Fuel usage at speeds and rpm's

[Comfortably Numb]

Sector 9 - I understand your frustration with centre lane hoggers. I have had them come up behind me when I am overtaking slow, lane 1 traffic at 70mph, and sit on my bumper, flashing their lights, rather than pull out into an empty lane 3, despite me giving plenty of indicated warning, and not pulling out if they are close, and lane 3 is busy. It's like they are playing " Let's see if i can do the whole journey in the middle lane"! Also, I find most of the time, I can't use lane 3 if I am only doing 70 + 7 for 10% speedo error; everyone is going a good deal faster than that!

[Supervinnie40]

My searches have lead me to a forum with several petrol heads. Although they are not very familiar with the Rover K series engine, they do realise that the theory behind is will not differ very much between engines.

The basic theory is that there is a linear relation between the rpm and the speed it will give you. All gears will begin at the zero-point, but will increase with larger steps for every increase in rpm because of the gear ratio. This linear relation does not follow the fuel usage, torque or bhp given by the engine.

It would be possible to calculate a very rough theoretical average fuel usage based on rpm, but this will not hold true to the real life version because of a few reasons:
- Engines are much less fuel efficient at low rpm's
- Engine are also much less efficient at maximum rpm's
- BHP increases with RPM, but lowers after its peak (the 1.8 peaks at 5500 rpm) which means that the energy from the combustion is more efficient at this point.
- Torque increases with rpm, but lowers after its peak (the 1.8 peaks at 4000 rpm) which would give the engine the
- Both torque and BHP don't increase/decrease in a linear motion as they are subjective to different variables that don't all function at the same rate at different rpm
- Even with the variables above, there will still be resistance from the tyres, the wind (which increases exponantially with speed) and accessoiries using electricity

Basically, there are 2 curves that grow at their own unique rate from 0 until they reach their own peak. After that, the line would decrease, making the engine benefit less from these forces. However, the perfect point does not have to be at the point between the peak for BHP or torque, as simply cruising might not require maximum torque or BHP.
While the 2 lines (torque and BHP) grow, the resistance grows with speed, but not at the same rate as torque and BHP grow.

In theory, the ideal line would be the intersection where both torque and BHP are fairly high, resistance would be relatively low and the distance travelled per hour would be reasonably high.
The guys at the other forum all agree that driving in 5th gear at the lowest revs will not be the most efficient speed. Not even close. The current guess is that somewhere around 70 to 100 km/h is the most efficient speed (on paper).

I'm going to continue the calculations and debat on the other forum, but I don't mind sharing some findings on here. Maybe other 1.8 owners are curious on how far we get.

[HarryM1BYT]

Originally Posted by Sector-9

I've found 55-60mph is probably best for economy in my CDT tourer but it's annoying driving at that speed as I'm overtaking lorries on the uphill motorway stretches then having them pass me on the downhill, only to repeat all over again at the next one. I'm sure it annoys them too having to keep pulling out to overtake the same car, so now I just drive at 50 so they only have to overtake me once.

I did try to do a whole journey last week at 70 on cruise control but it was impossible to maintain and my old road-rage demons came back after a few miles of trying to overtake lorries only to be boxed in by oblivious dimwits in the middle lane, so I invariably ended up flooring it and not doing 70 for much of the way anyway.

When towing on the motoway, that is the sort of speed I aim for. Except I tuck myself in behind an HGV which is able to maintain it's speed up the hills. I cannot recall the last time any driver came up behind me flashing their headlights, other than to flash me to pull out.

[Sector-9]

I think there are just too many variables to be able to calculate fuel consumption solely from engine capacity and speed. Just as one example, the electrical loads are never constant - even the battery's state of charge and temperature will affect the loading from the alternator. Invariably they measure fuel usage either through a flow meter (as in the old carburreted Sierras) or count fuel injection quantity per stroke (as rail pressure and injector flow rates are relatively fixed known values), and if you think about it, even the fuelling map for the ECM to decide how much to inject is only based on a limited number of sensors and relies on driver adjustment (the accelerator pedal) or cruise control to compensate for the unmeasured loads.

This is one of the things that make more modern engine management systems use less fuel and burn cleaner: they have a greater number of sensors measuring things like electrical loads, and actuators that can respond accordingly such as reducing alternator output when possible. Whether the benefits are worth the complexity and likelihood of faults occuring is a different matter...

[Supervinnie40]

Originally Posted by Sector-9

I think there are just too many variables to be able to calculate fuel consumption solely from engine capacity and speed. Just as one example, the electrical loads are never constant - even the battery's state of charge and temperature will affect the loading from the alternator.

That is true. And having the complete datasheet on all these tiny variables would make it a lot easier (even if that is nearly impossible and very inpracticle). But without these variables it should still be possible to have a calculated geuss with an acceptable error-margin.
I'm fairly confident I can eventually estimate the ideal cruising speed (under normal circumstances) within 10% margin. So it should be possible to tell the difference between 80 km/h and 90 km/h. It might not be perfect, and will only apply to normal circumstances (which is easy when you live in a flat country :P ), but it will still give a little bit more insight then not having the list at all.

And even if I'm absolutely wrong, I still enjoy trying to figure this out. I enjoy how complicated it gets and how to compensate for very difficult variables that are missing.

[Sector-9]

On an old Rover 820 I found best economy from the petrol engine was turning over at around 2000rpm. Not sure about the 1.8 75 but I'd say if you want to experiment then this would be a good starting point. Unless you just like maths in which case I'll be no help whatsoever to you...

[Comfortably Numb]

Originally Posted by HarryM1BYT

When towing on the motoway, that is the sort of speed I aim for. Except I tuck myself in behind an HGV which is able to maintain it's speed up the hills. I cannot recall the last time any driver came up behind me flashing their headlights, other than to flash me to pull out.

Had another one today (didn't flash his lights, though). I was in lane 2 of the M6 in Cumbria, doing over 70 and overtaking a long, well-spaced chain of vehicles in lane 1. A white Audi came up behind me and sat on my bumper for best part of a mile. Lane 3 was relatively empty for the whole time. Once I had a gap of 200yards or so, I pulled in, still doing over 70. The white Audi then rapidly accelerated to, I would guess 85 - 90, - still in the middle lane. What's the point?

[Comfortably Numb]

PS, at one point, I moved into lane 3 to overtake an overtaking truck in front of me, the white Audi stayed behind this truck until it had completed its manoevre, then came flying up behind me in lane 2 again. It's like for him, lane 3 didn't exist!

[neilll]

Hhhmm,
Started reading this thread at the beginning and thought what a load of twaddle (think no different now) move on!! Maybe time to sell your cars and get a eurobox (and join a different forum)if you want to talk so much rubbish!!
Neil

[Supervinnie40]

Originally Posted by neilll

Hhhmm,
Started reading this thread at the beginning and thought what a load of twaddle (think no different now) move on!! Maybe time to sell your cars and get a eurobox (and join a different forum)if you want to talk so much rubbish!!
Neil

Lol, thank you for that very usefull contribution to this thread . I whish more members would be like you.

Sector-9, It nevers hurts to add something to the list to test.