Toymods Car Club Inc: Tech & Conversions => PARALLEL Twincharging, whatever....

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oldcorollas
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Kita-Ku, Sapporo, Japan
Registered:
January 2003

PARALLEL Twincharging, whatever....	Sat, 20 August 2005 14:40

Heya Ppl, i've been pretty disappointed at the small amount of work that has gone into twincharge designs. so i thought it would be nice if i could share some of my ideas, and with a big collective group hug, get a workable design.. i haven't had the time to think through these designs thoroughly, so at first i'll just present the case for acceleration. The basic reason you would twin charge something, is so that you can use a much bigger turbo than would otherwise be practical from a lag or power delivery point of view. the drawbacks, apart from cost, are the complexity of having an arrangement that allows the SC to be turned off and still have a good transition from SC to turbo. there have been a few kits and installs that have them in series, with the turbo blowing into the SC. This works, but you stll have the parasitic SC loss, as well as the Roots blower inefficiency to deal with in terms of heat (I'm assuming use of SC12/14 etc). Ideally, you would have them in a parallel arrangement, whereby you could turn the SC off and have a great topend charge from the turbo, but still have bottom end torque from the SC. as i said, the issue of transition is the biggest hurdle to this setup..... So, my thinking cap went on, with the basic requirements that the transition from SC to be turbo had to be absolutely smooth, the SC must be able to be switched off, and that it could be MECHANICALLY controlled with minimal electronics. so first, i present the case for pure acceleration, the simplest case in terms of transitions. As shonw in dodgy diagram below, we have SC, turbo, and three throttles: one to throttle SC input, T1 one to throttle turbo output, T2 and one to throttle the air recirculating through the SC, T3. i will also assume that we have an ECU with programmable output to allow switching on, and perhaps off, of SC. IC will refer to any air that is after the chargers, ie from the IC. so we start from idle at WOT, and assume the SC is switched on T1 is open to allow SC to draw in air, T3 is closed to stop air recirculating from IC to before the SC. T2 is also closed because we don't want to force air out backwards thru the turbo.. so basically it runs like a normal SC car. for the transition, the problem is that the SC is positive displacement, ie it either pushes it's maximum volume, or none at all, whereas the turbo is a variable volume system, it can provide full pressure with nearly no flow. what we need to do is to balance the flow of both chargers so that at the transition there is a continuous increase in both flow AND pressure. we can use this at the transition point by referencing T2 to the pressure at the turbo exit, R1, and use some kind of wastegate actuator to open T2. we could either have the actuator as an absolute reference... to maybe 2psi above SC pressure, or as a relative reference, so that it begins to open when R1 is 2psi above R2 (the IC pressure) so basically, the turbo starts to spool, and the pressure between the turbo and T2 rises past the IC pressure (ie SC output) then T2 begins to open a little, bleeding air into the IC. we _could_ just leave the SC on also and be done with it, but what i want is to be able to switch the SC off, and turn higher rpm with the turbo alone... if we just switch it off now, the pressure drops as the turbo is not flowing any volume.. so what we do is to start throttling the SC's intake via T1, and allow an amount of air to recirculate back past the SC via T3. These could also be actuated by the turbo outlet pressure, and indeed for a rough effort, could be mechanically linked to T2 also!!! so what we would have is a system where the turbo reaches SC pressure and starts to contribute to the volume of the system. at the same time, the SC input is throttled and to allow SC to pump a constant amount, air is recirculated to the SC intake... by slowly closing T1 and opening T3, you can have the SC still turning full speed, yet not contribute any air to the system. in fact this is what we want.. by the time the turbo is properly spooled, and is providing almost all the air at a couple of Psi above SC pressure, T1 will be almost closed, and T2 fully open... so now, the SC is providing no new air to the system and can be swithced off completely, possibly using a microswitch on T1.... so now, only the turbo is providign air, T3 can remain open as long as T1 is closed (as both sides of SC are at equal pressure), and T2 is fully open... car is now running in turbo only mode so thats the basic idea for acceleration. initially only SC is running. turbo starts to kick in and SC is throttle back, but intake manifold pressure and flow continue to increase, but with the turbo providing an increasing proportion of air. the SC is throttled back and recirculated to the point where it has no contribution and is switched off. we are now in turbo only mode... i see that these three throtles can be simply actuatd using something like wastegate actuators, and given the right sizing/bell crank gemoetry, could even be mechanically linked so that only one actuator is needed. the only electronics needed are ECU signal to switch SC on (or manual switch), and then microswitch at T1 to turn the SC off when it is no longer contributing.... so... i'd like constructive comments and criticisms. this is not a thread to bitch and whine about the benefits or not of twin charging, but simply a "let's make it work" thread. Cya, Stewart [Updated on: Wed, 24 August 2005 05:47] [Report message to a moderator]

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