Deceleration surges (pipe-bangs) in 2-stroke engines, and how to cure them with the PowerCDI
The so called "pipe-bangs" in 2-stroke engines, especially large-displacement ones (250cc plus), are one of the most debated and unresolved topics on enduro forums.
The notorious pipe-bang is neither detonation nor pre-ignition, as it is sometimes claimed. Instead, it is a normal but very intense combustion ignited by the spark plug, which increases the engine speed 500-700 RPM in a single engine revolution. This is perceived as a sudden surge, during engine braking.
The cause of this very intense combustion is a considerable buildup of unburned fuel, unburned due to the repeated misfires caused by the stagnant exhaust gases (still trapped in the cylinder because of poor scavenging due to the very low pressure we have in the crankcase when the throttle is held closed) which does not allow normal ignition for many of the next engine revolutions. Every time a misfire occurs (we get a regular spark, but the mixture does not ignite because there are too many residual burned gases around the spark plug, or the combustion begins but the excess of burned gases quickly extiguish the flame front and we get only partial combustion) and imperfect scavenging, the percentage of "fresh" mixture in the combustion chamber will however increase, engine revolution after engine revolution, while residual exhaust gases (which prevent ignition and combustion) are diluted more and more. Finally, again, we get back to the condition that the spark is able to begin combustion and the latter will also complete, but there are such huge accumulations of fuel (meanwhile evaporated and mixed with air, thus now in ignitable form) that the new combustion will be very intense (it will produce a lot of pressure on the piston and thus of torque, in a single engine cycle, resulting in a surge due to the increase of 500-700 RPM in just one engine revolution). This combustion obviously also produces new exhaust gases, which will require many engine revolutions to be cleaned from the cylinder under the low pressure conditions in the crankcase in which we are due to the closed throttle, and thus begins a new long cycle of misfires with the engine not able to burn the mixture, until exhaust gases are sufficiently diluted and we finally have combustion again (very intense because of all the fuel deposits accumulated in the meantime), perceived as a surge, production of new exhaust gases, and so on.
The cause therefore is not directly rich jetting, although this certainly worsens the problem indirectly, as it is the source of both (additional) misfires and unburned fuel accumulation, as well as slowing down the combustion, all things that certainly do not help the engine to run cleanly.
It is not sufficient to lean (or rather to optimize, neither rich nor lean, as also lean jetting slows down combustion and is a source of misfires - in addition to being harmful for many other reasons) closed throttle jetting, because even with perfect closed throttle jetting, fuel accumulation may have occurred up to the moment before, eg. at part throttle (when the mixture is determined by the needle). By closing the throttle - even if from this moment on we had proper jetting - the engine will still have a previous accumulation of unburned fuel, and this phenomenon is self-feeding, ie the deceleration surge cycle itself (as its combustion quality is low) is also a source of stagnant unscavenged exhaust gases which cause further misfires, in a vicious circle. So it's important not only to have good jetting (neither rich or lean) at closed throttle, but also at all other throttle positions, in order to not inherit excess fuel from a previous situation, which would be impossible to dispose of even with the good closed throttle carburetor setting.
Getting rid of the annoying pipe-bangs can be done in several ways. One is to reduce the idle until the throttle slide is completely shut, this way on closed throttle too little air-fuel mixture will enter into the engine making combustion impossible to occur, thus even in the presence of previous fuel deposits we'll never have the conditions for combustion to arise, and consequent pipe-bang (given the fuel deposits) to happen anyway. This solution though doesn't allow the engine to idle, moreover during long braking at high revs it will not ensure adequate lubrication of crankshaft and rod bearings. Even worse would be an apparently smart system that inhibits ignition on closed throttle and RPM higher than idle (thus only during engine braking), because even providing the lubrication to the bearings and effectively curing the pipe-bangs, it would cause also an increase of deposits of unburned oil and fuel, with consequent disaster (fouled spark plug) re-opening the throttle after it has remained closed for a relatively long time.
The solution to the problem should therefore be sought not in the resolution of the symptoms but in the cause, which is the excess of misfires on closed throttle (due to insufficient scavenging) and consequent accumulation of unburned fuel mixed with residual exhaust gases. As a general rule, incoming fuel should always be burnt, as much as possible. The less misfiring we have, the better it is. The more effectively the exhaust gases are extracted, the better it is (thus when the power valve is open the problem is reduced, as it is true that restrictions in the exhaust path make it worse). Technically, the blowdown time-area (the amount of time / distance / degrees between the opening of the exhaust port and of the transfer ports, also in relation to the area of the exhaust port) should be as large as possible, this helps the expulsion of the exhaust gases and also facilitates the entry of the fresh air-fuel mixture (as on a well designed 2-stroke engine the exhaust pipe "sucks" the fresh air-fuel mixture in even more than the crankcase is able to "push" them), resulting in greater efficiency in scavenging. In addition, as mentioned earlier, it is necessary to avoid both excess fuel (that the engine can not burn, thus not only have good jetting but not least the quality of the atomization of the fuel is extremely important), and to avoid everything that slows down combustion or, even worse, that prevents it (misfiring).
Let's reassess, it is important to have the right carburetor setting (neither rich nor lean) throughout all the throttle openings, not only at closed throttle, to prevent accumulation of unburned fuel. Anything that favors rapid and complete combustion also decreases the accumulation of unscavanged exhaust gases in the engine and consequently the problem of deceleration surges. So not only perfect jetting, but also efficient head and exhaust surely help, but they do not completely solve the problem.
The voltage and the spark energy are much higher on the PowerCDI compared to the original Kokusan Denki CDI (voltage available for arc formation is increased from 25000V to 45000V and total spark energy from 60mJ to over 200mJ in the case less favorable to the PowerCDI, but in critical conditions (such as engine start and overrev) the difference is even higher. This significantly reduces misfires, and a further assistance can be obtained by using iridium spark plugs (such as the NGK BR7EIX) and increasing the gap between the electrodes (which is not advisable on the original CDI due to its low voltage, but on the PowerCDI it's possible to go from the original 0.6mm (0.024") to 1.0mm (0.039") without any problems) being careful not to exert any force on the central electrode but always on the side electrode (ground).
With PowerCDI we have a very high energy at the spark plug, and a total control over all aspects related to the management of the same, and this is reflected in a much more complete combustion that in turn makes the pipe-bangs completely disappear even on the most refractory engines, and the engine is considerably cleaner also for example after a long downhill.
In common enduro/MX/motard engines, the DynamicPowerControl (see paragraph "DPC Insights" on the main page of the PowerCDI site) is not able to control the butterfly / throttle valve directly (drive-by-wire) and can only act (to dynamically de-power the engine) by retarding ignition or in some cases inhibiting it altogether, thus deliberately causing the slowdown and/or postponement (or in rare cases even the inhibition) of the combustion to obtain the de-powering necessary to ensure the control of the engine on every single engine revolution. The introduction of the retard (only when needed) potentially also has a slight enrichening effect (increasing the chance for pipe bangs) on the engine, therefore it is essential to start with clean jetting so that the use of retard (by DPC) is not a cause of excessive accumulation of fuel and unburned gases itself, that are at the origin of the problem in question.
In our experience, a well setup engine (jetting, squish, etc..) runs very cleanly when using DPC, even in slippery terrain, where the DPC works a lot. But jetting must receive due attention, and that's why we consider jetting an integrating part of the system and we provide strict instructions, extensive technical support and assistance for it.
In conclusion, with a good general set-up (jetting, head and exhaust), the great spark energy produced by the PowerCDI which optimizes combustion in all operating conditions (thanks also to the use of a carburetor equipped with TPS) the deceleration surges of the 2-stroke engines are cured very effectively even during prolonged use of de-powering (DPC).
In this chart (please remember that on the PowerCDI the USB cable-interface and software are standard, you can then record and analyze your telemetry) we see an acceleration in 3rd gear, on the X-axis we have the time (in seconds), and on the Y-axis the RPM (blue line) and the TPS (purple line). It starts with about 20% throttle, which is then quickly opened; up to about 6000 RPM the engine accelerates smoothly without misfiring; from 6000 to 7000 RPM on the other hand, marked with an oval and the letter M, you can see misfires during acceleration due to rich jetting and imperfectly setup power valve (which partially hampers good scavenging); once fully opened, the power valve ceases to cause problems and the engine accelerates smoothly from 7500 to 8200 RPM, after which slight misfires return due to excessively rich jetting (which prevent overrev). Once the 9700 RPM has been reached, the throttle is completely closed and here the interesting and relevant part begins with the subject of the present discussion: while the power valve is open, the efficient exhaust scavenging keeps the combustion chamber sufficiently clean, but under a certain RPM the exhaust valve closes and the deceleration surges (in the chart there are 3, marked by arrows and numbers) occur and push the engine speed (and the bike) forward in a single, very intense combustion.
The load that the bike presents on the engine in 3rd gear doesn't permit the RPM to rise so much in a single engine revolution, but due to the rear suspension and the play of the chain we get a temporary increase of engine speed, which is then re-absorbed in subsequent engine revolutions, reflecting again the actual speed of the bike.
In neutral, however, we do not have the "cushioning" effect of the rear suspension and chain play, and the energy released during the deceleration surge and consequent increase of the engine speed is preserved, as can be seen from the following telemetry chart, where we briefly fully opened the throttle in neutral (the jetting is too rich and it can be seen also by the engine not being able to go beyond about 9500 RPM, and by the typical misfire pattern), then on closed throttle we have here 2 deceleration surges (see the arrows), but the increase of engine speed, as the engine is in neutral, is not absorbed by the bike (as was the case in the previous graph):
Also note the typical irregular idle of 2-stroke engines (due to the same physical phenomenon that is at the origin of the deceleration surges, ie. insufficient scavenging), where we have 1 combustion followed by many misfires, typically 1 cycle with successful combustion followed by 4-5 cycles on 125/144cc and up to 8-10 cycles on 250/300cc where combustion fails (due to too many residual exhaust gases), or even more misfire cycles if the jetting is rich at idle.
The "bursts" that are felt when idling (as in neutral on the sidestand) on a 2-stroke engine are not the engine revolutions, but the successful combustions, so (on average, on a 250/300cc engine) about one-tenth of the actual engine revolutions. If you want to "listen" to the real engine speed, you must listen to the intake (air-box) noise, not the exhaust.
Needless to say, if during one engine revolution we have combustion (and smoke) from the exhaust, while for another 9 we don't, in these 9 a portion of the air-fuel-oil mixture produced by the carburetor will go directly to the exhaust, with the emission of unburned hydrocarbons skyrocketing (in reality a good part of the fresh air-fuel mixture gets trapped into the cylinder anyway, but some will still inevitably escape directly through the exhaust).
The so called "fourstroking" is also due to insufficient scavenging: it is a very regular 1 cycle of combustion followed by 1 cycle of misfire, and is very typical of 2 stroke engines, especially when jetted rich on the needle (or if a truly big pilot jet is used).
At higher (from 30%-50% on, depending on jetting) throttle openings, instead, the 2-stroke engine scavenges better and is able to begin and complete the combustion at each and every engine revolution, quite efficiently (especially when "on the pipe").
Q: To the R&D guy: why do you prefer to call them "deceleration surges", although are they more are commonly known as "pipe-bangs"? A: A necessary premise is that this problem (however we want to call it) is not treated in the literature, so (as I really find them annoying and wanted to get rid of them) I was left alone trying to understand the cause of this phenomenon, to find a solution. So the old but always good scientific method came to the resque, in a iterative process of guess/theory, and verification (e.g. lean, enrich, retard, advance, add squish, remove squish, compress, decompress, etc..), to verify empirically that the theory under exam always held, and eventually refine it.
I am confident that the description I gave of this phenomenon in this technical article is correct and complete, however there's still an aspect that I would like to investigate more, and it is the amount of unburned fuel that gets into the pipe (which, notably, inspires also the "pipe-bangs" name). While many thus say/think that the combustion happens (or at least continues) in the exhaust pipe, this doesn't seem the case because if it were so, then the pressure coming from this combustion would not be able to push against the piston, but indeed it does, as shown by the telemetry (it happens in a single cycle, but the torque is very very high). Moreover, if the combustion happened (or continued) in the exhaust pipe then the gases would push where it's easier for them to do it, and this is the silencer side, not the engine side, as the latter is sealed. You would hear it. I have provoked combustion in the pipe and it was very, very noticeable from the silencer side, it does a strong "sputtering" sound.. this is not my video and it's just a small displacement engine but that's what happens when combustion happens (or continues) inside the pipe (it is very noticeable at 0:10, 0:12, 0:14 and 0:20):
Now, the final verification would be to make and mount an exhaust pipe with no expansion chamber, just a tube/pipe, and verify how the pipe-bangs change, of course I'd "provoke" them by retarding and jetting accordingly. But all that I observed so far make me think that, although the pipe sure acts as a "tank of gasoline" for the cylinder (and returns part of its fuel back into it), it's only in the cylinder that pipe-bangs happen. They are very intense combustions, you cannot get torque by combustion inside the pipe, that wouldn't generate any pressure on the piston, and even if it happened in the pipe it could only push on the piston if the exhaust port is open, and the piston would be too down low and geometrically not in the best condition to generate mechanical work (but we know instead that the surge is very intense and powerful), let away that the expanding gases would prefer to "push" towards the silencer, as it would be easier for them to escape in that direction than to push towards the cylinder. Moreover, combustion in the expansion chamber is very, very slow, "open space" and quite ineffective in generating a mechanical force. I am very confident that the pipe only acts as a tank of fuel, nothing more than that, but that the real phenomenon happens inside the cylinder, in a very traditional manner. If you cut the ignition, the pipe-bangs will completely disappear, this is too an indirect proof that the combustion is traditional, albeit very intense.