Basic notes about engine performance

1. KMJ36303: ENERGY &
POWER
IC ENGINE PERFORMANCE

2. 2
REVERSIBLE AND IRREVERSIBLE PROCESSES
Reversible processes deliver the most and
consume the least work.
Reversible process: A process that can be reversed without leaving any trace on
the surroundings.
Irreversible process: A process that is not reversible.
• All the processes occurring in nature are irreversible.
• Why are we interested in reversible processes?
• (1) they are easy to analyze and (2) they serve as
idealized models (theoretical limits) to which actual
processes can be compared.
• Some processes are more irreversible than others.

3. 3
Irreversibilities
Friction
renders a
process
irreversible.
Irreversible
compression
and
expansion
processes.
(a) Heat transfer
through a
temperature
difference is
irreversible, and
(b) the reverse
process is
impossible.
• The factors that cause a process to be irreversible
are called irreversibilities.
• They include friction, unrestrained expansion,
mixing of two fluids, heat transfer across a finite
temperature difference, electric resistance,
inelastic deformation of solids, and chemical
reactions.
• The presence of any of these effects renders a
process irreversible.

4. 4
THE CARNOT CYCLE
Reversible Isothermal Expansion (process 1-2, TH = constant)
Reversible Adiabatic Expansion (process 2-3, temperature drops from TH to TL)
Reversible Isothermal Compression (process 3-4, TL = constant)
Reversible Adiabatic Compression (process 4-1, temperature rises from TL to TH)
Execution of the Carnot cycle in a closed system.

5. 5
The Reversed Carnot Cycle
The Carnot heat-engine cycle is a totally reversible cycle.
Therefore, all the processes that comprise it can be reversed, in which
case it becomes the Carnot refrigeration cycle.

6. 6
BASIC CONSIDERATIONS IN THE ANALYSIS OF
POWER CYCLES
Most power-producing devices operate on cycles.
Ideal cycle: A cycle that resembles the actual cycle
closely but is made up totally of internally reversible
processes is called an Ideal cycle.
Reversible cycle such as carnot cycle have the
highest thermal efficiency of all heat engines
operating between the same temperature levels.
Unlike ideal cycles, they are totally reversible, and
unsuitable as a realistic model.

7. 7
The ideal cycles are internally reversible, but, unlike the Carnot cycle, they are not
necessarily externally reversible.
Therefore, the thermal efficiency of an ideal cycle, in general, is less than that of a
totally reversible cycle operating between the same temperature limits.
However, it is still considerably higher than the thermal efficiency of an actual cycle
because of the idealizations utilized.

8. 8
OTTO CYCLE: THE IDEAL CYCLE FOR
SPARK-IGNITION ENGINES

9. 9
The two-stroke engines are
generally less efficient than
their four-stroke
counterparts but they are
relatively simple and
inexpensive, and they have
high power-to-weight and
power-to-volume ratios.
Four-stroke cycle
1 cycle = 4 stroke = 2 revolution
Two-stroke cycle
1 cycle = 2 stroke = 1 revolution

10. 10
Air enters the cylinder through the open
intake valve at atmospheric pressure P0
during process 0-1 as the piston moves from
TDC to BDC.
The intake valve is closed at state 1 and air is
compressed isentropically to state 2. Heat is
transferred at constant volume (process 2-3);
it is expanded isentropically to state 4; and
heat is rejected at constant volume (process
4-1).
Air is expelled through the open exhaust
valve (process 1-0).
Work interactions during intake and exhaust
cancel each other, and thus inclusion of the
intake and exhaust processes has no effect on
the net work output from the cycle.
However, when calculating power output
from the cycle during an ideal Otto cycle
analysis, we must consider the fact that the
ideal Otto cycle has four strokes just like
actual four-stroke spark-ignition engine.

11. 11
In SI engines, the
compression ratio
is limited by auto-
ignition or
engine knock.

12. 12
AN OVERVIEW OF RECIPROCATING ENGINES
• Spark-ignition (SI) engines
• Compression-ignition (CI) engines
Compression ratio

13. 13
Mean effective pressure
The mean effective pressure can be used as a
parameter to compare the performances of
reciprocating engines of equal size.
The engine with a larger value of MEP
delivers more net work per cycle and thus
performs better.

14. 14
Indicated power (Ip), Friction power (Fp), Brake
power (Bp)
4-stroke engine
𝐼𝑝 =
𝑝𝑖𝐴𝐿𝑁𝑛
2
2-stroke engine
𝐼𝑝 = 𝑝𝑖𝐴𝐿𝑁𝑛
pi: indicated mean
effective pressure
A: Area of piston
L: Length of stroke
N:RPM
n: number of piston

15. 15
Indicated power (ip), Friction power (fp), Brake
power (bp)
Brake power (bp) = 2πNT
Friction power (fp) = ip - bp
Mechanical Efficiency =
𝑏𝑝
𝑖𝑝
Specific fuel consumption (sfc) =
𝑚𝑓
𝑏𝑝
* 𝑚 = mass flow rate of fuel consumed, Q = net heat value of the fuel
Brake thermal efficiency (ηbt)
=
𝑏𝑝
𝑚𝑓×𝑄
Indicated thermal efficiency (ηit)
=
𝑖𝑝
𝑚𝑓×𝑄

16. 16
Volumetric efficiency, 𝜂𝑣 and air-
flow rate, 𝑚𝑎
𝜂𝑣 =
𝑉𝑎
𝑉
𝑠
𝜌𝑎 =
𝑚𝑎
𝑉
𝑎
A x L
𝑚𝑎 = 𝑚𝑎𝑁
𝑚𝑎 =
𝑚𝑎𝑁
2
4-stroke engine
2-stroke engine
Piston Speed, Sp
𝑆𝑝
𝑠
=
𝜋
2
sin 𝜃 1 +
cos 𝜃
𝑅2 + 𝑠𝑖𝑛2𝜃
𝑅 = 2
𝑟
𝑠
Sp: Piston Speed
θ: Angle of Combustion
s: Stroke

17. 17
DIESEL CYCLE: THE IDEAL CYCLE
FOR COMPRESSION-IGNITION ENGINES
In diesel engines, only air is compressed during the
compression stroke, eliminating the possibility of auto-ignition
(engine knock). Therefore, diesel engines can be designed to
operate at much higher compression ratios than SI engines,
typically between 12 and 24.
1-2 isentropic
compression
2-3 constant-
pressure heat
addition
3-4 isentropic
expansion
4-1 constant-
volume heat
rejection.

18. 18

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