Diesel Basics

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.

A standard injection pump with cable operated throttle (even if it has added electrical parts), provides no real regulation for two of the three major parameters of the engine operation--it is usually adjusted for the worst case, which incidentally makes the engine far worse than it could actually be.

There are three major differences compared to a gasoline engine. First, there is no throttle; The constant compression results in high amounts of low-end torque; on the other hand, such an engine behaves as it had a fully open throttle at all times. Without another means of regulation, it would continue accelerating until self-destruction (this is called engine runaway). To keep control of the engine, the injection pump has to regulate the amount of fuel injected--this is the only real regulation a mechanically controlled injection pump can do.

A similar mode of operation would not be possible in a gasoline engine without producing huge amounts of pollutants. The gasoline engine has to keep its fuel to air ratio practically constant, this is why they have to rely on electronic injection systems, oxygen sensors and similar.

As for the second difference: as with every kind of engine, power output is limited by air (or air/fuel mixture) intake capacity primarily. This is not the same as the displacement of the engine--these two values would be equal if the engine ran very slowly. As gasoline engines operate on a constant fuel/air mixture, which is practically equal to the stochiometric ratio of 14.7 to 1 or, in other words, lambda = 1, the only way to vary the power output for a gasoline engine is to throttle it down, preventing more than a certain amount of mixture entering the engine.

On a diesel engine, as explained above, this is done by varying the amount of fuel injected. Unfortunately, although the stochiometric ratios for gasoline and diesel are almost the same, a diesel engine cannot operate with more than about half the ideal amount of fuel--threrfore it always has at least twice the needed air, that is, its lambda > 2. Why? The stochiometric ratio would produce perfect combustion, but diesel is a heavier oil than gasoline: it has more carbons. As a result, it burns down to H2O, CO2, and lots of elementary carbons--black soot. This would give a very black exhaust, and to avoid this, the free air flow to the engine has to be preserved.

The adequate combustion relies on the exhaust as well. If it is plugged up, more of the exhaust gases stay in the cylinder, allowing less fresh air to enter. This is where an electronically controlled injection pump can achieve more: a mechanically controlled one cannot tell how much air has actually entered the cylinder, and if the other end is restricted, the pump may end up injecting too much fuel, resulting in black smoke.

Mechanical pumps of turbo engines have a special device to estimate the added air when the turbo builds up pressure, rising the maximum fuel injection capacity. Recent systems control this so-called smoke limit by means of electronically regulated pumps and common rail injection. As they can adapt themselves to the actual condition of the engine at any speed, air temperature or density, they can optimise the engine output to achieve a usually very flat torque curve.

The smoke limit is the maximum amount of fuel to air ratio at which combustion just starts producing elementary carbon (black soot). It is usually around lambda = 2. A simple indicator how crytical the proper regulation of the smoke limit is: 1% increase in fuel over the smoke limit produces about 3 to 10 times the black smoke compared to no increase. Once elementary carbon is produced, it acts as a sort of catalyst which completely changes the nature of the combustion, it's almost like a chain reaction.

The fuel injection on a diesel is based on engine load. Within the pump, there is a spring that the accelerator cable actually pulls on. Opposing the force of a spring is the centrifugal regulator. The actual movement of the junction of the spring and the regulator determines the amount of the fuel injected. Thus, if the engine is kept loaded so it can't really increase its rpm, only a small pressure on the accelerator will actually open the fuel to the max. As soon as the revolution rises, the centrifugal regulator will start countering the added force to the spring and close the fuel, until the engine rpm matches the force of the spring. The maximum force on the spring (and thus the maximum rpm you can request) is simply limited by a limiter that prevents the accelerator cable to pull on the spring more than to a certain limit.

At first glance, you would think this means that the maximum fuel is simply limited below the smoke limit and that's all. The problem is, the smoke limit depends on the actual amount of air in the cylinder at injection time, and this depends on the rpm, althought not proportional to it. The mechanical design of the pump tries to track this dependency, although it cannot do so precisely. And even with a slight misalignment, the air-fuel to air ratio might go over the smoke limit at a certain rpm. This is especially true for turbos because they also modify the maximum fuel depending on inlet manifold pressure. This actually has an adiabatic relationship to air mass, again not linear, so the pump is again approximating. In addition, to make the turbo lag smaller, the pump is frequently adjusted so that it goes very close or even slightly into the smoke limit while the turbo is spinning up.

Under normal circumstances these exceptions happen very rarely, and the throughput of the engine is not very high at that point: thus. the soot ends up trapped in the exhaust mufflers. When you've driven in town for a while, there can be quite a bit in there. The next time you press the accelerator to the floor and allow the engine to really pump some gases into the exhaust at higher revs, your tailpipe will get cleaned out nicely, resulting in a (hopefully) small cloud of black smoke. If it stops smoking, all is well. I can tell you that cars that get driven around town a lot actually loose quite a bit of power as a result of the exhaust getting plugged up. Do a longer highway trip, and suddenly they will drive better.

The third major difference to gasoline engines is that in diesels, injection occurs blindly. While the spark timing in gasoline engines is very close to the combustion triggered by the spark, the actual moment of combustion depends on many factors in a diesel engine. In contrast to the gasoline, where there is a wide angle of engine rotation where the spark can occur (and is actually used in form of changing timing advance), most injection systems (even some electronic ones) do very little to regulate injection timing accurately. In spite of this, the accuracy of timing regulation itself accounts for 50% of the pollution, 20% of power output and 20% of engine noise.

As a consequence of all these, the diesel injection system relies on many things operating correctly, because the classical pump system has no feedback from the engine (expect the rpm, of course). This means that the cleanness of the air filter, a good exhaust system and the necessary adjustments (or replacement) of the injectors and pump are essential. As the pump and injectors age and wear, the timing slowly becomes late. This process is normal and does not mean the components should be renewed immediately, only to be adjusted to the correct static injection angle. Although this is a very simple procedure, a special diesel stroboscope is needed in order to do it accurately.