 20% of LEL.

These statements are true only for steady-state leakage and stated

concentration levels are attained after equilibrium conditions have

been established.

2 As the leakage flow rate is increased, maintaining 50 cm of vent

area, the next set of curves encountered on figure 23 are labeled

"Minimum Vent Area (high risk)". The H„ and CH. curves are almost

2 4

coincident at this point. Leakage flow rate is approximately 69 sees

for either H~ or CH • this means that under equilibrium conditions a

2 4

concentration level of 100% of LEL can be expected at sensor 5 in the

passenger compartment.

When the leakage flow rate exceeds 69 sees for the same vent area

2

(50 cm ) , the volume of combustible gas (> 100% of LEL) inside of the

vehicle increases. The curve labeled "Explosion hazard for entire upper

region of car interior" constitutes an arbitrarily chosen boundary at

which the region of the car interior from head-level to ceiling is

occupied by combustible gas (> 100% of LEL) . This limit is reached

at approximately 78 sees for CH, and 117 sees for H .

If increased leakage flow rates are realized (maintaining 50 cm vent area) we go from the shaded to the unshaded region on figure 23.

Therefore, an increasing portion of the vehicle interior volume is

being filled with combustible gas until, possibly, the total volume is

flammable (> 100% of LEL)

50
NBS-114A (REV. 7-73)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO.

NBS Technical Note 666

2. Gov't Accession

No.

3. Recipient's Accession No.

4. TITLE AND SUBTITLE

Efflux of Gaseous Hydrogen or Methane Fuels

from the Interior of an Automobile

5. Publication Date March 1, 1975

6. Performing Organization Code

7. AUTHOR(S)

J. M. Arvidson, J. Hord and D. B. Mann

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

8. Performing Organ. Report No.

10. Project/Task/Work Unit No. 2750154

11. Contract/Grant No. OF 4KE0 23, Case 66551

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Mr. R. L. Ullrich

Director, Motor Equipment Research & Technology Division

FZR; GSA-FS (FZR)

Crystal Mall Bldg. 4, Room 323, Washington, D.C. 20406

13. Type of Report & Period

Covered

Final

14. Sponsoring Agency Code

12. Co-Sponsor

Cryogenics Division, National Bureau of Standards, Department of Commerce

Bnnlfipr, Cnlnradn 8030?.

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

Gasoline-powered automobiles are being converted to operate

on gaseous fuels such as H~ or CH, . These fuels are commonly

stored in containers located in the trunk of the car. Potential

leakage of these gaseous fuels into the passenger compartment

of the vehicle constitutes a safety threat. Definitive experiments were performed to identify the explosion hazards, establish

venting criteria and obviate general safeguards for H_ or CH,

fueled passenger vehicles. Appropriately designed ventilation

systems significantly reduce the safety hazards associated with

accumulated combustible gases. Vents are recommended for all

autos converted to burn H„ or CH, and may possibly be eliminated

in new cars that are designed for gaseous fuel operation. Combustible gas warning systems are recommended, at least in the

interim, for all (converted and new-design) gaseous fueled vehicles.

H_ and CH, gases appear equally safe as vehicular fuels if used in

properly designed vehicles.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Automobile; detection; dispersion; explosion; fire; hydrogen; leakage; methane;

safety; vents.

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