1. Engine Cycles This presentation will explore: Engine Operation
Four-Stroke Cycle
Two-Stroke Cycle

2. Engine Operation
Expansion of a burning gas within a confined area is converted into rotary motion.
In an internal combustion engine, fuel (petrol) burns in a combustion chamber.
Expansion of the burning gases forces the piston and connecting rod down.
A crankshaft converts reciprocating movement into rotary motion.
Rotary motion drives the wheels via gearing.

3. Piston Stroke A piston moves in a cylinder.
The top of piston travel is called Top Dead Centre - TDC.
TDC
Stroke
Downward movement of the piston rotates the crankshaft.
BDC
Piston
The lowest point of piston travel is called Bottom Dead Centre - BDC.
The full range of travel from TDC to BDC is called the piston stroke.

4. Four-Stroke Cycle - Stroke One - Induction
At the beginning of the induction stroke the inlet valve opens.
The downward movement of the piston creates a low pressure region in the cylinder.
The air/fuel mixture is drawn into the cylinder.

5. Stroke Two - Compression
The inlet valve closes.
As the piston moves up toward TDC the air/fuel mixture is compressed in the cylinder.
Compression heats, mixes and evenly distributes the air/fuel mixture. This helps the combustion process.

6. Stroke Three - Power
Just before TDC is reached, a high voltage is applied to the spark plug causing a spark that ignites the air/fuel mixture.
Very rapid expansion of the burning fuel mixture creates pressure that forces the piston down, pushing on the connecting rod and turning the crankshaft.
This is the only stroke in the cycle that supplies energy to the engine.

7. Stroke Four - Exhaust The exhaust valve opens.
As the piston moves up, the exhaust gases are expelled into the exhaust system.
At the end of the stroke, the exhaust valve closes, ready for the inlet valve to open and the cycle to repeat.

8. Four-Stroke Cycle Compression Exhaust Power (Expansion) Induction
There is one stroke for each part of the operation, taking two complete revolutions of the crankshaft for the whole cycle.

9. Two-Stroke Cycle - Upstroke
Toward the end of the downstroke the piston uncovers the inlet transfer port.
Exhaust
port
Transfer port
The downward movement of the piston creates crankcase pressure that drives fuel mixture up into the cylinder, which is at low pressure (aided by port and piston design).
As the piston moves up, crankcase pressure is reduced, and the inlet port opens, allowing the air/fuel mixture to be drawn in via the carburettor.
Inlet
port
At the end of the upstroke both ports are closed, and the fuel mixture is compressed.

10. Two-Stroke Cycle - Downstroke
As the piston approaches TDC, the spark plug fires.
Exhaust
port
Transfer port
Combustion creates the power stroke, driving the piston down.
As the piston moves down, it uncovers the exhaust port, allowing the burnt gases to escape.
Inlet
port
When the piston clears the transfer port a new charge of fuel mixture starts to enter the cylinder, helping to force the exhaust gases from the combustion chamber. The cycle repeats.