This is the rating that shows how much of the power developed by the expansion of the gases in the cylinder is actually delivered as useful power. 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, then
During the transmission of ihp through the piston and connecting rod to the crankshaft, the mechanical losses which occur may be due to friction, or they may be due to power absorbed. Friction losses occur because of friction in the various bearings, between piston and piston rings, and between piston rings and the cylinder walls. Power is absorbed by valve and injection mechanisms, and by various auxiliaries, such as the lubricating oil and water circulating pumps and the scavenge and supercharge blowers. As a result, the power delivered to the crankshaft and available for doing useful work (bhp) is less than indicated power.
The mechanical losses which affect the efficiency of an engine may be called frictional horsepower (fhp) or the difference between ihp and bhp. The fhp of the engine used in
the preceding examples, then, would be 1343 (ihp) – 900 (bhp) = 443 fhp, or 33% of
the ihp developed in the cylinders. Then, using the expression for mechanical efficiency, the percentage of power available at the shaft is computed as follows:
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 indepen-dent 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. To show how mechanical efficiency is lower at part load, assume the engine used in preceding examples is operating at three-fourths load. Brake horsepower at three-fourths load is 900 × 0.75 or 675. Assuming that fhp does not change with load, fhp = 443. The ihp is, by expression, the sum of bhp and fhp.
ihp = 675 + 443 = 1118
Mechanical efficiency = 675/1118 = 0.60, or 60%; this is appreciably lower than the 67% indicated for the engine at full load.
Bmep is a useful concept when dealing with mechanical efficiency. Bmep can be obtained if the standard expression for computing horsepower (ihp) is applied to bhp instead of ihp and the mean pressure (p) is designated as bmep.
bmep = imep × mechanical efficiency
To illustrate this, the bmep for the engine in preceding examples at full load and three-fourths load is computed as follows:
Bmep gives an indication of the load an engine carries, and what the output is for piston displace-ment.
As the bmep for an engine increases, the engine develops greater horsepower per pound of weight. For a given engine, bmep changes in direct proportion with the load.