Combustion Confusion

Combustion confusion

TDC

Kevin Cameron

CONFUSION CAN ARISE AS TO WHETHER an unconventional engine is two-stroke or four-stroke. The big example is the Wankel engine. What’s a stroke? In a piston engine, a stroke is a piston movement, from one end of a cylinder to the other. In a Wankel, which has no cylinder, we need another definition; a stroke becomes a cycle of volume change, either increasing or decreasing.

If we trace out what the combustion space does as it travels around the engine, defined by the rotor, we see that there are, indeed, four distinct “strokes” or cycles: intake, compression, power and exhaust. Because Wankel engines lack poppet valves, and perform their valving function as the rotor slides across peripheral or end-plate ports, some people become confused because two-stroke engines also have wall ports. But the Wankel engine is definitely a four-stroke.

The confusion cuts both ways. Detroit Diesel has for years made two-stroke diesel engines with four poppet valves per cylinder. All of them are exhausts, while the intakes are cylinder-wall ports, rows of them, located down near bottom center. Count the strokes; there are only two, so the exhaust valves open every revolution of the crank, not every other as in a four-stroke.

Britain built thousands of sleevevalve aircraft engines in WWII. All the ports of these engines are in the cylinder walls-three intakes and two exhausts. They look exactly like twostroke ports, except that they are all located just below the plain, featureless, valveless head. The sleeve, which is driven in an oscillating-reciprocating path by a small crank, has four ports in it. As the piston rises on compression, all the sleeve ports have closed. The engine fires and the piston falls on the power stroke. Near its end, sleeve motion exposes the two exhaust ports, and the piston rises on its exhaust stroke to push the exhaust out. As the exhausts close, the intakes slide smoothly open and the piston descends on its intake stroke. Four distinct strokes-even though the engine has cylinder-wall ports just like a two-stroke.

German engineers built torpedo engines in WWII that had, in the interest of compactness, disc valves instead of

poppet valves. But disc valves do not a two-stroke define; the engines were four-stroke.

The Beare “six-stroke” engine described in this issue’s Roundup is another example. Its design incorporates several features that we have learned to associate with two-strokes: cylinderwall ports, reed valves and disc valves. However, the Beare is resoundingly and definitely a four-stroke. Its wide powerband results from the reed valves in its intake, which prevent back-pumping of mixture at low rpm. Alfa Romeo placed reed valves upstream from conventional poppet intakes on one of its car engines to achieve this same result.

The Beare engine is said to be very loud. This results from the unusual suddenness with which its exhaust ports open. A cylinder-wall exhaust port, exposed by a fast-moving piston, exposes flow area much faster than can one or two conventional poppet exhaust valves.

It’s also possible to make a sleevevalve two-stroke, and these were supreme in the speed of exhaust opening. Instead of exhaust ports in the cylinder sleeve as in the four-stroke, the entire sleeve slid out of the head at exhaust to expose essentially 360 degrees of the cylinder as exhaust port. Only prototype engines of this type were built (by Harry Ricardo and RollsRoyce), but they were, like the Beare, very loud thanks to the extreme suddenness of their exhaust opening.

The Beare is said to have good fuel consumption. So it should. With the

reed in its intake, it cannot blow back mixture at lower revs, making the next intake stroke overly rich and fuel consumption heavy.

Strangest of all is that real six-stroke engines have been built and tested. In the very early days of motoring, when compression ratios were limited to about 3:1 by very low-octane fuels, the combustion chamber volume above the piston at TDC was nearly equal to half the piston displacement. Therefore at the end of the exhaust stroke, there remained a large volume of hot combustion gas above the piston, which the exhaust stroke could not pump out. It would then contaminate the next intake charge, reducing power. If four strokes are good, reasoned the inventor, perhaps six will be better. By adding another exhaust and intake stroke, the purity of the charge finally fired could be improved-just like adding an extra rinse cycle on your clothes washer. Unfortunately, the engine was now spending 50 percent more time to get just a single power stroke.

At about this time, engine designers began to understand that by providing valve overlap, the volume of burned gas above the piston at TDC on the exhaust stroke could be allowed to follow the vacuum of departing exhaust out the pipe, chased across the head by fresh charge from the just-beginning-to-open intake. Overlap is the period of time near TDC, after the exhaust stroke, when the intake valves are beginning to open, yet the exhausts have not yet quite closed. To add yet more confusion, there have been those in the engine business who have called this overlap scavenge process a “fifth cycle.”

Heat-resisting stainless exhaustvalve steels did not appear until WWI, and before that time, exhaust valves regularly burned, warped and cracked. To postpone this deterioration, some engine tuners drilled holes through the cylinder walls of air-cooled four-stroke engines, near BDC, right where twostrokes usually have their ports. These, by starting the exhaust process early, probably took some of the heat off the poppet exhaust valves, and may have helped to cool the otherwise hot-running cast-iron or steel pistons. Wall ports or not, these were still four-stroke engines. I’m sure they were loud.