Jonathan S. Litt and Jeffrey C. Chin

Glenn Research Center, Cleveland, Ohio

Yuan Liu

N&R Engineering, Inc., Parma Heights, Ohio

Simulating the Use of Alternative Fuels in a

Turbofan Engine

NASA/TM—2013-216547

September 2013
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Jonathan S. Litt and Jeffrey C. Chin

Glenn Research Center, Cleveland, Ohio

Yuan Liu

N&R Engineering, Inc., Parma Heights, Ohio

Simulating the Use of Alternative Fuels in a

Turbofan Engine

NASA/TM—2013-216547

September 2013

National Aeronautics and

Space Administration

Glenn Research Center

Cleveland, Ohio 44135
Acknowledgments

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and Zachary O. Baker for their gracious help and guidance. This work was sponsored by the Aviation Safety Program at the

NASA Glenn Research Center.

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Hanover, MD 21076–1320

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Available electronically at http://www.sti.nasa.gov

Level of Review: This material has been technically reviewed by technical management. 
NASA/TM—2013-216547 1 

Simulating the Use of Alternative Fuels in a 

Turbofan Engine 

Jonathan S. Litt and Jeffrey C. Chin 

National Aeronautics and Space Administration 

Glenn Research Center 

Cleveland, Ohio 44135 

Yuan Liu 

N&R Engineering, Inc. 

Parma Heights, Ohio 44130 

Abstract 

The interest in alternative fuels for aviation has created a need to evaluate their effect on engine 

performance. The use of dynamic turbofan engine simulations enables the comparative modeling of the 

performance of these fuels on a realistic test bed in terms of dynamic response and control compared to 

traditional fuels. The analysis of overall engine performance and response characteristics can lead to a 

determination of the practicality of using specific alternative fuels in commercial aircraft. This paper 

describes a procedure to model the use of alternative fuels in a large commercial turbofan engine, and 

quantifies their effects on engine and vehicle performance. In addition, the modeling effort notionally 

demonstrates that engine performance may be maintained by modifying engine control system software 

parameters to account for the alternative fuel. 

Nomenclature 

ASTM American Society for Testing and Materials 

CD Drag force coefficient in the x direction 

CL Lift force coefficient in the y direction 

CP Heat capacity 

EPR Engine pressure ratio 

FAR Fuel-to-air ratio 

g Gravitational constant 

LHV Lower Heating Value 

P30 High pressure compressor exit pressure 

S Wing planform area 

T40 Burner exit temperature 

T50 Low pressure turbine exit temperature 

TSFC Thrust specific fuel consumption 

Velocity 

W Weight 

Wi Takeoff weight 

Wf Landing weight 

x Range 

Adiabatic index 

 Air density 
NASA/TM—2013-216547 2 

Introduction 

The worldwide interest in alternative energy is quickly gaining momentum as a result of political and 

socioeconomic concerns regarding the use of fossil fuels. The sheer magnitude of energy demand, in 

combination with sustainability and detrimental environmental effects of fossil fuels, makes the transition 

to alternative energy sources particularly attractive. The airline industry is responsible for 2 percent of 

annual global man-made CO2 emissions (Ref. 1) and there is a growing awareness that the use of 

alternative jet fuels could produce a net reduction in these flight-related carbon emissions. Significant 

reduction has already been demonstrated in tests utilizing fuel blends and pure biofuels in unmodified 

engines (Refs. 2 to 4). Biofuels are emerging as a popular alternative energy source because of their 

renewability and potential to mitigate fossil fuel dependence. 

Substituting aviation fuel is challenging, and stringent specifications set by the American Society for 

Testing and Materials (ASTM) help define that challenge. In addition to meeting tight quality standards, 

the fuel must serve multiple purposes within the engine. It is used as a hydraulic fluid in servo valves 

(Ref. 5), as a lubricant for valves and bearings, and as a coolant, which requires high thermal stability. 

Furthermore, standard fuels contain additives such as antioxidants and antistatic agents that prevent 

oxidation and static electricity generation, respectively, as well as corrosion and icing inhibitors. 

Biodiesel is an alternative fuel that can be acquired through the transesterification of vegetable oils, 

used cooking oils, and animal fats. Production and use of biodiesel has increased worldwide because of its 

compatibility with existing fuel distribution and storage infrastructures. Advantages of biodiesels include 

reduced exhaust emissions, improved biodegradability, higher flash point, and domestic origin, but 

developmental hurdles include oxidative stability and cold flow properties (Ref. 6). There are methods to 

overcome some of these drawbacks using secondary treatments such as hydrogenation. This process 

removes oxygen from the biodiesel, resulting in a kerosene fuel with a greatly improved freezing point. 

The growing interest in alternative fuels for aviation has created a demand to analyze the 

ramifications of their use. The availability of realistic dynamic turbofan engine simulations provides a 

way to evaluate these fuels from a propulsion system performance perspective. The introduction of the 

NASA-developed Modular Aero-Propulsion System Simulation (MAPSS) (Refs. 7 and 8), the 

Commercial Modular Aero-Propulsion System Simulation (C-MAPSS) (Refs. 9 to 11), and the 

Commercial Modular Aero-Propulsion System Simulation 40k (C-MAPSS40k) (Refs. 12 and 13) into the 

aerospace community has been a catalyst for research in diagnostics, prognostics, and control design for 

propulsion systems. Researchers are benefitting from the built-in capabilities and user-friendly features of 

these simulations that make them well-suited for controls and diagnostics algorithm development. 

However, these simulations were designed with specific uses in mind, and thus have limited capabilities 

outside of their preconceived domain. Still, the overall usefulness of the simulations has encouraged users 

to build on the baseline models to provide relevant features for their specific needs. Such features as 

compressor instability indication modeling, (Ref. 14) emissions modeling, (Ref. 15) and engine icing 

modeling (Ref. 16) are examples. 

This paper lays out a procedure for modifying the C-MAPSS40k model to allow the simulation of the 

use of alternative fuels that do not require the physical modification of the fuel system. Specifically, the 

thermodynamic equations that incorporate the use of standard jet fuel implicitly are replaced by 

equivalent lookup tables. The lookup tables themselves can be swapped out for others that represent 

properties of different fuels, thus enabling the simulation of any desired fuel. Note that because of the 

modularity of the simulations and because the source code is accessible and can be modified, a user can 

replace the fuel pump model, for example, if desired (Ref. 12). Reference 17 describes actual fuel system 

hardware and control software modified for use with hydrogen fuel. The fuel properties may affect the 

engine performance and operation, the impact of which can be evaluated through simulation. In some cases, 

baseline performance can be recovered through controller gain modification, as this paper will demonstrate. 

This paper is organized as follows. First, a modeling approach to enable the use of alternative fuels 

and the validation method are described. Next, the result of using such fuels on engine performance is 

demonstrated. This is followed by a sample analysis of the impact of using an example alternative fuel on 
NASA/TM—2013-216547 3 

aircraft performance. The paper concludes with some final comments regarding the use of alternative 

fuels beyond what is capable of being simulated in the turbofan engine model utilized here and a 

summary of results. 

Modeling Approach and Validation 

C-MAPSS40k is a faster-than-real-time, zero-dimensional (lumped parameter) simulation of a 

40,000 lbf thrust class engine that accurately models the engine dynamics to the fidelity appropriate for 

control system design. It has three inputs: fuel flow rate, variable stator vane (VSV) position, and variable 

bleed valve (VBV) position. It includes a realistic controller and enables the user to set initial and timevarying inputs to an engine operating in any part of the flight envelope. The engine model has been 

designed to be modular at a component level to allow for specific engine customization, which makes it a 

flexible platform that can be utilized for multi-disciplinary research. In this context, C-MAPSS40k should 

be able to accommodate the turbine engine’s system performance considerations involving the next 

generation of fuels. Since C-MAPSS40k represents a generic commercial turbofan engine, only “drop-in” 

fuels that do not require significantly redesigned engines can be analyzed using this simulation as is, 

although modification of the simulation is possible. 

Mechanical Fuel Properties: C-MAPSS40k models the physical combustion process using a 40-ms 

delay, combined with an efficiency parameter that encompasses minor resistances and losses at the liner 

and the mixer. When simulating the use of alternative fuel, it is assumed that any operational design 

issues regarding freezing, engine deposits and mixing are completely resolved. Biofuels have large carbon 

chain lengths and the presence of oxygen and fatty compounds result in higher kinematic viscosities than 

hydrocarbons of equal carbon length (Ref. 18). Additional differences can be found in Table 1. 

Thermodynamic Fuel Properties: C-MAPSS40k utilizes a Lower Heating Value (LHV) parameter 

and a set of four thermodynamic functions to calculate the thermodynamic properties of the gas within the 

engine components. The LHV parameter, which resides in the burner component, is defined as the net 

amount of heat released by combusting a specific amount of fuel. The thermodynamic functions, which 

are utilized by all of the major engine components, relate certain gas properties given a specific fuel-air 

mixture level: two are used to convert between temperature and enthalpy; the other two functions convert 

between temperature and entropy, given pressure. The LHV and thermodynamic functions in the baseline 

C-MAPSS40k correspond to Jet-A, a standard aviation fuel. 

The fuel’s chemical composition, including the carbon-to-hydrogen ratio and molecular weight, factor 

into the mixture’s heat capacity (CP	
		
which in turn affect engine performance. 

Both the LHV parameter and the set of thermodynamic functions are fuel-specific and hence must be 

replaced when simulating other fuel types. Since the thermodynamic functions included in C-MAPSS40k 

are in a format that is not particularly amenable to user modification, the functions are replaced with two 

multi-dimensional arrays that explicitly relate the necessary thermodynamic properties: (1) a twodimensional array of enthalpy values based on temperature and fuel-to-air ratio (FAR), and 

TABLE 1.—KEY DIFFERENCES BETWEEN FUEL TYPES 

Fuel properties Standard Jet-A Biodiesel, 

ethanol 

Biodiesel, 

ethyl stearate 

LHV (BTU/lb) 18,400 11,587 16,250 

Kinematic viscosity (mm2/sec) 1.39 1.07 3.7-5.8 

Average density (lb/gal) 6.84 6.58 7.3 

FAR (Stoichiometric) 0.068 0.111 0.074 

Freezing point (°C) 40 114 15 

Generalized chemical 

composition 

C10H19 C2H5OH C19H38O2

Average H/C ratio 1.9 3 2 

Total molecular weight of 

combustion products at a 

FAR = 0.02 (g/mol) 

28.968 28.820 29.197 
NASA/TM—2013-216547 4 

(2) a three-dimensional array of entropy values based on temperature, pressure, and FAR. Twodimensional and three-dimensional interpolation routines are used to traverse the arrays. The “inverse” 

relationships (i.e., enthalpy and FAR to temperature; entropy, pressure, and FAR to temperature) are 

calculated by applying the Newton-Raphson Method to the “forward” relationships (e.g., temperature to 

enthalpy). A thermodynamics software application is necessary to generate these arrays for different fuel 

types. For this work, the aforementioned enthalpy and entropy relationships were calculated using the 

Chemical Equilibrium with Applications (CEA) computer program (Ref. 19). The CEA program 

computes chemical equilibrium product concentrations from a given set of reactants and determines 

thermodynamic and transport properties for the product mixture. Since CEA accounts for the energy 

released due to combustion, the usage of LHV in the combustor model of C-MAPSS40k is redundant and 

removed when applying CEA-generated thermodynamic arrays. 

As a preliminary study, one can get a general idea of the effects of using different fuels with 

C-MAPSS40k through some simpler changes. Adjusting LHV to match that of the fuel type and, if 

running open loop, scaling fuel flow to achieve the FAR required to meet the thrust demand (the 

controller will adjust fuel flow automatically in closed-loop operation) should be sufficient model 

modifications. However, more detailed comparisons of fuels will require replacing the default 

C-MAPSS40k thermodynamic relationships, as outlined above. 

Validation: Utilizing multi-dimensional thermodynamic arrays to model different fuel types requires 

validation of (1) the interpolation routines and (2) the array generation procedure. First, to isolate the 

effects of interpolation, a nested-loop algorithm that repeatedly executes the original thermodynamic 

functions included in C-MAPSS40k was used to generate a 657-element two-dimensional array of 

enthalpy values and a 26,280-element three-dimensional array of entropy values. The performance of 

each major engine component using the original thermodynamic functions is then compared to that using 

array interpolation by applying the same inputs to the components (e.g., inlet pressure, temperature, mass 

flow rate) in both cases. The test case used for comparison corresponded to an operating condition of 

1000 ft altitude and Mach 0.2. The percent differences observed in relevant output parameters obtained 

using interpolation relative to those obtained with the original thermodynamic functions are shown in the 

“Interpolation Only” column of Table 2. The results show that the differences introduced by interpolation 

are relatively small. The discrepancies may be attributed to the tolerance of the interpolation algorithms 

and the size of the arrays, the latter of which is balanced against model execution speed and computer 

memory considerations. The accuracy of the results can be improved as desired by increasing the size of 

the tables. 

TABLE 2.—DIFFERENCES IN PRESSURE, TEMPERATURE, AND TORQUE BETWEEN 

THE BASELINE AND MODIFIED C-MAPSS40k FOR EACH MAJOR COMPONENT 

Component Parameter Interpolation only, 

percent 

Jet-A CEA maps, 

percent 

Fan Pressure 

Temperature 

Torque 

0 

0.23 

1.96 

0 

0.22 

1.80 

LPC Pressure 

Temperature 

Torque 

0 

0.083 

0.41 

0 

0.081 

0.46 

HPC Pressure 

Temperature 

Torque 

0 

0.42 

0.83 

0 

0.39 

0.81 

Burner Pressure 

Temperature 

0 

0.0014 

0 

0.55 

HPT Pressure 

Temperature 

Torque 

0.16 

0.00034 

0.00019 

0.44 

0.26 

0.032 

LPT Pressure 

Temperature 

Torque 

2.89 

0.0088 

0.48 

2.90 

0.42 

0.48 
NASA/TM—2013-216547 5 

Next, the thermodynamic arrays generated using the CEA computer program were investigated. To 

ensure their consistency with the default C-MAPSS40k thermodynamic relationships, the previously 

described procedure for validating the interpolation routines was repeated for identically sized enthalpy 

and entropy arrays generated using CEA. As before, engine component performance using the arrays was 

compared to that using the original thermodynamic functions. The results, listed in the “Jet-A CEA 

Maps” column of Table 2, show that the discrepancies introduced by using CEA are relatively small and 

on the order of interpolation errors for several performance parameters. Nevertheless, to control for these 

differences and prevent possible ambiguity due to error “compounding” when performing a full engine 

analysis, any reference in the remainder of the paper to engine performance using Jet-A fuel implies the 

usage of CEA-generated Jet-A arrays (as opposed to the original thermodynamic functions). The results, 

listed in the “Jet-A CEA Maps” column of Table 2, show that the discrepancies introduced by using CEA 

are relatively small and on the order of interpolation errors for several performance parameters. 

Nevertheless, to control for these differences and prevent possible ambiguity due to error “compounding” 

when performing a full engine analysis, any reference in the remainder of the paper to engine 

performance using Jet-A fuel implies the usage of CEA-generated Jet-A arrays (as opposed to the original 

thermodynamic functions). 

Effects of Different Fuels on Engine Outputs 

The impact on engine response as a result of using an alternative fuel with relatively low combustion 

energy is demonstrated in this section to emphasize the difference the type of fuel can make on engine 

performance. The C-MAPSS40k simulation with its baseline full-envelope controller was modified as 

described above to utilize thermodynamic tables. It was then used to simulate the response to a throttle 

doublet (5step up, 10step down, 5step up) using both ethanol and Jet-A, at an operating point of 

1000 ft altitude and Mach 0.2. Figure 1 shows the engine pressure ratio (EPR) response for the two cases. 

Figure 1.—EPR response to a doublet command in throttle (5about 70) to C-MAPSS40k with 

its baseline controller, using Jet-A and Ethanol. 

5 10 15 20 25

1.3

1.32

1.34

1.36

1.38

1.4

1.42

1.44

Time (s)

Engine Pressure Ratio

Demand

Jet-A

Ethanol
NASA/TM—2013-216547 6 

Ethanol’s Lower Heating Value (LHV) of 11,587 BTU/lb is significantly lower than that for Jet-A 

(18,400 BTU/lb from Table 1). As a result, the response time of the engine using ethanol is somewhat 

slower. However, while this result can be partially attributed to fuel properties, it is also due to using a 

controller with gains designed for a fuel type with more combustion energy. 

Thus, a redesign of the controller gains may result in recovery of the performance lost by using 

ethanol instead of Jet-A. To test this, linear models of the C-MAPSS40k engine using Jet-A and ethanol 

were first generated. The linearization routine in the baseline version of C-MAPSS40k utilizes a C-code 

version of the engine model with its default fuel properties, so it was modified to call the Simulink engine 

model1

 instead in order to account for the Jet-A- or ethanol-specific thermodynamic arrays. A modelmatching linear regulator design (Ref. 20) tool available in C-MAPSS (Refs. 9 and 11) was then used to 

create EPR point controllers for the two linear models. The same design parameters (closed loop system 

specifications) were used for both controllers; these design parameters were selected to give approximately the same response as the full-envelope baseline controller at the design point. Figure 2 shows the 

response to a doublet input of the nonlinear engine model using the newly designed linear EPR point 

controllers. The Jet-A-powered engine with the EPR controller designed using the Jet-A linear model 

(blue line) is the baseline case. The Jet-A EPR controller used with an ethanol-powered engine (red line) 

results in slower engine response. However, by implementing a new linear EPR point controller designed 

using the ethanol linear model, the ethanol-powered engine (dashed green line) is able to match the Jet-A 

performance. The lower portion of Figure 2 shows the fuel flow corresponding to these redesigned EPR 

controller test cases. While the difference in transient response between fuel type can be accommodated 

by controller redesign, the ethanol-powered engine requires more than 60 percent higher fuel flow2

 to 

generate the same EPR as an engine using Jet-A. This is a result of the difference in energy content 

between the fuel types, and has implications for aircraft weight and range, which will be discussed later. 

Modern commercial engines are designed to operate lean, i.e., the FAR is well below its 

stoichiometric ratio, or the ratio at which the products and reactants are balanced at a molecular level in a 

completely combusted reaction. This provides several advantages that were not modeled, but led to the 

decision to compare fuels at a lean design point. Operating lean can reduce NOx formation, increase fuel 

efficiency, and reduce combustor weight. It also reduces the chance of blade degradation and increased 

pollution caused by unburned hydrocarbons traveling downstream of the combustor. Additionally, 

operating with a FAR closer to stoichiometric results in excessive heat in the burner, which can also cause 

damage to the turbine blades. One advantage of using a fuel with a low LHV is that it burns at a lower 

temperature, even at the design FAR. Figure 3 shows the temperature of the hot gas entering the High 

Pressure Turbine (HPT) from the combustor. The HPT inlet temperature is over 50 R cooler for the 

ethanol-powered engine, with the maximum difference at the peaks of nearly 65 R lower. This can 

significantly extend the life of the components downstream of the combustor, particularly the HPT blades. 

It must be noted, however, that the higher fuel flow requirement of ethanol carries a weight penalty not 

only in fuel but potentially in the size of overall fuel system. 

So far, analysis was conducted comparing propulsion system control performance for standard Jet-A 

fuel with ethanol. The subsequent analyses compare Jet-A to Ethyl Stearate, a biofuel that matches it 

more closely in terms of thermodynamic properties. Ethyl Stearate was chosen in particular, due to its 

similarities in molecular composition and heating value. Both fuels contain long carbon chains with 

 1

It is anticipated that the ability to use the Simulink version of C-MAPSS40k to generate a linear model will be 

an option in future releases of C-MAPSS40k. 2

In C-MAPSS40k, the fuel flow is measured in lb/sec, with the upper limit of 4.557 lb/sec. For Jet-A, this 

corresponds to about 0.666 gal/sec using the density of 6.84 lb/gal given in Table 1, which is the true limiting 

quantity. Since the density of ethanol is about 6.58 lb/gal, the mass flow limit of ethanol would be about 4.38 lb/sec. 

The peak fuel flow rate of ethanol in the doublet shown in Figure 2 is 4.505 lb/sec, so when modeling its use, a 

larger fuel metering valve would have to be assumed, although it does not limit by mass flow rate as in the current 

implementation.
NASA/TM—2013-216547 7 

Figure 2.—Comparison of EPR response to a doublet in throttle for an engine 

using ethanol and an engine using Jet-A. The plots show EPR and 

corresponding fuel flow for a version of C-MAPSS40k with Jet-A (blue) and 

one with ethanol (red), using a point controller designed for Jet-A fuel. A third 

trace (dashed green) shows the same variables from C-MAPSS40k using 

ethanol, but with a redesigned point controller to match the Jet-A response. 

Figure 3.—Temperature at the inlet to the HPT for the doublet shown in Figure 2. 

For this transient, the temperature of the ethanol engine averages over 50 R 

lower than that of the Jet-A engine for the same EPR. 

5 10 15 20 25

1.3

1.35

1.4

Engine Pressure Ratio

Jet-A

Ethanol

Ethanol, re-design

5 10 15 20 25

1

2

3

4

5

Fuel Flow (lbm/s)

Time (s)

5 10 15 20 25

2350

2400

2450

2500

2550

2600

2650

2700

2750

2800

HPT Inlet Temperature (R)

Time (s)

Jet-A

Ethanol, re-design
NASA/TM—2013-216547 8 

similar hydrogen-to-carbon ratios, which indicate similar energy densities. Furthermore, the total 

molecular weights of their products after combustion differ by less than 0.8 percent when compared at a 

FAR of 0.02 using the chemical formulas listed in Table 1. This results in very similar heat capacities 

(CP) and adiabatic indices , which consequently affects turbine flow and overall Brayton cycle 

efficiency. 

The thermodynamic tables for ethyl stearate were created using CEA and inserted into the modified 

version of C-MAPSS40k with its baseline controller. A simulated doublet in PLA was used as input at an 

operating point of 1000 ft altitude and Mach 0.2; the results are shown in Figure 4 and Figure 5. The 

biodiesel-powered engine responses shown (EPR, net thrust, T40, T50, P30) closely match those of the 

engine powered with standard fuel, but with a slight damping effect. The fuel flow for the biodiesel is 

about 15 percent higher than for Jet-A to produce the same EPR, due to the lower combustion energy of 

the fuel, but it is significantly lower than that required by ethanol (Figure 2). The burner exit temperature 

(T40) is slightly higher for Jet-A since its LHV is higher. The low pressure turbine exit temperature (T50) 

also has this characteristic. Ethyl stearate’s lower peak temperatures are a result of using the baseline 

controller designed for use with Jet-A, not necessarily because of the different fuel properties. 

Figure 4.—Comparison of EPR, fuel flow, and net thrust responses for 

versions of C-MAPSS40k using Ethyl Stearate and Jet-A to a throttle 

doublet. 

5 10 15 20 25

1.3

1.35

1.4

1.45

1.5

Engine Pressure Ratio

Jet-A

Biofuel

5 10 15 20 25

1.5

2

2.5

3

3.5

Fuel Flow (lbm/s)

5 10 15 20 25

2.1

2.2

2.3

2.4

2.5

2.6

x 104

Net Thrust (lbf)

Time (s)
NASA/TM—2013-216547 9 

Figure 5.—Comparison of burner temperature (T40), LPT exit temperature 

(T50), and HPC exit pressure (P30) responses for versions of 

C-MAPSS40k using ethyl stearate and Jet-A to a throttle doublet. 

Fuel Consumption Impact 

In this section, a sample analysis of the effect of using the various fuels is performed in terms of their 

impact on aircraft operating range. The top plot of Figure 6 shows the difference in fuel flow rate for 

Jet-A and ethyl stearate corresponding to the case shown in Figure 4, which was covered in the previous 

section. The difference in fuel flow rate between the two fuel types increases and decreases with the 

actual fuel flow level. This means that fuel consumption is proportionally related. This becomes 

especially apparent in the bottom plot of Figure 6, which shows the cumulative fuel used over time. 

Biodiesel requires approximately 15 percent more fuel by weight, when compared in steady state. This 

makes sense since the biofuel has a lower heat of combustion, i.e., it has less energy per pound of fuel. 

Using fuel flow and thrust data, it is possible to calculate thrust specific fuel consumption (TSFC) as 

fuel flow TSFC

thrust  (1) 

5 10 15 20 25

2200

2400

2600

2800

3000

Burner Temp. (R)

Jet-A

Biofuel

5 10 15 20 25

1100

1200

1300

1400

LPT Exit Temp. (R)

5 10 15 20 25

300

320

340

360

380

HPC Exit Press. (psi)

Time (s)
NASA/TM—2013-216547 10 

Figure 6.—Instantaneous difference in fuel flow rate (top) and total fuel 

usage (bottom) for the engine using Jet-A and the engine using 

biofuel, for the doublet shown in Figure 4. 

With TSFC and the initial/final vehicle weight ratio, 

Wi

W f

, the aircraft range can be approximated, 

assuming constant velocity and lift coefficient (Ref. 21). The coefficients of lift and drag are defined as: 

2

Lift

1

2

CL

v S





(2) 

2

Drag

1

2

CD

v S





 (3) 

w

	
	
	v is the velocity, and S is the wing planform area. Since airplane weight 

decreases when fuel is consumed, one can develop the following range equation as a function of weight 

(Ref. 22). 

1 Range, ln TSFC

L i

D f

v C W

x

g CW

         	  	  	 	  	 
 
 
  
 

(4) 

5 10 15 20 25

0.3

0.32

0.34

0.36

0.38

0.4

0.42

0.44

Difference in Fuel Flow Rate (lbm/s)

5 10 15 20 25

10

20

30

40

50

60

70

80

Total Fuel Usage (lbm)

Time (s)

Jet-A

Biofuel
NASA/TM—2013-216547 11 

This equation is known as the Breguet Equation. Equation (4) can be applied to two identical aircraft, one 

with Jet-A and one with biofuel (ethyl stearate). Assuming they travel the same distance, the Breguet 

Equations for the two aircraft can be equated (assume that all fuel is consumed so that the final weight, 

Wf, is also the empty weight and therefore is the same for both aircraft): 

,biofuel ,Jet-A

biofuel Jet-A

1 1 ln ln

TSFC TSFC

i i

f f

W W

W W

     

 	 	  	 	 


  
 
  


(5) 

By rearranging Equation (5), the importance of the thrust specific fuel consumption becomes apparent. 

Jet-A

biofuel

TSFC

TSFC i i ,Jet-A ,biofuel

f f

W W

W W

   	  
    	


 

(6) 

An aircraft using biofuel will require more fuel as long as it has a higher TSFC, since the energy content 

is lower. In an attempt to reason why the difference in weight is governed by a power law, consider that 

the biofuel engine will consume fuel at a faster rate because of its lower energy density. Therefore, it 

needs to carry more fuel to travel the same distance, making the aircraft heavier. With a heavier aircraft, 

more fuel will be needed to generate enough lift. This compounding relationship results in the nonlinear 

relationship in fuel requirements. A longer flight necessitates a heavier biofuel tank and greater 

divergence between required amounts of fuel. To put this relationship in economic terms, analysis was 

performed for the least efficient scenario (i.e., maximum range as allowed by fuel tank capacity) using a 

757-300 aircraft with twin 40k pound thrust class engines. Assuming the vehicle is filled to its maximum 

take-off weight (272,500 lb) including its maximum fuel capacity (11,466 gal) of ethyl stearate, an 

equivalent aircraft with the same non-fuel payload running on standard fuel will only need 10,307 gal to 

reach the same range (gross take-off weight is different). Thus, at worst, an ethyl stearate-powered aircraft 

needs approximately 11.2 percent more fuel by volume. Following the same procedure, if these vehicles 

were flown at half the maximum range of the biodiesel aircraft, the one using standard fuel would need 

4746 gal, and the ethyl stearate craft would need 5209 gal, or 9.8 percent more fuel. The difference in fuel 

required by the two aircraft decreases as the flight range is shortened. 

Discussion 

As the push to make the use of biofuels more pervasive in the airline industry continues, it is 

important to understand their broader impact. Engine performance simulations using C-MAPSS40k have 

demonstrated the thermodynamic compatibility of biodiesel with existing engines. Biofuel engines will be 

able to produce equal amounts of thrust compared to conventionally powered engines with the same 

transient responses if the fuel flow controller is redesigned. From a system perspective, the most 

detrimental properties of certain biofuels are volume, weight, and freezing point. The volume and weight 

of certain biofuels will unavoidably reduce efficiency. Mixing standard fuel and biofuel would help 

reduce some of the aforementioned issues. From a component perspective, biodiesel’s higher viscosity 

could also lead to less desirable atomization characteristics, such as increased mean droplet size. 

Furthermore, ring sticking, injector coking, and injector deposit problems have all been known to lead to 

pump failure in biodiesel truck engines. A mix of biodiesel and standard fuel would optimize the 

properties while also alleviating fossil fuel dependency. 

In spite of viscosity issues, it has been found that biodiesel actually causes less wear in ground 

vehicle diesel engines (Ref. 23). This has been attributed to biodiesel’s high lubricity. A high lubricity 

means that biodiesel will be less corrosive, improving the life of fuel handling components. With all of 
NASA/TM—2013-216547 12 

these considerations together, it is difficult to determine how long term engine life trends will change. 

Biodiesel’s oxidation and engine deposit problems could mean that fuel system maintenance must occur 

more often. Conversely, the life of components downstream from the combustor may increase when using 

biofuels based on the lower combustion temperature, as shown in Figure 3. Incomplete combustion would 

also affect blade life. Testing actual engine rigs would be necessary to make accurate and conclusive 

engine life estimates. 

Tradeoffs exist in all engineering considerations, and alternative energy is characterized by its 

substantial entry costs. Nonetheless, the benefits may eventually prove to exceed the operating expenses. 

Biodiesel is much cleaner burning than petroleum, releasing fewer particulates into the air. It also emits 

60 percent less net CO2 than petroleum-based fuels since it originates from carbon dioxide absorbing 

plants (Ref. 24). As the demand for biofuel grows, it is anticipated that more companies will enter the 

market, which can help drive prices to a competitive level. 

Biofuels have been mostly associated with ground transportation; however advances in biofuel 

production technologies will make them more attractive to the aviation industry. The use of biofuels 

cannot completely replace fossil fuels, but they can be used most effectively in specialized niches such as 

short flights. Regardless of certain commercialization obstacles, simulations have confirmed that from a 

propulsion system performance perspective, biofuels are compatible with current turbofan engines. 

Summary 

A technique was outlined and demonstrated to model the use of alternative fuels in a nonlinear 

turbofan engine simulation to conduct propulsion system performance studies. The approach described 

here is specific to the engine simulation used—C-MAPSS40k—and the method used to generate the 

enthalpy and entropy tables corresponding to the type of fuel desired; however, the concepts are fairly 

general. The description is at a high level, but is sufficient to guide the experienced user. The results 

demonstrated the compatibility of biofuels with current turbofan engines from a propulsion system 

performance perspective. Even with a fuel such as ethanol, which has significantly lower energy content 

than standard jet fuel, simple controller gain modifications were shown to completely recover the engine 

response, albeit with a higher fuel flow rate. Therefore, the simulation modified to accept tables 

representing the properties of a variety of alternative fuels, can be used to evaluate the impact of using 

these fuels on thrust specific fuel consumption, temperatures, and other variables, and these in turn can be 

used to evaluate the impact on aircraft weight and range. 

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