EFFECT OF ENGINE CONDITIONS ON INDICATOR DIAGRAMS

Using the power (indicator) diagram, the following conditions affect engine performance:

• The turbine receives less exhaust gas flow/ energy, so the turbocharger speed and the scavenge/ charge air pressure fall.
• Effect on the diagram: The compression line is lower (lower compression pressure) because the charge air pressure is lower. The maximum (firing) pressure is reduced. The power output falls.
• Reason: Fouling of the turbine (gas side) reduces the gas flow through the turbine, reducing the turbocharger speed and the boost pressure, so less air is delivered to the cylinders, lowering the compression and firing pressures.

• The air filter restricts the air flow to the compressor, reducing the charge air delivery and the scavenge/ boost pressure.
• Effect on the diagram: The compression line is lower (lower compression pressure) and the maximum pressure is reduced; the power output falls.
• Reason: A fouled air filter increases the pressure drop on the compressor suction, reducing the air delivered to the engine, so the compression and firing pressures fall.

• Worn rings/ liners allow blow-by (gas leakage past the rings), reducing the compression and the effective combustion.
• Effect on the diagram: The compression line is lower (lower compression pressure) and the expansion line is affected (blow-by reduces the pressure during expansion); the maximum pressure is reduced and the power output falls.
• Reason: Blow-by past the worn rings/ liners allows gas to leak from the cylinder during compression and expansion, reducing the pressures and the work done.

In each case, the normal (healthy) diagram is deformed by a reduction in the compression and firing pressures and a loss of power, due to the reduced air supply (a, b) or the gas leakage (c).