Chapter 5—ENGINE PERFORMANCE AND EFFICIENCYThe ratio of length of stroke to cylinder boreUsing the factors which influence the engine’sis fixed in engine design; in most slow speedcapacity to develop power, the general orengines, the stroke is greater than the bore.standard formula for calculating ihp is as follows:Revolutions Per MinuteRevolutions per minute (rpm) is the speed atwhich the crankshaft rotates. Since the piston isconnected to the shaft, the rpm, along with thelength of the stroke, determine piston speed.Since, during each revolution, the piston com-pletes one up-stroke and one down-stroke, pistonspeed is obtained by multiplying the rpm by twicethe length of the stroke. This speed is usually ex-pressed in feet per minute (fpm). If the stroke is10.5 inches (or 10.5/12 of a foot), and the speedof rotation is 720 rpm, the piston speed is com-puted as follows:HORSEPOWER COMPUTATIONThe power developed by an engine dependsupon the type of engine as well as the speed ofthe engine. A cylinder of a single-acting, 4-strokecycle engine will produce one power stroke forevery two crankshaft revolutions, while a single-acting, 2-stroke cycle engine produces one powerstroke for each revolution.Indicated HorsepowerThe power developed within a cylinder can becalculated by measuring the imep and the enginespeed. (The rpm of the engine is converted to thenumber of power strokes per minute.) With thebore and stroke known (available in enginemanufacturers’ technical manuals), thehorsepower (hp) can be computed. This power iscalled indicated horsepower (ihp) because it is ob-tained from the pressure measured with an engineindicator. Power loss due to friction is not con-sidered in computing ihp.ihp =P × L × A × N33,000whereP = Mean indicated pressure, in psiL = Length of stroke, in feetA = Effective area of the piston, insquare inchesN = Number of power strokes per minute33,000 = Unit of power (one horsepower), orfootpounds per minute.To illustrate the use of this formula, assumethat a 12-cylinder, 2-stroke cycle, single-actingengine has a bore of 8.5 inches and a stroke of10 inches. Its rated speed is 744 rpm. With theengine running at full load and speed, the imepis measured and found to be 105 psi. What is theihp developed by the engine?In this caseSubstituting these amounts in the formula,you haveThis amount represents the horsepowerdeveloped in only one cylinder; since there are 12cylinders in this engine, total horsepower for theengine will equal 12 times 111.9, or approximately1343.5-3

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