[cycleworld.com] - Knock, Detonation, and Ping

[Ninjette Newsbot]

Detonation is no longer an issue in modern motorcycles, but in the earliest days of motorcycling the abnormal gasoline combustion was not easily understood.

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Kevin Cameron has been writing about motorcycles for nearly 50 years, first for Cycle magazine and, since 1992, for Cycle World. (Robert Martin/)Today the chemistry of how the abnormal gasoline combustion called detonation occurs is well understood, but thanks to modern sensors and capable engine ECUs it is no longer a problem in late-model motorcycle engines.

Yet only a couple of weeks ago a person commenting on a story on the Cycle World site described a problem that was almost certainly caused by deto. This rider revealed use of higher-than-stock-compression pistons and a preference for 87 (R+M)/2 gasoline, also revealing that one or two spark plugs had suffered broken porcelain insulators.

This is classic stuff! One common result of detonation is sandblast-like erosion of metal from piston edges, but another is cracked or broken plug insulators as a result of the impact of the sonic shocks generated by deto.



In the earliest days of motoring, back before World War I, detonation was not understood, save for the fact that somehow, hot objects in the combustion chamber (the exhaust valve, the hot center of the piston crown) were associated with audible knock and engine damage. Accordingly, pistons were made smaller to bring their dome centers closer to cooler cylinder walls, which resulted in most engines of the period being given great long strokes and itty-bitty bores. The same idea was also applied to exhaust valves; two or more small exhaust valves, because of their shorter heat paths, ran cooler than a single larger valve, so early engines were often given lots of valves in hope that this would protect them from knock.

It was also noticed that crude oil from different regions (California, Romania, the East Indies, Baku in Russia) produced gasolines that varied widely in their knock resistance. Hmm, must be something to do with chemistry…

After 1923 knock was controlled in three major ways:

1. Careful research based on engine knock testing with pure hydrocarbons revealed how to make more-knock-resistant fuels. Some, such as alkylates, would be synthesized in large volumes.
2. The discovery of the fuel additive tetraethyl lead (TEL) which could, within limits, suppress detonation chemically.
3. As engine power was increased, cooling had to improve in direct proportion.

All three of the above combined to make possible the high-power aircraft piston engines of WWII. For example, the air-cooled Wright R-3350 engines of the B-29 bomber would have been* useless without the 6cc of TEL added to each gallon of 115/145 “purple gas” provided for them.

Today TEL, which deactivates catalytic converters and is highly poisonous to people, is no longer legal in pump gasolines. That has required new technologies for suppressing detonation:

1. Affordable and highly productive methods of producing knock-resistant fuel components continue to be developed.
2. Because the heat-driven chemical changes that lead to detonation take time to occur, a major theme of modern design has been speeding up combustion by adopting the fast-burning flat four-valve “tumble” combustion chamber developed by the late Keith Duckworth. An example often cited is Cadillac’s Northstar line of auto engines, which cut the time required from ignition spark to peak combustion pressure by 20 percent. In effect, modern engines are being designed so that their combustion outruns detonation.
3. The development of affordable high-speed computing and sensors has made it possible to detect detonation and promptly retard the ignition timing to cause detonation to cease. Many modern engines thus carry cylinder-head-mounted accelerometers to detect knock, allowing the ECU to retard ignition timing enough to suppress it. If a bike equipped in this way happens to get a tankful of “bad” gas, the system will continue to provide protection until the next tank of normal fuel takes over that job.

Engines of large bore operating at lower rpm may be more vulnerable to knock, causing their manufacturers to mandate the use of 91 (R+M)/2 fuel.