Compression ratio influences the thermalefficiency of an engine. Theoretically, the ther-mal efficiency increases as the compression ratiois increased. The minimum value of a diesel enginecompression ratio is determined by the compres-sion required for starting; and this compressionis, to a large extent, dependent on the type of fuelused. The maximum value of the compressionratio is not limited by the fuel used, but is limitedby the strength of the engine parts and theallowable engine weight per bhp output.Mechanical EfficiencyThis is the rating that shows how much of thepower developed by the expansion of the gasesin the cylinder is actually delivered as usefulpower. The factor which has the greatest effecton mechanical efficiency is friction within theengine. The friction between moving parts in anengine remains practically constant throughoutthe engine’s speed range. Therefore, themechanical efficiency of an engine will be highestwhen the engine is running at the speed at whichmaximum bhp is developed. Since power outputis bhp, and the maximum horsepower availableis ihp, thenbhpMechanical efficiency = ihp × 100During the transmission of ihp through thepiston and connecting rod to the crankshaft, themechanical losses which occur may be due to fric-tion, or they may be due to power absorbed. Fric-tion losses occur because of friction in the variousbearings, between piston and piston rings, andbetween piston rings and the cylinder walls. Poweris absorbed by valve and injection mechanisms,and by various auxiliaries, such as the lubricatingoil and water circulating pumps and the scavengeand supercharge blowers. As a result, the powerdelivered to the crankshaft and available fordoing useful work (bhp) is less than indicatedpower.The mechanical losses which affect the effici-ency of an engine may be called frictionalhorsepower (fhp) or the difference between ihpand bhp. The fhp of the engine used inthe preceding examples, then, would be1343 (ihp) – 900 (bhp) = 443 fhp, or 33% ofthe ihp developed in the cylinders. Then, usingthe expression for mechanical efficiency, thepercentage of power available at the shaft is com-puted as follows:900Mechanical efficiency = 1343 = 0.67, or 67%When an engine is operating under part load,it has a lower mechanical efficiency than whenoperating at full load. The explanation for thisis that most mechanical losses are almost indepen-dent of the load, and therefore, when loaddecreases, ihp decreases relatively less than bhp.Mechanical efficiency becomes zero when anengine operates at no load because then bhp = 0,but ihp is not zero. In fact, if bhp is zero and theexpression for fhp is used, ihp is equal to fhp.To show how mechanical efficiency is lowerat part load, assume the engine used in precedingexamples is operating at three-fourths load. Brakehorsepower at three-fourths load is 900 × 0.75or 675. Assuming that fhp does not change withload, fhp = 443. The ihp is, by expression, thesum of bhp and fhp.ihp = 675 + 443 = 1118Mechanical efficiency = 675/1118 = 0.60, or60%; this is appreciably lower than the 67%indicated for the engine at full load.Bmep is a useful concept when dealing withmechanical efficiency. Bmep can be obtained ifthe standard expression for computinghorsepower (ihp) is applied to bhp instead of ihpand the mean pressure (p) is designated as bmep.bhp =(bmep) × L × A × N33,000or33,000 × bhpbmep = L × A × NFrom the relations between bmep, bhp, ihp,and mechanical efficiency, by designatingENGINEMAN 1 & C5-8
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