Diesel efficiency -- load vs. RPM

[bryson]

Tire selection will depend mostly on price, life, load range, and RR (if I can find info on it). Tire size is pretty flexible. I would like to take up more of the wheel well to improve aerodynamics, aesthetics, and gearing, but I'm not super picky.

Back to the original question: Load vs. RPM on diesel engine efficiency? Turbo diesels in particular....

There are a few threads on gassavers.org that are interesting, but I was looking for some more insight.

Rpm! - Gas Savers - Fuel Efficiency Forum
Diesel ecomony? - Gas Savers - Fuel Efficiency Forum

[slowmover]

It might help if one is more specific as to load & rpm. Highway, or stop & go in a regional metro? Then (as always) the comparison of truck spec, climate, terrain and specfic vehicle use.

Diesels are less weight-sensitive than gassers (high compression and higher torque versus higher horsepower), true, but determining optimal running spec is about the above factors plus keeping highway travel speed at or below 60-mph (as horsepower demand rises very fast from that point).

The mechanical baseline on a heavy pickup is highly important (brake drag, alignment, steering wander, tire pressure, bearing pre-load, etc), and for a T/C engine, being certain of no CAC leaks, etc.

.

[Diesel_Dave]

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:
==========================
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.
==========================

[euromodder]

Originally Posted by bryson

None so far -- just been trying to stay off of those brakes and keep it under 2000 rpm and 60 mph. I do end up rolling about 65 on the way to work, though -- I'll sacrifice some fuel economy to keep up with traffic

But you're sacrificing where the speed (squared !) really starts to hurt your fuel economy ...

Sticking to 60 should give way better FE, and it'll be better still if you stick to 55.

Try to keep the rpm even lower - it shouldn't be a problem on a 7.3L turbodiesel

[euromodder]

Originally Posted by bryson

My main question is whether load or RPM has a greater effect.

RPM is the killer.
While it requires more fuel to fill the cilinders more often, higher rpm also lowers the relative engine load, meaning that you won't get your HP as efficiently.


This works as long as you don't get near the max. engine loading at that rpm.
Then it'll just pour on more and more diesel to try to maintain (engine) speed.

I notice that towing my boat doesn't seem to affect the mileage nearly as much as I expected.

That's because once at constant speed, your engine isn't being used efficiently.
It may be getting (relatively) good FE, but gets bad BSFC-numbers at the same time.
The good FE comes from the basic fact that the engine isn't producing many HP, and as a result, doesn't need much gas.

Hooking up the trailer loads up the engine a bit more, improving the BSFC a bit. You will need a few more horses, but you're getting each of them more efficiently at the same speed, due to the higher loading. Net result : you see little difference.

[kerosene]

I am no pro. I came here looking for answers but here is what I know:

On boats even well matched diesels fitted with right props often get decent efficiency on certain narrow range where most of the HP for given RPM is truly needed.
With variable pitch propeller (essentially a tranny equivalent for cars) one can drastically improve the fuel burn.

(rough explanation)
What one does is set rpm with throttle, then adjust the prop (essentially upshift) until the engine is near struggling to keep the rpm. I think you can get a pretty good adjustment just by ear but often pyrometer / exhaust temp gauge is used to gauge if the engine is working at full load for the given RPM.

So this does confirm the original posters findings - a diesel engine does waste HP at given RPM and the only way to maximise thsi would be to always load the engine. Naturally because of engine design certain rpms can be more efficient than others but based on solely that fact "high gear" would always be the best.

Now where is the balance of straining the engine if lugging with low rpms etc. I do not know. That is why I came here - to confirm if it indeed is correct to always try to drive at the highest possible gear (reasonable rpm).