Increased speed increased fuel effiency ?

[Duck]

Well was wondering if anyone could shed some light on this for me. After Christmas I was driving home 1300k's, I have a v6 model & normally on the highway get around 11-12ltrs/100 ( no camp trailer on ) but was lucky enough to be able to be escorted in a manner of speaking or for the sake of this discussion for 350km's for the sake of this exercise lets just say I was driving on Private land with bitumen roads. I found that from about 100 - 115k's an hour the economy was average approx 12.5ltr/100, at 90k's an hour it was 11-12ltrs /100 but If I stayed at about 120 - 130 it dropped down to 8.9lts /100 & was getting the best economy I have got from this car even better than my diesel Triton. So looking at maybe better air flow which in turn gave this change ? I have no idea I would have thought that buy increasing my speed it would use more fuel not less. Looking forward to any suggestions, have been thinking about mounting a massive fan to the front of the car to keep the air speed up ! I have heard of snorkels giving efficiency gains ( I do not have a snorkel ) but nothing like this. Hoe you all ate to much drank to much & got good pressies over Christmas.

[wake jake]

I have found the same driving on a private road too 120kmh-130kmh is using less than 100kmh-110kmh besides more smoke haha. Someone could explain this.

[Notso]

Hmmm, I'd like to see an explanation too. Maybe the computer couldn't handle the speed?
Maybe a raging Tail Wind?

[Honky]

When I had the SSV V8 Sportswagon and driving along a private road at around 130 it used to show 8 l /100 k,s.
Around town it shows 18 l
Sitting on 100 or 130 did not make any difference.

Apart from town driving the V8 gets better mileage than the Triton
on the open road and thats with no turbo lag
Honky

[hvac guy]

The motor is more than likely in its peak torque at that speed vs rpm and the engine fan turns off at that sort of speeds.

[Quinny]

It's a V6 ... the engine that just keeps giving ...


... I'm a bit bored today

[Duck]

How can you get bored get the new Narva catalogue that will take up the rest of the day !!!

No real tail wind to talk of not that I really checked, But I have since been back to that same private road & it just seems to be the speed the Petrol likes to run at.

It's a V6 ... the engine that just keeps giving ...


... I'm a bit bored today

[anthraxx66]

I've got no idea about this... one experience I had was in the Northern Territory in early 2000 before the 130km/h limit came into play. I had a VS3 Commodore ute and once I hit 130km/h the fuel consumption stayed pretty constant at about 14l/100km... even travelling from Darwin to Tenant Creek on the way home averaging just a tad under 180km/h the consumption stayed the same. It was bog standard too...

Interesting!

[4wd26]

Just checked you are running the auto.

it would be locking up at those speeds, under the 100km mark I find the 4 speed to jump around (unlocking) using much more fuel.

really noticable when towing- I was better to travel/ tow at 110 than 100, or drop down to 85/90 and keep the auto in 3rd.

This could be what you are experiencing?

[NowForThe5th]

There is no real logic in this situation. Drag increases at a cube of velocity so running at a higher speed, even though this may be in the engine's "sweet spot", would require use of more fuel the faster you go.

This certainly is my experience. Under 100km/h can return under 10l/100km but at 130km/h on that private road it's up around 14/100km.

[irwazza]

I'll believe it when you do actual fuel used calculations

[gauci204]

were you travelling behind someone (maybe the owner of this private road )

[hvac guy]

yeah the logics there, a combustion engines output isnt linear to velocity or drag
so if the engine @ 100kmh or 2300rpm using 10ml fuel a sec with 2 units of drag with engine efficiency at that rpm about 50%.
lets change the speed to 130 kmh hour and rpm to 2700rpm or peak torque zone rpm of the engine with
100% efficiency even though drag is increased your power to drag ratio can be higher with the same 10ml fuel a sec used but more klm travelled.

hope this helps.

this same phyisics apply for prop selection on boats ie prop size/pitch vs max speed vs peak hp@rpm
also the same as fraz's fuel economy improved buy changing diff ratios.

[NowForThe5th]

Not really, hvac. As velocity doubles the amount of power required to overcome aerodynamic drag increases by 8 times. While I acknowledge what you say, there is an increase in power and efficiency from 2300rpm to 2700rpm, in a diesel that increase is actually quite small since they have such a flat torque curve. A petrol might be a better example but even that would not have an increase of 8 times, required just to overcome drag, not actually increase efficiency to the point where significant fuel savings were being achieved.

In aviation there is such a thing as the "sweet spot" where drag is minimised at a certain speed before increasing dramatically up to about Mach 1 from which it actually starts to decrease. If our Tritons could do Mach 1 I don't think we'd be having quite this conversation. I do understand, though, that for most cars that sweet spot where fuel efficiency vs drag are at the optimum is around 80km/h. Varies, obviously, according to design and accessories.

[hvac guy]

Life begins at mach 1 lol


but a combustion engines efficiency is not linear
To velocity or drag unlike a electric motor where
The formula your using which I use at work (fan laws)

[NowForThe5th]

That's correct, an IC engine's output is not linear, nor is the increase in drag as velocity increases which increases in line with the cube of the velocity.


There is a (reasonably) simple explanation and application of the formula here:

http://drive55.org/content/view/60/1/

Scroll down to the bottom of the page for a simplified explanation of the application of the formula.

There is also an online tool that will provide a table giving fuel consumption at varying speeds here:

http://ecomodder.com/forum/tool-aero-rolling-resistance.php#4

Lots of variables but if you use these you'll get kind of close
Vehicle weight = 2000kg
Cd = 0.45
Fuel = No. 2 diesel

Other variables can be changed too but, in the table produced, you'll see the effect pretty clearly.

The bottom line is that fuel consumption cannot decrease with an increase in speed.

[Duck]

Yes gauci204 I was travelling behind a group of Motorbikes ( quite a long way back ) but I am pretty sure they were the owners of the private road as no ne seemed to bother us. Maybe if I hired those guys to ride in front of me for the sydney trip probably save in fuel what they would charge me for the escort. Thanks for everyone's feedback, might have to get a snorkel & see if how that goes maybe the petrol is starving for air ? I don't know but I thought others might be experiencing the same thing.
Hvac & fifth you will need to talk to me in Engrish as you both seem to work at NASA what the bloody hell do you boys do for a living, I normally just yell at my computer I did understand the Mach bit ! Its a shaver I use
Thanks again

were you travelling behind someone (maybe the owner of this private road )

[snowman]

i reckon it was prevailing wind.

heading into a steady 25km/hr breeze vs a trailing 25km/hr breeze is a 50km/hr difference in wind speed and a significant factor to fuel use.

[Notso]

Imagine having a bunch of Bikies in front of you "Breaking Wind"

[Duck]

MMMMMMMMMMM Bacon

Imagine having a bunch of Bikies in front of you "Breaking Wind"