Pre-ignition/Detonation

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Presented to: By: Date: Federal Aviation Administration Produced by: The FAA Safety Team The National FAA Safety Team Presents Pre-ignition/Detonation Aviation Professionals Chuck Holsclaw April 2019 Federal Aviation Administration Welcome 2 • Exits • Restrooms • Emergency Evacuation • Breaks • Sponsor Acknowledgment • Cell Phones to Stun Federal Aviation Administration Pre-ignition and Detonation… …are Deadly 3 Federal Aviation Administration Pre-ignition and Detonation…are Deadly! 4 This engine is from a Beech S35 Bonanza Fatal accident. The #6 piston was eroded and begun to melt. The rings and piston skirt were compromised by thermal expansion and metal transfer. The red arrow is pointing at the deep pitting and erosion of the piston face. This caused combustion gases to bleed into and over pressurized the crankcase, forcing engine oil out of the breather. Connecting rods then failed in short order because of the lack of lubrication. The rods then smashed holes in the crank case. Causing lost of power and engine failure. Federal Aviation Administration What is Pre-ignition 5 Preignition Normal Combustion Federal Aviation Administration Normal Combustion vs Pre-ignition Normal combustion – is a progressive burning of the fuel-air mixture within the cylinders. The gasses within the cylinders are ignited from the top. The flame produced as a consequence travels down in an organized way. This combustive force, equally applied to the piston in a stable manner, pushes the piston down. The downward motion of the piston is then mechanically transferred to the propeller. This makes pilots very happy. "Pre-ignition" is an abnormal combustion event. Pre-ignition is the ignition of the air- fuel charge while the piston is still compressing the charge. The ignition source can be caused by a cracked spark plug tip, carbon or lead deposits in the combustion chamber, or a burned exhaust valve, anything that can act as a glow plug to ignite the charge prematurely. A cross over spark from an ignition system fault can also ignite the fuel air mixture while the piston is still compressing the charge. When this happens the engine works against itself. The piston compresses and at the same time the hot gas expands. This puts tremendous mechanical stress on the engine, and transfers a great deal of heat into the aluminum piston face damaging the piston. Engine failure can happen in minutes. 6 Federal Aviation Administration What is Detonation 7 Normal Combustion Detonation After TDC Federal Aviation Administration Detonation Detonation - Detonation, as the name suggests, is an explosion of the fuel-air mixture inside the cylinder. It occurs after the compression stroke near or after top dead center. During detonation, the fuel/air charge (or pockets within the charge) explodes rather than burning smoothly. Because of this explosion, the charge exerts a much higher force on the piston and cylinder, leading to increased noise, vibration, and cylinder head temperatures. The violence of detonation also causes a reduction in power. Mild detonation may increase engine wear, though some engines can operate with mild detonation. However, severe detonation can cause engine failure in minutes. Because of the noise that it makes, detonation is called "engine knock" or "pinging" in cars. A combination of high manifold pressure and low rpm creates a very high engine load, and can induce damaging detonation. In order to avoid these situations: • When increasing power, increase the rpm first and then the manifold pressure • When decreasing power, decrease the manifold pressure first and then decrease the rpm Other causes of detonation are improper ignition timing, high inlet air temperature, engine overheating, oil in the combustion chamber, or a carbon build up in the combustion chamber. High heat is detrimental to piston engine operation. Its cumulative effects can lead to piston, ring, and cylinder head failure and place thermal stress on other operating components. Excessive cylinder head temperature can lead to detonation, which in turn can cause catastrophic engine failure. Turbocharged engines are especially heat sensitive. 8 Federal Aviation Administration Why Are These Conditions Important? 9 Normal Combustion Detonation After TDC Federal Aviation Administration Pre-ignition - Detonation The explosion of pre-ignition and detonation is like hitting the piston with a sledge hammer. The automotive term for the sound it makes is “Ping” (Something pilots cannot hear in aircraft). The ping sound is the entire engine resonating at 6400 hertz. Sounds like a ping, but it is an explosion with enough power to make the engine resound like a gong. Both pre-ignition and detonation put tremendous mechanical stress on the engine, and transfer a great deal of heat into the piston deck. This can scour the protective layer of insulating air that separates the hot gasses from the aluminum piston surface causing the piston to melt (EGT is 1600 degrees. Aluminum pistons melt at 1200 degrees.) The force of these explosions can knock holes in pistons, bend connecting rods, overcome the lubrication film in the rod bearings, and hammer the babbitt out of rod bearings. Engine failure can happen in minutes. 10 Federal Aviation Administration Why Are These Conditions important? 11 The slide does not do the bearing shells justice, but you may be able to see the surface is rough and an uneven color. The light color at the edges is a babbitt coating. Babbitt is a soft alloy of tin, antimony and lead. These bearing were wiped by the forces of pre-ignition or detonation. This leads to one of the first indications that things are going badly. The extra clearance created by the vacating babbitt allows more oil flow, and lowers the oil pressure. Federal Aviation Administration Why Are These Conditions Important ? 12 This is a cylinder head showing signs of pre-ignition or detonation. The carbon coating that normally lines the head