Engine Detonation Science

Detonation: what it is, what it can damage, and how it affects fuel economy

The subject we have selected for this installment looks at the problems associated with engine efficiency, inasmuch as gasoline quality deterioration has become a factor in the economics of operating a vehicle. Lost fuel economy, damaged engine parts and reduced overall driveability all figure into the detonation picture. So let’s open the subject and see what’s inside.
First, we should probably make a distinction between detonation and preignition. Often, it seems, these two subjects are not clearly understood, and it is particularly important that we know the difference between them. Suppose we define preignition as any initiation of the combustion process by a source other than the spark plug. Glowing carbon, heated edges of an exhaust valve, or any other “hot spot” in the combustion chamber can begin the combustion process ahead of the spark plug function. Typically, this leads to a general loss in engine efficiency, much like excessive ignition timing advance.

Piston motion (moving toward top dead center [TDC] just prior to ignition by spark) sees additional work that is required to compress the already burning air/fuel mixtures since preignition has begun the combustion process. Actually, by strict definition, any combustion process begun before or after spark ignition can be termed preignition. As we’ll discuss in a moment, it is the result of preignition that can lead to detonation, even though preignition does not always set up detonation. And as we’ll also discuss a bit later, it is possible for detonation to create circumstances that will lead to preignition . . . thus the vicious circle.
Causes of preignition include carbon deposits that accumulate within the combustion space. Since such deposits do not conduct heat well, very high temperatures can be reached within such accumulations. This can trigger preignition. Spark plug heat ranges can also affect preignition if the electrodes are allowed to become too hot during normal combustion. And we should mention that any engine parts exposed to combustion heat can become sources for preignition, especially those with sharp edges.
Engines running with some degree of preignition tend to be rough, but not to the point of stalling, since in multicylinder engines it is typical that one or two cylinders will experience preignition while the others continue to function normally. If hot spots in the combustion space become sufficiently high in temperature, an engine will continue to run after the ignition switch is turned off. This so-called “after-running” is common to many modern-day engines. You may know of this as “dieseling.” It is also noteworthy that combustion temperature increases are more likely to cause preignition than increases in combustion pressure (this relates to detonation). Now let’s take a look at detonation.
Suppose we assume that spark ignition has occurred and the combustion flame has begun to move through the combustion space. Speed of this flame is relatively slow at first.

A. At the risk of oversimplification, here is the relationship between cylinder pressure conditions and time in a normal combustion process vs. detonation. Note that the rate of pressure rise vs. time is about the same for both conditions until the point of detonation (end of period “a”). At this stage of combustion, delay period shortness allows for spontaneous combustion of all remaining air/fuel mixture, resulting in the loss of small amounts of power, since there is not much mixture left to combust when detonation occurs. B. This is a graphic representation of how advancing the point of spark ignition tends to increase the amount of net “work” produced by an engine. If you accept that the area under the pressure vs. piston position (or time) curve relates to engine output, advancing the time of ignition can increase power (assuming we do not induce detonation). Up to the point at which power is lost or detonation is caused, an increase in ignition timing can aid both driveability and fuel economy.

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As flame velocity increases, heat developed by the combustion process causes the burned part of the air/fuel mixture to expand. This further increases the speed of the flame front, including a rise in cylinder pressure proportional to the amount of mixture that has burned (or oxidized, if you’re holding us to proper terminology). If, for some reason, the remaining unspent mixture ignites spontaneously (the result of combustion temperature and pressure), a second flame front begins movement through the combustion space. This second front causes such a significant rate of increase in cylinder pressure that, as the two fronts collide, pressure is excessively high and results in the “knock” or “rattle” associated with an engine running in detonation.
Specifically, combustion should begin at or near the spark plug and move outward toward the far side of the combustion space. Peak cylinder pressure normally occurs when the combustion flame has reached the last part of the air/fuel mixture. By examining the process of “end combustion,” we find that at these last stages of mixture burning, cylinder pressure is reaching a maximum. The last-to-be-burned mixture has already been subjected to compression by the piston (during the compression stroke) and by the pressure rise attending normal combustion. If the air/fuel ratio is reasonably homogeneous (uniform) at this point, all remaining mixture will ignite almost spontaneously at its self-ignition temperature. And while all this may seem a bit complex, it reveals that even during normal combustion there is a time when an engine experiences a low-energy form of detonation.
Taken to extremes, this condition of remaining mixture self-ignition results in a pressure wave (or flame front in severe cases) that causes rapid and excessively high cylinder pressure near the end of the normal combustion process. And as air/fuel mixtures are made more lean (or engine heat increases), the tendency toward detonation increases correspondingly. Detonation pressures are frequently as much as twice that of normal combustion pressure; and the mechanical damage that can be done to pistons, piston rings, ring lands and other parts or surfaces within the combustion space is severe.
Now, before we get any deeper into the subject, let’s give you a little food for thought in terms of applying some of what has just been discussed. Consider the possibility that an engine in detonation, while still burning about the same percentage of air/fuel mixture as an engine not detonating, will make about the same amount of power as it would if not in detonation.

C. The injection of water has been shown to reduce a given engine’s tendency toward detonation. Here you can see the relationship between maximum allowable cylinder pressure (before detonation) and air/fuel mixture ratio. Note that mixtures can be made more lean as the amount of injected water is increased. Note also that the level of cylinder pressure allowed prior to detonation is increased in proportion to injected water volume. What the graphs don’t show is how an engine’s thermal efficiency is reduced as a function of water injected. As discussed in the story, this has direct bearing in fuel economy, unless other additives are provided along with the water. D. This is the relationship between “normal” combustion and the spontaneous ignition of air/fuel mixtures in the combustion space. Secondary ignition of such mixtures results in a more rapid burning of air and fuel, resulting in a condition that gives rise to sharp increases in cylinder pressure and . . . detonation. And while piston dome and combustion chamber shape affect a given engine’s tendency toward detonation, the results are much the same: lost efficiency and reduced fuel economy, sometimes accompanied by wasted parts.

It has been shown that if there is no preignition, an engine running with best ignition spark timing will lose only a few percentage points of power if suddenly supplied with a fuel of reduced anti-knock quality. Further, if we were to slightly retard the spark timing of an engine in detonation, there is data supporting the fact that the same engine could make slightly more power than the engine not in detonation.
One particular factor affecting detonation (and we’re not saying detonation is good) is inlet pressure. Typically, an increase in inlet pressure (induction system flow rate) will increase combustion flame speed. This is partially because an inlet pressure increase causes an attending increase in air/fuel mixture density at the time of ignition (all else being equal) which speeds up flame travel. This means that there will be an accompanying increase in cylinder pressure during combustion, thus increasing the pressure level of the end combustion products. Depending upon related engine variables, an increase in inlet pressure tends to make an engine more prone to detonation. Case in point? Supercharged engines running on gasoline of poor anti-knock quality will normally detonate.
Another factor affecting detonation is compression ratio. As compression ratio is increased, so is the probability of detonation. This is particularly true of today’s pump gasolines, from which substantial amounts of antiknock chemical (usually tetraethyl lead) have been removed. Right about here, we should probably introduce another detonation-related term: the “delay period.” This is the time during the last stages of normal combustion when the remaining air/ fuel mixture is compressed beyond the point of self-ignition but has not yet self-ignited. If the delay period is long enough, normal combustion may progress to a detonation-free end. But if the delay period is short enough, self-ignition will occur and detonation result. We mention this because an increase in compression ratio usually shortens the delay period, leading to an increased tendency of the engine toward detonation.
Also, increasing the ignition spark advance will shorten the delay period and make an engine more likely to detonate. This is why a reduction in timing will reduce the problem of detonation.
Air/fuel mixture ratios have an effect on detonation. For conventional pump gasoline, the length of the delay period is about minimum at an air/fuel ratio of 12.5:1. However, combustion flame speed approaches a maximum at this same ratio.

E. Here you can see how net cylinder pressure (thus best power and best tendency to detonate) relates to air/fuel mixture ratio. As mentioned in the article, best power and highest cylinder pressure (in this case, indicated mean effective pressure) often occur at the same point, which frequently is the time at which detonation approaches a maximum. According to the graph, rich mixtures reduce power (and detonation) while lean mixtures can accomplish the same thing … all else being equal.

The odd part of all this is that an engine’s greatest tendency toward detonation appears to be at about its best air/ fuel ratio for maximum power.
Engine rpm also plays a hand in rhe detonation picture. Considering that we are using typical pump gasoline, the tendency toward detonation s decreased as engine speed is increased. This is probably because the combustion flame burns through the combustion space more rapidly as engine speed increases. There are exceptions to this, but it is generally the case. More rpm also mean greater levels of turbulence in the cylinders, and this tends to speed up the combustion process. But as a rule of thumb, detonation seems to occur more at low rpm (especially low rpm and high load) than at high.
Another factor of significance is the design of the combustion space, especially the combustion chamber. By shortening the distance of travel -equired for the combustion flame to reach the far “corners” of the com-oustion space, the problems of detonation are reduced. One way of accomplishing this is to have small combustion chambers. It is also good to locate the spark plug near the center of the chamber and not situated so that the end combustion mixtures will be close to some hot spot m the combustion space (edges of . alves, piston dome corners, etc.). Designing combustion chambers to increase the amount of mixture turbulence will also speed up the flame and help it traverse the combustion space quickly. This feature of turbulence is particularly useful in the design of engines intended to optimize fuel economy and/or wide-open-throttle power.
Type and quality of fuel also affect a given engine’s detonation characteristics. We mentioned before that the addition of tetraethyl lead helps supress detonation. And it does this by increasing the delay period. However, the lower the lead content and the higher the compression ratio, the greater the problem of detonation.
Which brings us around to water injection. Right off the bat, the introduction of water into an engine prone to detonation is not a panacea. A prime purpose for the addition of water is to reduce cylinder temperature and increase the delay period. Cooling the burning mixture of air and fuel means there is less tendency toward detonation. But the mere addition of water to the combustion process of a “heat engine” can reduce thermal efficiency and fuel economy during times when the water is injected. Of course, water volume and injection timing are important, since they should be used only during times when an engine is in detonation.
To regain some of the efficiency that can be lost to a water-injection system, other engine parameters (such as ignition timing) can be adjusted. For example, we stated that ignition timing affects a given engine’s detonation tendency. In such cases, advancing the spark timing will normally.increase this tendency.

F. As pointed out in the story, air/fuel mixture density can be related to the speed of flame travel during combustion. In figure a, mixture density is higher than in figure b. The result is faster movement of the combustion flame and a greater tendency toward detonation. Should detonation not be developed, combustion conditions in figure a would provide greater fuel economy and overall drive-ability than in figure b.

The addition of water in conjunction with slight increases in spark timing can rid the engine of detonation (or sharply reduce it) without a fuel economy penalty.
Another method involves the dilution of water with a chemical that can participate in the combustion process. Isopropyl alcohol (of the type you can buy at the corner pharmacy or grocery store) blended with water will restore some of the lost thermal efficiency while allowing the water to suppress detonation. Mixtures on the order of 50/50 will not only provide this sort of benefit but will also keep the water from freezing in cold weather. Not too bad, right?
In most engines used for cars, net efficiency involves a compression ratio sufficiently high that detonation will occur at low rpm levels at or near wide-open throttle. An exception to this can be found in engines of low compression ratio, unleaded fuel and lean air/fuei mixtures. Here, light-throttle detonation results from poor cylinder-to-cylinder air/fuel mixture distribution among an engine’s cylinders and a resulting extremely lean mixture condition that affects the delay period. In such cases, some cylinders receive less fuel than others (with about the same amount of air), resulting in an engine that not only may detonate at mid-rpm and light throttle conditions but is also lacking in fuel efficiency.
Water tends to reduce carbon residue in the combustion space, too. Over a long period of time, normal deposits of this type can contribute to preignition and a general loss in engine efficiency. For example, consider how an engine of given fuel economy (when new) gradually loses mileage as time passes and the miles accumulate. There is data indicating that a comparable engine fitted with water injection will not show the same amount of mileage deterioration as the engine ages. The belief is that by the reduction of carbon deposits within the combustion space, better combustion efficiency can be achieved. At any rate, it’s worth considering.
By way of a quick review, let’s collect some of the high points and throw in a couple of additional thoughts.
First of all, we’ve indicated that detonation is not good for an engine. While power and/or fuel economy performance may not be drastically reduced, parts damage normally results from prolonged detonation. Also, a distinction was made between preignition and detonation preignition being any initiation of the combustion process other than by ignition spark, and detonation defined as spontaneous combustion of air/fuel mixtures during the last stages of burning. The end-combustion period was defined as the time in which most detonation problems are developed. And the introduction of tetra-ethyl into gasoline was said to be an anti-knock or detonation suppressor.
Water also tends to reduce detonation by the decrease of combustion flame temperatures and subsequent drop in cylinder pressure. The allowance of hot spots in the combustion chamber can lead to preignition (carbon deposits, sharp edges of combustion space parts), and increases in the density of air/fuel charges can contribute to detonation. It was also mentioned that supercharged engines are more prone to detonation (using gasoline) than normally aspirated engines. Alcohol, used in conjunction with injected water, can help restore lost fuel economy in engines used for normal street transportation.
Finally, an increase in mechanical compression ratio tends to improve the chances for detonation. This is because end-mixture combustion time (the delay period) is reduced and detonation can occur more quickly. What wasn’t mentioned was the fact that valve overlap (time when both valves are unseated) can affect the amount of exhaust dilution of fresh air/fuel mixtures, resulting in a change in an engine’s tendency toward detonation. Rule of thumb here is: more dilution, less detonation.
Not a simple subject this time. But growing concern for engine efficiency and net fuel economy should make a short study of detonation worth your time. The compromise seems to include the fact that many vehicles now on the road are fitted with engines requiring gasoline of a quality not available at the pump. And for those of us who may be faced with the necessity of a premium fuel, the price has been increased accordingly. What might be worthwhile is developing an understanding of what can be done to adjust or build an engine for the gasoline of today. The characteristics of detonation are certainly a vital ingredient of such an understanding. After all, it can only save you money.

REVIEW QUESTIONS: True or False
1. Preignition is the result of combustion from overheated combustion space elements (spark plugs, carbon residue, etc.).
2. Unlike detonation, preignition does not affect net engine performance.
3. Preignition makes an engine act like there is excessive ignition timing retard.
4. Spark plug heat ranges have little or no effect on an engine’s tendency toward preignition.
5. When an engine goes into preignition, all cylinders experience this condition simultaneously.
6. So-called “after-running” and preignition are not related.
7. Combustion flame speed is initially quite high and decreases as the combustion process is completed.
8. If combustion pressure and temperature are sufficiently high, a spontaneous ignition of all remaining air/fuel mixture is called detonation.
9. Preignition causes an audible “knock” in an engine, and detonation does not.
10. As air/fuel mixture ratios are made rich, the tendency toward detonation increases.
11. Induction system pressure (inlet pressure) has little effect on detonation.
12. Compression ratio affects detonation, inasmuch as net cylinder pressure is made higher during the last stages of combustion, thus shortening the delay period.
13. Delay period is the time that elapses between the point of “self-ignition” of a given air/fuel mixture and the time of actual ignition spark.
14. Delay period is the time that elapses after a given air/fuel mixture is compressed beyond the point of self-ignition but before it has self-ignited.
15. Questions No. 13 and No. 14 are sorta tricky.
16. Rereading the text might aid in answering questions No. 13 and No. 14.
17. The tendency of an engine to detonate is increased according to increases in rpm.
18. The injection of water is intended to suppress detonation, but there is little or no effect on the amount of power produced or mileage achieved.
19. Carbon deposits within the combustion space can be reduced by the use of water.