| The diesel and gas powered engines are similar mechanically in that both have pistons that move up and down in cylinders. Valves are used through which air enters and leaves the cylinders. Both have a crankshaft by which rotary motion is created by connecting rods from the reciprocating pistons and this actuates the camshaft that causes the movement of the engines valves.
Although both engines burn mixtures of liquid fuel and air inside their cylinders the way in which each of the engines deals with the fuel is handled is the primary difference.
The fuel air mixture in a gasoline engine is compressed by the upstroke of the piston in the combustion chamber, and ignited by an electric spark igniting the gasoline and air vapor, in modern cars this is controlled by computer. In a diesel engine, only air is drawn into the cylinder and ignition is caused solely by the heat developed by the compression of the air in the cylinder. This is why glow plugs are required for a diesel engine to run during cold weather before enough heat can be generated for the combustion to continue automatically.
The heat developed in a diesel engine is so great that an electric spark is not required to produce ignition of the fuel air mixture. This makes high compression critical for a diesel powered internal combustion engine. The speed and power of a diesel engine is controlled by carrying the amount of fuel injected into the cylinders, air flow is not controlled in a diesel engine as it is done by the valve body in a gasoline engine.
Therefore the speed and power of a gasoline engine is controlled by varying the amount of the mixture of fuel and air admitted to the cylinders; the intake is throttled in varying degrees to admit more or less, as desired.The diesel does not have a throttle, using the literal sense of the word; it only has fuel control which in most cases controls a fuel governor rather than directly by the machine operator.