As you work your way through this month's column, be aware there are successful engine and parts manufacturers who do not subscribe to what is being shared—and that's OK. There are also numerous of both these groups of people who do and have benefitted from the information. We will let you draw your own conclusions. Wet vs. dry flow benches notwithstanding, and neither of these will replicate the results of unsteady-state, pulsing airflow of a running engine, you may find value in some of the following.
First, we know that air and fuel require mixing before combustion. The manner and extent to which this is accomplished foretells the level of combustion efficiency achieved.
At the risk of oversimplification, you could liken the so-called "burn" process to touching a burning match to one corner of a piece of paper. Generally, a uniform flame will consume the material. But if there are small spots of oil or gasoline on the paper, flame travel will not be uniform. Ideally, in a running engine, we'd like for all atomized fuel droplets to be the same in size (no oil or gasoline spots), but this isn't the case.
The process also requires more time to combust large droplets than smaller ones, so flame travel is correspondingly variable. Given the very brief time available to mix and burn air/fuel charges, conditions that affect droplet size uniformity tend to reduce combustion efficiency and power.
Combustion efficiency can vary for numerous reasons, in particular as a function of load and engine speed. In an environment of continually varying mixture speed and interrupted flow, air/fuel charge ratios will not be uniform in the combustion space that notably includes the chamber, piston crown, and inlet path. So because of these inequalities in mixture ratios, rich conditions tend to leave darker combustion residue than leaner ones (please refer to the accompanying pictures).
Of course, there are other elements in play that can "color" the surfaces of these parts (intrusive oil, coolant, and so on) but it's not too difficult to distinguish between residue from excessive oil and rich air/fuel ratios.
Turbulence is important in the combustion process. Whether it's deliberate as in "tumble" and "swirl," or coincident with trying to achieve higher levels of volumetric efficiency (or all of these), turbulence can aid keeping fuel properly mixed and suspended.
Just because fuel may pass from its point of delivery into the inlet airstream in a well-mixed fashion, it doesn't necessarily follow that the results of efficient mixing will remain that way in the combustion space. Virtually any disruption in what we'll call "mixture quality" can diminish combustion efficiency and decrease power.
Hopefully, as you try to visualize these various events, envision them in ultra-slow motion, because we need to remember that combustion is a process, not an event.
Probably one of the most common problems result from flame movement over, or past, sharp edges in the combustion space, and this includes piston crowns. These consist of valve notches or clearance "eyebrows" in the crown, sharp corners on piston domes, or other changes in surface direction or finish.
Not to be confused with intentional surface conditions (like dimpling intended to improve mixture quality), sharp edges can create unwanted eddies or vortices which, in turn, can cause mechanical separation of air and fuel. Evidence of this condition, in terms of combustion residue patterns, is the absence of color or "clean" areas on piston crowns and in chambers not associated with the quench area.
One area where "clean" spots occur is on line-of-sight paths from the intake port to combustion chamber wall. An example of this can be found in most small-block Chevrolet V-8 cylinder heads (OEM or aftermarket) where the "back wall" of the chamber is clean, as viewed through the intake port with the valve removed. In fact, based on experience, here is an area where benefits often result from placing small "dimples" on this portion of the chamber.