Diesel Engines (BX and XM)

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.

Željko NASTASIĆ

More on diesel combustion

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. This is called creation of ignition precursors, and it is responsible for a delay between injection and ignition. Once this happens, the temperature rises sufficiently for the rest of the injected fuel to ignite. This is the principle behind pilot injection, a technique that makes things like the HDi common rail possible and quiet compared to old style direct injection engines.

Incidentally, the ability to reliably create precursors with consistent timing is what the often misunderstood cetane number of diesel fuels is all about. The only reason it is measured is because this number and the timing accuracy strongly correlate. The higher the cetane number, the more consistent is the precursor creation across a varying range of temperatures and amounts of fuel injected. This is also why diesels misfire and sound much louder on bad quality fuel.

The cetane number is the measure of the volatility of the fuel. If this volatility is higher at the conditions present at injection time, the time from injection to combustion is shorter--and time is a valuable comodity in a diesel engine. Besides, self-ignition is a statistical event, the time interval elapsed between injection and ignition has properties of a chaotic event. The shorter the time, the narrower the statistical dispersion of the time of ignition, and the better controlled the engine timing. In practice, higher cetane fuels misfire less and result in a quieter engine. We would want both high cetane and octane at the same time but, unfortunately, they are mutually exclusive to an extent. Winter diesel, for instance, has a higher cetane number than summer one. There are some additives that improve but they are usually not worth their price, especially with Euronorm diesel.

As you can see, correct timing is very much chemistry dependent on diesels. This is why really accurate timing requires feedback using a knock sensor on the engine. Normal diesel strobes use injection time as a reference, and it is amazing how far they can be off for substandard fuel.

In theory, the fuel is always injected into the cylinder, so all of the diesels are, in a way, direct injection. In a similar manner, all gasoline injection engines except the newest GDIs are not really injection at all because the fuel is not injected into the cylinder, rather into the intake manifold--thermodynamically speaking that's not part of the engine. But that's a different story.

DI or direct injection occurs into a combustion chamber that is a part of the piston-cylinder combination (on small engines, it's a toroidal excavation in the face of the piston). II or indirect injection occurs into a combustion chamber which is part of the head, and is connceted to the cylinder through an orifice.

Why the difference? Well, they are really two different compromise solutions to the same problem--that the fuel does not burn up immediately. The combustion chamber has to be relatively small and swirl-shaped to ensure good combustion occuring away from the chamber walls, remaining that way until combustion is over. Short of a 5-stroke engine, this is not achievable. Indirect injection solves the problem of a controlled chamber in a simple way: the orifice creates high swirl in the small chamber as the piston compresses air in the cylinder and it rushes through the orifice. There are several types of chambers but only three are in widespread use, one is a toroidal swirl vertical chamber (patented by Mercedes), the second is a turbulance chamber (use by everyone else), and the third, a new, solitary and very late arrival is the spiral swirl chamber in the Fiat 1.9TD engine that replaced the old 1.9TD and already replaced by the 1.9JTD, after only being in service for about two years.

The solution also works backwards: the pressure drop in the chamber as the piston goes down is limited by the orifice, prolonging the time available for fuel injection and burning. The constriction in gas flow also makes it quieter; the small explosion acts onto a limited surface area. The disadvantage is lower efficiency (lost energy because of the gases passing through a constriction) and increased fuel consumption, and with turbocharging, the engine is required to give higher output with the same size prechamber, increasing the demands on the material of the prechamber (cracks in the prechamber orifice are a typical problem even on some newer II turbos).

Direct injection solves the problem by closing up the chamber when the piston is in the upper position, opening it up again as the gases expand and push the piston down. As a consequence, the chamber is initially the size of the whole cylinder, and swirl due to gas compression happens only as the piston reaches the very top. Once it's there, the time for injection and combustion is very short. And, when it happens, the chamber expands again, making the explosion act on a very big surface area, with a lot of fuel involved at once--hence the noise. However, there is no constriction, efficiency is excellent with low fuel consumption. Because the chamber is present only when the piston is in the top position, to have enough time for injection and combustion, these engines have been limited to low revs (all big diesels are DI). For small engines, the combination of low starting revs and requirement for good swirl (better at high revs) makes for difficult starting.

However, some new developments over the last decade made this principle practical even in higer rev, smaller engines:

Turbocharging plus accurate modeling of intake manifolds, channels, and valves. This enables the inward rush of the air as the intake valve opens, to create the initial swirl. Such modeling requires considerable computing power, and is still really in developemental phase; but it is nice to know that even with the advanced DI engines of today there is ample room for improvement...

High pressure injection systems shorten the time needed to inject the fuel, giving more time for combustion. In addition, injectors are multi-jet enabling combustion to start at several places at the same time, in effect sectoring out the combustion chamber to let the combustion to occur in several sectors in parallel. This is incidentally the same principle used on twin-spark gasoline engines.

Better control of the injection process by advances in pump and injector technology. This is essential because with the shortened interval for everything to happen, timing related error magnitudes are also smaller. A side developement of this is pilot injection: the starting drop of fuel, producing a sudden rise in pressure, temperature, and turbulence, makes the rest of the injected fuel burn almost immediately, uniformly and quickly.

Željko NASTASIĆ

Engine care (XM)

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. The user knows nothing of this until the very end--the constant tension actually improves compression, but wears out the cylinder and the rings, reducing the life of the engine at least ten times.

This is the reason why oil has to be changed more often in diesels. The oil itself would wear out slower than in a gasoline engines because the temperatures are generally lower than in those engines. For newer engines, oil change intervals have been increased because electronic regulation keeps the fuel/air ratio at full power accurately below the smoke limit, hence less particulates are created. In addition, the new common rail, higher pressure injection systems are less sensitive to injector wear because the injection timing is not derived from the injector opening pressure any more, and the very high pressures involved produce good atomization of the fuel even when the injectors are worn, again resulting in less particulate matter being created.

But even if the oil could remain operative for a longer time, black soot accumulating in the liquid will damage many parts of the engine, including the automatic valve adjusters. What's worse, this damage is actually hidden--at least initially--from the owner by the fact that the deposits cure the problems they have just caused: by filling and sealing the worn-out gaps, the problems remain unobserved until the next oil change. The new, fresh oil then washes out the soot, and due to the missing sealing effect, the valves end up having huge "clearances", which can be heard as a clicking noise at lower engine revolutions. As soon as the engine speed increases, the increased oil flow easily accounts for the leakage and the adjusters start working again. Non-working adjusters also influence airflow because the valves do not open completely--resulting in black exhaust smoke once more.

However, if there is a greater amount of soot deposits within the engine, after a few days the clicking might disappear as the new oil dissolves and distribute the deposits, which will again end up sealing up the adjusters. If, due to the symptoms described, you suspect that one or more of your adjusters fail, you should change all of them at the same time. It ouwld be possible to find out exactly which one failed, however, due to the nature of this failure, the rest have already had their share of the oil deposits, and they will also fail, usually in a short time (20-30,000 kms at most).

Before you replace the adjusters, it might be a good idea to change the oil and put an engine cleaning additive in it, as well as changing the oil filter (it's even better to change the filter twice while the cleaner is still in the engine; the cleaner can dissolve huge amounts of soot that can end up clogging the filter far earlier than normal, maybe only in 1,000 km or so). Then, after about 500-1000 km (sometimes even sooner), the adjuster(s) will start clicking again. At this point, drain the oil, change the adjusters, put in new oil and a new filter. As complicated as it may sound, you will doing the engine (and yourself, too) a great favor. The usual life expectancy of the adjusters is around 150,000 km; if they die prematurely, this is a sign of insufficient engine care.

The injectors also deserve some attention; if they are bad, you may end up changing oil very frequently and still end up with dead adjusters. When renewing the injectors, a typical and possibly fatal mistake is to forget to put new sealing rings between the injectors and the head. These are usually usually copper or brass profiled rings. When the injector is mounted, it squashes the profiled ring flat, which provides the seal. This means that whenever you take the injector out, you have to put in a new seal ring. If this is ignored, and either the old ring is put back or there is no ring at all, the hot combustion gases can leak past the injector, damaging its thread. The injector itself may overheat causing it to seize, which in turn may result in all sorts of bad things as the pump tries to compress uncompressable liquid into a closed volume: pump failure, intermittent engine runaway, and of course, injector destruction, leading to glow plug and even prechamber damage.

These seal rings not only seal but their thickness is accounted for in the positioning of the injector face. If they are missing, the injector face gets in the way of the direct gas path inside the prechamber. The prechamber is roughly spherical, and if you drill a hole in it to inject fuel inside, you would in theory have to fill out the hole by a piece of the sphere you have just drilled out--this means that the face of the injector, which is what fills the hole whould be concave. As you know, this face is actually flat. If you don't put the sealing rings in, the gases in the prechamber will actually wear out the flat face of the injector into a roughly concave space to restore proper gas flow and turbulence. While this is done, the turbulence will be inadequate, resulting in less than perfect combustion. Unfortunately, by wearing out the injector face, the action of the injector is compromised, so this is a lose-lose situation. With the seal in place, the injector face is slightly retracted from the wall of the prechamber. This makes the gases actually bypass the injector face, it's really in a 'wake' of the gases. Wear is far less (about 10%) and the geometry of the prechamber is actually less compromised this way.

Injectors that didn't have the seal ring fitted can be easily identified by being sooty and having a depression worn out of the injector face exactly the size of the opening towards the prechamber.

Glow plug failure is a sign that something is wrong with the injector inserts. The injectors themselves consist of an insert which is really a sort of a needle valve, and a body which holds the insert, plus a spring that determines the opening pressure. Unless there has been some sort of catastrophic failure (or a terrible case of rust because of abominable fuel quality) the body is never changed. The inserts should be checked at regular interval (about 50,000 kms, sometimes even more often if fuel quality is bad) and at the very least cleaned if not renewed. The more regulairly they are cleaned, the longer they will last. With good injectors, glow plugs last a very long time--at the very least 100,000 to 150,000 km.

We mentioned the injection pumps as well. Those manufactured by CAV will wear and eventually fail in due time, their construction is less than perfect. They have a sleeve bearing at the front which will wear out, then the shaft starts to roll around in the sleeve bearing, opening up too big a hole gradually, producing a beat. As the bearing wears out, it becomes roughly oval (it wears towards the force of the cam belt). This results in the shaft not being parallel with the axis of the pump. Bosch pumps are less sensitive to this but CAV ones start to wear their rollers and the cam ring out. The first symptom is a strange noise around 1,200 rpm, a ciclycal change in the noise of the engine that happens about once every three to four engine revolutions. It can be misleading but by using a stethoscope (or a wooden stick) you can find out that it actually comes from the pump. The pump itself will work for thousands or tens of thousand of kilometers after this problem develops, but sooner or later this uneven running will wear it out internally, making it more and more imprecise about fuel metering and injection timing.

Bosch pumps, on the other hand, seem to last forever, although they need a timing test (and possible adjustment) every 150,000 to 200,000 km. There are no problems as long as the fuel filter is changed and possible water drained regulairly. The CAV ones usually need a rebuild around 150,000 to 200,000 km which will change the cam ring, the rollers, the two radial pistons, the front bearing and a part from the uptake pump. CAV may also develop leaks on the manifold pressure sensor (for turbo) and the timing advance solenoid. Besides, CAV pumps are incredibly sensitive to air in the fuel uptake: this is because they use hydraulics to transfer certain forces within the pump, most notably to the speed regulator assembly and the turbo boost membrane. It uses the fuel for this, relying on its uncompressibility. Of course, if the fuel has air bubbles in it, they together become compressible which completely screws up the hydraulic stuff. The results are very annoying such as sudden shutouts, getting stuck at 2000 rpm, oscillations (sometimes really bad—from 1000 to 5000 rpm and back, once every second or so), even engine runaway. Very nasty, indeed.

Željko NASTASIĆ

Turbo failure

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. I can tell you that I looked at mine when I changed the head gasket, and even though the car has been tortured before I got to own it (I am not to certain it had its oil changed more than twice in 120,000 km, injectors were still the original ones, never touched), it was in excellent condition and could probably have gone at least that much longer without any problems even if I hadn't touched it.

To clean it, the turbo should be removed from the manifolds, dissasembled to pieces which are then cleaned thoroughly of carbon and crud deposits. This involves mostly the bearing case, the bearings themselves, the shaft, and the heat sheild on the hot side. The shaft should also be re-balanced when the whole unit is put together again, although with some attention to detail this can be avoided. It is essentially the same process a turbo rebuilder service would do except you need no spare parts.

If you have a turbo seizure, don't think of driving with it even temporarily. Also, for obvious reasons the turbo should not be removed (open manifolds), nor should the intake be bypassed and the turbo left to blow into the atmosphere (over-revving will occur because it will not reach waste gate pressure at any rpm). If a working turbo needs to be disabled it's actually safer to put a piece of pipe at the pressure output and plug it at the end, leaving a very small hole (say 0.5 cm, unfortunately this will result in lots of noise) thus creating a small reservoir and something for the turbo to do without revving itself apart. Merely pulling out the manifold pressure tube that goes to the compensator on the pump will not disable the turbo--it will still work, albeit at a much lower efficiency.

If exhaust gases end up hitting a seized turbo, they eventually burn off the blades of the turbine and the debris ends up in the converter, not to mention the overheating of the turbo itself. There are people who overhaul turbos: this will mean a new set of hydrodynamic bearings, oil seal rings, a new shaft and a new set of turbine (and possibly compressor) blades. In any event it will be cheaper than a new one.

Alternatively, you can look at scrapyards for the same basic model, and have the refurbishing people do a 'make one out of two' job, which actually only involves exchanging the parts that attach to the manifolds, plus cleaning the salvaged turbine.

The main reasons a turbo can fail are the following: it is not allowed to spin down before oil pressure drops. This results in the spinning shaft crashing into the hydrodynamic bearings. This in itself may not damage the bearing or the shaft, unless it happens repetitively. The oil that remains there offers some lubrication for a while until its temperature reaches a very high level because of drag to the shaft; this will produce a local carbonisation. This builds up until it either damages the soft bronze bearing the next time the shaft drops or until enough carbon builds up to seize the turbine. The situation is much worse if the turbo was under a great deal of strain just before this, the temperature of the shaft being quite high. Always allow then engine to run on idle for a minute or two to let the turbo cool down before you switch the egine off, especially after highway runs.

If the oil seal wears out on the hot side, the oil will pour out between the seal and the heat shield, getting burned. The resultant crud produces drag and more heat on the shaft, and as it builds up, it may actually end up pushing out the heat shield as it expands under heat. The resulting impact on the hot side rotor will brake it down. The added (signifficant) drag may then literally burn off the rotor blades. This is usually caused by contaminants in the oil carbonising on the oil seal and wearing it out, This is responsible for most turbo failures on a diesel. The occasional cleaning of the turbo will prevent this failure completely (unless the engine and the turbo are already on their last legs).

Željko NASTASIĆ

Fuel cut-off valve

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.

It is simple to check for--when you turn the ignition on, the valve makes a faint click. If you cannot hear it through the components blocking your way, use the stethoscopic method to listen to it. If you cannot hear the click but you can measure the voltage on the contact on top of the valve, it is positively dead. Replacing is straightforward.

Željko NASTASIĆ