The ability of an engine to burn a wide
variety of fuels depends upon many factors. A few of them are listed below:
i.
It should give good
combustion efficiency and heat losses from the engine should be minimum.
ii.
The temperature of the
engine should be comparatively higher because of low ignition quality of
petrol.
iii.
The engine must be able
to start under sub-zero temperature condition of the atmosphere without any
external aid.
iv.
The exhaust from the
engine must have low smoke and noise level.
To fulfil the above mentioned
requirements the following design features of the engine should be considered:
a. Compression Ratio:
The petrol have low
Cetane number (18 to 20) which is an indication of low ignition quality.
Therefore, if the petrol is used in CI engine, a compression ratio of 23:1 is
required to increase the temperature at the end of compression stroke.
b.
Stroke
to Bore Ratio:
It is necessary to
maintain the combustion chamber hot at all load and speed conditions. A high stroke
to bore ratio provides compact combustion chamber which reduces heat losses and
maintains the high temperature of combustion chamber. A spherical combustion
chamber gives lower-surface/volume ratio and reduces the heat losses to
minimum. A M-type spherical chamber is more suitable for multi-fuel engines
because it provides high degree of air utilization and smooth operation of the
engine.
c.
Performance:
The output of the engine is 15 to 20%
less with petrol as fuel compared with diesel oil at higher speed. The
difference narrows down at lower speed.
The bsfc for petrol engine is also
higher at lower speed compared with diesel fuel. The difference narrows at
higher speeds. This is because of lower combustion efficiency of diesel engine
at lower speed as swirl is weak.
Stratified
Charge Engine:
Conventional homogeneous charge engines
have exhibited various problems since their inception such as mechanical
loading; fuel economy, engine knock and combustion generated pollution.
The petrol engines possess good full
load characteristics, high degree of air utilization and lower weight power
ratio. But if has poor part load efficiency. The diesel engine has better part
load efficiency but poor air utilization, low smoke limited power and higher
weight to power ratio. The adoption of higher compression ratio for better
combustion imposes additional problems of maintenance and higher losses in
diesel engines. For automobile engines, part load efficiency and full load
power are very important as the engine works at part load conditions for about
90% total operating life.
Therefore, there is a need to develop an
engine which combines the advantages of both SI and CI engines and
simultaneously avoids the possible disadvantages. A Stratified charge engine is an attempt in this direction. The
stratified charge engine offers solutions to most of the problems mentioned
above. Despite, the attempts made by the engineers to develop stratified
engines, it has suffered several drawbacks as high hydrocarbon omission at part
loads.
Following are the major advantages and
disadvantages of Stratified Charge Engines:
Advantages:
i.
The rich mixture near
the spark plug and lean mixture near the piston surface provides cushioning to
the explosive combustion.
ii.
It gives good part load
efficiency.
iii.
It resists the knocking
and provides smooth combustion. Therefore, low Octane fuels can be used.
iv.
Stratified charge
engines have exhibited multi-fuel capability.
Disadvantages:
i.
More complex design to
supply rich and lean mixture and quantity is varied with load on the engine.
ii.
The Stratified engines
create high noise level at low load conditions.
iii.
Unthrottled stratified
charged engine emits high percentage of HC due to either incomplete combustion
of lean charge or occasional misfire of the charge at part load conditions.
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