Piston engine combustion
ALL ABOUT DETONATION, PRE- IGNITION AND BACKFIRING
Not so many years ago, before computers got involved, automobile engines ran using all sorts of ancient technology that made them prone to detonation and/or pre-ignition when the timing and fuel mixture got out of whack. These symptoms, usually called “engine knock,” were quite common and relatively easy to detect at automobile speeds, but not so easy to detect at airplane speeds and noise levels. Of course, the majority of the general aviation fleet still relies on some of that same ancient technology and even throws in some parts from a farm tractor (magnetos). Here’s a primer on how these conditions relate to aircraft engines.
Occurs when the fuel/air mixture ignites in the cylinder and burns progressively at a fairly uniform rate across the combustion chamber. When ignition is properly timed, maximum pressure is built up just after the piston has passed top dead center at the end of the compression stroke. The flame fronts start at each spark plug and burn in more or less wavelike forms. The velocity of the flame travel is influenced by the type of fuel, the ratio of the fuel/air mixture and the pressure and temperature of the fuel mixture.
There is a limit to the amount of compression and the degree of temperature rise that can be tolerated within an engine cylinder and still permit normal combustion. All fuels have critical limits of temperature and compression. Beyond this limit, they ignite spontaneously and burn with explosive violence. If the total compression exceeds the critical point, detonation occurs. The explosive burning during detonation results in an extremely rapid pressure rise. This rapid pressure rise and the high instantaneous temperature, combined with the high turbulence generated, cause a scrubbing action on the cylinder and piston. This can burn a hole completely through the piston. The detonation characteristic of the mixture can be controlled by varying the fuel-to-air ratio. At high power output, combustion pressures and temperatures are higher than they are at low or medium power. Therefore, at high power settings, the fuel-to-air ratio is made richer than is needed for good combustion at medium or low power output. This is done because, in general, a rich mixture does not detonate as readily as a lean mixture. Unless detonation is heavy, there is no flight-deck evidence of its presence. Light to medium detonation does not cause noticeable roughness, temperature increase or loss of power. As a result, it can be present during takeoff and high-power climb without being known to the flight crew.
As the name implies, it means combustion takes place within the cylinder before the timed spark jumps across the spark-plug terminals. This condition can often be traced to excessive carbon or other deposits that cause local hot spots. Detonation often leads to pre-ignition. However, pre-ignition may also be caused by high-power operation with excessively lean mixtures. Pre-ignition is usually indicated in the cockpit by engine roughness, backfiring and a sudden
increase in cylinder-head temperature. Any area within the combustion chamber that becomes incandescent serves as an igniter in advance of normal timed ignition and causes combustion earlier than desired. Pre-ignition may be caused by an area roughened and heated by detonation erosion. A cracked valve or piston, or a broken spark-plug insulator, may furnish a hot point that serves as a glow plug. The hot spot can be caused by deposits on the chamber surfaces resulting from the use of leaded fuels. Normal carbon deposits can also cause pre-ignition. Pre-ignition is caused by a hot spot in the combustion chamber, not by incorrect ignition timing. The hot spot may be due to either an overheated cylinder or a defect within the cylinder. The most obvious method of correcting pre-ignition is to reduce the cylinder temperature. The immediate step is to retard the throttle. This reduces the amount of fuel charge and the amount of heat generated, after which the mixture should be enriched to lower the combustion temperature. If the engine is at high power when pre-ignition occurs, retarding the throttle for a few seconds may provide enough cooling to chip off some of the lead or other deposits within the combustion chamber. These chipped-off particles pass out through the exhaust.
When a fuel/air mixture does not contain enough fuel to consume all the oxygen, it is called a lean mixture. Conversely, a charge that contains more fuel than required is called a rich mixture. An extremely lean mixture either does not burn at all or burns so slowly that combustion is not complete at the end of the exhaust stroke. The flame lingers in the cylinder and then ignites the contents in the intake manifold or the induction system when the intake valve opens. This causes an explosion known as backfiring, which can damage the carburetor and other parts of the induction system. Incorrect ignition timing, or faulty ignition wires, can cause the cylinder to fire at the wrong time, allowing the cylinder to fire when the intake valve is open, which can cause backfiring. Backfiring rarely involves the whole engine. Therefore, it is seldom the fault of the carburetor. In practically all cases, backfiring is limited to one or two cylinders. Usually, it is the result of a faulty valve clearance setting, defective fuel-injector nozzles or other conditions that cause these cylinders to operate leaner than the engine as a whole. In some instances, an engine backfires in the idle range but operates satisfactorily at medium and high power settings. The most likely cause in this case is an excessively lean idle mixture.