Fuel efficient driving.

cyclonebri1

tomstickland wrote: »

I really ought to go to bed.

Reading my original post, I actually disagree with some of it now! I was trying too hard to be provocative.

I'm going to write a new version next week based on the comments and feedback.

Tom, you made an excellant post. (90 odd percent accurate from my point of view).

I wouldn't bother rewriting as there will always be opposing experience and views, there has to be;)

At least it has made others consider economy and the effect there driving has on it, good man, :T:T:T

Strider590

jeferey wrote: »

Many studies have shown that any speed above 60mph starts to seriously dent mpg due the greatly increased drag.

This graph is for a 1986 Volkswagen Golf GTI.
Interesting to note the best mpg was obtained in the low 40's mph but as previously stated it varies from car to car.

It certainly does vary from car to car, both because of gear ratio's AND aerodynamics...
Large motorway cruising cars like the Mondeo and Vectra will perform better at higher speeds, because they were designed for that use.
I find in my Vec at 45mph im between gears, meaning im either labouring the engine or over-revving the engine (both are bad for fuel economy). Whilst 70mph feels comfortable.

A small town car will struggle to achieve good MPG figures on motorway usage, but the decreased weight/size will make it more nimble around town (less weight for the engine to pull around during stop/start driving).

The 1986 Golf was about as aerodynamic as a house brick, so I would use a large pinch of salt with that graph

Mark_Hewitt

I've found through lots of experimentation in my Diesel Fiesta that what gives by far the best fuel consumption is finding a big lorry and sitting behind it at 56mph. MPG of 70 or so is achievable using this technique. You just need to make sure you don't follow too close or you won't be able to react when the lorry suddenly stops!

tomstickland

A point to add:
I agree that driving only for economy and annoying other road users is bad practice. The other afternoon I was being caught up by another vehicle and could see that they assumed that I was a dawdler. So I ramped up my speed a bit to stop annoying them.

I think I confused them when I carried almost all of my speed round some corners and left them significantly behind.

Often there's going to be a compromise between what would give the best mpg and what is safest or least annoying to other people.
Meanwhile...

I've got another bit that I should have written at the start. It doesn't add anything new but gives the starting point.

Here's a bsfc plot from Wikipedia.


This shows the fuel consumption per unit of power output for a range of engine rpm and load levels.

The x axis is rpm ( min to the power -1 means 1/min i.e. number of events per minute, aka rpm)
The y axis is mean effective pressure measure in units of bar (1 atmosphere is approximately 1 bar). This is the torque divided by the cubic capacity. Consider it to be torque or throttle angle.

This particular engine makes peak efficiency at around 2,200rpm and 15/18ths of peak torque. That's quite startling really - it makes peak efficiency at 83% of peak torque output at 2,200rpm.

When accelerating then the driver can choose rpm and load level. The rpm is what is reported by the rpm dial in the car and changing gear up will cause the rpm to drop. The load level is selected by the driver's choice of accelerator pedal position.

The most efficient acceleration will keep the operating point near the peak bsfc. For this engine that means operating equally both sides of 2,200rpm and keeping the load at around 80% of maximum. ie: Short bursts of quite hard acceleration with a central rpm of 2,200rpm.

When cruising then the driver can choose a road speed and a gear. The most efficiency combination is one that occurs as close to the peak efficiency island as possible.

For a particular gear than increasing the road speed will increase the rpm and the load. The load will increase with speed squared. So there's a slightly curved line on the bsfc plot, getting steeper as it moves to the right. If the driver moves up a gear than the rpm will fall off and the load will increase. Hence another line can be drawn on the bsfc plot for this next gear. It will be higher than the first line. This can continue through each higher gear to give a set of results for each rpm point.

The best combination of speed and gear will be that which passes as closely as possible to the bsfc peak.

In practice most people don't have the actual bsfc plot for their engine, so they have to make some basic assumptions. Sensible choices would be to keep the rpm at around 2,000rpm (1,500 to 2,500) and the load at least somewhere in the middle.

tomstickland

More maps here: http://ecomodder.com/wiki/index.php/Brake_Specific_Fuel_Consumption_%28BSFC%29_Maps#Volkswagen_2.0L_5_cyl_diesel

Note that peaks are generally at around 2,000rpm and quite high load levels.

tomstickland

I've updated the original post to add the extra bits and try to keep it organised.

tomstickland

I'm currently at 420 miles with the needle just grazing the 5/8th point. By the 3/4 point I expect 504 miles. On my last tank this showed 450 miles and I got 51 mpg. So I'm looking at 504/450 * 51
which is around 57mpg.

tomstickland

Saturday morning.
615 miles and 47 litres (10.4 gallons).

Making 59.4mpg.

Harry_Flashman

My last fill up :

306.6 miles - 9.55 gallons = 32.1mpg

Not great, but not too bad for an L200

cyclonebri1

Mark_Hewitt wrote: »

I've found through lots of experimentation in my Diesel Fiesta that what gives by far the best fuel consumption is finding a big lorry and sitting behind it at 56mph. MPG of 70 or so is achievable using this technique. You just need to make sure you don't follow too close or you won't be able to react when the lorry suddenly stops!

That's not actually the real problem.

It's the lorry that I can quarantee will be sat "behind" you, that creates the issue.

Lorry sandwich anyone???:eek::eek:

Still bugs me tho' that folks really don't get what happens to fuel economy when you brake.
Clarkson actually got something right once when he was on that economy run on some big Audi or other, :

"never, never ever brake"