www.thecartech.com 3 Compression ratio (r) V C = Clearance volume V S = Swept volume = /4 D 2 L where: L (stroke) = 2 ρ, ρ is the crankshaft radius - Increasing the compression ration increases the thermal efficiency, compression is limited by the knock limit.
www.thecartech.com 4 Engine Displacement, Swept Volume or Engine Capacity (Ve): Ve = V S n Ve = ( /4) D2 L n Where: V e = engine capacity, V s = cylinder swept volume n = number of cylinders, L = stroke, D = bore diameter Stroke VSVS Bore VSVS VSVS VSVS TDC BDC
www.thecartech.com 6 Volumetric Efficiency V (cont.) Engines are only capable of 80% to 90% volumetric efficiency. Volumetric efficiency depends upon throttle opening and engine speed as well as induction and exhaust system layout, port size and valve timing and opening duration. High volumetric efficiency increases engine power. Turbo charging is capable of increasing volumetric efficiency up to 50%.
www.thecartech.com 7 Indicated mean effective pressure (imep) Factors affecting imep: Compression ratio Air/fuel ratio Volumetric efficiency Ignition timing Valve timing and lift Air pressure and temperature
www.thecartech.com 9 Pressure, Force, Work & Power p = imep (N/m 2 ) A (m 2 ) F= P.A (N) L (m) F (N) Work (W) = F.L (N m) Time (t) = 60 / (N e /k) (s) Indicated power (P i ) cylinder = W/t = F.L.N e /(k*60) (W) (P i ) cylinder = (imep.A.L.N e ) / (k. 60) (P i ) engine = imep. (A.L.n) N e / (k. 60) (P i ) engine = [imep. V e. N e / (k. 60)] (W) a b c k = 2 (four stroke) k = 1 (two stoke)
www.thecartech.com 10 Engine Indicated Power (P i ) Engine power factors: Engine capacity (V e ) Engine Speed (rpm) (N e ) Number of strokes “k” k=2, four stroke engine k=1, two stoke engine (imep): volumetric efficiency, compression ratio, ignition quality, mixture strength, temperature … P i = imep.V e.N e / (60. k)
www.thecartech.com 11 Engine friction Three types of friction- bearing surfaces in automobile engines: Journal Guide Thrust
www.thecartech.com 12 Engine Brake Power (P b ) -This is the power developed at the crankshaft or flywheel. -The term brake originated from the method used to determine an engine’s power output by measuring the torque using some form of friction dynamometer.
www.thecartech.com 13 Engine Mechanical Efficiency m P b = P i - P f Where: P i = indicated power P b = brake power P f = friction power m = P b / P i
www.thecartech.com 14 Engine Brake Power (P b ) P b = P i m P b = (imep V e N e / 60 k) m P b = (imep m )V e N e / 60 k P b = bemp V e N e / 60 k Where: bmep = brake mean effective pressure bmep = imep m * bmep is indication of engine efficiency regardless of capacity or engine speed, 1000 kPa represent high efficiency.
www.thecartech.com 15 Gross & Net Brake Power Gross brake power is measured without the following items: Cooling fan, coolant pump, radiator, alternator, exhaust system. (SAE) Net brake power is measured with all the above items. (DIN) Gross power is 10-15% more than net power.
www.thecartech.com 16 Engine Torque T e Torque and crankshaft angle: Work is also accomplished when the torque is applied through an angle. Distance xy = rθ W = F. xy = F r θ = T θ W per one revolution = T (2 ) P = W/t = T (2 )/t = Tω/1000 Where: ω = 2 N e /60
www.thecartech.com 17 Engine Torque T e (Cont.) P b = T ω = T e (2 N e /60) = T e N e / 9550 (kW) bmep. V e. N e / k 60 = T e (2 N e /60) T e = bmep. V e / 2 . K Where: P e = Engine power (kW) N e = Engine speed (rpm) T e = Engine torque (N m) bemp = brake mean effective pressure (Pa) V e = engine capacity (m 3 ) k = 2, for 4-stroke engines 1, for 2-stroke engines
www.thecartech.com 18 Engine Torque T e (Cont.) - There is a direct relationship between BMEP and torque output. - The torque curve with engine rpm is identical to the bmep curve, with different values.
www.thecartech.com 19 Engine Fuel consumption (FC) The amount of fuel an engine consumes can be measured by: volume (cm 3 or liter) per (sec. or mint, or hr) or mass (kg) per (sec, or mint, or hr).
www.thecartech.com 20 Engine Specific Fuel Consumption (SFC) Specific fuel consumption represents the mass or volume of fuel an engine consumes per hour while it produces 1 kW of power. Typical gasoline engines will have an SFC of about 0.3 kg/(kW.h). SFC is an indication of the engine’s thermal or heat efficiency. (kg/h)/kW or kg/(kW h)
www.thecartech.com 21 Engine Thermal Efficiency ( th ) The efficiency of an engine in converting the heat energy contained in the liquid fuel into mechanical energy is termed its thermal efficiency. The petrol engine is particularly inefficient and at its best may reach 25% efficiency. The thermal efficiency of a diesel engine can reach 35% due to its higher compression ratio.
www.thecartech.com 23 Thermal Efficiency ( th ) (Cont.) where: is the fuel consumption (kg/h) is the fuel consumption (L/h) CV is the calorific or heat value of 1 kg of the fuel (kJ/kg or MJ/kg). (CV for gasoline is 40000 kJ/kg) ρ is the relative density (kg/L) of the fuel.
www.thecartech.com 24 Specific Fuel Consumption (SFC) & Thermal efficiency ( th ) Where: th = thermal efficiency = fuel consumption (kg/h) P b = brake power (kW) CV = calorific value (kJ) SFC = specific fuel consumption (kg/(kW.h))
www.thecartech.com 25 Specific Fuel Consumption (SFC) & Thermal efficiency ( th ) A mirror reflection of the SFC curve shows the shape of the engine’s thermal efficiency curve. The lowest point on the SFC curve becomes the highest point on the thermal efficiency curve.
www.thecartech.com 26 Power Units BHP (bhp) = 550 ft lb/s PS = 75 kg m/s kW = 1000 (N m/s) BHP = British and American “horse power” PS ="PferdeStärke“ is "horse power“ in German PS = 0.986 bhp, BHP = 1.0142 PS kW = 1.36 PS, PS = 0.73529 kW kW = 1.341 bhp, BHP = 0.7457 kW
www.thecartech.com 27 Engine Performance Curves 1.Imep 2.Bemp and torque 3.Indicated power 4.Brake power 5.Indicated thermal efficiency 6.Brake thermal efficiency 7.Specific fuel consumption