shifting at the instructed speeds?

[sepp]

after falling onto a forum thread about someone asking the FE of a certain pickup, i was intrigued by someones answer. this person claimed to get 8.5 l/100km on his diesel pickup by driving at what i took for insane RPM's. he said he was just following the manual instructions about the ideal speed to shift to each gear, and that gave him his best FE.

so i jumped on my constructor manual and found the instructions for my 2.5 l turbodiesel nissan pickup. it mentions the numbers are for ideal fuel efficiency and power.
roughly:
1st to 2nd at 20 km/h
2nd to 3rd at 40 km/h
3rd to 4th at 60 km/h
4th to 5th at 70 km/h

i tried this for some time now and it means i need to shift when im between 2500 and 3000 rpm! this way i pick up the next gear at 2000 rpm.

this is basically how i normally drive on flat terrain:
1st to 2nd at 10 km/h
2nd to 3rd at 20 km/h
3rd to 4th at 30 km/h
4th to 5th at 40 km/h

my question is that could it be that i get better FE by following the constructor instructions? at those RPM you have a much higher efficiency. because im basically always driving in 5th and the pickup can perfectly take it, but it does react slower wich means it wastes a large part of the fuel when accelerating. so wich method causes you to waste the least fuel?

-driving always in highest possible gear, but being inefficient when accelerating?

-accelerating in the lowest gear possible and operating the engine in its optimal range, but using more fuel for the high rpm?

i found that i tend to accelerate slower than before when shifting at almost 3000 rpm, wich is good i suppose. its' because the engine has alot more torque it reacts instantly to you foot. wich i suppose means that the engine is more efficient. when accelerating in 5th at low rpm, the car can keep accelerating for like 5 seconds after i stopped pushing in the pedal deeper, wich means it's wasting fuel.

[dcb]

The few bmep maps of diesels I have seen put the peak power to fuel in the 2-3k rpm range with full torque(throttle). So even though it isn't fast acceleration, you are accelerating more efficiently getting the most power out of the fuel, and still not using as much fuel because the engine is turning slower and squirting less often (even though it is squirting a bit more per revolution).

Andyway, that's my guess.

[Bicycle Bob]

It isn't inefficient at low RPMs, just weak, because not much air is flowing through. Try to find some details of load, speed, and efficiency on a graph. As long as you are not blowing black smoke, a diesel is usually pretty good at low speed.

[sepp]

but when you press in the accelerator at a low rpm and high gear, the engine takes more time to react. doesn't that mean that it hasn't got the ability to burn all the fuel you are giving it? if you were in a lower gear, with the pedal at the same position the engine would be turning faster and be producing more HP with the same amount of fuel.
so my guess would be to accelerate in low gears and shift in the engine optimal ranges between 2500 and 3000 rpm (at least from 3 rd gear on, it can shift up to 3rd at under 1500 rpm no problem) and as soon as the cruise speed is reached, jump back to the highest gear the engine can take, but avoid accelerating?

i have been doing this on my current tank now, and im 3/4th down and i can't say that im noticing a positive or negative effect on my consumption. (though my dad has driven it alot on this tank and he does not hypermile at all, wich could imply that i have a possibly better FE than usual, since it should be lower in theory after my dad drove it so much.

[abcdpeterson]

Hi I am new here,

I looked into this same thing for Gas powered engines. I assume it is the same with diesel, but that is a guess

Everything I read states the lower rpm is your best fuel to power ratio. There is more power/torque to accelerate quickly at higher RPM’s. But… The fuel use goes up disproportionately fast.
That disproportionately high fuel burn rate more than eats up any saving you might be getting acceleration faster.

Here is a GREAT article on it.
Browser Warning

After reading up on that I have found my car’s sweet spot for fuel to power ratio looks to be shifting at around 2200 rpm’s. That drops my rpms in the next gear down to about 1850. I have a scangauge I will watch LOD during acceleration, I shoot to keep the engine at around 75 Load.

Any lower RPM’s and I lug the engine, not good for the engine and takes too long to accelerate.
Any High and I definitely accelerate faster but mileage goes down even faster.

[PaleMelanesian]

I run mine down to 1000 rpm, accelerating in 5th gear from as low as 25 mph. That's what works best for mileage. I'll run it up as high as 2000 sometimes before shifting.

[sepp]

ok thanks guys, i think i will get back to my previous driving style then.