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Discussion Starter #1
I've just been on the phone to my mate, Harvey, who's in the same field I used to be in - machine control. He came up with something interesting from a conversation with one of his colleagues the other day. He isn't into turbos as he reckons they complicate a car too much - his favourite car ever was a 1986 Volvo 245 estate from which he'd removed the fuel injection and electronic ignition and retro-fitted a carb and conventional ignition, then added a totally mechanical LPG conversion - lasted him 16 years and about 300K miles. However, he obviously knows I'm interested in turbocharging and from conversations we've had, he understands the technology very well - like me, he's an engineer.

So here's the thing. Bear with me, as the turbocharging connection will become clear eventually.

The original conversation, apparently, derived from a discussion of electrically-driven air pumps and vacuum pumps, similar to those found in domestic vacuum cleaners. He and a colleague were discussing the effect when you block off either end of a vacuum cleaner by putting a hand over the end of a pipe. As a youngster, I had noticed myself that blocking the pipe of a vacuum cleaner makes the motor sound very distressed. Like Harvey, I assumed this was caused by increased load on the motor, due to the strain of maintaining the pressure or vacuum (depending no whether the inlet or outlet pipe was blocked). Harvey's colleague maintained that this wasn't the case, citing the fact that the sound indicates an increase in motor speed, while the characteristic response of an electric motor to an increased load is a decrease in speed. He then went on to explain that the reason for this is that when the flow is blocked, the air in the impeller becomes more-or-less static, simply circulating with the impeller.

Harvey, being a sceptical Northern engineer, had to test the theory for himself. Having measured the electrical load on a blower motor for himself, he can confirm that blocking the flow actually reduces the load on the motor.

As related to a turbocharger, this would suggest that blocking the output of the compressor (with a closed throttle plate, for example) might present a smaller load to the compressor than making it provide a constant (non-zero) flow against the same pressure.

We know that the action of closing the throttle plate induces a shock wave that is known to stall the compressor. Now, the function of the turbo bypass valve ("dump" valve) is to provide an escape route for this shock wave, keeping the turbine and compressor spinning so boost can build rapidly once the throttle plate is opened again. However, this action will also reduce the pressure in the induction system, due to dumping of air to atmospheric pressure for the whole time that the throttle plate is closed.

The above experiment with the electric blower motor suggests that a constant pressure against which to act does not load the compressor significantly. Therefore, it can be said that any arrangement that will remove the shock wave without reducing the pressure in the inlet will allow the most rapid restoration of pressure once the throttle plate re-opens, providing maximum responsiveness of the engine.

So, if my reasoning works so far, then the bypass valve is a good thing, but not optimal. Even better would be any device that would remove (or absorb) the shock wave caused by the closing of the throttle body, while still maintaining as much as possible of the static pressure already generated by the turbo.

I'll leave speculation on implementation for later in this thread (if it develops into a thread), but can anyone see any flaws in the reasoning so far?
 

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Discussion Starter #2
One thing I've thought of since is that the pressure in the induction system would tend to rise with throttle plate closed and the compressor still spinning due to inertia, until the maximum static pressure is reached. It may be necessary to limit the pressure to something safe, say base boost, for when the throttle is opened again. Does this invalidate the above theory? Since the static pressure won't be reached, there will be some flow and the turbo will be loaded, but perhaps maintaining the pressure in the induction system at something higher than atmosphere will still make for quick recovery when the throttle is re-opened.
 

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he can confirm that blocking the flow actually reduces the load on the motor.
 [/b]
So, what you're trying to say is - I don't have to empty the vacuum cleaner bag to preserve the life of the motor. Excellent.
 

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Bill,

I think what you've just described could be summed up as a dump valve with a post-outlet Dawes type device set for, say 0.4bar such that you're not wasting time building up pressure when you get back on the gas.

So, the theory would be that on lift off, the dump valve opens, the pressure goes through to the "Dawes", which then opens until the pressure drops sufficiently.

Whether it will work will depend, I guess, on how much inertia there is in the compressor and, how much input it can draw from the exhaust gasses whilst not under acceleration in order to maintain the pressure.

My worry would be that the Dawes type device would not be able to react quickly enough.

I'll think some more about this and come back later...

PS I'm not for a moment suggesting using a Dawes as it simply couldn't flow the required volume of air!! It's a name used as an illustration of the concept of a pressure release valve that folk may be familiar with.
 

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been thinking about this one......
if all compressor designs were the same, yes you are right, however ....
On vacumm cleaners have to be designed to "stall" in a benign way so that when they get blocked up they do not burn out the motor..
turbo compressors are not designed this way.. they are not designed to "stall" at all so I would guess that if the output was restricted then it would put pressure on the input shaft .. and bang goes your turbo

I have given the area of dump valve design some thought and both designs that exist today (piston and diaphragm ) suffer from the same problem of relatively slow response
If you could have a elctronically controlled pnuematic flap valve where the pressure is vented instantaeneously as the throttle is closed
you could also tie it as a fail safe boost pressure regulator when a fast rising pulse is detected .
difficulties...need a air compressor and some tricky electronics grafted into the ecu..
maybe someone somewhere has already thought of this ?
 

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If you look into the double barrel exhaust system of the latest BMW cars, you'll notice that one pipe appears to have some sort of valve. I assume (but don't know for sure) that BMW is using a similar sort of technology to get their engine to perform better at low revs?
I remember that on my Yamaha 2 stroke motorbike the entry and exist ports on the cylinder were electronically controlled by valves in order to make those two stroke engines more efficient.

The theory from Bill's friend seems quite plausible, and worth looking into further.
 

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I don't know much about BMWs (long may it continue) but Ferrari use a valve in the exhaust system to get around the European regulations. I believe the regs require a certain max sound level (sound for Ferraris, noise for BMWs) at half max engine speed. I believe that at 1 rpm or so higher than half max engine speed, the valve opens, and bypasses part of the silencing arrangements, for the benefit of anyone standing nearby at the time, and increasing maximum power.
 

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If you look into the double barrel exhaust system of the latest BMW cars  [/b]
I'm sure I once read that the valve on a BM opens on tick over to make the engine sound better
but closed at high revs for quiet motorway cruising.But I may have got this the wrong way round
 

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I dunno, I've watched them whilst sitting in traffic and the valve seems to be closed whilst idling, then as the revs picked up to pull away the valve opened.

Not certain though.
 

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The Lotus Carlton has a convoluted double valve flap thingummy arrangement betwwen both it's downpipes about half way down the exhaust too...

Anyway, getting back to the original point...

The thing about the vacuum cleaner and less load...

The rotational speed of the cleaner is governed by the load applied to it- if the load is reduced, speed increases. The "load" is basically comprised of friction losses plus the work required to pump the air. Remove the air load and the speed increases- slightly. But there's still a large amount of electrical energy required to overcome the basic friction losses plus there will also be some load associated with the air that is in the compressor chamber, even if it is not being pumped anywhere.

The point of the above argument is that a turbo is similarly going to require energy input to keep it spinning and maintain a static pressue- which it won't have if there's no energy coming from the exhaust gasses, so I can't see how it will work.

However I do seem to remember that there's a technique been used on WRC involving fuelling between gear changes that effectively burns post manifold, providing energy for the turbo to keep it spinning. The downside IIRC is longevity...
 

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Add an electric motor to drive the turbo when not under load from exhaust gasses? I know this would result in an extremely expensive to manufacture turbo.
 

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Discussion Starter #13
Originally posted by Mark E:
[qb]The point of the above argument is that a turbo is similarly going to require energy input to keep it spinning and maintain a static pressue- which it won't have if there's no energy coming from the exhaust gasses, so I can't see how it will work.[/qb][/b]
It would be worth calculating the stored energy in the turbo to see if might be enough to maintain the pressure between gear changes, and the frictional losses in your ball-bearing turbo shoudl be a lot less than in my TD04.
 

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I believe it's been done.[/b]
Hmm, I guess it's such an obvious answer that it was impossible for me to be the first person to suggest it.

Thinking more about forced-induction generally; if your going to drive the turbo by means other than exhaust gasses, i.e. mechanically, why not just revert to old-fashioned mechanical supercharging like the Jags use? The supercharger is always spinning and therefore there's no lag. Q: Is it because traditional mechanically driven superchargers can't provide the boost levels that a turbine-supercharger (turbo) can produce? I don't know so could someone please enlighten me.


If I remember correctly, the SVC engine was going to be supercharged rather than turbocharged. My guess is that the current turbocharging technology has been explored to the point where no further advancements could be made to improve systems without incurring much greater manufacturing costs. Perhaps Saab are already on the cusp of producing mechanically aspirated engines which are more potent than turbine aspirated engines?

But I'm no engineer, so I don't want to spoil this thread with my lame-brained pondering.
 

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normally aspirated will never be as efficient as forced induction engines and the turbo is the most efficient of the lot
however there may be another reason for using a supercharger and that is response ..
as we all know the dreaded lag is something that cannot be totally engineered out ...with a supercharger you can have low down instantaeneous response (at the loss of top end grunt ) the svc may compensate for this..
The old lancia group b rally cars used both turbo and super for the best of both worlds
 

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Discussion Starter #19
Well, having got off the phone that night to Harvey, after a few drinks while we chatted, then starting the post, then being interrupted a couple of times by more phone calls, during each of which I also had "a few drinks", I'm surprised anything intelligible resulted at all.

While the conversation was sparked by the vacuum cleaner thing, it may be that the analogy with a turbo compressor is not valid. Having used this sort of vacuum cleaner motor in industrial applications, I know how to control them electronically, but I must confess I've never opened one up to see whether it is a centrifugal device like a turbo compressor.

However, the good thing about this sort of conversation is that it stimulates the thought process. Thinking about the shortcomings of current turbo systems (mine, at least), there are things that, if they could be addressed, would probably improve performance.

<ul type="square">
[*]The dump valve often doesn't open quickly enough to completely avoid the stalling effect on the compressor
[*]The boost pressure is completely lost when the dump valve opens (i.e. between gear changes)
[/list]

A data-logger monitoring the boost pressure before the throttle plate could produce interesting data. So would one monitoring the manifold pressure, to see whether there is a lack of boost pressure for a significant time after gearchanges.

Only when the problem has been quantified could a new design be put together and tested against the original results.

Come to think of it, how big a difference does one of those expensive piston-type valves make? We could measure that too...
 

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Don't the rally cars dump fuel to keep the turbo spun up on the overrun????

Couldn't you do that and then you could join the Gassy afterburner crew.

Andrew
 
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