Variable Compression Ratio (VCR) Engines

NYSpeed Automotive
1

So I started reading into it, and I guess there are a few companies working on this.

“A variable compression ratio (VCR) engine that can be produced commercially has been equated to the Holy Grail for automotive mechanical engineers.”- LINK

Nissan has just come up with this:

Nissan has developed a new engine mechanism that enables both high power of turbo engine and low fuel consumption of non-turbo engine.

Features of VCR piston-crank system

  1. The newly developed piston/crank mechanism (multi-link type) enables optimum compression ratio control.

  2. For example, the fuel economy of a non-turbo engine and the power output of a turbocharged engine can be achieved in one engine.

  3. By the ideal piston motion of multi - link, significant vibration reduction has been obtained.

http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/VCR/IMAGES/descriptionBlockD02_pic_01.gif

Technical explanation

New piston crank mechanism

By applying multi-link mechanism, significant improvement of engine vibration coupled with friction reduction (basic condition of engine mechanism) have been obtained.

And in addition, function of variable compression ratio control under engine running condition has been achieved.

http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/VCR/IMAGES/captionWithImageB01_pic_01.jpg

Example of CR control (turbo engine)

The highest compression ratio is used in city driving to increase thermal efficiency for improved fuel economy.

The system shifts to a low compression ratio for super charging when accelerating or climbing hills.

This enables also higher turbocharging for much power while suppressing engine knock.

http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/VCR/IMAGES/descriptionBlockD02_pic_02.gif

Low vibration nature of multilink mechanism

Piston motion in the conventional mechanism is essentially asymmetrical, with rapid motion occurring at top dead center.

That unbalance deteriorates engine vibration in the case of conventional 4-cylinder engines.

With the multilink VCR mechanism, the piston’s reciprocal action achieves ideal simple harmonic motion, resolving the unbalance.

This enables low vibration property of in-line 4 cylinder engine close to that of V type 6 cylinder engines.

http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/VCR/IMAGES/descriptionBlockD02_pic_03.gif

http://www.nissan-global.com/EN/TECHNOLOGY/INTRODUCTION/DETAILS/VCR/IMAGES/captionWithImageB01_pic_02.jpg

2

Saab made on in 2000 that worked by rocking the the engine 4 degrees.
http://www.popularmechanics.com/automotive/new_cars/1266656.html

Interesting concept, but can’t the same be accomplished by a variable vane turbo?

3

no, because this changes the compression ratio at engine speeds / loads where a turbo wouldnt be spooled anyways

if your cruising down the highway, cruise control set, your turbo wont be spooled because your at a constant load, this system would then have your CR set at the full 14:1, allowing you to get maximum power out of every drop of gasoline

if your getting maximum power out of the fuel consumed, you can run the engine at a lower rpm (via gearing) and therefor get maximum mileage out of the fuel consumed

bt since you also dont want 14:1 CR at full load / full boost, this engine automatically changes to the lowest CR under those conditions, allowing you maximum power for passing and such

and anwhere between no load and full load conditions it can set the CR for what ever is most effective, be it 9:1 or 13:1 or 10.2764:1

4

right now “displacement on demand” is the band aid

5

Wow now thats some cool engineering

6

What he means is it’s a highly cost effective solution.

7

What he means is it is a very cost effective solution as compared to the aforementioned. That multilink setup would probably add 50% to the cost of manufacturing the engine.

8

“Displacement On Demand” Is a great idea and system if implemented well.
It also practically limited to engines with more then 4 cylinders.

The SABB design is pretty simple and has a lot of potential for mass production with limited cost increases

9

The ‘Holy Grail’, is an appropriate nickname for this.

10

there was a lot of hype about this when they introduced it. it looked like they were going to put it into production, but it looks like it fell by the way side. chalk one up for GM.

11

so wait i must be missing something, the piston shaft gets longer and shorter? and with a multilink mech. wouldnt that be a weak point compared to the traditional connecting rod and shaft and bearing?

12

The piston shaft doesn’t physically get longer, but when the control shaft is rotated, it extends the piston further to increase compression, or it limits the extension to lower compression.

I’m sure there are weaknesses that need to be addressed, but it works on paper.

13

Just like communism!

14

When in doubt add more moving parts :slight_smile:

15

I think this is at least as significant as variable valve timing.

16

is there anything else on this?

it changes dwell and i would assume the r/s from that piston speed graph.