[tapeman]

That article is kind of shitty too.

Higher compression ratio = better combustion = more power with less fuel, regardless of turbo.

Higher compression will always mean better fuel efficiency. My moms Mazda CX-5 runs a 13.5 compression ratio and gets 27mpg. The power/efficiency trade off appears when you're talking about turbocharged engines.

Turbos compress air into the combustion chamber, so when you compress compressed air, you get exponentially compressed air. If you don't adjust the compression ratio for the turbo, you're going to get knock. An engine with a 13.5 compression ratio can run perfectly fine on regular gas, but an engine that runs the same ratio and 20 psi of boost with melt itself even on premium gas.

This is a problem for any kind of work vehicle. They almost always have turbos to give them the power to haul their loads with reasonable ease. But this forces the engineers to design the engine with a lower compression ratio, which means at light engine loads, when the turbo isn't doing anything, the engine isn't fulfilling its full potential. It's still compressing air at a low ratio, even though it's not taking in already compressed air. This is where Nissans innovation comes in. The compression ratio will change according to the degree with which the turbo is compressing air. Allowing the engine to run at peak efficiency through the entire range of engine load.

[usrusr]

So this only reduces inefficiency while the the engine revs too low for the turbo to kick in? Then it is competing with electrically assisted turbocharging (spin the turbo electrically when there is not enough exhaust pressure, basically a super/turbo hybrid using electric transmission), as both are addressing the same inefficiency.

I know where I would put my bets in terms of price, reliability and ease of development.

[hvidgaard]

At lower revs the turbo is more likely to boost the engine if you demand power, of course depending on the dimensions of the turbo, but it doesn't make sense to put a performance turbo that works in the upper rpm range, in a regular car.

In any case, an electrically assisted turbo is not a replacement for this. This is a way to have high compression at low loads, such as highway crusing, and ability to lower compression when boost is needed for performance.

[sandworm101]

>> while the the engine revs too low for the turbo to kick in?

This is a common misconception of 'turbo lag'. A modern turbo is actually operating at the low revs too. The boost is there. But a modern turbo engine, especially a diesel, has a very narrow powerband, giving the impression that the turbo isn't active until higher rpms. This isn't real 'turbo lag'. Real 'turbo lag' is the delay caused by the fact that the turbo depends on exhaust pressures, which rise only momentarily after throttle increases. But this problem has largely been solved via mutli-stage turbos, lighter turbo parts, waste gates and the like. Real turbo lag, where it is noticeable, occurs at all rpm ranges.

[LoSboccacc]

Which models are coming out with electrically assisted turbos?

[stonewhite]

Audi SQ7 is coming up with an electrical supercharger and two sequential turbochargers to deliver lagless and variable boost for high performance in all of the rev range.

http://www.caranddriver.com/news/2017-audi-sq7-tdi-diesel-ph...