Anti-lag in turbocharged cars 3

[Hurls]

I have been reading some of the posts on here for the last few weeks, and finally decided to join up. Lots of good info and smart people here! Here goes my first question...

I was wondering how the anti-lag system works in a turbo car...like the ones in the World Rally Championship cars.

I understand that fuel is ignited in the exhaust manifold before the turbo, causing the turbo to spool. Thus eliminating some turbo lag at low rpms. I don't know where this added fuel is coming from though. Would it be as simple as installing a fuel injector in the exhaust manifold? Obviously, you would need an ecu to control the injector as well.

 

[Fabrico]

Don't under estimate turbo sizing and mass reduction in regards to lag. To reduce smoke and increase throttle response on modern Diesel engines, turbo lag is addressed mainly by smaller, more efficient, lower mass designs. Some even use low mass ceramic impellers. Nowdays, because turbos are smaller and working harder, if you develop even small leaks in the air aftercooler or lines, the turbo can overspeed and self destruct. Years ago you could blow out a hump-hose and drive the rest of the day that way. That is, if you could find some earplugs.

 

[NickE]

Here are some links to how anti-lag is implemented by different makes.

thread71-55810

http://www.rallycars.com/Cars/bangbang.html

There you go.

 

[mattsooty]

Modern turbos are low inertia and they are more optimally designed due to the prevalence of good CFD packages. Modern diesels tend to use something more like Variable Nozzle or Variable Geometry turbochargers and do away with the wastegate - but that's a bit off topic.

The rallycar link above describes the "whys" of antilag but isnt 100% correct on the "hows" of antilag.

Although there is mention of spark retard during gearshifts this is actually a very common technique for allowing smooth gearchanges and pretty much all OEMs utilise it. Whereby when a gearchange is detected(or commanded in an Auto) the spark is retarded; then, when the throttle is opened again the spark is ramped back to the desired advance.

The above does not constitue antilag on its own because with the throttle closed the amount of air supplied by the Idle Air Control mechanism simply is not enough to keep the turbo spinning with gusto - even with all the retard in the world! Also by opening the throttle further during gearchanges one simply runs into the rev limiter before any sort of real boost can be made.

In a true antilag system, at least with a cable actuated throttle plate and the type used during gearchanges (such at the TTE ST205 WRC car) the setup is a little more cunning.

In the boost circuit there will be a recirculating dumpvalve that, instead of dumping to atmosphere, dumps boost into the exhaust manifold, between engine & turbine.

At WOT & high/max boost if the throttle is snapped shut (as in a gearchange) the dumpvalve actuates and bleeds air from the intake manifold into the exhaust manifold (the pressure in which will be low due to the closed throttle plate).

A lot of extra fuel is then injected, in the normal manner and the spark is retarded. As soon as the exhaust valve opens and the rich (often overrich) charge enters the exh manifold combustion occurs/contiues and you get the desired effect of increased exh manifold pressure, subsequent work on the turbine & more boost.

Quite often the charge will be so rich that combustion will not initiate until it mixes with the air in the exh manifold - this provides the chracteristic bang - bang sound.

MS

 

[Hurls]

Thanks for the response mattsooty. That makes sense during WOT, but do you know how it works at idle? It seems like it would be most beneficial at idle or low rpms in order to get a good launch at the start of a stage or accelerating out of a slow corner. At low rpms the turbo would not be making much boost. When you refer to the recirculating dumpvalve would that be equivalent to a wastegate being routed back into the exhaust manifold?

 

[patprimmer]

Please do a site search. This subject has been covered extensively before.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.