Spark Ignition Engine Combustion

Transcription

1 Spark Ignition Engine Combustion MAK 652E Introduction to Combustion Process in Engines Prof.Dr. Cem Soruşbay Istanbul Technical University - Automotive Laboratories Contents Course information Combustion process in engines Classification of engines Conventional engines Advance concepts in engine combustion 1

2 Information Prof.Dr. Cem Soruşbay İ.T.Ü. Makina Fakültesi Otomotiv Laboratuvarı Ayazağa Yerleşkesi, Maslak İstanbul Tel /sorusbay/si/si.htm Course Plan Introduction to engine combustion process Premixed combustion in engines Stratified charge engines, lean combustion, cyclic variations Combustion modelling in engines, thermodynamic models Multidimensional modelling of SI engines Chemical kinetics of HC combustion Autoignition in engines, knock modelling Exhaust emissions, kinetics of pollutant formation 2

3 Assessment Criteria Midterm examinations 2 x 15 = 30 % 16th March, th April, 2017 Project 20 % to be submitted by 11th May, 2017 Final examination 50 % References Soruşbay, C., Lecture Notes, İ.T.Ü., 2010 (Power Point presentation) Soruşbay, C. et al., İçten Yanmalı Motorlar, Birsen Yayınevi, İstanbul, Pulkrabek, W.W., Engineering Fundamentals of the Internal Combustion Engine, Prentice Hall, New Jersey, Mattavi, J.N. And Amann, C.A. (Eds.), Combustion Modelling in Reciprocating Engines, Plenum Press, New York, 1980 Ramos, J.I., Internal Combustion Engine Modelling, Hemisphere Publishing Corp. New York, Turns, S.R., An Introduction to Combustion - Concepts and Applications, Mc Graw - Hill, New York,

4 References Merker, G.P. et al., Simulating Combustion, Springer Verlag, Berlin, Heywood, J.B., Internal Combustion Engine Fundamentals, McGraw Hill Book Company, New York, Weaving, J.H., Internal Combustion Engineering : Science and Technology, Elsevier Applied Science, London, Stone, R., Introduction to Internal Combustion Engines, Macmillan, London, Arcoumanis, C and Kamimoto, T., Flow and Combustion in Reciprocating Engines, Springer Verlag, Berlin, Other references given in the list (see web page of the course) Combustion Process in Engines Internal Combustion Engines (IC-engines) produce mechanical power from the chemical energy contained in the fuel, as a result of the combustion process occuring inside the engine IC engine converts chemical energy of the fuel into mechanical energy, usually made available on a rotating output shaft. Chemical energy of the fuel is first converted to thermal energy by means of combustion or oxidation with air inside the engine, raising the T and p of the gases within the combustion chamber. The high-pressure gas then expands and by mechanical mechanisms rotates the crankshaft, which is the output of the engine. Crankshaft is connected to a transmission/power-train to transmit the rotating mechanical energy to drive a vehicle. 4

5 Combustion Process in Engines Most of the internal combustion engines are reciprocating engines with a piston that reciprocate back and forth in the cylinder. Combustion process takes place in the cylinder. There are also rotary engines In external combustion engines, the combustion process takes place outside the mechanical engine system Energy Conversion (Merker et.al, 2006) 5

6 Energy Conversion Classification of Engines Method of ignition Spark Ignition (SI) engines, ignition is by the application of external energy (to spark plug) mixture is uniform (conventional engines), mixture is non-uniform (stratified-charge engines) Compression Ignition (CI) engines, ignition by compression in conventional engine (Diesel engine), pilot injection of fuel in gas engines (eg, natural gas and diesel fuel > dual fuel engines) 6

7 GDI Engines History of Engines Huygens (1673) Hautefeuille (1676) Papin (1695) Modern engines Lenoir (1860) Rochas (1862) Otto Langen (1867) Otto (1876) Clark (1878) Diesel (1892) developed piston mechanism first concept of internal combustion engine first to use steam in piston mechaanism using same principles of operation as present engines previously no compression cycle driving the piston by the expansion of burning products - first practical engine, 0.5 hp, mech efficiency up to 5% four-stroke concept was proposed produced various engines, 11% efficiency Four-stroke engine prototype built, 8 hp Two-stroke engine was developed Single cylinder, compression ignition engine 7

8 History of Engines Otto Cycle Otto cycle (heat addition at constant-volume) p p 3 2 8

9 Diesel Cycle cut-off ratio (load ratio) V V 3 2 Diesel cycle (heat addition at constant-pressure) Comparison of Ideal Cyles Otto cycle th Otto 1 1 k 1 Diesel cycle th Diesel 1 1 k 1 k 1 k( 1) Dual cycle th Dual 1 1 k 1 k 1 1 k ( 1) 9

10 Comparison of Ideal Cycles For > 1 and k > 1 k 1 k( 1) term is greater than 1 therefore t-otto > t-dies for a constant value of compression ratio Also t-otto > t-dual > t-diesel efficiency of Dual cycle lies between Otto and Diesel cycles according to the value of Comparison of Engines In real engines, SI engines have a compression ratio between 10:1 to 12:1 this value is limited due to engine knock CI engines have compression ratio higher than 14:1 to provide temperature and pressure required for self ignition of the fuel compression ratio of 16:1 to 18:1 is sufficient for efficiency, but used for improving ignition quality high compression ratio increases thermal and mechanical stresses 10

11 Comparison of Engines SI Engines a) Full load b) Part load SI engine losses at full load (WOT) and part load (Merker et.al, 2006) 11

12 SI Engines a) Full load b) Part load SI Engines Engine map with lines of constant fuel consumption 12

13 Losses in Real Engine Losses in Real Engine 13

14 Equivalence ratio Conventional Diesel Engines Phases of combustion process in a Diesel engine, Ignition delay period Premixed combustion phase Mixing-controlled combustion phase Late combustion phase Conventional Diesel Engine Temperature Conventional Diesel engine combustion Temporal trajectories of equivalence ratio ( ) and temperature for injection timing of 25, 15 and 5 o CA BTDC 14

15 Soot Conventional Diesel Engine Vibe parameter (Mehdiyev, 2008) m = 0.65 CR injection m = 1.2 MR process m = 1.95 M engine NOx PM Trade-off 0.20 g/kwh 0.15 Euro II (10/95) Euro III (10/00) NO X -cat. part.-trap Low swirl combustion elevated injection pressure injection rate shaping partly EGR + cooled EGR part.-trap State of the art Euro V Euro IV NO x 15

16 NOx PM Trade-off Curve NOx vs. Soot Trade-Off 1,40 1,20 map=200 kpa AA9 map=200 kpa AA3 map=180 kpa AA9 map=180 kpa AA3 map=160 kpa AA9 map=160 kpa AA3 1,00 Soot [g/h] 0,80 0,60 0,40 0, NOx [g/h] NOx PM Trade-off PI MI Pol Pilot injection Main injection Post Injection 16

17 NOx PM Trade-off Curve (Sorusbay et al., Int J Vehicle Design, 2007) Split Injection Strategies MAIN INJECTION PRE INJECTION + MAIN INJECTION PRE INJECTION + SPLIT MAIN INJECTION 17

18 Operating Regions in Relation to NOx and PM Emissions of NOx and PM Operating Regions in Relation to NOx and PM Emissions of NOx and PM at various load conditions (Akihama, SAE ) 18

19 Limitations for SI Engines Compression and Combustion Friction Loss Heat Loss Incomplete oxidation Slow burning Knock limit Efficiency limited by CR Gas Exchange Process Heat Loss Pumping Losses Waste heat out Exhaust Inefficient valve timing at varying speeds (Source : Morey, 2011) Limitations for CI Engines Major limitations with Diesel engines today Controlling NOx and PM emissions Reducing fuel consumption, CO2 emissions Real-World exhaust emissions PEMS 19

20 Real-World Exhaust Emissions (Source : International Council on Clean Transportation, 2015) Advanced Combustion Concepts Compression Ignition (CI) engines have higher efficiency at part load operation, longer lifetime and relatively lower emissions of CO2, CO and unburned HC Spark Ignition (SI) engines have higher power density and lower combustion noise. In the history of engine design and development, there have been many attempts to combine the advantages of both CI and SI engines. 20

21 Gasoline Direct Injection Engines Conventional Spark Ignition Engines pre-mixed combustion homogeneous and stoichiometric mixture prepared in the manifold GDI Engines in-cylinder mixture formation stratified charge with a globally lean mixture Spray guided GDI Engine Gasoline Direct Injection Engines NEDC test Residency Points shift up for downsized GDI engine o 2.4 liter, V6 engine 1.2 liter, I3 engine (Source : Freeland et al., FISITA, 2013) 21

22 Advanced Combustion Concepts (Source : Ulas, PhD Thesis, TU Eindhoven, 2013) Low Temperature Combustion Heavy EGR application (Source : Wagner, SAE Paper No ) 22

23 HCCI Engines Homogeneous Charge Compression Ignition engines : lean and homogeneous mixture is compressed until p and T are high enough for autoignition to ocur HCCI ignition is governed by chemical kinetics and histories of cylinder pressure and temperature (inlet air temperature, compression ratio, residual gas ratio and EGR, wall temperature). Combustion starts simultaneously all over the cylinder Reaction rate is much lower than knock in SI engines due to a higher dilution of the fuel with air or residual gases (EGR) HCCI Engines High thermal efficiency due to high compression ratio, rapid heat release rate Low specific fuel consumption with lean mixture Low NOx emissions Difficulties in controlling ignition and combustion over a wide range of engine operating conditions 23

24 Diesel and HCCI Engines Comparison between conventional Diesel engine and HCCI engine Longer induction time for HCCI and lower cylinder temperatures HCCI Engines Homogeneous Change Compression Ignition engines have features from both SI and CI engines; Lean and homogeneous mixture is compressed until p and T are high enough for autoignition to occur Combustion starts simultaneously all over the cylinder Reaction rate is much lower than knock in SI engines due to a higher dilution of the fuel with air or residual gases (EGR) 24

25 HCCI Engines Autoignition process is controlled by time history of p and T during intake and compression stroke autoignition is mainly governed by the H (hydrogen), OH (hydroxyl), H2O2 (hydrogen peroxide) and HO2 (hydroperoxyl) radicals H2O2 and HO2 concentrations increase progressively during compression as p and T increases These two radicals also govern low temperature reactions (LTR), or cool flame At temperatures between 1050 and 1100 K, H2O2 decomposes and forms OH that quickly reacts with fuel molecules to produce heat and water -> autoignition starts -> high temperature reactions (HTR) take place concentration of H and OH radicals inc rapidly, while H2O2 concentration drops (Manente, V., PhD Thesis, Lund University, 2007) HCCI Engines Change of mole fractions OH (hydroxyl), H2O2 (hydrogen peroxide), HO2 (hydroperoxyl) radicals 25

26 HCCI Engines Autoignition temperature is controlled by the fuel used it is between K for fuels with no LTR such as natural gas, iso-octane, gasoline with high aromatics fuels with LTR, it is between K gasoline with high paraffin content, fuels containing n-heptane Advantages vs Disadvantages HCCI combustion has fast burning rate close to ideal Otto Cycle (higher thermal efficiency than Diesel cycle) lower fuel consumption (low CO2 emissions) compared to SI and CI engines of a given output power and CR lower pumping losses compared to SI engines at part load (no throttle), but CI engines have higher part load efficiency HCCI engines operate lean (with T below 1900 K) and due to fast combustion residence time of nitrogen at T higher than 2000 K is very short Fast burning results in high pressure rise rate noise emissions, structural damages difficult to control combustion phasing, ignition relies on spontaneous autoignition Low temperature combustion inc HC emissions from the engine 26

27 HCCI Engines Soot formation region is with equivalence ratio above 2 and temperature between K Thermal NOx formation region is with equivalence ration below 2 and combustion temperature above 2000 K Controlling HCCI Engines HCCI ignition is governed by chemical kinetics of mixture and histories of p and T in combustion chamber Controlling combustion and ignition; In-cylinder temperature inlet air temperature compression ratio residual gas ratio and EGR wall temperature, cooling water temperature, wall deposits Air-fuel chemistry dual fuel injection system (fuels with different ignition tendancies) Mixture composition fuel stratification in direct injection 27

28 Diesel and HCCI Engines solid lines : HCCI dotted lines : Diesel (Aceves et al. SAE ) Performance map for Volkswagen TDI 4-cyclinder engine HCCI engine is more efficient at low torque at any speed Higher efficiency is due to faster combustion (approaching to ideal Otto cycle) Diesel and HCCI Engines Performance map for Volkswagen TDI 4-cyclinder engine HCCI engine has, 72% of max torque obtained by Diesel 88% of the power obtained by Diesel while producing practically no PM and less than 100 ppm NOx under all operating conditions Needs no delay to reduce NOx Simulation results by Aceves et al., (SAE ) 28

29 Gasoline and HCCI Engines Saab SVC, VCR, HCCI, =10:1-30:1 General Motors L850, HCCI, =18:1 - SI, =18:1, ( =9.5:1 std) Scania D12 Heavy duty diesel engine, HCCI, =18:1 Fuel: US regular Gasoline SI engine (Johansson, Lund University, 2009) HCCI Engines Wiebe (Vibe) function (Yasar et al., Appl Thermal Engineering, 2008) Double - Wiebe function 29

30 HCCI Engines HCCI Engines 30

31 HCCI Engines PCCI Engines Premixed charge compression ignition engines : mixture of the fuel with air is provided prior to the initiation of combustion due to early injection of the fuel into the cylinder at low pressure and temperature conditions for autoignition to take place. Combustion is controlled by chemical kinetics. In cylinder temperature levels are controlled by applying heavy exhaust gas recirculation (EGR) rates to dominate ignition process, which also controls NOx and PM emissions. 31

32 PCCI Engines Misfire especially at low load conditions is a potential problem Rapid pressure rise at high loads and uncontrolled ignition timing are other issues which can cause low thermal efficieny and engine damage Dual Fuel Combustion Premixed Natural Gas induced into the cylinder during the intake stroke and ignited by pilot injection of Diesel Fuel Lean operation is possible providing reduction in emissions and improvement in fuel consumption City Bus fleet in Istanbul (1992) 32

33 RCCI Engines Reactivity Controlled Compression Ignition engine : mixture is formed with early injection of the fuel into the cylinder Low octane fuel injected earlier can be blended with high octane fuel with post injection Fuel blend ratio and injection timing are the parameters for control (Source : Reitz, 2011) RCCI Engines Varying the mixture ratio of fuel blends with different reactivity levels can provide considerably high thermal efficiencies. Relatively low injection pressures, in comparison to fuel injection systems used in modern diesel engines, provide energy saving. Low temperature combustion reduce NOx emissions while the level of uniformaty of the mixture reduce PM emissions. 33

34 Advanced Combustion Systems Injection timing and ignition Advanced Combustion Systems HCCI UNIBUS PCCI PREDIC MK LTRC PCI homogeneous charge compression ignition uniform bulky combustion system premixed change compression ignition premixed lean diesel combustion modulated kinetics low temperature rich combustion premixed charge ignition 34

35 Modulated Kinetics Basic MK concept, low temperature, premixed combustion system aimed at simultaneously reducing NOx and PM emissions (in a Diesel engine) Low T can be obtained by heavy EGR reducing O2 concentration -> but reduced O2 concentration increases PM in diffusion combustion Fuel and air is mixed before combustion, fuel injection time is retarded so that ignition delay is prolonged (Kimura et al., SAE ) Modulated Kinetics (Kimura et al., SAE ) 35

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