BUT MOSTLY NOT IN MOTORCYCLING
During the 1970s some motorcyclists wanted to make a moral issue of how many piston strokes an engine needed to complete its power-producing cycle—four strokes good, two strokes evil. And many a racing fan has referred to the long period of two-stroke dominance as the “forgotten era.”
I don’t see this as a moral issue. Twostroke or four, an engine must carry out the same four essential tasks of intake, compression, power, and exhaust. A twostroke fits two of those into each of two piston strokes, while a four-stroke provides (roughly) a separate piston stroke for each task. Either way, it’s the same air and the same fuel; combustion of a compressed fuel-air mixture releases heat that drives its pressure very high. That pressure acts on a piston to rotate a crankshaft. The rest is just housekeeping—sweeping out the exhaust gas, bringing in the fresh charge. It’s like cleaning up and restocking the kitchen after a meal. The meal’s the thing!
During WWII, Detroit Diesel produced some 57,000 two-stroke diesels to power tanks and landing craft, and very similar engines continue to power heavy trucks and equipment to this day. Each cylinder has four exhaust valves in its head, and as its piston approaches BDC it exposes a ring of fresh-charge ports served by a large Roots rotary blower (the six-cylinder 6-71 Detroit Diesel was the source for the aluminum-cased blowers that originally provided supercharge air for Top Fuel drag cars). Pure air was blown into the cylinder, chasing the departing exhaust gas up to the exhaust valves. Compression could begin once both the exhaust valves and fresh-charge ports had closed and near TDC fuel was injected, initiating combustion by the heat of compression.
Fairbanks Morse in the US and Napier in England produced two-stroke opposedcontrols piston diesels for rail and submarine use, and thousands of Soviat tanks also employed such opposed-piston two-strokes. In each cylinder there are two pistons, compressing fresh air between them. a ring of exhaust ports and the other uncovers fresh-charge ports, so the fresh “scavenge” air moves in one direction—hence the name “uniflow” for this engine type—from the fresh charge ports to the exhaust ports. This system has the considerable advantage of reduced combustion heat loss because it has no cylinder heads.
The obvious reason for developing the two-stroke principle is that such engines fire twice while a four-stroke fires once. That is the reason nearly all of marine international trade is carried by ships powered by large two-stroke diesels. The most efficient marine propellers are large slow-turning ones, and because reduction gearing is expensive and heavy, there is strong motivation to find a prime mover that can be directly coupled to a 30-foot propeller whose six blades require more or less 100,000 hp to drive it at 85 rpm. In case you’re wondering what torque that requires, it’s roughly 6 million pound-feet. Only a two-stroke could deliver that within the required engine volume and weight.
The simple crankcase-scavenged and carbureted two-strokes that many of us grew up with paid a high price for their light weight, low bulk, and simplicity. That price came in the forms of heavy fuel consumption (at least 30 percent greater than that of a four-stroke of equal power) and very high exhaust emissions of unburned hydrocarbons (UHC). Here’s why. As the piston rises, some form of valve opens to admit fuel-air mixture from a carburetor into the crankcase. As the piston falls, it slightly compresses this mixture in the case. Meanwhile, the cylinder has fired just before TDC and the resulting pressure forcibly
presses the piston down. By the time the piston has descended by half its stroke, it has taken about 85 percent of the usable pressure energy in the combustion gas, so the piston begins to expose one or more exhaust ports cut through the cylinder wall. Exhaust rushes out, and if a properly shaped exhaust pipe is present, a first return wave of low pressure invites any remaining exhaust gas to leave. At three-quarter stroke, two or more fresh-charge or “transfer” ports are exposed by the piston. These, fed from the compressed mixture in the crankcase, form jets entering the cylinder on a looping path. During this process, some fresh mixture short-circuits to the open exhaust port, causing the high fuel consumption and UHC. The rising piston closes the transfer ports first, and as the exhaust is about to close, a late wave of high pressure arrives from the exhaust pipe, stuffing back into the cylinder the leading edge of the freshcharge loop, preventing its loss.
Just as national emissions agencies were nixing further production of two-stroke road bikes in the 1980s, a new technology of direct fuel injection (DFI) was emerging. DFI crankcase-scavenged engines drew in pure air, and pure air jetted into the cylinder from the transfer ports. Then as soon as the exhaust port had closed (roughly 90 BTDC), fuel in the form of extremely small, fast-evaporating droplets (10 to 40 microns diameter) was rapidly injected. This was hard to do because with ignition occurring at about 20 BTDC, there are only 70 crank degrees in which to inject and evaporate the fuel (hence the need for droplets much smaller than those produced by four-stroke-type port injectors).
In a variant of this process, some Bombardier snowmobile engines inject their fuel into the transfer flows, far enough
THE SIMPLE CRANKCASESCAVENGED AND CARBURETED TWOSTROKES THAT MANY OF US GREW UP WITH PAID A HIGH PRICE FOR THEIR LIGHT WEIGHT, LOW BULK, AND SIMPLICITY.
upstream and timed such that almost none can reach the exhaust port before it closes.
Back in the 1980s DFI seemed complex and expensive, so it was not widely adopted. Since then, four-stroke injection—especially the “GDI” form that injects directly into the combustion chamber—has become equally complex and expensive. So, no, we cannot return to the days of simplicity and low cost, but, yes, the advantages of the two-stroke principle remain attractive because emissionscompliant two-strokes can be built. In snowmobiles and personal watercraft, low-emissions GDI two-strokes remain economically important. There are snowmobiles powered by large four-strokes, but reviewers coyly term them “suitable for groomed trail operation.” That means, if they get stuck in deep powder, you won’t enjoy dragging those heavy things out. Twostrokes still have their uses!