Variable compression ratio for aviation?

[RotorDude]

Does this have any potential for us flyboys/gals?

 

[Doggtyred]

Practically? No.

This technology optimizes the engine to be able to perform well in multiple regimes (acceleration, cruise, deceleration where the amount of power desired, boost, and other parameters may vary widely over a short period of time).. and while that might provide a SLIGHT benefit to aircraft, you are increasing the complexity of the engine, introducing points of failure and adding weight for an engine that typically is used to make a high, constant power output over much of its operating life (pretty much steady state with exception of ground ops, take off and descent). Very little return on any investment would be realized in performance.

 

[MAKG1]

Wow, that's a complex connecting rod.

And I thought valve timing mechanisms were getting ridiculously complex....

 

[mondtster]

Short answer, even if there was a possible benefit to this in an aviation application (which I don't see), I seriously doubt that any of the OEMs would pursue certification. That would cost far too much.

I know what kind of costs are involved with developing a new, high volume, non-aviation engine. I can't imagine what kind of expense it would be to certify an engine to satisfy the FAA...

Does anyone know what the last clean sheet aircraft engine design was? Most of the stuff I've worked on all appear to be variations on the same basic engine design.

 

[Dan Thomas]

Does anyone know what the last clean sheet aircraft engine design was? Most of the stuff I've worked on all appear to be variations on the same basic engine design.

As far as I know, the latest clean-sheet design is the SMA diesel. It looks like just another opposed four, but it's vastly different.

Hard to build a piston engine that doesn't look like something you've seen before. There are only so many ways to arrange cylinders, after all. The differences come inside the thing where you can't see. The four-banger in a new Toyota is a long ways from the Model T's, but it's still an inline four.

 

[mondtster]

As far as I know, the latest clean-sheet design is the SMA diesel. It looks like just another opposed four, but it's vastly different.

Had to build a piston engine that doesn't look like something you've seen before. There are only so many ways to arrange cylinders, after all. The differences come inside the thing where you can't see. The four-banger in a new Toyota is a long ways from the Model T's, but it's still an inline four.

I forgot about the SMA diesels. Probably because they aren't in wide circulation.

I should clarify my previous question. I'm not talking about some new space age design that deviates away from a typical four cycle piston design, or its layout. I'm just talking about something that uses little or no parts from something that was initially designed 50 or more years ago. There's nothing fundamentally wrong with old designs or basing a new engine on an old engine, it happens all the time. But basing something new on an old design cuts down on development costs since you wouldn't have as many new castings and parts to spend money on.

Back to the original topic, the variable compression discussion and development has been around for years now. I can think of several companies that have messed around with it, and none that I'm aware of have went to production. I'd say there is a reason for that...

 

[FormerHangie]

I forgot about the SMA diesels. Probably because they aren't in wide circulation.

I should clarify my previous question. I'm not talking about some new space age design that deviates away from a typical four cycle piston design, or its layout. I'm just talking about something that uses little or no parts from something that was initially designed 50 or more years ago. There's nothing fundamentally wrong with old designs or basing a new engine on an old engine, it happens all the time. But basing something new on an old design cuts down on development costs since you wouldn't have as many new castings and parts to spend money on.

Back to the original topic, the variable compression discussion and development has been around for years now. I can think of several companies that have messed around with it, and none that I'm aware of have went to production. I'd say there is a reason for that...

The closest thing to variable compression ratio current being used is the late intake valve closing used in the Atkinson cycle engines found (mostly) in hybrid cars. Those engines have a very high nominal compression ratio, but the intake valve closes later than in conventional engines, so some of the mixture gets blown back into the intake manifold, thus the effective compression ratio is somewhat less. The whole point of this is to have a greater expansion ratio, which allows the engine to capture more of the heat energy from combustion.

I'm not sure what the designers were hoping to get out of this variable compression ratio that would be an improvement on the late intake valve closing. I've seen some pretty big efficiency improvements claimed, but time will tell on that. From an automotive standpoint, it's pretty hard to beat the combination of a late valve closing engine and regenerative braking. I drive a hybrid on a daily basis, and it's not uncommon for a third of my miles to be powered by the electricity from regenerative braking while in town.

 

[Glen R]

Continental can barely make conventional cylinders that don't fail, forget about adding complexity.

 

[RotorDude]

Continental can barely make conventional cylinders that don't fail, forget about adding complexity.

Tell me about it.