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Figure 5 indicator diagramBrake powerThis is the power output of any engine measured at the engine's flywheel.The flywheel isa disc with teeth on it. It's connected to the rear of the engine and its purpose is for the smooth transfer of power to the transmission from the engine. Brake power is the measure of an engine's horsepower without the loss in power caused by the gearbox, generator, differential, water pump and other auxiliaries. The actual power delivered to the driving wheels is less. An engine would have to be retested to obtain a rating in another system. The term "brake" refers to the use of a band brake to measure 11
torque during the test (which is multiplied by the engine speed in revs/sec and the circumference of the band to give the power).The brake power is always less than the indicated power of an engine because of friction losses, the indicated power is the actual power developed in the engine’s cylinder.ip−fp=bpEquation 2: Indicated powerWhere ip = indicated powerfp = friction lossbp = break powerbrake power can also be expressed as:bp=(bmep)LAnK60×1000equation 3: brake power equationWhere: bp = brake power (Kw)Bmep = break mean effective pressure (kN/m2)L = length of stroke(m)A = cross-sectional area of piston (m2)n = number of power strokesk = number of cylindersbrake power obtained at output shaft can also be related as:bp=2π N T60×1000Where: bp = brake power (kW)12
N = crankshaft speed (rpm) T = engine torque (N-m)EfficiencyEfficiency is the relation between the power delivered and the power that could be obtained if the engine operates without loss of power. Several efficiency equations have been developed to determine how well engines avoid power losses. These efficiencies include:Brake thermal efficiencyVolumetric efficiencyMechanical efficiency.Brake thermal efficiencyThis is the ratio of brake power produced to the energy in the fuel burned to produce this power, it can be expressed as:ηbth=bpmfQfequation 4: brake thermal efficiencyWhere: bp=break powermf= fuel mass flow rateQf= calorific value of fuelVolumetric efficiencyThis is defined as the ratio of the mass density of the air-fuel mixture drawn into the cylinder at atmospheric pressure (during the intake stroke) to the mass density of the same volume of air in the intake manifold, it is expressed as:ηv=maρaVdequation 5: volumetric efficiencyWhere: ma= mass of air into the engine in one cycle (kg)ρa= air density at atmospheric conditions (kg/m3)Vd= displaced volume (m3) 13
Mechanical efficiencyThis is basically the ratio of the break power to indicated power, it is expressed as:ηm=bpipequation 6: mechanical efficiencyFuel InjectionFuel injection is the introduction of fuel in an internal combustion engine, most commonly automotive engines, by the means of an injector. All diesel engines use fuel injection by design. Petrol engines can use gasoline direct injection, where the fuel is directly delivered into the combustion chamber, or indirect injection where the fuel is mixed with air before