Just like it is the case with gasoline engines and carburetors, a mechanical device—even one as complicated as a diesel injection pump—cannot match the versatility and sensibility of a microcomputer coupled with various sensors, applying sophisticated rules to regulate the whole process of fuel injection.
The only input a mechanical pump can measure is the engine speed. The amount of air entering into the engine, unfortunately, is far from being proportional to engine speed, and the turbo or the intercooler disturbs this relationship even further. As the injection always has to inject less fuel than the amount which would already generate smoke, the mechanical pump—capable only of a crude approximation of what is actually going on in the engine—wastes a significant amount of air, just to be of the safe side.
The satisfactory combustion in diesel engines relies on the exhaust as well—if this is plugged up, more of the exhaust gases stay in the cylinder, allowing less fresh air to enter. A mechanically controlled injection pump has no feedback from the engine (except for the engine speed)—it will simply pump too much fuel into the engine, resulting in black smoke. An electronically controlled injection pump, on the other hand, can tell how much air has actually entered by using a sensor (although only the latest systems use such a sensor).
Diesel oil has been a contender to gasoline for many decades. Earlier diesel engines were not refined enough to win the hearts of many drivers but recent advances in technology made these engines not only worthy competitors in all areas but in some features—fuel economy or low end torque, to name just two—even exceeding the characteristics of their gasoline counterparts. And in addition to the general technological advantages, Citroën’s diesel engines have a widely accepted reputation—even among people blaming the quirkiness of its suspension or other features—of being excellent and robust.
As it is widely known, diesel engines have no ignition to initiate their internal combustion, they rely on the self-combustion of the diesel oil entering into a cylinder filled with hot air. Due to this principle of operation, the supply of the fuel has to comply with much more demanding requirements than it is necessary in the case of gasoline engines.
Unlike in the gasoline engine, not a mixture but air enters into the cylinders via the inlet valves. During the adiabatic compression all the energy absorbed is used to increase the temperature of the gas. The small droplets of fuel will be injected at high velocity near the end of the compression stroke into this heated gas still in motion. As they start to evaporate, they form a combustible mixture with the air present which self-ignites at around 800 °C.
This self-ignition, however, is not instantaneous. The longer the delay between the start of the injection and the actual ignition (which depends on the chemical quality of the diesel oil, indicated by the cetane number), the more fuel will enter the cylinder, leading to harsher combustion, with the characteristic knocking sound. Only with the careful harmonization of all aspects—beginning of injection, the distribution of the amount injected in time, the mixing of the fuel and air—can the combustion be kept at optimal level.
The jockey arm type auxiliary belt tensioner is no longer available from Citroën. If yours starts to rattle, warning of an impending failure, you only have the option to modify it. There are two possible ways to do it: either fix it permanently or use a replacement tensioner with the necessary modifications to fit it.
It usually dies either when you (try to) start the engine or when you stop it, rarely while it's running. The engine will not start because it will not be getting any fuel. The reason is usually overheating and deformations in the coil whichresult in the valve plunger getting stuck, or internal shorts or open circuits (due to heat) in the coil.
Clean combustion resulting in clean oil (assuming it has the proper grade, not some cheap concoction) is essential not only for the engine but for the turbo as well. Most people are not aware that a turbo can be--and actually should be--cleaned every 150,000 km or so. This procedure will make it last forever.
Most people are not aware that the main reason a diesel engine wears out is improper combustion, usually caused by worn out injectors, improper timing, or simply lack of air (influenced by several factors, one of which may simply be a dirty air filter). Anything which produces soot (eg. particulates), including the "black smoke" effect, results in particulates ending up in the engine oil. This soot then accumulates in areas where oil flow is low, like behind the piston rings. As the deposits grow, the ring is being forced outwards.
A lot of things happen at the moment the fuel is injected. If the fuel was somehow perfectly microscopically atomised on injection, it would not self-ignite at all. It would have trouble igniting even with a spark plug. Ignition needs a richer fuel-air mixture, and this is locally provided by the start of the injection being full of larger drops of fuel, because it occurs at a lower than maximum pump pressure (about 2-3 times lower). The evaporation of components from the fuel as it enters the hot compressed air is what makes it ignite.
Diesels are taken for granted in 99% of all cases--the truth is, most of them can take an incredible amount of "just driving", but the neglected maintenance results in slow decay. However, even engines treated extremely poorly can easily run 100k miles or more.
Most people come to a diesel from a gasoline car, and apart from the obvious differences, they do not realise how different the engine really is. Although it is very similar mechanically, the basic pricinples are very different.
CAV fuel pumps give an endless amount of trouble. Problems with idle speed, speed oscillation, stalling are among these typical failures. In other cases, the engine might idle normally but it gets stuck at higher or lower idle revolutions when the accelerator is first pressed and released. Or it can get stuck at a certain speed over idle regardless of accelerator position.
The most usual causes are:
A new type of head gasket was introduced in 1993. The older one was a mica press with metal inserts, the new variant a steel-aluminum-steel sandwich construction. The old one failed for me at 130,000 km: part of the gasket lining burned out, leaking gases into the coolant as well as the adjacent cylinder. Such failure is not possible with the newer gasket.