Premixed Internal Combustion Engines with Spark Ignition

exhaustintake

2-StrokeEngines

2-stroke

ReedValve

intake

Wankel Enginesintakeexhaust

+: No valves neededContinuous motion

less vibration

-: Leaks through seals

low compressionratiopollution (high levels of HCand CO)

4-Stroke SI Engines Emission characteristicsof air/fuel ratio

Otto cycle

Topics of Engine Combustion

• Brief introduction: - Spark Ignition (gasoline)- Fuel preparation, Ignition, Power analysis

(efficiency & losses) , Engine Knock, Modeling, Emissions

• Areas of improvement• Emerging new technologies

[Lean Burn, Homogeneous Charge Compression Ignition (HCCI) ]

SI Engine Performance VS Engine RPM

Engine RPM

Torque

Power

SFC

OutputPowerm

SFC fuel

•

=

Power

Torque

000,60*)(*2)( RPMNMTorquekwPower π

=

Specific Fuel Consumption

Spark Ignition (SI) Engines:• Control of combustion: ignition

timing• - Low efficiency (low

compression ratio, limited by knocking)

• + Cleanest emission with help of 3-way catalyst

• Advanced Technologies:--New fuel with higher octane number, improving combustion efficiency, reduces losses (pumping, heat, friction, exhaust & intake)

SI Combustion

-Spark initiates combustion-Turbulent flame propagation-Stoichiometric mixture-High combustion temperature-Compression ratioLimited by autoignition

Understanding Engine KnockSome physical models in mind:• Unburn gas is compressed by bunred turbulent flame leading

high temperature and pressure• Unburn gas auto-ignites or ignition triggered by hot spot.• Autoignition delay time is of critical importance Pressure waves bounce

back and forward throughout cylinder

Autoignition delays of large-molecule fuelsExhibit negative temperature behaviors

Low temp.ignition

Mainignition

Negative temperature behaviors:T ↑ delay ↓

n-heptane: C7H16

T ↑ delay ↓

Negative temperature behaviors complicate engine designs

1

100

10000IsooctaneMethanePRFethanol

PRF

Isoo

EthanoMethane

ition

Del

ay [m

s]Autoignition delays of large-molecule fuels

Exhibit negative temperature behaviors-Methane and ethanoldo not show “negative”temperature in the IC engine application regime•Are oxygenated fuels(biofuels) similar to ethanol?• Predictive models?

Fuel Octane number(RON)CH4 Methane 120C2H5OH Ethanol 107C8H18 Isooctane 100

PRF(80) 80

To avoid autoignition so that a higher compression ratio can be used higher thermal efficiency

• reduces time available for unburned gas-- higher flame speed (controlled largely by

turbulence)-- multiple ignition sources (limited by space)

• Keep unbuned gas as cool as possible • Fuel additives (small amount of ethanol and

others)…All these are explored by engine tests with test

matrix assisted by CFD simulations

Guidance to SI Engine Design

Multiple Site Spark IgnitionBenefits:• Combustion is initiated atouter and propagating inward• Fast combustion rate eliminatingend gas auto-ignition• Higher combustion efficiency• Shorter combustion period lessheat loss

3D-CFD Predictions of Engine Knock

Combustion Chamber Geometry• Enhances turbulent

combustion speed to decrease burning time

• Minimizes heat transfer & pollution formation

• Needs to be integrated with valve timing and intake manifold designs

• CFD calculations

Candidate Geometries

Velocity Field

TurbulenceIntensity

CFD of Intake Flows

Turbulent Premixed Flames

Internal Combustion Engine

FlameDevelopment

BunsenTurbulentFlame

Turbulent Flames

( )

TDC beforeignition spark pressure motoring

nsfluctuatiovelocity turbulent '

05.01'21.11 4.082.0

==

=

+⎟⎟⎠

⎞⎜⎜⎝

⎛+=

θ

θ

m

mLL

T

pu

pp

Su

SSEmpirical Relation:

Enhancing turbulence by multiple valve arrangement

Vertical Vortex – generates strongTurbulence as the tumble is brokenup near the top dead center

Fast burn Less time for heat transferPossible higher compression ratio10-20% better fuel economy

Various Losses from SI EnginesIdeal Otto Cycle

ExhaustBlowdown loss

Heat lossTime loss(slow combustion)

Pumping loss

Sketch of a Carburetor

ECU: ElectronicControl Unit

Throttle

Fuel InjectionSystem

Pressure data from a 1.8 Liter Pontiac Engine at Berkeley

Pumping lossesbecome significant

Overall Performance from SI Engines with/without Throttling

Current throttled SI engines

Self-ignition unthrottled

Effic

ienc

y

Effective Mean Pressure[bar]

Importance of valve timing

Gasoline Direct Injection (GDI)Lean Burn Engines

Advantages:-lean burn -Low (or no) pumping loss-High compression ratio-High efficiency

Disadvantages:-hard to maintain good combustion-high NOx an HC emissions-special NOx absorption catalyst

• CombustionChamber Geometry

• Combustion ofDiluted Mixtures

• Multiple source ignition

• Exhaust energyretention

Enabling Technologies:

• Friction Reduction

• Optimal control of Spark timing or Injection

•Variable Compression

• Variable Valve Timing (VVT)

Areas offering potentially largeimprovement in engine performancefor current & future fuels

HCCI Technologies

Modeling of MON, RON and HCCI Number

Complete Model with CFD good for engine designbut too expensive with detailed chemical kinetics

Chemical kinetics base model-- Single zone well-mixed reactor HCCI-- Two zone model (or multizone shell)

SI MON and RON

removed

Complete CFD with detailed chemistry exceedsCurrent computer capacities

a) gasoline engine b) diesel engine

Emission as functionof air/fuel ratio

Emission Characteristics from IC Engines

NOx Emission is highly sensitive to temperature

Sources of Unburned Hydrocarbon and CO

Unburned HC and CO are stored in crevices and released duringexpansion stroke as wall jets.

Spark Ignition (SI) Pontiac Engine

Horiba gas analyzers:CO,HC, NOx, O2, CO2

Engine performances:-- Pressure transducer-- torque-- SFC-- emission before and after catalyst

Comparison:3 Methods of Internal Combustion Engines

Diesel HCCIGasoline

(Premixed Spark) (Direct Injection) (Premixed Autoignitionof lean mixtures)

What is HCCI ?• Homogeneous-Charged Compression Ignition (HCCI) engine

-- has advantages of both SI and CI engines• Global autoignition of premixed fuel and air

(High compression ratio)• Heat release controlled by chemical kinetics • No flame propagation - operation possible at low Φ*

• Benefits: 1. Low combustion temperature (lean mixture < Φ~0.5)lower NOx emissions

2. Premixed charge no soot3. High compression ratio high thermal efficiency

• Challenges: 1. Control of Start of Combustion (SOC) is the main issue in HCCI (No direct control, i.e. spark or fuel injector)2. High unburned hydrocarbon and CO emissions

• Benefits and Challenges

* equivalence ratio for fuel-air mixture