Answer / karthick

since the diesel has the high self ignition temperature the
air inside the cylinder should be compressed at about 1000
deg.so that the diesel catch fire momentarily.so inorder to
raise the temp of air too high we have high compression
ratio for diesel engine

Answer / neeraj

The compression ratio of an internal-combustion engine or
external combustion engine is a value that represents the
ratio of the volume of its combustion chamber; from its
largest capacity to its smallest capacity. It is a
fundamental specification for many common combustion engines.

In a piston engine it is the ratio between the volume of the
cylinder and combustion chamber when the piston is at the
bottom of its stroke, and the volume of the combustion
chamber when the piston is at the top of its stroke.[1]

Picture a cylinder and its combustion chamber with the
piston at the bottom of its stroke containing 1000 cc of air
(900 cc in the cylinder plus 100 cc in the combustion
chamber). When the piston has moved up to the top of its
stroke inside the cylinder, and the remaining volume inside
the head or combustion chamber has been reduced to 100 cc,
then the compression ratio would be proportionally described
as 1000:100, or with fractional reduction, a 10:1
compression ratio.

A high compression ratio is desirable because it allows an
engine to extract more mechanical energy from a given mass
of air-fuel mixture due to its higher thermal efficiency.
High ratios place the available oxygen and fuel molecules
into a reduced space along with the adiabatic heat of
compression - causing better mixing and evaporation of the
fuel droplets. Thus they allow increased power at the moment
of ignition and the extraction of more useful work from that
power by expanding the hot gas to a greater degree.

Higher compression ratios will however make gasoline engines
subject to engine knocking if lower octane rated fuel is
used, also known as detonation. This can reduce efficiency
or damage the engine if knock sensors are not present to
retard the timing. However, knock sensors have been a
requirement of the OBD-II specification used in 1996 Model
Year Vehicles and newer.

Diesel engines on the other hand operate on the principle of
compression ignition, so that a fuel which resists
autoignition will cause late ignition which will also lead
to engine knock.

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