Spark Ignition Engine Combustion - İTÜ

1 Istanbul Technical University - automotive Laboratories Spark Ignition Engine Combustion MAK 652E Lean Combustion in Stratified Charge Engines Cem Soru bay Cem SORU BAY - ITU automotive Laboratories Contents Introduction Lean Combustion in engines Cycle-to-cycle variations Stratified charge engines Gasoline direct injection some applications Cem SORU BAY - ITU automotive Laboratories Engine Efficiency Thermal efficiency p ~ V diagram, compression ratio Heat losses cooling system , hot exhaust gases Pumping losses gas exchange process Frictional losses friction between moving parts Losses at = 1 best efficiency at = to Cem SORU BAY - ITU automotive Laboratories Specific Fuel Consumption Cem SORU BAY - ITU automotive Laboratories SI Engines a)Full load b)Part load Cem SORU BAY - ITU automotive Laboratories Diesel vs Gasoline Cem SORU BAY - ITU automotive Laboratories Flame Speed and Thickness Lean mixture Rich mixture Lean mixture Rich mixture Cem SORU BAY - ITU automotive Laboratories Excess Air Factor For a homogeneous charge Engine specific fuel consumption is minimum at around 10 20 % lean mixture ( = )

2 Slower Combustion - Ignition timing must be advanced when mixture is leaned cycle-to-cycle variations increase with lean mixtures Cem SORU BAY - ITU automotive Laboratories Cyclic Variations in Combustion For successive operating cycles, cylinder pressure versus time (or CA) shows substantial variations - due to variations occuring in Combustion process : cycle-to-cycle variations Each individual cylinder can also have significant differences in the Combustion process and pressure development between cylinders in a multicylinder Engine : cylinder-to-cylinder variations Cem SORU BAY - ITU automotive Laboratories Cyclic Variations in Combustion Cyclic variations are caused by variations in, mixture motion within cylinder at the time of Spark the amounts of air and fuel fed to the cylinder at each cycle the mixing of fresh mixture and residual gases within cylinder (especially in vicinity of Spark plug) at each cycle Same phenomena applies to cylinder-to-cylinder differences Cem SORU BAY - ITU automotive Laboratories Cyclic Variations in Combustion Cycle-to-cyle variations are important for, optimum Spark advance (effects Engine power output and efficiency) and extreme cyclic variations limit Engine operation.

3 Fastest burning cycles with over-advanced Spark timing have highest tendancy to knock - determine fuel octane requirement and limit compression ratio. Slowest burning cycles with retarded Spark timing are most likely to burn incompletely - set practical lean operating limits, limit EGR which Engine will tolerate. Variations in cylinder pressure correlate with variations in brake torque which is directly related to vehicle drivability Cem SORU BAY - ITU automotive Laboratories Measures for cycle-to-cycle Variations pressure related parameters max cylinder p, the crank angle at which max p occurs, max rate of p rise, crank angle at which (dp/d )max occurs, indicated mean effective pressure. burn-rate related parameters max heat transfer rate, max mass burning rate, flame development angle ( d), rapid burning angle ( b) Cem SORU BAY - ITU automotive Laboratories Measures for cycle-to-cycle Variations flame front position parameters flame radius, flame front area, enflamed or burnt volume all at given times, flame arrival at given locations Cem SORU BAY - ITU automotive Laboratories Coefficient of Variation The coefficient of variation (COV) in indicated mean effective pressure standard deviation in indicated mean effective pressure (pime) divided by mean pime expressed in percent (usually)

4 , vehicle driveability problems usually result when COVimpe exceeds about 10 % COV increases by leaning the mixture Cem SORU BAY - ITU automotive Laboratories Cyclic Variations in Combustion Cyclic fluctuations have a similar effect as the adjustment of Ignition timing Cem SORU BAY - ITU automotive Laboratories COV and Fuel Economy for GDI Engine Homogeneous charge PFI (Port fuel injection) Stratified charge DI (Direct injection) Cem SORU BAY - ITU automotive Laboratories SI Engine with Manifold Injection Single-point injection (replaces carburetor) Multi-point injection Cem SORU BAY - ITU automotive Laboratories SI Engine with Manifold Injection INTAKE MANIFOLD INJECTOR Combustion CHAMBER INTAKE VALVE . Homogeneous charge PFI (Port fuel injection) Solenoid injector Injection pressure of MPa (DI engines 15 MPa) Cem SORU BAY - ITU automotive Laboratories Stratified Charge Engines Since the first launch of DI gasoline Engine in 1996 (mass production), Japanese and European manufacturers introduced this concept into the market Advantages, improvement of fuel economy reduction of CO2 emissions due to higher compression ratio higher specific heat ratio pumping loss reduction (lean burn, EGR)

5 cooling loss reduction Cem SORU BAY - ITU automotive Laboratories Stratified Charge Engines Cem SORU BAY - ITU automotive Laboratories Stratified Charge Engines More effective at low load region Effect of lean burn is mainly due to higher specific heat ratio rather than reduction of pumping losses Effect of higher specific heat ratio is maintained at higher loads Higher specific heat ratio due to stable lean burn Higher CR due to higher knock resistance Pumping loss reductions due to lean burn (no throttling) cooling loss reduction due to lowered burned gas temperature and mixture stratification Cem SORU BAY - ITU automotive Laboratories Stratified Charge Engines Cem SORU BAY - ITU automotive Laboratories General features 1) Lean Burn Thermal efficiency stable Combustion at lean burn low pumping losses, low heat loss, high specific heat ratio low teperatures for burning gases NOx emissions in general depends on temperature, mixture ratio (available O2 and N2), time have to control equivalence ratio and temperatures for low NOx Cem SORU BAY - ITU automotive Laboratories General features 2)

6 Lower Amounts of Fuel Escaping Combustion Port injection engines fuel is captured at the wall oil film and scraped fuel by piston motion burns rapidly under unsuitable conditions during exhaust stroke not a direct source for unburned HC emissions but reduces thermal efficiency DI engines air around cylinder liner do not contain fuel Cem SORU BAY - ITU automotive Laboratories General features 3) Improved Anti-knock Characteristics Charge cooling effect by evaporating fuel charge cooled by 15 K and end of Combustion T reduced by 30 K Therefore, higher volumetric efficiency lower knock tendancy Lean mixture for reducing knock tendancy lean mixture at the end gas (away from Spark plug) reduces knock tendancy Cem SORU BAY - ITU automotive Laboratories General features Two-stage mixing Early first injection, during early intake stroke (lean mixture) Second injection at late stages of compression stroke (stratified charge) Cem SORU BAY - ITU automotive Laboratories General features 4)

7 Precise and Rapid Torque Management Port injection engines Engine torque is controlled by throttling air intake slow responce Direct injection engines torque controlled by the injected fuel quantity rapid control hybrid vehicles idle-stop is possible fast start from idle-stop and acceleration Cem SORU BAY - ITU automotive Laboratories Starting Process Cem SORU BAY - ITU automotive Laboratories Two-stage Combustion Main fuel injection is during compression stroke, additional fuel injection at a later stage (expansion stroke) increases exhaust temperatures catalyst conversion efficiency increase But fuel consumption also increase Cem SORU BAY - ITU automotive Laboratories Stratified Slightly Lean Combustion Light-off temp of CO is about 150 oC Heat released as a result of CO oxidation Then HC s are oxidized Cem SORU BAY - ITU automotive Laboratories Stoichiometric GDI Engines High pressure fuel injection ( 5 to 20 MPa ) and precise timing to prevent impingement of fuel on piston and cylinder walls for low HC Charge cooling by evaporating spray ( ~ 15K ) allows higer CR (~12:1) - increased power (up to 15%) and fuel economy (3 5%) Cem SORU BAY - ITU automotive Laboratories Honda CVCC system Honda CIVIC ( CVCC.)

8 Compound Vortex Combustion Chamber ) Cem SORU BAY - ITU automotive Laboratories Ford PROCO Combustion system Ford Programmed Combustion system Cem SORU BAY - ITU automotive Laboratories Texaco TCCS system Texaco Controlled Combustion system Cem SORU BAY - ITU automotive Laboratories DISC Combustion system Cem SORU BAY - ITU automotive Laboratories MAN FM Combustion system Cem SORU BAY - ITU automotive Laboratories Gasoline DI Concepts Wall-guided Air-guided Spray-guided Fuel economy o + ++ HC o + ++ PM o + ++ Power o - o to + Cem SORU BAY - ITU automotive Laboratories Wall-guided GDI Engines Cem SORU BAY - ITU automotive Laboratories Toyota GDI Engine Cem SORU BAY - ITU automotive Laboratories Mitsubishi GDI Engine

9 Cem SORU BAY - ITU automotive Laboratories Spray-guided GDI Engines Provide expanded speed-load range for stratified charge operation - better fuel economy in comparison to the first generation GDI (wall-guided) fuel stratification does not depend on piston cavity or in-cylinder flow fluctuations in spray properties, droplet size effect performance new injectors are developed for spray-guided GDI engines Cem SORU BAY - ITU automotive Laboratories Injectors First generation GDI engines based on wall-guided concept use mainly swirl type injectors New generation GDI engines use outward-opening and multi-hole injectors Cem SORU BAY - ITU automotive Laboratories Sprays Generated (a) Multi-hole (b) Outward-opening (c)

10 Swirl-atomizer Cem SORU BAY - ITU automotive Laboratories Piezoelectric Outward-opening Pintle Injectors Cem SORU BAY - ITU automotive Laboratories Solenoid-driven Multi-hole Injectors Simple and less expensive system than piezoelectric outward-opening pintle injectors Advantages in flexibility in adjusting spray configuration to Engine geometry, narrow cone angle of individual sprays, control of tip penetration and atomization through injection pressure and timing Cem SORU BAY - ITU automotive Laboratories Solenoid-driven Multi-hole Injectors Cem SORU BAY - ITU automotive Laboratories Multi-hole Nozzle Examples Cem SORU BAY - ITU automotive Laboratories Spray-guided GDI Engines Cylinder head configuration for spray-guided concept Cem SORU BAY - ITU automotive Laboratories Spray-guided GDI Engines Multi-hole injector for spray-guided GDI engines Cem SORU BAY - ITU automotive Laboratories Spray-guided GDI Engines Outward-opening injector for spray-guided GDI engines Cem SORU BAY - ITU automotive Laboratories DISI Engine Operation Modes Cem SORU BAY - ITU automotive Laboratories BMW Spray-Guided system Cem SORU BAY - ITU automotive