Diesel cars (toxic tax) announced

Sporky

hywelg said:

That's a 2014 report so it's isn't actually testing the latest EURO 6 engines.

I didn't even spot that bit. So not a relevant source any more.

Evilmags

Comparing cars (mobile) to powerstatons (generally away from populations) doesn't have any validity. The issue is in urban areas not the countryside. The higher levels of compression diesel requires to generate equivalent power is why it is more poluting. 

Nitefly

There was a discussion of the new testing regimes on "You and Yours", Radio 4, yesterday lunchtime, although it was centred more on MPG rather than emissions.  

A bloke from a green group said most cars don't get better than 40% of what they claim under current testing regime.

The woman from the EU department creating the new test regimes said they will get within 15 - 20% under new regimes incoming.  When asked why not closer than that, she said because driving styles differ, local driving conditions differ, etc.

Apparently the current regime (about to be superseded) doesn't allow for the fact that you might drive up a hill!  "Real-world", eh?

strtdv

Part of the reason newer cars get a better mpg is that a not insignificant part of the current test is done with the vehicle at a standstill, and if you have a car with stop-start then the engine won't actually be on at all for that bit

hywelg

Government report which was due to be published by Monday is now to be delayed because of the election. Great still no clarity.

Sporky

Nitefly said:

A bloke from a green group said most cars don't get better than 40% of what they claim under current testing regime.

That's provably untrue.

Nitefly said:

Apparently the current regime (about to be superseded) doesn't allow for the fact that you might drive up a hill!  "Real-world", eh?

Presumably most people who drive up a hill then drive back down, so it doesn't make a vast difference.

Sambostar

It won't happen.  Diesel cars will be phased out at source new, that is all that will happen.  If they put the road tax up by a grand on my Smiley Transit and make new plans to rejoin the EU they won't take the majority of this country alive and their heads will still be smiling on fence posts.

kaypeejay

Sporky said:

Presumably most people who drive up a hill then drive back down, so it doesn't make a vast difference.

It doesn't work like that. If you travel up an incline and accelerate at all, you will probably get 1/4 of your flat road mpg. Even if you then coast down the same length hill using no fuel, you will never match flat road mpg. 

ICBM

The drop in fuel economy caused by going up even a slight gradient is remarkable. My commute is across the central belt of Scotland, which although with local variations is essentially uphill to the middle, then downhill - but the average gradient must be tiny over that distance. The difference is about 5mpg between each half of the journey.

Wind speed and direction makes a big difference too - the overriding factor in fuel use when you're cruising at motorway speed is air resistance, so if you're driving into a 10mph headwind, that's the same in fuel terms as driving 10mph faster. You don't get all that back if it's a tailwind either - since air resistance is proportional to the square of the speed, an increase hits you more than you gain from a decrease of the same amount.

Sporky

kaypeejay said:

Sporky said:

Presumably most people who drive up a hill then drive back down, so it doesn't make a vast difference.

It doesn't work like that. If you travel up an incline and accelerate at all, you will probably get 1/4 of your flat road mpg. Even if you then coast down the same length hill using no fuel, you will never match flat road mpg. 

And if you go up the hill at a steady pace? Why are you accelerating, and why are you comparing accelerating up a hill to a steady speed on a flat road? Why not accelerating on the flat road?

I understand, of course, that going up hill uses more fuel, but I also understand that accelerating uses more fuel, and conflating the two doesn't give a fair picture. In the real world (which is the sort of testing we were talking about) most people slow down a bit going up a hill, and speed up a bit on the way down - which is the opposite of what you're claiming.

If you maintain a steady speed then - frictional losses aside - you only need to burn enough extra fuel to balance the gain in gravitational potential energy - and you get that back on the way down. The frictional losses are going to be pretty much the same no matter the gradient.

timmysoft

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

kaypeejay

Sporky said:

kaypeejay said:

Sporky said:

Presumably most people who drive up a hill then drive back down, so it doesn't make a vast difference.

It doesn't work like that. If you travel up an incline and accelerate at all, you will probably get 1/4 of your flat road mpg. Even if you then coast down the same length hill using no fuel, you will never match flat road mpg. 

And if you go up the hill at a steady pace? Why are you accelerating, and why are you comparing accelerating up a hill to a steady speed on a flat road? Why not accelerating on the flat road?

I understand, of course, that going up hill uses more fuel, but I also understand that accelerating uses more fuel, and conflating the two doesn't give a fair picture. In the real world (which is the sort of testing we were talking about) most people slow down a bit going up a hill, and speed up a bit on the way down - which is the opposite of what you're claiming.

If you maintain a steady speed then - frictional losses aside - you only need to burn enough extra fuel to balance the gain in gravitational potential energy - and you get that back on the way down. The frictional losses are going to be pretty much the same no matter the gradient.

To maintain a steady speed going uphill requires you to press the accelerator harder, which is accelerating by any other definition. The alternative is to slow down as you go up the hill and become a pain in the arse because you will immediately speed up again going down hill annoying any driver that may be overtaking you as you slow. 

And you you still use more fuel due to the "acceleration". 

hywelg

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. 

ICBM

It may not be phrased correctly, but the fundamental point is right - fuel consumption gets worse when going uphill by more than it gets better going downhill, on the same gradient. Not by much, but like the wind speed it does make a small difference that adds up over time.

Axe_meister

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.

VimFuego

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...

ICBM

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 .

VimFuego

it's odd, we call it the accelerator, but it activates a throttle. In the US they call is the gas pedal and despite their insistence on using the term gas for a liqued, they are (and this galls) more accurate than us.

Axe_meister

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.

hywelg

Axe_meister said:

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.

Again,  incorrect, acceleration (or deceleration) is defined as the rate of change of velocity with time. 

It is a product of the changing forces being applied.  Therefore if speed is constant acceleration equals zero. It is the force that is changing to compensate for the work being done to overcome the effect of gravity.