|
|
Good questions, Steve.
The A/r ratio is the ratio of the area (A) of the outlet of the turbine
housing to the radius(r) from the turbine wheel axis. Basically the
smaller the A/\r the smaller the area the exhaust gas flows through this
means that a relatively small amount of exhaust gas will move faster and be
more effective in spinning the turbine wheel. As you increase the A/r the
more area means less exhaust gas velocity and more area for the exhaust gas
to pass through. This results in less boost for a given exhaust mass flow.
So for low rpm, 0-60 mph sports car feel the manufactures almost all
(Porsche is one exception) use a smaller A/r to get the most out of the
lower exhaust gas mass flow in the lower rpm range. The down side is that
the lower A/r tends to overspeed the rotating assembly fairly quickly at
higher rpms (or decreases the engine's efficiency with excessive back
pressure). Corky Bell's book does a good job of explaining that larger A/rs
provide more top end HP - but you don't get the boost kick on the lower end.
You avoid back excessive back pressure, overspeeding of the rotating
assembly ,etc.
So for most aircraft application where we are winding high rpms for long
duration, a larger A/r would seem the way to go. Now, selection of the
compressor wheel size is fairly straight forward using compressor maps.
Unfortunately, its apparently not quite as exact a science in selecting the
right turbine housing (A/r) to give you the desire operating
characteristics. There are some rules of thumb - but, most custom turbo
fitters may end up trying several A/r sizes to get the ideal one. Somewhat
expensive for an individual to do.
I have read that the water cooling designed in to some automobile turbos is
not really for cooling the turbo during operation. It supposedly is to
continue to provide some cooling after the engine shuts down. Shutting down
the engine while the turbo temperature is high will result in the center
housing becoming "heat soaked" and the oil actually burning or more commonly
"coaking" into a sludge and harden mass of carbon which hastens wear of the
shaft and bearings. Now personally, I agree that it would seem that a water
cool turbo would be a great benefit if not completely necessary for aircraft
type application.
That is one reason that I think the Mazda turbo can be a successful aircraft
turbo - but again I would like to see the modifications made offer by the
Aussie turbonetics place I have referred to several times. They basically
increase the A/r ratio, enlarge the internal wastegate opening and drill
through the dividing wall between the two scrolls of the twin scroll turbo
(the older Mazda turbos did not have this twin scroll - which really made
them slightly better for the aircraft use in that respect than the later
turbos). The twin scroll is again designed to produce quicker and higher
boost at lower rpms -something we don't really need. So drilling through
the scroll wall divider will permit the exhaust gas to escape from the
scroll that normally does not get to vent through the waste gate. I would
presume that due to that modification that exhaust gas blowing through the
hole in the divider would also disrupt the flow of exhaust gas in the vented
scroll further reducing the effectiveness of the gas spinning the turbine.
I believe Todd has done at least some of these modifications to his turbo
which may account for his lack of turbo problems.
FWIW
Ed
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
----- Original Message -----
From: "Steve Brooks" <steve@tsisp.com>
To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
Sent: Friday, July 09, 2004 7:04 AM
Subject: [FlyRotary] Re: Turbo post mortem
> Ed,
> After recent events, I've been looking at T04 turbo's but I really don't
> know what the A/r ratio is, or what are the advantages to a low ratio vs.
a
> high ratio. I also noticed that most of those turbos are only oil cooled,
> while the stock turbo has both oil and water feeds to it.
>
> I would think that for aircraft use, you would need water cooling as well.
>
> Steve Brooks
>
> -----Original Message-----
> From: Rotary motors in aircraft [mailto:flyrotary@lancaironline.net] On
> Behalf Of Ed Anderson
> Sent: Friday, July 09, 2004 6:30 AM
> To: Rotary motors in aircraft
> Subject: [FlyRotary] Re: Turbo post mortem
>
> If he is using the stock Mazda turbocharger, I believe he can get that
kind
> of boost at altitude. The stock turbine out-let area is really inadequate
> for the amount of exhaust gas mass flow you can get at WOT. They have had
> continuos problems with boost creep on the Rx-7 (at least some model
years)
> due to this. So with the effective A/r of the turbo that small, 38" is
> probably attainable where it wouldn't be with a turbine housing sized for
> aircraft use.
>
> I personally believe getting the stock turbo modified as offered by the
> Aussies at this URL overcomes some of the limitations of the stock turbo
for
> aircraft use as I see them. Of course going to some of the T4/T3 series
> turbos can do the same as they have turbine housings with A/rs ranging
from
> around 0.4 to over 1.2.
>
> http://www.turbonetics.com.au/ATS_p&s_12a_13b.htm
>
> Ed
>
> Ed Anderson
> RV-6A N494BW Rotary Powered
> Matthews, NC
> ----- Original Message -----
> From: "Michael D. Callahan" <micallahan@worldnet.att.net>
> To: "Rotary motors in aircraft" <flyrotary@lancaironline.net>
> Sent: Friday, July 09, 2004 1:14 AM
> Subject: [FlyRotary] Re: Turbo post mortem
>
>
> > Hard to put out 38" at 11.5K with the gate wide open;-) I want an intake
> > like HIS if he can do that! Mike C.
> >
> >
> > > That doesn't sound right. How big is the wastegate, and what style
(ie,
> > > throttle plate or Delta-gate (or Race-gate)? With the wastegate fully
> > open
> > > the majority of the exhaust should be bypassing the turbo. Or am I
> > > misunderstanding your comment? WOuldn't be the first time. <g>
> >
> >
> > >> Homepage: http://www.flyrotary.com/
> > >> Archive: http://lancaironline.net/lists/flyrotary/List.html
> >
>
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
>
>
> >> Homepage: http://www.flyrotary.com/
> >> Archive: http://lancaironline.net/lists/flyrotary/List.html
>
|
|