Comparison Between Four- and Two-strokes Engine
| Four-stroke engines | Two-strokes engines |
|---|---|
| The thermodynamic cycle is completed in four strokes of the piston and two revolutions of the crankshaft. Thus, one power stroke is obtained in two revolutions of crankshaft.Turning moment is not so uniform during all the four strokes and hence heavier flywheel is required.The power produced for same size engine is less than two strokes engine due to one power stroke in two revolutions of crankshaft. Or for same power output engine required is heavier and bulkier.Lesser cooling and lubrication is required due to one power stroke in two revolutions and hence less wear and tear occurs.It consists of valves and valve actuating mechanism such as cam, camshaft, rocker arm, spring, valve and valve sheet.It has higher volumetric efficiency as the time available for induction of charge is more.It has higher thermal efficiency due to complete combustion of the fuel. | The thermodynamic cycle is completed in two strokes of the piston and one revolution of the crankshaft. Thus, one power stroke is obtained in one revolution of crankshaft.Comparatively, turning moment is more uniform and hence lighter flywheel can be employed.The power produced for same size engine is more than the four stroke engine due to one power stroke in each revolution of crankshaft.Larger cooling and lubrication is required due to one power stroke in each revolution and hence more wear and tear occurs.It has ports in place of valves.Volumetric efficiency is lower due to lesser time available for induction.It has lower thermal efficiency due to partial wastage of fuel through the exhaust port and incomplete combustion. |
Comparison Between SI and CI Engines
Table 6.2a Comparison Between SI and CI Engines
| SI engines | CI engines |
|---|---|
| It is based on Otto cycle or constant volume heat addition and rejection cycle.A high volatile, high self-ignition temperature fuel, gasoline is used.A gaseous mixture of fuel and air is inducted during suction stroke. A carburettor is necessary to provide the mixture.Throttle controls the quantity of fuel–air mixture introduced.For combustion of the charge, it requires ignition system with stark plug in combustion chamber.Compression ratio ranges from 6 to 10.Due to light weight and homogeneous combustion, they are high speed engines.It has lower thermal efficiency due to lower compression ratio but delivers more power for same compression ratio. | It is based on diesel cycle or constant pressure heat addition and constant volume heat rejection cycle.Comparatively low volatile and low self-ignition temperature fuel, diesel is used.Fuel is injected at high pressure at the end of compression stroke. A fuel pump and injector unit is used.The quantity of fuel is regulated in pump. Air quantity is not controlled. There is quality control.Auto ignition occurs due to high temperature of air resulting from high compression.Compression ratio ranges from 16 to 20.Due to heavy weight and heterogeneous combustion, they are comparatively low speed engines.It has higher thermal efficiency due to high compression ratio and delivers lesser power for the same compression ratio. |
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