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.
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
5 Volumetric Efficiency V
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%.
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
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)
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)
11 Engine friction Three types of friction- bearing surfaces in automobile engines: Journal Guide Thrust
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.
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
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.
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.
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
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
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.
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).
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)
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.
22 Thermal Efficiency (Cont.)
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 kJ/kg) ρ is the relative density (kg/L) of the fuel.
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))
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.
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 = bhp, BHP = PS kW = 1.36 PS, PS = kW kW = bhp, BHP = kW
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