Abnormal Combustion in SI Engines ME419 Thermal Systems Design

D. Abata

Normal Flame Progression

1100 r/m X 360 deg/r X 1/60 m/s X 1/1000000 s/μs = 0.0066 deg/μs

Abnormal Combustion

Effects of Engine ‘knock’

lowers engine efficiency

causes some (minor) damage to piston surface

(erosion) and cylinder walls (scuffing) over long

periods of time.

affects the oil surface layer

increases heat transfer

Precursors to knock

1. Time

2. Temperature

3. Density

Fuel Composition ◦ Fuel

◦ Fuel oxygen ratio

◦ Turbulence

◦ Other Inert gases

Deposits, lubrication oil, catalysts

Walls

Gradients in temperature

Spark plug placement, heat range

Intake and exhaust valve design

History

Knock was a significant problem with high power reciprocating engines

◦ 1940’s - aircraft rotary configuration engines

◦ 1960’s – ‘muscle’ cars

◦ 1970’s - emissions and fuel quality take priority

Today

Knock is less of a problem in modern engines

◦ Emissions has taken priority

◦ Compression ratios have decreased

◦ Fuels have improved

◦ Engine combustion chamber design has improved

◦ Electronic control (knock sensors) has reduced or

eliminated the problem (Note: shifting (retarding) the

spark lowers engine efficiency and increases exhaust

temperature)

Fuel Chemistry

1. Paraffins (alkanes) a. Increasing chain length b. Compacting the molecule with side

chains c. Adding methyl groups

2. Olefins (alkenes) a. Adding double bonds

3. Napthenes and aromatics a. Aromatics preferred over

napthenes b. Adding double bonds c. Lengthening attached chains d. Adding small branches

Chemical Kinetics

Modeling Knock

normalized radius

location of auto-ignition

at spark plug

in end gas region