abnormal combustion in si engines
TRANSCRIPT
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