The cylinder is the main working part of a diesel car's engine. It's where the fuel is converted to
the energy which drives the wheels. Since the conversion of fuel to energy is the intention and purpose
of the internal combustion engine, the cylinder can be considered the heart of the engine. In short, the
cylinder is the most important part of your car, and the concept of compression is key to the function of
a diesel engine. It makes sense then to learn about it and what it does, and that's the purpose of this
guide.
The Modern Cylinder
To understand the way modern cylinders work in diesel engines and why compression is important within
the cylinder, particularly to diesels, it helps to first look at each of the components of the cylinder
individually.
Cylinder Heads
As its name suggests, the cylinder heads are the tops of the cylinders. In a modern diesel engine
the cylinder heads are required to perform a few functions, including:
- acting as the seal for the cylinder, keeping the cylinder closed so the pressure can build,
- holding the exhaust valve, fuel injector, and all the necessary linkage.
Bore / Sleeve
This is the 'cylinder' part of the cylinder; a tube bored out of a lump of metal, in which the piston
slides up and down. The size of the bore regulates a few features of the engine, including the litre
capacity of your engine. For instance, if your car has a 3l V6 engine, that means each of the 6 cylinders
has 0.5l capacity.
Piston
The piston is the strong sliding shaft which travels up and down inside the cylinder. Its purpose
inside a diesel engine is to compress the air in the cylinder until it auto-ignites, at which point the
piston is forced back down, turning the crankshaft via the connecting rod.
Connecting Rod
The connecting rod is attached to the bottom of the piston and is hinged at both ends, since the
joints which connect it need to move. The connecting rod is attached to the crankshaft and it's here
that the rotational motion is produced. An upward movement of the piston will pull the connecting rod
up which will, in turn, exert force on the crankshaft by lifting up one side of it. The subsequent
downward motion of the piston forces the connecting rod down, which forces the crankshaft to complete
its rotation.
Intake Valve
With many names and in many different forms, the function of the intake valve is to load the cylinder
with diesel. It's through this valve that the diesel enters, and it's usually located in the cylinder
head.
Exhaust Valve
The exhaust is a valve in the cylinder which opens at a certain point in the stroke to allow the spent
exhaust created by the ignition to be extruded from the cylinder.
These are the primary components which make up the cylinder. Most diesel engines will have all or some
combination of these parts, although they maybe laid out differently. For instance, some internal
combustion engines don't use valves as such, but instead have slit-like intakes in the sides of the
cylinder sleeve which inject fuel when the piston moves below them in the stroke.
Compression
So where does the effect of compression come into all this? Compression is central to the function
of a diesel engine; perhaps more so than any other type of engine. The compression of the air-fuel
mixture in the cylinder is what gives the diesel engine all its advantages over other engines. Petrol
engines rely on spark plugs in the cylinder head to ignite the fuel mixture in the cylinder, which is
fine, but it's not really the most efficient way to harness the energy in fuel.
A petrol engine draws in a mixture of fuel and air into a cylinder during the intake stroke. This
mixture is then ignited by the spark plug, and the resulting energy pushes the piston back down the
cylinder. A diesel engine however draws in only air on the intake stroke. The piston then travels back
up the cylinder compressing the air inside, which makes the air very hot (as anyone who has held the end
of a bike pump after inflating a tyre knows). Once the piston is at the top of the stroke, the diesel is
sprayed into it, and the heat causes it to ignite on contact with the air. The efficiency of an engine's
compression is measured by the compression ratio.
Compression Ratio
The higher the compression ratio the better, because it means that more mechanical energy is being
extracted from the fuel due to the higher thermal efficiency created by the pressurisation. The molecules
which make up the fuel-air mixture are evaporated more efficiently and easily the higher the compression
ratio is, and the more effective the ignition (as long as the engine is strong enough to withstand the
pressures inside the cylinder, obviously).
The compression ratio is calculated as the volume of the cylinder when the piston is at the bottom of
its stroke compared with the volume of the cylinder when the piston is at the top of its stroke. If you
take a cylinder which has a volume of 1000cc when the piston is at the bottom of the stroke, and 50cc at
when the piston's at the top, then the compression ratio of your engine is 1000:50, or 20:1 when the
fraction is reduced.
A petrol engine's compression ratio is usually about 11:1 at the most. A diesel can comfortably
compress the air at a ratio of 25:1, more than twice as much as a normal petrol engine. The air inside
a diesel engine's cylinder can be as hot as 900 degrees Celsius when the diesel is injected, although it's
not necessary to be this hot since the auto-ignition temperature (the temperature at which a gas or liquid
will combust without the need of a spark) of diesel is around 210 degrees Celsius.
Cylinder & Compression Technology
Since the cylinder is the most central part of the engine, where the fundamental business of fuel
energy conversion goes on, it's here where minor changes can make the biggest differences. Sometimes
one engine might benefit from having multiple compression ratios. For example, a diesel engined car
might benefit from having a very high compression ratio for fast acceleration, but a lower compression
ratio for fuel economy at, say, motorway cruising speeds. Currently, there are no commercial diesel
engines which offer more than one compression ratio per engine. However, thanks to innovative leaps in
engine technology, and increasing attention and praise being applied to diesels, it's now known that Saab,
Nissan, PSA/Peugeot-Citroen, Renault, Mercedes and BMW amongst others are beginning to develop Variable
Compression Ratio engines. Most of these manufacturers are being very secretive about their research and
development, but one, Saab, has released details of a variable compression engine. It works by drawing in
the piston head closer to the crankshaft when the piston is on its intake stroke, thereby reducing the
overall volume of the cylinder so that the compression ratio is increased.
If the technology is viable, you can be sure that the other manufacturers will quickly jump on the
bandwagon, and the streets will quickly become full with exceptionally high performance, stunningly
economical diesel engined cars, all thanks to minute alterations made in the cylinder.