#### Transcript معاملات المحرك Engine Parameters

Engine Parameters www.thecartech.com 1 VC Combustion Chamber Gasket TDC Piston VS Stroke Cylinder BDC Connecting Rod Bore Crank Radius Crank Shaft Stroke Crank Radius (crank throw) www.thecartech.com 2 Compression ratio (r) Volume above piston at BDC r Volume above piston at TDC VC VS r VC VS r 1 VC • VC = Clearance volume • VS = Swept volume = /4 D2 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 3 Engine Displacement, Swept Volume or Engine Capacity (Ve): TDC Stroke VS VS VS VS BDC Bore • Ve = VS n • Ve = (/4) D2 L n Where: Ve = engine capacity, Vs = cylinder swept volume n = number of cylinders, L = stroke, D = bore diameter www.thecartech.com 4 Volumetric Efficiency V Air Entering the Engine ηV Engine Displacement www.thecartech.com 5 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 6 Indicated mean effective pressure (imep) Factors affecting imep: • • • • • • www.thecartech.com Compression ratio Air/fuel ratio Volumetric efficiency Ignition timing Valve timing and lift Air pressure and temperature 7 Factors affecting (imep) - Retarded ignition - Compression ratio www.thecartech.com - Weak mixture - Super charged 8 Pressure, Force, Work & Power a F (N) A (m2) c p = imep (N/m2) L (m) b F= P.A (N) Work (W) = F.L (N m) Time (t) = 60 / (Ne /k) (s) Indicated power (Pi) cylinder = W/t = F.L .Ne/(k*60) (W) (Pi) cylinder = (imep.A.L.Ne) / (k . 60) k = 2 (four stroke) k = 1 (two stoke) (Pi) engine = imep. (A.L.n) Ne / (k . 60) www.thecartech.com (Pi) engine = [imep. Ve . Ne/ (k . 60)] (W) 9 Engine Indicated Power (Pi) Engine power factors: • Engine capacity (Ve) • Engine Speed (rpm) (Ne) • Number of strokes “k” k=2, four stroke engine k=1, two stoke engine • (imep): volumetric efficiency, compression ratio, ignition quality, mixture strength, temperature … www.thecartech.com Pi = imep.Ve.Ne / (60. k) 10 Engine friction Three types of frictionbearing surfaces in automobile engines: • Journal • Guide • Thrust www.thecartech.com 11 Engine Brake Power (Pb) -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 12 Engine Mechanical Efficiency m • Pb = Pi - Pf Where: Pi = indicated power Pb= brake power Pf = friction power • m = Pb / Pi www.thecartech.com 13 Engine Brake Power (Pb) • Pb = Pi m • Pb = (imep Ve Ne / 60 k) m • Pb = (imep m)Ve Ne / 60 k • Pb = bemp Ve Ne / 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 14 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 15 Engine Torque Te 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 Ne/60 www.thecartech.com 16 Engine Torque Te (Cont.) • Pb = T ω = Te (2 Ne/60) = Te Ne / 9550 (kW) • bmep . Ve . Ne / k 60 = Te (2 Ne/60) • Te = bmep . Ve / 2 . K Where: Pe = Engine power (kW) Ne = Engine speed (rpm) Te = Engine torque (N m) bemp = brake mean effective pressure (Pa) Ve = engine capacity (m3) k = 2, for 4-stroke engines 1, for 2-stroke engines www.thecartech.com 17 Engine Torque Te (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 18 Engine Fuel consumption (FC) The amount of fuel an engine consumes can be measured by: • volume (cm3 or liter) per (sec. or mint, or hr) or • mass (kg) per (sec, or mint, or hr). www.thecartech.com 19 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. m SFC • (kg/h)/kW or kg/(kW h) . Pb www.thecartech.com 20 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 21 Thermal Efficiency (Cont.) www.thecartech.com 22 Thermal Efficiency (th) (Cont.) brake thermal efficiency (η th ) Pb . 60 .60 brake thermal efficiency (η th ) 3600 Pb . m . CV . V . ρ . CV where: . m is the fuel consumption (kg/h) V 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 23 Specific Fuel Consumption (SFC) & Thermal efficiency (th) 3600 Pb 3600 3600 η th . . m . CV (m /Pb ) . CV SFC. CV Where: th = thermal efficiency . m = fuel consumption (kg/h) Pb = brake power (kW) CV = calorific value (kJ) SFC = specific fuel consumption (kg/(kW.h)) www.thecartech.com 24 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 25 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 26 Engine Performance Curves 1. 2. 3. 4. 5. Imep Bemp and torque Indicated power Brake power Indicated thermal efficiency 6. Brake thermal efficiency 7. Specific fuel consumption www.thecartech.com 27