Variable Compression Ratio Engine Concepts
The modern automobile engine has fixed compression ratios, that currently usually hover at about 10.0:1 or 10.5:1 for normally aspirated engines and much lower for turbocharged engines. Cars with direct injection can go higher, but we’re quite limited because compression ratios are fixed. A turbocharged engine running off boost makes less power than a normally aspirated engine of similiar cubic capacity because of the reduced compression ratio. While this has been minimized with very fast spooling turbos these days, it is still not ideal.
The answer is a variable compression ratio engine. Saab showcased a concept version of a variable compression ratio engine back in the year 2000, and it was called the SVC, or Saab Variable Compression. The project never did go anywhere. The SVC was a turbocharged 1.6 liter engine that used a pivoting cylinder head to change compression ratios from 8.0:1 all the way up to a high 14.0:1 according to engine operating status. The pivoting head adjusted the slope of the upper part in relation to the lower part of the engine, thus changing the combustion chamber volume at piston top dead center. Because of this, the top and bottom parts of the engine were separated and each required its own cooling system. Thanks to turbocharging and SVC, the 1.6 liter inline-5 engine produced 225hp and 300Nm of torque using 2.8 bars of boost.
In 2003, Nissan took a different approach to implementing a variable compression ratio system. Nissan showcased its Variable Compression Ratio Piston-Crank System, or VCR for short. It uses a multi-link piston and crank mechanism to vary the compression ratio. Like the Saab SVC, it is also based on a turbocharged engine concept. Compression ratio varies from 8.0:1 to 14.0:1, and the highest compression ratio is used during slow and steady city driving when the car runs mostly off-boost. During hard acceleration when the turbocharger kicks in, the compression ratio is reduced up to 8.0:1. During medium engine loads and low to medium levels of boost, a middle ground such as 11.0:1 is used.
The conventional conrod is replaced by a multilink system that consists of three links An upper link is connected to the piston pin, a lower link revolves freely while connected to a crank pin, and a control link connects the lower link to a control shaft. The control shaft has bearing that has a certain amount of eccentricity to it. When the control shaft rotates, the fulcrum of the control link bearing changes, and this causes the top dead center position of the piston to be either moved up or down vertically. This modifies the combustion chamber volume at piston top dead center, this altering the cylinder’s compression ratio. Basically, the system is designed in such a way that the angle of the control shaft causes the top dead center position to move, so to vary the compression ratio you adjust the angle of the control shaft.
Nissan is also developing a similiar VCR system concept for diesel engines. I’m not sure if a production engine using this technology is in the works or not. VCR has probably been made outdated by direct injection - the Volkswagen Golf GTI’s 2.0 liter TFSI engine is turbocharged yet has a high compression ratio of 10.5:1!
Related Links:
Nissan Variable Compression Ratio Engine
Saab Variable Compression
February 7, 2008 @ 8:11 pm
Now everything in the engine can be variable. Variable displacement, var valve timing, etc. In future maybe we dont have to chose engine variant. Everything all in one and we can simply switch it on and off
February 7, 2008 @ 8:15 pm
Paul I really need your review comparison between Captiva, Santa Fe and Everest complete with test drive. Its a very hard decision esp between Captiva Diesel and Santa Fe diesel. I think Everest a little off the league since its based on pickup truck which comparable to Fortuner.
February 7, 2008 @ 9:21 pm
that is smart…
February 7, 2008 @ 9:40 pm
Hrmm…Variable compression ratio, interesting….and i thought that mankind has reached the peak in the evolution of the internal combustion engine…lolz…
February 7, 2008 @ 11:20 pm
we can have normal vtec and type R vtec engine in one package. Push R button to race
February 7, 2008 @ 11:25 pm
That is good news esp. in the light of the ever increasing price of fuel. Cars need not rely big cc engines for power.
With more introduction of electronic in controlling engine functionality, one day, all the old uncle/sifu mechanics in Malaysia will go out of business. Slowly but surely.
February 8, 2008 @ 12:42 am
I tot this technology have been talk for quite sometime now… Now a lot of them are talking about turbo diesels wif hybrid system, i think this will even save more fc.
February 8, 2008 @ 12:42 am
whoa…saab 1.6 engine var. compression can squeeze out 225bhp 305nm. yummy. How much actually our campro 1.6 capable. I believe campro also can do at least 150bhp 200nm….i mean, come on proton, you can do it
February 8, 2008 @ 12:56 am
I think when Proton did it, is beyond 2020… If i not mistaken the saab engine is turbocharged wif SVC so it can have massive output. Proton dont even have a simple turbocharge engine until now…
February 8, 2008 @ 9:00 am
I don’t think the additional complexities that come with the variable compression engines are worth the extra costs and complexities.
The future of internal combustion may as well lie with the Diesotto engine. Couple that engine with a hybrid system and you have an powertrain with a massive torque output as well as efficiency. Unfortunately however, Mercedes holds the patent to it.
If anything, the SAAB system is more efficient as it has less rotating mass.
However, the SAAB engine is designed in a time that direct injection petrol engines are just a rarity (only Mitsubishi’s GDI engine is in production) as the “GDI” has inherent emmisons issues due to the lean burn characteristics (high NOx). With the high NOx issue largely solved by manufacturers, we have a good selection of direct injection petrols (FSI/GDI..etc). I feel the VC engine has been made redundent by that development.
February 8, 2008 @ 10:03 am
Yeah I remember reading about the Saab thing way back … its like some pistons sort of rocked the whole block up to lower the C/R.
February 8, 2008 @ 12:47 pm
saab way of doing seem like,
1st draft, idea,
nissan bit batter ,and claim smoother though multilink con rod look kinda,
for me, we always talk about mechanical llightness ,but if efficiency over rule than might be just batter,
we ll see if any comes with simpler way of VCR
February 8, 2008 @ 5:53 pm
is this system did’n have any problem???when this engine will used in malaysia????
February 8, 2008 @ 5:53 pm
is this system did’n have any problem???when this engine will used in malaysia????
February 9, 2008 @ 11:18 am
Yo Paul, Happy New Motoring Year man!
February 9, 2008 @ 2:53 pm
anyway i do agree with paul,
VCR is too mechanical, and bulk,
DI is capable of a very lean combustion ..so,high efficiency enough and ,its a lighter solution for greater outcome,
February 10, 2008 @ 1:07 pm
Good to hear back the Variable Compresson Ratio engine is BACK, on production.
I’ve read read about this long time ago, i’ve think somewhere in 1999, as mentioned by Paul Tan by Saab, really good in paper but i think it’s not quite reliable cause the engine designed will have problem when switching the compression as top of piston is pivoting. Glad to hear at last Nissan had resolved this problem.
This engine is for sure a millenium revolutions, as what VTEC did it on nineties..
February 12, 2008 @ 4:47 pm
BRAVO…BRAVO….PROTON CAN DO IT..WELL DONE.