Topic: Twin Charging: An academic exercise
I've always been fascinated by twin charging. Mostly from a "that seems over-complicated and therefore amazing" point of view. A couple times a year the dumb idea pops into my head that I should try it, not for any particular reason, just to try it. Looking around my garage the other day it dawns on me that I have a whole crap ton of Turbo dodge stuff sitting around, and an electronically clutched supercharger..... Now, will I follow through on any of these thoughts? maybe. I have a good track record for starting projects, and a crap record for finishing them. Who cares? Lets talk theory.
There seems to be a couple ways to do twin charging.
1. Two separate intake tracks to the throttle body. One has a turbo, one has a supercharger. Find some way to close off one while the other works. Used very infrequently as it is flawed and doesn't work great.
2. Turbo feeding constantly into a supercharger. Supercharger works down low, and then as the turbo spools the supercharger acts as a "boost multiplier" as it compresses already compressed air. This works, just has some issues with making sure the supercharger can handle the compound pressure and additional heat load.
3. Turbo into supercharger with a by-pass valve. Basically the same as option #2, but there is a supercharger bypass that you open up when the turbo spools enough and essentially takes the supercharger out of the equation.
I like option #3. Mostly because I see it as a way to create fairly even boost levels all across the rev range. I'd want to set it up such that the supercharger makes ~8psi, and then the turbo can build all the way to ~12psi. In addition it seems easy to "turn off" and run like a normal turbo engine if the need were to arise. So, how would you set this up? I would do it like this. Turbo setup like normal. The turbo feeds into the supercharger in. The super charger feeds into an intercooler. The intercooler feeds to the throttle body. At the supercharger would be a bypass tube linking the supercharger inlet side to the exit side. When this valve is closed, all air goes through the supercharger. When it opens it allows air to skip the supercharger. Ultimately when the supercharger is fully bypassed it's clutch is switched off and you run purely on turbo. Basically looks like this, but without the intercooler between the turbo and supercharger. I think I'd throw in a water injection setup just before the supercharger to cool the lobes.
But layout is the easy part. Where I start hitting walls is control. Engine tuning is easy. The turbo dodge stuff is purely MAP based. So it only cares about intake manifold pressure. Adding a supercharger doesn't change anything, it just reads from other parts of it's fueling maps. It's control of the bypass valve and supercharger that scare me. I'm sure you could set it all up to run from basic vacuum/boost and pressure sensors. Something a diaphragm driven valve that starts to open once boost before the supercharger reaches a certain level, and then when the valve opens a certain percentage the supercharger is flicked off. The trouble I see is that you probably want the pypass to stay open on aggressive upshifts, or on light off throttle situations where you mean to be back in the throttle quickly, and I don't really know how to build that in.
So let's complicate more! Electronically control all the things! It can't be that hard to build an arduino controlled system that monitors pre-supercharger boost, intake manifold boost, throttle position, RPM, etc and have all that determine when to open and close a bypass. For the actual bypass you could use an electronically actuated throttle body, or something similar. With the Arduino you can build in those upshift delays and any other advanced control.
Oh right, I suck at programming. Who cares! I'll learn.
Here's my main hangups.
1. Writing the control program for the bypass and supercharger clutch logic
2. Changeover logic. How to you determine when it's time to start opening the bypass. You can't just say "when pressure into supercharger = pressure out" because the supercharger is multiplying the in. I guess you could use logic that says when you start seeing the boost pressure climb you know you're feeding pressure into the supercharger and it's time to start bypassing
3. Do you block off the supercharger as you bypass? If you don't block it everything equalizes as the bypass opens, so as long as the supercharger is turned off it just sits there with equal pressure on either side. But I imagine that you'd start opening that bypass before the turbo is making the same boost as the supercharger, so there would be a brief moment of backward flow as you open the valve. Does it have any benefit to partially block off the supercharger inlet to try and prevent a spike in compounding pressure?
Those are my madman ramblings so far. I need to think a lot of this through, then build up another engine to use as a guinea pig test bed. Who knows what i'll even use this for, other than just a "look what I managed to do" show piece.
Abandoned E36 Build
2008 Saab 9-5Aero Wagon
Retired - 1989 Dodge Daytona Shelby 2011-2015 "Lifetime Award for Lack of Achievement" IOE, 3X I got screwed, Organizer's Choice