It's really hard to separate speed from load -- they go hand in hand. In went into a bit of this in a previous post:
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Okay, you probably already know some of this stuff, but I'll start off with the basics (because that's what I always do to myself when I'm trying to figure something out).
The first thing to consider is the required power output of the engine. This is determined by:
1) Total vehicle & payload weight
2) Road grade
3) Aero drag
4) Rolling resistance of the tires
5) Mechanical efficiency of the drive train
6) Desired acceleration
For our purposes we can consider the first 5 things as givens--so we have a certain vehicle with a certain weight on a certain road. When you're driving the only thing you really have control over is your desired acceleration. Once you decide that, you know the required horsepower.
Now, look at that BSFC map I reffered to earlier (VW 1.9L TDI BSFC map--most modern turbodiesels have the same basic contour shapes).
There are blue lines. Each of those lines represents a different horsepower level. Up and to the right is the highest power level. So those 6 above factors dictacte which of those lines you will be on.
Now, even though you now know which line your on, there are various conbinations of engine speed/torque that can generate that horsepower (anywhere along the line). This is because horsepower = speed (rpm) x torque (lb-ft) / 5252. So you can see that 1000 rpm/ 200 lb-ft gives the same horsepower as 2000 rpm/ 100 lb-ft (38 hp).
Now, gearing will determine where you are on the line, because gearing determines the engine speed. The speed of your axle is dictated by your vehicle speed (and tire size) & gearing determines your drive ratio (between the engine crank and the axle). Thus, gearing determines the engine speed. For example, if the total drive ratio is 2:1 then your engine speed = twice axle speed).
So, going back to the chart. Let's say you have a 6-speed transmission. In theory, you have 6 different places you can be on that line. Your highest gear will put you the furthest to the left, the lowest gear furthest to the right. In practice you don't have 6 choices because some of the gears would put you completely off the map. If you picked your lowest gear at 70 mph, you engine might have to turn 15,000 rpm to generate enough power (which, obviously, the engine can't actually do). If you picked your highest gear a 10 mph, your engine might have to produce 1500 lb-ft of torque (which it can't do either). So realistically, in any given scenario, you probably only have 2 or 3 choices.
So, look at the BSFC map again. If you're at any given power requirement (any blue line), would you rather be down and to the right or up and to the left? You can see that, for this type of map, in almost every case, a lower rpm will give you better BSFC. (Note: if you look at a BSFC map for most gas engines, you'll see that this isn't true nearly as much for them).
So...all that leads me to my rule of thumb: unless you're getting up close to the torque curve (close to not being able to produce the required power), keep your rpms as low as possible.
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