Combustion• Detonation• Pre-ignition• Lean theory

Detonation and Detonation and PreignitionPreignition

Section Objectives

• Understand the importance of detecting detonation and pre-ignition early

• List the causes of detonation and pre-ignition

• Understand how to eliminate detonation and pre-ignition

Ka KnockKa Knock KnockKnock

Ka KnockKa Knock

KnockKnock

KnockKnock

Ka PingKa Ping

BangBang

KnockKnock

WAUKESHAWAUKESHA

Early damage on piston crown edge is common.

Advanced damage

Aluminum depositson the cylinder wall

Damage in the ring groove area

Total piston seizure

Aluminum depositsfrom the piston

DetonationAuto Ignition of the End Gas

The End Gas is the air fuel mixture inside the combustion chamber that is ahead

of the normal flame front.

Cylinder Wall

Direction OfFlame Travel

Flame Front Propagation

DetonationDetonation

NormalNormal

TDC TDC

Detonations pinging sound is caused by

pressure waves forcing the cylinder walls to

vibrate

OCTANE NUMBER

A FUEL STABILITY RATING

A MEASURE OF A FUEL’S ABILITY TO RESIST DETONATION

NO LONGER USED BY WAUKESHA

WAUKESHA KNOCK INDEX

Values • Pure Methane 100

• Digester 125

• Landfill 140

• Processed Natural Gas 90

• Propane 34

WKI Number

Btu

When a fuel’s Btu rises the Waukesha Knock Index Number drops

Detonation affects anengine’s potential power

output

Why?

Effect Of Operating ConditionsOn Detonation

• Spark Timing

• Compression Ratio

• Inlet Fuel / Air Temperature

• Coolant Temperature

• Waukesha Knock Index® Number

• Engine Speed

• Exhaust Back Pressure

• Atmospheric Humidity

• Combustion Chamber Deposits

• Fuel Air Ratio

• Engine Load

60° 50° 40° 30° 20° 10°

Degrees before TDC

Pre

ssu

reP

ress

ure

Ignition Timing

Compression Ratio

Detonation tendencies increase with the rising compression ratio

11:1

10.5:1

10:1

9.4:1

8.2:1

8:1

7:1

Final compression gas temperature increases

A .060” (1.52 mm) deposit can raise the compression by 1/4 of a ratio

DepositsDeposits

Pressure Higher combustionchamber pressurestend to increasedetonation tendencies

Increased temperature of the combustion chambercan increase detonation tendencies

HEAT

• Jacket Water • Air Induction

Hot generator air discharge

Air intake

Engine Speed

WK

I R

equ

irem

ent

140

130

120

110

100

90

80Low High

Effect of Back Pressureon Detonation

CombustionTemperature

CompressionTemperature

…decreases detonation tendencies

Rising intake air humidity…

Air/Fuel RatioRich Lean

Temp

16.09:1

Exhaust

WKI Number Requirements

Eng

ine

Load

HeatHeatPressurePressure

LOW

HIGH

HIGH

Engine design factors can influence an engine’s

tendency to detonate

• Spark plug location

• Combustion chamber shape

• Piston head shape

• Cylinder head size

Detonation SummaryDetonation Promoters Detonation Reducers

Higher Cylinder Temps Lower Cylinder TempsLower WKI® Fuels Higher WKI® FuelsMore Advanced Timing Less Advanced TimingHigher Compression Ratio Lower Compression RatioHigher Inlet Pressure Lower Inlet PressureHigher Coolant Temp Lower Coolant TempLower Engine Speeds Higher Engine SpeedsLower Humidity Higher HumidityHigher Engine Load Lower Engine LoadStoichiometric A/F ratio Lean or Rich A/F ratio

PreignitionIgnition before the timed spark

Preignition Promoters

• Incandescent cylinder deposits

• Spark plug with incorrect heat range

• Burning valve

• Overheated piston crown

Detonation - Preignition

Preignition - Detonation

Lean Combustion Theory

OOxygen

- 20.9% of atmosphere- Naturally found as 2 atoms- Chemical designation: O2

Nitrogen- 78% of atmosphere- An inert gas- Naturally found as 2 atoms- Chemical designation: N2

Major Components in the Atmosphere(98.9%)

O

NN

Water- Concentration depends on humidity and temperature- Chemical designation: H20

Carbon Dioxide- Makes up 0.034% of atmosphere- By-product of combustion- Major contributor to greenhouse effect- Chemical designation: CO2

Trace Compounds in the Atmosphere

OH

H

O

OC

Ozone- Bluish, pungent gas- Formed by ultraviolet light- Formed in upper atmosphere- By-product of photochemical - Chemical designation: O3

Regulated Compounds in the Atmosphere

OO

O

CH H

OC

Hydrocarbons:- Methane not included as toxic- Pollutant emitted by engines- Fuel in photochemical smog- Chemical designation: NMHC

Carbon Monoxide:- Colorless and odorless- Extremely toxic- Believed to promote

photochemical smog- Chemical designation: CO

Regulated Gases Found in the Atmosphere

H H

(Methane)

ONitric Oxide

- emitted by the engine- Oxidizes quickly to nitrogen

dioxide- Chemical designation: NO

Nitrogen dioxide- The catalyst in photochemical

smog- Yellowish brown in color (Haze)- Chemical designation: NO2

Regulated Gases Found in the Atmosphere

N

ON

O

Regulated Gases Found in the Atmosphere

Formaldehyde

- Higher levels produced in GL engines (0.2-0.3 GL, 0.05 GSI g/BHP-hr)

- Considered toxic and restricted by government agencies

- Chemical designation: CH2O

OO

HC

Toxic Gas Which May Be Regulated

OS

O

Sulfur Dioxide

- Occurs in combustion when gas contains H2S- Heavy, pungent, colorless gas- Can form sulfuric acid - Chemical designation: SO2

Two Types of Smog:

Particulate Smog (Cold wet days)Photo-chemical Smog (Hot dry days)

NO O(Nitric Oxide)(Atomic Oxygen)

NO2(Nitrogen Dioxide)

Ultra-violet light

NO2

O

NO

OO

O

Formation of Ozone

(Atomic Oxygen)

(Oxygen)

O O

O (Ozone)

NO2

NO

O

O2

O3OzoneMain

ToxicantPhotochemic

al Smog

HC

HCO

HCNO2

Stoichiometric Combustion

Chemically Ideal Combustion

CH4 + 202 + 2N2 CO2 + 2H20 + 2N2Combustion

OO

OONN

NN

CH

HH

H

CH

HH

H

OO

OO

Stoichiometric Combustion

OO

NN

NN

C

H

C

O

Stoichiometric Exhaust

OOH

H

OH

HO

H H

OH

OO

OONN

NNC

H

HH

H

OO

Rich Combustion

CH

HH

H

CH

HH

H

NN

NN

HO

Rich Exhaust

H

H

OH

HO

HH

OH

CH

HH

H

OC

OC

Cooler Flame

OO

OONN

NN

CH

HH

H OO

OO

OO

Lean Combustion

OO N NN

C

Lean Exhaust

H

OH

HO

HO

NOOO

OO

Cooler Flame

NOx ProductionTemperature vs. Oxygen

Max NOx

oF

EXHAUST O2

oC

807

670

642

615

587

559

53214 15 16 17 18 19 20

1000

1050

1100

1150

1200

1250

1300

0

1

2

3

4

5

AIR / FUEL RATIO

02 %

EXHAUST TEMP

EmissionsNOx, CO, & HC

CO

HC

NOX

% CO & NOX

AIR /FUEL RATIO

%HC

GL Emissions vs. Ignition Timing

NOX

CO

NMHC

G/BHP/HR

IGNITION TIMING (BTDC)

GL Operating Window

Too rich

AFR

Too lean

AFR

Timing Advance

Timing Retard

• High CombustionTemperature

• Detonation

• Power Loss

• Misfire

• Elevated emissions• Detonation

• Elevated BSFC

EmissionsNOx, CO, & HC

NOX

HCCO

%HC%CO & NOX

AIR / FUEL RATIO

Emission LevelsSeries II VHP

AFR NOx NMHC CO

Best Power(G/GSI engine out)

8.5 0.35 32

Best Economy

(G/GSI engine out)22 0.25 1.5

Catalyst (G/GSI exhaust out)

0.15 0.15 0.6

GL(G/GSI engine out)

1.5 1 2.65

Grams/BHP-hr