Friction

Introduction:
                Engine friction if defined as the difference between the indicated horse-power (power at piston top as produced by the combustion gases) and the brake horse-power (useful power) available at the output shaft, i.e.

                                                                                                f.p.= i.p.-b.p

TOTAL ENGINE FRICTION:

• Total engine friction, defined as the difference between ihp and bhp, includes the power required to drive the compressor or a scavenging pump and the power required drive engine auxiliaries such as oil pump, coolant pump and fan, etc.
• If the power to drive the compressor and auxiliaries is neglected, the total engine friction can be divided into five main components. These are:

                1. Crankcase mechanical friction.

                2. Blow by losses (compression-expansion pumping loss).

                3. Exhaust and inlet system throttling losses.

                4. Combustion chamber pumping loop losses.

                5. Piston mechanical friction.

Crankcase Mechanical Friction:  Crankcase mechanical friction can further be sub-divided into:

                (1) Bering friction,

                (2) Valve gear friction, and

                (3)Pump and miscellaneous friction.

Blow by Losses: Blow by is the phenomenon of leakage of combustion products past the piston and piston rings from the cylinder to the crankcase.

Exhaust and Inlet Throttling Loss: The standard practice for sizing the exhaust valve is to make them acertain percentage smaller than the inlet valves.

Piston Mechanical Friction: Piston mechanical friction can be sub-divided into:

                1. Viscous friction

                2. Non-viscous friction

                                a) Friction due to ring tension

                                b) Friction due to gas pressure forces behind the ring.

EFFECT OF ENGINE VARIABLES ON ENGINE FRICTION

• Effect of stroke/bore ration: The effect of stroke/bore ration on engine friction and economy is very small.

• Effect of cylinder size and number of cylinders: The friction and economy improves as a smaller number of larger cylinders are used. This is because the proportion between the working piston area and its friction producing area, i.e. circumference, is reduced.

• Effect of number of piston rings: The effect of number of piston ring is not very critical and this number is usually chosen on the basis of cost, size and other requirements rather than on the basis of their effect on friction.

• Effect of compression ratio: The friction mean effective pressure increases as the compression ratio is increased. But the mechanical efficiency either remains constant or improvers as the compression ratio is increased.

• Effect of engine speed: Engine friction increases rapidly as the speed increases. The best way to improve mechanical efficiency as the speed increases the number of cylinders.

Motoring Method:

In the motoring method, engine is driven with the help of an external motor. The power consumed by this motor, if corrected for mechanical and other losses of the motor, gives the fp of the engine.