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Location of carburettor

 
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jjuutinen



Joined: 13 Jul 2003
Posts: 180

PostPosted: Thu Aug 12, 2004 09:55    Post subject: Location of carburettor Reply with quote

I was just rereading Grahamīs R-2800 book and in the VDT section he mentions that use of pure turbocharging (i.e. no mech stage) would have required direct injection. I assume this is to mean that carburettor does not work if placed on the pressure side of the supercharger. However, Russian/Soviet engines like Mikulin AM and Klimov VK (Klimov had 6 diaphragm carbs) series had their carburettors on pressure, not suction side. Any idea on pros and cons of such location?
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szielinski



Joined: 13 Jul 2003
Posts: 96
Location: Canberra, Australia

PostPosted: Thu Aug 12, 2004 23:34    Post subject: Reply with quote

Blow-through carbs are quite possible with supercharging, mechanical or otherwise, and this has been done with many automotive applications. It's just that it IS harder, as any operations/functions based on pressure have to be referenced properly. Eg if you had a Holley carb on your blow-through turbo set-up, you would need to make sure the "power-valve" had the atmospheric side of the valve referenced to the boost pressure, not atmospheric pressure - else it would probably never see a low enough pressure difference to open. This goes for every single possible leak point. Would you like fuel leaking through the throttle blade bushes in a plane you're flying in ? Carbs set-up in this way often seem to have large amounts of external plumbing to prevent such pressure leakage.
Now, compare the cost of modifying a large expensive aero carb to 'blow-through' operation compared to having a DI system that 'does it all' - with mass airflow etc.
Add to this the complexity of a carburettor system that has to cope with varying start-up mixtures and turbo-lag at low rpm and the DI system probably would be heavier - but simpler to implement.
Also, does the chapter specifically mention the 2800 with respect to VDT, or does it potentially refer to a larger engine such as 4360 that may have greater mixture problems without an axial mechanical supercharger ?
The bottom line ?
Make everything as simple as possible - but not simpler. Just like Einstein said.
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gwhite



Joined: 13 Jul 2003
Posts: 58

PostPosted: Wed Aug 18, 2004 15:07    Post subject: Reply with quote

The reason direct fuel injection would be required for a purely turbo'd appliciton is for mixture distribution.
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