I'm studying ignitions at the moment, and I've come to wonder about high RPM power. It would seem that as RPM increases, the piston is increasing speed, while the flamefront remains at a constant speed. At a certain point, the pressure from the combustion is nullified by the piston moving faster than the flamefront can apply pressure to it, so the power curve could be reduced by this.

As octane decreases, the fuel mixture burns faster, so we have a faster-moving flamefront; i.e. the cylinder pressure increase occurs quicker. So it would seem to me - correct me if i'm wrong - but lower-octane gas would provide power at higher RPMs moreso than higher-octane gas. Even more so, diesel would be best since it burns the quickest.

Perhaps there could be some fuel crossover as RPM increases to get the most out of the powerband. Like say high-octane at low RPM, and switched to low-octane at a certain RPM above 6000. Any thoughts, or am I out of my mind?

 

High octane is to prevent knock/pinging/detonation from occuring. You don't want fast buring fuel with high cylinder pressure. And remember, at high rpm the engine have very low vacuum so it's hardly pulling in air. About diesel and gasoline, diesel have more BTU than gas, about 40k BTU more and burns slower.

 

Huh, this book I have says Diesel burns faster... So, what can be done to retain vacuum at high RPM? Besides turbocharging.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by lowdrive &raquo;</TD></TR><TR><TD CLASS="quote">As octane decreases, the fuel mixture burns faster, so we have a faster-moving flamefront; i.e. the cylinder pressure increase occurs quicker. So it would seem to me - correct me if i'm wrong - but lower-octane gas would provide power at higher RPMs moreso than higher-octane gas. Even more so, diesel would be best since it burns the quickest.</TD></TR></TABLE>That might make sense if there was some magical way to prevent the end-gas from autoignition. That's the mechanism for knocking... The end-gas is ignited before the flame-front gets there, resulting in detonation. Higher octane works to prevent that & slower flame front comes as part of the deal.

High vacuum happens a LOW load, so you don't want high vacuum. If you want to get high vacuum at high rpm, all you need to do is close the throttle...
The question about vacuum doesn't really make sense.

 

Water injection?

Yeah I still don't quite grasp all these concepts yet, but I'm trying.

 

I think what he meant was what can be done to maintain higher VELOCITY at high rpms, rather than vaccum....

 

RPMs do reduce the chance of detonation