Brake HorsepowerBrake horsepower (bhp), sometimes calledshaft horsepower, is the amount of poweravailable for useful work. Bhp is less than ihpbecause of the various power losses which occurduring engine operation.To determine the brake or shaft horsepowerthat is delivered as useful work by an engine, thesum total of all mechanical losses must bededucted from the total ihp.CYLINDER PERFORMANCELIMITATIONSThe factors which limit the power that a givencylinder can develop are the piston speed and themep. The piston speed, as stated before, is limitedby the inertia forces set up by the moving partsand by frictional heat. In the case of the mep, thelimiting factors are as follows:1. Heat losses and efficiency of combustion.2. Volumetric efficiency (the amount of aircharged into the cylinder and the degree ofscavenging).3. Mixing of the fuel and air.The limiting meps, both bmep and imep, areprescribed by the manufacturer or NAVSEA.They should never be exceeded. In a direct-driveship, the meps developed are determined by therpm of the power shaft. In electric-drive ships,the horsepower and bmep are determined by acomputation based on readings from electrical in-struments and from generator efficiency.CYLINDER LOAD BALANCEIn order to ensure a balanced, smooth-operating engine, the general mechanical condi-tion of the engine must be properly maintainedso that the power output of the individualcylinders is within the prescribed limits at all loadsand speeds. In order to have a balanced load onthe engine, each cylinder must produce its shareof the total power developed. If the engine isdeveloping its rated full power, or nearly so, andone cylinder or more is producing less than itsshare, the remainder of the cylinders will becomeoverloaded.Using the rated speed and bhp, it is possibleto determine for each INDIVIDUAL CYLINDERa rated bmep which may not be exceeded withoutoverloading the cylinder. If the ENGINE rpmdrops below the rated speed, then the cylinderbmep generally drops to a lower value. The bmepshould never exceed the normal mep at lowerengine speed. Usually, it should be somewhatlower if the engine speed is decreased.Some engine manufacturers design the fuelsystems so that it is impossible to exceed the ratedbmep. This is done by installing a positive stopto limit the maximum throttle or fuel control. Thispositive stop regulates the maximum amount offuel that can enter the cylinder and limits themaximum load of the cylinder.In order to meet emergency situations, enginesused by the Navy are generally rated lower thanthose designed for industrial use. The economicalspeed for most of the Navy’s diesel engines isapproximately 90% of the rated speed. For suchspeed, the best load conditions have been foundto be from 70% to 80% of the rated load or out-put. When an engine is operated at an 80-90 com-bination (80% of rated load at 90% rated speed)the parts last longer and the engine remainscleaner and in better operating condition.Diesel engines do not operate well at ex-ceedingly low bmep such as that occurring atidling speeds. You are well aware that idling anengine tends to gum up parts associated with thecombustion spaces. Operating an engine at idlingspeeds for long periods will result in thenecessity for cleaning and overhauling the enginemuch sooner than when operating at 50 to 100%of load.Symptoms of UnbalanceEvidence of an unbalanced condition betweenthe cylinders of an engine may be indicated bythe following symptoms:1. Black exhaust smoke. When this occurs,it is not always possible to determine immediatelywhether the entire engine or just one of thecylinders is overloaded. To determine whichcylinder is overloaded, you must open theENGINEMAN 1 & C5-4
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