Full expansion ic engine

1
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 Charneski3
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
DownloadedbyJayeshMehtaonJanuary10,2017|http://arc.aiaa.org|DOI:10.2514/6.2015-3780
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

2
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

3
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

5
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

13
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.

14
Appendix A
The Table below depicts the incremental benefit of introducing additional
technology feature into the conventional automotive engine.
References
1
Chen, 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.
2
Huang 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.
3
Huang 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.
4
Jain, 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

Full expansion ic engine

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