How A Diesel (Compression Ignition) Engine Works

Same four strokes as a petrol engine, but nothing lights the fuel except the pressure itself.

For those interested in engine history, Dr Rudolf Diesel took out patent 7241 in 1892, which detailed an engine design relying on the induction of coal dust for fuel. Prior to this, the English engineer Herbert Ackroyd-Stuart filed patent 7146 in 1890, which contained a design for an engine which ran on the timed, high pressure injection of fuel oil. Whichever camp you decide to pitch in, it cannot be denied that the work of Ackroyd-Stuart preceded that of Diesel. To avoid being dragged into the argument, the term compression ignition, or CI, is used throughout.

The induction and compression cycle

The basic four stroke cycle of induction, compression, power, exhaust used in the CI engine is exactly the same as that used in the petrol engine. Two main differences exist. First, engine construction tends to be heavier and more robust to handle the much higher compression and combustion pressures. Second, ignition of the fuel and air mixture does not require an external source — there is no spark.

Pressure and the burn process

So we have an engine which will run without having any external form of starting the burn process. How can this happen? Put simply, pressure. Cast your mind back to when you last used a bicycle pump and noticed the fixed end of the barrel becoming warmer. That was not entirely due to the warmth of your hand; it was caused by the air in the pump tube getting hotter as its pressure increased.

Now imagine this scenario magnified many, many times. Instead of generating 30 to 45 pounds per square inch (2 to 3 bar) you generate 500 psi, or 34 bar. At this pressure, created over a very short period of time, temperatures usually reach well over 600°C. Just before top dead centre, fuel oil is injected at very high pressure — around 2500 psi, or 175 bar — via an injector which produces a very fine mist. The fuel mist will self ignite at around 400°C, so its introduction into the cylinder produces almost instant combustion, and with it the power stroke.

Recent engine development

So why use a CI engine over a similar sized petrol engine? Until relatively recently the CI engine was the sole power unit of choice for heavier duty applications — trucks, buses, taxis. Its greater fuel efficiency over the petrol engine was guaranteed to give the operator lower fuel costs, and its better power characteristics produced high power output at lower engine speeds, promoting a longer service life.

The flip side to this was the CI engine's higher noise and harshness levels, plus its ability to belch black smoke under heavy acceleration. In more recent times, however, CI engine development has come on in leaps and bounds, offering noise, harshness and exhaust emissions levels virtually identical to its petrol driven cousins whilst still delivering the old attributes of excellent power output at relatively low engine speeds.