Chapter 9 Gas Power Systems
Ta-Hui Lin
1
2
Internal and External Combustion Engines (1 of 2)
►An internal combustion engine (ICE or IC engine) is a heat engine
in which the combustion of a fuel occurs with an oxidizer (usually
air) in a combustion chamber that is an integral part of the working
fluid flow circuit.
►In an internal combustion engine, the expansion of the hightemperature and high-pressure gases produced by combustion
applies direct force to some component of the engine.
►The force is applied typically to pistons (piston engine), turbine
blades (gas turbine), or a nozzle (jet engine).
►This force moves the component over a distance, transforming
chemical energy into useful kinetic energy and is used to propel,
move or power whatever the engine is attached to.
3
Internal and External Combustion Engines (2 of 2)
►In contrast, in external combustion engines, such as steam or
Stirling engines, energy is delivered to a working fluid not
consisting of, mixed with, or contaminated by combustion products.
►Working fluids for external combustion engines include air, hot
water, pressurized water or even liquid sodium, heated in a boiler.
►Two principal types of reciprocating internal combustion engines
are the spark-ignition engine and the compression-ignition engine.
4
Reciprocating Internal Combustion Engines
►The spark-ignition (SI) engine
 A mixture of fuel and air is ignited by a spark plug.
 This type is
- advantageous for applications up to about 300 hp (225 kW).
- lightweight and relatively low cost.
- predominantly used by automobiles.
►The compression-ignition (CI) engine
 Air is compressed to a high pressure and temperature.
 Combustion occurs spontaneously when fuel is injected.
 This type is
- preferred for high-power applications and when fuel economy is required.
- used in heavy trucks and buses, locomotives and ships, and auxiliary
power units.
5
Introducing Engine Terminology (1 of 5)
► The bore of the cylinder is its diameter.
► The stroke is the distance the piston moves in one direction.
► The piston is said to be at top dead center when it has moved
to a position where the cylinder volume is a minimum.
► This minimum volume is known as the clearance volume.
► When the piston has moved to the position of maximum
cylinder volume, the piston is at bottom dead center.
► The volume swept out by the piston as it moves from
the top dead center to the bottom dead center position is
called the displacement volume.
► The compression ratio r is defined as the volume at bottom
dead center divided by the volume at top dead center.
► The reciprocating motion of the piston is converted to rotary
motion by a crank mechanism.
FIG. 9.1
6
Introducing Engine Terminology (2 of 5)
►In a four-stroke IC engine, the piston executes four distinct strokes
within the cylinder for every two revolutions of the crankshaft.
►Intake stroke
 With the intake valve open, piston stroke draws
a fresh charge into the cylinder.
 For SI engines, the charge includes fuel and air.
 For CI engines, the charge is air alone.
►Compression stroke
 With both valves closed, piston compresses charge,
raising the pressure and temperature, and requiring
work input from the piston to the cylinder contents.
 For SI engines, combustion is initiated by the spark plug.
 For CI engines, combustion is initiated by injecting
fuel into the hot compressed air.
FIG. 9.2
7
Introducing Engine Terminology (3 of 5)
►Power stroke
 The gas mixture expands and work is done on the piston as it returns to
bottom dead center.
 For SI engines, heat addition (combustion) is
simulated at constant volume by the Otto cycle.
 For CI engines, heat addition (combustion) is
simulated at constant pressure by the Diesel cycle.
►Exhaust stroke
 The burned gases are purged from the cylinder
through the open exhaust valve.
►IC engines undergo mechanical cycles, but the
cylinder contents do not execute a thermodynamic
cycle – matter is introduced at one composition and
is later discharged at a different composition.
FIG. 9.2
8
Introducing Engine Terminology (4 of 5)
9
Introducing Engine Terminology (5 of 5)
►The mean effective pressure, mep, is the theoretical constant
pressure that, if it acted on the piston during the power stroke,
would produce the same net work as actually developed in one
cycle.
net work for one cycle
(Eq. 9.1)
mep=
displacement volume
►For two engines of equal displacement volume, the one with a
higher mep would produce the greater net work, and if the engines
run at the same speed, greater power.
10