Inside It: Reciprocating Engines

(Sources: Automotive Mechanics by Crouse and Anglin, Thermodynamics by Cengel and Boles.)

Air Standard Assumptions

Actual gas power cycles are complex. To reduce analysis we follow the air standard assumptions, which are:

• Working fluid is air, it continuously circulates in a closed loop and behaves like an ideal gas.
• All processes in the cycle are internally reversible.
• Combustion is replaced by heat addition from an external source.
• Exhaust is replaced by heat rejection to an external source.

Overview of Reciprocating Engines and Some Important Definitions

A reciprocating engine is basically a piston-cylinder device. It is quite verstile and has a wide range of applications. Eg. Automobiles, trucks, ships, light aircrafts, electric power generators etc.

The figure shows the basic components of a reciprocating engine.

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The piston reciprocates in the cylinder between two points called TDC and BDC

TDC or Top Dead Center is the position of piston when it forms the smallest volume in the piston
BDC or Bottom Dead Center is the position of the piston when it forms the largest volume in the cylinder.

Distance moved by the piston from TDC to BDC or vice versa is called a Stroke.
Bore is the diameter of the piston.

The whole arrangement contains two valves.
Intake Valve - through this the air-fuel mixture enters the cylinder.
Exhaust Valve - combustion products are expelled through it.

Clearance Volume - The minimum volume formed in cylinder when piston is at TDC.
Displacement Volume - Volume displaced by piston as it moves between TDC and BDC.
Compression Ratio - rotio of maximum volume formed in the cylinder to minimum(clearance) volume.

Mean Effective Pressure - a fictitious pressure that, if acted on a cylinder during the entire power stroke, would produce the same amount of work as produced during the actual cycle.