The problem of diesel engine 'runaway' is one which was dealt with long ago; it's a very rare incident in modern diesel engines. Runaway is the name given to the effect whereby lubricating oils are sucked into the cylinder of a diesel engine, causing it to accelerate uncontrollably until it digests itself in a satisfyingly cataclysmic destructive event.
Runaway is very uncommon today, and we've only mentioned it here to help illustrate an important point, a point which differentiates a petrol engine from a diesel engine. The point is that the speed of diesel engines is controlled by the amount of fuel being combusted in the cylinder, whereas the speed of a petrol engine is controlled directly by the throttle. This means that three components are particularly important in the diesel engine: the camshaft, the fuel injectors and the governor. These three components unite in a diesel engine to control its speed, and it's these that have eliminated runaway.
- The Camshaft provides the timing necessary to inject the fuel correctly
- The Governor controls the mount of fuel to be injected
- The Fuel Injectors inject the diesel into the cylinder
With all three of these components working in tandem, the amount of fuel being burned in the cylinder is controlled, which in turn controls and manages the speed of the engine. Every cylinder in the diesel engine has a fuel injector which injects the diesel at the correct time in the cylinder's stroke. This timing is determined by the rotation of the camshaft, and it's the camshaft which directly operates the fuel injector. The amount of fuel to be injected into the cylinder is controlled by a mechanical linkage called the 'fuel rack', and the position of the fuel rack (and therefore the amount of fuel) is controlled by the governor.
Engine Governors
The engine's governor adjusts the position of the fuel rack, which determines the amount of diesel necessary to maintain the engine speed. Petrol engines are self-speed-limiting, through the use of throttle control. Diesel engines are not self-speed-limiting; the speed of diesel engines is controlled purely and only the amount of diesel being injected. Because the throttle in a petrol engine is totally adjustable, it's easy change to the engine speed and to maintain a constant speed. A diesel engine doesn't have these functions naturally; diesels need a mechanism to control the amount of diesel being injected into the cylinders to control the speed, and the governors provide this mechanism. The engine's governor provides the feedback it needs to accelerate and to maintain a constant speed.
In your diesel car, the governor performs several different functions simultaneously. Here's a quick list of the more prominent roles:
- To maintain engine revolutions at a given steady velocity
- To control engine revolutions over a range of velocities
- To keep the engine revolutions above a minimum and below a maximum velocity
As you can see, the function of the governor can technically be classified as a speed-regulating one, since the control of the fuel directly controls the speed.
Fuel Injectors
The fuel injectors are essentially valves, which inject a regulated amount of diesel directly into the cylinders. The diesel enters the injector under pressure, through its filter cap and element. From there, the diesel travels down into the injector's supply chamber after which it is injected, still under pressure, into the cylinder to be burnt. The fuel injectors are probably less important than the regulators when it comes to controlling the speed of an engine, but the injectors can have a profound impact on a number of other factors. Subtle differences in the design of the fuel injection system can affect issues like power output, fuel efficiency, emissions, reliability, cost, and resistance to extreme temperatures. The manufacturers will decide early on in the design process what function they want to target (power output rather than fuel efficiency, for example) and develop the fuel injection accordingly.
Fuel injectors have traditionally been very mechanical devices, with fine, fiddly components. Quite often, they were the weak link in the diesel engine's chain, and if something went wrong it was usually the injectors. The newer injectors are digitally controlled electronic affairs, which precisely control the fuel injection system. The big European diesel makers like BMW, Mercedes, Volkswagen and others, use electronic fuel systems which are highly capable of getting the best of the injectors even on the fly, by making minute adjustments while the engine is running. As a result, diesel engines are more powerful, more efficient, smoother, cleaner and faster than ever before, and the technology continues to improve.