| Re: [GMCnet] apor lock - lost the battle but not the war, or: Ethanol is for drinking [message #100297] |
Sat, 18 September 2010 19:59 |
Gary Casey
Messages: 448
Registered: September 2009
Karma:
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Senior Member |
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Yeah, but...(all of us kids used to drive Dad nuts saying that. He always said
"yabit? What's a yabit?"). When there is flow, there will always be a pressure
drop and the longer the line the more the drop. How much? Don't know, but it
doesn't take much to cause a problem. The pressure starts at atmospheric in the
tank and drops continuously during the travel to the engine pump. As the fuel
goes up and down it also changes in pressure - dropping as it goes to the top of
the tank and then rising as it goes down to the frame. Finally drops to its
lowest pressure at the inlet of the engine pump. Moving the pump closer to the
tank will make it much less likely to vaporize at the pump inlet as the fuel is
at a higher pressure as well as cooler. Oh, and in the case of the soda straw
it is indeed at a higher pressure at the bottom than the top - both because of
the flow and because of the height. Not much, but it doesn't take much. Maybe
I'll dig out my books and figure out how much.
Yes, the time the fuel is in the line is not great as long as the fuel is
flowing - but at idle the fuel is flowing very, very slowly. In fact, the flow
rate at idle is typically only about 2% of what it is at full power. So the
fuel takes 50 times longer getting to the engine when idling. I've not done any
measurements, but I'll bet it takes very little time at idle for the fuel to
warm up to close to the local ambient temperature, which is very hot. According
to my extremely precise calculations (:-) it takes about 10 seconds for fuel to
get from the tank to the engine when running at high power (I assumed 150 hp).
At idle it will then take about 500 seconds(okay, my calculations are off -
I'll give you 100 seconds). That's a long time, and I'll bet that the fuel in
vicinity of the engine will be really hot after even a 30-second idle period.
The Toronado, as well as many big-engined cars, was equipped with a return line
that bypassed a portion of the fuel back to the tank, specifically to keep the
fuel in the pump cool. I wonder why that wasn't included in the GMC: I assume
it was because it got complicated with two tanks and all.
Gary
I don't think that the length of the hose from the tank to the pump makes any
difference in the pressure. The pump has a certain suction pressure and that
will transmit along the whole length of the hose. Just like sucking on a
straw. The suction at the bottom will be the same at that at the top.
As to the time the fuel in in the hose that doesn't make much difference either
as the fuel is flowing. If the engine were off then the fuel would absorb some
heat from the hose but when the fuel is flowing through it the temperature will
equalize very quickly.
Emery Stora
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