Spark Ignition Engine Combustion - İTÜweb.itu.edu.tr/~sorusbay/SI/LN04.pdf · Istanbul Technical...

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


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


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 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



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 (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



Cyclic fluctuations

have a similar effect as

the adjustment of ignition timing


COV and Fuel Economy for GDI Engine

Homogeneous charge PFI (Port fuel injection)

Stratified charge DI (Direct injection)


SI Engine with Manifold Injection

Single-point injection

(replaces carburetor)

Multi-point injection


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)


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



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


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 oC 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 wall-guided 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



Cylinder head configuration for spray-guided concept



Multi-hole injector for 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


Specific Power and Fuel Consumption


Fuel Consumption Reduction Potential

Spark Ignition Engine Combustion - İTÜweb.itu.edu.tr/~sorusbay/SI/LN04.pdf · Istanbul Technical...

Documents

Transcript of Spark Ignition Engine Combustion - İTÜweb.itu.edu.tr/~sorusbay/SI/LN04.pdf · Istanbul Technical...