Request for Data from TheHolyCow

[R.I.D.E.]

If your requirement is to maintain a 20% throttle position and a 2000 RPM engine speed in any gear, the only way to do that would be to have exactly the grade necessary to maintain that tps and rpm.

Imagine you have the magic ability to create a hill with the perfect grade that would be very steep in 1st gear, and less steep as you went to a higher gear. You would adjust the grade percentage so that at your 20% tps and 2000 rpm your vehicle maintained it speed exactly.

Then you would have a consistent vacuum reading regardless of the gear chosen, because you would be using more power for horizontal motion in higher gears, but less power for vertical climbing. in lower gears the opposite would be true. The load and fuel consumption would be exactly the same regardless of the gear you were in.

In reality such a magic mountain does not exist, so at a specific tps and rpm one or the other would not be sustainable, if that makes any sense.

If you were at 20% and 2000 rpm in top gear and happened to be climbing a grade that coincidentally was just the perfect amount to allow you to maintain 2000 rpm and 20% throttle, your vacuum would be lowest under those circumstances and your BFSC would be highest.

Your load would be matched to your power, and would be the maximum available at 20% tps position.

Otherwise you would have to go to a higher rpm or a lower tps to maintain one or the other (tps or rpm)

Hope that makes some sense.

regards
gary

[monroe74]

Originally Posted by R.I.D.E.

If your requirement is to maintain a 20% throttle position and a 2000 RPM engine speed in any gear, the only way to do that would be to have exactly the grade necessary to maintain that tps and rpm.

Imagine you have the magic ability to create a hill with the perfect grade…

I understand you, and I agree with you, and I don't think there's any fundamental difference between what I've said and what you're saying.

Let me start by pointing out that there are other ways to look at it, other than a magic hill (although that's a perfectly reasonable way to look at it).

Instead of a magic hill, imagine that you're dragging a giant adjustable parachute, and the drag it creates can be made to vary. Or imagine that you're fighting a headwind, and the weather keeps changing. Or imagine that there is a change in the pavement surface, which creates greater drag. These are all the equivalent of your magic hill, and any of these ideas can be used to help understand what we're talking about.

You're basically saying that a given throttle setting, combined with a given engine speed, will not remain constant except under exactly the right conditions. Of course you're right. They will remain constant only if the forces are in equilibrium (the power coming out of the engine, as compared with the power required to maintain the vehicle at that certain speed).

But the basic statement that's been made is still true: fuel rate per unit of time will remain constant if throttle and rpm are held constant, regardless of gear. You're basically saying this: 'well, you won't be able to hold those things constant, unless you're climbing just the right grade.' That's true, but it doesn't negate the statement.

Another way to think about this is to let go of the idea of holding vehicle speed constant, and just think about fuel rate at a particular instant. In the absence of the magic hill, a given state (e.g. 20% throttle and 2000 rpm) will only last for an instant. But in that instant, the fuel rate is the same as it would be if you were passing through 20% throttle and 2000 rpm in any other gear.

If you were at 20% and 2000 rpm in top gear and happened to be climbing a grade that coincidentally was just the perfect amount to allow you to maintain 2000 rpm and 20% throttle, your vacuum would be lowest under those circumstances and your BFSC would be highest.

I think that may or may not be true, depending on what you mean by "highest." This moment might not represent the best BSFC your engine can produce. It just represents the best BSFC that is possible, assuming certain conditions: you want to climb this hill, and you want to do it at this exact speed. For example, it could easily be the case that you could enhance BSFC by choosing, say, 80% throttle and 3000 rpm (i.e., a 50% increase in vehicle speed).