Retrotechtacular: The Free Piston Engine

We all know how a conventional internal combustion engine works, with a piston and a crankshaft. But that’s by no means the only way to make an engine, and one of the slightly more unusual alternatives comes to us courtesy of a vintage Shell Film Unit film, The Free Piston Engine, which we’ve placed below the break. It’s a beautiful period piece of mid-century animation and jazz, but it’s also  an introduction to these fascinating machines.

We’re introduced to the traditional two-stroke diesel engine as thermally efficient but not smooth-running, and then the gas turbine as smooth but much more inefficient. The free piston engine, a design with opposed pistons working against compressed air springs and combining both compression and firing strokes in a single axis, doesn’t turn anything  in itself, but instead works as a continuous supplier of high pressure combustion gasses. The clever part of this arrangement is that these gasses can then turn the power turbine from a gas turbine engine, achieving a smooth engine without compromising efficiency.

This sounds like a promising design for an engine, and we’re introduced to a rosy picture of railway locomotives, ships, factories, and power stations all driven by free piston engines. Why then, here in 2024 do we not see them everywhere? A quick Google search reveals an inordinately high number of scientific review papers about them but not so many real-world examples. In that they’re not alone, for alternative engine designs are one of those technologies for which if we had a dollar for every one we’d seen that didn’t make it, as the saying goes, we’d be rich.

It seems that the problem with these engines is that they don’t offer the control over their timing that we’re used to from more conventional designs, and thus the speed of their operation also can’t be controlled. The British firm Libertine claim to have solved this with their line of linear electrical generators, but perhaps understandably for commercial reasons they are a little coy about the details. Their focus is on free piston engines as power sources for hybrid electric vehicles, something which due to their small size they seem ideally suited for.

Perhaps the free piston engine has faced its biggest problem not in the matter of technology but in inertia. There’s an old saying in the computer industry: “Nobody ever got fired for buying IBM“, meaning that the conventional conservative choice always wins, and it’s fair to guess that the same applies anywhere a large engine has been needed. A conventional diesel engine may be a complex device with many moving parts, but it’s a well-understood machine that whoever wields the cheque book feels comfortable with. That’s a huge obstacle for any new technology to climb. Meanwhile though it offers obvious benefits in terms of efficiency, at the moment its time could have come due to environmental concerns, any internal combustion engine has fallen out of fashion. It’s possible that it could find a life as an engine running on an alternative fuel such as hydrogen or ammonia, but we’re not so sure. If new free piston engines do take off though, we’ll be more pleased than anyone to eat our words.

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Two Pots On Your Moped

The fastest motorcycle in the world is not some elite racer piloted across the salt flats at crazy speeds, instead it’s your first bike. Even if it’s a 50 cc moped, no other motorcycle you will own afterwards will give you that same hit as the first time you sit astride it and open the throttle. It has to be admitted though, that 50 cc mopeds are slow if it’s not your first ever ride. Really slow. How can they be made faster? Perhaps an extra cylinder will do the trick. In the video below the break, [LeDan] takes a single cylinder Simson moped engine and turns it into a 2-cylinder model.

The build has something of the machining porn about it, but who doesn’t like to sit down and watch as rough metal is transformed into a machined finish? A second Simson engine is used as a donor, and from it another crankcase section is fabricated. In that foes a newly enlarged crankshaft which we’re supprised not to see being balanced, and on the end of the whole assembly goes the Simson end casting. Two cylinders and their blocks the bolt on top, and the engine is complete. It’s a twin-carb model, and we have to admit curiosity as to whether small two-strokes need their carbs balancing. The result seems to work, though we don’t see it on a bike or at high revs. The kid with this engine really would have the fastest motorcycle in the world — compared to his mates.

As you might expect, this isn’t the first small engine build we’ve seen.

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Fridge Compressor To 2-Stroke Engine: JB Weld For The Win

We like this one because it has a real Junkyard Wars feel to it: turning a cast-off fridge compressor into a two-stroke internal combustion engine. [Makerj101] is doing this with tooling no more complicated than a hacksaw and a hand drill. And JB Weld — lots and lots of JB Weld.

[Makerj101]’s video series takes us through his entire conversion process. Despite the outward similarity between compressors and engines, there are enough crucial differences to make the conversion challenging. A scheme for controlling intake and exhaust had to be implemented, the crankcase needed to be sealed, and a cylinder head with a spark plug needed to be fabricated. All of these steps would have been trivial in a machine shop with mill and lathe, but [Makerj101] chose the hard way. An old CPU heat sink serves as a cylinder head, copper wire forms a head gasket and spacer to decrease the compression ratio, and the old motor rotor serves as a flywheel. JB Weld is slathered everywhere, and to good effect as the test run in the video below shows.

Think you recognize [Makerj101]? You probably do, since we featured his previous machine shop-less engine build. This guy sure gets his money’s worth out of a tube of JB Weld.

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Homemade Internal Combustion Engine – Sans Machine Shop

We’ve got a question for you:  If you were stuck in a basement, with nothing too much more than some copper pipe, solder, JB-Weld, and a few hand tools, do you think you could make a working 2-stroke motor? Well, [Makerj101] did just that, and the results are fan-freaking-tastic.

[Makerj101] began his journey like most of us do – with a full face-plant type failure. His first attempted at building an internal combustion engine wouldn’t run, due to a low compression ratio, and too small port sizes. So he did what most of us would do, and tore apart a small gas-power weed-whacker motor to see what he was doing wrong.

The type of engine he’s making is a 2-stroke. That makes the design much simpler as there are no mechanically controlled valves a like 4-stroke motor. The piston (along with the cylinder wall) does double duty by also directing the intake and exhaust gasses – along with a simple flap-type check valve.

For now, the ignition system is run off of mains power, but he has plans to change that – creating a self contained engine. We’re amazed that the entire build is made with such simple tools. Even the the piston is cast out of “JB Weld” epoxy putty. After seeing this, we think that the kid who took apart a clock is going to have to up his game a bit.

We’ve included all 6 parts after the break.

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Retrotechtacular: The Diesel Story

The diesel engine was, like many things, born of necessity. The main engine types of the day—hot bulb oil, steam, coal gas, and gasoline—were not so thermally efficient or ideal for doing heavy-duty work like driving large-scale electrical generators.  But how did the diesel engine come about? Settle in and watch the 1952 documentary “The Diesel Story“, produced by Shell Oil.

The diesel engine is founded on the principle of internal combustion. Throughout the Industrial Age, technology was developing at breakneck pace. While steam power was a great boon to many burgeoning industries, engineers wanted to get away from using boilers. The atmospheric gas engine fit the bill, but it simply wasn’t powerful enough to replace the steam engine.

hot bulb oil engineBy 1877, [Nikolaus Otto] had completed work on his coal gas engine built on four-stroke theory. This was the first really useful internal combustion engine and the precursor of modern four-stroke engines. It was eventually adapted for transportation with gasoline fuel. In 1890, the hot bulb oil engine was developed under the name Hornsby-Akroyd and primarily used in stationary power plants. Their flywheels had to be started manually, but once the engine was going, the bulb that drove combustion required no further heating.

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