Difference Between Petrol And Diesel Engine

Mechanically, 4 stroke and 2 stroke diesel engines work identically to their four-stroke and 2 stroke petrol engines counterparts in terms of piston movement and crank rotation the major difference being the inclusion of exhaust valves at the top of the cylinder in the case of the 2 stroke. (To be historically accurate, petrol engines are mechanically similar to diesel engines - diesel engines came first) It's in the combustion cycle where the differences come through. First, during the intake cycle, the diesel engine only sucks air into the combustion chamber through the intake valve - not a fuel/air mix as in the case of petrol engines. Second, there is no spark plug. Why? Put simply, Diesel engines work on self-ignition, or detonation - the one thing you don't want in a petrol engine. At the top of the compression stroke, the air is highly compressed (over 500psi), and very hot (around 700 °C - 1292°F). The fuel is injected directly into that environment and because of the heat and pressure, it spontaneously combusts (this system is known as direct-injection). This gives the characteristic knocking sound that diesel engines make, and is also why pre-igniting petrol engines are sometimes refered to as 'dieseling'.

Cycle of Operation of Petrol Engines

Petrol engines typically run compression rations around 10:1, with lower end engines down as low as 8:1 and sportier engines up near 12:1. Diesel engines on the other hand typically run around 14:1 compression ratio and can go up as high as 25:1. Combined with the higher energy content of diesel fuel (around 147,000 BTU per gallon versus 125,000 BTU for a gallon of petrol), this means that the typical diesel engine is also a lot more efficient than your common or garden petrol engine, hence the much higher gas-mileage ratings.

Because of the design of the diesel engine, the injector is the most critical part and has been subjected to literally hundreds of variations in both design and position. It has to be able to withstand massive pressures and temperatures, yet still deliver the fuel in a fine mist. One other component that some diesel engines have is a glowplug. From cold, some lower-tech engines can't retard the ignition enough, or get the air temperature high enough on startup for the spontaneous combustion to happen. In those engines, the glowplug is literally a hot wire in the top of the cylinder designed to increase the temperature of the compressed air to the point where the fuel will combust. These engines typically have a pictograph on the dashboard that looks like a lightbulb. When starting the engine cold, you need to wait for that light to go out - basically you're waiting for the glowplugs to get up to temperature. In really old diesel designs, this could be as long as 10 seconds. Nowadays it's nearly instantaneous, or in the case of advanced ECM systems, not needed at all.

Cycle of Operation of Diesel Engines

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