The document discusses the impact of variations in fuel cetane number on the combustion and performance of aviation diesel engines. Six specially formulated jet fuels with cetane numbers ranging from 30 to 55 were tested in a multi-cylinder turbocharged diesel engine. The results show that lower cetane number fuels cause significantly retarded combustion at low engine loads, with the effect being most pronounced at a descent idle condition. At high engine loads, differences between fuels were minimal. Lower cetane number fuels led to unstable and inefficient combustion at low loads that could damage the engine over time if not addressed.

1. American Institute of Aeronautics and Astronautics
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Combustion and Performance Sensitivity to Fuel Cetane
Number in an Aviation Diesel Engine
Michael T. Szedlmayer1
, Chol-Bum M. Kweon2
U.S. Army Research Laboratory, Aberdeen Proving Ground, MD 21005, USA
Kurt M. Kruger3
Environmental Research Group, Aberdeen Proving Ground, MD 21005, USA
Joseph A. Gibson4
, Ross H. Armstrong5
, Christopher A. Lindsey6
U.S. Army Aviation and Missile Research, Development and Engineering Center, Redstone Arsenal, AL 35898, USA
Rik D. Meininger7
Science Applications International Corp., Redstone Arsenal, AL 35898, USA
Newman B. Jackson8
Avion Solutions Inc., Redstone Arsenal, AL 35898, USA
and Andrew V. Giddings9
Westar Aerospace & Defense Group, Inc., Redstone Arsenal, AL 35898, USA
The objective of the following study is to quantify the impact of variations in fuel cetane
number on the combustion and performance of multi-cylinder, turbocharged, direct-
injection, diesel engines. In particular, the study focuses on engines designed for use in
unmanned aerial vehicles. For this study, the combustion properties of a typical engine are
evaluated based on in-cylinder pressure, heat release rate, and accumulative heat release.
Four different engine speed and load combinations are presented, matching the typical flight
profile of an aviation diesel engine. Engine tests are conducted using six different batches of
specially-formulated Jet A fuel, with cetane numbers ranging from 30 to 55. The fuel cetane
number is found to have the greatest impact on low load cases, such as Ground Idle and
Descent Idle. As the cetane number is decreased, the start of combustion is increasingly
retarded. In many cases the pilot fuel injection does not combust until after the main fuel
injection. In a small set of cases, combustion is nearly undetectable. The retarded start of
combustion at low-load, low-cetane number cases leads to enhanced fuel-air mixing, and an
increase in the premixed-phase combustion. This in-turn increases the peak heat release rate
to high levels, and has the potential to cause engine damage or shorten the engines time before
overhaul.
1
Mechanical Engineer, Engines Research Team, Propulsion Div., ARL VTD, Bldg 4603 APG, MD, AIAA Member
2
Team Lead/Research Aerospace Engineer, Engines Research Team, Propulsion Div., ARL VTD, Bldg 4603 APG,
MD, AIAA Member
3
Engineering Technician, Engines Research Team, Propulsion Div., ARL VTD, Bldg 4603 APG, MD
4
Project Engineer, Small and Certified Engines Team, Propulsion Division, RDECOM AMRDEC
5
Team Lead/Aerospace Engineer, Small and Certified Engines Team, Propulsion Division, RDECOM AMRDEC
6
Systems Engineer, Small and Certified Engines Team, Propulsion Division, RDECOM AMRDEC
7
Aviation Systems Engineer, Unmanned Aircraft Systems Division, RDECOM AMRDEC
8
Principal Engineer, Unmanned Aircraft Systems Division, RDECOM AMRDEC
9
Unmanned Aircraft Systems Subject Matter Expert, Contractor Supporting US ARMY UAS-PO

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Nomenclature
AC = Alternating Current
AHR = Accumulative Heat Release
CN = Cetane Number
DOD = United States Department of Defense
FADEC = Full Authority Digital Engine Controller
JP-8 = Jet propellant-8
NMEP = Net Indicated Mean Effective Pressure
NSFC = Net Indicated Specific Fuel Consumption
PQIS = Petroleum Quality Information Survey
SOC = Start of Combustion
UAV = Unmanned Aerial Vehicle
I. Introduction
urbocharged diesel engines offer several advantageous features for aviation propulsion applications, including
high efficiency, reliability, fuel flexibility, and reduced fuel consumption. Unmanned aerial vehicles (UAV) in
particular can benefit from these features due to their missions often being long in range and duration. Furthermore,
the fuel flexibility aspect of the engines allows them to meet the requirements of the US Department of Defense’s
(DOD) Single Fuel Forward Policy1
, which specifies that all vehicles must operate nominally on Jet Propellant-8 (JP-
8) and equivalent jet fuels. While jet fuel does allow diesel engines to function, the optimization of these engines often
suffers because the ignition quality of jet fuels is not regulated. Most aviation diesel engines were derived from
terrestrial applications, and have much of the same hardware and operating requirements. One such requirement for
diesel engines is that the ignition quality of the fuel, or cetane number (CN), must meet a critical minimum value. In
the case of diesel fuel for on-highway ground vehicles, the minimum cetane number is 402
.
A number of aviation diesel engines have recently experienced malfunctions in flight which were attributed to
erratic combustion. One suspected cause of the erratic combustion is poor ignition quality of specific batches of jet
fuel. This hypothesis was supported by the 2012 Petroleum Quality Information Survey (PQIS) findings on cetane
index3
, which is a calculated approximation of the cetane number. The PQIS showed that, depending on global location
and time of year, the cetane index at airfields ranged from as low as 30, to over 50, as shown in Figure 1. According
to a limited amount of existing literature, such a range would be expected to have a detrimental impact on the
combustion and overall performance on the engine.
An early study of the effect of fuel parameters, including volatility, 90% distillation temperature, and cetane
number, was conducted by McMillan and Halsall4
. Three different cetane numbers, 35, 40 and 45, were achieved for
diesel fuel from a single feedstock by controlling the aromatic content of the fuel. The impact of the cetane variation
was analyzed at three different engine speeds in a single-cylinder direct-injection diesel engine. As the cetane number
was decreased, the authors found that emissions of particulates, unburned hydrocarbons, carbon monoxide, and
nitrogen oxides all increased. Ignition delay, peak pressure and maximum pressure rise rate were found to increase.
T
Figure 1: Cetane index with respect to regions around the world

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Of the parameters investigated, cetane number was determined to have the strongest influence on engine emissions
and performance.
In a 2006 study, Schihl et al.5
, analyzed the impact of cetane number variation on liquid sprays of diesel surrogate
fuels. It was found that the lower ignition quality fuels lead to much higher pressure rise rates and the possibility of
spray over penetration. In a 2011 study, the same authors6
compared the performance of a single-cylinder engine using
JP-8 to the same engine using off-road DF-2 diesel. Even though the cetane number was maintained at 43 for both the
JP-8 and the DF-2, the JP-8 ignition delay was found to be slightly shorter. This was attributed to differences in the
evaporation rate of the fuel and the initial fuel injection rate. The paper calls for experimentation with low cetane
number fuels, in the range of 30-35, to quantify the differences between the impact of ignition quality and evaporation
rate on engines originally calibrated for use with diesel.
Murphy and Rothamer7
studied cetane variability by operating a single-cylinder engine at high load using a range
of different fuels. These included DF-2 with cetane number 43, Jet A with cetane number 47, and two mixtures of Jet
A and Fischer Tropsch JP-8, with cetane numbers 36 and 42. For the fuels with similar cetane numbers, maximum
pressure rise rate, ignition delay, and premixed burn fractions were found to be nearly identical, despite large
differences in the mass of evaporated fuel. The authors theorize that if the amount of air entrained into the fuel spray
and the cetane numbers are equal for two different fuels, then they will have similar premixed burn fractions and
maximum pressure rise rates. Previous work suggesting that the maximum pressure rise rate is proportional to the
mass of fuel evaporated conflicted with the author’s findings. The authors conclude that cetane number is the dominate
fuel property controlling the pressure rise rate.
The above literature, and a number of other sources8-10
, indicate several undesirable outcomes that result from low
cetane number. These include increased ignition delay, increased pressure rise rate, emissions, and over-penetration
of fuel spray. The following study will extend these results to a real multi-cylinder engine used by the DOD, and a
wider range of cetane numbers, matching the values that can be expected in operation.
II. Experimental Methods
Engine performance and combustion data was collected from an in-line 4-cylinder, 4-stroke, direct-injection,
turbocharged, diesel engine, representative of those typically used by the DOD. Testing was conducted at the new
Small Engine Combustion Research Laboratory at the US Army Research Laboratory. An alternating current (AC)
dynamometer was used to load the engine at four different operating conditions, representing a typical flight profile
of a UAV. Detailed engine specifications are not included because they contain proprietary information.
A schematic of the test bench is shown in Figure 2. Engine braking torque was measured by an in-line HBM torque
meter. The engine was instrumented with four Kistler 6058A in-cylinder piezoelectric pressure transducers, to provide
instantaneous measurements of cylinder pressure. Data acquisition was synchronized through use of a Kistler
2614CK1 crank angle encoder with a resolution of 720 pulses per cycle. In-cylinder pressure data was ensemble-
averaged over 110 cycles.
Figure 2: Schematic of the engine bench
Intake
Surge
Tank
Exhaust
Surge
Tank
Regulator
Flow
Meter
Exhaust
FanFiltered &
Dried
Compressed
Air Supply
Emissions
Bench
OpacimeterECU
Computer
Dynamometer
Workstation
Coolant Heat
Exchanger
Fuel
Bench
Charge Air
Cooler
Fuel
Tanks
National Instruments
Data Acquisition and
Control System
Vent
O2
Sensor
Signal Wire
Fluid Connection
Pulse
Multipier
300 hp
AC Dynamometer
Pressure
Charge
Amplifier
Accelerometers
Cyl Pressures
Thermocouples
Accel.
Signal
Conditioner
Exhaust
Throttle
Valve
Pressure
Control Valve
Optical Encoder
Turbocharger

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For isolating and quantifying the impact of cetane number, six specialty blends of Jet A fuel were acquired. This
fuel set varies from those of previous studies because the fuels were specifically formulated to vary cetane number
only. The majority of other studies use cetane improver or mix in other fuels to achieve a range of cetane numbers.
Consequently, parameters such as the density, viscosity, volatility or heating value may vary substantially. Table 1
describes the fuel properties of each of the fuels used in this study, which are labeled nominally CN 30, through CN
55. The first three fuel batches were created with a base stock of CN 30. Cetane numbers of 45, 50 and 55 were
achieved by lowering the aromatic content of the fuels from approximately 10.5% to 7%, without using cetane
improver. This indicates that for a fixed aromatic content, the cetane number can be expected to vary by approximately
10 points. Figure 3 shows the distillation profile of all of the tested fuels.
Engine, fuel injection and turbo-charger control was performed using the manufacturer’s original Full Authority
Digital Engine Controller (FADEC). The FADEC accepts a single voltage signal representing throttle command and
uses that to determine fuel rail pressure, fuel injection timing, fuel quantity, and intake manifold pressure according
to a power curve programmed into the FADEC. Four points along this curve were selected for the study at hand. These
include: Ground Idle; Full Power Ascent; Cruise; and Descent Idle. Ground Idle, Cruise and Descent Idle each use a
preinjection and single main fuel injection. The Full Power Ascent case uses only a main injection. The main
difference between the test conditions and real flight conditions was that the tests presented here were all conducted
at sea level ambient conditions.
TEST
ASTM
METHOD
UNITS CN 30 CN 35 CN 40 CN 45 CN 50 CN 55
LHV D3338 [MJ/Kg] 43.6 43.6 43.4 43.6 43.6 43.6
Density @ 15 °C D4052 [g/cc] 0.785 0.777 0.796 0.788 0.788 0.790
Viscosity @ 20 °C D445 [mm2
/s] 4.01 3.84 4.8 4.31 4.04 4.73
HCR
D3343 /
D5291
[] 1.985 2.024 1.968 2.010 2.009 2.009
CN D613 [] 30 34 40 44 51 54
Aromatics D1319 [vol. %] 10.5 9.1 11.6 7.4 6.6 7.3
Table 1: Fuel properties of test fuels
Figure 3: Distillation temperatures of test fuels

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III. Results and Discussions
A. In-Cylinder Pressure Analysis
Figure 4 presents the ensemble-averaged pressure traces for one cylinder at each of the four engine operating
conditions for the range of cetane numbers. In some cases only CN 30, 40 and 55 are shown because the intermediate
cetane values do not provide any additional information. The actual values of pressure have been normalized by the
peak pressure obtained while using CN40 fuel at that engine condition. This protects proprietary information while
still allowing for detailed analysis of the impact of the cetane variations. In the Ground Idle condition it is immediately
obvious that the preinjection combustion was significantly affected by the fuel cetane number. The higher CN fuels
autoignite dramatically better than the lower CN fuels, which leads to shorter ignition delay. This is because the
preinjection is small and becomes lean quickly as the fuel is injected; thus the higher CN fuel exhibits better ignition
quality than the lower CN fuel under very lean conditions. In the case of CN 30, the preinjection combustion was
barely detectable and burned only a small portion of the preinjection fuel. The quality of this preinjection combustion
stage had a significant impact on the main injection combustion. The peak in-cylinder pressure was not significantly
influenced by the preinjection combustion, due to the small fuel volume and short preinjection separation time.
However, the location of peak pressure was noticeably retarded. This indicates retarded combustion phasing.
For the Full Power Ascent case, which uses only a main fuel injection with no preinjection, differences between
the various cetane blends are difficult to detect. Ignition delay, peak pressure, peak pressure location, and combustion
duration are all very similar. This indicates that cetane number does not have a significant impact at high load cases,
such as when the vehicle is taking off from the runway.
The Cruise condition is one at which the vehicle is likely to spend the majority of its operational hours. In this case
the preinjection combustion is again significantly affected by the cetane number. Little or no fuel from the CN 30
Figure 4: Ensemble-averaged in-cylinder pressure vs. CAD for four different engine conditions and six fuels (Ground
Idle) and three fuels (Full Power Ascent, Cruise, and Descent Idle)

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preinjection burns initially, and is instead consumed during the main combustion event. This leads to a slightly higher
peak pressure, and small change in peak pressure location.
The Descent Idle case exhibits the most pronounced effect from low cetane number fuels. This is a case where the
aircraft is descending. The FADEC dramatically cuts the quantity of fuel, and injects only enough to allow the
turbocharger to maintain the manifold air pressure at 1 atm. The preinjection and main fuel injection quantity are both
very low relative to the other cases. Even at the highest cetane number, the preinjection does not appear to burn. In
the worst case, at CN 30, the combustion of the main injection is almost undetectable. This results from the long
ignition delay allowing for overmixing of the small quantity of fuel. Erratic or severely retarded combustion such as
this would lead to highly unstable engine operation. At high altitudes, when the air density and temperature are greatly
reduced, this problem is likely to be exacerbated. To alleviate some of the problems observed at low fuel quantity,
low cetane number operation, it may be possible to optimize the timing and quantity of the preinjection.
B. Heat Release Rate Analysis
Heat release analysis is one of the best tools to understand engine combustion in internal combustion engines. It
provides good representation of combustion processes because it contains a derivative of in-cylinder pressure with
respect to time. Equation 1 shows a representative net heat release rate equation derived from the 1st
law of
thermodynamics.  is time and temperature-dependent specific heat ratio, V is time-dependent combustion chamber
volume, P is time-dependent in-cylinder pressure, and Q is heat or energy. θ is engine crank angle, and is used in place
of time for the heat release data presented here.  uses a first-order polynomial equation as shown in Eq. 2 where T is
obtained from the equation of state 𝑃𝑉 = 𝑚𝑅𝑇.
𝑑𝑄
𝑑𝜃
=
𝛾
𝛾−1
𝑃
𝑑𝑉
𝑑𝜃
+
1
𝛾−1
𝑉
𝑑𝑃
𝑑𝜃
(1)
𝛾 = 1.392 − 7.35 × 10−5
× 𝑇 (2)
Figure 5 shows the heat release rate as a function of crank angle for the four engine conditions, at each cetane
number. The Ground Idle condition includes two red dashed lines to indicate the approximate start of the preinjection
command and the main injection command, to provide a frame of reference. The figure demonstrates again that
preinjection combustion is extremely sensitive to cetane number, and that the preinjection combustion significantly
impacts the subsequent main combustion. The lower CN fuels with longer ignition delays generated less combustible
mixture with the preinjection. Thus, their peak heat release rates of the preinjection combustion were lower than those
of the higher CN fuels. Higher peak heat release rate indicates higher combustion efficiency; therefore, higher CN
fuels had higher combustion efficiency for preinjection combustion. It demonstrates that for small fuel quantity, more
mixing time leads to lower combustion efficiency due to over mixing. Unburned fuel during the preinjection
combustion for the lower CN fuels was mixed with the fuel injected during the main injection and burnt during the
main combustion. Therefore, the lower CN fuels have higher peak heat release rates for the main combustion than
higher CN fuels. Although other minor losses existed, the increase in peak heat release rate was induced primarily
because energy must be conserved.
For the Full Power Ascent case, the lower CN fuels exhibited higher peak premixed-phase heat release rate. This
is a result of the longer ignition delays for the lower CN fuels, which provide more mixing time. Premixed-phase heat
release rate is defined as combustion of the fuel which has had time to mix with air during the ignition delay period11
.
Once the premixed fuel-air mixture is consumed, the heat release rate depends on the fuel-air mixing rate. This phase
is referred to as mixing-controlled combustion. Although the ignition delay variation was small for the different CN
fuels, it significantly influenced premixed-phase combustion which affected the mixing-controlled combustion. Peak
heat release rates during the mixing-controlled combustion seem insignificantly influenced.
In the Cruise condition, the heat release rate traces showed premixed and mixing-controlled phase combustion
regardless of the ignition quality. At this mid-load condition, the mixing-controlled combustion started becoming
dominant for the high CN fuels. For the lower CN fuels, the mixing-controlled phase combustion exhibited significant
oscillations in heat release rate, induced by the magnitude of the peak heat release rate. The higher CN fuels also began
showing small oscillations after the premixed-phase combustion. Such oscillations can cause a significant amount of
thermal and mechanical stress on an engine, and dramatically reduce the expected time before overhaul. These stresses
are analyzed in more detail in Meininger et al.12
.
For the Descent Idle case, even the CN 40 fuel experienced poor combustion. The volume of fuel injected was at
the lower limit of the fuel system’s control range, and often resulted in misfires and inaccurate quantities of fuel

7. American Institute of Aeronautics and Astronautics
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injected. Additionally, the fuel was injected too early at low thermal conditions in the combustion chamber.
Preinjection combustion did not occur, but small cool flames were observed. This operating condition needs to be
optimized to run without erratic combustion behavior as the cetane number of fuel supplies varies.
Figure 6 presents the peak heat release rate (either premixed or mixing-controlled phase combustion) versus CN
for all operating conditions. The influence of cetane number on the Cruise condition is made very clear. The poor
combustion at low cetane numbers for the Descent Idle case is also made clear as the max heat release rate curve
approached zero. The change in slope of the curves as the cetane numbers increase shows the combustion shifting
from premixed phase to mixing-controlled phase. This indicates that at the high cetane numbers, the peak heat release
rate is not a strong function of cetane number.
Figure 7 shows the accumulative heat release (AHR) for the Ground Idle and Descent Idle cases. AHR is an
integration of heat release rate with respect to crank angle. The dotted black and red lines show the CA10 and CA50
points, where 10% and 50%, respectively, of the energy contained in the fuel has been released. AHR is useful to look
for combustion phasing, and thus it has been used as a control parameter for closed-loop engine control, especially
CA5013-15
. The start of combustion (SOC) has also been determined from the AHR, based on the widely used CA10
point. SOC is then used with the start of injection to determine the important ignition delay parameter. Although CA10
is very useful, it can miss combustion phasing for multiple-injection combustion due to inflection point(s). The number
of inflection points depends on the number of injections per cycle.
For the Ground Idle condition, AHR is extremely sensitive to fuel CN up to the peak of the preinjection combustion
AHR. Thus, different CNs can provide huge variations in the measured ignition delay if the CA10 is in the region of
the preinjection combustion. However, the CA10 at this condition was just above the preinjection combustion AHR;
thus, the CA10 variation was insensitive to fuel CN. The CA50 point is also shown to be insensitive to the fuel CN at
this condition. Thus, although the combustion is influenced by the fuel CN, neither CA10, nor CA50 can be used for
combustion control at this engine condition.
Figure 5: Heat release rate vs. CAD for four different engine conditions and six fuels (Ground Idle) and three fuels (Full
Power Ascent, Cruise, and Descent Idle)

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For the Descent Idle AHR, the huge ignition delays observed at low CN in the pressure traces become obvious as
the curves shift to the right. The AHR values are initially negative as a result of heat transfer through the cylinder wall
during the long ignition delay. No discontinuities are observed in the heat release rate around the time of the
preinjection, which indicates that preinjection combustion did not occur. Due to high ignition delay, premixed-phase
combustion is dominant. Under these conditions, it is observed that CA50 combustion-phasing control may work.
However, at higher throttle conditions, mixing-controlled combustion becomes dominant, and the CA50 point
becomes insensitive to CN. This indicates that the control parameter would need to be changed with increasing engine
load. For example, to capture the sensitivity to the fuel CN or other parameters with influence on the combustion
process, the control input would have to be changed from CA50 to CA20 to CA10.
Figure 8 shows the start of combustion calculated from the typical CA10 value. Some of the physics, particularly
at the Ground Idle case, are lost when SOC is calculated this way. However, it does accurately display that the SOC,
and therefore the ignition delay, at the high load Ascent case is largely insensitive to CN. For the Cruise condition,
the SOC decreases as the CN was increased, as expected. The low-load Descent Idle case shows the most dramatic
impact, as described earlier, resulting from an overly lean fuel-air mixture that barely burns at the lowest CN.
Figure 6: Peak heat release rate vs. cetane number for four different engine conditions and six fuels (Ground Idle)
and three fuels (Full Power Ascent, Cruise, and Descent Idle)
Figure 7: Accumulative heat release with respect to CAD at the Ground Idle and Descent Idle conditions

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C. Engine Performance Parameters
Figure 9 shows the overall engine performance parameters of net mean effective pressure (NMEP) and net specific
fuel consumption (NSFC). Net terms were used in this study because the focus is on the useful work produced by just
the fuel, as opposed to the entire engine. Braking performance parameters were more difficult to interpret because the
Descent Idle case produced a negative torque value as the engine allows the UAV to lose altitude. The performance
parameters at each engine condition have been normalized by the value using CN 40 fuel to allow for comparisons
between engine conditions.
The NMEP shows a minimal impact from changing the CN, as most of the values are close to 1.0. The exception
occurs at the Descent Idle case. As previously described, combustion at this engine condition was erratic or severely
retarded, and further deteriorated as the cetane number decreased. In the worst case, at CN 30, NMEP decreased to
just 25% of the value at CN 40, which is consistent with the poor combustion observed in the heat release rate plots.
Figure 8: Start of combustion for all engine conditions and test fuels
Figure 9: Engine performance parameters NMEP and NSFC for all engine operating conditions and test fuels

10. American Institute of Aeronautics and Astronautics
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Analysis of the NSFC produced similar findings to the NMEP plot. For the Ground Idle, Ascent, and Cruise cases,
the normalized NSFC remained close to 1.0. For Descent Idle, the fuel consumption began to climb steeply as the CN
decreased, to the extent that it was not suitable to show the values on the same scale. At CN 35, the NSFC is 2.4 times
higher than the CN40 value. At CN 30, it is 6.7 times higher. This indicates that the amount of useful work produced
by the fuel in the engine at this condition is very small. At this case it is also likely that some of the fuel is not burning,
and is ejected from the cylinder during the exhaust stroke.
IV. Conclusions
A representative aviation diesel engine was successfully tested at various operating conditions for six different CN
fuels, at sea level ambient conditions. The fuels all met the MIL-DTL-83133 specification16
, so a similar range of
ignition quality can be expected for jet fuels at airfields. The following conclusions were made:
1) As expected, the higher CN fuels ignited more promptly that the lower CN fuels.
2) Preinjection combustion exhibited strong sensitivity to the fuel CN, with the higher CN resulting in more
intense combustion and shorter ignition delay.
3) Small quantity preinjection of fuel led to an overly mixed fuel air mixture, which produced extremely lean
and erratic combustion. Most of the preinjection occurred too early in the engine cycle when the thermal
conditions were unfavorable for autoignition.
4) Combustion phasing was retarded with lower CN fuels due to longer ignition delays. This resulted in higher
premixed-phase combustion and increased pressure oscillations in the subsequent mixing-controlled
combustion phase. Additional analysis is available in Meininger et al.12
.
5) It was found that the conventional combustion phasing control parameter CA50 would be insufficient for
closed-loop combustion control with these fuels. While CA50 showed effectiveness at low loads, its
effectiveness as a control parameter decreased as the load was increased.
6) Engine performance parameters NMEP and NSFC were found to be insensitive to fuel cetane number, except
at the Descent Idle condition.
With the exception of the low load cases where ignition quality effects become obvious, typical engine
performance parameters do not sufficiently characterize the impact of variations in fuel cetane number on the
representative engine. Without analyzing the combustion parameters in detail, it would not be possible to detect issues
such as poor preinjection combustion, heat release rate oscillations during the mixing-controlled combustion phase,
or surges in the peak heat release rate. Therefore, it is recommended that detailed combustion characterizations be
undertaken for any new diesel engine calibration, aviation or terrestrial, which will be fueled with any fuels for which
the ignition quality is not regulated.
V. Future Work
The engine characterized in this study was an aviation diesel engine. To fully quantify the response of such an
engine to cetane number variations, it must be studied under realistic ambient conditions. To this end, another
representative aviation diesel engine has been installed in the new US Army Research Laboratory Small Engine
Altitude Research Facility. The facility will expose the engine to intake air pressure as low as 30 kPaA, and intake air
temperature as low as -40 °C, to match the ambient conditions found at an altitude of over 8,000 meters (25,000 feet).
The engine will be fueled with a variety of Jet A fuels to determine the combined impact of altitude and fuel properties.
Three variable-cetane fuels from the current study will be included, as well as additional Jet A batches where the
aromatic content is independently varied, and the cetane number is held constant.
Acknowledgments
The authors wish to thank Mr. Bernard Acker, AMRDEC UAS Division Chief for his technical and financial
support of this work, Mr. Richard Gerdom from ERG and Mr. David Gondol from Engility for their support in
laboratory operation.

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References
1
US Department of Defense, Directive 4140.25, DOD Management Policy for Energy Commodities and Related Services,
Section 4.2: Fuel Standardization, Washington, D.C, 2004.
2
ASTM Standard D975, 2015, "Standard Specification for Diesel Fuel Oils," ASTM International, West Conshohocken, PA,
2015, DOI: 10.1520/D0975-15C.
3
Defense Logistics Agency. PQIS 2012 Annual Report. Petroleum Quality Information System, 2012.
4
McMillan, M. L., and Halsall, R., “Fuel Effects on Combustion and Emissions in a Direct Injection Diesel Engine,” SAE
Technical Paper Series, 881650, Jan. 1988.
5
Schihl, P., Hoogterp, L., and Pangilinan, H., “Assessment of JP-8 and DF-2 Evaporation Rate and Cetane Number
Differences on a Military Diesel Engine,” SAE Technical Paper Series, 2006-01-1549, Mar. 2006.
6
Schihl, P., and Hoogterp-Decker, L., “On the Ignition Behavior of JP-8 in Military Relevant Diesel Engines,” SAE
International Journal of Engines SAE Int. J. Engines, vol. 4, 2011-01-0119, Dec. 2011, pp. 1–13.
7
Murphy, L., and Rothamer, D., “Effects of Cetane Number on Jet Fuel Combustion in a Heavy-Duty Compression Ignition
Engine at High Load,” SAE Technical Paper Series, Dec. 2011.
8
Olree, R. M., and Lenane, D. L., “Diesel Combustion Cetane Number Effects,” SAE Technical Paper Series, Jan. 1984.
9
Groendyk, M. A., and Rothamer, D., “Effects of Fuel Physical Properties on Auto-Ignition Characteristics in a Heavy Duty
Compression Ignition Engine,” SAE Int. J. Fuels Lubr. SAE International Journal of Fuels and Lubricants, vol. 8, 2015, pp. 200–
213.
10
Kidoguchi, Y., Yang, C., Kato, R., Miwa, K., “Effects of fuel cetane number and aromatics on combustion process and
emissions of a direct-injection diesel engine,” JSAE Review, vol. 21, 2000, pp. 469–475.
11
Heywood, J. B., Internal combustion engine fundamentals, New York: McGraw-Hill, 1988.
12
Meininger, R.D., Kweon, C.M., Szedlmayer, M.T., Dang, K., Jackson, N.B., Lindsey, C.A., Gibson, J.A., Armstrong, R.H.,
“Knock Criteria for Aviation Diesel Engines,” International Journal of Engine Research, (to be published).
13
Olsson, J.-O., Tunestål, P., and Johansson, B., “Closed-Loop Control of an HCCI Engine,” SAE Technical Paper Series, May
2001.
14
Willems, F., Doosje, E., Engels, F., and Seykens, X., “Cylinder Pressure-Based Control in Heavy-Duty EGR Diesel Engines
Using a Virtual Heat Release and Emission Sensor,” SAE Technical Paper Series, Dec. 2010.
15
Flynn, Patrick F., Hunter, Gary L., zur Loye, Axel O., Akinyemi, Omowoleola C., Durrett, Russ P., Moore, Greg A.,
Muntean, George G., Peters, Lester L., Pierz, Patrick M., Wagner, Julie A., Wright, John F., Yeager, Jackie M., “Premixed
charge compression ignition engine with optimal combustion control,” Cummins Engine Company, Inc., May 15, 2001.
16
Detail Specification Turbine Fuel, Aviation, Kerosene Type JP-8 (NATO F-34), NATO F-35, and JP-8 þ 100 (NATO F-37);
MIL-DTL-83133F, April 11, 2008.