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oldcorollas
Forums Junkie

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

Re: PARALLEL Twincharging, lets make it work

Tue, 23 August 2005 18:20

VeeP wrote on Wed, 24 August 2005 02:43

i still dont understand why you would want to persevere with a system that uses a shitload of piping and at least 3 or four control poitns with teh possibilty of a recirc unit too.
What do you propose this system will gain over a series SC/turbo system.
Henn -
thanks d00d, i meant to say 1 cyccle not one revolution Very Happy

oldcorolla -
you sorta confused me when you said that a turbo producing 14psi might move zero volume but a supercharger at 14psi has to move all its air volume. How does that work, a supercharger moving real slow might be making 14psi, but speed that same supercharger up and the pressure after it will increase all else being equal. So how does a turbo and SC differ in pressure/moved volume?

it might look like a lot of piping but it can be made very compact... SC and turbo are on opposite sides,
for a series, it has to go to turbo, then across to SC, the forward to IC, then acrossat the front, then back across to the plenum on the oposite side... thats three trips across the engine bay.

for parallel, the turbo can go straight from the outlet to the IC, across the front then into plenum... this is where the most power is made.. when turbo is on full boost.. this is just one trip across the engine bay.
the SC goes from SC, across to turbo outlet/IC inlet, and then follows same path... ie two trips across engine bay... the bypass is from plenum to SC.. luckily (as mentioned previously) the plenum is directly above the SC, so the bypass pipe might only be 20cm long.....

so the parallel actually has LESS piping and less pipe volume than the series one Wink

and a more direct path for the high power turbo inlet path Very Happy

i dunno what advantage it will have apart from keping SC output at lower temps perhaps?, and reducing the length of the path from turbo to plenum.. and also increasing efficiency of turbo piping (less length/bends = less friction losses)

as for complexity, yes it is more complex, and that's why it is interesting.

okay..
go to here and play the little movies near the top
http://club.corrado.free.fr/prepa/eaton/eaton.htm

the SC is a positive displacement air pump, meaning that each revolution, it Positively... Displaces.. the set volume of air. ie Definitely Moves the air... what goes in comes out _unchanged_. the compression occurs in the plenum itself...

you could kind of think of it like..... hmm... positive displacement is like filling a bucket with water using a cup. each time you you fill the cup with a set amount, and each time you pour out the same amount.. thats positive displacement....

if you ke filling the cup and pouring it out, you will keep moving water..

same goes for the SC, if you keep turning it, it will keep moving air, no matter what.. it doesn't get compressor stall or anything, it just keeps moving air....and the reason is that there is no way for the air to go back, since the lobes seal against each other...

now a turbo.. a turbo is just a fan... have a look at the impeller of a turbo.. there is actually a path for the air to go backwards thru the turbo... if the turbo is not moving fast enough, and there is too much pressure in the turbo outlet, air WILL go backwards..... it is only the movement of the blades, and the throwing of the air from the middle of the turbo to the outer edges that causes compression... think of a turbo like a merry-go-round (or a gravitron ride Razz

).... if it's not moving you can easily move to the centre, but if it is spnning quickly, it's hard to move from the outside to the middle as you are being thrown outward.. but if you try hard enough (have enough pressure Wink

) you can get there...

so basically a turbo is like a fan.... if you take a normal room fan, or PC fan or whatever, and covr the exit with a piece of cardboard, the air between the cardboard and the fan will be slightly higher pressure than the surounding air, but there is NO flow.... if you moved the fan really fast, you could get some compression of the air, but no flow still...

in a turbo, if the outlet is blocked, the air is still trying to be thrown outwards, but it has nowhere to go, so the pressure increases, but there is no flow... of course there is a point at which the impeller will experience stall/cavitation,

try blowing air back thru a turbo with your mouth.. and then try doing the same with a supercharger.... which one is harder???

hmmmm, lets make it arbitrarily simple example. you have a motor that uses 1L of air per revolution.
an SC making 1 bar boost means it moves 2L of air per engine rev, so the outlet air is compressed 2:1. since the SC and the motor are mechanically linked, and the SC and the engine (alo a positive displacement air pump Wink

) move fixed amounts of air each time, regardless of what speed the engine goes (assume 100% efficiency in everything), there will ALWAYS be 1 bar boost...

a turbo does not move a fixed amount of air, the faster it spins, the more air it moves and it throws it out with greater force..... at low rpm, the turbo compressor is moving LESS air than the engine needs, so air is SUCKED thru the turbo... at some higher rpm, the turbo is spinning more, and so it will cease to become a restriction, and is effectively freewheeling... at higher rpm, the compressor is moving fast enough to spit air out with enough force/velocity to be more than the engine requires, and so you start making boost.... the amoutn of air move increases with the square of the compressors impeller speed...

hmm.. i guess it's hard to explain....i guess a vacuum cleaner on BLOW mode is similar as it uses a centrifugal compressor.... if you cover the end of the pipe, you feel the pressure, but there is no flow... you can probably hold your hand on the end for a while as the pressure will only increase until the compressor cannot provie any more force to the air, and flow stops.. but there is still pressure..

if the vac had a PD device moving air, you put your hand over the end and each revolution of the pump puts more air in... the pressure gets higher and higher and higher since there is nowhere else for it to go...eventualy either your hand comes off or the pipe explodes Wink

geez, i can't explain it any simpler than to say that, at a given fixed speed:
a PD SC is a Fixed Volume-Variable Pressure device.
a Centrifugal Compressor is a Fixed Pressure-Variable Volume device.

you could also search for it on google Razz

ummm

http://www.lextreme.com/centrifugal_supercharger_diagram.jpg

http://www.lextreme.com/roots_blower.jpg

http://www.lextreme.com/twin-screw-diagram.jpg

[Updated on: Tue, 23 August 2005 18:25]

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