The effects of fuel prices on driving styles

[Dand]

After posting on another topic its got me thinking about how relaxing my drive was the other day. I had to travel to the family business which is roughly 30 miles away from home. I mostly use the A55 which normally is a giant ball of aggression due to people driving to fast, to slow, or in the wrong lane but the other day it was totally different. Everyone was sat in lane 1 traveling at 60-65 leaving the overtaking lane clear to overtake if you wish to. No one was flashing or tailing and the lorries werent playing silly bug*ers either. It was also nice to leave the roundabout without having some silly chav in a Civic tryin to get some kind of youtube style rolling pull run out of me. IMO the only thing i can put this down to as there were no police about to calm things is everyone trying to get the best economy out of there car.

Has anyone had a similar experience?

 

[wemorgan]

I remember 10 years ago during the big fuel protests that many cars were doing 40mph on dual carriageways.

I'm not sure I do enough of that sort driving now to know if the same is happening.

 

[davidjpowell]

I remember that as well. Motorways were dead quiet, and at 60 MPH (I was lucky in that I had just filled up my relatively frugal pug 406 at the start) I was one of the fastest cars on the road...

Not sure prices affects my driving, but do affect what I drive.

 

[KillerHERTZ]

So, overall what is meant to be the best/most eco speed to travel at? is it still 55Mph?

 

[renault12ts]

56, I think.

 

[wemorgan]

I think it will depends from car to car. Some older technology automatics only lock-up in 5th, new autos can lock-up in lower gears, so I've read. Manuals naturally don't have this issue.

In 5th at 2000rpm my car does 55mph. Peak torque also begins at 2000rpm. So 55mph could well be the most efficient.

 

[renault12ts]

The pity is, it doesn't tell you in the car's manual when or if lock up occurs, so you don't know what's the best speed.

 

[Bellow]

Motor magazine throughout the 70s used to include a graph showing mpg vs mph in their road tests.

Mainly aero dependent, so the slope of the line is going to vary mostly on the overall drag, ie Cd x A. Obviously very different for a car compared to a bulky 4WD.

Surely TC lock up is obvious from the revcounter.... Or by ear..

 

[st4]

The pity is, it doesn't tell you in the car's manual when or if lock up occurs, so you don't know what's the best speed.

You can tell, lift you feet from the pedals, does the RPM drop disproportionately to the drop in speed.

For example, in my E220cdi if you were on the go pedal at 65mph the rpm was 2000 if you lifted it would instaneously drop 100rpm. It was not fully locked up.

The most efficient speed for a car is with very little exception is the LOWEST speed that can be attained, in the HIGHEST gear. In a 7g E320cdi, thats 55mph. In my E220cdi, it was 45mph.

 

[Top Cat]

You can tell, lift you feet from the pedals, does the RPM drop disproportionately to the drop in speed.

For example, in my E220cdi if you were on the go pedal at 65mph the rpm was 2000 if you lifted it would instaneously drop 100rpm. It was not fully locked up.

The most efficient speed for a car is with very little exception is the LOWEST speed that can be attained, in the HIGHEST gear. In a 7g E320cdi, thats 55mph. In my E220cdi, it was 45mph.

Thanks, you just explained that very well and saved me writing it.

I have an 8 mile dual carriageway run to work, and over recent weeks I've noticed more and more cars sat in the inside lane doing approximately 60MPH, myself being one of them, and I'm talking about some big exec models as well as smaller runabouts. I guess a lot of VW & Audi TDI owners are now finding that they need try harder to get their MPG up to the oft quoted 60+MPG.

 

[Mercington]

The most efficient speed for a car is with very little exception is the LOWEST speed that can be attained, in the HIGHEST gear. In a 7g E320cdi, thats 55mph. In my E220cdi, it was 45mph.

Not true I'm afraid, (although I'm sure thats what the powers that be would prefer you to think).

Engines are running most efficiently where they produce maximum torque, so obviously different for each vehicle. My Monaro was much more economical at 90 than 70, the CLK is similar, 85 seems to be about best.

People may believe they're saving gas by driving slower, but the driving standards are so poor these days they would do much better by using a bit of forward vision, and not stomping on the brakes every 40 seconds

Random braking seems to be the latest craze, gas, brake, more gas, brake. Tw*ts...

And I haven't started on the fact that the additional ethanol they're trying to add will give you less to the gallon! More money for the coffers though...

 

[Bellow]

Engines are running most efficiently where they produce maximum torque,

Except that would coincide with the throttle being wide open and produce maximum acceleration - hardly economical cruising!

What is needed is the highest gear that still provides enough torque at the wheels, and at that reduced engine speed the throttle opening will effectively be very close to full throttle for that engine speed, avoiding unnecessary throttling losses.

 

[Mercington]

Except that would coincide with the throttle being wide open and produce maximum acceleration - hardly economical cruising!
QUOTE]

Confused.com..

I think maybe you're confusing maximum torque with maximum revs?

For example, an engine that revs to 7500 may produce max torque at 5000, so keeping it at 5000 is hardly full throttle! And you can get there as slowly as you like

 

[renault12ts]

Turbo diesels produce max torque over a range of revs. Or have I got that wrong (often am you know ).

 

[Bellow]

I think maybe you're confusing maximum torque with maximum revs?

For example, an engine that revs to 7500 may produce max torque at 5000, so keeping it at 5000 is hardly full throttle! And you can get there as slowly as you like

No confusion at my end.
The initial statement referred to maximum torque - which obviously requires full throttle.
If the torque peaks at 5000 rpm, and you run at 5000 rpm but with say, half throttle, you will only achieve half the torque.

At approx 2500 rpm though, half throttle will yield as much torque as can be obtained at that engine speed - further opening of the throttle will have no effect.
Effectively the engine sees full throttle and will therefor be operating more efficiently with less throttling pumping losses.
Allied to much reduced internal frictional losses, low engine speed at cruise is preferable.

All clear now?

 

[cathalferris]

It comes down to an engine parameter that is very rarely publicised, but it is the true measure of how efficient the engine is
It's fuel consumption per horsepower ( or any fuel per power output ), and is measurable on an engine dyno. The resulting graph is usually high at low speeds. This is accounting for fluid friction losses, lack of exhaust scavenging, turbo losses and other low-speed inefficiencies. At high engine speeds the graph reaches higher again, as there are different fluid losses and other friction elements start coming in to play more as the lubrications behave slightly differently.
For most cars this graph when done with the car as a unit (not just the stationary engine) can be graphed against road speed in each gear.
It's not always correct to say that 55mph is the most economical speed, as the actual maximum fuel efficiency point may differ greatly depending on the number of available gears, the aerodynamic efficiency of the design etc. For some cars the efficient point may even be as low as 35-40 mph (old landrovers) or above 70mph in the case of some audi diesels I've been in.
It's an interesting intellectual exercise to know these figures for you own car and engine.

Personally I'l lstill drive at or slightly above the speed limit, but I'll not accelerate too hard if I can avoid it when I 'm paying lots for the fuel.

 

[Shude]

Cruised to work today at 56mph (1750 rpm in 5th) on the motorway. Annoyed a lot of people I suppose but the economy looked fractionally better based on what the IC claimed.

 

[Bellow]

It comes down to an engine parameter that is very rarely publicised, but it is the true measure of how efficient the engine is
It's fuel consumption per horsepower ( or any fuel per power output ), and is measurable on an engine dyno. The resulting graph is usually high at low speeds. This is accounting for fluid friction losses, lack of exhaust scavenging, turbo losses and other low-speed inefficiencies. At high engine speeds the graph reaches higher again, as there are different fluid losses and other friction elements start coming in to play more as the lubrications behave slightly differently.

Which is just another way of saying

Engines are running most efficiently where they produce maximum torque,

while hinting at SFC but not quite mentioning it but still leaves this

Except that would coincide with the throttle being wide open and produce maximum acceleration - hardly economical cruising!

unanswered...

 

[renault12ts]

Experimented today.

Reset the obc whilst driving at 70 mph, with CC. After 20 miles got to 45mpg.
Reset again, at 56mph, with CC. After 20 miles got to 50mpg.

Less than scientific, but as a broad indicator, travelling at 56mph is more fuel efficient (in my car) than 70 mph.

 

[cathalferris]

still leaves this unanswered...

Not really.
That point at which an engine has the lowest SFC, that rev point is (usually) the lowest SFC for any chosen throttle setting along the rev range.

To maintain the cruising speed, a car requires an amount of power available to stay at that speed. This is naturally always a lot less than the actual power available if at WOT at that rev point. The efficiency when measured in fuel per horsepower, when off WOT, is still valid for efficiency determination - it is to do with the fuel used to generate the horsepower required to cruise at that speed - nothing to do with whether it's at WOT or not.

It may sound a bit illogical, but the numbers work out. I completed a module on internal combustion as part of my time doing mechanical engineering in uni, and this point was made during that course with the diagrams and maths to back it up.
I'm sorry I'm not the best at explaining hte how and why, but then I suppose that's why I'm not currently in academia

One of the examples given (iirc) was the difference between a 4 gear manual car and the same car with a 5th gear, all other ratios being the same. For the example chosen, it was apparent that going over 40 mph 3rd gear became less fuel efficent than 4th gear, so it was a good point to shift at when driving for max economy. When 5th gear was added in to the mix, it was when going over about 55mph that it became more economical to cruise in 5th gear. If there was a longer 6th gear, it could have been taken to about 65mph where the crossover between 5th and 6th was.

For modern cars with near-realtime readouts of the fuel economy, it's often worth cruising at various speeds on quiet roads in different gears at different speeds once the engine has warmed up, and seeing how the numbers read. For my Audi a4 tdi, my economy point in 5th is about 1900rpm, at about 56mph or so. Below this speed, 4th is a better choice as I'm in the turbo efficiency range.