Physics thread

rjsterry

As (entirely reasonably) requested. I'm thinking something like CS's answer to the XKCD What If... Blog. We can start with the effect of reducing a 30mph urban speed limit to 20 on fuel consumption or go for something else as we've flogged that one already.

bompington

... what, you mean a thread where every answer ends with "... and collapses into a massive black hole"?

Wheelspinner

I'll assume this is pointed at me since I started it... :-) Apologies for another long post.

@Pross very helpfully posted some links on the other thread. I appreciated the effort and went to take a look...

The "20splenty" report in the link opens with this table:



That shows for a (smallish) petrol engined car, driving "slower" (on the 20mph max sections) actually generated an increase in vehicle emissions of CO2 and NOx per km driven compared to the figures for the 30mph sections.

Wait, what?

CO2 emissions increased by 2.1% per KM, and the NOx almost an 8% increase.

Where do those emissions come from? I can't understand how that is possible, because AFAIK those emissions come from.... burning fuel. So, I am guaranteed to use less fuel if I drive slower (according to @rick_chasey) and simple physics tells me that is the case too. I think.

Hmm.

I figured I should look deeper, so in Footnote 1 of their article they reference the actual study commissioned by City of London, here:
https://cityoflondon.gov.uk/business/environmental-health/environmental-protection/air-quality/Documents/speed-restriction-air-quality-report-2013-for-web.pdf

It's 71 pages. Lots of statistical stuff. Plenty of things I will assume are correct, but what I was looking for was the data that 20splenty referenced.

On pages 49 and 50, there are two tables, and some text.

First, on p49, the data summary for petrol engined cars:



In a "lobby group cherry picks data to suit their argument" shocker, it appears that 20splenty actually quoted the least bad sample for petrol cars.

What this table shows is that in ALL engine size classes of petrol engined vehicles, driving slower actually INCREASED the "per KM" emissions of CO2 and NOx, by quite significant percentages too.

I am shocked I tell you. Shocked.

How can driving slower actually generate more pollutant gases per km driven?

I look forward to someone clearing that up for me, because that research report offered no explanation I could find.

Particulates are reduced (good) but it does note elsewhere in the report that this includes (maybe) such things as tyre wear and brake pad residue, which are lower with lower speed. Fair enough.

Even more weirdly in the text below the table on p50 they make this claim:

"In general it is concluded that it is incorrect to state that a 20mph speed restriction will lead to greater pollutant emissions for vehicles."

Given their own data on the preceding page appears to claim exactly the opposite at least for petrol engined cars, I am confused.

Help?

laurentian

I suspect the word "efficiency" will be part of the considered and calculated answer though I am unable to provide that answer

Darius_Jedburgh

If you are driving slower you may well be in a lower gear but using higher revs.

Just a thought.

morstar

There is an optimum speed where all of the varying losses through internal friction, rolling resistance and aerodynamic drag facilitate the best fuel efficiency.
It is widely accepted that this is around 56mph.
However, that’s cruising speed. Harder accelerating and decelerating then comes into the equation to obtain such speeds.
In a frictionless system it should all be equal. It isn’t a frictionless system.
I’d have thought all those stats are probably very laboratory tested. You’d need to put analysers on thousands of cars being driven up and down real streets with all the random stops and starts plus creative driving techniques employed in order to get real world data.

Pross

Wheelspinner said:

The "20splenty" report in the link opens with this table:



That shows for a (smallish) petrol engined car, driving "slower" (on the 20mph max sections) actually generated an increase in vehicle emissions of CO2 and NOx per km driven compared to the figures for the 30mph sections.

Wait, what?

CO2 emissions increased by 2.1% per KM, and the NOx almost an 8% increase.

Nice try, you accuse the 20s Plenty site of cherry picking (I linked to it as it had links to the research behind the figures that I thought would be useful) then do the same to prove it wrong. However, if you take a closer look you'll see using diesel made a lot more sense as the NOX levels are about 10 times greater.

I haven't been able to find a split of petrol to diesel vehicles on UK roads but let's assume it's 60% petrol to 40% diesel (I think it's closer to 50/50 but sales of new diesel are dropping sharply). Let's take an identical sample of 100 vehicles travelling 100km at 20mph and 30mph respectively on the basis of that 60/40 split. The total NOX at 30mph would be 3645.4g whilst at 20mph 3410.4g over the 100km. That gives a reduction of 235g or 6.4%.

You already admitted that particulates are lower at 20mph but again diesel is more than double petrol. CO2 emissions are higher at 20mph but it's pretty marginal.

Even if you insist on using the petrol figures you're a hell of a long way off your initial claim that reducing speed by 50% increases journey time by 50% and therefore emissions by 50% that the others questioned.

veronese68

Whilst studying for my physics A level there was a girl that thought if she drove everywhere as fast as she could she would use less petrol as it took less time. I tried to explain why that didn’t work but she wouldn’t have it.
She failed, I scraped through.

Wheelspinner

Pross said:

Wheelspinner said:

The "20splenty" report in the link opens with this table:



That shows for a (smallish) petrol engined car, driving "slower" (on the 20mph max sections) actually generated an increase in vehicle emissions of CO2 and NOx per km driven compared to the figures for the 30mph sections.

Wait, what?

CO2 emissions increased by 2.1% per KM, and the NOx almost an 8% increase.

Nice try, you accuse the 20s Plenty site of cherry picking (I linked to it as it had links to the research behind the figures that I thought would be useful) then do the same to prove it wrong. However, if you take a closer look you'll see using diesel made a lot more sense as the NOX levels are about 10 times greater.

I haven't been able to find a split of petrol to diesel vehicles on UK roads but let's assume it's 60% petrol to 40% diesel (I think it's closer to 50/50 but sales of new diesel are dropping sharply). Let's take an identical sample of 100 vehicles travelling 100km at 20mph and 30mph respectively on the basis of that 60/40 split. The total NOX at 30mph would be 3645.4g whilst at 20mph 3410.4g over the 100km. That gives a reduction of 235g or 6.4%.

You already admitted that particulates are lower at 20mph but again diesel is more than double petrol. CO2 emissions are higher at 20mph but it's pretty marginal.

Even if you insist on using the petrol figures you're a hell of a long way off your initial claim that reducing speed by 50% increases journey time by 50% and therefore emissions by 50% that the others questioned.

There’s a joke that says if you hire a consultant to tell you the time, the first question they ask is “what time do you want it to be?”

This study reads a lot like that - they were commissioned to provide evidence that reducing speed limits would be a good thing from emissions perpsective. Just my view, obvs.

Actually, I read through most of it. That was done in 2013, and 20splenty notes that diesels comprised 37% of passenger cars in UK. No idea whether that climbed sharply to your guess of 50/50 in the interim but I would also expect the sales to be dropping as you note since they are to be banned anyway. Has it dropped sharply from 37 or 50 then?

And a side note even the larger engined diesels generated marginally more emissions at the lower speed. Would assume that’s a small proportion of them, but in the interests of not cherry-picking data it’s worth noting. 😀

So, my first claim: reducing average speed from 30 mph to 20 will cause a 50% increase in travel time.

Data:
If I drive 10 miles at average speed 30mph, it takes 20 minutes.
If I drive 10 miles at average speed 20mph, it takes 30 minutes.
That’s an additional 10 minutes, which means 30 is 50% longer than the original 20 it took me.

I don’t care if you use physics, maths or voodoo to do your calculations; if you get a different answer you’re wrong.

My second claim was that driving slower as above would make you use more fuel, between 30 and 50% maybe! I was apparently (possibly quite) wrong with my guess of how much. I used my car, a 3 litre petrol engine, for reference. I was told that simple physics would prove that was *not even possible*. I was guaranteed I would use less fuel because reasons.

And yet, right there in the opening summary of the report, in bullet point #6 on page 7, they say...

“CO2 emission factors follow the same pattern as NOx, although with smaller percentage changes, demonstrating increased fuel consumption when travelling at lower speeds”

Even if they forgot to say that applied to petrol and large diesel only, it still applies.

So I was correct that slower can mean worse fuel consumption. I just maybe got the percentage wrong, and I don’t own a small diesel car, ... but..

OK, I got two out of 3 correct. Sorry. Now...

Using the summary data in the report, that means for somewhere between 63 and say 70% of vehicles (based on that 37% are diesels and some will be more than 2 litre ones), the result of the lower speed limit is they produce more CO2 and NOx emissions per km. Am sure they’ll appreciate that. Drive slower, pay more.

I don’t know (or care to be honest) whether the NOx of CO2 change is a more accurate indication of the change in fuel quantity burnt, but two of the four petrol classes were over 7% increase in CO2 which is I think the more accurate measure as a by product of combustion. Is that a direct reflection of fuel consumption? Doesn’t say.

If you read further you find the analysis excluded a whole lot of data, and instead was only comparing emissions data for the segments where vehicles were in “cruise mode”, i.e. not sitting idling, nor accelerating or decelerating.

Further still, the average speeds for the segments used in the comparison are rather less than you think. For segments in the 20mph zones, the average speed was 15 mph. For segments in the 30mph zone, the average was 18mph.

So, go back to the increased emissions data again, and see that for at least two classes of petrol cars, cutting your average speed by just 16% (from 18 down to 15) leads to increased CO2 emissions of over 7%. That correlates to fuel consumption?

It’s a pity they didn’t model actual impact of 30 down to 20. I wonder how different the result would look? Better or worse?

Now look at the data in there which records time spent in each speed profile zone, and note that for vehicles in the 30mph limits, the average time spent sitting idling (speed = 0) was 17%. For vehicles in the 20mph limit zones, the average was 23.3%.

So, lower the speed limit and you get to spend 35% MORE of your time sitting there idling. Result!

An interesting side note in that is the later attempt to assess whether lower speed limits would change the brake and tyre wear factor. The conclusion was weak - drive smoother (not necessarily slower) and the impact is less. Duh. They ignore the fairly obvious point that sitting there idling going nowhere - now increased on average by 35% - requires you to sit with your foot on the brakes, and lots of trickle stop/start stuff. But let’s not even mention or consider that in the brake tyre wear discussion.

After all that, remind yourself the emissions data excluded any segments where a vehicle is sitting idling, or accel/decel, even though the report notes these are especially significant in terms of emissions performance. But let’s ignore them anyway.

Also for full disclosure, that City of London document on p50 notes that the analysis excludes any allowance for the other impacts that the lower speed limit may cause such as “reduced flow” (congestion and route diversion). Presumably because including that might make the data look worse still because the amount of time people spent idling and stuck there, or turning off and driving further. That’s really going to help, isn’t it?

Even more convincing is the other bit where it notes that some of the measures used to reduce speeds (“traffic calming” ha!) such as speed humps or cushions actually significantly *increase* the emissions because they cause much more stop/start accelerations, and increased brake and tyre wear emissions.

However that data was excluded from the comparative emissions analysis too because it was difficult to quantify. To do so would require (presumably) removal or installation of the speed humps etc.

The bit I really like - a genuine LOL moment - was this statement on page 50, para 2:

“In general it is concluded that it is incorrect to state that a 20mph speed restriction will lead to greater pollutant emissions from vehicles.”

The two data tables 19 and 20 immediately preceding this actually show that in 4 out of 6 vehicle classes studied that the reduction in speed does exactly that - increase some pollutant emissions for CO2 and NOx!!

@Pross I didn’t choose this report - you did. Even it’s authors say in the opening summary on pages 7 and 8 that the results were “mixed” from an emissions perspective. As a (retired) consultant of more than 25 years, I’d suggest that is a fairly euphemistic term for “not really valid conclusion”, but YMMV. I am a cynic and sceptic about consulting reports in general. They do make a nice pitch for more work though late in the piece.

There are some seriously confusing data discrepancies, odd choices of data charts to show, and then text which seems to contradict the pictures. The Units of measure are inconsistent between mph and kmh. In short, it screams cut and paste filler, to be blunt, but hey, I only wrote this sort of stuff for a living for years.

It’s more than 35 years since I racked up Distinctions in maths and physics at school and University, and I left Engineering immediately on finishing the degree because the reality of it as a career bored me senseless. So, I’ve likely forgotten or got stuff wrong. My maths is still good though. 😀

if this is the standard of research that you would base literally tens (or hundreds) of millions of pounds work on changing roads infrastructure on, with the resulting impact on overall benefit described as “mixed”, good luck. Colour me unconvinced.

Pross

I lost the will to live after the first paragraph but here are the numbers I found for relative percentages of new cars registered from 2013-2018. The rate steadily dropped from over 50% and dropped sharply in 2018 due to new legislation on emissions and talks of diesel cars being banned from cities.

https://www.statista.com/statistics/299031/fuel-types-of-new-cars-registered-in-the-united-kingdom/

As the average age of a car on UK roads is over 8 years old I would say my 40/60 split was conservative.

Do you want to go and move the goalposts again?

By the way, I didn't say you chose the report I said you accused the 20s Plenty interpretation of cherry picking then cherry picked yourself. If you take cars on the road holistically it seems reasonable to say that reducing speeds from 30 to 20 doesn't increase pollution and that there's nothing hypocritical for environmental campaigners to support lower speed limits which was your original claim.

Pross

Oh, and I had already acknowledged the impact of traffic calming and that the numbers assume smooth driving. It's one of the reasons that traditional traffic calming (humps) are less common these days. On new build residential roads the aim is to keep speeds low by a layout that makes driving faster feel uncomfortable although it's not easy when you also have to design to allow for a monster refuse vehicle that wouldn't be able to travel down the existing roads to reach the site.

I also originally said that getting people to drive at 20mph simply by lowering the speed limit is impossible and I suspect the aim is often just to encourage some reduction in speed (as well as the reduction in pollution 😉).

rjsterry

Worth pointing out that pollution is only one of the reasons for advocating reduced speed limits in urban environments

veronese68

rjsterry said:

Worth pointing out that pollution is only one of the reasons for advocating reduced speed limits in urban environments

This, in a 30 lots of people are doing 40. Make it a 20 and they tend to be closer to 30.
I admit I find it difficult to keep to 20 in the car, but I do enjoy the 20 limits when I’m on my bike.

Wheelspinner

Pross said:

I lost the will to live after the first paragraph but here are the numbers I found for relative percentages of new cars registered from 2013-2018. The rate steadily dropped from over 50% and dropped sharply in 2018 due to new legislation on emissions and talks of diesel cars being banned from cities.

https://www.statista.com/statistics/299031/fuel-types-of-new-cars-registered-in-the-united-kingdom/

As the average age of a car on UK roads is over 8 years old I would say my 40/60 split was conservative.

Do you want to go and move the goalposts again?

By the way, I didn't say you chose the report I said you accused the 20s Plenty interpretation of cherry picking then cherry picked yourself. If you take cars on the road holistically it seems reasonable to say that reducing speeds from 30 to 20 doesn't increase pollution and that there's nothing hypocritical for environmental campaigners to support lower speed limits which was your original claim.

I'm impressed you got that far

In all honesty, I'm not that fussed what policy is in place in any large city anywhere these days - I quite happily left a big city, a decision influenced to a large degree by the fact the traffic was already appalling and only going to get worse.

The only thing in that whole report that *really* stood out to me as dodgy was the fact they ignored the impact on emissions of the time spent idling - going nowhere. I can appreciate that it's difficult to test and quantify the impact of acceleration and deceleration as that's dependent on whose foot is on the pedal, but their GPS data seems quite accurate for time spent standing still.

And that's what makes me think the conclusions are both biassed and invalid.

The chart on page 47 (Fig 28) shows that for diesels in that Euro IV 1.4 - 2.0 L class in particular, as the speed drops to very low, the emissions factor for the NOx climbs fast then really skyrockets. In the range of speeds that the data set considers (average 15 to 18 mph) this curve still shows an increase at the lower speed, yet the data presented in the tables later shows NOx for this class of engine as being *lower* at the lower speed.

That does not make sense?

And had they included emissions for the time spent "idling" the diesel gains posted for the reduced speed might well have been completely wiped out by the increase in idle time, and the report would have shown no emissions benefit at all.

Anyway I'll shut up. I promise. In the spirit of the departed Frenchie...

rick_chasey

If the issue is idling cars why are you bellyaching about reducing the speed limit from 30 to 20?

Wheelspinner

rick_chasey said:

If the issue is idling cars why are you bellyaching about reducing the speed limit from 30 to 20?

You do realise an idling car still produces emissions, don't you? Even your Polo?

Pross

Wheelspinner said:

rick_chasey said:

If the issue is idling cars why are you bellyaching about reducing the speed limit from 30 to 20?

You do realise an idling car still produces emissions, don't you? Even your Polo?

I think the point Rick is making is an idling car is an idling car and therefore not related to the relative speed of travel.

Personally I think that widespread 20mph zones are unrealistic on most existing roads but as RJS said they will hopefully mean the majority of drivers drive below 30mph.

I do think at some point that engine management systems will be linked to either GPS or smart systems that read speed signs and will limit speeds automatically though. We already have 'softer' versions of the technology in cars so that the driver gets a warning they are exceeding the speed limit.

Wheelspinner

Pross said:

Wheelspinner said:

rick_chasey said:

If the issue is idling cars why are you bellyaching about reducing the speed limit from 30 to 20?

You do realise an idling car still produces emissions, don't you? Even your Polo?

I think the point Rick is making is an idling car is an idling car and therefore not related to the relative speed of travel.

Personally I think that widespread 20mph zones are unrealistic on most existing roads but as RJS said they will hopefully mean the majority of drivers drive below 30mph.

I do think at some point that engine management systems will be linked to either GPS or smart systems that read speed signs and will limit speeds automatically though. We already have 'softer' versions of the technology in cars so that the driver gets a warning they are exceeding the speed limit.

There's already more intrusive systems in use than that. I did some work for a mining company which had data loggers in the company cars, with a key fob you had to plug in to start it. If the system decided you were an "unsafe" driver it locked you out altogether.

One guy new to it took a car to a production site several hours drive away. On the way he apparently broke a couple of speed limits for longer than he was allowed, didn't stop often enough for rest breaks, and was a bit hard on the throttle and brakes. When he went to *leave* the work site the car would not start - he was already reported by the data logger via a 4G phone update in the background and was stranded out there for 2 days till his manager persuaded OH&S to unlock his ID.

Darius_Jedburgh

veronese68 said:

I admit I find it difficult to keep to 20 in the car, but I do enjoy the 20 limits when I’m on my bike.

I find 20 mph limits on my bike a challenge. Either to get up to it, or to keep down to it.

Stevo_666

Driving at 20mph is really boring.

I prefer slightly more esoteric physics - was reading an article the other day about neutron stars and their extraordinary physical properties - like how they are so dense that a teaspoon full of neutron star material would weigh a billion tonnes. And their gravity is 2 billion times that on earth.
https://space.com/22180-neutron-stars.html

And some rotate so fast that the equatorial speed is around 25% of the speed of light.

Mind blowing stuff if you like that sort of thing.

TheBigBean

I know Physics is not renown for its riveting discussions, but I had higher hopes for thread.

morstar

But at least it removed a very OT discussion from where it was polluting another thread.
Maybe we need an Interesting physics thread.

rjsterry

So we have reduced pollution

I too was hoping we would move on from speed limits.

rjsterry

Stevo_666 said:

Driving at 20mph is really boring.

I prefer slightly more esoteric physics - was reading an article the other day about neutron stars and their extraordinary physical properties - like how they are so dense that a teaspoon full of neutron star material would weigh a billion tonnes. And their gravity is 2 billion times that on earth.
https://space.com/22180-neutron-stars.html

And some rotate so fast that the equatorial speed is around 25% of the speed of light.

Mind blowing stuff if you like that sort of thing.

Funnily enough I had just looked up whether it was practical for human action to alter the rotational speed of the Earth.

Pross

TheBigBean said:

I know Physics is not renown for its riveting discussions, but I had higher hopes for thread.

Nothing stopping you starting something about the creation of the Universe or Newton's Laws if you want 😉

Stevo_666

rjsterry said:

Stevo_666 said:

Driving at 20mph is really boring.

I prefer slightly more esoteric physics - was reading an article the other day about neutron stars and their extraordinary physical properties - like how they are so dense that a teaspoon full of neutron star material would weigh a billion tonnes. And their gravity is 2 billion times that on earth.
https://space.com/22180-neutron-stars.html

And some rotate so fast that the equatorial speed is around 25% of the speed of light.

Mind blowing stuff if you like that sort of thing.

Funnily enough I had just looked up whether it was practical for human action to alter the rotational speed of the Earth.

And the answer was....?

The other neutron star weird bit of info I Iiked was that if you could drop something from 1m above the surface it would hit the ground at 1,400km/sec. Falling over could be painful...

Stevo_666

Pross said:

TheBigBean said:

I know Physics is not renown for its riveting discussions, but I had higher hopes for thread.

Nothing stopping you starting something about the creation of the Universe or Newton's Laws if you want 😉

I'm trying to nudge it in the right direction without being too techy?

rjsterry

Stevo_666 said:

rjsterry said:

Stevo_666 said:

Driving at 20mph is really boring.

I prefer slightly more esoteric physics - was reading an article the other day about neutron stars and their extraordinary physical properties - like how they are so dense that a teaspoon full of neutron star material would weigh a billion tonnes. And their gravity is 2 billion times that on earth.
https://space.com/22180-neutron-stars.html

And some rotate so fast that the equatorial speed is around 25% of the speed of light.

Mind blowing stuff if you like that sort of thing.

Funnily enough I had just looked up whether it was practical for human action to alter the rotational speed of the Earth.

And the answer was....?

The other neutron star weird bit of info I Iiked was that if you could drop something from 1m above the surface it would hit the ground at 1,400km/sec. Falling over could be painful...

170bn Saturn V rockets to generate the required force, so, err, no.

I'm making my way through Season 1 of The Expanse at the moment, and one of the nice details is that people who have grown up living in the Asteroid Belt (i.e. in a low gravity environment) can barely tolerate Earth's gravity.

Charlie_Croker

Wheelspinner said:

…One guy new to it took a car to a production site several hours drive away. On the way he apparently broke a couple of speed limits for longer than he was allowed, didn't stop often enough for rest breaks, and was a bit hard on the throttle and brakes. When he went to *leave* the work site the car would not start - he was already reported by the data logger via a 4G phone update in the background and was stranded out there for 2 days till his manager persuaded OH&S to unlock his ID.

It’ll be when insurance companies can acquire and use this data to set their premiums that you’ll see speed limits adhered to more often. However there’s always an exception to the rule.
Sorry not physics so OT

TheBigBean

Pross said:

TheBigBean said:

I know Physics is not renown for its riveting discussions, but I had higher hopes for thread.

Nothing stopping you starting something about the creation of the Universe or Newton's Laws if you want 😉

There should be a sticky on Newton's greatness.

briantrumpet

TheBigBean said:

Pross said:

TheBigBean said:

I know Physics is not renown for its riveting discussions, but I had higher hopes for thread.

Nothing stopping you starting something about the creation of the Universe or Newton's Laws if you want 😉

There be a sticky on Newton's greatness.

I just wish I was clever enough to understand quite what a genius Richard Feynman was.