Engines – Part Two.

Previously I described an engine as having pistons that travel up and down inside a cylinder.  The piston is attached to the engine’s crankshaft via a conrod.  Each piston in a four stroke engine has four distinct phases through which it travels.  For two of these “strokes”, the piston travels downwards. The other two strokes, the piston travels upwards.

1 - Intake Stroke The first downward stroke is referred to as the Intake stroke.  At the top of the cylinder head, there are valves that open and close at different points in time.  The inlet valve (or valves) open during the intake stoke, allowing a fuel and air mixture to enter the cylinder.  Because the piston seals the cylinder when it travels downwards, it creates an area of low pressure above it.  This helps draw the fuel-air mixture in.
2 - Intake Stroke completed At around the time the piston reaches bottom dead centre (BDC) the inlet valve closes, sealing the gasses in the cylinder. The piston then starts its second stroke: the compression stroke.  The piston travels back up, compressing the fuel-air mixture at the top of the cylinder.  The difference in volume between when the piston is at the bottom of its stroke and the top of its stroke, is commonly referred to as the engine’s compression ratio.
3 - Compression Stroke Compressing a flammable gas in the presence of oxygen is somewhat fraught with problems.  Compressed gas gets hot.  Hot flammable gas can combust!  The octane rating of a fuel denotes how stable it is.  Engines with high compression ratios need high-octane fuel to prevent uncontrolled detonation during the compression stroke.
4 - Compression Stroke completed Once the piston reaches top dead centre (TDC) the compressed fuel-air mixture is ignited by the spark plug. This causes the gas to ignite and expand rapidly.
6 - Power Stroke The valves in the cylinder head remain closed at this stage, so the only way the gas can expand is by forcing the piston back down the cylinder. This is referred to as the Power stroke.
7 - Power Stroke completed All things going well, by the time the cylinder reaches BDC again, all the gasses have been burnt.
8 - Exhaust Stroke No further kinetic energy is to be gained from them and they need to be removed from the cylinder, ready for the next cycle. The outlet valve (or valves) open and the now rising piston forces the exhaust gasses out through them. This is known as the exhaust stroke. At the completion of the exhaust stroke, the exhaust valve has closed and we are ready to repeat the entire process.

Two stroke engines work in a similar manner, but combine the intake /power strokes together and the compression / exhaust strokes.  How this is done, is a story for another time.