A national laboratory of the U.S. Department of Energy

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National Renewable Energy Laboratory 

Innovation for Our Energy Future 

Fuels Performance Technologies: 

Milestone FY06 9.1 

Using IQT measurements, develop 

simplified kinetic expressions for 

ignition of fuels that could be used in 

HCCI engine models

J.D. Taylor

Milestone Report 

NREL/MP-540-40755 

November 2006 

NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 
Fuels Performance Technologies: 

Milestone FY06 9.1 

Using IQT measurements, develop 

simplified kinetic expressions for 

ignition of fuels that could be used in 

HCCI engine models

J.D. Taylor

Prepared under Task No. FC06.8000 

Milestone Report 

NREL/TP-540-40755 

November 2006 

National Renewable Energy Laboratory

1617 Cole Boulevard, Golden, Colorado 80401-3393 

303-275-3000 • www.nrel.gov 

Operated for the U.S. Department of Energy 

Office of Energy Efficiency and Renewable Energy 

by Midwest Research Institute • Battelle 

Contract No. DE-AC36-99-GO10337
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Table of Contents 

Table of Contents .........................................................................................................................iii

List of Figures............................................................................................................................... iv

List of Tables ................................................................................................................................ iv

List of Acronyms.......................................................................................................................... iv

Executive Summary...................................................................................................................... v

Introduction................................................................................................................................... 1

Experimental ................................................................................................................................. 3

Results ............................................................................................................................................ 4

Discussion and Conclusions ......................................................................................................... 5

 iii
List of Figures 

Figure 1: Temperature-pressure history through the adiabatic compression stroke of 

an engine ........................................................................................................... 2 

Figure 2: Experimental data of n-heptane ignition delay at 27 test point conditions 

using the IQT .................................................................................................... 4 

Figure 3: Regression of rate parameters to 27 data points of ignition delay data for 

n-heptane........................................................................................................... 5 

Figure 4: Comparison of experimental values with predictions from the simplified 

rate model.......................................................................................................... 6 

List of Tables 

Table 1. Kinetic Parameter Values Determined for Heptane ............................................. 6 

List of Acronyms 

CIDI compression ignition direct injection 

DCN derived cetane number 

EGR exhaust gas recirculation 

HCCI homogeneous charge compression ignition 

IQT Ignition Quality Tester 

 iv
Executive Summary 

A new fuel characterization metric, based on a simplified kinetic expression, was 

developed to quantify ignition quality for low-temperature combustion applications. The 

ignition delay for a given fuel is measured over a range of temperatures, pressures, and 

concentrations of oxygen using the Ignition Quality Tester (IQT). An initial matrix of 27 

test points was selected for testing on the IQT. Three parameters are derived from these 

measurements including the derived cetane number, the activation energy (sensitivity to 

temperature), and the reaction order on concentration of oxygen [sensitivity to pressure 

and exhaust gas recirculation (EGR)]. Since intake air temperature and EGR are both 

common control methods for low-temperature combustion, these fuel index parameters 

can be used to adjust the relative controller gain response in advanced combustion 

engines. Data from engine testing at Oak Ridge National Laboratory 1

 corroborated 

results measured at the National Renewable Energy Laboratory for increased temperature 

sensitivity of biodiesel for homogeneous charge compression combustion. 

 

1

 Szybist, J., B. Bunting, C. Wildman, and J. Tassitano, “Diesel Range Fuel Effects on Single Cylinder 

HCCI” Presented at the 2006 DOE Merit Review of Advanced Combustion, Emission Control, and Fuels 

Research at National Laboratories, May 15-18, 2006. 

 v
Introduction 

In light of strict emission requirements and demand for high fuel economy, vehicle and 

engine companies are focusing research efforts on advanced combustion engines. 

Advanced combustion engines, such as homogeneous charge compression ignition 

(HCCI) engines, offer the potential of high fuel efficiency and extremely low emissions 

due to better fuel-air mixing and lower peak combustion temperatures. However, these 

engines are difficult to operate due to challenges in controlling combustion phasing (i.e., 

ignition timing) and rate of pressure rise. Furthermore, it is difficult to assess which fuel 

properties are desirable in HCCI engines, and there is no agreed-upon index to rank fuels 

for HCCI. 

In a compression ignition, direct injection (CIDI) engine, fuel is injected just slightly 

before top dead center, and the fuel ignites rapidly in the hot air environment. Fuels, then, 

can be ranked based on how quickly they ignite under a single set of representative 

conditions. This is the basis for ranking fuels by cetane number or derived cetane number 

(DCN) test methods. Even so, we have shown in past work that this single-point ignition 

measurement is not sufficient to predict the performance of a fuel under cold-start or 

other extreme variations in operating conditions for a CIDI engine. A benefit of CIDI 

engines over HCCI engines is that injection timing can be adjusted to correct ignition 

timing based on calibration maps. During the calibration phase for a CIDI engine, 

separate injection timing maps can be used for cold-start or other conditions. While 

variations in fuel properties will cause slight shifts in ignition timing, the effects are 

generally small in these types of engines. 

A challenge for implementing HCCI engines is that any variations in fuel ignition quality 

have a much larger impact on ignition timing. The rate of pre-ignition chemistry is not 

only important at a single representative condition, but rather at all temperatures and 

pressures throughout the compression stroke. This is shown in Figure 1, where the 

temperature-pressure conditions are shown during the compression stroke where each 

point represents one crank angle degree. Because the fuel is mixed with the air 

throughout the compression stroke in an HCCI engine, the chemistry begins when the 

temperature-pressure conditions are sufficient for appreciable reaction rates. For this 

reason, it is important to understand how the ignition rate varies with temperature and 

pressure. Additionally, intake air temperature and EGR are both used as “knobs” to adjust 

combustion phasing in HCCI engines. Therefore, it is important to understand how much 

the ignition timing changes with temperature and composition. 

 1
0

100

200

300

400

500

600

700

800

900

1000

0 10 20 30 40 50

Pressure (bar)

Temperature (K)

CIDI

HCCI

Fuel ignition also 

varies with % EGR

Figure 1: Temperature-pressure history through the adiabatic compression stroke 

of an engine 

In this research, we aim to develop an index of fuel ignition quality that is applicable to 

HCCI and other advanced combustion engines. Our approach is to rank fuels based on 

their ignition delay at a representative condition (e.g., the DCN), but also to determine the 

sensitivity of the ignition delay to temperature and oxygen composition. These sensitivity 

parameters are derived from a simplified reaction rate expression for ignition delay based 

on activation energy for temperature dependence and reaction order of oxygen for EGR 

and pressure dependence. 

 2
Experimental 

Ignition delays are measured using the Ignition Quality Tester (IQT), which is a constant 

volume combustion apparatus; this experimental apparatus has been explained in detail in 

a previous report 2. Each experiment consists of 47 separate injections, of which 15 serve 

to equilibrate all temperatures, and 32 are used for a statistical measure of the ignition 

delay. The standard conditions to measure DCN are a temperature of ~550°C, a pressure 

of 22 bar, and 21% oxygen charge air. 

In this set of experiments, conditions were selected to span a range of temperatures, 

pressures, and compositions. The conditions selected were as follows: T = 450, 500, and 

550°; P = 10, 20, and 30 bar; and [O2] = 15%, 18%, and 21%. These compositions of 

oxygen correspond to EGR rates of 28%, 14%, and 0% respectively. In these 

experiments, exhaust gases were simulated by using compressed gas bottles that 

contained reduced concentrations of oxygen with the balance of nitrogen. Since the IQT 

does not operate using a compression stroke (where differing heat capacities affect the 

conditions), the effect of substituting nitrogen for CO2 and H2O is negligible. A full 

factorial design of the conditions was tested, resulting in 27 test points in addition to the 

DCN measurement. 

Normal heptane was used to demonstrate the derivation of the sensitivity parameters for 

the fuel index. This approach can be applied to any real fuel or pure compound as long as 

the ignition delays are within the range measurable by the IQT. Additional testing will 

determine what those limits are, but it is expected that only very low cetane fuels (i.e., 

high-octane gasoline) will be excluded from this test method. 

 

2 Taylor, J., R. McCormick, and W. Clark, “Report on the Relationship Between Molecular Structure and 

Compression Ignition Fuels: Both Conventional and HCCI” NREL Report #MP-540-36726

(http://www.nrel.gov/vehiclesandfuels/pdfs/mp36726.pdf) August, 2004.

 3
Results 

The results of the ignition delay measurements are shown in Figure 2. The data are shown 

on an Arrhenius-like plot where the representative reaction rate is plotted versus the 

inverse of temperature. A more appropriate reaction rate would be the inverse of ignition 

delay, which would result in the values all being multiplied by negative 1. The use of an 

Arrhenius plot was chosen because it facilitates the derivation of the activation energy, 

which is proportional to the slope of the plot. 

21% oxygen (0% EGR)

0

0.5

1

1.5

2

2.5

3

3.5

1.2 1.25 1.3 1.35 1.4

1000/T (K)

ln (delay [ms])

10 bar

20 bar

30 bar

18% oxygen (~14% EGR)

0

0.5

1

1.5

2

2.5

3

3.5

1.2 1.25 1.3 1.35 1.4

1000/T (K)

ln (delay [ms])

10 bar

20 bar

30 bar

15% oxygen (~28% EGR)

0

0.5

1

1.5

2

2.5

3

3.5

1.2 1.25 1.3 1.35 1.4

1000/T (K)

ln (delay [ms])

10 bar

20 bar

30 bar

Figure 2: Experimental data of n-heptane ignition delay at 27 test point conditions 

using the IQT 

These data show the expected trends in how ignition delay varies with each of the three 

variables. As the temperature decreases from 550°C to 450°C, the ignition delay 

increases. Although the curves are not straight lines, the activation energy can be 

approximated from the slope of the lines. As pressure increases from 10 to 30 bar, the 

ignition delay decreases. Finally, as the concentration of oxygen decreases from 21% to 

15% (i.e., increasing EGR), the ignition delay increases slightly. 

 4
Discussion and Conclusions 

A simplified global rate model was fit to the data in an attempt to deconvolute the effects 

of temperature and oxygen concentration on the measured ignition delay. In this model, 

the effects of charge pressure and oxygen fraction are coupled into a molar concentration 

of oxygen rather than treating them separately. The rate expression is shown in equation 

(1): 

[ ] b 

2 exp delay ignition

1 O

RT

E Rate A a ⋅ ⎥

⎦

⎤ ⎢

⎣

⎡ = ⋅= (1) 

where A is the pre-exponential constant, Ea is the activation energy, R is the universal gas 

constant, and b is the reaction order for oxygen concentration. In equation 1, the 

concentration of oxygen is calculated by: 

[ ] 2 O2 x

RT

P O ⋅= (2) 

where xo2 is the mole fraction oxygen in the charge air. 

By applying this rate model, the ignition delay of a given fuel can be characterized by a 

three parameter fit. At first glance, it would appear that the three parameters should be A, 

Ea, and b. However, since A has little practical meaning, it is more convenient to use the 

DCN value as the constant. The two approaches are identical; and A can be calculated 

from DCN, Ea, and b using the correlation for DCN in the IQT literature. 

The value of b is regressed by maximizing the R-squared value of the linear fit shown in 

Figure 3, and Ea is determined from the slope of the best fit line. It is clear from Figure 3 

that the model is sufficient to allow for a good fit to the data, resulting in an R-squared 

value of 0.98. 

y = -6.04x + 2.86

R2

 = 0.98

-6

-5.5

-5

-4.5

-4

-3.5

-3

1.2 1.25 1.3 1.35 1.4

1000/T

ln(rate/[O2]^b)

Figure 3: Regression of rate parameters to 27 data points of ignition delay data for 

n-heptane 

 5
The values determined for heptane are shown in Table 1. 

Table 1. Kinetic Parameter Values Determined for Heptane 

DCN Ea b 

52.5 50.2 kJ/mol 0.74 

A plot of the experimentally measured ignition delay versus the predicted value using the 

model is shown in Figure 4. These parameters are a quantitative metric of how the fuel 

ignition responds to changes in temperature, pressure, and EGR. The activation energy 

can be used to adjust the gain value on an intake air temperature controller for a given 

fuel. Similarly, reaction order (b values) can be used to adjust relative boost (intake air 

pressure) or EGR levels. Furthermore, using the rate expression and correcting the preexponential value for engine specific conditions, it is feasible that this type of formulation 

can be used to predict ignition timing over a wide range of conditions in an HCCI engine 

model. 

y = 0.99x

R2

 = 0.99

0

5

10

15

20

25

0 5 10 15 20 25

Experimental Delay (ms)

Model Delay (ms) .

Figure 4: Comparison of experimental values with predictions from the simplified 

rate model 

Currently, we have only applied the complete model approach to n-heptane, but we will 

use this approach for other pure compounds and real fuels. In past research, we have 

shown that different fuels (or pure compounds) can have significantly varied activation 

energies, such as the higher activation energy shown by biodiesel (which was confirmed 

in engine experiments at Oak Ridge National Laboratory ). This work shows that we can 

now quantify the effects of activation energy independent from the change in oxygen 

density that is inherently coupled in the IQT. 

 6
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4. TITLE AND SUBTITLE

Fuels Performance Technologies: Milestone FY06 9/1—Using IQT 

measurements, develop simplified kinetic expressions for ignition of 

fuels that could be used HCCI engine models 

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NREL/MP-540-40755 

5e. TASK NUMBER

FC06.8000 

6. AUTHOR(S)

J.D. Taylor 

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14. ABSTRACT (Maximum 200 Words)

A new fuel characterization metric, based on a simplified kinetic expression, was developed to quantify ignition 

quality for low-temperature combustion applications. The ignition delay for a given fuel is measured over a range of 

temperatures, pressures, and concentrations of oxygen using the Ignition Quality Tester (IQT). An initial matrix of 27 

test points was selected for testing on the IQT. Three parameters are derived from these measurements including the 

derived cetane number, the activation energy (sensitivity to temperature), and the reaction order on concentration of 

oxygen [sensitivity to pressure and exhaust gas recirculation (EGR)]. Since intake air temperature and EGR are both 

common control methods for low-temperature combustion, these fuel index parameters can be used to adjust the 

relative controller gain response in advanced combustion engines. Data from engine testing at Oak Ridge National 

Laboratory corroborated results measured at the National Renewable Energy Laboratory for increased temperature 

sensitivity of biodiesel for homogeneous charge compression combustion. 

15. SUBJECT TERMS

fuels; fuel characterization; simplified kinetic expression; ignition quality; low-temperature combustion; exhaust gas 

recirculation; IGR; IQT 

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