Mechanical Efficiency

We have seen that part of the gross indicated work per cycle or power is used to expel exhaust gases and induct fresh charge .

An additional portion is used to overcome the friction
of The bearings , Piston , and other mechanical
components of the engine , and to drive the engine
accessories . All of these power requirements are
grouped together and called friction power Pf .

                  P ig = Pb + Pf

Friction power is difficult to determind accurately.One
common approach for high speed engines is to drive
or motor the engine with a dynomometer ( i.e,operate
the engine without firing it ) and measure the power
which has to be supplied by the dynomometer to
overcome all these frictional losses.

Note : The engine speed , throttle setting,oil and
                 water temperatures,and ambient conditions are kept the same in the motored test as under firing
                 conditions .

The major sources of inaccuracy with this method test that gas pressure forces on the piston and rings are lower in the motored test thanwhen the engine is firing and that the oil temperatues on the cylinder wall are also lower under motoring conditions .

The ratio of the brake (useful) power delivered by the engine to the indicated power is called the mechanical efficiency

                                                                      Brake power        Pb
                             Mechanical efficiency = -------------------------- = ---------
                                                                      Gross power         Pig

Since the friction power included the power required to pump gas into and out of the engine ,mechanical efficiency depends on throttle position as well as engine speed and design .