Re: [DMCForum] Re: DeLorean MPG (Martin G)

[Martin Gutkowski <martin@xxxxxxxxxxxxxx>]

Spot on, Matt, but I think your numbers are a bit off,. but I don't know 
what they should be either

In a simple strait line graph - C moves the graph up and down, M changes 
the gradient. In real terms, C changes the offset of the flap vs the 
plunger, and M changes the amount of fuel the flows in proportion to the 
position of the plunger.

Woops - I think I've gaffed - the control pressure sets the actual 
deflection of the flap, not the primary pressure.

And it does state in the engine manual that the fuelling graph is 
as-near-to-strait-line-as-makes-no-odds. Well maybe not like that.

Martin

Matt Spittle wrote:

> Hey there Bill,
>
> If I may interject here, maybe I can try to clarify what Martin tried
> to say with the math example.  I don't claim to be a PRV expert, so if
> this analogy is flawed to begin with; I have no idea. 
>
> y = Mx + C
>
> y --> the total fuel used
> M --> proportion set by the primary pressure regulator
> x --> engine RPMs (we'll just say x goes from 75 to 600)
> C --> baseline fuel consumption set by the mixture screw
>
> Now if you play with the numbers a bit, you can see the trend...
>
> If we assume at idle, that C is 750, M is 10 and x is 75, then both C
> and the primary pressure regulator are contributing about 50% to the
> total fuel used.  (this is a *very* crude explanation).  This would
> support the idea that backing off the mixture screw would save you
> fuel.  It's also interesting to note here, that if your mixture
> adjustment is way off, you would really notice it here. 
>
> Assume then we're cruising at 2500 RPM (x is now 250).  Now C is only
> contributing to 23% of the total fuel used.  Again, if your mixture
> adjustment is way off here, you wouldn't notice it as much.  
>
> This would seem to support the experimental result that when your
> mixture screw is a little "off" you usually can still nurse the car to
> the nearest garage as long as you can keep it from dying (by keeping
> the RPMs a little higher).  When you do this, you're simply moving
> yourself further along the y = Mx + C graph to where the effect of C
> (the mixture screw) becomes less and less. 
>
> Now there is a bit of a problem with this example, and it's mostly due
> to the fact that I don't know the baseline relationships between the
> primary press. regulator and the mixture screw.  Maybe the mixture
> screw accounts for 99.9% of the total fuel used at idle?  The only way
> to really get a "feel" for how much it matters is by setting the screw
> to a point where the engine BARELY runs.  In mostly all cases, opening
> the throttle will make it run better, and that seems to be consistent
> with what I've been saying (that the effect of the mixture screw
> decreases as the RPMs increase). 
>
> Maybe that will help.
>
> Matt
> #1604

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