Fundamentals of Piston Engine Operation

021-10-01Pablo Asensio Martínez2026-03-242 min

Basic Principles & Components

The reciprocating engine (piston engine) consists of:

  • Cylinder: Closed at one end (Cylinder Head) containing inlet/exhaust valves and a spark plug.
  • Piston: Slides up and down inside the cylinder.
  • Connecting Rod: Connects the piston to the crankshaft.
  • Crankshaft: Converts the reciprocating (linear) movement of the piston into rotary movement.
  • Camshaft: Operates the valves (driven at half crankshaft speed).

Piston vs. Gas Turbine

Both engines use a cycle of induction, compression, combustion, and exhaust.

  • Piston Engine:
    • Process is intermittent.
    • Combustion occurs at constant volume.
    • Reciprocating motion.
  • Gas Turbine (Jet) Engine:
    • Process is continuous.
    • Combustion occurs at constant pressure.
    • Entirely rotational motion (smoother operation).

Manifold Pressure (MAP)

  • Definition: Measures the absolute pressure in the intake manifold just downstream (behind) of the throttle valve.
  • Indication:
    • Engine Stopped: Indicates atmospheric pressure.
    • Engine Running (Idle/Cruise): Indicates lower than atmospheric (due to suction).
    • Gauge Leak: If the gauge reads atmospheric pressure while the engine is running, it indicates a leak in the pressure gauge line.
  • Significance: Provides a reliable indication of power being generated.

Engine Volumes and Ratios

  • Swept Volume (Displacement): The volume displaced by the piston moving from Bottom Dead Centre (BDC) to Top Dead Centre (TDC).
    • Calculation: $\text{Stroke Length} \times \text{Piston Crown Area}$.
    • Total Displacement = Swept Volume $\times$ Number of Cylinders.
  • Clearance Volume: The volume remaining above the piston when it is at TDC (smallest volume).
  • Total Volume: Volume above the piston at BDC (largest volume).
  • Compression Ratio: The ratio of Total Volume to Clearance Volume.

Power Definitions

  • Indicated Horsepower (IHP): The total power developed in the cylinder (theoretical potential). Depends on cylinder pressure, swept volume, and RPM.
  • Friction Horsepower (FHP): Power lost to friction and driving accessories.
  • Brake Horsepower (BHP): The actual power available at the crankshaft (output power).
    • $BHP = IHP - FHP$
    • $Power = Torque \times RPM$
  • Mechanical Efficiency: $(BHP / IHP) \times 100\%$.