Efficiency

EFFICIENCY – This is, for engineering purposes, expressed always by the ratio of the output, from a conversion of energy, to the input to the system. It is often used incorrectly, instead of the word effective.

Hence Efficiency =

Energy output
Energy input

Thus combustion Efficiency =

Heat Energy output
Chemical Energy input

Such a value can only be 100% if the combustion process is complete, i.e. perfect. Only steam and carbon dioxide can result from the complete burning of a pure hydrocarbon. Hence unburned hydrocarbons and carbon monoxide, in the exhaust emissions indicate that the combustion efficiency is not perfect.

Also an engine’s thermal efficiency =

Engine Power Output
Fuel Power Input

A diesel can develop up to 50% thermal efficiency. A spark ignition engine can develop up to about 35% thermal efficiency. Such values are drastically reduced when the power output is measured at the vehicle road wheels particularly at high road-wheel speeds (i.e. above 100km/hr).

Also Mechanical Efficiency =

Mechanical Power Output
Mechanical Power Input

For an engine, the output power is delivered by the crankshaft (in kw) whereas the input power is provided by the pistons (in kw).

Therefore Engine Mechanical Efficiency =

Crankshaft Power
Piston Power

Typical values are up to 90% for a well run-in engine with correct lubrication and operating temperatures, for both diesel and spark-ignition engines.

Transmissions systems, i.e. gearbox and final drive gears can produce up to 95% mechanical efficiencies.

All mechanical systems are affected by friction and hence poor lubrication, tight bearings and brakes (shoes pads) not running clear (drums or discs) can drastically lower the transmission efficiency and therefore waste energy instead of applying it usefully.