Four stroke diesel engine working principle, merits and demerits

4- STROKE DIESEL ENGINE

Diesel Engine: Operates on theoretical diesel cycle.

•        Four stroke: The cycle completes in two revolution of the crankshaft or 4 strokes of piston

•        Constant pressure combustion cycle: combustion of the fuel takes place at constant pressure

•        Compression ignition (C.I. engines): Fuel ignites due to high temperature of compressed air

Suction stroke: A TO B

•        Inlet valve: open , exhaust valve: close

•        Piston is at TDC and start to move downwards.

•        The inlet valve is open and exhaust valve is closed.

•        Suction created by the downward motion of the piston draws in the air from the atmosphere

•        Suction stroke end when the crank completes half a revolution and piston reaches BDC

•        Inlet valve closes down at the end.

•        Line A to B on PV Diagram

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Compression stroke: B TO C

•        Inlet and exhaust valves: remain closed.

•        The piston moving up BDC to TDC compresses the air adiabatically

•        Pressure and temperature rises to as high as 4MPa and 1000 °C respectively.

•        Line B-C: reversible Adiabatic Compression Process

•        Just before the end of the stroke: fuel injector sprays a fine spray of fuel into the hot compressed air

•        Crank completes one revolution.

Working /Power stroke

•        Inlet and exhaust valve: remains closed

•        Expansion occur in two stages:

•        I^st stage (C-D): Hot gases (combustion) expand at constant pressure

•        II^nd stage (D-E): Hot gases expand in reversible adiabatic way

•        Exerts force on the piston and pushes it from TDC to BDC as gases expand and pressure decreases

•        At the end of the stroke: exhaust valve opens and hot gases exit out due to high pressure inside compared to the atmosphere pressure, Process E-B

•        The crank completes one and half revolutions.

Exhaust stroke

•        Inlet valve: close, exhaust valve: open

•        The piston returns from BDC to TDC: pushes the remaining burnt gases to the atmosphere at constant, Line B-A on PV Diagram.

•        At the end of the stroke, exhaust valve closes and small quantities of gas gets trapped in the clearance volume which mix with the fresh air entering the cylinder in the next cycle.

•        The crank completes two revolutions.

Comparison of Petrol Vs Diesel Engines: