Fuel price discussion thread (was ‘chaos’)

[jpaul]

Yeah it's down to aerodynamic drag. So van, pickups and SUVs are going to generate more aerodynamic drag than a "normal car' at 50mph. I'd expect they their optimum efficiency is at at a lower speed.

yes the drags got to predominate on trucks (maybe cyber truck will be a revolution - tv programme on musk last night was interesting - unboxing his first mcclaren f1)

My pickup the optimal point is getting up to around 66-67MPH and sitting there (which is pretty much lowest RPM in top gear) - it quickly drops off either side of that for highway speeds.

it's 4wd drive, google suggests a navara(?) has 7 speeds, isn't 50-55mph in 6th more efficient - given their target use, optimising the gearing for dual/motorway speeds sounds odd ?

 

[Rroff]

it's 4wd drive, google suggests a navara(?) has 7 speeds, isn't 50-55mph in 6th more efficient - given their target use, optimising the gearing for dual/motorway speeds sounds odd ?

This gets very complicated - the Navara is not full-time 4WD it is rear-wheel drive for road use. There are (in the UK) essentially two variants of the D40 - the 2.5L V6 which is designed as a workhorse and the 3.0L V6 (V9X) like mine which were designed as "lifestyle" vehicles (with fancy seats and a Bose premium sound system, etc. heh - so not your typical builders truck).

The 2.5L behaves like you are saying, the V9X is a "luxury SUV" engine and uses transmission and electronics based off (but not the same as) what is in the 370Z - it uses silly software tricks and different gear profiles low vs high to try and combine the best of both worlds and while it mostly works isn't flawless - for instance if I get stuck behind a cyclist power delivery becomes very bursty as it can't quite determine what I'm trying to do.

Then it gets all different again for the older and newer models than the D40.

 

[BUDFORCE]

The sweet spot for different cars could be 40mph in one car and 60mph in another.

100% this.

 

[jpaul]

[

Wait, you actually talk to your neighbours?

learned I had, earlier, unknowingly taken in the amazon jubilee flags they ordered - need to find petrol to get out of dodge that weekend.
neighbour opposite seems to have a new e-tron 55 - what a behemoth it looks every bit its 2500Kg
]

thats the key, Every car has an optimal maximum mpg speed based on its aerodynamics and its top gear. It varies greatly between cars especially with modern cars with 9 gears and perhaps ridiculous long gearing in the top gear. Its really brought home when I am driving an old car like my MGB with 4 gears. At 70mph the engine is running at more than 3,500 rpm. In my 3l bmw , the engine is barely ticking over at less than 1500rpm.

It's a pseudo science article - the behaviour of the gearbox in enabling the engine to work at stochiometric/efficient fuel/air point for different roads speeds/load is giving non linearity in the drivetrain friction component ...let's have some critical thinking, apart from anything else red-flag = the site has sections on entertainment , quizzes etc.
lol ".... So, for most cars, the "sweet spot" on the speedometer is in the range of 40-60 mph " - lot's of hand waving, how do their workings out show that.
Hopefully the govt internet harms bill will include such sites.

 

[Simon]

Ah yes. Gears have the ability to reduce air drag

 

[BUDFORCE]

Ah yes. Gears have the ability to reduce air drag

Oh man, you are not getting this......

Yes drag increases the load on the engine, which in turn increases fuel consumption, to a point.

Trying to drive at 100mph even at very low rpm in highest gear I'd 100% agree would be terrible for fuel economy.

But driving at 10mph at low rpm, with basically no drag, is terrible for fuel economy.

Mpg = miles per gallon. Guess what, the faster you are moving, the more miles you are covering.

 

[Simon]

We are talking about real world. No one is driving 10mph.

Not really sure what I’m not getting? All I said was 50mph is always more efficient than 70mph. Don’t try and create a new point (that I didn’t say) just to try and win some argument when we are talking physics. 10mph is obviously not the ‘sweet spot’ as no ICE car will run efficiently at 10mph in top gear. But once past 50-55mph gearbox friction and drag start to be the main factor. Going faster doesn’t reduce any friction anywhere in a car.

Even EV the motors aren’t running efficient until there sweet spot. Most EV peak efficiency is 45-50mph but again depends on the motor efficiency and gear reducer chosen

 

[Simon]

Mpg = miles per gallon. Guess what, the faster you are moving, the more miles you are covering.

You seem to be confusing miles per hour with miles per gallon here. I have no idea what this statement even means. Is it an error or something you actually believes is relevant here ?

don’t think you can somehow outrun physics if you drive faster

Mpg. Guesswhat. The slower you are moving. The more miles you cover PER GALLON

 

[Deleted User 298457]

Oh man, you are not getting this......

Yes drag increases the load on the engine, which in turn increases fuel consumption, to a point.

Trying to drive at 100mph even at very low rpm in highest gear I'd 100% agree would be terrible for fuel economy.

But driving at 10mph at low rpm, with basically no drag, is terrible for fuel economy.

Mpg = miles per gallon. Guess what, the faster you are moving, the more miles you are covering.

Worryingly don't you do mortgage advice for a living?

 

[touch]

Yeah, they should be increasing the speed limit not decreasing it. If you can drive faster you can get to your destination quicker before your petrol guage drops as much.
It would save everyone money.

 

[MagicBoy]

Yeah, they should be increasing the speed limit not decreasing it. If you can drive faster you can get to your destination quicker before your petrol guage drops as much.
It would save everyone money.

Aww. Bless.

 

[FerretBoy]

We are talking about real world. No one is driving 10mph.

Not really sure what I’m not getting? All I said was 50mph is always more efficient than 70mph. Don’t try and create a new point (that I didn’t say) just to try and win some argument when we are talking physics. 10mph is obviously not the ‘sweet spot’ as no ICE car will run efficiently at 10mph in top gear. But once past 50-55mph gearbox friction and drag start to be the main factor. Going faster doesn’t reduce any friction anywhere in a car.

Even EV the motors aren’t running efficient until there sweet spot. Most EV peak efficiency is 45-50mph but again depends on the motor efficiency and gear reducer chosen

The internet suggests 20-25mph for EVs generally. It mainly comes down to the energy required for the car to be on, if you have the heating on full blast then you'd want a faster mph.

 

[BUDFORCE]

Ok fine, apologies for my comment last night was a bit aggressive.

So humour me then, because maybe I am being special.

A petrol or diesel engine needs to constantly rotate to keep running, which uses a little fuel regardless on whether you are moving or not.

A. If you are in first gear, driving at 1500 rpm, you will moving at X speed using X amount of fuel.

B. If you are in 6th gear, at 1500 rpm, the same applies.

So I would assume, the car will be using a bit more fuel at 1500 rpm in 6th than 1st because of aero drag, friction etc, fine.

But you are also travelling faster, so every revolution, youve covered more distance.

So which is more efficient?

 

[Greebo]

Ok fine, apologies for my comment last night was a bit aggressive.

So humour me then, because maybe I am being special.

A petrol or diesel engine needs to constantly rotate to keep running, which uses a little fuel regardless on whether you are moving or not.

A. If you are in first gear, driving at 1500 rpm, you will moving at X speed using X amount of fuel.

B. If you are in 6th gear, at 1500 rpm, the same applies.

So I would assume, the car will be using a bit more fuel at 1500 rpm in 6th than 1st because of aero drag, friction etc, fine.

But you are also travelling faster, so every revolution, youve covered more distance.

So which is more efficient?

The second one.

The most efficient any ICE car is, is the speed you are going in the highest gear with the lowest revs without the engine labouring. FACT. Now that varies a lot between gears.

As I said with my mgb. I am doing 3500 rpm in 4th gear at 70 mph. The same car with overdrive can do 70mph in overdrive at 2700 rpm. Which car is doing better fuel economy?

Nobody is saying drag and rolling resistance arent factors in fuel economy. They are. But now we have cars with 9 gears, modern cars will have a higher top speed where they are on maximum efficiency compared with older cars with only 5 gears.

Biggest savings can be made with accerlation rather than dropping speed limits. Not going above 2500rpm in a petrol car and above 2000rpm in a diesel before changing up a gear saves 15% fuel economy on average. Boring but economical.

 

[jpaul]

Yeah, they should be increasing the speed limit not decreasing it. If you can drive faster you can get to your destination quicker before your petrol guage drops as much.
It would save everyone money.

funny you should say that, but subsequently reading an article on top gear selection

Optimisation of the vehicle transmission and the gear-shifting strategy for the minimum fuel consumption and the minimum nitrogen oxide emissions​

Traditionally, the top-gear ratio for a vehicle is selected to provide the maximum speed. This is limited by the engine power and the resistive forces, predominantly the aerodynamic drag, when travelling at high speeds.14 The aim of optimisation is to reduce the fuel consumption and the NOx emissions. Therefore, the selection criteria for the selection of the top-gear ratio are changed to achieve these aims, noting the maximum legislated speed limit. Here, the top-gear ratio is selected so that it provides the maximum efficiency at the maximum motorway legal speed. For the UK, the legal speed limit is 70 mile/h (31.3 m/s).

so if our cars have same gearing as eu cars adapted for faster motorways, the top gear on uk cars maybe less efficient for our 70 (perhaps that's why people drive at 80), and the diesel or petrol is not at its optimum efficiency/torque output.

 

[BUDFORCE]

Worryingly don't you do mortgage advice for a living?

Not quite, I underwrite, assessing risk basically.

Funnily enough this topic doesn't apply to what I do for a living.

 

[Deleted User 298457]

Aww. Bless.

It's just physics!!!

 

[alec]

car will be using a bit more fuel at 1500 rpm in 6th than 1st because of aero drag, friction etc, fine.

But you are also travelling faster, so every revolution, youve covered more distance.

As I said with my mgb. I am doing 3500 rpm in 4th gear at 70 mph. The same car with overdrive can do 70mph in overdrive at 2700 rpm. Which car is doing better fuel economy?

you are touching the area of brake specific fuel consumption, BSFC
a curve of how much fuel is burned per horsepower produced over range of rpm and throttle positions.

Fascinating plots that may explain it better when you see one

Figure 2. BSFC of the diesel engine.

Download scientific diagram | BSFC of the diesel engine. from publication: Performance Analysis of the Vehicle Diesel Engine-ORC Combined System Based on a Screw Expander | To achieve energy saving and emission reduction for vehicle diesel engines, the organic Rankine cycle (ORC) was employed...

www.researchgate.net

The Best Way To Compare Engine Efficiency - BSFC

Brake Specific Fuel Consumption - How Massive Engines Can Be EfficientComparing Engine Performance: BMEP - https://youtu.be/bE_1JYrlYYUSubscribe for new vide...

www.youtube.com

What this tells you, is that to get best fuel consumption it is not enough to keep it in "optimal" rpm, but also optimal load.

Modern cars generally have oversized ICE engines, inefficient at low load, thats why the general rule of "top gear, low rpm" holds to achieve best mileage. Which should be around 40-50 mph for most cars, maybe 60 for some outliers.

At motorway speeds the load is approaching the optimal BSFC area of the engine. But thats optimal fuel per horsepower... And because power required grows so high, it is burning more fuel per mile.

so if our cars have same gearing as eu cars adapted for faster motorways, the top gear on uk cars maybe less efficient for our 70

Anecdotally I noticed on couple models where EU and UK models have different gearbox ratios, it is UK version that has taller gears.

 

[Doobedoo]

so if our cars have same gearing as eu cars adapted for faster motorways, the top gear on uk cars maybe less efficient for our 70 (perhaps that's why people drive at 80), and the diesel or petrol is not at its optimum efficiency/torque output.

I doubt people are driving at 80 because of efficiency it’s because they either drive a BMW or Audi basically. They own the roads and at some point you are in their way so move.

 

[jpaul]

A petrol or diesel engine needs to constantly rotate to keep running, which uses a little fuel regardless on whether you are moving or not.

A. If you are in first gear, driving at 1500 rpm, you will moving at X speed using X amount of fuel.

B. If you are in 6th gear, at 1500 rpm, the same applies.

So I would assume, the car will be using a bit more fuel at 1500 rpm in 6th than 1st because of aero drag, friction etc, fine.

But you are also travelling faster, so every revolution, youve covered more distance.

So which is more efficient?

B, - my take, even though you are using more fuel to generate more torque in B , the frictional engine losses predominate in A so the fuel used to travel a given distance ends up being greater than travelling in 6th at a higher speed, despite the increase in fuel for aero drag at higher speed
so to travel a mile in 1st @1500 say 2000J friction + 50J aero (driving that mile might take 10minutes?)
6th @1500 50J friction + 1000J aero