A regulation problem with the two-stroke carburetor engine
Original: Ein Regulierproblem des Zweitaktvergasermotors
Von Dr.-Ing. Otto Holm, Hamburg, 1942
It is well known that two-stroke petrol engines with carburettor cannot idle evenly with regular ignitions at every revolution. At best, the engine will switch to regular four-stroke operation. Often, individual ignitions follow several misfires at more or less regular intervals. The ignition intervals are generally greater the lower the idle speed. It is impossible to achieve an idle speed as low as that of a four-stroke engine, which still operates without misfires. How can this behavior of the two-stroke engine be explained?
There is a very obvious and plausible explanation for the irregular running at no or low load. According to this, the misfires are due to the fact that the relatively small amount of purge gas required for one working cycle is not sufficient to completely purge the cylinder chamber. As a result, a lot of unpurged exhaust gas remains. This mixes with the fresh gas. This dilutes it so much that it is no longer ignitable. The fresh gas flushing of the following cycles means that the cylinder contents are increasingly enriched with fresh gas until they are ignitable. An ignition occurs, which is followed by several misfires, with the above processes repeating themselves.
This explanation contradicts the empirical fact that a very slow idle is much easier to achieve with a large idle jet in the carburettor, i.e. with high fuel consumption, than with a small fuel jet, i.e. with lower fuel consumption. If, for example, the fuel supply is shut off when the engine is idling safely and slowly, the speed initially increases significantly, before it then drops again a little later and the engine stops due to a lack of fuel. During the transition period, less fuel is sucked out of the jet than before due to the falling fuel level in the float chamber. The mixture formed is therefore less fuel-efficient and the engine tends to run over. In fact, the irregular running of the engine at idle and under light load is caused by the fact that the ignition failures occur as a result of excess fuel and lack of air, and ignitions occur when the mixture becomes somewhat thinner in fuel.
The periodic fluctuations in the fuel content of the mixture are caused by the speed fluctuations, which in turn are caused by the periodically uneven ignition sequence. When idling, the carburetor throttle or slide is almost completely closed. The air drawn in flows at a relatively high speed through a narrow gap that is still open, sucking the fuel out of the idle jet. The air speed depends not only on the size of the gap but also on the engine speed.
As the air speed increases, the mixture becomes richer in fuel; as the speed decreases, it becomes poorer in fuel. This phenomenon is often even more pronounced when idling is regulated than when the engine is under load, when the idle jet is higher above the fuel level in the float chamber than the main jet. The well-known fact that a fuel-air mixture formed in a simple spray carburetor becomes richer in fuel as the air speed increases, unless compensation is provided in some way, can be traced back to this. that fuel delivery only begins at a certain minimum air speed. This minimum air speed is required in order to suck the fuel level in the nozzle up to the outlet. In the idle state, the fuel level is a little below the outlet. This prevents it from overflowing accidentally in the event of vibrations - for example in a vehicle.
The main result is that it can only escape when the speed of the air passing by is sufficient to entrain and atomize the fuel. It can be shown that of the two possible methods of idling regulation at the upper and lower ignition limits of the mixture, the one with a fuel-rich mixture results in a far more stable equilibrium. For this reason, a two-stroke carburettor engine is always regulated to a fuel-rich mixture even when idling, not least because when working with a fuel-poor mixture, the ignitions can easily backfire into the intake line and the carburettor. A stable operating state is always present then. if deviations from it create forces or moments that tend to restore the original state. If the speed of a two-stroke engine idling at the fuel supersaturation limit increases, the suction resistance of the largely throttled carburettor increases.
This increases the internal resistance of the machine that has to be overcome. In addition, the mixture becomes richer in fuel, as explained above, and therefore less ignitable. The fuel winding is reduced. These two effects of the temporary increase in speed mean that the speed drops again as quickly as possible until equilibrium is restored. If the speed is temporarily reduced, the opposite effects are triggered. The throttling effect and thus the internal resistance decrease and the mixture formed becomes less fuel-rich and therefore more effective. Its burning speed and power development increase. As a result, the engine is relieved of load and accelerated strongly, so that the original higher speed of the equilibrium state is quickly reached again.
When idling near the lower limit of the mixture's ignitability, the forces acting to maintain a state of equilibrium are significantly smaller. The throttling effect, which increases the load when the speed increases and reduces the load when the speed decreases, is the same as in the case discussed first. But the change in the mixture composition now works in the opposite direction. When the speed increases, the fuel-rich mixture becomes more ignitable, burns faster and produces a greater output. As a result, there is a tendency to accelerate further. When the speed drops, the poorer mixture loses even more ignitability and burns more slowly, so that the engine shows a tendency to reduce its speed even further. Only when the throttling effect exceeds the effect of influencing the mixture at a sufficiently high speed is a stable control state present and possible. Therefore, adjustment to a low idle speed is only possible with a large idle jet, i.e. with a fuel-rich, so-called rich mixture! If the fuel nozzles become inadvertently narrowed due to contamination, the idle speed will immediately increase significantly! Such an increase in speed is a direct indication that the fuel nozzles are dirty. On the other hand, the automatic control in the area of fuel supersaturation of the mixture is so stable that it is possible to limit the speed at any throttle position by installing fuel nozzles that are too large - at the expense of fuel consumption - so that the engine does not run over! This feature cannot be exploited in practice, but it is interesting from a control engineering perspective.
When assessing the control properties of engines equipped with centrifugal governors, the influence of the size of the fuel nozzles must be taken into account. It is possible to meet particularly stringent requirements for the limits of the control speeds by choosing a somewhat larger size of the fuel nozzles. If the nozzles are slightly clogged, the control limits will then deviate noticeably. In this case, the centrifugal governor is too weak. Its adjustment energy between the prescribed control limit speeds is not sufficient. These are usually determined by suddenly putting the engine under full load and just as suddenly unloading it, and using a tachometer to observe the highest and lowest speeds that the instrument shows. Whether the speed limitation observed in this way is actually caused by the centrifugal governor alone or also by the mixture being influenced by the fuel nozzles can be determined in the manner described at the beginning by closing the fuel valve in the supply line. The unloaded engine then briefly increases its speed to the true control limit speed. If this increase in speed does not occur, the centrifugal control is OK and the controller is sufficient. Otherwise, a controller with greater adjustment energy must be installed or the harmful frictional resistance of the adjustment rod and the throttle element must be reduced.
It is also interesting to answer the question of how a two-stroke ignition engine with fuel injection will perform. Does it also have an idle control problem or does it not occur as a result of the fuel injection? The answer is: "Yes, the control problem exists here too!" Slow two-stroke idling is also not possible. With an injection ignition engine, the intake air quantities and the injected fuel quantities must be regulated simultaneously and adjusted to one another. If the amount of purge air is reduced by throttling, the amount of exhaust gas remaining in the working cylinder after purging increases accordingly, since the pressure in the working cylinder adjusts to the external atmospheric pressure until compression begins through the exhaust slot. The mixture only becomes ignitable from a certain minimum amount of air and a corresponding amount of fuel. As soon as the ignitability is definitely achieved, the work output of the burning mixture is already greater than desired. The engine does more than the idle work and would run through without ignition failures if the load was completely relieved.
Idling in four-stroke or six-stroke mode is achieved in such a way that the amount of scavenging air is more strongly influenced by deviations from the equilibrium speed than the amount of fuel. If the speed increases, for example, the amount of scavenging air for each working stroke becomes smaller due to the throttling effect of the almost closed throttle valve, while the delivery rate of each fuel pump stroke is hardly affected, and in some pump designs even increases. As a result, the fuel-air mixture becomes richer in fuel. If the amount of fuel for the idle range was already generous, the mixture becomes less or no longer ignitable, so that the speed drops again and the equilibrium state is restored. If the speed deviates downwards, the effect is the opposite. The mixture becomes poorer in fuel, thus more ignitable, and accelerates the engine so that the equilibrium speed is reached again. The engine therefore runs in four-stroke mode despite practically uniform fuel injection at every revolution! This is a surprising observation at first glance, which one makes during the idle test on the test bench.
Incidentally, you can also achieve regular two-stroke idling in a two-stroke engine if you set the ignition so late that the mixture only ignites after the piston has passed the internal dead center. The energy released during combustion is then converted into mechanical work with such poor efficiency that you can process a readily ignitable mixture at every revolution without the engine running over. It also stays warm without giving off any power. This regulation option is generally not used because the usual throttle control does not result in ignition distortion. In addition, the ignition regulation is not entirely satisfactory because the ignition has to be very late when idling and the engine therefore starts to run hard.