A major portion of the power developed by the expansion of the gas in the cylinders is used for overcoming friction between piston and cylinder and in the bearings of the connecting rod and crankshaft and for driving the water-circulation pump, oil pump, dynamo, camshaft and valves (Fig.1). Hence only a certain proportion of the theoretical power output is available as effective output. This proportion is termed the mechanical efficiency of the engine. Depending on the type and design of the engine and on its state of maintenance, the mechanical efficiency is usually between 0.75 and 0.85.

More than half the loss of power is due to friction of the pistons and bearings. The piston friction depends on the pressure developed in the cylinder and on the piston speed, which is determined by the stroke and speed of rotation. Generally speaking, the rotational speed should be as high as possible. Therefore the only possible means of reducing the friction is to shorten the piston stroke. The friction developed at the piston rings depends on the number of rings per piston. To reduce the loss of gas, it is necessary always to have two compression rings; in addition, each piston has an oil-scraper ring (Fig.2).

Friction in the crankshaft bearings can be reduced by the use of lighter connecting rods. This also reduces the lubricant requirement of the bearings, so that the oil-pump power input is lessened. A crankshaft rotating at high speed causes frictional losses due to turbulence and foaming of oil in the sump. For this reason high-speed engines have dry-sump lubrication (Fig.3). In this system, oil entering the crankcase is immediately extracted by suction and is returned through a filter and a cooler to the oil tank. A second pump delivers the oil from the tank to the bearings.

For E.g. A water cooled engine is usually equipped with a fan. The fan is necessary only when the cooling-water temperature is high. For a substantial proportion of the engine’s running time the fan is absorbing power without performing any useful function. For this reason fans have been developed that are switched on and off automatically, controlled by the temperature of the cooling water or air.

The factor which has the greatest effect on mechanical efficiency is friction within the engine. The friction between moving parts in an engine remains practically constant throughout the engine’s speed range. Therefore, the mechanical efficiency of an engine will be highest when the engine is running at the speed at which maximum bhp is developed. Since power output is bhp, and the maximum horsepower available is ihp.

When an engine is operating under part load, it has a lower mechanical efficiency than when operating at full load. The explanation for this is that most mechanical losses are almost independent of the load, and therefore, when load decreases, ihp decreases relatively less than bhp. Mechanical efficiency becomes zero when an engine operates at no load because then bhp = 0, but ihp is not zero. In fact, if bhp is zero and the expression for fhp is used, ihp is equal to fhp.