I learned a long time ago that getting things done wasn’t about finding new blocks of time but using the available time to best advantage. This week I discovered that the laundromat is a great place to learn about spark advance and incipient detonation while watching my work clothes go ’round and ’round.
Detonation, “knock” or “spark knock” is the near constant volume combustion of combustion end gasses within the cylinder of an internal combustion engine. It is a primary performance limiting factor of internal combustion engines because it can cause life-limiting damage to engine components, especially pistons. Pushing back the constraints of detonation is an important part of engine development, especially for a racing engine.
While at the laundromat I learned is that the octane requirement of an engine is related to it’s speed and the notion that a high speed engine is an engine which requires high octane fuel is patently false. Actually, the opposite is true.
Combustion end gas temperature, pressure and end gas soak time at a given temperature and pressure are the primary predictors of detonation. As the soak time is reduced, the onset of detonation is reduced. Therefore, as engine speed increases the octane requirements of the engine are reduced, all other factors being constant.
That explains why so many early spark ignition engine designs were knock limited at low levels of spark advance and Brake Mean Effective Pressure (BMEP). The engines operated primarily at low speeds, providing a long soak time, were air-cooled promoting high cylinder head and charge temperatures and burned low octane fuels which detonated more readily. The resulting engines had low Knock LImited Mean Effective Pressure (KLIMEP) and thus low power output
It also explains the modern trend towards very high speed engines in sport bikes. A liquid-cooled high-speed engine can generate much higher KLIMEP than a low-speed engine because the end gas soak time is much shorter. What does all this mean? A small high-speed engine can have a higher displacement specific power output than a larger engine because it produces more combustion events in a given time at a higher mean effective pressure. The pervasive notion that “There’s no replacement for displacement” has long been old hat.
I’m already looking forward to building a research engine and studying some of these variables. A basic design for that engine is beginning to swim around in my head and will need to be sketched out soon.
Finally for today, here’s a little on-topic levity. I suspect the laundromat owner wouldn’t find it so funny though: