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Free-piston linear generator
It's already been around for a long time. Basically, it's even older than the four-stroke engine. First of all, we'd like to call it a free-piston engine. It's main task seems to be, to do without the crank drive. As mentioned, even
the atmospheric gas engine was, in principle, constructed this way. In this engine the piston-rod on the right was laid out as a geared-rack, which
on it's way to B
ottom Dead Center, engages with a gearwheel with freewheel drive.
This engine has appeared over and over again. It became well known as the 'Stelzer engine'. In this case, it has two cylinders, alternatively as either an air-, hydraulic pump or as a generator. The latter application would
appear to be the most interesting of them all. Perhaps this possibility and the expired patent-rights, have caused the German Aerospace Center (Deutsche Zentrum für Luft- und Raumfahrt) to once
again, take an interest in the free-piston engine.
Now it is called the free-piston generator and may well be the range-extender of the future. Imagine the above shown engine part as a two-stroke engine with direct injection. Not only for Diesel fuel. This is a so-called
multi-fuel engine, with a variable filling and compression, it makes a regulated self-ignition possible.
How this would be realised, depends on the (in our picture not shown) right hand part of the engine. There could e.g., be a changeable air-volume, which would drive the piston from BDC to TDC. Thus, not only can
pressure for the compression stroke be determined but also the available stroke travel. This results in an almost interruption-free conversion, eg., from Diesel fuel to hydrogen, whereby, here we have chosen the most
extreme contrast.
Neither should one have any trepidations concerning the two-stroke principle. As far as the lubrication is concerned, there is only one critical part and that's the piston. The DLR-engineers are thinking about a carbon-
construction, which completely excludes the possibility of having oil in the exhaust gas. A decision has also not yet been made concerning piston- or rotary in- and outlet control. For a long time there have already been two-strokes with valves. What is important, is only the linear generator on the right hand side, which from the to-and-fro
movement and without a great deal of loss through friction, creates electricity.
Alledgedly, the engine uses, compared with other combustion engines, only one third of the amount of energy needed to produce electricity. The promise that it gets the maximum efficiency from every type of fuel, is
however, understandable, this is definitely not the case with our customary engines. Neither does one need to worry about the exhaust gases, since the four-strokes also have Exhaust Gas Recirculation and in the meantime one is even thinking about a procedure where the exhaust gases from one cylinder are once again,
burnt in one of the other cylinders.
Any further questions? Yes, because we've not yet cleared up, e.g., how such an engine would be started. To do this, one would have to take the air-volume from the other side, which through pumping in a supply of air
while the engine is running, this would stockpile enough energy to start the engine, if necessary, from either side. Alternatively, the linear generator would, for a short time, become part of the drive-train. Nothing is known
about the performance of the free-piston engine yet. Indeed, the nominal speed, compared with one piston stroke, amounts to only 1500 RPM, so there's still plenty of room for developement work. 03/13
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