Most Economical Motorway Speed?

On a car ecu i have read about a 'closed loop' state that an engine enters into and maintains 15-1 air fuel ratio once on constant throttle

Does an sv with efi use this?

[Sid Squid]

Peak torque = max efficiency, not max economy.

Do not confuse economy with efficiency. In essence max efficiency is getting the most back for whatever you put in, max economy is simply using the least amount.

56mph is simply the Imperial equivalent of 90kmh - sort of the NSL for metric users - and thus the speed used when calculating the out-of-town fuel consumption so that various manufacturers vehicles can be compared.

[ophic]

Originally Posted by Sid Squid

Do not confuse economy with efficiency. In essence max efficiency is getting the most back for whatever you put in, max economy is simply using the least amount.

That's a bad description, Sid, although you're right.

Max efficiency = max power output for least fuel used.
Max economy = furthest distance travelled for least fuel used.

The difference is that at peak efficiency, the engine is chucking out quite a lot of power - you'd be accelerating quite hard or maintaining a high top speed, where the effects of drag would prevent good economy.

I spose it could be argued that you should accelerate using best efficiency to the best economic speed.

[yorkie_chris]

Efficiency for heat engine = what you want / what you pay for = power out / fuel used

It's always going to be a trade off between getting there cheaply and getting there sometime before next christmas. I get pretty decent economy with 85-90ish cruising so I stick with that...

[Sid Squid]

Originally Posted by ophic

That's a bad description, Sid, although you're right.

I think calling it bad is rather harsh .

But it was intended to be a simplistic explanation.

Originally Posted by Ceri JC

In my experience, it's the acceleration and braking of stop-starting that burns fuel. I've had better mpg on runs with an average speed nudging 3 figures when the roads are clear, compared to ones where my speed has fluctuated between 35-80 (probably averaging 60mph) due to heavy traffic.

I similarly find that in practice keeping a constant speed is more important than the actual speed itself (within reason), years ago when I was a poor student trying to squeeze as much as possible out of every drop of fuel in my Metro 1.1 I found it my mpg was better at 60 than 50 or 55 simply because I found it easier to keep a constant speed. At the lower speed I found I was having to accelerate harder for hills and similar to keep my speed reasonably constant.

My current car seems to be much the same, it can sit at around 70-80mph at 2,000 revs and there's no noticeable increase in economy going down to 60 where it's not much above idling speed. The mpg takes a noticeable hit if I'm spending most of my time at 50-60mph and having to overtake frequently.

John

[embee]

A couple of things -

As Sid said, the old "56mph" economy for cars thing was purely a legislative speed for fuel economy testing (Euro used to be "urban", 56mph (=90kph), 75mph (=120kph), weighted 40/50/10% to give a combined fuel economy, USA used to be city/highway weighted 55/45), it is now done on a drive cycle consisting of "urban(x4) plus extra-urban" as here

but that's by-the-by, and doesn't apply to bikes anyway. It just happened to be that of the test conditions used, the constant 56mph was the best economy, but didn't pretend to equate to the best economy point for the vehicle.

As for economy generally, it depends primarily on where in the load/speed range the engine is working and what the demand is.
Brake Specific Fuel Consumption (BSFC) graphs show how the amount of fuel required to give each kW output varies, it is a function of the various efficiencies (volumetric, thermal, mechanical, chemical) involved.
This one is for a VW 90PS diesel, but shows the principles (I chose this map because it already had the constant power lines shown).


If the road load of the vehicle is, say, 20BHP (15kW, typical of around 60mph), it could run at nearly 4000rpm in a low gear and use 400g/kW.hr of fuel, or it could run at 1750rpm in a high gear and use 230g/kW.hr and get much better economy, 6kg/hr vs 3.5kg/hr, and if it was indeed 60mph that would be something like 36mpg vs 58mpg.

Road load is the total power required to overcome drag. Rolling resistance for a car is typically something like 1.5% of the weight, so a 1500kg car needs about 20-25kgf (200-250N) to roll it and is roughly constant, then the aerodynamic drag rises with the square of the speed,
F=(½.r.v2)Cd.A ,
where Cd is drag coefficient (car around 0.3) and A is frontal area (car around 2m2), r is density of air (1.2).
So at 60mph or around 25m/s a typical car road load is around
P(kW)= 25m/s x (250N + (0.5 x 1.2 x 25 x 25 x 0.3 x 2)
Road Load = 12kW which might not sound much but is about right. Roughly half and half rolling resistance and aerodynamic.
A bike has much worse drag coefficient, more like 1.0 I believe.

Basically unless you know the BSFC map and road load curve, you're guessing, but as the others say, lowest speed comfortable in top gear usually gives about the best economy.

max fuel economy? whats your top speed? and i beleive thats the best on motorways