- Excessive cylinder
temperatures cause detonation and pre-ignition.
- Maximum power amd economy
is achieved just prior to detonation.
- Detonation is uncontrolled
combustion after the spark plug has fired.
- Pre-ignition is
uncontrolled combustion before the spark plug has fired.
Most engines perform best
when cylinder pressure peaks at 12-14deg after TDC and performance is usually
acceptable with a pressure peak as late as 18-20deg after TDC. Piston
acceleration away from TDC is relatively slow, so at the time when combustion
pressure is peaking, the combustion space is not rapidly increasing in volume.
Because of this, pressure holds at a high level and gives the crankshaft a good
shove, making high power.
when the charge temp is excessive, detonation occurs and a violent explosion
occurs instead of a steady, progressive burn. Obviously this can lead to engine
destruction and spark retard is often used to protect against this. But spark retard in itself can cause engine destruction so a good understanding
of detonation and spark retard is essential for any engine tuner.
The positive effects of spark retard as follows…
The fuel burn is started as late as possible to ensure the cylinder peak
pressure is reached well past 20deg after TDC. At the time peak burn pressure is
reached, the piston has begun fast acceleration, thereby enlarging the
combustion space. Cylinder pressure quickly tapers off as the fire chases the
piston down the cylinder, which wards off harmful detonation – but also reduces
combustion slows so much that as the exhaust valves open, the fire is still
going. This increases exhaust valve temps and the exhaust side of the chamber
becomes hotter along with the piston crown.
After this the inlet port opens and
allows fresh fuel and air to enter but all the added heat affects the air
charge, and it quickly expands, filling the chamber with low density air and
leads to lost horsepower at best.
At worst, combustion continues on the down stroke and the piston crown, exhaust
valve, and chamber have insufficient time to cool and soon reach dangerous temps
and causes combustion occurs long before the plugs get fired. The intense heat
generated by this pre-ignition situation acts like a blow torch and melts
through the crown of the piston.