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Knock-Knock, Who’s There?...Detonation and Pre-ignition
By Ted Tunnecliffe
Detonation and pre-ignition – both can give you headaches and a sour stomach.
There are a lot of similarities between the two, but detonation is not pre-ignition and pre-ignition is not detonation. It is not unusual for a valve failure to be attributed to one of these causes when it was actually the other. It is important to know one from the other so that you can assign responsibility correctly and make corrections.
A good one-word definition of detonation is "autoignition." In other words, it is ignition caused by something other than the spark plug. (Unfortunately that’s true of pre-ignition as well.) That’s not to say that the spark hasn’t taken place, because one of the key points in the distinction between detonation and pre-ignition is that detonation occurs after the spark. Parts of the fuel charge ignite the mixture of fuel and air in the combustion chamber due to the pressure and temperature increases from the expanding flame front. This is important because pre-ignition takes place because of ignition before the spark even goes off.
When detonation takes place, the temperatures and pressures in the combustion chamber go very high but are irregular and are only present for microseconds in each instance. There can, in fact, be several of them going off in different parts of the combustion chamber at the same time. In some texts, the types of detonation are considered too different if they are explosive or non-explosive. That is, in explosive detonation, the flame speed may be 2,500 to 3,000 feet per second as opposed to 200 to 300 for the normal burning rate. So what is happening is an actual explosion, not a controlled burn as it should be.
Maybe a good way of getting to this is to use an example: the charge comes into the cylinder through the intake valve as the piston is moving down, creating the vacuum to pull it in. The intake valve closes about when the piston reaches bottom center and then the piston starts moving back up. With both valves closed, the piston begins to compress that charge and heats it up. Just before top dead center, the spark plug fires and ignites the charge.
Now – and you’ve got to think of this in slow motion – the flame front moves across the combustion chamber as it burns. This movement causes additional compression of the unburned portion of the mixture because it is being squeezed even more. If, for example, the octane rating of the fuel is not high enough, it will ignite all by itself just from the heat of that compression – and that is detonation.
Technically, octane is C8 H18 or an isomer that is present in petroleum. We generally refer to octane number by simply calling it "octane". It is the property of a fuel that allows it to resist combustion, in particular, combustion induced by the heat of compression – so it is really resistance to autoignition. In other words, it doesn’t catch fire as easily as a lower octane fuel would. Therefore, the low octane stuff is more dangerous than the high – just the opposite of what you might expect. So when that guy on TV says there was a big fire at the airport with "high octane aviation fuel" you know that perhaps it could have been more dangerous if regular low octane automotive gasoline was involved. One way to avoid or reduce spark knock – detonation – is to use a higher-octane gasoline, and that’s why the big boys in NASCAR use it in their high performance applications.
Destructive Versus Non-Destructive Detonation
The definition of these two terms is pretty obvious but just what is happening? It may not be too unusual to hear some "pinging," spark knock or detonation – whatever you want to call it – in an engine on occasion, and if it is random, you can pretty much ignore it. That is "non-destructive" detonation, and you can live with it. For example, if you are temporarily driving through high elevations, you will be much more likely to hear it than at lower elevations – assuming your vehicle is set for those lower altitudes. That is because under those conditions, you are running a different air-fuel ratio than your engine is used to – "leaner" than normal. That is, of course, because at the higher elevations, the air is thinner. If that condition persists, however, and you stay in that environment for awhile, it may become "destructive" and start to tear things up.
So just what is it tearing up? Although the valves rarely show much distress, the top edges of an aluminum piston can be corroded away as shown in Figure 1.
Other Engine Factors
In addition to the properties of the fuel, other factors can influence detonation. As you might expect, the higher the engine compression ratio, the higher the susceptibility to spark knock will be – more compression, more heat – right? The same is true of spark advance. If you set the ignition to go off early in the compression stroke, it means that the fuel will be compressed more by the time the spark goes off, so it will be hotter and more prone to detonation. Those are two biggies, but other factors such as rpm and fuel-air ratio (including variation of that ratio from cylinder-to-cylinder) can have effects as well. The use of port injection on an engine, however, can and does reduce that cylinder-to-cylinder variation significantly. There can still be variation but much less than in a carburated engine.
Compression Ignition Engines
So what about diesels? Since compression ignition engines like diesels don’t have a spark, they can’t very well get into detonation. So, how do you explain all the knocking that they do? It certainly is a form of "spark" knock, or detonation, but there is not much that can be done to avoid it. Essentially, such applications are operating in detonation all the time. But the heavy crank and rod bearings are designed for those heavier loads and also most diesels use cast iron pistons rather than low melting point aluminum.
If you really want to get into this subject and have a good background in physical chemistry, try reading Edward F. Obert’s Chapter 4 in his book Internal Combustion Engines and Air Pollution (Intext Educational Publishers, 1973). This has been my Bible for many years when it comes to trying to understand engines.
Detonation Valve Failures
The failures resulting from detonation, at least with valves, are usually associated with the exhaust side. They are relatively rare, but you might see incipient melting or chipping type corrosion at the O.D. corners of the exhaust in particular.
Let’s switch gears now and talk pre-ignition. As we said earlier, one important characteristic is that pre-ignition takes place before the spark. As a result, the temperatures and pressures are much higher than they are in detonation. Another characteristic is the short durability of pre-ignition. It typically lasts only a few seconds and really tears things up in that short period. The reason is that, with the extreme temperatures everything in the combustion chamber feels it and can’t last very long.
Ordinarily, pre-ignition takes place in spark-ignited engines and is characterized by one of several failure modes. Those that I have seen for the most part are the ones typically, but not exclusively, associated with valves. It is not uncommon for an intake valve to "dish" (or "tulip") if the cylinder has been in pre-ignition. The reason is that the intake valve, since its normal operating environment is a temperature of perhaps 800° to 1,000° F, is made of an alloy that is very strong at that temperature level but very weak if exposed to high temperatures that take place during pre-ignition. The combination of extreme temperatures and pressures from pre-ignition therefore make it very susceptible to this kind of collapse. Figure 2 shows an example of such intake valve distress. The dishing increases the combustion chamber volume, essentially reducing the compression ratio, and that is sometimes enough to stop pre-ignition.
Exhaust Valve Localized Melting
This is the other common failure mode for valves. If the intake valve is operating so cool that it does not overheat sufficiently to where it collapses, then the exhaust valve may be the weaker point and can melt in a localized area.
A couple of factors come into play here. Although the intake valve is typically made of an alloy that is strong at low temperatures but weak at high temperatures, the exhaust valve alloys typically used are stronger at those high temperatures but actually melt at a lower temperature range than will the intake material.
The intake alloy is weaker but still has a higher melting point. In addition, if the exhaust valve has a hard facing applied to it in the seat face area, it will melt at an even lower temperature than does the base alloy. When you stop and think about it, it would have to be that way or you would never be able to weld a hard facing on an exhaust valve.
But exhaust valve alloys are still pretty strong even at temperatures approaching those of pre-ignition so they don’t usually dish.
Once you see an exhaust valve that has failed after the cylinder has been in pre-ignition, you will recognize it next time. The melting that takes place is always at the point where the valve is in contact with the cylinder seat, the point where it seals with the cylinder head.
You can typically see a groove melted across the seat face of the valve at that point and the melted valve material is obvious on the valve underhead fillet immediately adjacent to the seat face. Sometimes it is even carried down onto the stem surface.
But the key point is the melting. That distinguishes it from a corrosion type of failure where "guttering," or a relatively slow corrosion of the seat area, has taken place. Remember, pre-ignition takes place in only a few seconds. The temperatures and pressures are so high that something in the combustion chamber has to give. The picture in Figure 3 shows the results of an exhaust valve that has failed due to pre-ignition.
Other Pre-ignition Failures, Characteristics
In addition to failures of valves, holes melted through pistons – especially aluminum pistons – are fairly common pre-ignition failures. We said especially aluminum because it melts at about 1,300° F as compared to valve alloys that melt at anywhere from about 2,400° to about 2,900° F.
Another common pre-ignition failure is the melting off of the spark plug outer electrode. Actually that electrode is the hottest spot in a spark-ignited engine, so it is well on the way without a lot of encouragement from the extreme temperatures of uncontrolled combustion.
Another characteristic I have noted over the years of looking at such failures is the absolute cleanliness of a combustion chamber that has been in pre-ignition. There will be no deposits – they have all evaporated from exposure to those temperatures.
In my experience, some of the most common causes of pre-ignition are sharp corners, as on the outer edge of an exhaust valve or possibly a metal fin from a gasket. I have seen spark plug outer electrodes, that are too thin, initiate pre-ignition and this is probably the most common cause.
The sharp edges from metal corrosion scaling in the center of the head of an exhaust valve can do the same thing. But, one I’ve never seen is pre-ignition caused by combustion deposits – no matter how heavy those deposits might be.
This is an impossibility. By definition, in order for pre-ignition to take place, there must be a charge – a fuel air mixture – that is being compressed as it is trying to expand from having been ignited by some source other than the spark plug. In a diesel, the "charge" that is being compressed is only air, not a fuel/air mixture.
But the situation can get confusing if you see a valve whose seat area looks as if it had melted. Figure 4 shows such a valve. In cases like this you typically will see a similar appearing "washed out" zone but you’ll notice that there is no evidence of residual melted metal in that area.
This one frankly has always been a mystery to me; apparent melting. In fact, metallographical analysis of areas at the sides of the washed out zone confirmed that it did melt, but showed no melted metal residue. I know the temperatures to cause the melting had to be extremely high, but it’s difficult for me to imagine that they are high enough to vaporize the metal. That would especially be true at points somewhat downstream where the temperatures would drop significantly. But what other explanation is there? If you have any ideas I would sure like to hear them.