It is a free-piston engine. It has been done. (McLaren does not use it ... with the explanatory note that we in the outside world have no idea what they have in their research labs).

http://www.extremetech.com/extreme/185789-toyota-d...

Well thanks for the reply, and appreciate the link.

Sterling engines have been built this way for years.
The only reason to have magnets is if you want to integrate the generator.
Otherwise you just use gas as the 'return spring'.
So picture the Toyota engine, with valves and injectors on the sides so that you could put two of these together without a head. Your power to weight numbers would be impressive.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

After looking at the OP I was left rather flabbergasted. Just what are they teaching kids at universities now days? I'd like to think that even a freshman engineering student would be able to figure out on their own what sort of practical problems this engine concept presents.

Seems there are some interesting "free piston" engines in development.
It will be interesting to see what actually shows up in production.
In the linked article they seemed to obsess about the (allegedly) three-phase output.
The illustration in the article had flow arrows that look wrong. They show the flow exiting through the valve solenoids.

I wonder if you could control the pistons as in a linear motor. Control position and velocity of the piston(s), might make it easier to control emissions and improve power extraction.

Jay Maechtlen
http://www.laserpubs.com/techcomm

Controlling the pistons to anything outside of whatever the natural vibration frequency is would certainly require excessive current and give poor efficiency. You have a mass (the piston and the magnets for the "alternator") and you have what amounts to a gas spring in each end. It is going to have a certain frequency range that it wants to run at. Complicating matters is that gas springs aren't linear. The motion profile will not be sinusoidal, and that's why I said frequency "range", because it's likely to vary. The compression ratio won't necessarily remain constant, either.

Free-piston engines have been built in the past which act as gas compressors with the useful power output being delivered by a turbine.

I always wonder about the "gas spring" part, and how it is managed.
Anyway, managing the resonant frequency could to some extend be done by adding or subtracting magnetic force to that of the gas spring.
And driving the position of the piston needn't be too lossy, considering the efficiency we see from the best motors, generators, and power supplies nowadays.

'power output delivered by a turbine' - if you need shaft power, that's certainly an avenue.
- I see that Ford build an experimental farm tractor (!) in the - 50's?

cheers
Jay

Jay Maechtlen
http://www.laserpubs.com/techcomm

Think about the gas spring taking the place of the crank and rods in a normal engine. In a normal engine, a piece of the "excess energy" from a power stroke gets recycled into the compression stroke of the next cycle, and the rotating inertia of the flywheel handles this very efficiently over a reasonably wide range of operating speeds, as long as the flywheel is heavy enough. If the engine is driving an alternator to generate power, that alternator doesn't have to handle any of the internal "recycling" of mechanical energy from one power stroke into the next compression stroke. And that is a good thing, because the instantaneous amount of power going in and out between the flywheel and the piston can be large - potentially a few times larger than the engine's normal power output, during the final stages of compression and the first stages of expansion. The alternator doesn't have to handle that, the flywheel takes care of it very efficiently.

Now, if you use a gas spring, if you want to use that concept of recycling mechanical energy from one stroke to the next, you are relying on the power stroke turning its energy completely into kinetic energy of the piston, then into compressed gas at the opposite end, then back into kinetic energy in the opposite direction, then into the compression stroke. As long as this is happening at the resonant frequency, it is all well and good, and the alternator need only bleed off the "excess" energy just as it does with the normal engine. But if you try to force this system at a speed other than the resonant frequency, now you are going to have to deal with instantaneous power going in and out of the magnetic side of this several times larger than the engine's power output.

That alternator might be 97% efficient but when you start compounding a 97% generating efficiency with a 97% motor efficiency for several times the engine's normal output because you are trying to force operation at an abnormal speed, that 3% loss repeated over and over again is killer.

These engines have to operate at their resonant frequency. Period.

Changing the gas pressure will change the resonant frequency, but it might also change the stroke of the piston. One way or another, the approach speed of the piston on the compression stroke will have to be in a pretty narrow range in order to achieve the next compression stroke without going too far.

Turbocharging, to increase the overall system pressure under load, would also increase the resonant frequency when running under boost, which is not a terrible relationship.

I suspect that controlling an engine of this sort is a nightmare, which could be largely why we haven't seen them in real world use.

Oh, they are used in the real world, but only in a very ugly fashion. Pile drivers.

Cheers

Greg Locock


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Why would it need to operate at different frequencies anyways? Using a fixed frequency is fine for generating electricity and picking the right frequency would help would help with efficiency. The Toyota design would certainly rely on a controller on that PM linear generator.

These FP engine concepts always look great on paper, but they never work very well in practice. It is very difficult to accurately control the motion of a free piston using a pneumatic return spring. The conversion efficiency of a linear alternator coupled to a 2T FP engine is nowhere close to 97%. Plus a 2T FP engine is not as simple as most people think. The engine still requires many subsystems like scavenge pumps, oil pumps, coolant pumps, fuel pumps and injectors, turbochargers, intercoolers, oil and water heat exchangers, etc.