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American Institute of Aeronautics and Astronautics

An Innovative Turbo Compound Internal Combustion Engine Concept for UAV Application

Jack Taylor1

Excel Engines Engineering Company, Mainville, Ohio, 45039

Dr. Jayesh Mehta.2, Joe Charneski

3

Belcan Corporation, Cincinnati, Ohio, 45242

Conventional Internal Combustion Engines (ICE) are limited in compression ratio by the

detonation or pre-ignition. This limits the thermal efficiency of these engines, resulting in higher

SFC and toxic emissions. In contrast, diesel engines with direct fuel injection operate at much

higher compression ratios and leaner fuel-air ratios resulting in much higher efficiency. However,

as the fuel is not pre mixed, it results in significant combustion delay time and lower efficiency.

Furthermore, Diesel engines do not use spark plugs to ignite the fuel. This also results into

inefficient fuel ignition and combustion. The proposed concept, herein, addresses some of these

short comings of conventional IC Engines and provide an innovative solution that has the

potential to provide low Specific Fuel Consumption (SFC), higher efficiency, and lower emissions.

Nomenclature

BDC = Bottom Dead Center

CO = Carbon Monoxide

HC = Hydrocarbons

IC = Internal Combustion

NOx = Nitrous Oxide

TBC = Thermal Barrier Coating

UAV = Unmanned Aerial Vehicle

CA = Crank Angle

EVO = Exhaust Valve Opening

EVC = Exhaust Valve Closing

I. Introduction

For Internal Combustion Engines, there are a number of design configurations that lead to

better engine performance, including lower SFC, and reduced emissions. Some of these

configurations feature reduced thermal losses, improved fuel atomization/fuel-air mixing

schemes, improved ignition, and optimized inlet/exhaust valve movements. In addition,

combustor flow features also impact the IC Engine performance. For example, premixed

combustion yields better efficiency and lower emissions, while stratified combustion with cooler

1 President, and owner

2 Manager, Advanced Thermal Systems, and Principal Engineer, Associate Fellow - AIAA

3 Manager, Thermal and Fluid Systems

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51st AIAA/SAE/ASEE Joint Propulsion Conference

July 27-29, 2015, Orlando, FL

AIAA 2015-3780

Copyright © 2015 by the American Institute of Aeronautics and Astronautics, Inc. All rights reserved.

AIAA Propulsion and Energy Forum

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American Institute of Aeronautics and Astronautics

flame front near the walls results in lower thermal losses. In addition, timing and the degree of

scavenge are also critical as they impact the overall combustor performance.

At Excel Engineering, we have conducted a series of studies that evaluate the impact of

these parameters on engine performance. For example, we have developed combustion designs

that feature exhaust system with minimum heat losses, offer significantly improved fuel air

mixing, and in general operate at a higher mean temperature due to the use of CMC materials on

the cylinder head, piston, and side walls. Furthermore, the proposed design features high swirl

velocity, lean direct injection, stratified spark ignition, and overall intense fuel air mixing. Based

on these studies, we have developed a concept that has potential for higher combustion

efficiency, with Specific Fuel Consumption (SFC) that is less than 0.3 lbs/hp-hr. This is about half

of the fuel consumption compared to existing conventional turbo props.

Unmanned Aircraft Vehicles (UAVs) are becoming increasingly acceptable in civil as well

as in Military applications. The platforms vary in size and shape from Micro Air Vehicles (MAVs)

with wing span of inches, to behemoths with wingspans greater than 50 feet. The UAV missions

are equally disparate, and they range from intelligence – to – Surveillance – to- Reconnaissance.

For all of the missions, a common requirement for the power plant is high altitude operation,

better SFC, and often low emissions. The IC engine based concept to be described here

addresses these requirements, where it offers an innovative option to conventional turboprops or

gas turbines in the 200 HP to 2000 HP range.

II. A Brief History of Turbo-Compound IC Engines

One of the earliest turbo-compound IC engines was the Napier Nomad. This turboprop was

designed and tested by Napier Aircraft Engines in England in 1950 [1, 2]. The Napier Nomad

turboprop was a 3,000 HP 12 cylinder two-stroke Diesel engine with an axial flow compressor to

supercharge it, and an axial flow turbine to drive the supercharger. A second power turbine was

geared to one of the engine propellers. The engine crankshaft was geared to a counter-rotating

propeller. From published engine specifications, this engine had tested specific fuel consumption

(SFC) of only 0.345 at full power. In comparison, the Excel Engine cycle analysis shows an SFC of

0.350 for the equivalent supercharged design. The Curtiss-Wright R3350 was a very successful

turbo-compound concept that was developed and produced in the 1950's and 1960's [3]. This

engine was an 18 cylinder radial engine, where a centrifugal compressor was geared to the

crankshaft and three exhaust turbines were spaced 120 degrees apart around the engine, which

were also geared to the crankshaft.

III. The Uniflow 2-stroke Turbo Compound Engine with Swirl Stratified Combustion

The proposed design features two-stroke, Uniflow, IC engine concept. The design

Equivalence Ratio is 0.6, with attendant lean combustion and lower peak flame temperatures. With

the stratified charge fuel injection, compression ratios can be very high without effecting

detonation or pre-ignition. In addition, we propose to coat inside of the cylinder head, cylinder

walls, and the piston crown with Aviation Industry Grade Thermal Barrier Coating (TBC), or in

another version use Ceramic Matrix Composite (CMC) material for these components. This

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coupled with lean combustion, allows also air cooling that results in extremely low heat losses

through the walls. The turbo-compound engine, Figure 1A has an exhaust driven turbine that

powers a compressor, and also has a geared exhaust driven power turbine that drives the engine

crankshaft. Thus, supercharger supplies combustion air to the Uniflow cylinder through tangential

inlet ports at the bottom of the piston stroke with uniform high velocity swirl. The exhaust gases

leave the cylinder through a single valve in the cylinder head. Furthermore, in contrast to a

conventional two-stroke engine, this engine features pressure lubrication with an oil sump, an oil

pump, an oil filter, and typical oil lubrication passages throughout the engine. The turbo

compound engine is essentially a gas turbine with an IC engine in place of the combustion

system. Figure 1B shows TS diagram for the engine scheme of Figure 1A. As shown in the

Figure, atmospheric air, with small boost from the propeller, enters the supercharger compressor

and exits at point 1 at higher temperature. It then flows through an intercooler (Point 1c), and is

then allowed to enter engine cylinders through one or more ports at the bottom of the cylinder.

Next, the air compressed to point 2 wherein the fuel is injected, ignited and burned at point 3. As

the piston moves down from TDC (point 3), during the power stroke the exhaust valve opens until

the cylinder pressure is slightly lower than the inlet port pressure. At this point (Point 4c), the

burned gases then enter supercharger turbine which drives the compressor. Next, the flow enters

the power turbine which is geared to the engine crankshaft through a reduction gear train.

A typical two-stroke engine features a loop scavenged cylinder with air intake ports at the

bottom that allow non-swirled combustion air from a pressurized crankcase. As the piston moves

upward, the top exhaust port is closed and the air compressed until near TDC, where fuel is

injected and ignited a few moments later. For the power stroke, as the piston moves down, the

cylinder pressures and temperatures are still very high – whereas the exhaust port is still open.

This results in significant heat loss due to high temperature and pressure gases being vented off

to the atmosphere. At the same time, some of the air also loops around the cylinder, and pushes s

exhaust gasses out through the exhaust ports. Thus, with this classical design, a substantial

amount of burned gasses are lost resulting in lower overall efficiency.

Figure 1A Supercharged

Turbo Compound Scheme Figure 1B TS Diagram

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Figure 2 Piston and Crank Movements

for the Proposed Design

In contrast, Figure 2 depicts the crank shaft movement that is designed to avail maximum

advantage of compression stroke, fuel injection and ignition times, maximum scavenge, and the

power stroke. First, the inlet port is configured in a series of tangential ports such that they impart

a strong swirl to incoming air when the piston is at the Bottom Dead Center. The exhaust port is

also open at this piston position in order to allow scavenging of the cylinder air by incoming high

pressure inlet air. As the piston moves upward, closing the inlet ports, the exhaust port is also

closed allowing the pressure and temperature to rise in the cylinder. At the near end of the

compression stroke, the fuel is injected in the same tangential direction as the swirl such that

fuel/air mixture retains circumferential stratification as rich mixture in the center disperses radially

outward. The design overall fuel air ratio is 0.04 to 0.05 – thus maintaining overall lean

combustion. By injecting fuel – just prior to the Top Dead Center (TDC), we also make a provision

for the fuel to atomize, evaporate and then mix prior to igniting it at the TDC, which also coincides

with the beginning of a power stroke. As the piston moves downward, the exhaust port is opened

just prior to it reaching the BDC. This initiates the scavenging which is further augmented by

opening of the inlet port at the BDC.

In the further embodiment of this design, we also anticipate that the design compression

ratio will be about 10:1, the piston crown, head, and cylinder walls to be coated with high

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temperature Thermal Barrier Coating material, and fuel injector providing atomized fuel droplets in

the range ten to fifteen microns.

A. Texaco Controlled Combustion Stratified Charge Engine:

Since stratified combustion is a major feature of the proposed engine, the following paragraphs

describe some of the salient features of the engine operation and the attendant benefits. As

described in details by Jain, Rife, and Keck [4], Figure 3, they attain swirling flow through the use

of a toroidal cavity formed on the piston head. The closed end of the cavity is in the form of a

toroid, while the open end is cylindrical in shape. The nominal diameter at the open end is almost

half that of the piston diameter, while the depth to diameter based aspect ratio of the cavity is

nearly one. As shown in Figure 3(a), the fuel injector is placed slightly upstream of the spark plug,

thus allowing the injected fuel spray sufficient evaporation and mixing times. Due to judicial

positioning of the injector, and the spark plug the hot gases tend to gravitate toward the center,

while the cold unburned fuel air mixture swirls around the core in a radially stratified fashion.

Figure 3(b) depicts the SFC variation as a function of mean effective pressure, and as shown it

varies between 0.25 lbs/HP-Hr to 0.40 lbs/HP-Hr. The results of the current design are compared

with this model in order to elucidate the salutary aspects of the proposed design.

Figure 3 Texaco Stratified Combustion Model

(a). TSCC Piston Model (b). SFC predictions

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IV. Thermodynamic Cycle Analysis

Figure 4 depicts the schematic of a base two stroke internal combustor that is configured

in the turbo compound engine as the proposed power plant. It features the standard two stroke

engine components, though with the following augmented features.

The engine features a high pressure super charger inlet air compressor system

that introduces high pressure, highly turbulent compressed air into the engine

inlet ports,

The inlet port consists of multiple tangential passages, such that, they impart

strong swirl to incoming air,

The piston crown is optimally designed to provide additional stratification to

incoming air,

The piston crown, cylinder side walls, and cylinder head – all are coated with high

temperature Thermal Barrier Coating (TBC), or use Ceramic Matrix Composite

(CMC) as the use material in order to minimize wall heat losses,

Figure 5b Engine cylinder head design

Figure 5a. Engine cylinder cross section

Exhaust Valve

Fuel

Injector

Spark Plug

Exhaust

(a) Inlet, Piston, and Exhaust

Configuration

(b) Engine Cylinder Head

Configuration

Swirl Rotation

Inlet

Piston

Fuel Injector

Spark Plug

Figure 4 A Schematic of the Base two-Stroke Engine

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Downstream of the combustor exhaust a high pressure inflow turbine is provided,

through which high pressure combustor exhaust gases pass. This step allows extraction

of additional power from the high pressure/high temperature exhaust gases,

Fuel is injected just prior to the end of the compression stroke, and is ignited when the

piston is at the Top Dead Center (TDC). This permits the formation of fuel rich core in the

center with lean mixture fanning radially outward. The fuel rich mixture burns readily,

when ignited. Thus, the proposed design offers a multi fuel use, where

Finally, a low power turbine is provided downstream that drives the crank shaft of the IC

engine, extracting more power out of the system.

Thus, the proposed design offers a Uniflow, stratified combustion for the lean fuel air

mixture with equivalence ratio ranging from 0.45 to 0.8. With the use of supercharger at the front

end, and an expansion turbine at the back end, it offers improved efficiency and lower SFC.

Furthermore, the Uniflow design pushes nearly all of the burned gases through the exhaust valve

resulting in Scavenge efficiency as high as 90%. With excess air, fresh fuel-air mixture, and with

very small amounts of burned gasses, there is an ample supply of Oxygen for the fuel to burn

quickly. These results in reduced combustion delay times, higher combustion efficiency, and

reduced emissions.

A. Preliminary Cycle Analysis:

The proposed two-stroke supercharged turbo compound engine is an in-line, 4-cylinder,

two-liter displacement engine with compression ratio of ten. The direct injection engine is

designed such that it is equipped with a supercharging compressor and turbine. The

compressor’s drive energy can be obtained from either the crankshaft or the turbine, and the

output of the turbo-compound engine as a system is calculated as the total of the crankshaft

output and the turbine output, minus the drive power of the compressor.

In order to evaluate the system performance that includes: number of cylinders,

displacement volume, compression ratio, heat generation, heat transfer characteristics,

supercharging characteristics, etc., a thermodynamic model was developed. In order to simplify

the analysis process, the following assumptions were made.

Each cylinder, intake manifold, and exhaust manifold gases were considered as separate

Entities and in thermal equilibrium at all the times,

The gases permeated in each direction across the boundary though they were considered

Uniformly mixed with thermal diffusion times being significantly lower than the mixing

times. Thus, the heat exchange across the boundaries occurred instantaneously,

The physical property data, such as, enthalpy, density etc., were obtained from JANAF

Thermo-Chemical Table, and were assumed to vary with temperature and mean gas

composition only.

The engine performance was evaluated for Sea Level Standard conditions: flow inlet

temperature and pressure being at 520 deg. R., and 14.7 psi, respectively. For the analysis, engine

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compression ratio was assumed to be ten while the inlet compressor ratio was assumed to be

five. The in-cylinder combustion was assumed lean with fuel equivalence ratio being 0.6.

For the cycle analysis of the turbo-compound engine, enthalpy changes for the

compressor and turbines and internal energy changes for the engine energy balance were

calculated using standard equations, with corrections applied for the component efficiencies. The

heat release was calculated from the heating value of the fuel and the fuel-air ratio. To reduce

combustion temperatures and NOx emissions, Lean burning, with an equivalence ratio of 0.6 of

was assumed for the full power operating condition.

Table 1 above shows the engine operating conditions for the design point, and results of

the model for three fuel/air equivalence ratios, and a given system airflow of 1.01 pps. As the table

shows, the brake SFC varies from 0.297 to 0.32, and has an inverse relationship with fuel to air

equivalence ratio. Evidently, as the fuel air ratio is increased, maximum cylinder temperature is

increased. This helps more efficient combustion, increased flame speed, and better stratification

where hotter core is surrounded by progressively cooler gases. Furthermore, the table shows

results only for one engine compression ratio, and one turbine expansion ratio. In the attendant

parametric study, it was also found that the SFC was significantly dependent on the compression

pressure ratio, while it was less dependent on turbine expansion parameter. In this case, SFC

improved with higher compressor pressure ratio.

Input Parameters -

Temperature ( R ) 520

Pressure (psia) 14.7

Fuel Heating Value (Btu/lb) 18650

Stoichemetric Fuel/Air Ratio 0.067

Engine Compression Ratio 10

Inlet Compressor Pressure Ratio 5

Fuel/Air Equivalence Ratio 0.8 0.6 0.4

Max Cylinder Pressure (psia) 4953 4521 3832

Max Cylinder Temperature ( R ) 4788 4370 3704

Brake Horsepower (with 1.01 lb/sec Airflow) 664 500 308

Brake SFC (lb Fuel/hp-hr) 0.297 0.298 0.32

SLS Standard Day Conditions

Cycle Analysis Results for Three Fuel/Air Equivalence Ratios

Table 1. Cycle Analysis Results for a Turbo-Compound IC Engine

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In order to validate the model results, the analysis was carried out for several comparable

turbo compound designs for which engine test data were available in the open literature. That

comparison is shown below in Table 2. For all of the following calculations the cycle conditions

were assumed to be the same above, the cylinder geometry was identical, the turbo charger and

turbo compound compressor and turbine parameters were assumed to be the same. As the Table

shows, the computed SFC for the proposed engine is 0.285, significantly lower than engines that

have been tested thus far, indicating potential for its UAV application.

IC Engine HP Test SFC Current Model SFC

Typical Auto Engine 200 0.55 0.567

Typical Diesel Engine 400 0.45 0.451

Napier Nomad 1 Inline Compound

Turboprop 3000 0.345 0.35

HAECO-Baker Compound Turbo Diesel 718 0.365 0.386

HAECO-Detroit Diesel Compound

Turbo Diesel 400

No

Results 0.392

Proposed Compound Turboprop

Engine 800

Not

Tested 0.285

B. Swirl Stratified Charge Combustion:

During the course of this study, it was realized that the nature of in-cylinder stratified

combustion is a critical phenomenon that controls overall engine efficiency, In particular, the

charge stratification features the following:

As the engine switches between part and full load quite frequently, the engine needs to

operate efficiently at various engine conditions, i.e. from lean to rich fuel air mixture

levels,

In this case, the f/a distribution is radially diffused, such that, it is rich near the center and

is lean moving radially outward. Thus, if the ignition is applied at the center, the mixture

ignites readily resulting in the highest temperature there, and lower temperature

elsewhere. This results in the case where the mixture is rich only in the center, while it is

lean in an averaged sense,

Table 2. Cycle Analysis Results for Several Engines

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Petrol IC engines have higher full load efficiencies, while the Diesel engines have higher

part load efficiencies. Swirl stratified combustion results in uniformly better overall

efficiency throughout full range of the engine operation,

Overall leaner combustion results in lower emissions.

There are several means of achieving fuel flow and so temperature stratification. These

include: pre chamber charge stratification, stratification thru’ structural changes in the piston

head, and the swirl stratification. From Among these methods, we have selected a combination of

air flow swirl and tangential fuel injection, such that, the fuel flow is co-swirling with the

combustion air. In this case, the fuel particles atomize – the smaller particles tend to coagulate

near the center – while the larger particles tend to move away from the igniter. Thus, the smaller

particles at the center burn more readily than the particles in the outer periphery providing a

temperature induced radial flow stratification.

Considering the fact that stratified charge combustion is a key factor in optimizing the

design, we have initiated detailed CFD studies in order to elucidate underlying fuel air mixing,

ignition, and combustion mechanisms. The following paragraphs describe some of the results of

an ongoing study.

V. Numerical Simulation of a Single Cylinder IC Engine

Cylinder Bore 3.67 in

Cylinder Stroke 3.68 in

Piston Stroke 4.68 in

Inlet port height 1.0 in

Engine speed 4000 rpm

Connecting Rod Length 9.0 in

Valve diameter 1.5 in

Valve maximum lift 0.5 in

Intake flow rate 0.062 lbm/s

Operating pressure 17.64 psi

Table 3. Engine dimensions

In this study, two transient simulations were performed using the CFD code, FLUENT. The

first case is a cold flow In-cylinder simulation with injected fuel (no combustion), while the second

case involves fuel ignition and combustion of n-heptane as the fuel in a simplified geometry. The

objectives of these analyses were to study air flow characteristics inside the engine cylinder, the

interaction of the air-fuel mixture, as well as resulting combustion.

B. Initial CFD Simulations:

The 3-D model is built using Unigraphics NX 6.0 version with the dimensions and

specifications in Table 3. The grid is generated using the ANSYS ICEM software. The initial grid

is shown in Figure 5 below. The mesh is 418348 cells in size containing hexahedral and

tetrahedral elements. At this point in the design evaluation, we are interested only in the

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qualitative behavior of the fluid flow. As a result, no particular attention was paid to optimize the

grid size, turbulence model, or turbulence - chemistry interactions. Hence, turbulence model used

is the standard two equation k-epsilon model, and the chemistry assumes fast chemistry

behavior.

Particle diameter 25 microns

Fuel flow rate 0.0017 lbm/s

Equivalence Ratio 0.4

Fuel / Air Ratio 0.0268

Fuel Temperature 520° R

Table 4. Fuel property

Since this is a transient solution, care needs to be taken in mesh generation. Towards that

end, first the fluid volume is ‘Chunked,” Figure 5, such that different meshing is implemented in

different chunked volumes. The hybrid approach involves layering and remeshing grids to

appropriately model the dynamic mesh. The layering zones require hexahedral cells and the

HEX Cells

Layering Zone

TET Cells

Remeshing Zone

TET Cells

Remeshing Zone

HEX Cells

Layering Zone

Small gap allowed between valve

seat and wall when the valve is closed.

Exit valve

Figure 5 Implemented Meshing Scheme in the Combustor Fluid Volume

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remeshing region requires tetrahedral cells. Stationary zones can be meshed using either

hexahedral or tetrahedral cells, where we have used tetrahedral elements for this configuration. It

should be noted that closing of the valve needs to be treated with care. In this case. the closing

without degenerating the wall cells was accomplished by specifying a minimum valve lift to

change the sliding interface to a wall, such that, cell faces do not actually come in contact with the

wall.

Figure 6 above shows results of initial CFD modeling. It renders the total velocity contours

inside the cylinder during the power stroke, after the exhaust valve opens: crank angle 110 deg. In

particular, it shows the flow movement during scavenging of the burned gases from the cylinder

into the exhaust. As shown in the Figure, at about 110 degree crank angle – the exhaust valve

opens allowing the combustion products to exhaust through the exit opening at the top. As the

piston moves downward, exhaust valve is nearly fully open at Crank Angle of 150 degrees

allowing most of the cylinder scavenging to complete before the piston reaches the BDC. At

Crank angle of 190 degrees, the exhausts as well as the inlet valves are open that facilitates still

further scavenging of the combustion flows from the cylinder. At crank angle of 220 degrees the

exhaust valve is closed, while the inlet port is fully open allowing the new air to enter the cylinder.

Figure 7 depicts some of the early combustion results. For the model, n-heptane fuel featuring 5-

species reaction was simulated. No valve motion was included in this analysis, with focus being

only on the compression stroke, fuel injection, assumed PDF turbulence chemistry interaction,

and eddy dissipation option for volumetric reactions. As Figure 7(a) shows, fuel is injected just

prior to piston reaching TDS (CA = 340 Deg.) in the direction of the swirl. The fuel seems to spread

in the gap between the cylinder head and the piston, ignites readily in the center of the cylinder.

As shown in Figure 7(b), the combustion occurs in the radially stratified fashion with highest

temperature being in the center.

(CA=110 Deg)

(d)

(CA=150 Deg)

(CA=190 Deg) (CA=220 Deg)

EV – Opens

IV - Closed

EV – Open

IV - Closed

EV – Open

IV - Open

EV - Closing

IV - Open

Figure 6 Total Velocity Contours During Power Stroke

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VI. Concluding Observations and Future work:

In this paper we have presented a novel IC engine concept that can find application in 200

HP to 2000 HP power plant range. It features combination of various SFC improvement ideas,

such as, near-full expansion engine, near full scavenging, CMC cylinder walls and piston head,

and stratified charge. From among these, the stratified charge seems to have the dominant

influence, and that is going to be the area of further investigation at Belcan.

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Appendix A

The Table below depicts the incremental benefit of introducing additional

technology feature into the conventional automotive engine.

References

1Chen, Gong. In-Cylinder Combustion and Output Performance and Emissions Influenced by

Split Fuel Injection Input Parameters of Liquid-Fuel Combustion Engines. 10th Annual

International Energy Conversion Engineering Conference, May 2012. ID: 1284328. 2Huang Z H, Wang H W, Chen H Y. Study on combustion characteristics of a compression

ignition engine fueled with dimethyl ether. Proc. Inst . Mech. Eng, Part D, J Automobile Eng, 1999,

213 (D6): 647-652. 3Huang Z H, Jiang D M, Zeng K, Liu B, Yang Z L. Combustion characteristics and heat release

analysis of a DI compression ignition engine fueled with Diesel-dimethyl carbonate blends. Proc.

Inst. Mech. Eng, Part D, J Automobile Eng, 2003, 217(D7): 595–606. 4Jain, B.C., Rife, J. M., and Keck, J. C., A Performance Model for the Texaco Controlled

Combustion Stratified Charge Engine, SAE Paper No. 760116, Automotive Engineering Congress

and Exposition, Detroit, Michigan, 1976.

Turbo Compound Full Expansion

1 Conventional Automotive Engine 0.58 0.58

2 Swirl with Stratified Charge 0.5 0.48

3 Reduced Cooling Heat Loss 0.43 0.4

4 Exhaust Turbine or Full Expansion 0.3 0.31

SFC

Table A-1 - IC Engine Efficiency Improvement

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