Energy Conversion Chapter 3 Internal Combustion Engine Internal Combustion Engines Rotary Engine • Wankel engine Reciprocating Engine • Spark Ignition Engine (SIE): gasoline • Compression Ignition Engine (CIE): diesel • Dual-cycle engine Open-cycle Gas turbine Wankel Engine Wankel Engine Reciprocating Engine Reciprocating Engine Combustion processes: 1. Intake 2. Compression 3. Power 4. Exhaust Diagram of a cylinder as found in 4-stroke gasoline engines.: C – crankshaft. E – exhaust camshaft. I – inlet camshaft. P – piston. R – connecting rod. S – spark plug. V – valves. red: exhaust, blue: intake. W – cooling water jacket. Gray structure – engine block. Nomenclature for reciprocating engines TDC : top dead center BDC : bottom dead center Displacement and clearance volumes of a reciprocating engine Vmax VBDC Compression ratio r : r Vmin VTDC Energy Flow Energy Flow Energy Flow Indicated Horsepower ( ihp / Ni ) • The total horsepower actually developed on the piston in the engine (ICE & AP Edward F. Obert) Brake Horsepower ( bhp / Nb ) • The power from an engine (ICE & AP Edward F. Obert) Friction horsepower ( fhp / Nf ) • The power is spent in overcoming friction of the bearing, pistons, and other mechanical part of the engine and also in induction of the fuel-air charge and delivery of the exhaust gas. ( ICE & AP Edward F. Obert ) ihp = bhp + fhp Motor efficiency Theoretical Efficiencies : ηsiklus • Thermodynamic Efficiency is a function of the compression ratio and method of combustion. Since theoretical cycles are based on air as working substance , their efficiency is also called “Air-standard Efficiency”. • Air-standard Otto Cycle : ηsiklus = 1 - ( 1 / r k-1 ) • Air-standard Diesel Cycle : ηsiklus = 1- (1/r k-1)(rck-1)/(k(rc-1)) r = Compression ratio rc = Cutoff ratio Motor efficiency Combustion Efficiency : ηcomb • Combustion Efficiency designated the ratio of heat generated to the heat value of fuel . ηcomb = Qgenerated / HV Charge Efficiency : ηch • Is the ratio of the weight Wch of the actual fresh charge to the weight of the piston displacement Vs if it were filled with fresh charge having the outside pressure and temperature . ηch = Wch / Vs x ρa dimana : ρa = pa / R Ta Motor efficiency Volumetric Efficiency : ηv • Due to the short cycle time and flow restrictions less than ideal amount of air enters the cylinder. • Is the ratio of the volume of fresh charge taken in during the suction stroke to the full piston displacement . ηv = Vch / Vs Volumetric efficiency Volumetric efficiency (v) = (mass of air actually drawn into cylinder) / (mass of air that ideally could be drawn into cylinder) air (measured) m v airVd N / n where air is at ambient = Pambient/RTambient and R - 287 J/kgK for air Volumetric efficiency indicates how well the engine “breathes” what lowers v below 100%? • Pressure drops in intake system (e.g. throttling) & intake valves • Temperature rise due to heating of air as it flows through intake system • Volume occupied by fuel • Non-ideal valve timing • “Choking” (air flow reaching speed of sound) in part of intake system having smallest area (passing intake valves) Volumetric efficiency normally around 80% - supercharging and turbocharging Engine Geometry s(q ) a cosq l a sin q VC 2 1/ 2 Cylinder volume when piston at TC (s=l+a) defined as the clearance volume Vc BC The cylinder volume at any crank angle is: 2 L l 2 TC B s 2 B V (q ) Vc (l a s(q )) 4 Maximum displacement, or swept, volume: 2 q a B Vd L 4 Compression ratio: For most engines B ~ L (square engine) VBC Vc Vd rc VTC Vc Mean and Instantaneous Piston Speeds VC TC B s a cosq l a sin q 2 2 2 1/ 2 Average and instantaneous piston speeds are: L S p 2 LN BC l s Sp ds dt Where N is the rotational speed of the crank shaft in units revolutions per second cos q sin q 1 Sp 2 l / a 2 sin 2 q Sp q a 1/ 2 Average piston speed for a standard auto engine is ~15 m/s. Ultimately limited by material strength. Therefore engines with large strokes run at lower speeds those with small strokes can run at higher speeds. Engine Torque and Power Torque is measured using a dynamometer. b Stator Force F Rotor N Load cell The torque exerted by the engine is: T = F b with units: J The power P delivered by the engine turning at a speed N and absorbed by the dynamometer is: P = T = (2 N) T w/units: (rad/rev)(rev/s)(J) = Watt Note: w is the shaft angular velocity with units: rad/s Indicated Work Given the cylinder pressure data over the operating cycle of the engine one can calculate the work done by the gas on the piston. Wi pdV The indicated work per cycle is WA > 0 WB < 0 Compression W<0 Power W>0 Exhaust W<0 Intake W>0 Indicated Power Pi = Wi N / nR w/units: (kJ/cycle) (rev/s) / (rev/cycle) where N – crankshaft speed in rev/s nR – number of crank revolutions per cycle = 2 for 4-stroke = 1 for 2-stroke Power can be increased by increasing: • the engine size, Vd • compression ratio, rc • engine speed, N Mechanical Efficiency Some of the power generated in the cylinder is used to overcome engine friction. The friction power is used to describe these losses: Pf = Pi - Pb Friction power can be measured by motoring the engine. The mechanical efficiency is defined as: m = Pb / Pi = 1- (Pf / Pi ) Mechanical efficiency depends on throttle position, engine design, and engine speed. Typical values for car engines at WOT are 90% @2000 RPM and 75% @ max speed. Power and Torque versus Engine Speed Rated brake power 1 kW = 1.341 hp Max brake torque There is a maximum in the brake power versus engine speed called the rated brake power. At higher speeds brake power decreases as friction power becomes significant compared to the indicated power There is a maximum in the torque versus speed called maximum brake torque (MBT). Brake torque drops off: • at lower speeds do to heat losses • at higher speeds it becomes more difficult to ingest a full charge of air. Mean Effective Pressure (mep) The Mean Effective Pressure is the theoretical constant pressure that , if it acted on the piston during the power stroke, would produce the same net work as actually developed in one cycle. Mean Effective Pressure (mep) Indicated Mean Effective Pressure (IMEP) imep is a fictitious constant pressure that would produce the same work per cycle if it acted on the piston during the power stroke. imep = Wi / Vd = (Pi nR) / (Vd N) so Pi = imep Vd N / nR = imep Ap Up / (2 nR) imep does not depend on engine speed, just like torque. imep is a better parameter than torque to compare engines for design and output because it is independent of engine speed, N, and engine size, Vd. Mean Effective Pressure (mep) Brake Mean Effective Pressure (bmep): bmep is defined as that theoretical constant pressure which can be imagined exerted during each power stroke of the engine to produce power ( or work ) equal to the brake power ( or work ). Wb 2 T nR bmep Vd Vd bmep Vd T 2 nR Maximum BMEP Wb 2 T nR bmep Vd Vd • The maximum bmep is obtained at WOT at a particular engine speed • Closing the throttle decreases the bmep • For a given displacement, a higher maximum bmep means more torque • For a given torque, a higher maximum bmep means smaller engine • Higher maximum bmep means higher stresses and temperatures in the engine hence shorter engine life, or bulkier engine. • For the same bmep 2-strokes have almost twice the power of 4-stroke Specific Fuel Consumption • In engine testing the fuel consumption is measured in terms of the fuel mass flow rate. • The specific fuel consumption, sfc, is a measure of how efficiently the fuel supplied to the engine is used to produce power, m XSFC bsfc = mf / Pb f PX isfc = mf / Pi (w/units: g/kW-hr) • Clearly a low value for sfc is desirable since at a given power level less fuel will be consumed Brake Specific Fuel Consumption vs Size •BSFC decreases with engine size due to reduced heat losses from gas to cylinder wall. •Note: cylinder surface to volume ratio increases with bore cylinder surface area 2rL 1 diameter. cylinder volume r 2 L r Brake Specific Fuel Consumption vs Speed • There is a minimum in the bsfc versus engine speed curve • At high speeds the bsfc increases due to increased friction • At lower speeds the bsfc increases due to increased time for heat losses from the gas to the cylinder and piston wall •Bsfc increases with compression ratio due to higher thermal efficiency Performance Maps Performance map is used to display the bsfc over the engines full load and speed range. Using a dynamometer to measure the torque and fuel mass flow rate you can calculate: bmep = 2 T nR / Vd Pb = 2 N T . bmep@WOT bsfc = mf / Pb Constant bsfc contours from a two-liter four cylinder SI engine Combustion Efficiency • The time for combustion in the cylinder is very short so not all the fuel may be consumed or local temperatures may not support combustion • A small fraction of the fuel may not react and exits with the exhaust gas. The combustion efficiency is defined as actual heat input divided by theoretical heat input: . . c = Qin/ (mf QHV) = Qin / (mf QHV) Where Qin = heat added by combustion per cycle mf = mass of fuel added to cylinder per cycle QHV = heating value of the fuel (chemical energy per unit mass) Thermal Efficiency t = work per cycle / heat input per cycle t = W / Qin = W / (c mf QHV) or in terms of rates… t = power out/rate of heat input . . t = P/Qin = P/(c mf QHV) • Thermal efficiencies can be given in terms of brake or indicated values • Indicated thermal efficiencies are typically 50% to 60% and brake thermal efficiencies are usually about 30% Arbitrary Efficiency (aka fuel conversion efficiency) . f = Wb / (mf QHV) = Pb / (mf QHV) Note: hf is very similar to ht, the difference is that ht takes into account only the actual fuel combusted in the engine. . Recall that sfc = mf / Pb Thus f = 1 / (sfc QHV) Volumetric Efficiency • Due to the short cycle time and flow restrictions less than ideal amount of air enters the cylinder. • The effectiveness of an engine to induct air into the cylinders is measured by the volumetric efficiency which is the ratio of actual air inducted divided by the theoretical air inducted: . v = ma / (a Vd) = nR ma / (a Vd N) where ra is the density of air at atmospheric conditions Po, To for an ideal gas ra =Po / RaTo and Ra = 0.287 kJ/kg-K (at standard conditions ra= 1.181 kg/m3) • Typical values for WOT are in the range 75%-90%, and lower when the throttle is closed Air-Fuel Ratio • For combustion to take place, the proper ratio of air and fuel must be present in the cylinder. •The air-fuel ratio is defined as . . AF = ma / mf = ma / mf • The ideal AF is about 15:1, with homogenous combustion possible in the range of 6 to 19. • For a SI engine the AF is in the range of 12 to 18 depending on the operating conditions. • For a CI engine, where the mixture is highly nonhomogeneous and the AF is in the range of 18 to 70. Fuel-Air Equivalence Ratio Actual Fuel- Air Ratio : m fuel F A act m air,act Stoichiometric Fuel- Air Ratio : Fuel Air Equivalence Ratio: m fuel F A sto m air, sto F A act F A sto Fuel properties Fuel Heating value, QR (J/kg) f at stoichiometric (f=Fuel mass fraction in mixture (---)) Gasoline 43 x 106 0.0642 Methane 50 x 106 0.0550 Methanol 20 x 106 0.104 Ethanol 27 x 106 0.0915 Coal 34 x 106 0.0802 Paper 17 x 106 0.122 Fruit Loops 16 x 106 Probably about the same as paper Hydrogen 120 x 106 0.0283 U235 fission 82,000,000 x 106 1 Example Problem Calculating performance Other performance parameter Heating Value / Calorific Value kJ/kg.fuel ; kcal/kg.fuel ; Btu/lbm.fuel Higher Heating Value (HHV): The higher heating value (also known as gross calorific value or gross energy) of a fuel is defined as the amount of heat released by a specified quantity (initially at 25°C) once it is combusted and the products have returned to a temperature of 25°C, which takes into account the latent heat of vaporization of water in the combustion products. Lower Heating Value (LHV): The lower heating value (also known as net calorific value) of a fuel is defined as the amount of heat released by combusting a specified quantity (initially at 25°C) and returning the temperature of the combustion products to 150°C, which assumes the latent heat of vaporization of water in the reaction products is not recovered. Spark Ignition Engine Four-stroke engine Two-stroke engine Four-stroke SIE Two-stroke SIE Two-stroke SIE Theoretical Otto Cycle Actual Otto Cycle Combustion stages in SIE Flame Propagation in SI Engine After intake the fuel-air mixture is compressed and then ignited by a spark plug just before the piston reaches top center. The turbulent flame spreads away from the spark discharge location. Flow N = 1400 rpm Pi = 0.5 atm Flame Development Mass fraction burned Flame development angle Dqd – crank angle interval during which flame kernal develops after spark ignition. Rapid burning angle Dqb – crank angle required to burn most of mixture Overall burning angle - sum of flame development and rapid burning angles Mixture Burn Time vs Engine Speed How does the flame burn all the mixture in the cylinder at high engine speeds? The piston speed is directly proportional to the engine speed, up ~ N Recall the turbulent intensity increases with piston speed, ut = ½ up Recall the turbulent burning velocity is proportional to the turbulent intensity St ~ ut, so at higher engine speeds the turbulent flame velocity is also higher and as a result need less time to burn the entire mixture Combustion duration in crank angles (40-60 degrees) only increases a small amount with increasing engine speed. = 1.0 Pi =0.54 atm Spark 30o BTC Heat Losses During Burn During combustion the cylinder volume is very narrow. Heat loss to the piston and cylinder head is very important In order to reduce the heat loss want burn time to be small (high flame velocity) accomplished by either increasing a) laminar burning velocity, or b) turbulence intensity. Highest laminar burning velocity is achieved for slightly rich mixtures (for isooctane maximum Sl = 26.3 cm/s at 1.13) Optimum F/A Composition Maximum power is obtained for a F/A that is about 1.1 since this gives the highest burning velocity and thus minimum heat loss. Best fuel economy is obtained for a F/A that is less than 1.0 Spark Timing Spark timing relative to TC affects the pressure development and thus the imep and power of the engine. Want to ignite the gas before TC so as to center the combustion around TC. The overall burning angle is typically between 40 to 60o, depending on engine speed. Engine at WOT, constant engine speed and A/F WOT=Wide open throttle motored 56 Maximum Brake Torque Timing If start of combustion is too early work is done against piston and if too late then peak pressure is reduced. The optimum spark timing which gives the maximum brake torque, called MBT timing occurs when these two opposite factors cancel. Engine at WOT, constant engine speed and A/F MBT=maximum brake torque Effect of Engine Speed on Spark Timing Recall the overall burn angle (90% burn) increases with engine speed, to accommodated this you need a larger spark advance. WOT Brake Torque Fixed spark advance MBT 2600 rpm N Fuel-air Ratio AFR POWER ECONOMY CO/HC NOx LEAN LOW HIGH LOW HIGH RICH HIGH LOW HIGH LOW Performance curve SIE Variable Speed Test of Automotive SIE at Full Throttle (CR = 9) Power and mep SIE Efficiency and sfc SIE Example: Eff and sfc SIE Gross indicated, brake, and friction power (Pi, Pb, Pf), indicated, brake, and friction mep, indicated and brake SFC, and hm for 3.8dm3 six cylinder automotive SIE at wide-open throttle. Bore = 96.8 mm, stroke = 86 mm, rc = 8.6. Abnormal Combustion in SI Engine Knock is the term used to describe a pinging noise emitted from a SI engine undergoing abnormal combustion. The noise is generated by shock waves produced in the cylinder when unburned gas ahead of the flame auto-ignites. Knock cycle Exhaust valve Spark plug Normal cycle Intake valve Observation window for photography 69 Knock As the flame propagates away from the spark plug the pressure and temperature of the unburned gas increases. Under certain conditions the end-gas can autoignite and burn very rapidly producing a shock wave flame P,T end-gas time shock P,T time The end-gas autoignites after a certain induction time which is dictated by the chemical kinetics of the fuel-air mixture. If the flame burns all the fresh gas before autoignition in the end-gas can occur then knock is avoided. Knock is a potential problem when the burn time is long. Parameters Influencing Knock i) Compression ratio – at high compression ratios, even before spark ignition, the fuel-air mixture is compressed to a high pressure and temperature which promotes autoignition ii) Engine speed – At low engine speeds the flame velocity is slow and thus the burn time is long, this results in more time for autoignition - However at high engine speeds there is less heat loss so the unburned gas temperature is higher which promotes autoignition. Some engines show an increase in propensity to knock at high speeds while others don’t. iii) Spark timing – maximum compression from the piston advance occurs at TC, increasing the spark advance makes the end of combustion crank angle approach TC and thus get higher pressure and temperature in the unburned gas just before burnout. Assignment Oktane number Cetane number Cooperative Fuel Research (CFR) engine Fuel Knock Scale Octane number: a standard measure of a fuel’s ability to resist knock. The octane number determines whether or not a fuel will knock in a given engine under given operating conditions. The higher the octane number, the higher the resistance to knock: Normal heptane (n-C7H16) has an octane value of zero and isooctane (C8H18) has a value of 100. Blends of these two hydrocarbons define the knock resistance of intermediate octane numbers: e.g., a blend of 10% n-heptane and 90% isooctane has an octane number of 90. A fuel’s octane number is determined by measuring what blend of these two hydrocarbons matches the test fuel’s knock resistance. Cooperative Fuel Research (CFR) Engine Two methods have been developed to measure ON using a standardized single-cylinder engine developed under the auspices of the Cooperative Fuel Research Committee in 1931. The CFR engine is 4-stroke with 3.25” bore and 4.5” stroke, compression ratio can be varied from 3 to 30. Cooperative Fuel Research (CFR) Engine Two methods: RON and MON Note the motor octane number is always higher because it uses more severe operating conditions: higher inlet temperature and more spark advance. Octane Number Measurement Testing procedure: • Run the CFR engine on the test fuel at both research and motor conditions. • Slowly increase the compression ratio until a standard amount of knock occurs as measured by a magnetostriction knock detector. • At that compression ratio run the engines on blends of nhepatane and isooctane. • ON is the % by volume of octane in the blend that produces the stand. The antiknock index which is displayed at the fuel pump : RON MON Antiknock index 2 Fuel Additives Chemical additives are used to raise the octane number of gasoline.
