Twin Charging

cammmy · Post by **cammmy** » Thu Apr 04, 2013 11:03 am

Hi Guys

Done a bit of reading on twin charging (Supercharger + turbo) and just for the sake of something to daydream about...

It seems the best way is a supercharger that feeds into a turbo, at higher rpm the charger is bypassed and ideally disengaged by an electromagnetic clutch. My chargers have clutches and bypass gear as they were originally switched on or off in relation to manifold vacuum.

Could it be worth feeding the outlet of each charger into a turbo and then having that go through an intercooler to the motor? I believe the chargers I have could be the limiting factor for HP. I could spin them faster in relation to the motor for increased boost at lower rpm and then cut them off as the turbo's spool up (so as not to over speed them)? Even if they only provided the same amount of air as the chargers higher up, the parasitic load would be gone, freeing up 30hp or so without actually increasing the internal stresses on the motor?

As lag wouldn't be an issue, I could also use a less restrictive A/R on the turbo's and use cheaper units that don't have special design features to decrease the time it takes them to spool?

Worth thinking about?

DEVONMAN · Post by **DEVONMAN** » Thu Apr 04, 2013 11:54 am

When the chargers are cut off, is there any drag/resistance to the airflow into the turbos?

kiwicar · Post by **kiwicar** » Thu Apr 04, 2013 12:01 pm

Hi
Personally I think it is either
1 use a pure turbo system and spend the money saved on the blowers sorting out the set up so it does not lag.

or

2 don't bother with the turbo and spend the money on a supercharger that is a better match for the engine.

The only way to gain real benifit from a turbo + supercharger set up is in a marine of aeronortical environment where you run at a pretty high boost steady state condition. Generally these engines are Miller cycle, not otto cycle http://en.wikipedia.org/wiki/Miller_cycle and running in the 3 to 5 maybe even 6 bar of boost region.
You really need an intercooling stage between supercharger and turbo charger or you get too much heat at the output of the second stage to remove with a single charge cooler. The whole idea of 2 stages is to enable you to controle charge temperature (you would otherwise run a high risk of ignition in the inlet tract as you injected the fuel if you did not direct inject). They tried it with the group B pugeot rally cars in the '80s and with one or two of the 1500cc F1 cars but there were too many reliability issues, the set up ended up very heavy and the engine charictoristics did not really suite the application.
Best regards
Mike

cammmy · Post by **cammmy** » Thu Apr 04, 2013 12:06 pm

Not much I would have thought Devon, the air is routed around the charger. There might be a bit of resistance compared to a straight pipe but it's not dragging it through the charger or anything.

I just found a diagram of what I was thinking:

The original setup for these chargers is the same except that instead of going to a turbo inlet, it merges back in and goes to the manifold. The clutch is also built into the charger pulleys

Okay cool Mike. I was just thinking about how VW have started using this setup and about the Delta S4 (which was very successful rally car).

I was thinking about the temperatures but thought that they might not get too high as when the chargers are running, the turbos aren't doing much and when the turbos takes over, the chargers are off. There's only a brief overlap between the two

kiwicar · Post by **kiwicar** » Thu Apr 04, 2013 12:39 pm

Hi
Sorry I wasn't implying it did not work, what I was trying to say was that it was complicated and heavy and there were better ways to use a limmited budget (of money and time) yes the S4 worked, the Pug was less successfull, the big successes were boat and aircraft engines and parciculally miller cycle ones that benifit the most from very high boost.
Best regards
Mike

cammmy · Post by **cammmy** » Thu Apr 04, 2013 1:40 pm

Cool. Just thought that since I already have chargers and the bypass gear that it would be reasonably cheap way to free up 30+ HP without having to deal with the stresses of extra boost; as it's not really compound charging because one takes over from the other.

kiwicar · Post by **kiwicar** » Thu Apr 04, 2013 2:03 pm

Hi Mate
It would be a very interesting project. . . especially on the control side, infact great fun. . . just not sure I would take it on myself

I knew I saved this link for a reason http://store.arduino.cc/eu/index.php

Best regards
Mike

Post by **katanaman** » Thu Apr 04, 2013 4:47 pm

I know a couple of guys that are doing this with varying success. It seems to be pretty difficult getting the timing right for the bypass gear. One of them is doing it with an arduino and the other is doing it with wastegate actuators on boost. The more successful is a 3rd person who is running 2 turbos, one small and one large. This works pretty good and a few manufacturers are starting to go this route. You will still have bypass valve control issues but for some reason it seems to be easier to setup. Might be something to do with suddenly loosing the large extra drag of the supercharger but I really don't know.

cammmy · Post by **cammmy** » Fri Apr 05, 2013 9:41 am

Thanks guys, I've done some reading and various people who have done it are very happy with the results. It will be interesting to see what I could do with the gear I have as the valves already operate based on manifold pressure. I could perhaps reverse it so that they bypass the chargers when boost begins to rise above what they alone can provide?

Like I said, it's really something to day dream about/pub talk

Edit: Turns out that the standard gear is designed to gradually open and bypass the supercharger at 8psi to limit boost; so the control mechanism I need is already built in.

kiwicar · Post by **kiwicar** » Fri Apr 05, 2013 12:26 pm

Hi
I am sure it can be got to work, bt just thinking about the control side

I would have thought the tricky one to sort out would be the situation where you are driving along a at fairly rapid rate, with the super charger clutched out and bypassed, your slightly bigger than nomally matched turbo is spinning round giving say 1or 2 psi boost and say 240 bhp some annoying pleb in a Bently beatles up behind you . . You obviously have to put the little tick in his place so you boot the throttle. . . Then what does your ECU do? Presumably the first thing it does is slam the waste gate shut to spool up the turbo, but that will not give you instant enough throttle response as you matched it a bit bigger for efficiancy. . . so do you drop the clutch in on the super charger? seems like a yes, however if you do then the load of accelerating the blower and the asociated air column is going to pull out the 30 or so brake horsepower you wanted to save by having the clutch and bypass set up there in the first place and it feels like you lifted off not floored the throttle. After a second or so the clutch is in and the supercharger is producing boost and off you go, now the extra exhaust gass has hit the turbo and it is spinning up nicely and we are really beginning to make boost, what happens next, open the waste gate again. . ? ok but with the supercharger now shoving in all that boost you run the risk of surge, do you drop the clutch out on the supercharger again and add 30bhp to the mix but instantly dropping out the blower's contribution just as the turbo is opening it's waste gate anyway. . .yes I can see slowing down all the valves will help but that somewhat further negates the point of adding the blower, the instant throttle response. To me it just seems a control theory nightmare, you can probably get it to work, but can you get it to work better than a better matched turbo or blower?
I can see the 2 differnt sized turbos working well as their response is in the same control loop dependent on the same energy input, the exhaust gas, and they do not add in the +- 30 bhp penalty load on start up.
I know the setup of turbo charger and supercharger worked very well on post war radial aero engines where silly levels of break mean efficiency were avaliable over a very wide range of operating altitudes but . . .
Best regards
Mike

cammmy · Post by **cammmy** » Fri Apr 05, 2013 1:21 pm

Fair point. I'm going to have to re-read it when at home to fully understand.

Quickly though. What if I had a switch to turn the chargers on or off. While it's on the chargers spin regardless of what the turbos are doing. When the boost rises and the by-pass opens around the chargers, the pressure difference on either side should be very low (as the chargers are not pushing air against against anything). This means they are not doing much work and so the parasitic load reduces, just not quite as much as completely disabling them.

This blows up my theory of overspeeding the blowers and turning them off but should still reduce parasitic losses at high rpm.

I know it would be better to use either the correct turbos or blowers. I already have two blowers and I'm going to use them, it's just whether I could get some of the benefits of of both systems for less cost. My chargers already have by-pass and control gear and I have to buy new headers anyway. It's a question of whether it could be done for the cost of buying cheaply available (but normally unsuitable) turbo's and the extra plumbing.

teamidris · Post by **teamidris** » Mon May 27, 2013 4:36 pm

I wrote a boat load there and it ate it

What about a valve on the supercharger inlet, to stop them sucking air to compress. Suction offloaders work well on screw compressors?

I was told the turbo doesn't like hot air intakes, so it needs a charge cooler or intercooler after the supercharger.

I think the clutch has a few issues. For an easy life I'd avoid it, if only for mechanical drive issues of, BANG it's in

cammmy · Post by **cammmy** » Wed May 29, 2013 11:41 am

I hate it when that happens. With a long post I always copy the contents before submitting, just in case.

There's a few different ways it could work. I think I need to find someone who has done it to see what they think. May end up being more work that it's worth but is fun to investigate

teamidris · Post by **teamidris** » Thu May 30, 2013 8:43 am

The problem I find with "find someone who has done it to see what they think" is I can never find that same person

On my RV8 dry sump there were a few who had done it, but no one who could confirm it would sit nose up or down at 45 dgerees for 20 mins without killing the high-and-dry pump segment. So I got a pump made that self lubricated, for a belt and braces job

And the clutched supercharger is proving the same sort of task. With a rotex blower and an air con clutch I'm taking a customers word that they interact okay? I was also told the belt driven pulley could sit on the 12mm spindle, yet on the air con package the pulley sits on a much larger bearing and hollow ally stub shaft? So I'll need a new support stub making, as I believe the clutch might be okay on drive, but I don't believe a little drive shaft will support the side loads of a belt

So I'm trying to figure what's right from folk who might have been guessing? But I know if I do a solid adapter between the two parts, (a good foundation if you will), we can build on that if the clutch needs a bit more ooomf.

In the mean time I'll continue with my home project of mini dozer blade on a landrover, because your right, you need some low risk daft projects on the go, to keep the Fun

SuperV8 · Post by **SuperV8** » Thu May 30, 2013 9:29 am

With a rotex blower and an air con clutch

I'm sure you've calculated your blower speed with the crank pulley dia and the aircon pulley dia but I'm just a bit surprised an aircon pulley is small enough to give you enough supercharger speed? also do you have a source of different sized aircon pulleys, because you you can bet will always want more boost!

Tom.