have always been seen as noisy, smelly and
of little use other than in trucks, taxis and vans. But as
diesel engines and their injection system controls have become more refined,
the 1980s have seen that situation change. In the UK in 1985 there were almost
65,000 diesel cars sold (about 3.5 per cent of the total number of cars sold),
compared with only 5380 in 1980.
The main advantage of diesel engines over petrol engines is their lower
running cost. This is partly a result of the greater efficiency of the high
diesel engine and partly because of the lower price of diesel
- although the price difference varies, so the advantage of running a
diesel car will be slightly reduced if you live in an area with high-priced
diesel fuel The service intervals are often longer too, but many diesel models
require more frequent oil changes than their petrol counterparts.
How diesel engines work
A diesel engine works differently from a petrol engine, even though they
share major components and both work on the four-stroke
. The main
differences are in the way the fuel is ignited and the way the power output is
In a petrol engine, the fuel/air mixture is ignited by a
. In a diesel
is achieved by
of air alone. A typical compression
for a diesel engine is 20:1, compared with 9:1 for a petrol engine.
Compressions as great as this heat up the air to a temperature high enough to
ignite the fuel spontaneously, with no need of a spark and therefore of an
A petrol engine draws in variable amounts of air per suction
exact amount depending on the throttle opening. A diesel engine, on the other
hand, always draws in the same amount of air (at each engine speed), through an
unthrottled inlet tract that is opened and closed only by the inlet
(there is neither a
nor a butterfly valve).
reaches the effective end of its
stroke, the inlet
valve closes. The piston, carried round by the power from the other pistons and
the momentum of the
, travels to the top of the
the air into about a twentieth of its original
As the piston reaches the top of its travel, a precisely metered quantity of
diesel fuel is injected into the
. The heat from compression
fires the fuel/air mixture immediately, causing it to burn and expand. This
the piston downwards, turning the
As the piston moves up the cylinder on the
, the exhaust valve
opens and allows the burned and expanded gases to travel down the
At the end of the exhaust stroke the cylinder is ready for a fresh
The major components of a diesel engine look like those of a petrol engine
and perform the same jobs. However, diesel engine parts have to be made much
stronger than their petrol engine equivalents because of the much higher loads
The walls of a diesel
are normally far thicker than a block
designed for a petrol engine, and they have more bracing webs to provide extra
strength and to absorb stresses. Apart from being stronger, the heavy-duty
block can also reduce noise more effectively.
, crankshafts and
caps have to be made
stronger than their petrol engine counterparts. The
design has to
be very different because of the
and also because of the shape
and swirl chambers.
internal combustion engine
to operate smoothly and efficiently, the
fuel and air need to be properly mixed. The problems of mixing fuel and air are
particularly great in a diesel engine, where the air and fuel are introduced at
different times during the cycle and have to be mixed inside the cylinders.
There are two main approaches direct injection and indirect injection.
Traditionally, indirect injection has been used because this is the simplest
way of introducing
so that the injected fuel spray mixes well with
the highly compressed air in the combustion chamber.
In an indirect injection engine there is a small spiral swirl chamber (also
called a pre-combustion chamber) into which the
squirts the fuel
before it reaches the main combustion chamber itself. The swirl chamber creates
turbulence in the fuel so that it mixes better with the air in the combustion
The drawback with this system is that the swirl chamber effectively becomes
part of the combustion chamber. This means that the combustion chamber as a
whole is irregularly shaped, causing combustion problems and hampering
A direct injection engine does not have a swirl chamber into which the fuel
is injected - the fuel goes straight into the combustion chamber instead.
Engineers have to pay very careful attention to the design of the combustion
chamber in the piston crown to ensure that it creates enough turbulence.
A diesel engine is not throttled like a petrol engine, so the amount of air
sucked in at any particular engine speed is always the same. Engine speed is
regulated purely by the amount of fuel squirted into the combustion chamber -
with more fuel in the chamber, combustion is fiercer and more power is
pedal is connected to the metering unit of the engine's
injection system rather than to the throttle butterfly as with a petrol
Stopping a diesel still involves turning off the `ignition' key but, rather
than cutting off the sparks, this closes an electric
that cuts off the
fuel supply at the injector
of the fuel metering and distribution unit.
The engine then only has to use a small amount of fuel before it comes to a
halt. In fact, diesel engines come to rest more quickly than petrol engines
because the much higher compression has a greater slowing-down effect on the
Starting a diesel
As with petrol engines, diesel engines are started by being turned with an
, which begins the
cycle. When cold,
however, diesel engines are difficult to start, simply because .compressing the
air does not lead to a temperature that is high enough to ignite the fuel.
To get around the problem, manufacturers
. These are small
electric heaters, powered from the car's
, which are switched on a few
seconds before attempting to start the engine.
The fuel used in diesel engines is very different from petrol. It is
slightly less refined, resulting in a heavier, more viscous and less
. These physical characteristics often lead to it being
referred to as 'diesel oil' or 'fuel oil'. On diesel pumps on garage
forecourts it is often called 'derv', short for diesel-engined road
Diesel fuel can begin to stiffen slightly or even solidify in very cold
weather. This is compounded by the fact that it can absorb very small
quantities of water, which may freeze. All fuels absorb tiny amounts of
water from the atmosphere and leakage into underground storage tanks is
quite common. Diesel fuel can handle a water content of up to 50 or 60
parts in a million without problems—to put this into perspective this is
about a quarter of a mug-full of water for every ten gallons of fuel.
Any freezing or 'waxing' can block
and injectors and prevent
the engine from running. This is why, in very cold weather, you will
occasionally see people playing blow lamps on their truck's fuel lines.
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In many ways a diesel engine is less complicated than a petrol engine, with
greater reliability and a longer working life. There is little difference
between servicing a diesel and a petrol engine except that the diesel has no
ignition system to worry about. However, the fuel system needs very careful
attention, as part of the regular service, to prevent dirt and water in the
fuel damaging it.