engines, speed and economy....

[E38Ross]

a friend of mine and i were talking earlier about speed and economy, and how it obviously changed depending on what speed you go; as well as engine rpm, gearbox ratios, engine torque and power at said rpm etc etc.

he was of the belief that basically the slower you go, the more economical the car; which i don't think is quite right. if that were so, my car would be more economical at a cruise at 20mph than it is at 55-60mph, and it's not. i know it must have something to do with ideal engine speed etc because obviously, the faster you go there is more air resistance, rolling resistance etc so engines are more efficient at certain speeds.

anyone have any comments?

this question came up after i commented that the (not entirely accurate, i'm sure) consumption computer in my car had the economy at 80 the same as at 70mph (the car does between 2000 and 2100rpm at 70mph)

cheers

ross

[E38Ross]

Ozzie Osmond said:

It's not difficult and there most definitely are applicable general rules,

• The faster you go the more work you have to do pushing air out of the way (rises as the square) so the worse your fuel consumption.
• Once the engine is going fast enough to run efficiently and provided the gearing is suitable you will get best mpg at slowest speed.

Anyone who doubts this should get on a flat road, stick the car in 5th gear at 40-45 mph and watch the fuel consumption.

in auto mode my car won't go into 5th unless i'm doing at least 50mph IIRC

but agree with what you're saying.

but...why is it more efficient at 2k rpm in 5th, and not 2k rpm in 3rd?

i've noticed the economy in my car between 70 and 80 really doesn't seem to change a great deal, in 5th 85mph works out at 2500rpm - redline is 6500rpm. seems to be very happy with running at 85 and economy isn't too bad, still over 30mpg no problems. does 27mpg at 100mph on cruise

[E38Ross]

OldSkoolRS said:

cheddar said:

Back in the real world I average 40.

From a 123d? I'd have thought it would be higher than that. I've been getting a fraction under 50mpg out of my 320d touring, though it's mostly journeys of at least 50 miles minimum. I do cruise at approx the proposed speed limit so not crawling along.

is that a late E92 or a very early F30?

[E38Ross]

McSam said:

es - I touched on this with mentioning the complications for working out an ideal cruise speed. Increase speed by x%, gaining efficiency of y%, but at cost of drag of z% etc. and that's the important bit.

I was using the BSFC stuff mostly to say where the ideal sort of point for most engines is and why it's there, but the really interesting one is what it implies about the most efficient way to accelerate or deal with hills and things. If you're coming up a steep incline in 5th and at 2000rpm and are having to give it quite a lot of boot, you sort of instinctively think that's inefficient and you should change down to get a lesser throttle position. In fact, that makes it much worse, and being almost wide open in the higher gear at 2000rpm would be better!

that's very interesting and useful to know! with my car, when it's on cruise control i sometimes leave it in auto, if it has to climb a steep hill it will often shift into 4th gear to maintain the speed, sometimes i leave it in manual mode and bung it in 5th and leave the cruise on to stop it shifting down, for the sake of a couple of mph it makes for a slightly more relaxed drive. it rarely happens (only on pretty reasonably steep hills on the motorway to be honest) but now it would appear leaving it in 5th on manual would be the more efficient method. as said, 70mph comes in at around 2050rpm so it's geared very tall.

thanks!!