how a turbo hif works?
Turbo Mini Forums

Just wondered, when the trottle is opened and compressed/charged air comes through is the air after the throttle still a vacuum. Cause how I understood forced induction is that the air/fuel mixture is forced into the cylinders, so that would be a positive pressure, but if that is the case how does the fuel get drawn through? Doesn't that rely on vacuum to draw fuel through the jet ? Or is it still the case that compressed air/ fuel mixture is still being sucked/drawn into the engine?
Cheers in advance.

Pretty sure the carb sees boost as the turbo carbs need to be sealed differently.

My build thread..

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=542985

Thats what I dont understand, yes the carb sees boost, but hif44 carbs rely on vacuum to draw fuel through the jet.

Simples. The float bowl is pressureised around 3psi above the venturi. So fuel is drawn through owing to the pressure differance.

The difference in pressure is is created by the restrictor in the plenum. This reduces the pressure in the venturi but the float bowl is pressurised through a tube that is fed from an unrestriced part of the plenum.

The fuel pressure regulator increases fuel pressure to compensate for the float bowl being pressurised, otehrewise you'd run out of fuel in the carb.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

The bernoulli principle.

increase the velocity across the venturi profile, and reduce the local pressure at the jet (fuel inlet) creating entrainment.

increase velocity, increase fuel entrainment.

The fuel is not pushed out of the jet.

The positive pressure in the float bowl is to give the same delta p (pressure drop) over the inlet and suction.

Thats grest. Cheers all

I design venturis for multi phase streams for a living

The fuel is not under higher pressure than the upstream motive pressure. Half your comments I will agree with.

Have a look at this. Hopefully it will help
http://transvac.co.uk/howanejectorworks.php

Edited by wil_h on 8th Feb, 2016.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

The fuel will be higher than the plenum pressure pre restrictor by what ever you set it to at idle

The fuel pressure regulator is referenced from this point and will go up by the same amount.

Fuel pressure in the float bowl will not be the same as the pressure of the fuel in the feed line post regulator
The pressure in the float bowl is created by the restrictor only and to my knowledge and testing should be around atmospheric at idle and then increasing with boost
But importantly the fuel pressure in the float bowl will be imo the same as the boost pressure pre restrictor

This is all assuming the lean off pipe is blocked

If this is used the pressure in the float bowl does some funky stuff at part throttle

Yes i moved to the darkside

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upstream motive pressure = air inlet in this case. the main energy source in the venturi calculation.

The fuel cannot be at a higher pressure than the inlet or back flow will occur.

on balance, in a dynamic application, using multiphase, the pressures will have to be Inlet = high pressure, suction (fuel) = low pressure (at least lower than the inlet), discharge (mixture or air and fuel) medium pressure.

The technology is getting on a bit now and this could be designed a lot better in terms of efficiencies and mixing capabilities.

It would be a good project to create individual port venturis, which in theory should work. You could even have a second stage to add some water into the mix.

Back flow? The normal flow of fuel is from the float bowl to the carb throat. Back flow would occur of the inlet was a higher pressure than the float bowl; surely?

Remember, it will only be pressurised when boost is made and fuel is required. The amount of fuel is metered by the needle profile.

How does fuel get into the carb if it's not at a higher pressure than the inlet? I can assure you is the restrictor is removed, there is insufficient pressure difference and the fuelling goes lean.

We must be talking cross purposes here.

Fastest 998 mini in the world? 13.05 1/4 mile 106mph

www.twin-turbo.co.uk

So if the restrictor, or for that matter a pitot tube, does nothing to increase the float chamber pressure, why have them? You would simply connect the float chamber to the 'inlet' air pressure at the throat of the carb.

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=402578

http://www.turbominis.co.uk/forums/index.php?p=vt&tid=272999

A turbo carb works along with a plenum and turbo fuel pressure regulator to create a pressure differential, ensuring that the fuel is at a slightly higher pressure than boost pressure at all times. It's very very basic stuff. A pitot tube instead of a plenum mainly uses pressure waves instead of just a restricion. Used on a lot of turbocharged bikes. Don't try and overcomplicate it with fancy stuff, it is what it is, and it simply works, and works well!

The fuel pressure regulator serves one purpose - to ensure fuel is at a high enough pressure to be able to enter the float chamber. It has no influence over float chamber pressure, and does not help fuel to be drawn up the jet.

If it didn't reference pressure from the plenum, then it wouldn't be able to provide sufficient pressure to fill the float bowl with fuel, so its part of the system as a package.