® The Mysteries of Combustion

Combustion within an engine is not explosion. Combustion is a rolling flame across the piston's top, which continues steadily, relentlessly, outwardly moving like the ripples in a pond when a rock is dropped in, until all the intaked air/fuel mixture is consumed. While this is going on, the heat produced is turned by the enclosed cylinder into pressure which pushes down the piston.1 Combustion's flame is the laziest part of the engine's power operation; it takes its own sweet time to traverse the cylinder. A valve can be opened as many as a hundred times a second. The piston reaches speeds of nearly as many feet per second. But the flame's speed never changes. It can't be hurried. Its importance is such that everything else is subservient to it; thus combustion must begin even before the piston reaches the top of the cylinder, so that the burn can start and pressure can be near peak by the time the piston is poised for its rapid descent.

However, there is a problem. Because of combustion's flying start the piston is rising against an already-burning air/fuel mixture. The problem is that there are always pockets of fuel at the combustion chamber's edges that are the last to burn, and while they wait for the flame's arrival they are being pressurized and heated by the rising piston. It is understandable then that these pockets can reach ignition point on their own. When that happens, they don't burn, they explode. The result, unlike combustion, is not a smooth push on the piston but instead a tremendous shock that breaks the piston. Not push but a hammer. This shock is called detonation. The difference is not one of force--the fuel releases the same thermal energy in either case.2 What is different between normal combustion and detonation is time. While normal combustion might be spread over some 70 degrees of crankshaft rotation, allowing the engine to safely and efficiently absorb and channel the force, detonation hits intantaneously and rapidly, spending itself in a fraction of the time, and with that concentration of force comes damage. Incidentally, one of the ways you detect detonation is loss of power. Detonaton robs power because it steals energy that should be taking place over a longish, useful time, and instead pinpoints it into a very brief, destructive moment.3

While on the subject, beware of confusing detonation with preignition. While they can occur in combination, they are not at all the same thing. First-semester technical school students know that three things distinguish preignition from detonation. First, detonation is spontaneous, that is, the fuel's own heat ignites it. Preignition on the other hand is caused by an outside heat source, usually a hot spot in the combustion chamber (typically a glowing piece of carbon). Second, detonation occurs after the spark plug's spark, preigniton always before. And third, the results of detonation are a shock-hammering of the piston (resulting in broken ring lands, typically), while preignition more often melts the piston crown. There are more differences, but these are the three rules of thumb.

In motoring's earliest years, detonation was the obstacle to engine power. For a reason we'll explore shortly, initial attempts to get beyond this barrier focused solely on the fuel. As gasoline began to be refined, General Motors engineer Charles Kettering (who later founded Delco) was tasked with finding ways to make gasoline behave. His engineering team experimented with idodine, various metallic salts, and all sorts of chemical additives to gasoline until finally, in 1922, chemical lead was tried with astounding results. What lead did was to insulate the air/fuel molecules from each other, preventing them from rubbing on one another, so that their self-ignition point could be delayed until combustion's flame reached them.

That such an abundant and inexpensive additive could do so much was a boon to engine power development. It was nothing less than a watershed event. But it also very ironically at the same time stymied the development of engine efficiency. Lead's discovery, while it heralded in a new era of engine performance, actually stunted the development of engines because then and for decades afterward it was simply too easy to make engines survive by use of a chemical bandaid rather than to improve combustion chamber design. Conversely, the late 1960s' start of the war against lead, while it caused angst among a lot of us gearheads, ultimately proved to not only be a good thing for human health, but it rather perversely was also the best thing to happen to the internal combustion engine. The removal of lead whisked off the blinders engineers had put over their own heads and forced them to make engines combust far better than they would have otherwise, and should have for the previous almost 50 years.

As mentioned, the time required for the flame to get across the chamber and consume all of the intaked air/fuel charge is fixed. The amount of time taken never varies. No matter the engine rpm: a lazy 1,000 revs per minute or a very rushed 10,000, the burn stll takes its required time. It should become clear then that the ignition timing (the head start) that works at 1,000 rpm is not going to work at 10,000 rpm. At the higher rpm, the time alloted for combustion is reduced to just one-tenth the original time. Combustion can't be allowed to extend into the next cycle, and the burn can't be sped up. Something has to give. The solution is to start combustion earlier as rpm increase. If at 1,000 rpm the ignition fired the spark plug at 10 degrees before top dead center (BTDC), then at 5,000 rpm it might have to start at 20 degrees instead, and at 10,000 rpm it might need to start at 30 degrees. This is necessary just so combustion has all the time it needs to be completed. This thing of starting combustion earlier and earlier as rpm increase is called "ignition advance", and it's facilitated by a system that makes spark happen earlier as the engine revs. In the beginning days of internal combustion engines this advance was done by the rider, who moved a lever on the handlebar. But as motorcycles got faster and more sophisticated, automatic forms of ignition advance were developed. The most familiar of these to vintage Japanese motorbike riders is the spring-loaded centrifugal advancer whose flyweights rotate a small cam, and ignition advance is taken care of with no intervention from the rider.

How early ignition timing has to be depends on a number of things: how well the air and fuel are mixed, the air's speed as it enters the chamber, whether the air enters in a steady stream or tumbles and whirls, and some other considerations. The most important single factor however is the volume and shape of the combustion chamber. Consequently, every combustion chamber design requires a different amount of ignition advance. Combustion chambers in fact are rated by the timings they require. Fairly early on, engineers discovered that the purely hemispherical combustion chamber, that is, one that could nearly swallow a baseball, had an ideal property: since the arc of the chamber roof was continous, that is, its angle never changed, the flame's speed was therefore also continuous and thus the time required for it to complete its pass across the chamber could be accurately predicted. Unfortunately, continuous isn't the same as fast, and the hemi proved of little real benefit as engine development moved forward. Oddly, the hemi combustion chamber retains its mystical fame for other reasons entirely, most of them related to the sizes of the engines that employed them.4 Though misunderstood today due to folklore, the rather ancient hemi combustion chamber was one of the worst combustion-wise, requiring more than 50 degrees of total ignition advance at high rpm. Later partial hemis were better, topping out at some 40+ degrees. (The pentroof chamber popularized by the earliest four-valve designs, despite its promises, was unfortunately no better than the partial hemi.) But gradually combustion chambers were reshaped, usually acquiring in the process more vertical valve angles, until liquid cooling became common. Liquid cooling meant valve angles could be made radically more vertical because airflow over the top of the chamber was no longer important, and these designs brought us what was first called the "flying saucer" (Yamaha Genesis) combustion chamber: very flat and disc-like, and nearly optimum for combustion's flame, as can clearly be seen in its sub-30 degree ignition advance requirement. Today, none of the best production engines time later than 30 degrees and many are in the 20s.

Reduced optimum ignition timing is therefore the indicator of a better combustion chamber. Shorter timings, where possible, offer three significant benefits. One, they free up engine power. Engines with larger timings actually give away power because remember that for much of the combustion event the piston rises against an already-burning mixture. With less of this internal push-back in engines with shorter timings, more of the engine's work comes out as power. Two, the best, very shallow combustion chambers offer a smoother, easier path for the rolling flame, and most importantly, no pockets that trap mixture, so that detonation is virtually eliminated. This is huge. Detonation remember is generated by trapped pockets of mixture that become overheated and self-ignite before the flame can reach them. Flying saucer chambers don't create these pockets. Thus, production engines with 12:1 and even 13:1 compression are now common, and this on 87 octane. Three, the flatter chamber is superior because it doesn't need the high-topped pistons in order to get reasonably high compression. Flat top pistons will do instead, and this is a boon to the engine on numerous levels: freer air pumping is merely one of them. Flat topped pistons are best when their choice is possible.

Speaking of octane, this is an interesting thing. First, in a nutshell, octane is simply a system that rates a fuel's compressibility to the point of self-ignition. Though a common misconception, octane has nothing to do with power--that comes from the fuel's thermal energy.5 Second, octane is a laboratory invention. That is, fuel chemists took the worst detonation resistant test fuel, heptane, and compared it with the best, iso-octane, and devised a percentage system. An actual gasoline that exhibited detonation resistance equal to a heptane/iso-octane mix of 80 percent was dubbed an 80 "octane" fuel. The numbering system was later revised in two ways. First, room was made for gasolines performing over 100 percent, for example fuels needed in supercharged warplanes. Second, the white-coated lab guys argued with the greasy-handed test technicians, and a compromise was formed that resulted in real-world "pump" octane designations. You're familiar with these formulas on the faces of gas station pumps.

Of course, alcohol and other oxygenates are part of gasoline to increase octane. But oxygenated gas actually has a more important role, the reduction of exhaust emissions. With older design engines not as efficient as modern ones, oxy fuel forces such engines into later levels of emissions compliance by the gasoline bringing more oxygen into the combustion event, thus resulting in leanness that will reduce CO emissions. It's a broad-reaching solution. If you want gas, you pretty much get oxygenated gas. Though it would seem otherwise, oxygenated gas does not harm the older engines because motor vehicles are universally designed to run a bit richer than absolutely necessary for driveability reasons. There is a built-in margin there, as long as the vehicle is in good mechanical condition and tune. However, there is the catch. With the richness margin eliminated with oxy fuel, any little misadjustment can tip an engine into a performance problem. The bottom line is, oxy fuel makes properly running but simply older-designed engines emit polutants on a par with their more modern counterparts, satisfying the feds. The beauty is, while it may be non-elective, compulsary "updating" to older ones, oxygenated gas is actually invisible to modern engines. Their computers adjust fuel and ignition to compensate for the added oxygen, producing a zero net effect on the engine.

Speaking of older engines, engine modifiers have sought for ways to make older combustion chamber designs with largish, complexed shapes as efficient as the newer, flatter ones. Large amounts of squish have been tried. Squish is the band of tight spacing around the edge of the piston. Carefully designed, it wedges any mixture pockets away from the chamber's edges and toward the center, largely eliminating those pockets and greatly reducing detonation. Swirl is another tactic. This is merely taking advantage of intake air speed and manipulating it (through angles and carefully planned shapes in the port and chamber) so that the mixture is agitated when it enters the combustion chamber. The idea, again, is to reduce mixture pockets forming at the chamber's edges.

All of these are good and some are still used to varying degrees today. But one of the more enterprising solutions used when partial hemi chambers were common was to add a second spark plug to the combustion chamber. Wired to the ignition system so that both plugs fired as one, this arrangement, by sending across the chamber two flame fronts instead of just one, essentially halved the chamber's volume. The mixture was consumed in a shorter time, greatly reducing the partial hemi chamber's tendancy to detonate. Compression could then be increased, with the result greater power and greater detonation resistance at the same time. A two-valve pushrod BMW winning the inaugural AMA Superbike race in 1976, and a two-valve Kawasaki in 81, proved demonstrably that the dual-plug idea indeed could make older combustion chamber designs work far beyond their original limitations. Eventually, even the Big Four manufacturers would incorporate dual plugs into production, most notably in their large displacement cruisers.


1 Piston's push varies considerably depending where in the stroke the piston is at a given moment. Thus it is calculated as a mean pressure, usually referred to as "mean effective pressure".

2 Fuel heat energy is measured in BTUs, just like your air conditioners'. This its source of power.

3 Similar to the difference between your wife standing on your foot in her bare feet versus doing the same thing in her high heels.

4 The racing hemi's claims to fame included spark plugs in the center of the cylinder, valves so nearly horizontal that intakes up to 52 percent of the cylinder bore diameter could be fitted, and resultingly huge ports so large and so horizontal a broom handle could be pushed through from intake to exhaust. And big honking displacement. But burning? Very very poor.

5 There are two ways a higher octane fuel can deliver added power, but neither contradicts what I have said above about octane and power having no connection. One, if the higher octane fuel is race gas. Racing fuel is racing fuel categorically by being processed for engine use, and nothing else. No emissions-driven ingredients, either exhaust or evaporative, no compromises anywhere in refinement. None of that. By virtue of this it simply works better. The second way is if the engine in question is in such a poor state that it is detonating frequently (thus low on power). Even pump gas, if higher than the octane you normally use, will often remedy the detonation and thereby restore normal power.

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