Piston deposits as precursors of detonation in Lycoming and Continental aircraft engines

In 1994 Chevron sold avgas contaminated with jet fuel. Mechanics were requested to look inside cylinders and examine the tops of the pistons for evidence of detonation. This type of examination is not routinely done and it soon became clear that there were no guidelines what to look for.

(Since detonation causes preignition and preignition creates detonation, damage is often caused by both)

After spark ignition the burning fuel causes a pressure wave that travels outward from the plug gap. The surface of the pressure wave, which we shall call the flame front, expands outward typically 40 to 50 centimeters per second. The flame front is ragged and "fractal" as turbulence and swirls in the mixture disturbs the front. This turbulence increases the surface area of the flame front causing more unburned fuel to be in contact and thus increasing the flame front velocity and hence the speed at which the fuel burns. The faster the fuel burns, the faster the pressure rise in the combustion chamber. Typically a pressure rise of 20-30 psi per degree of crankshaft rotation.

But combustion is never complete. There remains an unburned boundary layer of air-fuel mixture insulating the metal components of the combustion chamber from the propagating flame front. This boundary layer is in thermal contact with the cool metal whose surface temperature is well below the ignition temperature of the fuel/air mixture and does not burn when the flame front passes over it. This boundary layer is roughly the same temperature as the metal below and acts as an insulating layer preventing direct contact of the metal to the flame. If the flame front touches the aluminum it melts. Your EGT temperature out the exhaust is up to 1600 degrees F. while the pouring temperature of aluminum is approximately 1380 deg. F.

Deposits from unburned or partially burnt fuel collect in the boundary layer especially in isolated pockets causing at times significant build-up of carbon deposits. Typical areas are between the seat and the cylinder wall and at the juncture of the combustion chamber and the cylinder wall. Turbulence, by speeding up the flame front, reduces the thickness of the boundary layer and reduces the buildup of combustion deposits. Turbulence increases efficiency as liquid fuel doesn't burn

Hemispherical combustion chamber (rounded) found in engine such as the Continental 470 and 520 angle-head engines produces fewer combustion chamber deposits. Notice stop spark plug points toward smaller exhaust valve. This prevents detonation by directing the flame front toward the hotter exhaust valve thereby burning the gases in the hot region first before the temperature and pressure rises.

Hemi heads have valves angled into the combustion chamber and use two separate rocker arm shafts. view photo

Standard combustion chambers, such as this Lycoming 235, produce greater amounts of combustion chamber deposits.

Standard combustion chambers have the valves going straight into the combustion chamber and use one rocker arm shaft.

see Combustion Chamber Cleaning of Lycoming aircraft Engines

Combustion chamber designs, such as the hemispherical type used in angle head Lycoming and Continental engines, force the fuel/air mixture to enter the combustion chamber in a swirling motion. Swirl:

Parallel head engines lack swirl and have greater carbon deposits (especially the Lycoming O-235). Thus one can check the relative combustion efficiency of different engines by the amount and distribution of carbon deposits. Engines that leave more carbon or irregular build-ups of carbon leave more unburned fuel in the combustion chamber than do combustion chambers that leave slight but uniform deposits. Pilot technique influences the amount of carbon build-ups as a rich mixture allows more deposits to form.

The amount and location of carbon deposits depends on may factors including combustion chamber design, fuel distribution, fuel fractional distillation in he intake tubes causing the separation of light and heavy ends to go to different cylinders, power settings, fuel type, oil consumption, to name a few. However, with each engine model a typical build-up occurs that is relatively consistent from one engine to another of the same model. This requires the examiner to observe normal deposit patterns in many engines so that abnormal patterns are identifiable.

Preceding each flame front is its sonic pressure wave. Colliding sonic pressure waves concentrate on the irregular shapes present (edges of pistons, valves, even the spark plug) or at the edges of the piston dome where reflecting pressure waves from the piston or combustion chamber walls can constructively recombine to yield localized high pressures and temperatures.

Detonation may start from the fuel pre-igniting from a hot spot in the combustion chamber. One reason why Continental points one spark plug toward the exhaust valve is to propagate the flame front across the hot exhaust valve early before the pressure (and temperature) in the combustion chamber has risen very far. Later in the combustion cycle when temperatures of the fuel become hotter and more likely to be ignited from a hot surface, only unburnt gasses remains in the area of the hot exhaust valve.

During detonation the almost instantaneous ignition of the fuel/air mixture causes such a rapid pressure wave that shock waves pound against the insides of the combustion chamber and piston. These shock waves produce the knocking sound in your automobile engine but are not heard in our more noisy aircraft engines. Shock waves that are strong enough to mechanically "ping" the walls of the combustion chamber are strong enough to sweep away any unburned boundary layer of fuel/air mixture near the metal surfaces of the combustion chamber.

Without a boundary layer protecting the aluminum piston, the surfaces are exposed to the combustion flame which melts through the piston. As we mentioned earlier, melting occurs on the edge of the piston next to the cylinder wall where pressure waves reflecting off the wall combine and amplify the pressure at specific locations. Sharp bends in the metal such as at the edge of the piston and along valve cut-outs in high compression pistons are difficult for boundary conditions to provide protection and are usually first damaged by detonation.

If the detonation is minor, or doesn't last long, the result is that carbon deposits in the area of boundary layer failure are burnt off the surfaces leaving bare aluminum. If the conditions that cause detonation are quickly terminated, such as a power change made by the pilot, little or no engine damage may occur.

In more severe or prolonged detonation, local temperatures melt the aluminum causing "termite holes" in the piston. With any engine type, burning of holes into or through the piston firsts require the destruction of the boundary layer and a burning off interceding carbon deposits. Evidence of boundary layer destruction by detonation is best early identified by the complete removal of carbon deposits in areas normally observed to contain carbon. This removal often is localized and close to the outside edge of the piston dome.

Detonation damage is not limited to burning of holes in pistons or the combustion chamber (typically between the valve seats). The rapid increase in pressures can over-load the rod bearings causing lubrication failure and quite possibly bearing failure. There could be other more hidden damage caused by detonation such as detuning of engine counterweights and resultant over stressing of the crankshaft or propeller.

Inspecting for Early Signs of Detonation

Inspect the spark plug white ceramic for tiny black specks or shiny specks of aluminum that have fused to the porcelain.
Inspect the spark plug ceramic for any cracks or missing material.
Compare the free diameter of the top piston ring with a new piston ring. A piston ring is a simple spring, when a spring is over-heated it looses tension.

A dangerously lean air/fuel mixture burns with most efficiency, so much that the insulating boundary layer also gets consumed and the flame front touches the metal walls. At those locations, there is a dramatic rise in temperature, high enough to cause subsequent charges of air and fuel to spontaneously ignite resulting in multiple flame fronts. This is pre-ignition.

If a hot spot in the combustion chamber is igniting the fuel/air mixture it would do so immediately during the intake stroke resulting in backfiring or inlet charge combustion. If you suspect preignition (or preignition induced detonation) and there was not any inlet charge combustion, than the hot spot alone was not sufficient to ignite the inlet charge. Additional energy was required, usually from the following sources:

since 1940

Piston deposits as precursors of Preignition in Continental and Lycoming Aircraft Engines

Preignition damage to top of piston from continental engine.