Tdc

TDC

Knock, knock

Kevin Cameron

ALTHOUGH THE PHENOMENON OF ENgine knock was identified and described as long ago as 1918, there is still confusion about it.

Normal combustion is not an explosion; it is a smooth, steady process of burning in which a flame front expands from the sparkplug at a combustion velocity of tens of feet per second, until the entire charge has been burned. The rate of pressure rise during normal full-throttle combustion is along the order of 30 to 40 psi per degree of crank rotation. Engine parts experience this pressure rise as a firm push rather than as an impact.

Engine knock is actually the onset of supersonic combustion-called detonation-in the cylinder, after the spark has ignited the mixture. Rates of pressure-rise in detonation are extremely high, and can damage parts just like hitting them with hammers. The explosive pressure wave of detonation strikes the inside walls of the combustion chamber so sharply that it sounds like a metallic impact. Hence the rattling, pinging sound. Shock waves hitting the combustion-chamber walls also scour away the normally present boundary layer of stagnant gas molecules. Robbed of the insulating effect of this boundary layer, heat flow from combustion gas to chamber walls increases, and engine-coolant temperature rises. Because more heat is now flowing into the coolant, less is going out the exhaust, so exhaust-gas temperature falls.

Supersonic combustion can arise in the cylinder in either of two ways. First, as normal combustion proceeds, the unburned part of the charge ahead of the flame front is both compressed and heated. This induces so-called pre-flame reactions in the unburned charge; fuel molecules, vibrating violently from thermal collisions, break into simpler pieces. Where normal fuel molecules are quite stable (we ride around with tanks full of them), these fragments are extremely unstable. As cylinder temperature and pressure rise during combustion, these pre-flame reactions in the unburned charge may proceed far enough for it to ignite spontaneously.

Detonating combustion can also arise from excessive turbulence in the cylinder, leading to a runaway flame speed. This is the “entrainment theory” of detonation. Some level of charge turbulence is essential to fast, normal combustion, but excessive turbulence can also cause detonation.

Fortunately, we have defenses against detonation. One is to select fuel molecules that resist pre-flame reactions better. Instead of long straight-chain molecules that break up easily, we pick shorter, stabler, branched chains or ring structures. These stabler molecules are characteristic of knock-resistant, or high-octane, gasolines. A second defense is to use anti-knock additives. The bestknown of these, tetra-ethyl lead, is no longer legal for use in motor gasoline, but there are others still in use. When heated, these release heavy metal atoms whose electric fields immobilize unstable fuel-molecule fragments long enough for normal combustion to be completed without knock.

Detonation is time-dependent. The longer combustion takes, the longer we hold unburned mixture at high temperature and pressure, the more likely it is to detonate. This is why engines knock when lugging; at low rpm and high throttle, there’s plenty of time for detonation to occur, and plenty of heat and pressure to encourage it.

Fast burn is another weapon against detonation. The smaller the chamber, the quicker a normal flame can reach all parts of it, preventing pre-flame reactions from going too far. Turbulence also speeds combustion. We get it from (1) the velocity of the fresh charge, rushing into the cylinder and (2) squish-the provision of regions of the piston that closely approach the head. Charge caught in such areas is squirted out near TDC, producing supplementary turbulence. A fastburn chamber can run knock-free at a higher compression ratio than can a slower-burning design.

High compression and excessive spark advance encourage detonation by subjecting the unburned charge to higher pressure, for a longer time period. A fuel’s octane number tells us its resistance to knock, not its energy content. All motor gasolines have closelyequivalent energy content per pound. A gasoline stove boils a quart of water no faster burning high-octane fuel. The value of high-octane fuels is that they can tolerate higher compression ratios-which in turn extract more of the fuel’s energy as mechanical work.

Pre-ignition is often confused with detonation. Where detonation occurs after the spark, pre-ignition is lighting-off of the charge before the spark. The cause is hot, glowing bodies in the combustion chamber. One common cause of pre-ignition is use of too hot a sparkplug heat range; the center-wire and ground-wire of the plug, being thin, run too hot-hot enough to act as an ignition source. In supercharged engines that run continuously at heavy throttle, the exhaust valve itself can run hot enough to cause pre-ignition. Or, in highmileage engines, combustion-chamber deposits can themselves be made to glow red-hot.

Because pre-ignition is equivalent to operation with very advanced ignition timing, it usually leads shortly to detonation as well. When the cause of engine damage is detonation, the sparkplug electrodes are usually undamaged. In two-stroke engines, detonation damage is typically seen at the outer edges of the squish band. The resulting piston overheating leads to seizure. When an overheated sparkplug is the cause, the electrodes are usually blown away, and that region of the piston nearest the plug is cratered.

For engines, detonation is a lifeand-death matter. Without it, they may run 100,000 miles or more. With heavy detonation, they may not last 10 seconds. □