turbo - why improving LOW-end torque

[Talus1]

I think the real answer to the question is emissions fuel economy regulations based on unrealistic testing conditions. To improve economy, you need a small, low reving engine with high compression that is capable of running very lean. Because you need some power while accellerating but not much at any other time, at least on the typical emissions or fuel economy test protocol, manufacturers are pushed towards turbocharging. Small turbos combined with high compression ratio, very high pressure direct injection and sophisticated and precise fuel control allow very good numbers to be achieved in the Euro cycle and US EPA tests. Real world economy is generally worse than in these "idealized" test scenarios, but the gap is much larger for modern turbocharged engines than for NA.

I suspect that is part of why Mazda has mostly resisted the turbocharged approach and focused on weight reduction combined with sky high compression with their Skyactive concept.

TLDR: Manufacturers are forced by emissions and economy regulations to replace larger NA engines with smaller turbo engines with lots of low end boost.

[wbradley]

Note: Most newer performance oriented brands have switched to smaller displacement (1.4L to 2.5L) turbocharged engines using small twin scroll turbines. From an emissions and fuel economy perspective, hitting peak torque down low (1600 to 2000 RPM) is ideal and will provide good acceleration. This combined with properly calculated gear ratios. Another strategy is switching from Otto cycle to Atkinson cycle or having the ability for the ECU to switch on the fly. Finally, variable valve lift , duration and ignition timing and sometines variable intake runner length.

The sweet spot for displacement seems to be 2 L with premium forced induction engines now.

[serialk11r]

Originally Posted by wbradley

This combined with properly calculated gear ratios. Another strategy is switching from Otto cycle to Atkinson cycle or having the ability for the ECU to switch on the fly. Finally, variable valve lift , duration and ignition timing and sometimes variable intake runner length.

The sweet spot for displacement seems to be 2 L with premium forced induction engines now.

"Atkinson cycle" is already done to some extent on any car with VVT. There's limits to how much you can go without variable lift profiles (such as the Honda R18), but the latest generation of Toyota VVT-iW and VVT-iE is essentially as good as you can do with a single cam profile. For a higher revving engine with a bigger intake cam this is more effective. I don't really see variable lift becoming popular because Honda is the only company that has much experience doing it, and turbocharging is probably simpler in R&D since it's completely external to the engine.

Variable intake runner length is also only able to do so much. The 2AR Camry engines already have it, and they're not much of a performance boost over the 2AZs. The LFA has 3 stage variable runners, but the low end torque is still lacking.

2L is the sweet spot in the US today, because fuel here is cheap enough that no one really cares about the savings that you would get from going down to the 1.3-1.5L range that's the norm in other countries. With the EPA standards going up quickly though, I think engines like the Hyundai 1.6 turbo and Ford 1.6 turbo are going to become more common.

[wbradley]

^^ While most people realize the US is a big market, most global manufacturers design for a global market.

Toyota for one, has something like 10 different small displacement engines ready for or scheduled for production. Brands like Jaguar are using 2L turbo engines in their new models. Mazda 9, their flagship SUV has a "huge" 2.5 L turbo for a 7 passenger vehicle.

Too bad gas is cheap in the US, it just makes their CO2 emisions that much worse.

[ITmushishi]

Originally Posted by serialk11r

"Atkinson cycle" is already done to some extent on any car with VVT. There's limits to how much you can go without variable lift profiles (such as the Honda R18), but the latest generation of Toyota VVT-iW and VVT-iE is essentially as good as you can do with a single cam profile. For a higher revving engine with a bigger intake cam this is more effective. I don't really see variable lift becoming popular because Honda is the only company that has much experience doing it, and turbocharging is probably simpler in R&D since it's completely external to the engine.

Just another reason why electrically controlled valves should be in normal production engines already.... Basically it moves the cam profile into software(!).

[serialk11r]

Originally Posted by ITmushishi

Just another reason why electrically controlled valves should be in normal production engines already.... Basically it moves the cam profile into software(!).

Very very high power requirement, no failsafe. I live/work in SF and drink silicon valley Koolaid everyday, but actually, software is not the solution to everything.

Cams are a reasonable solution to opening poppet valves. The only things that really matter are duration (can be fudged by moving the cam around) and lift, but there's no way to get more lift without more duration, because the faster you open the valve the more power it takes (you need a stronger spring to resist that force too) and the more stress there is on the valve. It only takes 2 or 3 profiles to get a reasonable approximation of the ideal profile for 95% of driving circumstances, which is what Honda figured out.

[xyborg]

Originally Posted by ITmushishi

Just another reason why electrically controlled valves should be in normal production engines already.... Basically it moves the cam profile into software(!).

Pneumatically controlled valves are the future, koenigsegg already has working prototypes.

[ame="https://www.youtube.com/watch?v=f4p-55a3WV8"]Koenigsegg engine innovations - YouTube[/ame]

[mav1178]

Originally Posted by mdm

OK, so it may be a stupid question, but I just don't understand it.


So people say that turbo helps in particular with low-end torque. And if you look at car spec sheets, it seems to be the case, NA cars tend to have max torque between 4.5 and 6 k rpm, and turbocharged have max torque from 1.7 - 2 k.


But turbo is driven by exhaust, so it should be particularly effective at high rpm, because high rpm equals a lot of exhaust to spin the turbo to high speed.


What am I missing here?

What you are missing is velocity.

Smaller turbos have smaller (turbine) wheels, thus they spool up faster.

Given the same amount of mass (exhaust gases), a smaller wheel will spool up faster than a larger wheel. Kind of like a small garden hose versus a large mainline pipe. Given the same mass of water, the velocity coming out of the small hose is much faster.

As for when a turbo is efficient, that depends on how the wheels are sized. Bigger and faster is not better... and much like most tuning and modding questions in general, the right answer for turbos depends on what questions you are asking and the context.

-alex

[Captain Snooze]

There's been a handful of cars with sequential turbos: Porsche 959, Mazda RX-7, Toyota Supra, Subaru Legacy.

[wbradley]

I think OP's question is how is it that turbos increase low RPM torque. Simple answer is small turbine spools up at lower RPM. The reason this design is widely used is to make useful power for acceleration whilst maintaining low fuel consumption.