Diesel cars (toxic tax) announced

Axe_meister

Yes force is changing (well being equalised) but force is mass x acceration. We had many a debate at uni about this. It all depends on your frame of reference. 

hywelg

The frame of reference is clear 
Velocity is measured relative to the road surface, and is constant, therefore acceleration is zero. Force is applied to overcome rolling and frictional resistance and the work to overcome gravity when going uphill.

So since acceleration = zero, mass is constant  the only changing parameter is force. And the portion of force applied to create acceleration is zero.  The additional force is required to overcome gravity and add potential energy to the vehicle. 

Axe_meister

Mathematically you have acceleration backwards (due to gravity) and acceleration forward they cancel each other out.
When calculating all forces on the car you have to take both +ve -ve acceation into account (in all three dimensions).
I spent many years doing this. Our mechanics lecturer would always say never assume acceleration is zero, even if it appears to be cancelled out.

Sporky

What if you were driving up a conveyor belt?

To clarify a bit, if you are maintaining a constant speedometer speed up a hill then yes, you're using more fuel than if you were maintaining a constant speed on the flat, because you're exchanging chemical energy in the fuel for gravitational potential energy.

On the way back down, assuming the hill isn't so steep that you need to use the brakes, you will be getting that gravitational potential energy back, and won't use as much fuel as on the flat.

The frictional losses in each scenario (hill vs not-hill) will be pretty much identical as they're related to speed vs the road and speed vs the air.

This does change if the hill is much steeper on one side than the other, but as many trips are there-and-back you'll probably get the reverse in the other direction.

Thus I maintain that if you stick to a steady speed on hills (up and down) then they don't overall make a big difference to fuel consumption on the journey as a whole. Obviously it takes more fuel to go up a hill than to drive on the flat, but equally obviously it takes less fuel to go down a hill than it does to drive on the flat.

Regarding the acceleration arguments, I think we're getting a bit into semantics, and both views are correct for slightly different definitions. I have been working on a midway definition, that acceleration means a change in velocity, because I believe that's probably the most common usage.

crunchman

Sporky said:

Thus I maintain that if you stick to a steady speed on hills (up and down) then they don't overall make a big difference to fuel consumption on the journey as a whole. Obviously it takes more fuel to go up a hill than to drive on the flat, but equally obviously it takes less fuel to go down a hill than it does to drive on the flat.

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.  On the way up the hill you will need extra fuel to gain GPE but because you are only using 40% of the energy in that fuel to move the car then you will need a lot extra.  That energy is used to overcome wind resistance, rolling resistance, resistance in bearings etc.  On the way down, 100% of that GPE will be used to overcome all of those forces (which should remain constant).  In other words the process of energy conversion is 2 and half times as efficient going down so the savings in fuel are much smaller than what you use on the way up.

@Sporky, changing the subject slightly, I believe you and @Hywelg commented on a link I posted about real world emissions from Euro 6 diesels using data from 2014.  The fact is that those vehicles are registered as Euro 6 compliant.  If Euro 6 diesels are allowed to be driven into cities then those vehicles will be part of that group.  Yes some of the new ones in the last couple of years might be better but maybe they need to come up with a new designation for them - or retrospectively remove it from the older ones - which would cause a lot of issues with people who have bought them, and massive legal complications.  Realistically, a Euro 6 designation for a diesel means very little in terms of air pollution.  I'm not even sure that they are better than the older diesels.  The particulates from the older ones aren't as small and don't get absorbed into the body in the same way.

Sporky

crunchman said:

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.

And does that efficiency change just because it's on a hill?

crunchman said:

@Sporky, changing the subject slightly, I believe you and @Hywelg commented on a link I posted about real world emissions from Euro 6 diesels using data from 2014.  The fact is that those vehicles are registered as Euro 6 compliant.

The point that Hywelg made is that Euro6 only came into force in September 2014. That report was published on the 10th of October, so it only applies to a very small proportion of Euro6 vehicles, most of which weren't even on sale at that point. At most they had what, 41 days to do all their testing, all their analysis and publish the report. I believe the word "shonky" can be fairly applied to that.

ICBM

Sporky said:

crunchman said:

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.

And does that efficiency change just because it's on a hill?

No - but because it's less than 100% efficient you need to burn more extra fuel to go up a hill than you save coming down the same hill. So your overall journey efficiency is worse if there's a hill in the middle than if it was flat all the way, even if the start and end points are at the same height.

crunchman

ICBM said:

Sporky said:

crunchman said:

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.

And does that efficiency change just because it's on a hill?

No - but because it's less than 100% efficient you need to burn more extra fuel to go up a hill than you save coming down the same hill. So your overall journey efficiency is worse if there's a hill in the middle than if it was flat all the way, even if the start and end points are at the same height.

To expand on this, down the hill it's not the engine doing all the work to overcome all the drag forces.  You are using the stored gravitational potential energy - which will not have the inefficiency of the engine which wastes around 60% of the energy as heat.

Sporky

ICBM said:

Sporky said:

crunchman said:

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.

And does that efficiency change just because it's on a hill?

No - but because it's less than 100% efficient you need to burn more extra fuel to go up a hill than you save coming down the same hill. So your overall journey efficiency is worse if there's a hill in the middle than if it was flat all the way, even if the start and end points are at the same height.

Ah!

Yes, I see what you mean. You do get some back on the way down, but not as much as you spend on the way up.

I do not dispute that there being a hill means you use more fuel than if there isn't a hill. I do dispute the idea that going up (and then down) a hill necessarily uses four times as much fuel as being on the flat, which is the assertion with which I originally disagreed.

I also maintain that any attempt at "real world" testing will, of course, be subverted by the manufacturers, because it'll have to be on a fixed regime or consumers won't be able to make worthwhile comparisons. That said, I do think it's worth a more rigorous test than the current one, which makes small turbo engines and hybrids look exaggeratedly efficient.

ICBM

Sporky said:

I do not dispute that there being a hill means you use more fuel than if there isn't a hill. I do dispute the idea that going up (and then down) a hill necessarily uses four times as much fuel as being on the flat, which is the assertion with which I originally disagreed.

No, it certainly doesn't. I must have missed that. It depends on the gradient for a start.

As I said, I commute across a 'hill' of sorts - an average of a little under a thousand feet up over about 20 miles, then the opposite. Even this small gradient - less than 1% - is enough to make the fuel economy over the first half up to 5mpg worse than over the second half, in either direction. I was actually surprised it made as much difference as that, but I've tracked it several times and it's quite consistent.

hywelg

Sporky said:

The point that Hywelg made is that Euro6 only came into force in September 2014. That report was published on the 10th of October, so it only applies to a very small proportion of Euro6 vehicles, most of which weren't even on sale at that point. At most they had what, 41 days to do all their testing, all their analysis and publish the report. I believe the word "shonky" can be fairly applied to that.

Indeed. I also made the point that this is a campaigning group so they have an inbuilt bias. They need headlines so they're going to push the tests to make their point for them.Euro6 compliance wasn't required until Sept 2016. I know this because I was interrogating the Merc commercials dealer, They were only fully Eu6 compliant from July 2016. It didn't apply to new cars as stated it applied to new models.

Witness the other link which shows that VW 2.0 150ps engines, introduced late 2015, are pretty much bang on the money as regards ACTUAL emisssions. And of course they havn't tested petrol engines with quite the same enthusiasm. Its quite possible they are piss poor too.

Sporky

ICBM said:

As I said, I commute across a 'hill' of sorts - an average of a little under a thousand feet up over about 20 miles, then the opposite. Even this small gradient - less than 1% - is enough to make the fuel economy over the first half up to 5mpg worse than over the second half, in either direction. I was actually surprised it made as much difference as that, but I've tracked it several times and it's quite consistent.

I find that quite believable. I'd be interested to know (though it'd be hard to test) what you'd get over an equivalent flat journey.

timmysoft

VimFuego said:

timmysoft said:

People should be charged for driving diesels. Diesels are shit and they sound shit. Diesel should be used for commercial vehicles only. 

good argument, well presented...

It wasn't an argument, it was a statement of fact. 

Agricultural bullshit, throw it in the bin.

VimFuego

timmysoft said:

VimFuego said:

timmysoft said:

People should be charged for driving diesels. Diesels are shit and they sound shit. Diesel should be used for commercial vehicles only. 

good argument, well presented...

It wasn't an argument, it was a statement of fact. 

Agricultural bullshit, throw it in the bin.

lol,

ToneControl

Axe_meister said:

ICBM said:

Axe_meister said:

hywelg said:

kaypeejay said:

To maintain a steady speed going uphill requires you to press the accelerator harder, which is accelerating by any other definition. ". 

I might have left school 45 years ago but acceleration causes an increase in velocity. Therefore your statement is bollocks. 

Remember F=ma.
When going up hill you have to fight gravity. Well a proportion of it anyway (the amount depended on the gradient) This will be a force acting backwards. 
To maintain an equal speed to have to apply a forward force of the same about. So you have to accelerate at the same amount as you are being de-accelerated.

I think the point hywelg is making is that you're not accelerating, because speed is constant. However, you definitely are having to put extra energy into the system when you go uphill, and you don't get the same amount back when you come downhill because the engine isn't 100% efficient. On a typical motorway gradient the difference is small, but if you're as obsessive as I am about watching the fuel consumption you can easily tell .

Just being pedantic, in engineering terms you are accelerating and the same rate you are being decelerated at.

no

ToneControl

Axe_meister said:

Yes force is changing (well being equalised) but force is mass x acceration. We had many a debate at uni about this. It all depends on your frame of reference. 

at Uni?
this is O level maths

ToneControl

Axe_meister said:

Mathematically you have acceleration backwards (due to gravity) and acceleration forward they cancel each other out.
When calculating all forces on the car you have to take both +ve -ve acceation into account (in all three dimensions).
I spent many years doing this. Our mechanics lecturer would always say never assume acceleration is zero, even if it appears to be cancelled out.

well your lecturer was shite my friend
a jet engine in a test lab firing at full power kicks a lot more power out than a 2 litre diesel, but nevertheless, acceleration is zero, since its velocity does not change from zero (relative to the planet it is anchored to)

you can't "cancel out" acceleration, you are either accelerating or not

F=ma

For example, on a "rolling road" you are not accelerating, you are just testing the power output of the car against friction, with zero air resistance

You take the net force vector and that determines acceleration. Going up a hill, if you open the throttle, Force is applied forwards, at the tyres' interface with the road. If you put it in neutral, the force is in the direction back down the hill (I'm ignoring the forces keeping you on the ground)

ToneControl

ICBM said:

Sporky said:

crunchman said:

As @ICBM said, you need to consider the fact that an engine is not 100% efficient.  A modern engine has an efficiency in the region of 40%.

And does that efficiency change just because it's on a hill?

No - but because it's less than 100% efficient you need to burn more extra fuel to go up a hill than you save coming down the same hill. So your overall journey efficiency is worse if there's a hill in the middle than if it was flat all the way, even if the start and end points are at the same height.

especially if you need engine braking coming down the other side

Axe_meister

ToneControl said:

Axe_meister said:

Yes force is changing (well being equalised) but force is mass x acceration. We had many a debate at uni about this. It all depends on your frame of reference. 

at Uni?
this is O level maths

Degree in Aero Engineering. Of course our equations would take into loss of mass due to fuel consumption, drag, friction on the road, side wind change in angle of attack if the gradient changes.

crunchman

Today's Torygraph (who are not on a campaign on this issue like the Standard or Guardian):

http://www.telegraph.co.uk/science/2017/04/26/toxic-diesel-particles-penetrate-right-heart-scientists-warn/

The newer diesels give off more of these smaller particulates.  The older ones were actually better for particulates because the particulates were bigger and didn't get into the body.  The problem is they were even worse on NO2.

For those who can't be bothered to read the whole article here are a couple of key paragraphs:

While petrol particles are also able to penetrate the lungs, a petrol engine will throw out roughly 50 times fewer particles than a diesel engine of equivalent size, the researchers said.

The particles are also capable of penetrating the masks worn by some cyclists to avoid pollution.

What worries me is that all the low emissions zones at the moment are proposing to let Euro 6 diesels in for free.

Diesels in cities do not work.