| Subject: | Re: Adjustable rate fuel pressure regulator? anybody try one? |
| From: | David Bjorkman |
| Date: | Wed, 16 May 2012 10:45:22 -0500 |
<div style="FONT-FAMILY: Arial; COLOR: #000000; FONT-SIZE: 24px"><DIV><FONT size=2 face=Tahoma>Well, it is still early in the week, and you have to count Friday, but ya gotta love the posts David puts up. That guy knows many things about many things and much about our Vanagons (worming? on this list? I knew, but I live on the ocean and have some sailing experience). He is not shy about helping others out and he is not alone in that regard.</FONT></DIV><DIV><FONT size=2 face=Tahoma></FONT> </DIV><DIV><FONT size=2 face=Tahoma>This is a great list</FONT></DIV><DIV><FONT size=2 face=Tahoma></FONT> </DIV><DIV><FONT size=2 face=Tahoma>David B. (the other one)</FONT></DIV><DIV> </DIV><DIV> </DIV><DIV style="MARGIN: 5px 0px; BORDER-TOP: #bcbcbc 1px solid"></DIV><SPAN style="FONT-FAMILY: arial; COLOR: #000000; FONT-SIZE: 12px">On 05/16/12, <SPAN>Rocket J Squirrel<camping.elliott@GMAIL.COM></SPAN> wrote:</SPAN><DIV> </DIV><DIV style="FONT-FAMILY: arial; COLOR: #000000; FONT-SIZE: 12px">I nominate this "best read of the week."<BR><BR>--<BR>Jack "Rocket j Squirrel" Elliott<BR>Bend, Ore.<BR>1984 Westfalia. A poor but proud people.<BR>1971 "Ladybug"-brand utility trailer ca. 1972 from a defunct company in<BR>San Clemente, Calif., now repurposed as The Westrailia.<BR><BR>On 05/16/2012 02:37 AM, David Beierl wrote:<BR>> At 02:40 AM 5/16/2012, Scott Daniel - Turbovans wrote:<BR>> [If the ECU is running closed-loop (listening to the oxygen sensor in<BR>> the exhaust) it will dial back the pulse time until the mixture is<BR>> correct again. ]<BR>><BR>>> within a fairly narrow range I think.<BR>><BR>> No, it's quite a wide range.<BR>><BR>>> our era vans use narrow range 02 sensors, I believe.<BR>><BR>> This is true. However it doesn't affect the amount of adjustment the<BR>> ECU can apply, only the method by which it does it. And it means<BR>> that the ECU can only adjust to one specific mixture, the one at<BR>> which the fuel and air charges are balanced so that each one is<BR>> completely used up. That mixture is called the stoichiometric<BR>> mixture and is about 14.7 parts of air by weight (mass if you're<BR>> picky) to one of gasoline. Since that's the mixture that the<BR>> catalytic converter needs to work properly, the narrow-band sensor<BR>> can succeed. It is in between the best mixture for power (richer)<BR>> and the best one for economy (leaner).<BR>><BR>> The control method is called the "bang-bang" method of servo<BR>> control. With that method, the ECU is constantly changing the<BR>> mixture. It enriches the mixture until the O2 sensor "bangs" over<BR>> into a rich reading, and then leans it out until the sensor "bangs"<BR>> back into lean, then starts enriching again, rinse and repeat. The<BR>> ECU *always* thinks the mixture is wrong and is trying to correct in<BR>> whichever direction will make the sensor flip the other way.<BR>><BR>>> they run wonderfully without an oxygen sensor anyway, and the 02's<BR>>> limited influence is why our vanagons can run so well without a<BR>>> working 02 sensor.<BR>><BR>> Well...the engines run well without the sensor because they're<BR>> designed to, just like any engine. It's the catalytic converter that<BR>> requires this very precise mixture control, not the engine. I<BR>> haven't kept up with recent engine-control techniq and it may well be<BR>> that nowadays wide-band o2 sensors are really integrated into the<BR>> function of the engine - I don't know. But with engines of our era<BR>> the O2 sensor is strictly an afterthought to keep the CAT<BR>> happy. Aside from that it's a two-edged sword. It can compensate<BR>> for certain engine problems and make other ones worse. It turns an<BR>> exhaust leak from a noise problem into a too-rich mixture. It turns<BR>> a misfiring cylinder into a too-rich mixture. It turns its own<BR>> failure or a ground fault on its lead into a *really* too-rich<BR>> <cough, cough> mixture, so rich that the engine can have trouble<BR>> getting away from idle.<BR>><BR>>> 'normally' ..on our engines , fuel pressure is held within a fairly<BR>>> narrow range...roughly 32 to 38 psi ..something like that..<BR>><BR>> Actually it's held very closely at a constant pressure. It only<BR>> seems to be varying because you're measuring from a different point<BR>> than it cares about. Specifically it cares about engine vacuum,<BR>> except when you call it that it becomes impossible to talk about<BR>> it. So instead we talk about MAP or Manifold Absolute Pressure.<BR>><BR>> Engine vacuum says how much less pressure there is in the manifold<BR>> than in the air outside the car, and it's measured typically in<BR>> inches of mercury, because that's how people first measured it.<BR>><BR>> MAP says how much *more* pressure there is in the manifold than no<BR>> pressure at all (i.e. a perfect vacuum), and it's measured in<BR>> whatever unit is convenient, which is seldom inches of mercury. So<BR>> assuming that we're at sea level and measuring in psi, when the<BR>> engine is not running, the MAP is about 14.7 psi. If we hadn't<BR>> already specified that we were talking about absolute pressure, we'd<BR>> say 14.7 psia, for absolute; which is equal to 0 psig, for gauge. In<BR>> other words, psia is referenced to complete vacuum, and psig is<BR>> referenced to whatever environment is outside the gauge at the time<BR>> of measurement.<BR>><BR>> Back to injectors and the point of all this. Leave the O2 sensor out<BR>> of the system, this engine is running open-loop because it's in an<BR>> '84 1.9l Westy in Edzell Scotland in 1991 and they're still using<BR>> leaded gas (and it has a cracked 3-4 synchromesh hub but that's<BR>> another story). So no CAT and no sensor. The ECU has to do the job<BR>> with no hand-holding, which means it has to know how much fuel gets<BR>> injected into the cylinders with each injector pulse. The only thing<BR>> it can directly control is how long it holds them open, so it's<BR>> necessary that they flow at a constant rate no matter what else is<BR>> going on with the engine; and the thing that controls flow rate is<BR>> the difference in pressure between the two ends of the injector. So<BR>> we have to keep that difference constant within fairly small<BR>> tolerance. I'm guessing +/- 2% or better.<BR>><BR>> One end of said injector is in the fuel rail and the other end is in<BR>> the manifold, so if we're going to keep the pressure difference<BR>> across the injector constant, the fuel pressure is going to have to<BR>> follow the manifold pressure very closely; and that's what the fuel<BR>> pressure regulator does. It's got a spring-loaded flexible diaphragm<BR>> operating a blowoff valve that vents excess pressure (above 2.6 bar?)<BR>> into the return line going back to the fuel tank. It's always<BR>> venting because the fuel pump shoves a *lot* more fuel through there<BR>> than the engine can use. Circulates a full tank about once an hour<BR>> IIRC. On one side of the diaphragm is fuel pressure. But on the<BR>> other side, the reference side, instead of outside air the diaphragm<BR>> is connected into the manifold, and the regulator keeps *that*<BR>> difference the same. We've got our little pressure gauge hooked up<BR>> to the test tee and it's measuring the difference between the fuel<BR>> system and the local atmosphere; so every time something changes in<BR>> the engine that gauge jumps around. But if we built an airtight box<BR>> around the gauge and connected the box to the manifold, the gauge<BR>> should stay rock-steady at 2.6 bar any time the fuel pump is running<BR>> - engine on, engine off, full throttle under load, idle, you name<BR>> it. It's just inconvenient to do that and expensive to buy a gauge<BR>> that's already set up that way, so Bentley gives you a sort of<BR>> generally-probably set of ballpark readings. But if you *really*<BR>> want to know what the fuel pressure regulator is doing, that's how<BR>> you find out.<BR>><BR>> Yours,<BR>> David<BR></DIV></div>
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