Four stroke internal combustion (IC) engines have been used for over 100 years and their design has not changed significantly since this time. Each of a four stroke engines strokes is used for one stage of the combustion cycle i.e. there is one stroke for each of the suction, compression, power and exhaust stages.
Four Stroke Engine Animation
Compared to two stroke engines, four stroke engines have more components and weigh more, but are more efficient. Four stroke engines may be fired on many different fuels, including gasoline/petrol, diesel, gas (methane) and bio-oils (to name a few fuel types).
Four Stroke Engine Components
Four stroke engine designs vary, so the number and type of components used per design also varies. For example, common rail engines employ different engine parts compared to non-common rail engines.
Common four stroke engine components include:
- Connecting Rod (Con Rod)
- Plain Bearings
- Combustion Space (Cylinder Liner)
- Inlet Valves and Exhaust Valves
- Rocker Arms
- Fuel Injectors
Access the below 3D model if you would like to learn all of an engine's main components and some engine terminology.
Note: The type of engine shown in this 3D model utilises direct fuel injection with common rail fuel injectors.
The below video is an extract from our Internal Combustion Engine Basics Online Video Course.
How Four Stroke Engines Work
A four stroke engine requires four strokes to complete one combustion cycle. The strokes are:
- Suction (Intake)
- Power (Ignition)
Another way to remember the strokes and their order is to change the wording to:
Stroke 1 = Suction (suck) Stroke 2 = Compression (squeeze) Stroke 3 = Power (bang!) Stroke 4 = Exhaust (blow)
The suction stroke draws air into the cylinder liner (combustion space) as the piston moves down towards bottom dead centre (BDC). When the piston reaches BDC, the inlet valves close and the piston travels back upwards towards top dead centre (TDC); this is the compression stroke.
Four Stroke Engine With TDC and BDC Indicated
As the piston moves towards TDC, the air in the cylinder is compressed (volume reduces) and its temperature and pressure increases. Shortly before TDC, fuel is injected into the combustion space. The fuel ignites and a controlled explosion occurs.
Pressure and Volume Graph
Once ignition occurs, the power stroke begins. The increase in pressure and temperature created by combustion pushes the piston towards BDC. Once at BDC, all the fuel within the combustion space has been burnt and the final engine stroke is ready to begin.
The exhaust stroke is the fourth and final stroke. The piston moves from BDC to TDC and expels exhaust gas from the combustion space via the exhaust gas valves. Once the piston reaches TDC, the air inlet valves open and the exhaust valves close a short time later (there is some valve overlap to ensure all exhaust gas has been removed from the combustion space). The combustion cycle is now complete as all four strokes have occurred.
Spark Ignition and Compression Ignition Engines
Petrol/gasoline engines use spark plugs for ignition, whilst diesel engines use only the heat generated due to compression. For this reason, petrol engines are known as spark ignition engines and diesel engines are known as compression ignition engines.
3D Model Details
This 3D model shows each stage of the four stroke engine cycle. Blue indicates suction and compression whilst red indicates expansion (power) and exhaust. The valves and other components are all correctly timed to show the entire four stroke process.