Hypothetical mpg question fo a 90 mile journey.

Joe_Horner

Ultrasonic wrote: »

It isn't quite that simple I'm afraid, you need to consider engine load not just revs when considering what is optimal. (See my comment re. BSFC above.)

Ideally you want peak torque revs at full throttle for maximum efficiency. The trouble is, in alost all cases that won't give you constant speed. Aiming to cruise at the peak of the torque curve is probably as close as you can hope for in practice

vaio

full throttle? Surely not?

Ultrasonic

Joe_Horner wrote: »

Ideally you want peak torque revs at full throttle for maximum efficiency. The trouble is, in alost all cases that won't give you constant speed.

Normally optimal engine efficiency is at a little below maximum load actually, as you'll see from the plots I posted the link to. But you are of course right that this point is not useful for driving at a constant speed (unless I guess going up a serious hill!) but I wasn't actually suggesting this.

What Hezlos posted was incorrect though, because they are ignoring the variation of load with speed.

Joe_Horner wrote:

Aiming to cruise at the peak of the torque curve is probably as close as you can hope for in practice

Not quite sure what you mean by 'as close as you can hope for' there. Driving at the rpm at which max. torque is achievable will likely NOT give optimal mpg (driving slower in top gear probaby will), and at lower load this rpm may also not correspond the minimum BSFC for this load. But this is another discussion really, and not relevant to the OP!

Ultrasonic

vaio wrote: »

full throttle? Surely not?

Maximum engine efficiency (minimum BSFC) occurs at close to max. load at the rpm where the torque peak is achievable. If you actually need this power then this is optimal. For normal driving this is only really relevant when accelerating though.

Edit: if you're designing something like a diesel generator that runs at a fixed rpm/load then this near max. load condition is what you'd go for though. (Yes I know diesel engines don't have a throttle but the high load condition is comparable...)

Ultrasonic

I keep forgetting to say, if the OP was driven purely by financial considerations then the increased tyre wear at 90 mph compared to 30 mph is another factor. Although one I have no idea how to even approximately quantify!

Joe_Horner

Ultrasonic wrote: »

Normally optimal engine efficiency is at a little below maximum load actually, as you'll see from the plots I posted the link to. But you are of course right that this point is not useful for driving at a constant speed (unless I guess going up a serious hill!) but I wasn't actually suggesting this.

Not quite sure what you mean by 'as close as you can hope for' there. Driving at the rpm at which max. torque is achievable will likely NOT give optimal mpg (driving slower in top gear probaby will), and at lower load this rpm may also not even correspond to the local minimum BSFC. But this is another discussion really, and not relevant to the OP!

Simply that the chances of driving everywhere near peak torque, given relatively few ratios in most transmissions, is likely to be a problem in normal traffic.

I agree that theoretical optimal efficiency will often be a higher gear and slightly lower revs, but that's at least partly offset in practice by the loss of flexibility - as traffic speed varies the slightly higher gear will need less change of throttle to adjust.

The problem (as with the original question) is that there are so many variables involving the car, weather, traffic, topography and so on that a definitive answer is impossible, and may even vary for the same car on different days. But, as you say, that's a whole 'nuther (and fascinating) discussion :beer:

WTFH

Ultrasonic wrote: »

I keep forgetting to say, if the OP was driven purely by financial considerations then the increased tyre wear at 90 mph compared to 30 mph is another factor. Although one I have no idea how to even approximately quantify!

Would it be higher wear? I'm not certain. Also depending on the car's aerodynamics, will the car have more or less downforce at higher speeds.

Apropos of nothing, when studying A level physics I remember my teacher giving the class a question about tyre wear, based on the level of tread on a new tyre and the level at which it needed replacing. Basically, you end up seeing that evey revolution of the tyre it loses 1 molecule of thickness. (there are a whole series of caveats along with that, but it was quite an interesting one to learn at the time)

reeac

WTFH wrote: »

Would it be higher wear? I'm not certain. Also depending on the car's aerodynamics, will the car have more or less downforce at higher speeds.

Apropos of nothing, when studying A level physics I remember my teacher giving the class a question about tyre wear, based on the level of tread on a new tyre and the level at which it needed replacing. Basically, you end up seeing that evey revolution of the tyre it loses 1 molecule of thickness. (there are a whole series of caveats along with that, but it was quite an interesting one to learn at the time)

I agree with that result - did the calculation myself once [a long time ago].

accidentalspender

bsms1147 wrote: »

With the hypothetical scenario proposed, the car traveling at 30mph will use three times the fuel as the one travelling at 90mph, minus whatever things you want to factor in, such as air resistance.

I doubt the air resistance (and other unaccounted for factors) could treble the fuel-use of the 90mph option so that it matched the fuel use for the 30mph option.

Above is the short answer. I read down the thread up until this post to check I wasn't repeating anyone. The average car will experience roughly 30 times as much drag at 90 vs 30MPH;

BUT...

Aerodynamic drag is only one of the inefficiencies of a car. Amongst others is heat production, engine & transmission friction, electrical ancillaries' drag, road wheel friction, etc. All of these are experienced by both cars.

AND...

There is the stoichiometric fuel/air mix aspect. Petrol engines need a certain ratio of fuel to air injected into the cylinder per engine cycle to produce an output. The graph of fuel injected to power produced is not linear. Diesel's stoichiometric mix is more favourable, hence their suitability for motorway cruising, but that's a different lecture.

AND...

With the above in mind, the 30MPH car's engine will have turned over 240,000 times more than the 90MPH car in the same journey. Every time it turns over, it has fuel injected & experiences all the same inefficiencies as the 90MPH car, other than air resistance.

westwood68

For anyone who thinks that Air Resistance is *not* the major factor in assessing the max sped of a vehicle; I suggest you all watch this:-
http://www.channel4.com/programmes/speed-with-guy-martin/4od

"Guy attempts, with the help of slipstreaming, to break the British record for outright speed on a bicycle: an incredible 110mph. Guy recruits an unlikely team made up of a truck racer, an Olympic gold medallist, a bicycle builder and a design engineer more used to working on next-generation military aircraft.

Together they work out how to modify a 1000-horsepower racing lorry to create a large enough slipstream for Guy to cycle in, and build a unique bicycle gearing system capable of triple-figure speeds."

The *only* resistance removed in this case is air resistance. All other factors stay the same (including all mechanical resistance). In fact for the final attempt, the rolling resistance is increased by quite a margin as the attempt is made on sand rather than a smooth road.