On higher
volumes
Dr Anupam Dewan examines the pros and cons
of using combustion and emission engines
in petrol and diesel vehicles
An internal combustion engine is one in
which the combustion of fuel occurs
inside a chamber producing hot
expandable gases which in turn provide mechanical
power. Such engines can be classified as reciprocating
or rotary, spark ignition or compression ignition,
two-stroke or four-stroke—the latter is present in
most automobiles. Petrol engines usually deliver
higher output compared to diesel ones, though the
latter are known to be more efficient.
Petrol vehicles
Characteristics
Octane number of petrol is a measure of the
maximum compression ratio that can be achieved
without the fuel-air mixture auto-igniting, which
results in knocking and can often be undesirable. The
auto (self)-ignition of the air/fuel mixture in the
cylinder results in a loss of power. As the
compression ratio of the engine increases, the
required octane number of petrol also needs to be
increased, to avoid engine knock. The performance of
a fuel is also dependent on the engine's operating
conditions. To account for differences in the
performance quality of a fuel under different
operating conditions, two types of octane numbers are
used. Research Octane Number (RON) corresponds
to low-speed mild knocking condition and Motor
Octane Number (MON) simulates more severe hightemperature
knocking conditions at high-speed and
high-load. The reported octane number of petrol is
the average of RON and MON.
Carburettor, MPFI and GDI
Three techniques for preparing air-fuel mixture in a
petrol vehicle are used. Earlier, carburettors were used
for mixing of petrol and air and the mixture was
carried to the combustion chamber via an inlet
manifold before being ignited by a spark plug. In a
Multi Point Fuel Injection (MPFI) system, petrol is
injected directly into the inlet manifold of an engine
cylinder based on an electronic engine control system.
Gasoline Direct Injection (GDI) is a recent technique,
where the fuel is injected directly into the cylinder. A
GDI engine controls injection timings according to
the load on the engine. It can provide low fuel
consumption and high power output by using two
types of combustion. For load conditions required of
typical city driving, fuel is injected late in the
compression stroke. Thus, a very lean (with low fuel
content) combustion is achieved resulting in
considerable fuel savings. During high performance
driving conditions, fuel is injected in a normal way
during the suction stroke. This enables a
homogeneous air-fuel mixture similar to that in a
conventional MPFI engine to deliver a higher output
resulting in a cooler air-fuel mixture minimising the
chances of an engine knock out.
Combustion and knock
A petrol engine uses spark ignition and the
combustion begins in the form of a flame near the
spark plug. The flame front moves in the combustion
chamber and burns the fuel-air mixture. The
maximum pressure in the cylinder is usually produced
....CONTD
