Some Enginology columns ago, we spoke about the functionality and benefits derived from in-cylinder pressure measurement in increments of crankshaft angles. In that discussion, we noted several useful data streams. One of them related to a continuous measurement of cylinder pressure from the beginning of combustion to its end, cycle to cycle in a running engine. Hold that thought for a moment.
We've also spent some space in this column talking about air/fuel mixture quality, as we have about the quality of inlet airflow. In particular, we've noted that cycle to cycle (in any given cylinder), it's possible to have differing air/fuel charge quality, based upon the efficiency by which fuel is blended with air. Fundamentally, air/fuel separation and how this can affect a range of fuel droplet sizes was a focal point we discussed. Because of the problems associated with poorly-mixed air and fuel, total working cylinder pressure can vary as displayed by changes in crankshaft torque.
The bottom line is decreased power. It is these changes in working cylinder pressure, cycle to cycle, that can be defined as "cyclic dispersion." Interestingly, exhaust gas analysis for unburned fuel (hydrocarbons or HC levels) has helped validate what the in-cylinder pressure variations suggested as lost power from poorly mixed or combusted air/fuel charges. In other words, as air and fuel tend to separate (either during the inlet cycle, as the combustion flame travels, or both), there's an increase in unburned fuel that's accompanied by a power reduction.
What can cause cyclic dispersion? Of the possibilities, separated air and fuel and overall mixture motion in the combustion space rank pretty high. And, as you might expect, these two conditions are related. For example, although two principle types of motion (swirl and tumble) have been employed in both stock and racing engines, it's possible to have too much of both. Either can be the cause of fuel being mechanically separated from air, somewhere along the path of time before combustion, as well as reducing net volumetric efficiency or cylinder filling. And, as previously discussed, there are separation causes that can materialize during the inlet cycle, not only among an engine's cylinders but in a random fashion, cycle to cycle, in individual cylinders.
Given the nature of how cyclic dispersion can develop, it doesn't require much imagination to see that an engine fitted with a carburetor could be more problematic than one with sequential, multi-point electronic fuel injection (MPEFI). Even a "batch-type" EFI (fuel delivered to four cylinders at a time in a V-8 configuration, for example) appears to offer a reduction in cyclic dispersion more than a carburetor layout and the customary wet-flow issues that can develop between it and the combustion space. In fact, in-cylinder pressure data I've seen comparing carbureted engines to those with EFI clearly shows a reduction in both overall cyclic dispersion patterns and those on a cycle-to-cycle basis for individual cylinders.
Furthermore, if we shift our focus over to how power can be reduced by what we'll call "typical" cyclic dispersion conditions, data has shown percentages of power reduction in a range of 5-8 percent. So by simply reducing this condition, given the same amount of fuel consumed, it's possible to increase power by this percentage. Translation? A reduction in cyclic dispersion can lead to improved combustion efficiency that nets an increase in crankshaft torque. That means more power.