Petrol And Diesel Engine Theory Flashcards
When the piston is moving up in a 2 stroke
petrol engine explain what is happening.
As the piston moves up it compresses a charge already in the cylinder. At
the same time a depression or partial vacuum is created below the piston in
the sealed crankcase which allows air-fuel mixture from the carburettor to
flow through a valve into the crankcase.
When the piston reaches T.D.C (Top Dead Centre) the compressed charge
trapped on top of the piston is ignited and the piston moves down on a
power stroke.
When the piston is moving down in a 2
stroke petrol engine, explain what is happening.
The downward moving piston creates pressure in the crankcase which now
closes the intake port (One way valve) and because the crankcase is
sealed, the previous in-coming air-fuel mixture is now being compressed in
the crankcase. (If the crankshaft seals are worn the air fuel mixture can
escape from the crankcase and combustion force will be reduced).
As the piston approaches Bottom Dead Centre (B.D.C) it uncovers the
exhaust port, in the cylinder allowing the exhaust gas to escape from the
cylinder.
The partially compressed fresh charge is also allowed to pass from the
crankcase into the cylinder by the transfer port and some of this fresh airfuel mixture flows over the top of the piston, forcing out any remaining
exhaust gas from the cylinder. This is known as scavenging.
It is through this port that the air-fuel mixture enters the crankcase. The
port is opened and closed by the movement of the piston skirt.
Inlet Port
This is a passage way between the crankcase and the cylinder. The air-fuel
mixture is transferred from the crankcase to the cylinder when the downward
movement of the piston uncovers the top of this port.
Transfer Port
This port is nearest to Top Dead Centre (T.D.C.) and is uncovered as the
piston descends, allowing the burnt gases to leave the cylinder.
Exhaust Port
List the four strokes of a four stroke engine.
Intake
Compression
Power
Exhaust
This is the first movement of the piston and is a
downward movement of the piston that develops
a vacuum in the cylinder. The intake valve is open
as the piston starts to descend. This allows an air/fuel mixture to fill the empty cylinder. When the
piston reaches the bottom of the stroke the intake
valve will close trapping the mixture inside the
cylinder.
First stroke (Intake)
On the second stroke the mixture is
compressed.
Both the inlet and exhaust valves are closed.
The weight of the crankshaft and flywheel drive
the piston up to TDC compressing the mixture.
The temperature of the mixture has now
increased to a highly flammable charge.
Second stroke (Compression)
Third stroke Combustion (Power)
On the third stroke the engine produces its
power. Just before the piston reaches the top
of the compression stroke TDC.
(Top Dead Centre) the spark plug produces a
high voltage spark. As the mixture explodes the
temperature and pressure inside the enclosed
cylinder increases. The force of hot gas and
pressure (combustion gases) drives the piston
downward in the cylinder, and by means of the
connecting rod produces movement of the
crankshaft.
Fourth stroke (Exhaust)
On the fourth and final stroke the burnt gas is
discharged from the cylinder. As the piston
starts moving upwards from BDC (Bottom
dead centre) the exhaust valve opens and
the burnt gases are forced from the cylinder
through the open exhaust valve. The cycle
repeats itself after the exhaust stroke. The
crankshaft has made two revolutions, and
the engine has completed one cycle
consisting of four strokes.
True
True or false. In a four stroke cycle the crankshaft rotates faster than the camshaft
True or false. In a four stroke cycle the crankshaft and camshaft rotate at the same speed.
False
True or false. In a four stroke cycle the camshaft rotates faster than the crankshaft
False
Explain the term Torque
Turning force that is produced by the engine.
Explain the term Power.
Rate or speed at which work is done
