In an ideal, 100 per cent efficient
internal combustion engine
would burn to give just
and water vapour. In practice, of
are far from efficient and the
oxides of nitrogen
, as well
dioxide and water vapour.
These by-products of combustion are expelled as part of the car's exhaust
gases into the atmosphere where they cause pollution.
In recent years, public concern about atmospheric pollution, and imminent
pollution-control laws, has led to car manufacturers trying to find ways of
reducing the level of these gases in car exhausts.
There are two basic approaches to reducing harmful exhaust
lean-burn engines or attaching
Lean-burn engines are designed to produce a lower level of harmful emissions
by better combustion control and more complete burning inside the engine
Catalytic converters clean up the exhaust gases coming from the engine.
Catalysts are the older of the two systems, and have been used in the US and
Japan for some years.
Catalytic converters are fitted by the car manufacturer downstream of the
engine in the exhaust system. It looks like a slightly swollen silencer and
contains a fine metal or ceramic honeycomb, coated with
or a related
metal, across which the exhaust gases flow.
The platinum initiates a chemical reaction in which the harmful exhaust
constituents are converted into harmless nitrogen, carbon dioxide and water
The problem with catalytic converters is that they sap engine power and
reduce fuel economy. They also lead to increased maintenance costs.
Another drawback is that the catalytic system needs unleaded petrol to work
properly, because any lead in the exhaust gases quickly ruins the catalyst's
efficiency. And some European countries, such as Britain, have none or very few
outlets for unleaded petrol, with little hope of establishing a comprehensive
network for distributing the new fuel in the near future.
Inside the CVH combustion chamber
Ford's lean-burn engine, based on the CVH, has a
which is kidney shaped — it looks rather like an off-centre hemispherical
This type of design ensures good breathing, and the enhanced 'squish'
effect means that the fuel and air will be well mixed for
mixture is forced up and sideways into the kidney shape of the chamber,
rather than just being pushed into the more regular hemisphere of the
These problems have forced car manufacturers to look elsewhere for ways of
reducing exhaust emissions. The most obvious avenue for reducing emissions is
to burn less fuel in the first place.
This requires an improvement in
, which is now very
difficult to achieve because all the readily available routes have already been
One remaining possibility is to produce a 'leaner' mixture, namely to reduce
the proportion of fuel in the fuel/air mixture entering the engine.
Petrol burns best in a standard car engine when it is mixed with air in the
proportions 14.7:1 - nearly 15 parts of air to every one part of petrol. In
practice the mixture strength varies between about 13:1 and 16:1, depending on
the speed of an engine and its load at the time. At these mixtures, engines
produce fairly high levels of harmful exhaust
during initial acceleration.
When you try to move away from the ideal fuel/air
, the engine's
running is affected - if the engine is fed too much fuel it produces smoke,
wears out quickly and is expensive to run. If the engine is made to run too
lean, combustion becomes extremely variable from one
to the next, exhaust
gas temperatures rise because of the persistence of flames from 'late-burn'
cycles, and the engine starts to misfire frequently. All of these result in
high levels of hydrocarbons in the exhaust gases.
To overcome the difficulties in making an engine run well on leaner
mixtures, the air/fuel mixture needs to be more intimately mixed and the actual
timing and combustion process needs to be very finely controlled.
To this end, some car manufacturers are fitting engine management systems
where sophisticated electronics control both the ignition and the fuel delivery
systems. This makes it possible to make sure that the
fire at just
the right moment to ignite a fresh fuel/air
, which may otherwise be
reluctant to ignite.
Also under development are engine parts made of new materials that have
, such as
made of ceramics. But most development
is going into ensuring that the air and fuel are well mixed.
Stir the mixture
In reducing the proportion of fuel in the mixture entering the engine, car
manufacturers have encountered problems with
combustion which have, in some cases, increased rather than decreased fuel
To get round these problems, the industry has tried different ways of
`stirring' the mixture just prior to ignition, with the aim of promoting faster
burning and more complete combustion.
There are three main ways of stirring the mixture. First, the engine's inlet
ports can be shaped to cause swirl - a technique borrowed from direct injection
. Second, a deflector, or 'fence', around which the mixture has
to flow, may be positioned near the inlet
or valves. And third, the
combustion chamber itself can be made smaller than the cylinder
what is known as a 'squish' effect - under
from the upcoming
piston, the fuel / air mixture has to squeeze itself into the combustion
chamber, and this increases the
of the fuel droplets in the
Working out how best to design the engine so it can cope with very lean fuel
mixtures is a very difficult process. Part of the problem is trying to see what
actually goes on inside a combustion chamber when the fuel/air mixture is
burning, particularly when the throttle is rapidly opened or closed.
So researchers are now using a quartz window in the combustion chamber,
combined with a cine camera and complex computer programming, to see exactly
what is going on inside. From this they can tell how and where the flame is
spreading, which gives an indication of how fully the mixture is burning.
The way ahead
generation of lean-burn engines run on ratios of around 17:1 or
18:1, and the next generation should run with ratios averaging 20:1 or
But lean-bum technology still has some way to go before it fully meets the
proposed EEC laws. Some manufacturers are proposing to use a combination of a
catalyst and a lean-burn engine to meet the demands of the new regulations.
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resembles the simpler variable-jet type (See
How variable-jet carburettors work
) in having a
- a constricted neck - through which air flows on its way to the
Almost all car
work on the four-stroke
, so called because it
and exhaust -
to produce one firing of the
/air mixture. This means that the
rotates twice to complete each cycle.