Combustion speed limits

Hi all,

Contrary to popular belief the combustion event is a progressive burn, not an explosion. In the realm of high performance, it is generally accepted that a fast burning chamber is a positive thing. There are many contributing variables to list, but one of the more tangible manifestations of a fast burn chamber is needing little ignition advance to achieve MBT. I have also heard that fast burning chambers are tolerant of more cylinder pressure because they can "outrun" the onset of detonation. My question is what is the limit (if there is one) on how fast we can burn the mixture? I know different fuels have different burn characteristics, so the scope of my question is focused primarily on gasoline for the sake of simplicity.

Thanks in advance for your insights.

When I was trying to figure out the exact flame speed for gasoline I could not find that data very easy. General rule is that alcohol fuels burn faster than gasoline and that is pretty much it. However, I found out that sport high octane rating blends of gasoline fuels (VP Q16, VP Import) were made in such way, that their burn rate is the same with alcohol fuels although no concrete figures were given.

Anyways I attached two pictures to give you an idea...

PS fast flame speed is also very good for very high revs engine where the piston moves away from TDC very fast and faster flame speed allows to keep more pressure in the cylinder.

I agree that a higher combustion speed is usually preferable for all the reasons you've listed. This will be influenced by the properties of the fuel as well as the combustion chamber (and likely piston crown) design. As for a limit, it's not something I've given much thought to since it's going to be hard for us to influence dramatically beyond mixing your own fuels. Since I'm only ever tuning on commercially available fuels, for me it's basically a case of 'it is what it is'. If we take things to extremes however, detonation is essentially a case of the fuel releasing all it's energy almost instantly and of course the effects of this can be catastrophic. On this basis we can say that is too fast. So I guess at that point what we want is a steady and fast rise in cylinder pressure after combustion is initiated, but not an instantaneous jump as we'd see with knock. Sorry beyond that (which I assume you've already figured out, I can't add much more.

A common factor I've noticed on well-known slow burning chambers is lack of mixture motion before the combustion phase of the 4-stroke cycle. Things like the Yamaha 5 valve and earlier Hemispherical heads come to mind. What becomes obvious when you look at these chambers is the lack of a quench area, which has historically been very effective at increasing effective knock resistance (by increasing the burn speed and the resulting benefits mentioned above). Curiously, the M139 Mercedes chamber (which came on to my radar because of the podcast, thanks Andre) does not have quench pads, but is one of the most knock resistant chambers out there right now (is it because of burn speed?). My current theory is that they create very aggressive tumble in the port just before the mixture goes into the chamber. Using tumble inducing shapes in the port (in my rather limited research on the topic) seems to reduce the port energy and cylinder filling, but as the Ilmor engineer said in the podcast, they just compensate for that by increasing the air density with a few bar of boost. I've attached a picture of the chamber for your reference. Let me know your thoughts.

As you say, the quality of the mixture - how well it's mixed - is a critical factor, whether it's using "quench", port and seat design, or both.

With DI petrol/gasoline engines, and diesel engines, there is a further method which is how the incomming fuel is directed to the piston crown, to break up the fuel.