Swing-piston engine
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A swing-piston engine is a type of internal combustion engine in which the pistons move in a circular motion inside a ring-shaped "cylinder", moving closer and further from each other to provide compression and expansion. Generally two sets of pistons are used, geared to move in a fixed relationship as they rotate around the cylinder. The basic concept is very similar to the Wankel engine, the "traditional" rotary, but predates it by some time. It also bears a strong lineage with the free-piston engine.
It is unclear whether or not any swing-piston engine has ever reached production, but the closest attempt appears to be the German World War II-era design by Otto Lutz. His design had six pistons in total, three each attached to two disks. They were geared to form six chambers between the pistons, such that at any one time one set of three chambers were "close together" while the other set of three was "wide apart", or some point between those two extremes. The timing was such that the chambers reached their "close together" point over the spark plug, and their "wide apart" point over the intake and exhaust ports. This action is similar to the Wankel, the primary difference being that Wankel creates compression and expansion via the shaping of the engine, as opposed to the motion of the pistons.
Lutz's engine was being designed as an experimental gas generator for a new type of aircraft engine, one that replaced a traditional centrifugal or axial compressor with his swing-piston design. Utimately the exhaust would be used to drive a turbine, that power being used to drive a propeller to produce a turboprop engine. For this role the exhaust gas was too hot to be used effectively in a turbine, so the design added a second "exhaust port" that vented cold air into the hot exhaust. During this stage the compression of the cold air was not used to later produce power. This is not a strict requirement, for direct power use (as opposed to driving a turbine) this "third area" of the engine can simply be left open to avoid losing power to compression that will not be used.
The initial test engines had some minor problems, notably with sealing, but these were worked through and the engines were under test during 1944. One particularly nice feature of the engine is that they can be bolted back to back along a common crank shaft to make a larger engine, and with each additional stage the running becomes smoother and the only part that needs to be made larger is the crankshaft. A similar arrangement with a radial engine is generally more difficult to arrange, and ones with inline engine arrangements soon become so long that keeping the crankshaft from vibrating becomes a serious problem.
Each "cylinder" from Lutz's design was 0.70 m in diameter and only about 30 cm in depth, providing 445 hp from 140 kg, an excellent power-to-weight ratio compared even to jet engines of the era. A five-block version was proposed for his turboprop concept, providing 3,450 hp from an engine about 2 m long. While the power-to-weight was good, the density of the engine was simply superb.
The overall turboprop looked much more like a jet engine than a piston one. The swing-piston gas generator was located in the middle of a long nacel, with a five-stage axial compressor in front and a three-stage compressor behind. The compressor was used both to act as a supercharger for the piston engine, as well as provide cold air to cool the turbine. The actual power to the propeller, combining both the pistons and the turbines, was 4,930 hp at 10,000 m altitude, far greater than any German wartime project.
Why all this complexity to produce a new version of an engine that's primary advantage was simplicity? The primary problem with conventional jet engines is that the combustion takes place at constant pressure, which is considerably less efficient than in a piston engine, where it has constant volume (or close to it). The constant volume cycle extracts more power from the fuel, and therefore has better specific fuel consumption. Lutz's design was intended to power very long-range bombers and patrol aircraft, where fuel economy was more important than simplicity and outright performance.
Lutz's design is not the only way to produce such an engine, BMW experimented with a traditional engine with poppet valves on the combustion chambers, which had been used a number of times previously in experiments. Another approach entirely is to recover some of the heat of the exhaust in a heat exchanger and use that instead of fuel to heat the compressed air, a concept used by General Motors in a series of automobile turbines. Generally, however, improvements in the basic piston engine in the "low power" roles have kept any of these advanced designs out of the marketplace.
