During the intake stroke, the
piston moves downward, drawing
a fresh charge of vaporized fuel/air
mixture. The illustrated engine
features a 'poppet' intake valve
which is drawn open by the
vacuum produced by the intake
stroke. Some early engines worked
this way, however most modern
engines incorporate an extra
cam/lifter arrangement as seen on
the exhaust valve. The exhaust
valve is held shut by a spring (not
As the piston rises the poppet
valve is forced shut by the
momentum drives the piston
upward, compressing the
At the top of the
compression stroke the
spark plug fires, igniting
the compressed fuel. As
the fuel burns it
expands, driving the
At the bottom of the
power stroke, the
exhaust valve is opened
by the cam/lifter
upward stroke of the
piston drives the
exhausted fuel out of the
The fuel/air mixture is first drawn
into the crankcase by the vacuum
created during the upward stroke
of the piston.
During the downward
stroke the poppet valve is
forced closed by the
At the top of the stroke
the spark plug ignites
the fuel mixture. The
burning fuel expands,
driving the piston
downward, to complete
Toward the end of the stroke,
the piston exposes the intake
port, allowing the
compressed fuel/air mixture
in the crankcase to escape
around the piston into the
main cylinder. This expels
the exhaust gasses out the
exhaust port, usually located
on the opposite side of the
some of the fresh fuel
mixture is usually expelled as
The piston then rises,
driven by flywheel
compresses the fuel
mixture. (At the same
time, another intake
stroke is happening
beneath the piston).
Since the two stroke engine fires on every revolution of
the crankshaft, a two stroke engine is usually more
powerful than a four stroke engine of equivalent size.
This, coupled with their lighter, simpler construction,
makes two stroke engines popular in chainsaws, line
trimmers, outboard motors, snowmobiles, jet-skis,
light motorcycles, and modelairplanes.
Unfortunately most two stroke engines are inefficient
and are polluters due to the amount of unspent fuel
that escapes through the exhaust port.
Petrol Engine Diesel engine
(1) Petrol is used as fuel
(2) Mixture of air and petrol is used as charge
(3) It is a spark ignition (5.1.) engine.
(4) Spark plug is used for fuel ignition and
(5) Have lower compression ratio (7:1 to 10:1)
(6) Carburetor is used to supply air- petrol
mixture in proper ratio.
(7) Low thermal efficiency.
(8) Low weight engine
(9) Can run at higher speeds (5000 rpm)
(10) Low maintenance cost
(11) Low initial cost and high running cost
(12) Starting is easy
(14)Fuel is wasted in scavenging process
(15)Can not be rough used
(16)Exhaust temperature is high
(17)Occupies less space
(18)Normally used for light duty vehicles.
(1) Diesel is used as fuel
(2) Only air is used as charge
(3) It is a compression ignition engine
(4) Fuel injector is used to spray diesel into the
cylinder for burning.
(5) Have higher compressor ratio (14:1
(6)Fuel pumi5 is used to force Diesel in
to fuel injector.
(7)High thermal efficiency
(8)High weight engine
(9)Can not run above 3500 rpm
(10) Starting is difficult
(11) High maintenance cost
(12) High initial cost and low running cost
(13) Huge size engine
(14) Fuel is not wasted in scavenging process.
(15) Can be rough used
(16) Exhaust temperature Is low.
(17) Occupies more space
(18) Normally used for heavy duty vehicles like
trucks, tractors etc.
Two Stroke Four Stroke
(1)one working cycle is completed in two
(2) one revolution of crank provides one
(3) Higher fuel consumption.
(4) Ports are used for Intake and exhaust.
(5) Compact and simple constructions.
(6) Light weight engine.
(7) Fuel compression ratio is less.
(8) Consume more lubricating oil.
(9) Low thermal efficiency.
(10) Produce more noise
(11) Can be easily reversed
(12) Normally used for scooters &
(13) Better balancing & low vibrations
(14) Require small flywheel since less
variation in torque.
(1)one working cycle is completed in four
(2) two revolution of crank provides one
(3) Lower fuel consumption
(4)Valve are used for intake and
(5)Larger and complicated
(6) Heavy weight engine.
(7) Fuel compression ratio is high.
(8) Consume less lubricating oil.
(9J High thermal efficiency.
(10) Produce less noise
(11) Can not be reversed
(12) Normally used for, car, truck, cran and
heavy duty vehicles.
(13) Gives more vibrations
(14) Require larger flywheel due to uneven