Diesel engine
The diesel engine is an internal combustion engine in which ignition of the
fuel is caused by the elevated temperature of the air in the cylinder due to
the mechanical compression; thus, the diesel engine is a so-called
compression-ignition engine.
Working principal
Diesel engines work by compressing only the air. This increases the air
temperature inside the cylinder to such a high degree that atomised diesel
fuel injected into the combustion chamber ignites spontaneously. Diesel
engines may be designed as either two-stroke or four-stroke cycles.
DIESEL ENGINE RELATION FOR ITS THERMAL EFFICIENCY
Diesel engines have approximately 42% brake thermal efficiency at full
load, with 28% of fuel energy wasted in exhaust gas (including 4% going to
pumping loss), 28% of fuel energy dissipating to cooling media as heat
rejections to the ambient (including 4% going to mechanical friction and
parasitic accessory power), and 2% going to miscellaneous heat losses.
There are strong interactions between heat rejections and exhaust gas
energy, mainly depending on EGR rate and pumping loss. When EGR rate
increases, heat rejections become higher and exhaust energy becomes
less.
PETROL ENGINE
Gasoline engine is internal combustion engine that generate power by
burning a volatile liquid fuel (gasoline such as ethanol) with ignition initiated
by an electric spark. Gasoline engines can be built to meet the
requirements of practically any conceivable power-plant application, the
most important being passenger automobils.
Working principal
The fuel is mixed with air and then inducted into the cylinder during the
intake process. After the piston compresses the fuel-air mixture, the spark
ignites it, causing combustion. The expansion of the combustion gases
pushes the piston during the power stroke.
THERMAL EFFICIENCY of Petrol Engine
Petrol engines give petrol engines lower efficiency than diesel engines.,
most petrol engines have approximately 20%(avg.) thermal efficiency which
is nearly half of diesel engines. However some newer engines are reported
to be much more efficient (thermal efficiency up to 38%) than previous
spark-ignition engines.