Spark Ignition Engine Combustion MAK 652E Lean Combustion in Stratified Charge Engines Prof.Dr. Cem Soruşbay Istanbul Technical University - Automotive Laboratories
Contents Introduction Lean combustion in engines Cycle-to-cycle variations Stratified charge engines Gasoline direct injection some applications
Engine Efficiency Thermal efficiency Heat losses Pumping losses Frictional losses p ~ V diagram, compression ratio cooling system, hot exhaust gases gas exchange process friction between moving parts Losses at = 1 best efficiency at = 1.1 to 1.3
Specific Fuel Consumption
SI Engines a) Full load b) Part load
Diesel vs Gasoline
Flame Speed and Thickness Lean mixture Rich mixture Lean mixture Rich mixture
Excess Air Factor For a homogeneous charge engine specific fuel consumption is minimum at around 10 20 % lean mixture ( = 1.1 1.2) slower combustion - ignition timing must be advanced when mixture is leaned cycle-to-cycle variations increase with lean mixtures
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
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
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. 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
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)
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
Coefficient of Variation The coefficient of variation (COV) in indicated mean effective pressure standard deviation in indicated mean effective pressure (p ime ) divided by mean p ime expressed in percent (usually), COV imep p imep ime.100 vehicle driveability problems usually result when COV impe exceeds about 10 % COV increases by leaning the mixture
Cyclic Variations in Combustion Cyclic fluctuations have a similar effect as the adjustment of ignition timing
COV and Fuel Economy for GDI Engine Stratified charge DI (Direct injection) Homogeneous charge PFI (Port fuel injection)
SI Engine with Manifold Injection Multi-point injection Single-point injection (replaces carburetor)
SI Engine with Manifold Injection INTAKE VALVE INJECTOR. INTAKE MANIFOLD Solenoid injector Injection pressure of 0.5 1.5 MPa (DI engines 15MPa) COMBUSTION CHAMBER Homogeneous charge PFI (Port fuel injection)
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) cooling loss reduction
Stratified Charge Engines
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
Stratified Charge Engines
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
General features 2) 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
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
General features Two-stage mixing Early first injection, during early intake stroke (lean mixture) Second injection at late stages of compression stroke (stratified charge)
General features 4) 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
Starting Process
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
Stratified Slightly Lean Combustion Light-off temp of CO is about 150 o C Heat released as a result of CO oxidation Then HC s are oxidized
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%)
Honda CVCC System Honda CIVIC ( CVCC : Compound Vortex Combustion Chamber )
Ford PROCO Combustion System Ford Programmed Combustion System
Texaco TCCS System Texaco Controlled Combustion System
DISC Combustion System
MAN FM Combustion System
Gasoline DI Concepts Wall-guided Air-guided Spray-guided Fuel economy o + ++ HC o + ++ PM o + ++ Power o - o to +
Wall-guided GDI Engines
Toyota GDI Engine
Mitsubishi GDI Engine
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
Injectors First generation GDI engines based on wallguided concept use mainly swirl type injectors New generation GDI engines use outward-opening and multi-hole injectors
Sprays Generated (a) Multi-hole (b) Outward-opening (c) Swirl-atomizer
Piezoelectric Outward-opening Pintle Injectors
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
Solenoid-driven Multi-hole Injectors
Multi-hole Nozzle Examples
Spray-guided GDI Engines Cylinder head configuration for spray-guided concept
Spray-guided GDI Engines Multi-hole injector for spray-guided GDI engines
Spray-guided GDI Engines Outward-opening injector for spray-guided GDI engines
DISI Engine Operation Modes
BMW Spray-Guided System
BMW 3L I6 HPI Engine
Mercedes Spray-Guided DISI Engine
Mercedes Spray-Guided DISI Engine
Specific Power and Fuel Consumption
Fuel Consumption Reduction Potential