Detonation and preignition are your engine's trouble twins
Imaginations run amok. Mention the term detonation and the average maintenance-aware pilot will conjure truly perilous conditions inside the cylinders. Searing heat burns the oil film from the heads and barrels. Massive pressure waves pound the now-molten piston dome and twist the connecting rods like Slim Jims in a teenager's grasp. Cylinder head temperatures skyrocket as the engine loses power. Complete destruction of the reciprocating members, followed by shards of steel and aluminum grinding into bearings and tearing through the rest of the otherwise undamaged engine, is only the visualized outcome.
So much for imagination. While it's true that protracted operation with heavy detonation will damage the engine, from talking to pilots and mechanics it seems that the foreboding associated with detonation and its cousin, preignition, is overblown. Simply put, neither happens very often in properly maintained airplanes. But, because these anomalies are often mentioned in defect reports filed by mechanics — and pilots very often take the blame for its occurring — it helps to understand the mechanism.
In the normal cycle of things, the combustible mixture burns in an orderly fashion from the point of ignition outward until all of the fuel/air mixture is consumed. In this scenario, the temperature and pressure in the cylinder rise smoothly to a peak of between 500 and 800 psi. Detonation occurs when a pocket of the precombusted fuel/air mixture ignites ahead of the normal flame fronts. These fronts are started by the spark plugs; in aircraft, this timing point is fixed by the magneto orientation. Keep in mind that detonation occurs after the spark event as a result of so-called "end gases" spontaneously igniting from the heat of compression created by the already-burning gas. This spontaneous combustion creates dramatically greater pressures in the combustion chamber; lab tests suggest that the figure is close to 2,800 psi. In automobiles, detonation results in audible knocking or pinging, but because of the high noise environment in aircraft, it's unlikely that you'll ever hear detonation taking place.
In and of itself, detonation (particularly mild, or incipient, detonation) is not likely to do much damage. It can lead to ablation of the oil film in the cylinders and squeezing out of the oil in the main bearings. Even though the theoretical pressure rise is some 3.5 times the nondetonating event, detonation in test engines does not seem to inflict serious mechanical damage directly. However, detonation does lead to a snowball effect in cylinder integrity by increasing the transfer of heat from the combustion chamber to the heads. Normally, there's a boundary layer of cooler air in contact with the cylinder head and walls, as well as the piston head. When detonation occurs, the high pressures present allow the flame front to penetrate this boundary layer, greatly increasing conduction of heat to the head and piston. And, as the head temps rise, the structural integrity of the aluminum heads and pistons goes way down.
Perhaps more treacherous than detonation itself is something called preignition. When detonation continues in sufficient measure to radically raise the temperature of the combustion chamber, spark plug tips and carbon deposits can begin to act like spark plugs, setting off the fuel/air charge well in advance of the nominal ignition timing. This is where the real mechanical mayhem takes place. With the piston still rising on the compression stroke, preignition can light off the charge and work forcefully against the piston and reciprocating mass, jackknifing connecting rods and deforming crankshafts.
Several factors influence a given engine's proclivity toward detonation and preignition. First is compression ratio; simply put, higher compression ratios require fuel that is more resistant to spontaneous combustion. Next on the list — and at the head of the line for maintenance-related issues — is ignition timing. Depending upon the engine, magnetos advanced as little as five degrees more than specified might do nothing at all, or the miscue might set the engine up for serious trouble.
Inlet-air temperature and pressure are also primary among the reasons for detonation — watch the use of carb heat on takeoff and keep tabs on induction-air temperature in turbo airplanes. Higher temperatures increase the heat in the end gases, promoting volatility — according to the seminal text on internal-combustion engines, Charles Fayette Taylor's The Internal-Combustion Engine In Theory and Practice. Lab tests indicated an eightfold increase in reaction time of the end gases when the inlet temperature was increased from 150 degrees Fahrenheit to 225 degrees. Similarly, higher intake pressures increase the likelihood of detonation. Cylinder-head temperature is also among the big influences of detonation; the higher the temperature, the more likely is detonation. Finally, the fuel/air ratio enters the picture, with detonation most likely to occur at the chemically correct ratio, or about peak EGT.
So, the theory's fine. What's in it for me, the pilot? According to a five-year listing of the FAA's service difficulty report database — an admittedly uneven source for such data, but the only one we've got — there were far more cases of upper-end distress stemming from cylinders that separate at the head and barrel than from cases where detonation and preignition are clearly indicated. Does this mean that detonation isn't a part of the head separations? No, but it shares the rostrum with dissimilar-metals corrosion and plain old age.
We did find one case that speaks to the cumulative effects of temperature, advanced timing, and detonation. The pilot of an airplane powered by a TIO-540 Lycoming (rated at 270 horsepower using parallel-valve cylinder assemblies) felt a roughness at 16,000 feet and landed safely. Upon inspection, the number-two cylinder had no compression — this was because of a 3/4-inch-long hole in the cylinder head adjacent to the exhaust port. Another cylinder was cracked, too. Maintenance personnel found the magnetos advanced by 3 and 6 degrees over spec. This particular installation tends to run hot, and pilots generally push this engine to high specific power settings leaned to peak turbine-inlet temperature. Did the mistimed magnetos cause detonation and engine damage? Probably. That this installation does not have epidemic detonation events suggests that the advanced timing eroded the detonation margin to nil.
How do you know if you've got detonation or preignition? First of all, those of you with normally aspirated engines can settle back into the armchair. The historic record of detonation in non-turbo applications suggests that it happens very rarely except when some combination of factors is involved — like poor baffling leading to high CHT; inadequate fuel flow during high-power (read: takeoff) operations; advanced ignition timing; poor turbo or intercooler performance leading to high induction temperatures; or poor ignition harness conditions that lead to cross-firing. A plugged fuel injector or intake leak can drive a cylinder toward detonation at high power. All of this is in addition to the chance that the fuel is incorrect or tainted.
For turbo owners, there's a bit more to be watchful of. High temperatures coupled with aggressive leaning (or insufficiently aggressive leaning) can knife into an already shallower detonation margin. High-altitude operation exacerbates the problem — cylinder heads run hotter and the induction air is likely to be toasty in the flight levels — so pay strict attention to high-power fuel flows and baffling condition.
Unless you have all-cylinder engine monitoring, it's unlikely that you'll notice detonation occurring because it's not a given that detonation will take place on all cylinders. (Preignition's another matter — the roughness and loss of power are unmistakable.) Typically, during a serious detonation event, the CHT will rise dramatically and individual EGTs may fall; performance will also suffer. By far the best initial reaction to apparent detonation is to reduce power with the throttle — for most engines the detonation margin at 65 percent is much greater than at 75 percent or more. Increase prop rpm next. This will, given the fixed timing of magnetos, effectively retard the timing. Naturally, if you can't restore normal readings on the analyzer, consider finding a place to land so that trouble-shooting can take place in the comfort of a hangar.
If you suspect that you've had a detonation event — or want to see if one's already taken place — there are easy inspections. Check the spark plugs. Plugs that are white — not tan — can indicate a detonation event, but don't stop there and assume you've had trouble. Borescope the cylinder and look for evidence that the chamber or piston dome has been washed clean of its normal carbon deposits. A sparkling-clean chamber is cause for concern. Also look at the valves for signs of overheating or mechanical distress. If nothing else, this inspection will keep your imagination from running away with you.
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