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 will be used throughout.
Look At 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 of such makes as Alfa Romeo. Two main differences exist; 1) engine construction tends to be heavier and more robust to handle the much higher compression and combustion pressures, and 2) ignition of the fuel/air mixture does not require an external source (a 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, notice the fixed end of the barrel becoming warmer, this 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 and instead of generating 30-45 pounds per square inch (psi) or 2-3 bar you generate 500psi 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 2500psi 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 this cylinder will produce almost instant combustion thus producing 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 etc, the 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 thus promoting a longer service life. The flip side to this was the CI engines 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.