Category Archives: Current Reading

DETONATION AT THE LAUNDROMAT

laundromatstudying

Studying at the laundromat. A new favorite study spot.

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:

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CURRENT READING: “THE INTERNAL COMBUSTION ENGINE IN THEORY AND PRACTICE” VOL. 2

For someone planning to build an entirely new racing engine from scratch, it isn’t until you pick up a book like Taylor’s “The Internal Combustion Engine in Theory and Practice” that you realize just how much you’ve bitten off.  I purchased both volumes about 6 years ago during a fit of boredom while on a road trip out west.  I was actually amazed when I found both volumes in a small Borders (remember them?) in California.  It’s true!   I get bored on vacations and end up in book stores…

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Charles Fayette Taylor’s “The Internal Combustion Engine in Design and Practice” Volume-2

The two volumes can be purchased from Amazon.com for about $60 each.  So far, they’re the best value I’ve found for the amount of knowledge and data they contain.  I can say even at this early stage that my volumes will become tattered and broken before this is all over.

Speaking as a visual thinker, I’ve found both volumes follow the same format:  Concepts outlined in the text are almost always followed up with charts, illustrations and photographs.  This makes the information so much easier to digest as I can’t remember a jumble of words, but charts and illustrations become etched into memory.

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A page from Charles Taylor’s “The Internal Combustion Engine in Theory and Practice” Volume-2

As an aircraft mechanic and machinist who has been working with internal combustion engines my whole life, I’ve had a number of “ahah!” moments browsing through both volumes.  Once I started to delve into Volume 2 last week, it was like opening a book of answers.  Many things I’ve observed in the design, running and operational qualities of engines and the questions they posed have been answered and I’m not yet a quarter of the way through.

A full report will follow once I’ve had a chance to internalize all 800 pages.  The quest for knowledge is so much fun!

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