Ways to get rid of turbo lag 1

[inultus]

I was wondering if this would be feasible:

We all know that turbo "lag" is due to the process of waiting for the turbo(s) to spin up via the exhaust gases flowing through them.

Why couldn't you simply hook electric motors up to the turbos to spin them forcefully, in compliance with the amount which the gas pedal was depressed.

It seems to me that this would get rid of turbo lag. Of course, it would take some tuning and such...

 

[kikiturbo]

warpspeed..... I think there might be a terminology problem here.... variable nozzle turbines (VNT) have been used as OE on road cars before, like peugeot 405 T16 for example... it is the nes generation of variable geometry (with those little movable airfoils around the turbine wheel - which is what you are using I believe) that are used on diesels only.... from what I understand, it is the problem with exhaust backpressure of those turbos and very difficult mapping that is making them unattractive to OE users..

the problem of turbo lag is most evident in engines with high specific power, like from 150 hp/litre and above... turbo lag is also very evident in underpowered cars... same engine will give very different feeling to the driver in two cars with a large weight difference...

one thing that is often overlooked, is exhaust manifold design... propper design, and not using too large pipe diameters will give noticable improvements in lag...



cheers

vlado

 

[Tmoose]

Porsche published some data about quicker spooling when using air injection. This was the days before boost recirculation. I think the cars were CIS, not electronic injection too.

Heat Wrapping the many feet of naked pipe between my Corvair's engine and turbo got noticeably more and quicker boost in the lower gears.

 

[Artsi]

What I've read, the Chrysler appears to have been the very first OE manufacturer to adobt VNT turbocharger in their petrol (gasoline) engine. (Apparently they had plenty of problems with VNT charger.)

http://www.allpar.com/mopar/22t.html

Exhaust header dimensioning differs from freely aspirated counterpart according to one publication I happened to read. It suggested that well tuned naturally aspirated engine has larger inside diameter header tube than equivalent turbo engine of same size. This may not hold ground in racing applications, but I reckon such dimensioning would make low end boost developement more brisk.
Say in a 4-cyl engine one cylinder in it's exhaust stroke will pressurise all adjacent header tubes as well, before working on the turbine wheel - right?
In this light, the smaller dia header should enable higher exhaust pressures (and flow as well) in the turbine housing in lower engine operating range.

 

[kikiturbo]

equal length 4 cyl exhaust headers for turbo applications I saw had ID of as low as 34 mm... which was good enough for almost 500 HP...

small dia exhaust tubing will keep gas flow at high speed, and if you have a single collector right before the turbo you will minimise the effect of preassurising the other exhaust ports at the same time..

 

[GregLocock]

Back on topic, here's an electric supercharger

http://www.autoindustry.co.uk/news/industry_news/day-1_4_2005#7

I'd like to see it combined with turbo driven alternator, but it is the right idea

Cheers

Greg Locock

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips.

 

[NickE]

Greg- I just wonder where they are going to get the power to spin the compressor at 225,000rpm and do work. (compress air)

Are you suggesting spinning a turbine off the exaust, that turning an alternator, battery storage, delivering enough reserve to spin-up the compressor.... Wow, why even use a recip-piston engine then, wouldnt turbine generator to motor wheels be more efficient?

Nick
I love materials science!

 

[kikiturbo]

I think that the main question is specific power... I have driven a number of low and moderate pressure turbocharged engines (let's say 0.4 to 0.8 bar) that deliver between 85 and 100 hp/litre, that have no lag whatsoever, and deliver usable torque almost from idle to max power.... so somplicating an already very usable engine with added complexity of electricaly driven turbos is not an isse for me...

cheers

vlado

 

[Warpspeed]

Greg, that electrically boosted turbo is a splendid idea.

I had thought of using a mechanically driven centrifugal supercharger driven via an epicyclic power splitter. Supercharger rotor speed could be controlled by a water cooled eddy current brake. It might be good because, it could also reduce shock loading on the drive during gear changes, but still respond much faster than an exhaust turbine at low engine Rpm.

An eddy current brake can typically hold about fifty times the electrical power required to energize it, whereas a motor requires more electrical input than you get mechanical output.

Heat output from a small water cooled brake could probably be absorbed into the engine cooling system without much drama. That might be less objectionable than finding hundreds of peak amps to run an equivalent motor.