variable compression engines

[stevej37]

Infiniti is offering it's 4 cyl turbo'd cars with variable compression ratio engines starting in 2018....per Car and Driver.
Just when you think there is no more room for improvements, some new tech comes out!
They say the comp ratio can be changed from 8.8:1 to 12.0 in .2 second...giving better mpg and allowing more boost to be added.
According to their preview..an eccentric bushing is used on each connecting rod at the piston end. A small ram turns the bushing to control the compression ratio.

[pavulon]

Seems prone to catastrophic failure...but would imagine that was accounted for in the R&D.

[arcsine]

I think Volvo was messing around with variable compression a while ago. In their system the block itself was on a cam that would move the block/head in relation to the static crankshaft position. Did not hear much about it since.

[arcsine]

Sorry. It was Sabb and their SVC system. Bankruptcy killed it along with the company.

[stevej37]

They never said what the small piston/ram is controlled by. My Honda has var valve timing and it's controlled by oil pressure. I imagine with todays tech, they could prob control it electronically.

Seeing that it's set to be in all their 4 cyl engines, I'm sure it went thru lots of testing.

[unclebilly]

Ummm, isn't every engine with forced induction (Turbo / Supercharged) variable compression?

Another way to think of forced induction is variable displacement. At 1 bar of boost, you are burning twice as much fuel and air as at ambient temperature / pressure so it is equivalent to having double the displacement.

[stevej37]

Originally Posted by unclebilly

Ummm, isn't every engine with forced induction (Turbo / Supercharged) variable compression?

Another way to think of forced induction is variable displacement. At 1 bar of boost, you are burning twice as much fuel and air as at ambient temperature / pressure so it is equivalent to having double the displacement.

I prob should have said that as..variable compression "ratio" engine ...meaning the engine is able to vary the distance of the piston from the cylinder head.

[stevej37]

This is a better explanation of how it works. I didn't know Porsche has the patent on it.

Porsche patents Variable Compression Ratio Engine Technology - Ferdinand

[Flieger]

Originally Posted by stevej37

This is a better explanation of how it works. I didn't know Porsche has the patent on it.

Porsche patents Variable Compression Ratio Engine Technology - Ferdinand

That's a different mechanism. There are many ways to achieve a variable compression ratio. Including valve timing.

[stevej37]

Originally Posted by Flieger

That's a different mechanism. There are many ways to achieve a variable compression ratio. Including valve timing.

Same as the Infiniti system in the OP

How could valve timing vary the compression ratio?

[Flieger]

Originally Posted by stevej37

Same as the Infiniti system in the OP

How could valve timing vary the compression ratio?

No, it is not the same. Porsche one is a 2 position, Inifiniti is an analog adjustment.




You could also vary the height of the cylinder head which is how a CFR engine works (that is the engine used as the standard test for octane rating).

Valve timing- late closing of the intake valve aka Miller Cycle. You push some of the fresh air back out and this gives you a longer expansion ratio than the effective compression ratio, at the cost of some displacement.

I have no idea how the extra complexity and mass gives a big enough thermodynamic benefit over the Miller Cycle to justify itself.

Personally, I find this more interesting.
http://www.thedetroitbureau.com/2017/01/mazda-aiming-to-first-with-breakthrough-hcci-engine/

[Nickshu]

Saab made one back in the late 1990's and showed it off in 2000. They were going to put it in the 9-5 but that was right about the time GM bought the remaining 50% of Saab and shelved the engine due to cost.
Saab's version used a tilting cylinder head with the goal of matching CR to forced induction boost. Interestingly (and very non-Saab like) this engine used 5 cylinders instead of 4 and used a supercharger instead of a turbocharger.







https://en.wikipedia.org/wiki/Saab_Variable_Compression_engine

[stevej37]

This is from the Car and Driver Mag


Selectable Squish

Raising an engine’s compression ratio improves fuel efficiency during low-load operation, but it can also lead to catastrophic detonation when maximum power is requested. Engineers have long sought to navigate this challenge with a variable-compression-ratio engine, and Infiniti promises to implement its solution in early 2018 with a complex system of links in the crankcase.

There’s more than one way to skin this cat, though. German powertrain engineering company FEV proposes a piston wrist pin that passes through an eccentric bushing at the top of the connecting rod. As the bushing rotates, the piston moves vertically relative to the rod, in turn adjusting the compression ratio. The position of the bushing is determined passively by a pair of miniature pistons inside the connecting rod that rotate the cam in reaction to combustion and inertia forces. FEV’s 1.7-liter turbocharged inline-four demonstrator can adjust its compression ratio from 8.8:1 to 12.0:1 in 0.2 to 0.6 second.

[Charles Freeborn]

An old family friend has been working on that concept for some time (15+ years)
VCREngine
He holds a patent of some sort. His has the crank main bearing journals in an eccentric carrier. Not sure how it's actuated.

[island911]

Originally Posted by Flieger

That's a different mechanism. There are many ways to achieve a variable compression ratio. Including valve timing.

This.^

Toyota has been doing this for a while with an Atkinson cycle engine. (Prius)

edit: here we go...

Our Point of View: Atkinson Meets Otto: Why the Prius is So Efficient



by Toyota Open Road Blog

Folks interested in our Prius may have noted that its engine is referred to as an Atkinson-cycle engine. Some of you probably don’t care about that, and are just happy that the Prius is rated by the EPA at 48 city/45 highway mpg.

But maybe some of you wonder just what in the dickens an Atkinson-cycle engine is, and how it’s different from the standard engine – also known as the Otto-cycle engine. So here’s a quick primer.

There are two definitions of an Atkinson engine. The first is that in very early examples of the Atkinson design, all four strokes of the Otto cycle (intake, compression, power, exhaust) happen in a single revolution of the crankshaft, rather than the usual two. This is accomplished via a complex linkage between the piston and the crankshaft.

But there’s a subsidiary and more modern definition, and that’s the one in which we’re interested. This says an engine that uses the Atkinson-cycle is one in which the post-combustion expansion ratio is different from the effective compression ratio. In other words, the compression stroke of the engine’s piston(s) is, by whatever means, shorter than the power, or combustion, stroke.

This imbalanced compression/expansion ratio results in a reduction of what are called pumping losses. It produces a difference between how hard the engine works and how much power it develops.

In the case of the Prius engine , the effective compression ratio is about 8:1, while the expansion ratio is about 13:1. As a result, it is 12% to 14% more efficient, in terms of power output per fuel consumed, than the non-Atkinson engine upon which it is based.

But there’s no free lunch. Use of the Atkinson cycle results in improved efficiency, but it also results in a significant narrowing of the rpm range in which the engine makes useable power.

There are two ways to solve this problem. One way is to couple the engine to a continuously variable transmission (CVT) so that the engine always will run in its optimal rev range. The other is to give the engine supplemental power such as an electric motor. We do both those things.

Even better, by using our Variable Valve Timing-intelligent (VVT-i) system to continuously adjust intake-valve timing between Atkinson-cycle valve timing and conventional valve timing, the Prius engine can maximize fuel efficiency while still producing maximum power.

The result is the Prius Hybrid, which provides sprightly acceleration, more than sufficient highway speed and the best fuel economy ratings of any automobile available in the U.S. today. Seems like the best of all possible worlds.

Originally published on the Toyota Open Road blog on Sept. 08, 2008.

[stevej37]

Available now in the 2018 Infiniti's

[svandamme]

Originally Posted by unclebilly

Ummm, isn't every engine with forced induction (Turbo / Supercharged) variable compression?
.

not really.

The term compression has never been used to describe forced induction.
Merc is the only brand that use the term Kompressor as a marketing name for it's superchargers.. but they didn't use the term compression.. it's just a brand name that nobody else used.

Turbo/supercharge just forces more air in the chamber
And then more fuel is injected via a seperate process.
Sure on a physics level, the air is compressed. the Fuel isn't. Not by the Forced induction it ain't.

It's just forced induction.. Nobody calls it compressed induction
Forced induction is about air only.


Compression points to the piston compressing the Air/fuel mixture.

[wdfifteen]

The Saab system seems so simple. I assume it hasn't been adopted is that moving all that mass gives a slow response time.

[stevej37]

Originally Posted by wdfifteen

The Saab system seems so simple. I assume it hasn't been adopted is that moving all that mass gives a slow response time.

from C&D
Back at the turn of the 21st century, Saab revealed a supercharged 1.6-liter inline-five engine with a hinged two-piece block. The engine lowered its compression ratio from 14.0:1 to 8.0:1 by pivoting the top of the block, including the cylinders and the head, around a beefy hinge on the intake side. A set of mini connecting rods riding on an eccentric shaft on the exhaust side did the lifting. Since the crankshaft position is fixed, tilting the block’s top changed the combustion-chamber volume and thus the compression ratio. Saab claimed 225 horsepower and a fuel-economy bump of 30 percent over an engine with similar output. Development costs and Saab’s redheaded-stepchild status within GM meant the capital and interest weren’t there to bring the technology to production.

[wdfifteen]

Originally Posted by stevej37

from C&D
Back at the turn of the 21st century, Saab revealed a supercharged 1.6-liter inline-five engine with a hinged two-piece block. The engine lowered its compression ratio from 14.0:1 to 8.0:1 by pivoting the top of the block, including the cylinders and the head, around a beefy hinge on the intake side. A set of mini connecting rods riding on an eccentric shaft on the exhaust side did the lifting. Since the crankshaft position is fixed, tilting the block’s top changed the combustion-chamber volume and thus the compression ratio. Saab claimed 225 horsepower and a fuel-economy bump of 30 percent over an engine with similar output. Development costs and Saab’s redheaded-stepchild status within GM meant the capital and interest weren’t there to bring the technology to production.

I understand all that. But the concept is so simple I wonder why it has not been adopted by Lexus or anyone else interested in VCR. putting two pistons in each connecting rod seems like an overly complicated way to accomplish the same thing. Either Saab wanted too much money for the patent or there is some technical problem with it. I suggest that rocking the the mass of the block back and forth would not happen fast enough to respond to the changes in compression required by a modern computer controlled engine.