For a variety of reasons, race engines can experience either or both these conditions involving uncontrolled combustion. And, we all know either or both can cause power losses and potentially damaged parts, depending upon the frequency and severity of the events. Experience has shown that there is sometimes a misunderstanding about how these two events are created, what actually takes place in the combustion space when they do occur, and ways to avoid both problems. First, let’s examine each of the events to determine how they transpire.
During normal combustion and following spark ignition, the flame front travels through the combustion space, burning air/fuel charges while compressing and raising the temperature in the remaining, unburned air and fuel. Tests have shown that during this process, the pressure in the burning and unburned charges are relatively equal. If we focus our attention on what is occurring with and within the remaining unburned air/fuel charge, we find that as the flame front is approaching, the unburned charge has had its temperature and pressure increased not only by the advancing flame front but also by the piston ascending on its compression stroke. It’s during this phase of the combustion process that the remaining unburned charge will reach a temperature and pressure that exceeds the “self ignition” point, causing the unburned charge to spontaneously ignite.
Studies have shown it’s because of this excessive and rapid rise in unburned air/fuel charge and the fact self-ignition occurs so rapidly that it is considered combustion at a constant volume. Since the cylinder pressure rise associated with the first part of the combustion process is much lower than the cylinder pressure that occurs at self-ignition, a pressure wave resulting from the latter travels back and forth across the combustion space, creating audible “knock” or the sound of detonation. So by way of definition, we could say that detonation is the result of self-ignition (virtually instantaneous combustion) of the unburned air/fuel charge brought about by excessive temperatures and pressures following the normal initiation and progression of the burn.
But as Smokey often said, “There’s just one more little item.” Digging a bit deeper, it has been discovered experimentally that even when air/fuel charges are compressed and heated above the point of self-ignition (spontaneous ignition), there is a time period that passes before this ignition occurs. By definition, this is called the “delay period” prior to combustion. Further, such experiments revealed that even when such charges are conditioned beyond the point of self-ignition, this event doesn’t occur every time. In fact, by sufficiently increasing the delay period (experimentally), the normal flame front may pass through the last portions of the air/fuel charge without detonation. Therefore, in a running engine, if the delay period is so short that the end-portions of the air/fuel charge are not exposed to the normally-advancing flame front, the previously described condition of excessive pressure and temperature leads to spontaneous combustion (detonation).
From a practical standpoint, the list of factors that can affect flame speed include piston displacement, piston speed, air/fuel charge ratio, the presence of combustion byproducts, compression ratio and rpm. Of these, at least within the scope of this discussion, there are two we’d like to point out and identify their effects on flame speed; rpm and combustion residue in the combustion space.
First, let’s examine the effects of rpm. As engine speed increases, so does airflow velocity, independent of how fuel is introduced. Whether by intended or unintended reasons, so does airflow turbulence. Sometimes this is controlled, sometimes not. Short of turbulence that causes a mechanical separation of air and fuel (obviously not good), air/fuel charge atomization can be improved, and we’ve previously discussed the benefits from this; e.g., improved atomization, improved flame speed and generally increased combustion efficiency as evidenced by lower brake specific fuel consumption readings.
Here’s where detonation wasn’t...
Here’s where detonation wasn’t noticed and the engine ended up grenading. Detonation first, then something broke and caused the mechanical damage you see to the piston.
Combustion residue in the combustion space? Not good at all. Exhaust gas does not combust. Any presence of combustion residue interrupts smooth and rapid flame travel. By whatever process causes inefficient evacuation of combustion byproducts (typically from reversion), flame speed suffers. And when you factor in the influence of a poor intake manifold design, cylinder-to-cylinder mixture distribution becomes an added problem against having near equal flame speed in each combustion space. It turns out that distribution problems among an engine’s cylinders, caused by an intake manifold, can vary as a function of rpm and manifold design. And sorting out mixture distribution issues on an engine dynamometer often bears little resemblance to the patterns that occur on the track. But we digressed.
The results from detonation, aside from parts damage, include such factors as loss of power, efficiency and….pre-ignition. As a rule, pre-ignition is the initiation of the combustion process by something other than a controlled spark, typically a hot-spot in the combustion space. Pretty straightforward. But there’s more.
If pre-ignition occurs early in the normal combustion cycle, additional work must be done by the engine working against combustion pressure, as the piston would normally be completing its compression stroke ahead of a controlled spark. Much like an over-advanced ignition system, power is reduced. Further to this, if a hot-spot is remotely located from the spark plug and initiates combustion from that location early after normal combustion begins, the two flame fronts can elevate cylinder pressure to the point of creating detonation. Simply stated, detonation and pre-ignition aren’t the same, although pre-ignition can lead to detonation, if the circumstances are right.
A Few Hands-on Tips
In terms of applying some of this information to your parts selection, modification or use, there are two areas of particular importance. The first deals with air/fuel charge mixture quality, as it relates to detonation suppression. Previous discussions in this column have emphasized the need to homogenize and mix the two as well as possible. Flame speed is also a function of atomization efficiency; e.g., the smaller the droplets (all else being equal), the faster the burn, the higher the total force on the pistons (power) and the less the tendency toward detonation. Specifically, roughened intake paths, improved carburetor booster efficiency, and “roughed-up” piston crowns can all contribute to increased atomization. You may also find less total ignition spark is required for optimum power, a direct result of a quicker burn as evidenced by reduced exhaust gas temperatures.
The other area has to do with reducing exhaust residue in the combustion space to an absolute minimum. Early exhaust events (relative to BDC piston position on the power stroke) place an added burden on reducing this inert gas. Two suggestions come to mind.
Intake valve jobs that help reduce back-flow into the inlet path. Such modifications can be evaluated by flowing intake ports in reverse direction. And finally, you may find that exhaust port modifications measured with the exhaust valve “parked” at about 85 percent of maximum lift (on the flow bench) will yield more efficient total exhaust gas elimination from the combustion space. Successful modifications here will also show up as reduced brake specific fuel consumption (b.s.f.c.) numbers on the dyno. Clearly, fresh air/fuel charges diluted with exhaust gas not only reduce flame speed and power but can lead to detonation problems already discussed.