olefud wrote:
I don’t have actual four-cycle experience in the 12,000-18,000 RPM range so it’s possible that the differences are in kind rather than degree. If the combustion kinetics are such that the fuel mixture hasn’t fully burned by Strad’s 90 degree point, detonating it could be useful if not optimum. Given that the charge would be expanded to half the cylinder displacement at this point (offset gudgeon pins aside), it would be difficult to generate detonation. And prior to this point there’s still time to burn the charge in a more efficient manner. IMO of course.
With relatively low RPM, boosting –which enhances flame propagation- and rule room to configure the combustion chamber AFAIK, I would be most surprised to see planned detonation.
Planned detonation may also be a once necessary expedient.
ideally spark ignition combustion adds heat at a rate related to expansion of combustion chamber volume with piston movement such that the pressure stays only just below the level that would produce detonation
heat added at lower pressure is (relatively) wasted
so combustion speed is critical, and ideally combustion should add all its heat long before the 90deg point
if SI cannot do this, then in principle a safe amount of detonation at a safe time would add the remaining heat early enough to avoid the above waste
the whole basis of ignition timing is to eliminate detonation (this is done empirically)
(it's because detonation has a (relatively constant) delay time the ignition timing must vary with rpm)
at very high rpm the delay gives less scope to detonation, so a semi-safe level of detonation can occur naturally via ign timing
F1 etc engines were showing this exactly 50 years ago, an unprecedented 10000+ rpm was driving timing tradeoff between reliability and power (semi-continuous low-level detonation was later shown in lab investigation, BRM won the WC this way)
with 2014 F1 the (direct) injection is capable of any injection-rate strategy to facilitate maximising the heat rate relative to volume/piston position ie to avoid detonation only minimally and in combination maximise work recovered from all heat
(DI always gained CR or boost etc due to detonation delay starting only on injection - nothing new)
(if combustion rate is still non-ideal, injection rate could give/manage later detonation (SCCI) of the remaining fuel mix ?)
this strategy (controlled detonation) might benefit current F1 engines (ie if they had 2014 injection)
(also the 24000 rpm engines that we would have without the freeze on bore & stroke ?)
