Engineology: Engine Cycle Analysis

Using Engine Cycle Analysis to monitor the combustion process

A number of years ago, during the early life of this column, we touched on a subject that remains an issue, particularly when building or modifying an engine intended for racing. It was also early in the availability and affordability of in-cylinder combustion pressure testing as a function of incremental crankshaft angles. “Engine cycle analysis” (ECA) had evolved from the academic community and was further explored among the higher-end motorsports, such as F1 and NASCAR. This technology provides combustion pressure measurement in virtually real-time while linking the data to high-resolution crankshaft angles. From these data, a variety of thermodynamic-related calculations give particular insight into how a given engine is converting fuel into heat during the combustion process.

This approach to combustion analysis provides a way to look inside the combustion space, particularly with respect to cyclic dispersion and how frequently it occurs. Let’s take a closer look into what this condition affects.

First, let’s discuss what the term means. We have on multiple occasions discussed the importance of air/fuel charge quality as it pertains to both air flow and fuel atomization efficiency. It’s a frequent topic because the quality of both is critical to optimizing combustion efficiency and power. Further, we’ve suggested a poor man’s way of monitoring combustion efficiency is through the study and optimization of brake-specific fuel consumption (BSFC) data. Not only does ECA enable measure the pressure history from beginning to end of each combustion cycle, it can also measure such history’s cycle to cycle in a running engine. Hang in with us. This’ll all come together soon.

Since in any given cylinder of a running engine, it’s not only possible but also probable that on a cycle-to-cycle basis, the same combustion efficiency level will vary due to all the previous conditions we’ve listed about air and air/fuel charge quality. Combustion cycle to combustion cycle, there can be variations in the actual air/fuel ratio in the spark plug’s gap, again based on the conditions we’ve attempted to describe. Based on the amount of combustion residue (exhaust gas) that remains in the combustion space, turbulence at the beginning of the burn and separated fuel can cause variables in the air/fuel ratio at the gap from cycle to cycle in the same cylinder. Regardless of the cause, it is this cyclic dispersion that results from cycle to cycle changes in combustion efficiency. OK, we’ve now come full circle. We began by noting conditions that can affect combustion efficiency and introduced the fact that it can subsequently impact cycle-to-cycle power levels.

What are additional causes of cyclic dispersion that you must consider and can address? Well, separated air and fuel in the combustion space is pretty high on the priority list. Of course, these two are related. For example, mixture motion (swirl and tumble) has been used in both stock and racing engines, with the possible exception of the racing engines that operate at significantly higher rpm. By definition, swirl motion is air (also with fuel) that is rotating about a vertical axis upon entering the combustion space, while tumble (also with fuel) that is rotating about pretty much a horizontal axis as it meets the combustion space. Taken to excess, it’s possible to generate too much of either (or both), causing fuel to be centrifuged out of its suspension (mechanically separated—and you know the results of that. There can be, and often is, a corresponding reduction in volumetric efficiency. Of course, the consequence from this is reduced torque.

During the period when this column was presenting commentary from a number of noted engine builders, one of the questions consistently asked was about the advantages or disadvantages of EFI in racing, particularly at the local level. That question was included for a reason. We wanted to canvas that particular group of engine builders who were and are in relatively consistent contact with the weekend, or possibly touring, racer. Consensus opinion seemed to favor carburetors instead, for reasons that we don’t need to consider here.

But think about this: The difference in atomization efficiency between EFI and carburetors is significant. And guess what? This leads us back to improvements in cyclic dispersion, whereby the carburetor (remember Smokey’s comment about it being a controlled leak) does a poor job of atomizing fuel, as compared to EFI. While we are not advocating the use of EFI (with its cost and learning curve), the comparison illustrates the benefits from, and importance of, good fuel atomization, especially when using carburetors.

How much power loss can unattended cyclic dispersion cause? We’ve seen a fair amount of data that indicates a range of 5 to 8 percent. Stated another way, if we can reduce the amount of cyclic dispersion and use more of the fuel supplied an engine, you can expect about this amount of power gain, over baseline power that’s experiencing the problem. Bottom line? A reduction in cyclic dispersion tends to improve combustion efficiency while translating to increased crankshaft torque.

Serious students of internal combustion engines say not only does the reduction of cyclic dispersion net more power, but it also helps assure combustion efficiency during the early stages of a given burn, particularly since peak cylinder pressure occurs just past TDC on the power stroke and varies somewhat as a function of revolutions per minute. Initiating a quality burn early in the process helps overall combustion efficiency and power.

To circle back to other discussions about air/fuel charge quality and ways to improve combustion (once again in an attempt to compensate for a carburetor’s poor atomization efficiency), recall that we discussed mechanically aiding the break-up and attending suspension of fuel in the inlet air stream. The texturing of certain areas along the intake path (post carburetor), properly done, can help the overall cylinder-filling event with more combustible air/fuel charges. In fact, some cylinder head modifiers currently employ their own brands of this by the practices they follow themselves. We even recall some favorable results that came from small dimples placed on the backside of intake valve, but that’s an entirely different topic.

We are typically dealing with engines of multiple cylinders, connected in some fashion that allows them to influence the overall volumetric efficiency of the entire package. Understanding the phenomena and seeking ways to minimize their effects can net an increase in power. The use of ECA can produce revealing and useful data. However, it’s possible to watch for and experiment with much less-expensive techniques, which only require fundamental understanding about the telltale signs and for which you can apply comparatively simple tools.

Source: hotrod.com

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