Have you ever wondered what was the process that goes on in your car engine to create the driving force (
torque) to move the vehicle?
A complicated but simple process called the four stroke cycle that uses air, fuel and electricity to generate the power
in the engine which is then transferred to the drivetrain to move the vehicle
The fuel is pulled from the fuel tank, air is pulled from the atmosphere and the battery provides the electricity for
the engine to go through these four (4) strokes:-
This process, as long as it sounds happens very quickly. In fact at around 750 rpm (revolutions per minute)
which is approximately idling speed the crankshaft rotates around 12.5 times every second which means that each piston
is pushed up and down around twelve times every second resulting in each piston having around six power strokes every
- Intake Stroke (downward movement)
In the petrol engine a mixture of air and fuel is injected into the cylinder as the piston goes down but in the
diesel engine only air is injected into the cylinder.
- Compression Stroke (upward movement)
In both engines this stroke is the same as the pistons is forced up and compresses the contents inside the cylinder but the
diesel engine has a higher compression ratio which raises the temperature of the air to a higher temperature than the air
in the petrol cylinder.
- Power Stroke(downward movement)
This is the stroke with the major difference as in the petrol engine the air/fuel mixture is ignited using the electricity
through a spark plug. In the diesel engine the fuel is injected under high pressure and is ignited by the hot,
This explosion pushes the piston down and and rotates the crankshaft which provides the power for the engine,
This stroke also provides the force for the other pistons in the engine to move up or down and carry out their
stage of the cycle.
- Exhaust Stroke (upward movement)
The burnt air/fuel mixture is pushed out of the cylinder through the exhaust valves as the piston goes
up. After pushing out the burnt mixture the piston can only go down and this will be an intake stroke to start
another cycle for that piston and cylinder.
Now lets base this discussion on a regular four cylinder engine for simplicity. There are four
pistons and four strokes so each piston will be on a different stroke from the rest of pistons in the four
stroke cycle at any given time.
In a larger engine such as a six cylinder or V8 there will be more
than one piston on the same stroke on some occasions. This four stroke cycle is called the Otto cycle in a
petrol engine and the Diesel cycle in a diesel engine.