Intake /Suction

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 illustrated here).

Compression As the piston rises the

poppet valve is forced shut by the increased cylinder pressure.  Flywheel momentum drives the piston upward, compressing the fuel/air mixture.

Power At the top of the

compression stroke the spark plug fires, igniting the compressed fuel.  As the fuel burns it expands, driving the piston downward

Exhaust At the bottom of the

power stroke, the exhaust valve is opened by the cam/lifter mechanism.  The upward stroke of the piston drives the exhausted fuel out of the cylinder.

Intake

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 increased crankcase pressure.   Power.

At the top of the stroke the spark plug ignites the fuel mixture. The burning fuel expands, driving the piston downward, to complete the cycle.

Transfer/Exhaust.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 cylinder.  Unfortunately, some of the fresh fuel mixture is usually expelled as well.

Compression.The piston then

rises, driven by flywheel momentum, and 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 burning. (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 (13)Compact construction (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 to 22:1) (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 stroke (2) one revolution of crank

provides one power stroke. (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 &

motorcycles. (13) Better balancing & low

vibrations (14) Require small flywheel since

less variation in torque.

(1)one working cycle is completed in four stroke

(2) two revolution of crank provides one power stroke.

(3) Lower fuel consumption (4)Valve are used for intake and (5)Larger and complicated

constructions. (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 torque