Detonation versus Pre-Ignition
The subject of detonation in engines has been covered in countless articles over the years but it is still a major cause of engine failures. As an Automotive Technical Consultant I have examined and identified numerous failed engine parts over the years and many were the result of detonation. This article explains two types of engine failures, detonation and pre-ignition.
So, what is Normal Combustion?
To understand what detonation is we first need to review normal combustion. It is the burning of an air/fuel mixture charge in the combustion chamber. It should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber, similar to throwing a pebble in water with the ripples spreading out. The flame front should progress in an orderly fashion. The combustion burn moves all the way across the chamber and, quenches (cools) against the cylinder walls and piston crown. Note that the air/fuel mixture does not “explode” but is a controlled burn.
Ideally the burn should be complete with no remaining fuel-air mixture left in the chamber, however, in practice the boundary layer against the combustion chamber walls and piston surface are too cool for combustion to take place and a very small percentage of unburned fuel exits through the exhaust system. In turn the catalytic converter burns this remaining fuel to lower the emissions of the engine.
There are many factors that engine designers look for to quantify combustion. One of these is called “Peak Cylinder Pressure” (PCP). It is measured by a pressure transducer inside the combustion chamber. Ideally, the PCP should occur at 14 degrees after top dead center (ATDC). Depending on the chamber design, spark plug location, burn rate, fuel and engine speed the time of spark ignition advances and retards to optimize Peak Cylinder Pressure which should occur at 14 degrees after top dead center. If the PCP occurs too soon or too late, it won’t give the engine optimum performance.
To summarize, normal combustion is the combustion process that is initiated by the spark plug. The combustion burn moves all the way across the chamber in an even and orderly fashion and peak pressure occurs at 14 degrees ATDC. Note: The PCP is always 14 degrees ATDC for any engine.
A lot of confusion exits between detonation and pre-ignition. Detonation and pre-ignition are both abnormal combustion. The two, as I will explain, are somewhat related but are two distinctly different phenomenon and can induce different failure modes.
What is Detonation then?
Detonation is the spontaneous combustion of the end-gas (remaining air/fuel mixture) in a combustion chamber. It always occurs after normal combustion and is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. During detonation the end gas in the chamber combusts spontaneously. The key point here is that detonation occurs after the spark plug has fired.
Pre-ignition is defined as the ignition of the air/fuel mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition.
During detonation conditions the unburned end gas is under increasing pressure and heat (from the normal progressive burning process) and spontaneously combusts. The end gas is ignited solely by the intense heat and pressure. The remaining fuel in the end gas simply lacks sufficient octane rating to withstand self-combustion.
Detonation causes a very high and very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace measured by a pressure transducer inside the combustion chamber, you would see the normal burn as a normal pressure rise, followed by a sudden sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much like when hit by a hammer. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise of detonation is also known as spark knock or pinking. This noise changes only slightly between iron and aluminum. This noise or vibration is what a knock sensor picks up.
Controversy exists around whether the knocking or pinging sound is the result of “two flame fronts colliding” or comes from the vibration of the engine structure reacting to the pressure spike. It is generally agreed that the noise comes from the vibration of the engine structure reacting to the pressure spike.
Over the many years as an Automotive Technical Consultant I have learned that detonation is not necessarily destructive. Some engines can sustain long periods of light detonation without incurring any damage. It’s not an optimum situation but it is not a guaranteed instant failure.
The higher the specific output of an engine, the greater the sensitivity to detonation. An engine that is making 25kW/Litre or less can sustain moderate levels of detonation without any damage. An engine that is making 75KW/Litre or more and exposed to detonation will probably be damaged very quickly.
Detonation can cause three types of failure; Mechanical damage, abrasion and localized overheating. The impact nature of the spike can cause engine parts to fail. It can break the spark plug electrodes, the porcelain around the plug, cause fracture of the piston ring lands, break piston rings, erode head gaskets, cause bearing distress and can cause fracture of intake and exhaust valves.
The crown of the piston will show sandblasted appearance. The detonation mechanically erodes material out of the piston. You can typically expect to see that sanded look at the hottest part of the combustion chamber usually most distant from the spark plug.
In four valve twin cam engines with a pent roof chamber with a spark plug in the center, the chamber is fairly uniform in distance around the spark plug. Detonation in these engines usually starts close to the exhaust valves because that area is usually the hottest part of the chamber. Where the end gas is going to be hottest that is where detonation will likely occur.
The pressure spike during detonation is so severe and of very short duration, that it can shock and break through the boundary layer of gas that surrounds the piston. Combustion temperatures exceed 2000 degrees C. If you subjected an aluminium alloy piston to that temperature it would melt instantly. The reason it doesn’t melt is because of thermal inertia and also because there is a boundary layer next to the piston top. This thin boundary layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold piston material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shock wave from the detonation spike can cause that boundary layer to break down which leads to excessive heat transfer into those surfaces. The boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the piston crown and cylinder head and into the coolant. Therefore engines that are detonating will tend to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat. This is a self-worsening snowball effect. That’s why an overheating engine wants to detonate and that’s why engine detonation tends to cause overheating.
To summarize; the detonation pressure spike is very sharp and very brief and occurs after the spark plug fires. In most cases that will be well after ATDC, when the piston is moving down. You have high pressure in the chamber anyway with the normal combustion burn. The pressure is pushing the piston like it’s supposed to, and superimposed on that you get a detonation spike that rings the engine.
Detonation is influenced by the combustion chamber design (shape, size, geometry, spark plug location), compression ratio, ignition timing, camshaft timing, intake charge temperature, cylinder pressure and fuel octane rating. Too much spark advance ignites the burn too soon so that it increases the pressure too greatly, and the end gas spontaneously combusts. Backing off the spark timing will stop the detonation.
The octane rating of the fuel is very important. Octane is the ability to resist detonation. Production engines have optimized compression ratios for the type or grade of fuel that the marketplace offers. The design compression ratio is adjusted to maximize efficiency/power on the available fuel. Many times in the aftermarket the opposite occurs. A compression ratio and cam profile is “picked” by the end user who then tries to find good enough fuel.
If the fuel you used was of a lower octane rating than that demanded by the engine’s compression ratio and spark advance, detonation could result and cause engine failure. Equally the same, if you increased the compression ratio by re-surfacing the cylinder head or block without increasing octane rating, detonation could also result. Some fuels have a higher octane quality. For instance, methanol as fuel has a much better octane rating just because it cools the mixture significantly due to the extra amount of liquid being used.
Another solution to suppress detonation is increase the burn rate of the combustion chamber. That is why many modern engines utilize fast burn or quick burn chambers. Fast burn chambers use special improved swirl design to enhance the burn rate. The faster you can make the charge burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can’t sit there and soak up heat and have the pressure act upon it, it can’t detonate.
If, however, you have a chamber design that burns very slowly, like many old 1960’s designed engines, you need to advance the spark and fire at 38 degrees BTDC. Because the optimum 14 degrees after top dead center (PCP) hasn’t changed the chamber has far more opportunity to detonate as it is being acted upon by heat and pressure for much longer. If we have a fast burn chamber, with 15 degrees of spark advance, we’ve reduced our window for detonation to occur considerably. It’s a mechanical phenomenon. Fast burn combustion chambers are more resistant to detonation.
There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build up, the more efficient the engine becomes. Pumping losses are minimized. If the spark plug fires 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If the spark fires 20 degrees before top dead center, it’s only acting against it for 20 degrees. The engine becomes more mechanically efficient.
The best indication of detonation is the pinging sound that cars, particularly old models, make at low speeds and under load. It is sometimes very difficult to hear detonation in well insulated luxury interiors of today’s cars. Also an engine running open or straight through exhaust can easily mask the characteristic spark knock. The point is that often it is very difficult to hear that detonation is going on.
The definition of pre-ignition is the ignition of the air/fuel charge prior to the spark plug firing. Pre-ignition caused by some other ignition source such as an overheated spark plug tip, carbon deposits in the combustion chamber or a burned exhaust valve, can all act as a glow plug to ignite the charge.
It is generally believed that during pre-ignition the charge ignites 5 or 10 degrees before the spark plug fires. Keep in mind the following sequence when analyzing pre-ignition. The intake charge enters the combustion chamber as the piston reaches Bottom Dead Centre (BDC). As the piston travels past BDC it starts to compress the charge. You have to accept that the most likely point for pre-ignition to occur is a full 180 degrees BTDC. (140-160 degrees before the spark plug fires) Since the spark voltage requirements to fire the intake charge increase in proportion with the amount of charge compression, almost anything can ignite the proper air/fuel mixture at BDC. Around BDC is the easiest time to light that mixture and becomes progressively more difficult as the pressure starts to build. Therefore we are talking some 145-160 degrees of burn being compressed that would normally be relatively cool. The result is the engine is trying to compress an expanding charge. This puts an extreme load on the engine and adds tremendous heat into the chamber and piston. Engine damage will occur very quickly. A piston will only take a few revolutions of pre-ignition before it fails.
Remember, with detonation the spark plug ignites the mixture followed by a sharp pressure spike. With pre-ignition, the ignition of the charge happens far ahead of the spark plug firing. There is no sharp pressure spike to resonate the block and the head to cause any noise.
An engine can sometimes live with detonation occurring for considerable periods of time. There are no engines that will live for any period of time when pre-ignition occurs. A melted hole in the middle of the piston is always due to the extreme heat and pressure of pre-ignition. This occurs because the piston is trying to compress the already burned air/fuel mixture and the piston soaks up a tremendous amount of heat very quickly. Parts that survive are the ones that have a high thermal inertia, like the cylinder head and cylinder wall. The piston, being aluminium alloy, has a relatively low thermal inertia (aluminum soaks up heat very rapidly). The crown of the piston is relatively thin, and when it gets too hot it can’t reject the heat. With tremendous pressure loads against it, the result is a hole in the middle of the piston where it is weakest.
The other signs of pre-ignition are melted spark plugs showing melted and fused looking porcelain. Sometimes times a “pre-ignited plug” will melt away the ground electrode. This is a typical sign of incipient pre-ignition.
Spark plugs should be carefully matched to the recommended heat range. Many engine tuners use colder spark plugs and relatively rich mixtures. The spark plug heat range is also affected by coolant temperatures. The spark plug may be getting hot, starts to melt and starts to act as a pre-ignition source. The plug can actually melt without pre-ignition occurring. However, the melted plug can cause pre-ignition the next time around. Also, a loose plug that can’t reject sufficient heat through its seat can cause pre-ignition. Pre-ignition damage is almost instantaneous. Without a high pressure spike to resonate the chamber and block, you would never hear pre-ignition. The only sign of pre-ignition is when damage has already occurred.
Detonation induced pre-ignition
Just when you start to understand the difference between detonation and pre-ignition, there is also a situation called detonation-induced-pre-ignition. It may sound a bit like double Dutch but it does happen. Imagine an engine under heavy load starting to detonate. Detonation can occur for a long period of time. The plug overheats because the pressure spikes break down the protective boundary layer of gas surrounding the electrodes. The plug temperature starting to elevate until it becomes a glow plug and induces pre-ignition. There would not have been any danger of pre-ignition if the detonation had not occurred. Damage attributed to both detonation and pre-ignition would be evident.
Henry van Vugt
Automotive Technical Consultant