CHECKING MAXIMUM CYLINDER PRESSURES AND THEIR EFFECTS

Methods of checking maximum cylinder pressures:

• The maximum cylinder pressure (Pmax, the peak firing pressure) is measured using an engine indicator (a mechanical/ electronic pressure indicator) connected to the cylinder via the indicator cock. The indicator draws a pressure-volume (indicator) diagram, from which the maximum pressure is read.
• Electronic/ digital indicators and the engine's electronic management system (if fitted) also measure/ display the Pmax.
• The Pmax is measured at a set load and compared with the maker's specified value.

Effects of too high and too low maximum cylinder pressures:

Too high Pmax:

• Excessive mechanical loading on the piston, connecting rod, bearings and crankshaft, causing accelerated wear and possible failure.
• Increased thermal loading/ overheating of the piston, liner and head.
• Increased risk of detonation/ knocking.
• Higher fuel consumption and emissions.
• Possible damage to the running gear.

Too low Pmax:

• Reduced power output and efficiency.
• Poor combustion (incomplete combustion, higher fuel consumption, smoke).
• Lower exhaust temperature (may affect the turbocharger/ exhaust gas boiler).
• Possible misfiring/ uneven running.
• Indicates a problem (e.g. low compression, fuel/ injection fault, or low charge air).

How the maximum cylinder pressure can be increased or decreased:

• Increased: Advance the injection timing (inject earlier), increase the fuel quantity/ index, increase the charge air pressure (boost), or increase the compression ratio (e.g. by reducing the clearance volume).
• Decreased: Retard the injection timing (inject later), reduce the fuel quantity/ index, reduce the charge air pressure, or reduce the compression ratio.
• The Pmax is adjusted by setting the fuel pump timing/ VIT and the fuel index to the maker's specification.