ALT-2 What are the Differences between Diesel and Gasoline Engines
The main difference between a diesel and a gasoline engine is the fuel. Gasoline is a volatile liquid with high vapor pressure, low boiling point, and low viscosity. Diesel fuel is an oil with a low vapor pressure, high boiling point, and medium viscosity. Because of the difference in fuels, the engines operate in different ways.
A gasoline engine is generally of the four stroke type, as in the common automobile engine. On the first stroke air is taken into the cylinder, along with a charge of atomized fuel either from a carburetor or a fuel injector. The fuel air mixture is compressed by a factor of eight or ten on the compression stroke. Slightly before the piston reaches the top of the compression stroke a spark plug fires, igniting the fuel air mixture in a controlled explosion. The heat from the burning fuel causes the pressure in the cylinder to rise, which forces the piston down on the power stroke. The spent fuel mixture and burned gasses are exhausted to the atmosphere when the piston again rises in the cylinder on the exhaust stroke. Fuel/air and exhaust gasses are flowed through poppet valves in the cylinder head.
A diesel engine works in a similar fashion. Air is taken through an intake valve into the cylinder. The stroke of a diesel engine piston is longer than a gasoline engine piston. The air is compressed by about a factor of twenty in the cylinder on the compression stroke and heats up to a high temperature. Slightly before the piston reaches the top of the cylinder, a high pressure pump sprays an amount of diesel fuel into the hot compressed air. The heat of the air causes the diesel fuel to burn, which causes the air to expand in the cylinder, the pressure rises and forces the piston down on the power stroke. Exhaust gasses are vented to the atmosphere on the next upstroke of the piston.
Diesel engines tend to be more massive than gasoline engines because of the higher compression ratio. Throttling a diesel engine is done by varying the amount of fuel injected into the cylinder, while throttling the gasoline engine involves restricting the air/fuel mixture intake to the engine with a butterfly valve. Diesel engines tend to be more efficient when run at a constant output near full power. Gasoline engines are better suited to conditions of varying load, as in city driving.
