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REMEMBER those wailing banshee motorcycles with plumes of acrid blue smoke billowing behind them? Your correspondent came of age astride one. But few two-stroke engines survived the draconian pollution measures of the latter part of the 20th century. Today, the little two-stroke smoker lives on largely in lawnmowers, chainsaws, hedge-trimmers and the like—where its cheapness, lightness and willingness to run in any orientation has won it a loyal following among gardeners everywhere. From the road, though, it has all but vanished.
Yet, the traditional two-stroke petrol engine—so-called because it had two strokes per cycle instead of the more usual four—had a lot going for it. With no complicated poppet valves and cam shafts, no oil reservoir and far fewer parts all round, it cost less than half as much to make as a comparable four-stroke engine, and was far lighter into the bargain. Also, because it fired once every revolution instead of once every other revolution, it put out considerably more power than four-strokes of similar size.
Simplicity remains the two-stroke’s greatest virtue. The engine still has to perform the same four separate processes (“suck”, “squeeze”, “bang”, “blow”) as a four-stroke. But it does so by making the exhaust stroke (“blow”) at the end of one cycle and the induction stroke (“suck”) at the start of the next cycle happen simultaneously while the piston is travelling through the bottom half of the cylinder. The other two strokes—compression (“squeeze”) and combustion (“bang”)—are carried out sequentially while the piston is in the cylinder’s upper half.
In its basic form, a two-stroke consists of a specially shaped piston rising and falling in a cylinder that has an exhaust port on one side and an inlet port lower down on the other. As the piston falls, it first uncovers the exhaust port, which allows most of the spent gases from the previous cycle to be expelled. It then uncovers the inlet port, where fresh air-fuel mixture is admitted. The compression stroke occurs as the piston rises back up the cylinder, with the mixture being ignited by a spark plug as the piston nears the top of its stroke and both ports are covered.
Sadly, such simplicity comes at a price. The fact that the inlet and exhaust ports are, for part of the stroke, open simultaneously means that the engine’s scavenging (getting rid of the burned gases before fresh fuel is admitted) is less than ideal. Inadequate scavenging was one of the reasons for the two-stroke’s poor economy. Over the years, various attempts have been made to improve matters. The method most widely used today—loop scavenging—was invented in Germany during the 1920s. A carefully shaped inlet port causes the incoming mixture to swirl around the cylinder rather than make a bee-line for the open exhaust port. Not only does this permit better scavenging, the turbulence also promotes combustion. The result is greater power and better fuel economy.
The two-stroke’s downfall is usually attributed to its “total loss” lubrication system. Instead of being contained in the engine’s sump, the lubricant was pre-mixed with the fuel (traditionally as one part of oil to 16 of petrol, though up to 50 parts of petrol later became possible). Because oil is less combustible than petrol, as much as a third of it can survive the process—escaping into the atmosphere as unburned hydrocarbons and soot.
In truth, this was not the only reason for the two-stroke’s disappearance. Most of the leading motorcycle makers of the day stopped building two-strokes not because of concerns about pollution, but because they wanted to focus on their pricier and more profitable four-stroke models. A rule change finagled by the manufacturers, which forced 250cc two-strokes to compete on the track against 450cc four-strokes, did the trick. The two-stroke’s demise—at least on the road and the track, if not in the dirt, the snow and the forest—followed swiftly.
And there the technology rests. Or it did so until recently. Two-strokes are back in the news, thanks to the success of pilotless planes like the Predator and Reaper in Iraq and Afghanistan. A new generation of air-cooled two-strokes that look like grown up versions of the baby diesel engines aeromodellers have used for decades are being hurried into production for military duty by firms such as Cosworth, Desert Air, Evolution, Graupner, OS Engines and Zenoah. They range in size from 10cc to over 200cc, and can run on a variety of fuels, including avgas and jet-fuel as well as petrol and diesel.
Surprisingly, however, it is on the road that two-strokes look set to make their most dramatic comeback. Two new, and radically different, designs are causing the biggest stir.
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