AER 710 Aerospace Propulsion
Introduction
Propellers
 Internal Combustion Engines
Gas Turbine Engines
Chemical Rockets
Non-Chemical Space Propulsion Systems
Rolls-Royce 12-cyl. Merlin XX
Caproni CA.36 using Isotta-Fraschini 6-cyl. V.4B
Introduction to Internal Combustion Engines
• In an IC engine, the fuel is burned in
reacting with incoming air, within the engine,
to produce the required heat energy
• A number of different powerplants (sparkignition, compression-ignition, rotary) under
the IC engine category that can be used for
driving a propeller shaft
• Each share the characteristic of intermittent
combustion in their cyclic operation
Spark-Ignition Engines
• A conventional spark-ignition (SI) IC engine
is commonly associated with a 4-stroke
operational cycle, with reciprocating pistons
in cylinders used to drive (rotate) the main
driveshaft (crankshaft) that in turn rotates
the propeller shaft (directly, or indirectly
through a propeller speed reduction unit
gear-box, if the driveshaft speed is too high)
• Some variants may have fuel injection, and
perhaps turbosupercharging, to improve
performance
Four-stroke operation of spark-ignited internal combustion engine (NASA report, 1985)
Cross-section of four-stroke cycle SI engine showing engine components; (A) block, (B) camshaft, (C)
combustion chamber, (D) connecting rod, (E) crankcase, (F) crankshaft, (G) cylinder, (H) exhaust
manifold, (I) head,(J) intake manifold, (K) oil pan, (L) piston, (M) piston rings, (N) push rod, (0) spark
plug, (P) valve, (Q) water jacket
From: W.W. Pulkrabek, “Engineering Fundamentals of the Internal Combustion Engine”, 1997
Chair of Department
Dr. Kamran Behdinan
Dr. David Greatrix
Dr. David Greatrix
Assoc. Chair of Department
Cutaway diagram of an in-line four-cylinder Chrysler 4-stroke sparkignition engine for automotive applications. A connecting rod connects the
reciprocating piston (up-and-down motion) to the rotating crankshaft. The
intake-compression-combustion-exhaust process is illustrated at the right
In-line
SI Piston Engine Configurations
Radial
V
Horiz.-opposed
From: W.W. Pulkrabek, “Engineering Fundamentals of the Internal Combustion Engine”, 1997
Schematic cutaway diagram of front view of Chevrolet Corvair
horizontally-opposed “flat-six” turbosupercharged air-cooled
six-cylinder engine, for automotive and aircraft applications.
• present-day lighter aircraft are powered by
two, four or six air-cooled cylinders in a
horizontally opposed configuration for a
given engine
• lower-cost normally-aspirated engines likely
using the older carburetion approach for
fuel-air vapor delivery to the cylinders via a
carburetor and intake manifold, versus fuel
injection directly or almost directly into the
cylinder
• higher performance engines are likely
turbosupercharged and fuel-injected
Throat of carburetor with secondary venturi. The small secondary venturi
gives a large pressure drop and good fuel flow control, while the larger
primary throat offers less resistance to the main air flow. At higher altitudes,
the venturi is susceptible to icing (not a good thing), thus the use of
carburetor heating
From: W.W. Pulkrabek, “Engineering Fundamentals of the Internal Combustion Engine”, 1997
• SI engines generally run on gasoline (automotive
gasoline sometimes called mogas), and for aircraft
piston engines, aviation gasoline (avgas), which
has a higher octane level than mogas to prevent
detonation (knocking) at higher cyl. pressures
• Fuels are characterized in part by their flash point;
the flash point is the lowest temperature at which
they will ignite in air or oxygen under standard
conditions
• combustion process needs some fuel vapor
present to proceed, i.e., some vapor pressure; the
higher the volatility of the given fuel (i.e., its vapor
pressure), the lower the flash point
• Fuel is typically stored in the wing or in wing-tip
tanks, for greatest safety in the event of a crash
• Tanks can be constructed of aluminum, since that
lighter metal does not chemically react significantly
with aviation fuels. Synthetic rubber or nylon fuel
bladders may be used for containing fuel in some
applications, e.g., for tanks constructed of
composite materials having micro-pores that
would leak fuel, otherwise. Bladders, while adding
weight to the aircraft (not a good thing), do provide
additional safety in the event of a crash
Thermodynamic Cycle, SI Engines
• ideal air-standard (air as working medium)
Otto cycle approximates the open-cycle
performance of a spark-ignition IC engine
• actual 4-stroke cycle, beginning with outward stroke 1 from TDC to
BDC, one has a fresh fuel-air mixture being taken into the cylinder.
Inward stroke 2 from BDC to TDC is compression of this gaseous
mixture, followed by ignition via a spark and subsequent burning.
Outward power stroke 3 from TDC to BDC results in the expansion
of the hot combustion products as the net cylinder volume
increases. Inward stroke 4 from BDC to TDC results in the
exhausting of these products out of the cylinder
• typically less efficient 2-stroke cycle combines the fuel-air intake and
exhaust strokes from the 4-stroke case into a shorter transitional
process near BDC. This process is called scavenging, and utilizes
some variant of a pump/port mechanism to implement the intake of
fresh fuel-air mixture and exhaust of combusted gas. One
advantage of 2-stroke engines is that they do have a higher powerto-weight ratio, given the fact that intermittent power delivery is
every driveshaft revolution, as opposed to every two driveshaft
revolutions for the 4-stroke case
Illustration of operational cycle of 2-stroke spark-ignition piston engine.
Flapping reed or leaf valve commonly employed for intake of fresh fuel-airoil mixture from carburetor into cylinder. The schematic shows the finning
about the cylinder walls and head, as an efficient means for dissipating
heat from the metal structure (more surface area) in an air-cooling
approach. Diagram courtesy of HowStuffWorks.com
Common scavenging geometries for two-stroke cycle engines. (a) Cross scavenged with intake
ports and exhaust ports on opposite sides of the cylinder. (b)Loop scavenged with intake ports and
exhaust ports on the same side of the cylinder.(c) Through-flow scavenged, with intake ports in cylinder
walls and exhaust valve in head. Other variations and combinations of these types exist, depending
on the placement of slots and/or valves
From: W.W. Pulkrabek, “Engineering Fundamentals of the Internal Combustion Engine”, 1997
Operations in a 2-cycle spark-ignition engine
Thermal efficiency of Otto cycle:
T4
1)
mCv ( T4  T1 )
QH  QL
QL
T1 T1
 th 
 1
 1
 1
QH
QH
mCv ( T3  T2 )
T2 T3
( 1)
T2
(
Via observation of the two cycle diagrams:
T1
1
 th  1   1  rv1  1   1
T2
rv
rv  V1 / V2  V4 / V3
Thus, efficiency is a function of compression ratio; 7 to 9 is
the typical SI detonation (knock) limit with avgas.
Net ideal useful work as fcn. of mean effective pressure:
wnet  mep ( v3  v1 ) ,
Also:
J/kg of air
wnet  q H  q L   th q H
Ideal shaft power for 4-stroke cycle (1 power stroke every
two engine shaft revolutions; 2-stroke cycle would change
½ to 1):
1
V h = head displacement
PS ,i  mep  Vh  ( N  )
2
Brake specific fuel consumption:
BSFC 
m f
PS
Shaft power as a function of intake manifold air pressure (MAP; in
inches of mercury, where 1 atm = 29.9 in Hg = 760 mm Hg) and engine
shaft speed (rpm) at full and part throttle, at sea level. The data is
representative of a small 4-stroke normally-aspirated carbureted sparkignition piston engine, the air-cooled horizontally-opposed four-cylinder
Lycoming O-320-A2B
Shaft power as a function of altitude and engine shaft speed at full
throttle, Lycoming O-320-A2B. Corresponding dashed curves for MAP
are also shown
Fuel consumption rate in U.S. gallons per hour as a function of MAP and
engine shaft speed, Lycoming O-320-A2B
Compression-Ignition Piston Engines
• For CI (diesel) engines, fuel injection (vs.
carburetion) directly into the cylinder is required,
and for higher performance for aircraft
applications, turbosupercharging is also likely
required
• can operate at higher compression ratios of 15:1
up to 20:1, without too much concern about
diesel fuel detonation (knock)
• 2-stroke variant is actually more popular for
aircraft, given the higher power-to-weight ratio
discussed earlier
Illustration of a turbosupercharged 160-hp two-stroke four-cylinder (V-4)
DeltaHawk aircraft diesel engine (DH160A4/V4/R4). The engine can run
on diesel or jet fuel. Diagram courtesy of DeltaHawk Engines, Inc.
Illustration of internal workings of a 2-stroke compression-ignition piston
engine. Diagram courtesy of HowStuffWorks.com
4-stroke diesel engine operations
2-stroke diesel engine operations
Thermodynamic Cycle, CI Engines
• ideal air-standard (air as working medium)
Diesel cycle approximates the open-cycle
performance of a comp.-ignition IC engine
•
•
Process 12 represents an isentropic compression of air as the piston
moves from BDC to TDC. Process 23 is representative of heat being
added at a constant pressure while the piston is moving downward from
TDC to some intermediate point in the power stroke, until combustion is
completed. In the actual case, one has fuel injection with subsequent,
relatively slower burning (note: a glow plug heat source may be required for
starting the engine initially, but thereafter would typically not be required for
sustained operation), and further resulting pressurization. Process 34
represents an isentropic expansion, as the piston moves further downward
until reaching BDC. Process 41 is representative of heat being rejected
from the air and fresh air in turn being take in, while the piston is in the
vicinity of BDC
Thermal efficiency of Diesel cycle:
T4
( 1)
1 T
 th  1   1 1
T3
rv
( 1)
T2
Net ideal useful work as fcn. of mean effective pressure:
wnet  mep ( v1  v2 )
Also:
, J/kg (of air)
wnet   th q H  q H  q L
On the negative side, higher diesel engine weights are
required due to the higher operating cylinder
pressures. On the other hand, BSFC is potentially
quite low, on the order of 0.35 lb/hrhp (0.21
kg/hrkW) or less for existing aircraft diesels, thus
there is some incentive to continue research and
development on such items as lighter weight engine
materials.
Rotary Engines
• Piston rotary approach used in the early
20th century, having the crankcase and
cylinders in a radial configuration rotating
in conjunction with the propeller, while the
crankshaft was nominally fixed to the
airplane; high roll torque and aerodynamic
drag were undesirable characteristics
Gnome 100-hp 9-cyl. piston rotary engine
Fokker E.III Eindecker using an Oberursel U.I 9-cyl. 100-hp piston rotary engine
• The most common pistonless rotary design is the
Wankel engine, which uses an offset cam-shaped rotor
(vane) rotating about a center output driveshaft, which
also rotates in conjunction
• Because their volumetric displacement versus total
engine size is quite high relative to the reciprocating
piston engines, the power-to-weight ratio, PS/WE , can be
quite high, on the order of 1 to 3 hp/lb (as compared to
0.5 to 1 hp/lb for SI piston engines). These engines
typically use spark ignition, with the intake, compression,
ignition, expansion (power) and exhaust processes
occurring smoothly in going around one cycle of the rotor
• Sealing and maintenance are traditional design issues
• Run at higher engine shaft speeds, likely necessitating a
PSRU for the propeller
1. Engine inlet
2. Exhaust outlet
3. Housing
4. Combustion chambers
5. Stationary gear
6. Rotor
7. Internal gear
8. Eccentric shaft
9. Spark plug
Wankel rotary engine schematic
Photo at top left of opened rotary engine, courtesy of Mazda Motor
Corporation. Photo at top right of AR731 air-cooled single-rotor 38-hp
Wankel rotary engine for unmanned aircraft applications, courtesy of
UAV Engines Ltd. Photo at bottom right of AR682 water-cooled twinrotor 95-hp Wankel rotary engine for unmanned aircraft applications,
courtesy of UAV Engines Ltd.
Illustration of internal operations of a Wankel rotary engine.
Diagram courtesy of HowStuffWorks.com
Turbosupercharging
• performance of an IC engine (spark- or
compression-ignition) can be enhanced by
supercharging, i.e., compressing the air
entering the intake
• Superchargers are gear-driven off the
main engine driveshaft, and as a result,
their performance, while effective, is
somewhat limited and inefficient
Schematic diagram of conventional gear-driven supercharger setup
Rolls-Royce Merlin 62, with two-stage (compressor),
two-speed (gear-box), intercooled supercharger at rear
of engine
• To avoid the issue of bleeding off the main
shaft’s power to run a front-end
compressor, turbosuperchargers (TSCs)
were introduced
• TSCs employ a turbine in the exhaust
stream, exploiting available energy that
would otherwise be lost; the back-end
turbine in turn drives a front-end
compressor for mechanical air
pressurization, and thus bypasses the
need to extract power from the driveshaft
• fuel-air detonation pressure limit in the
downstream combustion chambers should
not be exceeded; similarly, pressure on
engine structure should not exceed
allowed threshold
• An intercooling (aftercooling; charge air
cooling) approach is sometimes used to
enhance performance (via increased
combustor intake air density), by using a
heat exchanger to remove heat (cooling
the air) that is leaving the TSC, prior to
entering the combustion chamber
Schematic diagrams of turbosupercharger setup
Schematic diagrams of turbosupercharging setups
Diagram courtesy of University of Notre Dame
Front view of the Boeing B-17 Flying Fortress bomber of WWII fame.
The three-bladed variable-pitch propeller in view (one of four) is being
driven by a 1200-hp Wright R-1820 Cyclone 9 air-cooled spark-ignition
piston engine (turbosupercharged; radial 9-cylinder arrangement). The
General Electric designed turbosupercharger allowed the engine to
maintain sea-level power up to 25000 ft in altitude. Radial engines always
come in an odd number of cylinders (like 9) due to the alternate cylinder
firing order sequence requirement (e.g., 1-3-5-7-9-2-4-6-8)
Shaft power as a function of altitude and MAP at an engine shaft speed
of 2700 rpm. The data is representative of a small 4-stroke
turbosupercharged fuel-injected spark-ignition piston engine, the aircooled horizontally-opposed six-cylinder Teledyne Continental
TSIO-520-WB
Fuel consumption rate as a function of percentage of maximum rated
shaft power (max. rated MAP = 39 in Hg), turbosupercharged
Teledyne Continental TSIO-520-WB
Note on Pulsejets and PDEs
• The pulsejet engine can be considered a
transitional development between the older
reciprocating piston engines and the newer gas
turbine engines
• intermittent fuel-air burning cycle, as compared
to the continuous burning seen with gas turbines
• exhausting a hot jet to provide thrust (as
opposed to using a propeller)
• PJ has no mechanical compressor or turbine;
constant-volume combustor (vs. constantpressure combustor of gas turbine)
Photo of an American “copy” of a German pulsejet-powered V-1 “buzzbomb” cruise missile, reproduced in large measure from reverseengineering those seized by American forces towards the end of WWII, for
post-war flight testing and technology demonstration by the USAF.
Eventually, a lot of these copies were used, and destroyed, as target drones
Schematic diagram of conventional valved pulsejet
t cyc 
Illustration of operational cycle of conventional pulsejet
4 c / p
a1

1
f cyc
Illustration of representative wave activity in various locations of the
pulsejet
F
Illustration of idealized (sinusoidal) combustor pressure, and
corresponding thrust, as a function of time
Fmax

Valveless PJs
• Valveless (aerovalved) PJ designs remove the need for
a front-end combustor intake valve (valves tend to wear
out quickly). These designs can vary, but typically
depend on accurate tuning of the pressure wave
passage in the applicable duct (combustion tube) system
in order to operate efficiently. These designs allow for
backflow of air out of the intake, which can be a problem
if excessive, whereas the valved designs prevent
backflow altogether with a mechanical valve
• Valved or valveless, not much interest currently in direct
usage of PJs as the principal propulsion system;
elements of PJ technology, like wave rotors and PDEs,
may have more promise
Temperature distribution in one type of valveless pulsejet, V = 50 m/s
Courtesy of Applied Energy Research Laboratory, North Carolina State University (Dr. William Roberts)
Pulse Detonation Engines
• PDE operation depends more on higher-speed, higherheat- input, higher-strength supersonic detonation waves
moving downstream (rightward) in the
intake/combustion/exhaust (detonation/combustion tube)
duct system (flame front moves with the compression
shock front), as opposed to weaker slightly supersonic
compression and sonic rarefaction waves moving
significantly faster than the subsonic deflagration front of
the flame within the PJ combustor and tailpipe
• Candidate propulsion system for high-speed flight
applications (competing against more established
ramjet, and scramjet, engines for consideration)
Illustration of operational cycle of conventional pulse detonation engine.
Diagram courtesy of Aerodynamics Research Center, University of Texas
at Arlington (Dr. Frank K. Lu)
Diagrams of p-v and T-s profiles for ideal Humphrey cycle,
applicable to constant-volume combustor
Thermal efficiency, Humphrey cycle
T3 1
( ) 1
T
T
 th  1   1  { 2
}
T3
T2
1
T2