Pre-Ignition and Detonation

The information (and harrowing pictures!) that follows is reprinted courtesy of the FAA.

Pre-ignition. Detonation. Both can be deadly. But what’s the difference? And how can you avoid them?

This engine is from a Beech S35 Bonanza’s fatal accident. The #6 piston was eroded and began to melt. The rings and piston skirt were compromised by thermal expansion and metal transfer. Note 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 the breather. The connecting rods then failed due to the lack of lubrication and smashed holes in the crank case, causing loss of power and engine failure.

Normal combustion vs. pre-ignition

Normal combustion is an orderly, progressive burning of the fuel-air mixture in the cylinders. The gasses within the cylinders are ignited from the top. The flame then 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.

In a pre-ignition event, combustion is abnormal. It happens when the air-fuel mix ignites before the spark plug fires, while the piston is still moving up in the compression cycle. The ignition can be caused by a cracked spark plug tip, carbon or lead deposits in the combustion chamber, a burned exhaust valve, an ignition system fault, or anything that can act as a glow plug to ignite the charge prematurely.

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.

Detonation

As the name suggests, detonation 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 excessive 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.

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 temperatures can lead to detonation, which in turn can cause catastrophic engine failure. Turbocharged engines are especially heat sensitive.

Some causes of detonation include:

  • improper ignition timing
  • high inlet air temperature
  • engine overheating
  • oil in the combustion chamber
  • carbon build-up in the combustion chamber

A combination of high manifold pressure and low rpm creates a very high engine load, which can also cause 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

Pre-ignition and detonation results

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. It 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 cause 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.

The bent connecting rod at the start of the article is a good example of the damage pre-ignition and detonation can do.

These cylinder #2 spark plugs are packed with melted piston material.

Here’s what happens

This is a cylinder head showing signs of pre-ignition or detonation.

The carbon coating that normally lines the head dome is knocked off. There is melted piston material in the head and the cylinder sleeve is scored by the overheated piston.

This is the same piston , but note that the piston deck is eroded.

The rings are broken. The piston skirt is scuffed from rubbing on the cylinder wall. A piston in this condition allows combustion gases into the crank case. This over-pressurizes the crankcase and blows engine oil out of the crank case breather — all of the engine oil, in minutes.

Soon after the engine oil departs the connecting rods try to make a break for it, resulting in giant holes in the crank case.

How do I detect pre-ignition?

A rough-running engine can be the first sign of pre-ignition or detonation. High EGTs or CHTs can be a sign of a problem so be sure to keep an eye on that if you can.

Below are common indications of detonation. You should have increasing oil temperature, not pressure. The top left gauge is RPM. The top right is manifold pressure.

What do I do when it happens?

Since excessive heat can be so damaging, your main priority is to cool the engine:

  • Reduce power
  • Increase airspeed
  • Enrich the fuel mix
  • Open the cowl flaps.
  • Land immediately!

Preventing Pre-ignition

  • Do not take off unless the run-up is perfect
  • Maintain the ignition system
  • Pay attention to cylinder compression tests
  • Use the proper heat range spark plugs
  • Make sure cooling baffles are in good repair

Preventing Detonation

  • Lean the engine per the flight manual
  • Keep engine load to a minimum
  • Do not over boost
  • Use only the recommended fuel grade
  • Make sure engine timing is properly set
  • Make sure cooling baffles are in good repair
  • Be wary on hot, dry days
  • If in doubt, run rich

By |2024-09-18T13:48:58-04:00July 18, 2023|Aircraft, Articles|Comments Off on Pre-Ignition and Detonation

Aircraft Problems: Should I Be Worried?

One of the main purposes of oil analysis it to find problems that might be developing in an engine, and after doing this for a lot of years, I can say without a doubt that it works. However, some problems are more urgent than others, and part of our job is to determine if a problem is a major one or not. Most engine problems start out minor but if left unchecked can lead to major issues, which eventually result in an engine’s demise.

Minor problems

Abrasive Contamination

Dirt getting past the air filter will cause a lot of problems in an engine, and piston scuffing is the primary concern. Fortunately, most air filters do a really good job even when they are dirty. If you change your air filter on a regular basis, then this type of problem is pretty easy to avoid, but remember, it’s also important to check the whole air induction system down-stream of the air filter to make sure no cracks or other problems exist that could be letting dirt in.

Fuel dilution

This generally includes any fuel level between 1.0% and 3.0% that keeps showing up again and again. This is not a normal situation, but it doesn’t necessarily cause an engine problems in the short term. Still, since fuel is a contaminant, it will cause the oil to oxidize faster that it normally would. That typically causes problems like stuck oil control rings, which leads us to our next minor issue.

Oil Consumption

This one isn’t really a problem at low levels because all engines are designed to use some oil. What you really want to watch out for is a change in how much oil is being burned. If you always use 1 quart every 10 hours and it suddenly jumps to 1 quart every 3 hours, then you know something has changed. That’s part of the reason we ask about oil added between changes. If you’re not losing oil due to a leak, it’s either getting past the rings or the valve guides. Granted, you can buy a lot of make-up oil for the cost of a top overhaul, but there will probably come a time you’ll have to bite the bullet and fix the issue.

Corrosion

If you fly around 5 hours per month, that should keep this minor problem off you mind, though we all know that life doesn’t necessarily allow this. Still, if corrosion is minor it should easily disappear once the engine is back to flying regularly. If corrosion gets so bad that it causes pitting on the parts, that’s when the problem elevates to major status.

Major problems

Cam spalling

This one is often directly related to corrosion getting out of hand, though it can also be related to oil starvation on things like cold starts and high RMP starts. It takes time for that thick oil to get circulating through the engine and if it doesn’t get to the cam and followers fast enough, metal-to-metal contact happens. Problems of this nature won’t necessarily cause an engine to fail, but can lead to loss of some power, which might be needed to clear that 50’ obstacle at the end of the runway.

Excess Heat

This is really a pretty broad category and is often due to operational factors, though it’s almost always avoidable if you are paying attention to your cylinder head temperatures. If those are getting too hot, then maybe the cooling baffles aren’t quite working like they should. Maybe you have a crack in an air induction tube. That could allow abrasive dirt into the combustion chamber, but would also cause that one cylinder to run leaner than others (due to extra air being sucked in) and likely hotter. Excess heat causes the parts to expand more than they were designed for and that’s when wear starts getting heavy.

Stuck or Burned Valves

Abrasive contamination, fuel dilution, and oil consumption will all contribute to this type of problem. Sticking valves can be identified by things like morning sickness (not necessarily in the morning), intermittent rough running, and high mag drops (not due to a fouled spark plug). Burned valves are usually pretty easy to spot with a borescope though might not necessarily cause major operational problems until they burn to a point where compression has significantly degraded.

Detonation

This issue develops in an engine when the combustion process is not completed correctly, usually when an engine is under a heavy load and producing a lot of heat. It can easily burn a hole right through the top of a piston, resulting in all of the oil in your engine being pushed out the breather tube and oil starvation (see below). If your engine had a good muffler, you would hear a ticking or pinging noise, but since those don’t exist in general aviation, this problem can often go unnoticed without the help of oil analysis and/or engine monitor data. If this problem exists, running a richer fuel/air mixture to keep the engine cooler should help.

Instant failure

Oil starvation

Whether it’s caused by oil consumption left unchecked or severely worn bearings not letting oil get to all of the parts, this type of problem will cause an engine to fail in short order and it’s usually accompanied by the worst sound your engine can make — silence.

Spun bearings

When the babbit is worn off your bearings, either due to hard use, abrasive oil, or lack of oil, you will start to lose oil pressure. If the problem gets severe enough, the spinning shaft will actually weld to the bearing itself and spin in place. Once this happens, the engine is pretty much shot, though amazingly enough it might still run (but not for long).

Outside causes

Of course there are lots of other things that can cause instant engine death — see the first cartoon on this link for an example, or, although it’s not a plane, this picture of my flooded MINI. Unfortunately, outside factors probably take more engines down than anything else.

It’s pretty rare for engines to fail suddenly due to minor issues, so when we see something going on, that doesn’t necessarily mean you need to get out the wrenches or head straight to the engine builder and demand a repair. Usually, you’ll have some time to see if the problem persists or is getting worse. Once that has been established, then some action will likely be required to keep the engine going, but the cost should be minor compared to the hassle and expense of having to replace the whole engine. So test your oil every now and then. Chances are good your engine will look perfect, but if it doesn’t, you’re better off knowing about it sooner rather than later.

By |2024-09-18T13:49:41-04:00July 18, 2023|Aircraft, Articles|Comments Off on Aircraft Problems: Should I Be Worried?

Aircraft Oil: Go Ashless or Go to the Repair Shop

Any of you flying air-cooled aviation engines should be familiar with the phrase AD oil or Ashless Dispersant oil. These are special oils designed for aircraft engines and their use will help protect again pre-ignition and detonation. But what exactly does ashless mean?

Basically when you burn the oil, it will completely disappear and leave no significant ash behind. For non-ashless oils, most of the ash left behind when you burn the oil is from additives in the oil itself. Automotive and diesel engine oil designs for liquid cooled engines will contain a lot of additive that are is ashless and it’s not a problem in those types of engines because they don’t typically run hot enough to burn the oil.

Air-cooled aircraft engines are another story. In those engines, it is common for certain engine parts to reach temperatures at which the oil can burn. If a non-ashless oil was used, then deposits from the left-over ash could end up sticking to valves and ring lands. Those deposits could lead to hot-spots in the combustion chamber and those commonly cause pre-ignition.

The dispersant part of AD means there is additive present that is meant to hold solids in suspension so then can either be filtered out, or drain when the oil is being changes. If this additive is not present, it would be a lot easier for sludge to build-up in your engine during normal operation.

While the use of ashless oils won’t necessarily prevent all of the problems associated with pre-ignition and detonation, it is one easy way to help protect your engine from these dangers.

By |2024-09-18T14:04:07-04:00July 18, 2023|Aircraft, Articles|Comments Off on Aircraft Oil: Go Ashless or Go to the Repair Shop

About Aircraft Oil

Lots of people want to know: what’s the best type of oil to use in an aircraft engine? We see wide variations in engine wear depending on a variety of things: the cylinder type, how the engine is operated, and the environment it’s flown and stored in. What we don’t see a making a difference is oil brand. There might be a correct grade of oil, depending on how and where you operate your engine, but there is no correct brand.

When you change the oil in an air-cooled aircraft engine, the only oil you can safely use is an aircraft-use oil. To use any other type of oil is to invite premature failure of the engine due to detonation. Beyond that, it matters very little what brand of oil you’re using.

All aircraft-use engine oils on the market today (that we know of) are mineral oils, i.e., refined, petroleum-based oils. Some of them have an additive in them to aid in scavenging debris and carrying it to the filter or screen. These are called ashless dispersant (AD) oils. Without the additive, they are called mineral oils.

We measure the viscosity at 210°F, which is in the neighborhood of your engine oil at operating temperature at cruise. W100 oil is an SAE 50 oil at operating temperature, and so are 15W/50 and 20W/50. The only difference in the multi-grade oils is the addition of long-chain polymers (viscosity improvers) that cause them to be more viscous at higher temperatures. At ambient temperatures the oils act as an SAE 15W or SAE 20W oil to allow your engine to spin over more easily, but at operational temperature, the oil behaves as an SAE 50W.

Tradition would have you using mineral oil during wear-in of a new or overhauled engine, and then changing to an AD oil after two or three oil changes. While we aren’t exactly sure of the reason for this procedure (some theories suggest it helps with ring seating, though it could also just be held over from the days of yore), it’s fine to follow the engine manufacturers’ recommendations. After that, it doesn’t much matter which brand of oil you select. As long as you’re running an aircraft engine oil, the brand and type of oil makes very little difference in your engine’s wear patterns.

There are many variables that determine how an aircraft engine wears. We consider the oil type to be the least of these variables (if it has any significance at all).

By |2024-09-18T14:24:34-04:00July 13, 2023|Aircraft, Articles|Comments Off on About Aircraft Oil
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