Powering Transportation: The Internal Combustion Engine
An internal combustion engine (ICE) is a heat engine where the combustion of a fuel occurs with an oxidizer (usually air) in a combustion chamber that is an integral part of the working fluid flow circuit.
The Four-Stroke Gasoline Engine (Otto Cycle)
Most cars use a four-stroke spark-ignition engine. The cycle consists of four piston strokes:
1.Intake: The piston moves down, drawing a mixture of fuel and air into the cylinder. The intake valve is open.
2.Compression: The intake valve closes, and the piston moves up, compressing the fuel-air mixture. This raises its temperature and pressure.
3.Power (or Combustion/Ignition): The spark plug ignites the compressed fuel-air mixture. The resulting explosion forces the piston down with great force. This is the stroke that produces power to turn the crankshaft.
4.Exhaust: The exhaust valve opens, and the piston moves up, pushing the burned gases out of thecylinder.
Diesel Engines vs. Gasoline Engines
Gasoline Engine (Spark-Ignition): Uses a spark plug to ignite a pre-mixed fuel-air mixture. They generally have a lower compression ratio.
Diesel Engine (Compression-Ignition): First, only air is drawn into the cylinder and is compressed to a much higher pressure and temperature. Then, fuel is injected into the hot, compressed air. The heat of the compressed air is high enough to ignite the fuel spontaneously, without the need for a spark plug.
Compression Ratio
The compression ratio is the ratio of the volume of the cylinder with the piston at the bottom of its stroke to the volume with the piston at the top of its stroke.
A higher compression ratio generally leads to higher thermal efficiency and more power output.
Diesel engines have much higher compression ratios than gasoline engines, which is one reason they are more efficient.
Gasoline engines are limited in their compression ratio to avoid 'knocking' or pre-ignition, where the fuel-air mixture ignites from compression heat before the spark plug fires.