Finally, the long, narrow shape of the combustion chamber means the flame front has a long way to travel, and sometimes it doesn't make it all the way to the ends. Partially because the motion of the rotor sweeps all the condensed fuel to the back of the combustion chamber, there tends to be a pocket of fuel-rich gases near the trailing apex seal. This pocket of unburned gas turns into a spurt of HCs as it gets pumped out the exhaust. The side exhaust port helps tremendously here, for the simple reason that it closes before the trailing end of the rotor gets there. The pocket of fuel-rich gases simply get swept around into the next intake stroke for a second chance at complete combustion. In the aforementioned research engine, moving the exhaust port to the side cut hydrocarbon emissions by 35 to 50 percent!

It turns out that even in a well-balanced charge of fuel and air, there are highly localized "pockets" of varying mixtures at the "local level." By "local level" you should think of a bunch of little fuel molecules huddling together "over here" and "over there" in different places inside the cylinder as the piston is rising up towards top dead center and starting down. Some of these pockets may be so lean (or so rich) that they won't burn at all, some may be in the combustible range, and some may be perfectly mixed, "ready to go," so to speak.

1. We have nice cool induction air and fuel entering a cylinder;

2. The cylinder happens to have very hot walls. Those hot walls cause some of that nice cool induction air to start to heat up. And it doesn't all happen uniformly.

3. Further, shortly after the sparks go off, we have a couple of flame fronts, giving off lots of infrared heat, adding to the continuing heat load being absorbed by some of those little remote pockets of fuel and air that are waiting for the flame front to arrive and consume them;

4. The unburned mixture is experiencing a very rapid increase in pressure, because of two things: A) The piston is rising rapidly during the compression stroke; and B) the flame front combustion products are creating a huge increase in released energy and resulting bulk gas pressure, all of which is neatly measured on the pressure traces you see in the accompanying graphics.

5. At least some of those little "local pockets" of unburned combustion mixtures have exactly the right mixture of fuel and air to be just a hair-trigger away from exploding.

6. And … if the fuel is the wrong octane, or the spark advance was set too soon, or the manifold pressure was too high, or the cylinder head temperature was too high ... then one or more of those little "local pockets" of unburned fuel do just that ... they "explode."