OVERVIEW
The gasoline engine, also known as petrol engine, is a
class of internal-combustion engines that generate
power by burning a volatile liquid fuel, in this case
gasoline, with ignition initiated by an electric spark.
The most commonly used power cycle in a gasoline
(petrol) engine is the Otto Cycle.
ABOUT THE OTTO CYCLE
Nikolaus August Otto, a German Engineer, involved himself in the
study of heat engines and built his first working four-stroke engine
in 1876. Coal gas and air mixture were used as working fluid.
Wilhelm Maybach joined Otto to revamp the model based on
Otto’s theory and made an engine called Otto Engine.
It involves a series of pressure, temperature, volume, and heat
exchange changes, ultimately converting fuel into motion. This
cycle is utilized in both 2-stroke and 4-stroke engine
configurations.
TWO-STROKE VS. FOUR-STROKE ENGINE
TWO-STROKE FOUR-STROKE
• Completes the Otto Cycle in four • Completes the Otto Cycle in two
piston strokes or two revolutions of piston strokes or only one revolution
the crankshaft, namely: Intake, of the crankshaft. One stroke
Compression, Power, and Exhaust combines compression, intake, and
Stroke. exhaust, while the latter combines
power, exhaust, and intake stroke.
• Simpler, lighter, and more powerful
but less efficient and produce more • More efficient, durable, and
emissions. environmentally friendly, but are
heavier and more complex.
ANIMATED PROCESS
TWO-STROKE VS. FOUR-STROKE ENGINE
TWO-STROKE ENGINE FOUR-STROKE ENGINE
STEP-BY-STEP PROCESS
TWO-STROKE VS. FOUR-STROKE ENGINE
TWO-STROKE ENGINE
STEP-BY-STEP PROCESS
TWO-STROKE VS. FOUR-STROKE ENGINE
FOUR-STROKE ENGINE
STATE POINTS AND PROCESSES
OTTO CYCLE
1→2 : Isentropic Compression 3→4 : Isentropic Expansion
2→3 : Isometric Heat Addition 4→1 : Isometric Heat Rejection
P-v Diagram of Otto Cyle T-s Diagram of Otto Cyle
STATE POINTS AND PROCESSES
INTERNAL COMBUSTION ENGINE (ICE)
WITH RESPECT TO THE OTTO CYCLE IN A FOUR-STROKE ENGINE
At Pt. 1 : Before Compression, After Heat Rejection
At Pt. 2 : After Compression, Before Heat Addition
At Pt. 3 : Before Expansion, After Heat Addition
At Pt. 4 : After Expansion, Before Heat Rejection
NOTE: Combustion occurs at 2→3, while the maximum temperature and
pressure is achieved at Pt. 3.
NOTE: The strokes in the ICE does not equate to the four state points in
the Otto Cycle. The inlet stroke is essentially considered as Pt. 0.
APPLICABLE FORMULAS
BASED ON PROCESSES AT STATE POINTS
ISOMETRIC ISENTROPIC
(V = C) (S = C)
Pv - T Relation
Heat Transferred, Q
Ideal Gas Law
PROPERTIES OF AIR
FOR OTTO CYCLE AT STANDARD CONDITION
S.I. UNIT (METRIC SYSTEM) ENGLISH UNIT
Specific Heat
Constant Volume, Cv
Gas Constant, R
Gas Ratio, k
PARAMETERS FOR
POWER MEASUREMENT
• Clearance Percentage
• Compression Ratio
• Displacement Volume
• Mean Effective Pressure (MEP)
• Net Work
• Thermal Efficiency
• Volumetric Efficiency
CLEARANCE PERCENTAGE, C
The percentage of the total volume that remains when
the piston is fully at the top of its stroke. It indicates how
much of the cylinder volume remains uncompressed at
Top Dead Center (TDC).
Mathematically expressed as:
COMPRESSION RATIO,
It measures how much the fuel-air mixture is compressed
before ignition. Higher compression ratio means lower
clearance percentage, thus more of the cylinder volume
is used for compression which leads to higher thermal
efficiency, and vice versa.
Mathematically expressed as:
DISPLACEMENT VOLUME,
The volume of an engine's cylinder that the piston moves
through when it goes from one dead center to the other.
It is therefore, the volume swept by the piston from
Bottom Dead Center (BDC) to Top Dead Center (TDC) in a
single stroke.
Mathematically expressed as:
GRAPHICAL REPRESENTATION
DISPLACEMENT AND CLEARANCE VOLUME
From the figures, we can observe that:
NET WORK,
Following the second law of thermodynamics, it is the
difference between the heat energy added during combustion
and the heat energy rejected during the exhaust process. It
represents the useful work done by the cycle to produce
mechanical power.
Mathematically expressed as:
(if mass flow rate is involved)
NOTE:
MEAN EFFECTIVE PRESSURE, MEP
The average pressure needed to act on the piston as it
moves one displacement in order to produce work. It is a
theoretical constant pressure that, if applied uniformly
over the entire power stroke of an internal combustion
engine, would produce the same work output as the
actual cycle.
Mathematically expressed as:
THERMAL EFFICIENCY,
Represents how effectively the cycle converts the fuel’s
chemical energy into mechanical work. Since the Otto
cycle is an idealized thermodynamic cycle for spark-
ignition internal combustion engines, its thermal
efficiency depends on the compression ratio and the
specific heat ratio of the working fluid.
Mathematically expressed as:
VOLUMETRIC EFFICIENCY,
Measures how effectively the engine fills its cylinders with
fresh air-fuel mixture during the intake stroke. It is defined
as the ratio of the actual air intake to the theoretical
maximum air intake at the given conditions. Higher
volumetric efficiency leads to better combustion
efficiency and more power output.
Mathematically expressed as:
SAMPLE PROBLEM #1
What is the efficiency of an Otto cycle with a compression ratio of 6:1?
Note: the gas used is air.
Given: Solution:
Req’d:
SAMPLE PROBLEM #2
What is the clearance of an ideal Otto engine if the efficiency is 51.16 %
and k= I.3 ?
Given: Solution:
and: thus:
Req’d:
SAMPLE PROBLEM #3
The conditions at the beginning of compression in an Otto Engine
operating on hot-air standard with k = 1.35 are 101.325kPa, 0.05 cubic
meters and 32°C. The clearance is 8% and 15kJ are added per cycle.
Determine the mean effective pressure.
Given: Req’d: solving for volume, V 2
Solution: solving for compression ratio,
and:
hence: thus:
and: (not stated)
SAMPLE PROBLEM #4
A four-stroke Otto cycle engine has a displacement volume of 2000
cm³ and operates at 3000 RPM. The actual air intake mass flow rate is
measured to be 0.020 kg/s, and the theoretical mass flow rate
(assuming perfect volumetric filling) is 0.025 kg/s.
Given: Req’d: Solution:
SAMPLE PROBLEM #5
In an ideal Oto cycle, the pressure at the beginning of the compression
process is 15 psia, the temperature is 70° F and the volume is 13.08
cubic feet. The maximum temperature of the cycle is 2804° F, and the
heat supplied is 372 Btu. Using constant specific heat, calculate the
temperature at the beginning of the combustion, the compression
ratio, the net work done, and the thermal efficiency.
Given: Req’d:
Solution:
A.) Temperature at beginning of combustion,
solving for mass, m:
and,
B.) Compression Ratio, thus:
solving for volume, V 2
for isentropic process:
C.) Net Work Done, Solving for Temperature, T4
for isentropic process:
Solving for heat rejected, Q R
and:
hence:
thus:
D.) Thermal Efficiency,
REFERENCES
Bongcales, G. C. M. (2025). Power cycles [Slide show; Microsoft Powerpoint].
Testbook. (2024, November 14). Otto Cycle: Know Principle, PV & QTS
Diagram, Advantages, Disadvantages & Applications. Testbook.
https://testbook.com/mechanical-engineering/otto-cycle-process-diagram
Shubham Kola. (2022, January 6). Engine Terminology: Stroke, Bore, Swept
volume, Compression ratio, Mean effective pressure [Video]. YouTube.
https://www.youtube.com/watch?v=pW7qwy-61X8
The Internal combustion engine (Otto Cycle). (n.d.).
https://web.mit.edu/16.unified/www/FALL/thermodynamics/notes/node26.h
tml
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