Engine Development And Cylinder Pressure - Enginology

Written by on January 18, 2010 What Is "Cyclic Dispersion" And Why Should It Be Understood?
Engine Development And Cylinder Pressure - Enginology View All 2 Photos

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.

Ctrp 1004 01 O+engine Development Cylinder Pressure+peak Cylinder Pressure 2/2

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.

There's another little wrinkle involved here in further support of why a reduction in cyclic dispersion is good. Hard-core students of internal combustion engines will tell you that cyclic dispersion all but guarantees there'll be a different air/fuel mixture in a spark plug's gap, every firing cycle. Sometimes, based upon the extent of the condition (simply stated), the air/fuel charge will be rich, other times lean. Residual combustion byproducts, separated fuel, or the conditions of turbulence in the combustion space can affect what the plug sees at the time of combustion.

Regardless, what we'll call the quality of the initial combustion (flame) and the speed with which it traverses the combustion space is affected by the air/fuel ratio in the spark-plug gap at ignition. Even though the combustion process itself creates a measure of turbulence (early in the burn) that is overridden by subsequent activity as the flame continues, cyclic dispersion can affect the rate of initial combustion and net cylinder pressure. All this brings us back to in-cylinder pressure measurement to determine the extent of the problem.

Although somewhat exaggerated for purposes of discussion, note the accompanying illustration. While not taken directly from a pressure/crank-angle test trace, it illustrates how peak working cylinder pressure (nettorque) can vary as a function of cyclic dispersion. As previously discussed and illustrated in this column, peak pressure typically occurs slightly after TDC on the power stroke and varies in proportion to crankshaft speed. At any rate, the sketch illustrates the relationship among peak working pressures and crank angles, as affected by cyclic dispersion.

Techniques we've previously discussed related to ways of improving air/fuel charge quality tie into all this, particularly for engines fitted with a carburetor. In fact, if you'll take a moment to think about it, we who have worked on or designed parts for engines with carburetors have long been focused on dealing with poor mixture quality and the compromises involved that lead to reduced power. It's been the nature of the beast. However, with the advent of EFI and how this technology offers a distinct opportunity to reduce cyclic dispersion and its negative impact on engine performance, it only remains for the motorsports sanctioning bodies to step into contemporary times and allow (perhaps even require) this concept to be utilized.

There are pockets of circle track engine development already refining how EFI can bridge into this category of racing, addressing issues that might otherwise be of concern. Of them, how to deal with high-pressure fuel delivery systems and the potential for on-board fires when race cars are involved in accidents is one.

Given the innovative and creative capacity that historically abounds in the motorsports parts development community, there will be solutions to this and similar issues. The fact of the matter is that problems linked to how air/fuel charges are handled in engines fitted with carburetors can be materially improved by incorporating ways the OEM have addressed both emissions reductions and fuel economy requirements for on-road vehicles. EFI, aside from any concerns about dealing with associated electronics, presents a clear path to resolving some basic problems with the internal combustion engine, including those that turn left, right, or a combination of the two. Cyclic dispersion is only one issue that stands to be diminished.


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