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Elastic piston 5

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globi5

Mechanical
Oct 10, 2005
281
I heard that Ford was working on elastic pistons at some point, but couldn't find any information online. Does anyone know more about this? What was it exactly and what were the results?
(There's supposed to be an elastic connection between the piston crown and the actual piston.)

Advantages:
* You can increase the compression ratio without increasing peak pressure. At peak pressure some of the heat energy (pV) would be transfered into spring energy which would then transfered back again at a lower pressure.
* Torque should go up, because pressure is reduced at TDC, but increased at a larger crank angle.

Disadvantages:
* The forces are tremendous so it might only work with smaller pistons (small diameter pistons).
* Material fatigue could be an issue.
* The mass and the complexity of the piston would increase.
 
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Try modelling this using a cycle simulation code with a flexible piston and you will find the effect is similar to reducing the compression ratio. Talk about "increased torque" and "energy storage" is largely hogwash. Garage inventors often think that you can increase torque by somehow using a longer lever, but you have to bear in mind that torque is a cycle-averaged quantity and local maxima and minima are irrelevant.
 
Doesn't a longer lever mean a larger crankshaft and therefore more stroke and more torque anyway? Or what do you mean with longer lever?
Assuming you have detonation at TDC. How are these pressure peaks transfered to the crank (other than damaging the bearings?)
 
'just realized how the energy is stored. It's stored in the gas anyway. The gas is the spring, there's no need for an elastic piston other than possibly protecting the bearings or the rod.
 
And even then how much protection could an elastic piston offer?

If the spring rate is low enough to compress, you lose compression and hence power. If the spring is stff enough to resist compression during the compression stroke it will just transmit the force directly to the wrist pin bearing.
 
You could have an initial (preset) tension that needs to be overcome before the elastic element moves. You know the maximum force the rod and bearings can bear and you can adjust the preset tension just below that force.

I realize that this is not a solution of striking simplicity. I was curious whether anybody heard about this, but maybe this was only some rumor.
 
Doesn't a longer lever mean a larger crankshaft

What the garage inventors say is that pressure is somehow recovered later in the expansion stroke so that it creates more torque, because the crank angle is nearer to 90 degrees. As I said, hogwash.
 
I agree it's not possible to generate more torque this way. But you could reduce the maximum forces on your rod and bearings if you had some mechanism that allows you to increase pressure at 90 degrees (and reduce it at TDC).
 
maybe the theory could be that the elastic piston creates a high compression ratio at low throttle settings with the spring under the cap strong enough to keep the crown in its upper position. but moves lower when the cylinder pressures gets higher under higher loads and throttle settings.thereby giving a lower compression ratio just under the knock limit. maybe 15 to 1 at idle and 10 to 1 at full throttle.
malbeare

A tidy mind not intelligent as it ignors the random opportunities of total chaos. Thats my excuse anyway
Malbeare
 
Yes I think this could have been the idea behind an elastic piston with a preset tension.

(As SomptingGuy correctly pointed out before, a truly elastic piston wouldn't do anything other than simply decrease the compression ratio.)

Btw, if knock occurs, is the increased pressure the main damaging part or does the vibration caused by the high frequency of the pressure peaks pose a problem as well?
 
At one particular engine speed it would be possible to arrange it so that the spring is compressed at the peak pressure point, reducing the the peak pressure, and then imparts that energy to the main piston 90 crank degrees later. This would improve the imep and reduce peak pressure, both are good things to do.

Whether the gains outweigh the many other losses and disadvantages is up for discussion.

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.
 
inertia of the piston crown would also have an effect on the piston crown height as the piston passes over TDC.

This effect would increase with the square of the speed.

How much effect this has on preload would also be up for debate.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.
 
Since the crown can only be pushed and not pulled (preset tension) the inertia of the piston crown would only be a problem because it would increase the effective force that needed to be overcome with higher rpm.
I guess this would be another reason why it would only work with smaller pistons (less mass).

If you had an engine with sleeve valves you could place an elastic (static piston) on top (cylinder head) and then you wouldn't have to worry about the inertia and a relatively complex part moving up and down. However the elastic piston on top would need to be ring shaped to allow space for a spark plug or an injection nozzle.
 
Would the inertia of the crown also effectively increase the stroke at BDC at high revs?

Jeff
 
Yes but the inertia of the crown would only increase stroke at BDC, if the inertia is higher than the preset tension of the spring in the piston.
The preset tension has to be higher than the inertia, otherwise you'd deal with a stroke that increases with rpm.
 
I have seen various schemes for spring loaded heads on two-stroke engines, like this:

(see US pat.,5,476,072)

However, an undampened or uncontrolled spring-loaded system is not a very good idea in a high speed recip engine. It would be noisy and would have a limited life span.
 
Hey folks, I noticed you guys hitting my site, so I figured I'd stop in and say hello. I'm Evan Guy, the guy with the patent in the post directly above this.

My patent is for a VERY specific area of internal-combustion reciprocating-piston operation. Specifically, a VERY low octane fuel in a spark-ignited realm.

Let's first look at what's happening with this concept. Constant-volume combustion is MUCH favorable to constant-pressure combustion, from both a power output AND cycle efficiency standpoint!! Unless you have some REALLY strange reasons for doing what I've done, the best scenario is to CLOSELY approximate Constant-Volume combustion!! Burn the fuel and make a LOT of pressure! (In a substantially-constant volume of a reciprocating piston AT TDC)

The stuff I was doing is NOT presently viable (although it's REALLY FREAKING CLEVER if one was to look into it deeply enough!!)

A non-elastic combustion chamber WILL give better results, in terms of power output, specific power output, specific fuel consumption, hardware design/implementaion, and patent attorney FEES!!!

An elastic chamber wishes to allow constant-pressure combustion. My concept is (was) really neat and I have exemplary prototype hardware, to boot. But in the long run, doesn't make sense. More on this, if queried....Best, Billet142....
 
But wouldn't it at least make sense to have an elastic chamber as long as the elastic chamber is fixed at low throttle settings (as malbeare mentioned earlier) and become flexible at high throttle settings?

This way you could have constant volume combustion at low throttle settings and constant pressure combustion at high throttle settings. This should allow a very high compression ratio at low throttle settings that would otherwise not be possible without damaging the engine at full throttle or shouldn't it?

Also constant volume combustion would basically mean to build an engine that can handle constant detonation or would it not? (Are there engines that can handle this?)

Evan it would be nice if you could elaborate a little more. Thanks.
 
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