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For those who’re nerdy sufficient to note, you see that typically, compression ratios on air-cooled engines (particularly if they’ve massive bore) are usually decrease than on liquid-cooled engines, particularly these with reasonable to small bores.
The upper the compression ratio you possibly can safely run in an engine, the higher the torque it would produce. Why? As a result of as a rule of thumb, peak combustion stress is 100 instances the compression ratio. Engine torque outcomes from that combustion stress urgent your pistons down to show the crankshaft.
What units the restrict right here is the diploma of heating the recent fuel-air cost suffers because it enters the cylinder and is heated by contact with it, then by compression, and eventually, as yet-unburned combination out close to the cylinder wall is compressed and heated by the increasing combustion fuel.
If that unburned combination out on the cylinder wall—which the engineers name the “end-gas”—is sufficiently heated earlier than the flame can attain it, bits of it might autoignite, burning at or above the pace of sound in a phenomenon known as “detonation” or engine knock. The knocking sound we hear throughout detonation is sonic shock waves, hitting the steel surfaces of the combustion chamber.
Compression ratio needs to be set low sufficient that even on the most popular day, with the worst gasoline you might be prone to discover on the pump, your engine won’t detonate. Larger compressions heats the fuel-air combination extra and produces larger peak stress, so the upper the compression ratio, the extra probably detonation turns into.
As a result of it’s more durable to chill properly and constantly with air (summer time air? winter air?), head temperature of air-cooled engines tends to be larger than in a liquid-cooled engine, so fuel-air combination will get heated extra, probably resulting in detonation.
Regular detonation (versus the “occasional tinkle”) is damaging, inflicting overheating, piston warmth softening, and eventually, erosion of piston steel.
As a result of typically, the bigger the cylinder bore the longer it takes to finish combustion, detonation is extra probably in big-bore engines, which expose their combination to warmth longer. Because of this air-cooled engines and plenty of engines with massive bore are given decrease compression ratios than liquid-cooled and/or smaller-bore engines.
There could be exceptions. I spent the summer time of 1963 in Denver, Colorado, the place I labored in a motorcycle store. One in all our clients was a really severe evening warrior who had constructed an air-cooled Triumph 650 twin with a sky-high 12-to-1 compression ratio (that’s not sky excessive in the present day, when each sportbike engine has 13:1, however in 1963 it was loopy excessive).
Why didn’t his engine detonate itself into aluminum gravel? His mom had a again shed, and in it he stored a secret 55-gallon drum of purple aviation 115/145 gasoline. It was wealthy in extremely knock-resistant alkylates and contained 6 grams per gallon of the highly effective anti-knock compound tetraethyl lead. That was the gas that powered many supercharged and air-cooled World Warfare II plane engines, all of which had large bores between 5-3/4 and 6-1/8 inches (146 to 155.6mm).
On this glorious however now unavailable gas his Triumph didn’t knock, however left the competitors for lifeless. Drag racers in the present day pay $60 a gallon for racing gasoline having related knock resistance.
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