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How A Diesel Engine Works

How A Diesel Engine Works

How does my diesel engine work?

I was asked this the other day. How does my diesel engine work, and what's so different between it and a petrol engine?

Well, first of all, fuel.. Err, obviously. No seriously, there are a few differences apart from that, so I thought it might be an idea to give you a quick idea of how it works.

Okay, I'm sure you are all aware of the four-stroke system? There are four stages in the combustion cycle, intake, compression, combustion, and exhaust. (I heard a student of engineering refer to this once as 'suck, squeeze, bang, blow,'…which is, I'm sure you'll agree, easier to remember! Least, I think he was referring to the combustion cycle!)

Unlike petrol, diesel is not 'flammable'. It would be better to describe it as 'combustible'. It will not ignite instantly like petrol would if you dropped a match on it…in fact, it would in all probability put the match out! Rather, it would be better to describe it as fuel that self ignites under high pressure.

Unlike a petrol engine, a diesel engine does not have a spark plug to ignite the air/fuel mixture…they rely on the heat of compression to cause combustion. A good example of this would be if you were to put your finger over the nozzle end of a foot pump…you press the pedal, and as the air is compressed it becomes hot, which you feel on your finger. This is because the air molecules are compressed, have less space to move around, and collide with each other more frequently. The resultant friction causes heat.

The same thing happens in a diesel engine! As the piston moves up, the air in the cylinder is compressed, and it heats up. It gets up to about 400 deg. C!! As the piston reaches the top of its stroke, and is as far away from the crank as it can get (TDC, or Top Dead Centre) the pump and injectors spray a very fine mist of fuel into the piston chamber. The intense heat of the compressed air in the cylinder causes the diesel to combust, which forces the piston back down. That movement is converted by the crankshaft into drive to the gearbox, which in turn makes your wheels go round.

So, it is quite simple really…though it gets more complicated when you start to look at the timing involved.

Okay, let's go into a little bit more depth.

What are the main differences between a petrol engine and a diesel engine?

A petrol engine sucks in a mixture of fuel and air on the intake stroke, compresses the fuel mixture and then ignites it with a spark. The combustion forces the piston down, and then the next upward stroke forces the exhaust gases out. We know that the diesel injects the fuel into the compressed mixture…

The petrol engine compresses at around 8:1 and in some engines up to 12:1. The diesel engine compresses at a much higher pressure, between 14:1 and sometimes as high as 25:1! The higher compression ratio of the diesel engine leads to better efficiency.

Petrol engines use either 'carburetion', in which the air and fuel is mixed via a carburettor well in advance of it reaching the cylinder, or by 'port fuel injection', where the fuel is injected into the cylinder (from outside) just before the intake stroke. It is injected in to the inlet port of the manifold, and the air rushing through helps to mix the fuel. We already know the diesel injects its fuel directly into the cylinder.

The fuel pump….

Older diesel engines use a mechanical pump and valve assembly driven by the crankshaft, usually by a belt in the case of the 2.5 diesels in 90's, or a chain and skew gears in the earlier 2.25's.

These engines use injectors which are basically spring-loaded valves which open and close at a specific fuel pressure.

The distributor pump assembly consists (simply!) of a pump which pressurises the fuel, and a disc shaped valve which rotates at half the crankshaft speed. This valve has a single aperture which allows the pressurised fuel in on one side, and an opening for each injector on the other. As the engine turns, the openings line up and deliver a burst of pressurised fuel to the injector about to enter its power/combustion stroke.

The injector 'valve' is opened by the pressurised fuel and the diesel is injected until the disc shaped valve rotates out of alignment, and fuel pressure to that injector is cut off. The engine speed is controlled by another disc in the pump, which rotates only a few degrees, controlled by the throttle pedal/lever. This disc alters the width of the aperture through which the fuel enters, and thereby the length of time the injector is held open before the fuel supply is shut off, hence controlling the amount of fuel injected.

An electronic system works in a different fashion… It has a separate fuel pump which supplies constantly at high pressure to each injector. Each injector has a solenoid operated by an ECU (electronic control unit) which enables more accurate control of injector opening and closing times which depend on other engine control conditions, such as engine speed, and load. This makes for a more economic engine, with better performance. This is also a much simpler system than the mechanical distributor pump, making it generally more reliable, and certainly a lot quieter!

Different types of injection….

Indirect injection

An indirect system delivers fuel into a chamber (a pre-chamber) off the combustion chamber where combustion begins, and then spreads into the main combustion chamber. This system allows smoother and quieter (honest) running, and because the combustion is aided by turbulence injector pressure can be lower.

The pre-chamber has the disadvantage of allowing some heat dispersal into the cooling system, which decreases the efficiency a little, and most indirect injection engines require some form of cold start device, such as glow plugs.

Direct Injection

The first types of direct injection systems used a rotary pump, like the indirect systems. The main difference was that the injector was mounted in the top of the combustion chamber, rather than in a pre-combustion chamber. The main problem with this type was the noise they make, and the smoke/particulate levels they produced. Fuel consumption levels were about 15% lower than the indirect types though.

A good example of a mass produced direct injection engine is the 200Tdi 2.5 litre Land Rover engine, which has an aluminium cylinder head, Bosch two-stage injection and multi-phase glow plugs, whilst still utilising mechanical fuel injection.

Common Rail

In a common rail system, the injector distributor pump is eliminated. The fuel is constantly pressurised (typically up to 26,000 p.s.i.!) in a common rail, which is basically a tube with branches to each injector. The injectors are typically controlled electronically by solenoids, which are in turn computer controlled by the ECU.

And Finally, a WARNING!

The injection of fuel is at a very high pressure, as you have seen. It is VERY IMPORTANT to be aware that if you remove an injector from the engine, and attempt to operate it to check the spray pattern you risk serious injury.

This is what is called 'hypodermic injection injury' which can result in severe blood poisoning, and in some cases may require amputation. In fact, the spray pattern is enough to amputate body parts on its own!

In short, it is far better to get your injectors checked by a specialist.
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