[Diy_efi] Small A/R, big WG (was: Throttling intake air --

[Axel Rietschin]

----- Original Message -----
From: "Lasse Langwadt Christensen" <langwadt at ieee.org>
>
> Axel Rietschin wrote:

> > Modern state-of-the-art turbo engines (WRC and CART) have port
throttles,
> > very small turbine housings, massive (and aggressively opened)
wastegates,
> > no lag and wide powerbands.
>
> If BMW  did ~1000bhp on 1500cc and the WRC does ~300bhp on 2000cc could
> that be part of the explanation for the smaller turbo and lag?

I was trying to point out that driveability (no lag, wide powerband) is more
of a concern than raw peak power today (at least in some applications) than
it was in the past on race engines.

> Could the wide powerband be that the output is "railed" at ~300BHP
> because of the restrictor so that you just can't "see" the peak?

Yes, the unrestricted 2L rally engine would produce 500+ hp with good
driveability without intake restrictor. Max torque is produced at around
3500 rpm and the engine would pull happily to 7000+ without the damn thing,
making the useable band even larger. Of course more power could be produced
by shifting up and narrowing the powerband (mainly by using a larger turbo)
at the expense of driveability.

> Is the big waste gate needed to prevent the turbo from spinning so
> fast it falls apart?

My point is they use an unusually small A/R exhaust housing, with its
potential overrevving and big backpressure issues, and a unusually big
(separate) wastegate to fix thing up, I see this as an unusual approach
worth to be mentionned.

Clearly, the wastegate is used to control the turbine speed - not the
boost - to maintain the compressor at its best efficiency and, as a welcome
side effect, avoid overspeeding the turbo. Some management systems use a
turbo speed sensor for that purpose, other use a differential pressure
sensor to measure the pressure drop across the restrictor.

On such setups you can expect the wastegate to be more or less open most of
the time in race driving, it is not the exception as in "normal" turbo
setups. The goal is to get a very quick spoolup when needed without getting
too much backpressure at high engine speeds; this lower backpressure helps
work around the large boost drop due to the restrictor and produce a little
more power at the top end despite the clamping of the intake air. Coupled
with an anti-lag system which further reduces the lag, modern rally engines
have instant throttle response and are pleasant to drive, they really feel
like very powerful NA engines and this was not the case only a few years
ago.

> > The current 12'000 rpm Cosworth CART engine also
> > has a 9th by-wire throttle just before the plenum chamber, not sure what
it
> > is used for.
> >
>
> could it be used to keep the boost right below what will trigger the
> pop-off valve ?

I've no idea, maybe could you explain more?

--Axel


_______________________________________________
Diy_efi mailing list
Diy_efi at diy-efi.org
http://www.diy-efi.org/mailman/listinfo/diy_efi