A petrol engine, also known as a gasoline engine, works by burning fuel within cylinders to create motion. It uses the four-stroke cycle of intake, compression, power, and exhaust or the two-stroke cycle. In a four-stroke engine, the piston completes the cycle over two revolutions of the crankshaft, while a two-stroke engine completes the cycle in one revolution. Petrol engines are commonly used in automobiles but have lower efficiency than diesel engines due to their lower compression ratios.

2. INTRODUCTION
• An engine or a motor is a machine designed to
convert energy into useful mechanical motion.
Heat engines, including I.C. engines and E.C.
engines burn a fuel to create heat, which then
creates motion. Motors convert electrical energy
into mechanical motion.
• "Engine" was originally a term for any mechanical
device that converts force into motion. The word
derives from Old French engin, from the Latin
ingenium, which is also the root of the word
ingenious, meaning ability.

3. Exploded view of petrol engine

4. PETROL ENGINE
• A petrol engine (also known as a gasoline
engine) is an internal combustion engine with
spark-ignition, designed to run on petrol and
similar volatile fuels. It was invented in 1876 in
Germany by German inventor Nicolaus August
Otto. In most petrol engines, the fuel and air are
usually pre-mixed before compression. The pre-
mixing was formerly done in a carburetor, but
now it is done by electronically controlled fuel
injection, except in small engines where the
cost/complication of electronics does not justify
the added engine efficiency.

5. COMPRESSION RATIO
• The compression ratio is a value that
represents the ratio of the volume of its
combustion chamber from its largest capacity
to its lower capacity.

6. SPEED AND EFFICIENCY
• Petrol engines run at higher speeds than diesels, partially
due to their lighter pistons, connecting rods and crankshaft
(a design efficiency made possible by lower compression
ratios) and due to petrol burning faster than diesel. They
also tend to have a much shorter stroke and therefore
petrol engines pistons can move up & down much quicker
than a diesel engines.
• However the lower compression ratios of a petrol engine
give a lower efficiency than a diesel engine. To give an
example, a petrol engine is like operating a bicycle in its
lowest gear where each push from your feet adds little
energy to the system, but you still expend energy to move
your legs back to the TDC position.

7. APPLICATIONS
• Petrol engines have many applications,
including:
• Motor cars
• Motorcycles
• Aircraft
• Motorboats
• Small engines, such as lawn mowers,
chainsaws and portable engine-generators

8. DESIGN
• WORKING CYCLES
• Petrol engines may run on the four-stroke
cycle or the two-stroke cycle. For details of
working cycles :
• Four-stroke cycle
• Two-stroke cycle

9. FOUR-STROKE ENGINE
• A four-stroke engine (also known as four-cycle) is an
internal combustion engine in which the piston completes
four separate strokes—intake, compression, power, and
exhaust—during two separate revolutions of the engine's
crankshaft, and one single thermodynamic cycle.
• There are two common types of four-stroke engines. They
are closely related to each other, but have major
differences in design and behaviour. The earliest of these to
be developed is the Otto cycle engine developed in 1876 by
Nikolaus August Otto in Cologne, Germany, after the
operation principle described by Alphonse Beau de Rochas
in 1861. This engine is most often referred to as a petrol
engine or gasoline engine, after the fuel that powers it.

10. • The four strokes refer to intake, compression,
combustion (power) and exhaust strokes that occur
during two crankshaft rotations per power cycle
• 1.INTAKE or INDUCTION stroke
• 2.COMPRESSION stroke
• 3. POWER stroke
• 4. EXHAUST stroke

11. INTAKE or INDUCTION stroke
• On the intake or induction stroke of the piston,
the piston descends from the top of the cylinder
to the bottom of the cylinder, increasing the
volume of the cylinder. A mixture of fuel and air,
is forced by atmospheric (or greater) pressure
into the cylinder through the intake port. The
intake valve(s) then closes. The volume of air/fuel
mixture that is drawn into the cylinder, relative to
the maximum volume of the cylinder, is called the
volumetric efficiency of the engine.

12. COMPRESSION stroke
• With both intake and exhaust valves closed,
the piston returns to the top of the cylinder
compressing the air or fuel-air mixture into
the combustion chamber of the cylinder head.
During the compression stroke the
temperature of the air or fuel-air mixture rises
by several hundred degrees.

13. POWER stroke
• This is the start of the second revolution of
the cycle. While the piston is close to Top
Dead Centre, the compressed air–fuel mixture
in a petrol engine is ignited, by a spark plug.
The resulting pressure from the combustion of
the compressed fuel-air mixture forces the
piston back down toward bottom dead centre.

14. EXHAUST stroke
• during the exhaust stroke, the piston once
again returns to top dead centre while the
exhaust valve is open. This action expels the
spent fuel-air mixture through the exhaust
valve(s).

15. GRAPHS FOR OTTO CYCLE
P-V GRAPH T-S GRAPH

16. 1.Process 1-2 is an isentropic compression of
the air.
2.Process 2-3 is a constant-volume heat
transfer to the air from an external source.
3.Process 3-4 is an isentropic expansion (power
stroke).
4.Process 4-1 completes the cycle by a
constant-volume process in which heat is
rejected from the air.
PROCESSES IN OTTO CYCLE

17. first law is rewritten as:
•
Applying this to the Otto cycle the four process equations can be derived:
•
Thermal Efficiency
net work

19. TWO-STROKE ENGINE
• A two-stroke, two-cycle, or two-cycle engine is a type of
internal combustion engine which completes a power cycle
in only one crankshaft revolution and with two strokes, or
up and down movements, of the piston in comparison to a
"four-stroke engine", which uses four strokes to do so. This
is accomplished by the end of the combustion stroke and
the beginning of the compression stroke happening
simultaneously and performing the intake and exhaust (or
scavenging) functions at the same time.
• Two-stroke engines often provide high power-to-weight
ratio, usually in a narrow range of rotational speeds called
the "power band", and, compared to 4-stroke engines, have
a greatly reduced number of moving parts, are more
compact and significantly lighter.

20. ADVANTAGES AND DISADVANTAGES OF
TWO STROKE ENGINES
• Advantages:
• It has no valves or camshaft mechanism, hence simplifying its
mechanism and construction
• For one complete revolution of the crankshaft, the engine executes
one cycle—the 4-stroke executes one cycle per two crankshafts
revolutions.
• Less weight and easier to manufacture.
• High power-to-weight ratio
• Disadvantages:
• The lack of lubrication system that protects the engine parts from
wear. Accordingly, the 2-stroke engines have a shorter life.
• 2-stroke engines do not consume fuel efficiently.
• 2-stroke engines produce lots of pollution.

21. CONCLUSION
• After reading the paper, we come to the know that Petrol engines
are about 30% efficient; in other words, 30% of the energy
generated by combustion is converted into useful rotational
energy at the output shaft of the engine, while the remainder
being losses due to waste heat, friction and engine accessories.
• The maximum amount of power generated by an engine is
determined by the maximum amount of air ingested. The amount
of power generated by a piston engine is related to its size
(cylinder volume), volumetric efficiency, losses, air-to-fuel ratio,
the calorific value of the fuel, oxygen content of the air and speed
(RPM).
• The speed is ultimately limited by material strength
and lubrication. Valves, pistons and connecting rods suffer severe
acceleration forces. At high engine speed, physical breakage
and piston ring flutter can occur, resulting in power loss or even
engine destruction.
• The Thermal efficiency of a Petrol engine is directly related to the
compression ratio.