Compound forced induction.
Discussion
Does a turbo charger generate a pressure differential or does it shift a particular volume of air ? To put my question another way if the input air supply is already above atmospheric is the output boosted or will it still shift the same amount of air ?
I was wondering about mildly supercharging and intercooling the supply to an already turbo charged small engine. The intention would not to be to massively increase the boost but to negate losses due to induction piping and supply cooler air. To ensure the turbo never starves the pressure should, I would think always be above atmospheric, how would it cope ?
I was wondering about mildly supercharging and intercooling the supply to an already turbo charged small engine. The intention would not to be to massively increase the boost but to negate losses due to induction piping and supply cooler air. To ensure the turbo never starves the pressure should, I would think always be above atmospheric, how would it cope ?
steve-V8s said:
Does a turbo charger generate a pressure differential or does it shift a particular volume of air ? To put my question another way if the input air supply is already above atmospheric is the output boosted or will it still shift the same amount of air ?
I was wondering about mildly supercharging and intercooling the supply to an already turbo charged small engine. The intention would not to be to massively increase the boost but to negate losses due to induction piping and supply cooler air. To ensure the turbo never starves the pressure should, I would think always be above atmospheric, how would it cope ?
er, i think you're understanding of how an engine and forced induction work is somewhat wrong!I was wondering about mildly supercharging and intercooling the supply to an already turbo charged small engine. The intention would not to be to massively increase the boost but to negate losses due to induction piping and supply cooler air. To ensure the turbo never starves the pressure should, I would think always be above atmospheric, how would it cope ?
A centrifugal compressor (driven by an exhaust turbine (turbocharger) or direct mechanical drive (supercharger) simply adds "energy" to the air charge. The air enters the compressor wheel, gets "caught" in the blades and accelerated, and then it thrown out of the outer edge of the compressor wheel at high velocity. At this point it is at LOW pressure (because it is moving very rapidly) but has lots of energy in it. The compressor scroll (the snail shell shaped part surrounding the compressor wheel) then guides this high speed air, and it decellerates it rapidly, which increases the pressure (and hence density) of the air.
Because of this, centrifugal compressors are generally classed as "compressors" and use "internal" compression (bit of a misnomer, as the "compression" doesn't actually occur in the compressor wheel.....)
Mechanical superchargers that don't use a centrifugal compressor wheel, but large, slow moving rotors (rootes, volumex etc) can be both volumetric devices (where the compression occurs after the device) and internal compressors (where there is a volume change through the mean flow path) but both achieve the same end result. (although with very different efficiency's)
In either case the engine plenum, assuming the throttle is fully open, is then or course full of this same high density pressurised air. When the inlet valve opens, the piston descending on it's intake stroke "sucks" in this dense air.
A big mistake is to assume that a turbo "blows air through the engine" as it does not. In fact, quite the opposite. The restriction of the exhaust turbine results in the engine pressure ratio being LOWER than for an Normally Aspirated engine. It is the increase air charge density that provides the extra power potential.
In your case, you cannot reduce intake duct flow loses by adding more restrictions! You simply move the restrictions to a different location!
The question you are asking is really:
"Will i increase the efficiency of my turbocharger by enough to offset the losses i add with the extra compressor device"
And the answer is NO.
DVandrews said:
A friend runs a compound charged engine in his sprint/hillclimb Elise. The supercharger runs directly until around 2500 at which point it is switched via a gate to just run the turbocharger.
Dave
Is that the green one which resides in Abbingdon ?Dave
Can I present my question differentially and for the time being remove the engine from the picture.
Imagine I have a centrifugal compressor running at a fixed speed blowing air down a pipe which is open to atmosphere at both ends and I measure the flow. If I now feed the compressor with air at above atmosphere will the amount of air shifted increase or will it remain the same because the compressor moves a fixed amount of air for a given speed ? Compressor charts refer to pressure ratio which suggests it would increase.
A crude experiment with two cooling fans suggests the flow is increased provided the first fan moves more air than the second, if the two fans are equal there is no effect.
DVandrews said:
A friend runs a compound charged engine in his sprint/hillclimb Elise. The supercharger runs directly until around 2500 at which point it is switched via a gate to just run the turbocharger.
Dave
Guess ^^^^^ that's useful if you want to do the hill climb / sprint in completely the wrong gear then???Dave
I have followed this guy in Australia, with a twin charged Alfa (super chrger and turbo) - its a long thread, but intresting, he is now doing the same thing again but looking for 700HP....
http://www.alfabb.com/bb/forums/engine-conversions...
http://www.alfabb.com/bb/forums/engine-conversions...
In 2014, i have no idea why you would want to go to all the effort, complexity and added mass of a compound boosting solution, unless you have some very specific issues to solve by that route?
(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
Max_Torque said:
In 2014, i have no idea why you would want to go to all the effort, complexity and added mass of a compound boosting solution, unless you have some very specific issues to solve by that route?
(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
Tell that to VAG....(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
It doesn't matter how far technology has come on, physics hasn't changed and you won't get a 700bhp turbo to give much power below 4000rpm.
http://www.dyno-plot.co.uk/dyno/dynoplot/id%3D1043...
Is a dyno plot from a Twin charged K series 2.1 litre at the hubs.
Evoluzione said:
Max_Torque said:
In 2014, i have no idea why you would want to go to all the effort, complexity and added mass of a compound boosting solution, unless you have some very specific issues to solve by that route?
(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
Tell that to VAG....(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
You'll also note the carefully chosen phrase i used "unless you have some very specific issues to solve". In the 1.4 TFSi case, that issue was to make a low capacity fuel efficient petrol engine drive at low speed like a much larger capacity engine. However, in the "aftermarket" arena, it's simply better to either make better use of your gearing (to go fast, you need to have high BMEP @ high rpm) or just fit that larger engine. (the cost of adding an extra supercharger will outweigh any fuel economy savings by 10x or more).
Max_Torque said:
Evoluzione said:
Max_Torque said:
In 2014, i have no idea why you would want to go to all the effort, complexity and added mass of a compound boosting solution, unless you have some very specific issues to solve by that route?
(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
Tell that to VAG....(back in the 1980, when turbo's where much less efficient and poorly optimised, engines didn't have the benefit of variable everything and precise EMS control, and transmissions were slow and ponderous, then, perhaps, they had a role to play)
You'll also note the carefully chosen phrase i used "unless you have some very specific issues to solve". In the 1.4 TFSi case, that issue was to make a low capacity fuel efficient petrol engine drive at low speed like a much larger capacity engine. However, in the "aftermarket" arena, it's simply better to either make better use of your gearing (to go fast, you need to have high BMEP @ high rpm) or just fit that larger engine. (the cost of adding an extra supercharger will outweigh any fuel economy savings by 10x or more).
The newer tech is great, but not available to the aftermarket tuners, nor will be for a long time yet so that's why they are still using good old twincharging.
chuntington101 said:
Max, do you see VNT / VGT turbos becoming more popular in petrol engines? Garrett has a gt35 and gt40 based turbo that uses the technology (upgrade for the GM v8 turbo deisel engines I think).
To a small degree yes, but with modern "wide operating point" fixed geometry turbocharging and 8 (or more) speed rapid shifting transmissions, allied to significantly more electrification of all types of drivetrain, it's not really that important any more.Gassing Station | Engines & Drivetrain | Top of Page | What's New | My Stuff