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.