Half a croissant, on a plate, with a sign in front of it saying '50c'
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NSFPE Genset

Not-So-Free-Piston-Engine Genset.
  [vote for,

This idea is for an electrical genset that has the advantages of a free piston engine combined with the precision and operational flexibility of a reciprocating engine. Basically a linear-generator is mounted between the cylinder and the crosshead guide, so the conrod/crankshaft/flywheel only receives and releases barely enough energy to push the piston back to TDC and operate the valves so what would normally be part of the powertrain is just the rebound mechanism for an almost-free-piston engine.

A "crosshead" is a device used in very large engines to completely eliminate piston side-loading <see link>. As a result the piston-rod has no angular motion, making it an ideal place to put a linear generator. The generator converts all the combustion stroke energy into electricity, except the small amount used to drive the crankshaft/flywheel.

Picture a 50hp cylinder attached to the crankcase and internals of a 5hp motor. Or, most precisely, a "free piston" engine with a small flywheel return mechanism.

Over a "real" FPE...
• precise, consistent TDC piston placement
• not limited to 2-stroke operation
• very wide power range
• no critical feedback control and management.
Over a Reciprocating Engine...
• *much* lighter
• no noticeable vibration save piston-travel
• cylinder can be larger because of lack of vibration.

Compared to a FPE...
• dwell time is more fixed.
• barring mechanicals, CR is fixed.
Compared to a RE...
• Unlike a rotary generator which is constantly in use, this one is only operating half the time (in a two-stroke).


The NSFPE has two mechanisms:
• structural: in case of generator failure, the exhaust valve opens immediately to avoid stressing the rebound mechanism. (or you could put an emergency brake on the piston rod)
• operational: in case of a misfire, the linear generator kicks in and helps the flywheel through the next stroke. (or you could just use a bigger flywheel)

The Generator...
is a linear brushless field-coil type which, as well as enabling the generator to be "off" during non-powerstrokes, gives a much broader powerband without requiring increased RPM.

The Engine-Mount...
is on the cylinder where all the weight is, since the crankcase is little more than a dustcover. The engine would best be mounted inverted resulting in a low CG and, for a serial-hybrid vehicle, a shorter exhaust path.

One Big Cylinder...
means better thermal efficiency and a lower operating RPM. The kicker is that, in contrast to a normal reciprocating engine of many small displacement cylinders, one big displacement cylinder isn't going to result in a heavier engine. While the "crankshaft" needs to be a bit heavier, one cylinder will weigh much less than an equal displacement in smaller cylinders. It might even be lighter. In fact...

Air Cooling...
is viable since there's no engine block to crack: the "crankcase" is almost the consistency of a valve cover.

Parasitic Devices...
such as the valvetrain, oil-pump, fuel-pump, waterpump, could be either run from the crankshaft, electrically, or in some cases from the (almost completely monolinear) vibration of the engine itself.

Variable Geometry...
isn't totally impossible. While the stroke can't be changed without switching out some crank parts, it would be pretty easy to move the lightly-loaded crankshaft a bit out or in, relative to the cylinder, to give a variable CR, even on the fly. This would result in the piston rod, thus the center of the generator core, moving a bit out or in as well.

FlyingToaster, Jun 27 2011

http://en.wikipedia.org/wiki/Crosshead [FlyingToaster, Jun 27 2011]


       I don't believe free piston engines suffer from the issue that you're trying to address.   

       As a free piston approches TDC, it is compressing gas. The piston's kinetic energy is converted into pressure & heat which are used in the combustion cycle.   

       Using a free piston with an armature (i.e. a linear alternator) it is relatively straightforward to accurately control the piston's location and velocity, by controlling load and feedback in the coils exactly like a servo.   

       Here at work, we make hydraulic damper dynamometers which use closed loop electrical control of a hydraulic valve to accurately drive an actuator with a 20kg 'piston' driving a damper generating up to 30kN force at frequencies up to 50Hz while maintaining position control to within 0.2mm.
Twizz, Jun 27 2011

       //I don't believe that free piston engines suffer from...//   

       Okay, find me a commercially available FPE genset :)   

       This idea combines almost *all* the possible advantages of a free-piston engine generator with the spot-on accuracy of mechanically governed piston travel. And it's just a large engine cylinder with a crosshead, grafted onto a small engine's crankcase with a linear-generator on the piston rod.   

       All non-piston-travel vibration is that of a 5hp lawnmower engine, not a 40kW generator engine.   

       (Note that at some point in time I really should find out how much power it takes to push a piston around and run some parasitic stuff, and not just toss out "5hp". Also note that I reworded the post considerably since [Twizz]'s anno, in a less annoying fashion, to emphasize that the idea is for a genset.
FlyingToaster, Jun 27 2011

       I have nothing of any value to contribute other than to say that I want one for my next motorcycle. [+]
Alterother, Jun 27 2011

       //motorcycle// >_<
Put it on your deuce. Glue a couple 71-Series cylinders to a lawnmower crankcase, toss the old engine, propshafts and differentials, and stick a differential-less electric motor into each axle.
FlyingToaster, Jun 27 2011

       Absolutely! Still looking for those pics, btw. Will post on 'maximum traction' when I find them. Would go take new ones if my leg wasn't still in a cast (my truck is on loan to a friend in the next county for the summer/duration of my convalescence).
Alterother, Jun 27 2011


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