Istanbul Technical University - Automotive Laboratories Spark Ignition Engine Combustion MAK 652E Lean Combustion in Stratified Charge Engines Prof.Dr. Cem Soruşbay
Istanbul Technical University - Automotive Laboratories
Spark Ignition Engine Combustion MAK 652E
Lean Combustion in Stratified Charge Engines
Prof.Dr. Cem Soruşbay
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Contents
Introduction
Lean combustion in engines
Cycle-to-cycle variations
Stratified charge engines
Gasoline direct injection – some applications
Prof.Dr. Cem SORUŞBAY - 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 = 1.1 to 1.3
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Specific Fuel Consumption
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
SI Engines
a) Full load b) Part load
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Diesel vs Gasoline
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Flame Speed and Thickness
Lean mixture Rich mixture
Lean mixture Rich mixture
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
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
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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.
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
Prof.Dr. Cem SORUŞBAY - 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)
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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),
vehicle driveability problems usually result when COVimpe exceeds about 10 %
COV increases by leaning the mixture
100.ime
imep
imepp
COV
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Cyclic Variations in Combustion
Cyclic fluctuations
have a similar effect as
the adjustment of ignition timing
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
COV and Fuel Economy for GDI Engine
Homogeneous charge PFI (Port fuel injection)
Stratified charge DI (Direct injection)
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
SI Engine with Manifold Injection
Single-point injection
(replaces carburetor)
Multi-point injection
Prof.Dr. Cem SORUŞBAY - 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
0.5 – 1.5 MPa
(DI engines 15MPa)
Prof.Dr. Cem SORUŞBAY - 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)
cooling loss reduction
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Stratified Charge Engines
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Stratified Charge Engines
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
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
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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)
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
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
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Starting Process
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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%)
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Honda CVCC System
Honda CIVIC ( CVCC : Compound Vortex Combustion Chamber )
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Ford PROCO Combustion System
Ford Programmed Combustion System
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Texaco TCCS System
Texaco Controlled Combustion System
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
DISC Combustion System
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
MAN FM Combustion System
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Gasoline DI Concepts
Wall-guided Air-guided Spray-guided
Fuel economy o + ++
HC o + ++
PM o + ++
Power o - o to +
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Wall-guided GDI Engines
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Toyota GDI Engine
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Mitsubishi GDI Engine
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Sprays Generated
(a) Multi-hole (b) Outward-opening (c) Swirl-atomizer
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Piezoelectric Outward-opening Pintle Injectors
Prof.Dr. Cem SORUŞBAY - 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
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Solenoid-driven Multi-hole Injectors
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Multi-hole Nozzle Examples
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Spray-guided GDI Engines
Cylinder head configuration for spray-guided concept
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Spray-guided GDI Engines
Multi-hole injector for spray-guided GDI engines
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Spray-guided GDI Engines
Outward-opening injector for spray-guided GDI engines
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
DISI Engine Operation Modes
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
BMW Spray-Guided System
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
BMW 3L I6 HPI Engine
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Mercedes Spray-Guided DISI Engine
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Mercedes Spray-Guided DISI Engine
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Specific Power and Fuel Consumption
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories
Fuel Consumption Reduction Potential
Prof.Dr. Cem SORUŞBAY - ITU Automotive Laboratories