The Triumph Dolomite Club - Discussion Forum

I appreciate that a supercharger draws power from the engine to drive it, quite a lot of power! This is the argument for the turbocharger, the power it uses is effectively "free" since it was going to be wasted anyway. But supposing you were to chop a turbocharger in half so that only the induction side remained, then drive it with an electric motor, would that really need a 10HP motor? Somehow I doubt it, my workshop compressor only has a 3HP motor - and it's MASSIVE and very inefficient, being based on the reciprocating engine principal, I once had a fan compressor (that once drove dentists drills) this had less than a .5hp motor, built up more pressure, more quickly, used less electricity doing it and was almost totally silent. I wish I still had it today! Surely there is drag in a turbo shaft and fan, but not THAT much and it is already being helped by manifold depression. I think the reason it has not been done so far is more likely to be control, since this would have to be adaptive rather than linear, ie not only from amount of throttle opening , but also speed of that opening too. I don't know enough about modern computer control to know if this is possible yet - but I have one or two ideas over how it could be done!

Steve

I understand some of the e-blowers that actually work use a DC-DC converter, and charge a high-voltage supercapacitor on the overrun, rather than draw off the battery.

But I can't believe it needs owt like 15 or 25 hp (11 or 19 kW) to move 0.1 cubic meters of air per sec, against 50 k Pascals of overpressure. So if a mechanically driven one takes that, it has to be due to inefficiencies. I think I remember we worked it out for someone's car in R&D at work a long time ago, and got something like a kilowatt. But I can't remember the car, its capacity, the overpressure, or the impeller efficiency we used. So I'd have to have a think about how to work it out again. But even that's 83 amps at 12 volts.

Graham

Need to double check, but 0.1 m cubed per sec - 6 m cubed per min (2 ltrs every other turn at 6000 rpm) - against 50k Pacals needs 5kW = 6.7 hp. Then there's the efficirncy of the fan and motor to add in. More than I thought, so I'll do it again a few times.

Graham

Was slightly wrong, using 1kg per m^3 of air, it should have been 1.2 kg/m^3, which makes it nearly 6 kW (500 A at 12 v) to push 6000 rpm's worth of air (0.1 m^3/s) at 50 kP (just under 0.5 atmospheres, 7.5 psi), and so about 8 hp.

You can divide that by whatever fraction of 7.5 psi you want, and by what ever fraction of 6000 rpm you want to boost to, so 4 hp at 3000 rpm and 7.5 psi, 5 1/3 hp at 6000 rpm and 5 psi, and 2 2/3 hp at 3000 rpm and 5 psi, etc.

Still needs the losses from the electric motor and the impeller to factor in though. Should be able to get some estimates for those off the Web. I'd expect a motor to be rather good - 80 - 90 percent. But I'm not sure about a fan. A very quick look suggests 25 percent might be a highish figure, but the context will matter so much, it's no better than a guess. But, if right the 15 to 25 hp is looking suprisingly (to me at least) accurate.

Graham

I think the 15-25Hp Jonners is referring to as supercharger losses are bhp losses rather than horsepower losses which are, I think, not the same thing at all. then there is the 1bar atmospheric pressure (14.7psi?) always present, aided by manifold vacuum. Sensible boost pressure for normal road work is about .7 bar (1.7 bar total) so that is a minimum to aim for.
Sorry to be ignorant, but what is a kilopascal and what does it measure?

Kilopascal is a pressure measurement and 10kpa is 14.5 psi (approx)
Tony

tony g wrote:Kilopascal is a pressure measurement and 10kpa is 14.5 psi (approx)
Tony

No, 100 kPa is 1 bar, is 14.7 psi, is 760 mmHg, is 760 Torr, is 1 atmosphere, as far as mere mortals are concerned. I'm sure some theorists somewhere would like to take task with this, but as far as we're concerned, it's all close enough.

Yes you are correct I missed a 0

Tony

GTS290N wrote:

tony g wrote:Kilopascal is a pressure measurement and 10kpa is 14.5 psi (approx)
Tony

No, 100 kPa is 1 bar, is 14.7 psi, is 760 mmHg, is 760 Torr, is 1 atmosphere, as far as mere mortals are concerned. I'm sure some theorists somewhere would like to take task with this, but as far as we're concerned, it's all close enough.

Why so many different descriptives for the same thing? I can understand not wanting to use Imperial measures, no matter what you measure with Imperial, it always comes out to some peculiar and inconvenient fraction - but the rest -

By the way, even an ignorant git like me knows that "atmospheric pressure" is an arbitrary figure placed on something which actually varies according to altitude and what the weather is like.

It seems to me (and I HAVE been wrong before) that what is needed here is not so much pressure but air SPEED. In a NA engine this is produced by atmospheric pressure filling a vacuum made by the descending piston. The aim of forced induction is to get more air into the cylinder in the restricted amount of time when the inlet valve is open which simply means speeding it up a bit. And I still can't see how helping air go a bit faster, when it's going that way anyway, can be such a power hog, after all, noticeable gains can be made, just using a ram trumpet! That is why the Carledo has its intake facing forward and just behind the grille - every little helps!

Steve

GTS290N wrote:No, 100 kPa is 1 bar, is 14.7 psi, is 760 mmHg, is 760 Torr, is 1 atmosphere, as far as mere mortals are concerned. I'm sure some theorists somewhere would like to take task with this, but as far as we're concerned, it's all close enough.

GrahamFountain wrote:50 kP [sic] (just under 0.5 atmospheres, 7.5 psi)

It's exact on some dull days.

There's even an SI unit of beauty, the millihelen: that amount of beauty needed to launch excatly one ship.

Graham

Carledo wrote:By the way, even an ignorant git like me knows that "atmospheric pressure" is an arbitrary figure placed on something which actually varies according to altitude and what the weather is like.

1 atmosphere is meant to mean the "standard" pressure in STP. In propper units, its defined as 101.325 kPa. But 100 is always near enough for engineering, particularly in relation to ground vehicles.

But you're right, it's all distracting from the point. You wouldn't use an electic fan to continualy boost air pressure right accross the rev range.

You might use one just for a bit at low rpm, and then only if you have the right kind of electical storage, and can charge it at the right time. But when is that going to be cheaper than a mechanical solution?

Graham

Carledo wrote:no matter what you measure with Imperial, it always comes out to some peculiar and inconvenient fraction

Actually, I still have a soft spot for the furlong, hundredweight, fortnight system of units that I learned about at school in the 1950s. They give Science such a lovely baroque feeling.

Graham

GrahamFountain wrote:

Carledo wrote:
You wouldn't use an electric fan to continually boost air pressure right across the rev range.

You might use one just for a bit at low rpm, and then only if you have the right kind of electrical storage, and can charge it at the right time. But when is that going to be cheaper than a mechanical solution?

Graham

This is why a turbocharger is so successful - despite it's drawbacks! And I suspect that a turbo is not as "free" as everyone seems to think, but still much more efficient in absolute terms than a mechanical supercharger, which is a large and bulky lump of metal with heavy frictional losses.

Well I guess if the big brains say it won't work then I guess I have to accept it! But i'm tempted to ape Clarkson and co in saying "how hard can it be?"

Steve

GrahamFountain wrote:

Carledo wrote:no matter what you measure with Imperial, it always comes out to some peculiar and inconvenient fraction

Actually, I still have a soft spot for the furlong, hundredweight, fortnight system of units that I learned about at school in the 1950s. They give Science such a lovely baroque feeling.

Graham

Well yes, they are the same measures I grew up with too, ounces, pounds and stones, a ton being 2240 lbs, feet and inches. Lovely!
But in mathematical and engineering terms, a massive PITA!
Privately i'm surprised we don't have metric time yet, with 100 seconds to the minute and 100 minutes to the hour!

Steve

And yes I did get the "face that launched a thousand ships" reference, just not entirely sure it's a joke!

Imperial measurements are intuitive and natural. They are derived from experience. Metric units are supposedly
scientific, but they are more about luck than anything else in their origin.

They just got lucky that 1cc of water weighs a gram for example...

Imperial units just work in real life. We all instinctively know how far a mile is.
A mile is derived from mille passum - a thousand paces of a roman marching soldier. Useful that.
We all have a foot of our own and an inch as well.

Also in cooking Imperial units work. 1 egg 4oz of flour half a pint of milk - it just works.
A pint is the right amount to drink. A litre of gassey euro beer is enough to give anyone the hump.
You get 4 decent apples to a pound or 6 small ones. How many apples is a kg? I have no idea.
1/2 lb of carrots feeds a familly of four for sunday lunch. Brilliant.

A 10 stone bloke is a weed and a 15 stoner is a big bloke or a fat git. What on earth is that in kg? It means nothing.
There is the odd fortunate convergence like a ton is very close to a tonne. Funny that 100 hp is about what you need to
give acceptable ooomph in a 1 ton car.

So there you are. Imperial works - metric is just unfathomable cods. Its what made us great.

The only difference between bhp and hp is that the bhp is supposed to be measured under load - against a brake.

25 bhp was the figure oft quoted for the mechanical losses in the Jag XJ-R supercharger, so is clearly not far wrong.

I personally think metrication alone is sufficient justification for leaving the EU and is why all our kids now cant do mental arithmetic like we all could cos we had to think in x12 and x14 rather than just 10's which any twit can do.

How far could you throw an education or health secretary? About 1 meter...

Jonners