In a spark-ignition internal-combustion piston engine, the fuel-air mixture normally burns like a forest fire: The heat of the advancing flame front raises the temperature of the unburned material ahead of it enough to ignite it. Engineers and chemists call this kind of burning deflagration.
In abnormal circumstances, parts of the unburned fuel-air mixture can be chemically changed by heat such that an abnormal and violent form of combustion—detonation—can occur within it. In detonation, combustion takes place in a very thin zone that is just a few molecules thick, and it is driven by a step in pressure occurring in that zone, which is a shock wave. As this high-speed wave advances, the compression that occurs within it is vigorous enough to initiate combustion, no progressive heating required. This is a detonation wave.
Power in a piston engine is delivered by the expansion of high-pressure combustion gas against the piston. For maximum power, we want combustion to reach peak pressure just as the piston significantly accelerates downward on its power stroke. Reaching peak pressure sooner than this wastes power through heat loss that occurs the longer the hot, high-pressure gas is confined between piston and cylinder head. Reaching peak pressure later reduces peak pressure itself because the piston, having moved farther, decreases the pressure above it by expansion.
As a rule of thumb, combustion in a piston engine is pretty much centered on TDC. If for best torque we need to set the ignition to spark at 35 degrees BTDC, then combustion continues for roughly the same time after TDC, making the whole combustion event about 70 degrees. At 10,000 rpm, this is roughly 0.001 second. Although people casually refer to combustion in engines as “an explosion,” it is not. It is smooth progressive burning at a low speed of 50 to 150 feet per second. By contrast, dynamite reacts at 8,800 fps and more modern explosives such as PBX or HMX combust at nearly 30,000 fps.
The slower combustion is, the earlier we have to set the spark timing to achieve peak pressure just as the piston begins significant movement. That creates two problems. One is that the longer we confine hot, high-pressure gases between piston and head, the more heat we will lose from those gases and the less net power can reach the crankshaft (the missing energy goes into the engine’s cooling system). The second point is that although compression ratio is the ratio of the volume above the piston at BDC to the volume above the piston at TDC, in real engines much of the fuel-air charge is burned while the piston is fairly far from TDC, either before or after.
Let’s see what this means in numbers. If our engine with 12:1 compression needs ignition at 35 degrees BTDC, then mixture that burns right at the end of combustion is burning not at 12:1 but at 6:1. If, like Yamaha’s five-valve FZR750, our engine needs ignition at 45 degrees BTDC, then mixture burning at the end of combustion (when the piston has descended through 18 percent of its stroke) is burning at an even lower compression ratio of 4.7:1. And, in the extreme case of certain racing engines needing ignition at 60 degrees BTDC, then the tail end of combustion is acting at a very low 3.6:1 compression ratio (because at 60 ATDC the piston has already moved through 30 percent of its power stroke).
Why do we care at what compression ratio the mixture burns? We care because the rule of thumb for estimating peak pressure is 100 times the compression ratio. You bet we want higher combustion pressure because that’s what exerts force through the pistons to turn the crankshaft. But as we see from the above, the slower our combustion is, the more fuel-air mixture is burned at lower compression ratios because the piston has already moved a considerable distance into its power stroke. Mixture burned at TDC produces maximum pressure, but the later it burns the farther the piston has moved and the lower the pressure contribution of that late-burning mixture.
These are the reasons why Keith Duckworth’s DFV Formula 1 engine of 1967 was able to give more power and accelerate harder than competing engines with more cylinders and more valve area operating at higher rpm. While the slow combustion in competitors’ engines forced them to set their ignition at 45 or more degrees BTDC, Duckworth’s DFV needed only 27 BTDC at peak torque because its combustion was so rapid. Because it confined its hot combustion gas for a shorter time before taking power out of it by expansion (the piston’s power stroke), the DFV suffered less heat loss from its combustion gas, leaving a higher pressure to drive its eight pistons. Because more of its fuel-air mixture was burned closer to maximum compression at TDC, the pressure initially generated by combustion was higher.
How did Duckworth achieve such fast combustion? He saw that the tall piston domes used by other builders to achieve high compression in deep combustion chambers obstructed rapid flame propagation. Therefore he created a quite shallow combustion chamber above an essentially flat-topped piston. Between them was an open volume in which turbulent charge motion could produce very fast combustion. Fast combustion, by reducing the time available for heat loss, sent more power to the pistons and less into the cooling and exhaust systems. The result was greater power from a given amount of fuel burned. That is greater efficiency.
Why not speed up combustion even more? There are mechanical limits to how much we can safely accelerate combustion. Harry Ricardo found that a rate of pressure rise greater than 40 psi per crank degree made engines feel unpleasantly rough. Even faster combustion, like that known as detonation, can cause rapid failure of parts such as pistons and crankshaft bearings.
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