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Variable compression

Variable compression

Variable compression

How practical will a variable compression engine be?
Seems kind of fragile.


RE: Variable compression

I always wonder if the immense design effort, engineering, complex manufacturing and energy spent in producing such engines will ever be compensated by its (allegedly reduced) consumption over its lifetime.
I have my doubts.


RE: Variable compression

Interesting point Greg. The claim is this is the worlds first production VCR engine. Have there been others?

je suis charlie

RE: Variable compression

The extra joints in the mechanism see similar loading and motions as wrist-pins do. Given the amount of time that Nissan has been working on this, I suspect that they've done their homework.

RE: Variable compression

Does seem overly complex but many people have worked on this for a long time so the benefits must be substantial. Kudos to Nissan for finally bringing this to production, to bad they don't make anything I would buy.


The Help for this program was created in Windows Help format, which depends on a feature that isn't included in this version of Windows.

RE: Variable compression

A much simpler approach to the same end result (or better) is a high compression engine that runs on regular fuel, and doses anti-detonation fluid as needed in the upper load range (which is rarely used by most drivers under most conditions).
This was the practice on highly rated reciprocating aero-engines in the 40s and 50s, albeit using already high octane gasoline. But then again, they took advantage of all the octane also at cruise power, which is a much higher fraction of full load than typical automobile engines.
While these aero-engines aren't high compression by typical modern automotive standards, keep in mind they are highly supercharged at take-off power.

In a modern automotive application, there would be a fail-safe derate in the event of lack of fluid. Improper fluid would be detected via excessive knock, with attendant mitigation steps and trouble codes.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

RE: Variable compression

There have been extensive tests on DI engines with variable valve timing. By delaying the closing valves you can reduce the effective CR, and since the fuel is injected after closing you don't have that issue.

= = = = = = = = = = = = = = = = = = = =
P.E. Metallurgy, Plymouth Tube

RE: Variable compression

VVT doesn't allow the "effective" compression ratio to be changed independently of the amount of load on the engine, though. This mechanism does.

If you are running at half rated torque output, with VVT trickery you are stuck with running at half (ish) of whatever the mechanical compression ratio is, because you are intentionally drawing only a half (ish) intake charge (whether by throttling or by VVT trickery). This mechanism allows the effective compression ratio to be maintained (ish) under those conditions.

Use of the variable compression mechanism in no way affects the possible use of VVT to simulate the Atkinson cycle.

RE: Variable compression

I think Saab's VCR engine with its hinged-block approach was a better system.

RE: Variable compression


Intake and exhaust manifolds that shift relative to the rest of the powertrain because they're attached to the cylinder head?

Sealing the two parts of the block together in a way that will last 300,000 km?

Variable-length timing chain?

RE: Variable compression

They need an animation of this thing in action. Not clear to me how it operates from static diagrams. Maybe I'm just having a dense moment..

RE: Variable compression

I think the SAAB engine was close to production.

The movement of the head is very small, the cam chain or belt could pivot without any length change. The exhaust flex would be truly tiny if the pivot is on the exhaust side (unlike the SAAB), -much less than that from engine movement on its mounts and way less than exhaust track bounce under normal driving. Intake snorkel is flexible anyway.

In contrast, the Nissan monkey links look unduly complicated.

RE: Variable compression

The Nissan linkage has the secondary benefit of making the up-and-down motion of the pistons closer to sinusoidal, which eliminates the need for twice-crank-speed counter-rotating balance shafts on an inline-4, so there's that. The Saab system wouldn't have done anything about this.

RE: Variable compression

The star of the show with that is the ability to achieve Atkinson cycle operation. The increase in geometric compression ratio mainly serves to better accommodate the Atkinson cycle, in my opinion.

Its an interesting design that looks like it was a very fun project for that team.

"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin

RE: Variable compression

Hemi said. "A much simpler approach to the same end result (or better) is a high compression engine that runs on regular fuel, and doses anti-detonation fluid as needed in the upper load range (which is rarely used by most drivers under most conditions)."

Bosch developing water injection for modern cars. Link

je suis charlie

RE: Variable compression

Very interesting gg, thanks for the link. I was wondering when interest in this approach would re-awaken in mainstream automotive thinking, following its abortive introduction by Oldsmobile in 1962 on their 1hp/cu.in. 215 CID turbocharged V8.
Automotive engines have been going down almost exactly the same development path followed by reciprocating aero engines; just several decades later. Of course, development of mainstream high power density aero engines stopped suddenly in the mid-1950s, due to the advent of gas turbines; but I don't foresee a similar cliff event for automotive reciprocating engines.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

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