Patent Page: Two-Stroke Engine

By Staff
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'Drawings from Patent No. 675,796 show a cross-section of the engine (above) as well as suggested plumbing for tanks holding compressed air and air/fuel (right). '

Two-stroke developments

While Nicolaus August Otto’s name will forever be associated with the invention of the four-stroke or “Otto cycle” engine, neither he nor his legion of engineers was immune to the pull of emerging two-stroke technology.

As the term “four stroke” implies, Otto’s engine used four strokes to complete its process; intake, compression, ignition and exhaust. In the two-stroke engine (patented by Sir Dugald Clerk in 1878 but almost certainly in development in 1876 when Otto patented his four-stroke technology), that formula was reduced to a combined intake/compression stroke and igntion/exhaust stroke, effectively doubling power output by giving a power stroke every revolution, instead of one power stroke every second revolution.

In contrast to the two-stroke engine powering, say, a small Maytag, which uses a crankcase-scavanged, piston-ported intake/exhaust system, early two-stroke engines used intake and exhaust systems any four-stroke fan would recognize, with intake and exhaust valves, and a camshaft. The crankcase-scavanging scheme would come later, courtesy one Joseph Day, and was limited, at least initially, to small engines of generally less than 5 HP.

One problem with two-stroke engines was getting spent gasses out of the combustion chamber before the next fuel/air charge was admitted, without igniting the new mixture. With no separate exhaust stroke, a fresh fuel/air charge was drawn in at the same time hot exhaust gasses from the last ignition phase were flowing out. This led to premature ignition of the new charge, making it difficult to engineer a fuel efficient two-stroke engine that could be timed to deliver reliable performance.

More than a few engineers addressed the problem, including a pair of Germans working for the American Otto Gas Engine Works in Philadelphia, who came up with the scheme shown here in Patent No. 675,796, granted June 1901.

According to its authors, a standard two-stroke engine could be greatly improved – better efficiency, cooler running – by forced induction during the intake/compression stroke and during the ignition/exhaust stroke, the latter to sweep spent gases out of the engine and cool the cylinder, and the former to guarantee complete mixing of the incoming fuel/air mixture.

To achieve this, the design used a pair of reservoirs, one filled with a compressed fuel/air mixture for ignition and the other with fresh air for “clearing” and cooling the cylinder. A third valve in the intake allowed the admission of “clearing” air at the very end of the exhaust phase, and the two reservoirs could be filled and compressed either by separate means or by a pump run off the engine itself.

Just how well the Otto Gas Engine Works patent worked isn’t known, but we do know that other manufacturers used similar schemes to improve two-stroke engine efficiency, so a reasonable guess could be made the design should have performed as intended.

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