supercharging and fuel consumption
Discussion
When you're on the power, yes of course, but on a steady motorway cruise i would have thought not.
The TPC kit, which is available from 9m (Colin), uses a bypass valve so that the supercharger is not "boosting" when you are on light throttle openings.
I would expect the fuel consumption at a contstant 75mph then to be near-as-dammit the same as the n/a equivalent vehicle.
A bit like a turbo'd car cruising along off boost.
VS
The TPC kit, which is available from 9m (Colin), uses a bypass valve so that the supercharger is not "boosting" when you are on light throttle openings.
I would expect the fuel consumption at a contstant 75mph then to be near-as-dammit the same as the n/a equivalent vehicle.
A bit like a turbo'd car cruising along off boost.
VS
verysideways said:
When you're on the power, yes of course, but on a steady motorway cruise i would have thought not.
The TPC kit, which is available from 9m (Colin), uses a bypass valve so that the supercharger is not "boosting" when you are on light throttle openings.
I would expect the fuel consumption at a contstant 75mph then to be near-as-dammit the same as the n/a equivalent vehicle.
A bit like a turbo'd car cruising along off boost.
VS
Eh? Surely that defeats the whole advantage of a SC over a TC....???
No, cos when you load the s/c the bypass valve closes - if you are on the throttle from 1k rpm in 5th you will feel boost all the way through.
With my 993tt, if i floored it at 1k rpm i would accelerate very slowly to about 2.5k rpm and then the boost would build very quickly and by 3k rpm i'd be getting the full 0.8 bar.
With my 993tt, if i floored it at 1k rpm i would accelerate very slowly to about 2.5k rpm and then the boost would build very quickly and by 3k rpm i'd be getting the full 0.8 bar.
I've got a supercharged 993. Funnily enough fuel consumption has gone *down* since I had the kit fitted.
I have two theories for this result, though I'm no engineer:
1) Much improved low end torque means I don't need to rev it as hard and as often;
2) Optimum cylinder filling across the entire rev range increases the efficiency of the engine.
When on boost, more fuel is injected along with the extra air - the 9m kit uses a single additional injector to supply extra fuel. In this case, more fuel will be used, but you will be going faster!
As to steady-state cruising, etc. I'd expect at least some increase in consumption, purely because of the drag created by the supercharger. However across an entire journey, I use less fuel.
I have two theories for this result, though I'm no engineer:
1) Much improved low end torque means I don't need to rev it as hard and as often;
2) Optimum cylinder filling across the entire rev range increases the efficiency of the engine.
When on boost, more fuel is injected along with the extra air - the 9m kit uses a single additional injector to supply extra fuel. In this case, more fuel will be used, but you will be going faster!
As to steady-state cruising, etc. I'd expect at least some increase in consumption, purely because of the drag created by the supercharger. However across an entire journey, I use less fuel.
OEM superchargers use bypass valves for part load work. Jaguars, for instance do.
The Bypass only works effectively on Roots blowers.
This is because a Roots blower doesn't do any internal compressive work but supplies a volume of air.
If a vastly more efficient Screw Compressor type SC is used (such as a Lysholm compressor) then a bypass has less of an effect on fuel economy. A Lysholm compressor consists of "male and female" rotors and these DO do internal compressive work. The advantage is that a Lyholm compressor is MUCH more efficient. I've heard them on dynos and they're also much quieter too.
The other source of poor fuel economy on a SC engine is the fact that the compression ratio has been lowered.
Mercedes Benz in the past have played with Clutching- the-supercharger-out on part load operation- however for an OEM that cares about driveability- this isn't practical to calibrate at anything other than perhaps idle operation. The associated "shunt" associated with it coming in would probably be deemed unacceptable to most customers.
At Full load/Wide open throttle-(as already touched upon) modern Supercharged cars can munch the fuel - often due to over fueling. Overfueling may be neccessary to keep the catalysts cool- and usually alot of overfueling is required at WOT on boosted engines.
>> Edited by Marquis_Rex on Friday 19th August 17:16
The Bypass only works effectively on Roots blowers.
This is because a Roots blower doesn't do any internal compressive work but supplies a volume of air.
If a vastly more efficient Screw Compressor type SC is used (such as a Lysholm compressor) then a bypass has less of an effect on fuel economy. A Lysholm compressor consists of "male and female" rotors and these DO do internal compressive work. The advantage is that a Lyholm compressor is MUCH more efficient. I've heard them on dynos and they're also much quieter too.
The other source of poor fuel economy on a SC engine is the fact that the compression ratio has been lowered.
Mercedes Benz in the past have played with Clutching- the-supercharger-out on part load operation- however for an OEM that cares about driveability- this isn't practical to calibrate at anything other than perhaps idle operation. The associated "shunt" associated with it coming in would probably be deemed unacceptable to most customers.
At Full load/Wide open throttle-(as already touched upon) modern Supercharged cars can munch the fuel - often due to over fueling. Overfueling may be neccessary to keep the catalysts cool- and usually alot of overfueling is required at WOT on boosted engines.
>> Edited by Marquis_Rex on Friday 19th August 17:16
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