QUESTION: There’s a warning sticker on the Kawasaki Concours C14: “Use minimum of 90-plus octane gasoline to prevent severe engine damage.” Are knock sensors (of any type) impracticable on a motorcycle engine? Robert Bodenhamer Woodstock, GA ANSWER: Yes indeed, detonation sensors can be and are used on motorcycle engines. Honda used a detonation counter on its NSR500 two-stroke Grand Prix bikes beginning in the 1990s. This system not only detected that detonation was occurring, but counted the number of detonating cycles in relation to throttle opening and rpm. Detonation and pre-ignition are not the same. Detonation occurs after the spark has ignited the fuel-air mixture, not before. As the flame front moves away from the spark plug at 50 to 250 feet per second, it heats the unburned mixture ahead of it. This heat drives so-called pre-flame reactions in the mixture, which generate an increasing population of highly reactive molecular fragments, called radicals. Gradually, this population converts the remaining unburned mixture into a sensitive explosive. If the unburned mixture reaches a particular temperature threshold, it can (1) auto-ignite, and (2) burn not at its normal rate (see above) but at or above the local speed of sound. This generates shock waves that make the knocking sound of detonation, drives more heat into head and piston, and ultimately knocks heat-softened metal out of those parts. Light detonation gives the very edge of the piston a sandblasted look, while heavy detonation can blast away so much piston metal that the rings are exposed and combustion gas enters the crankcase. In pre-ignition, the obvious symptom is a hole through the center of the piston dome. In detonation, the effects begin at the outer edges of the piston. 
Detonation detectors are basically very rugged microphones or accelerometers, from whose signals the presence of detonation may be inferred. In one implementation, the detector consists of a diaphragm, tuned to the engine’s range of detonation frequencies. When detonation occurs at one point, its pressure wave bounces rapidly from one side of the cylinder to the other, so the frequency detected is the sound velocity in the hot combustion chamber, divided by twice the cylinder bore. In a non-detonating engine, the detector diaphragm vibrates very little, but when detonation begins, the diaphragm vibrates in resonance with it, emitting a strong signal. If the engine ECU has a detonation control system, this strong signal triggers ignition retard until detonation ceases. Then the ignition timing is again advanced in a continuing cycle that keeps timing just short of values that cause detonation. In a variant of this, no tuned diaphragm is employed but the ECU filters the signal to measure the sound energy in the detonation range. An accelerometer system essentially uses the cylinder head as its vibrating diaphragm. Before 1975, detonation was controlled mainly by adding chemical anti-detonation elements to the fuel in small amounts. The horribly poisonous tetraethyl lead (TEL), added in quantities of one to four grams per gallon of gasoline, suppressed detonation by acting as a negative rate catalyst against the production of the radical OH-. Tetraethyl lead was of immense value during World War 2 because it enabled Allied aircraft piston engines to safely run increased supercharger boost, thereby producing more power. Because TEL poisons emissions-control exhaust catalysts, a phase-down of its use began in 1975. Today, more selective refining and use of synthesized fuel components are used to achieve today’s reduced level of gasoline knock resistance. Today's (R + M)/2 = 93-octane pump fuel is inferior in knock rating to the fuel adopted as standard by the US Army Air Corps in 1935. A higher octane number does not in itself boost power. Increased power is given only when such a fuel is combined with an increase in compression ratio. Send your “Ask Kevin” questions to cwservice@cycleworld.com. We cannot guarantee a reply to every inquiry.Gadget Reviews: mamaktalk.com
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