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Author Topic: Emissions  (Read 4860 times)
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peterbaker
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« on: 04 October, 2007, 02:42:32 PM »

The government wants lower speed limits because reduced speed equal less emissions. How do they know that? Combine a slower running engine with a longer journey time and surely the emissions must rise? Everybody knows high revving engines run cleaner and cats are more efficent, so this, allied to shorter moments of atmospheric exposure must win the day. Or have I missed something? Another thing. Which vehicle is better for the enviroment. A Prius driven from London to Brighton with just the driver or a integrale, five up?   
« Last Edit: 04 October, 2007, 02:51:14 PM by peterbaker » Logged

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« Reply #1 on: 04 October, 2007, 06:54:51 PM »

Typical Government approach, they don't actually know as half of them don't drive (Just like Ken Livingstone), but it sounds like an awfully good idea Angry why bother with facts, when you can go by gut feelings.
See in America Toyota are being sued as owners can't achieve anything like the claimed fuel consumption figures for the Prius.

Not to do with cars but just had a typical case around here, a public Enquiry costing £150,000 was held recently,  Centre Parcs had appealed against a project being turned down, enquiry agreed with the councils decision to reject; Hazel Blears, the Minister, has now overridden that decision, why bother spending that sort of money, when a minister can second guess a decision Huh?

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« Reply #2 on: 04 October, 2007, 09:59:54 PM »

Well, its just mpg really.  If you do a journey at 53 mpg (my return trip to Guildford today) you will produce less emissions than if you do the same journey at an average of 35 mpg.  I was doing about 70-80mph on the motorways and had a relatively clear run until I hit the M42 on the return leg - so I was able to run at steady speeds.  Had a I decided to travel at such illegal speeds that my mpg fell to 35 (tough in a turbo-diesel), then I might well have got there quicker (though perhaps less safely, and with serious risk to my licence).  It follows that if I had travelled at 56 mph, I may well have returned a higher mpg still, and hence reduced my emissions further for the whole journey.  Its ultimately a trade off between the convenience of a shorter journey time and emissions.  I understand that the journey is most efficient in emissions terms at a steady 56 mph - much below this and you become less efficient per mile travelled - for example at 30 mph you will almost certainly not be in the highest gear with your engine running at its most efficient speed.  The other issue is of course congestion - stop start or low speed crawling in traffic is not efficient, steady speed running is far more so - hence the theory that the speed control measures that I encountered today on the M25 and M42 encourage steady speeds rather than stop-start (I've tended to find it works until the traffic becomes so heavy that even a steady 40 is a pipe dream - its also less stressful).

There is logic to the lower speeds, lower emissions argument, to a point.  Its when issues of convenience, journey time and personal freedom are brought in that it becomes less clear cut.  The best advice is to drive a more fuel efficient car when you can  - 53 mpg feels good for the wallet and the soul.
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Chris Owen
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« Reply #3 on: 04 October, 2007, 11:29:58 PM »

You have been brainwashed. 56mph relates directly to speed versus fuel consumed, based on an incremental focused document issued by the government as far back as 1967. Conditions have changed, for example diesel powered vehicles now account for 72% of all moving transport. In carcagenic terms this means slower moving transport, certainly in built up areas giving off unpresidented pedestrian damaging particles ( breathing disorders) as that politically paid scientists are not allowed to challenge as the whole freight and passenger infrastructure would come to a standstill if the facts were made public. New generation petrol engined cars are now so clean the air that is emitted from their exhausts is cleaner than the air consumed. In London, and in Oxford Street in particular, there is a continual queue of diesel buses and taxis moving at less than 2mph average because of pedestrian traffic continually crossing the road. At these speeds a bus is averaging no better than a Boeing 737 ( 100 litres of kerosine per 500kms) cruising at 35,000 ft carrying 138 passengers. You may have read that Ken Livingstone has ordered that taxis now have to fit a a catalyst at a cost of £250 per vehicle, laugh is it only brings them up to Euro 3 standard, a level exceeded by Ford and their competitors five years ago. In pure enviromental damage terms it costs less to pump a barrel of oil and refine it into petrol than it does to extend the process to manufacture the equivilent amount of diesel fuel. And there is still no limit to the amount of oil yet to be discovered. Its cheap, even at todays record prices of around 80$ Brent crude.
What ever anybody says, at the moment, a mass produced four cylinder petrol engine is much cheaper to produce than its diesel rival and still provides the best compromise, it is very cheap to produce, cheap to service, and now gives around 45mpg against a nominal advantage of 55mpg, on average although new light, low capacity, turbo charged petrol units now promise 70+. Finally, end of life dismantling costs are much lower than any hybrid fitted with complicated and intricate electric motors.  But whatever the method of propulsion, the faster a car is allowed to travel, the less pollution it creates.
Interesting discussion. 
« Last Edit: 04 October, 2007, 11:37:04 PM by peterbaker » Logged

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« Reply #4 on: 05 October, 2007, 01:18:09 PM »

I don't think I was making any case for petrol vs diesel, and certainly not for hybrids.  And I think what you say about London traffic simply backs up my comment about congestion.  But 'the faster a car travels, the less pollution it creates' I'm afraid flies in the face both of all statistical evidence, and fundamental physics.  Any car, petrol or diesel, at steady speeds uses more fuel and produces more CO2 for a given distance travelled the faster that steady speed (above the most fuel efficient speed) - its exacly the same for any form of transport, trains and buses included.  Moreover the relationship is not linear - increasing speed requires more fuel than a linear relationship would imply.  Its exactly the same physics that makes it impossible for space travel at the speed of light incidentally - the energy requirements for such speeds are to all intents and purposes infinite (sorry, this is rocket science  Wink ).  Certainly it is true that up until an optimum speed, which will differ from car to car, but is typically in the mid-fifties mph, going faster is more fuel efficient.  That's part of why town driving is so fuel inefficient - low speeds, lots of fuel used to speed up, lots of energy lost slowing down again.

On the other pollutants it all cuts every which way.  Some older diesels are very dirty (PM10s), newer ones with particulate traps etc are considerably less so - hence the value of forcing progress from all manufacturers on standards via Euro IV, Euro V.   If petrol engines were more fuel efficient than diesels I would likely be driving one - so if new technology reverses the current situation, I'm sure we'll all be back to petrol (it smells nicer for a start...).  But bear in mind that my company car tax is higher in relative terms because I drive a diesel - there is a 2% penalty on diesel versus petrol - and a considerable argument as to wether than penalty should apply to Euro IV/V compliant vehicles.  That is a response to the fact that diesels are dirtier on particulates.

There is a weakness in policy in addressing all the environmental costs of human activity - the whole life costs of a vehicle (taking into account the energy used in manufacture) and the costs of refining particular fuels for example.  So it is not necessarily bad to drive an older, relatively less fuel efficient car if mileage is relatively low, and the environmental savings on manufacturing a replacement exceed the environmental costs of emissions.  But its a long way from there to justifying driving anything simply because you can make an argument for almost anything.  In environmental terms, if you're driving an older car, its still best to drive a relatively fuel efficient one with a small, efficient engine.  As Lancia fans, with most of our cars with small, well-engineered engines we are at least better placed to justify ourselves than those driving Jensens and Rover P6 3.5s...

There is a finite limit to the oil supply, we just don't know what it is.  The real issue is not how much there is, but at what point we start producing less rather than more each year, against a background of ever-rising demand - so called peak oil.  When that happens, we are in for one very big economic shock, because the cost of oil will then skyrocket.  Before then we need to find replacements, and biofuels aren't it, because there simply isn't enough capacity on the planet to produce all the feedstock we would need to produce enough biofuel to replace current, let alone future, fossil fuel use.

The problem with air travel is not just the efficiency or otherwise of the energy consumption of the planes, but where they dump their pollutants - they do a lot more damage at 30,000 feet than they do at ground level.

And I've not been brainwashed by anybody - I just happen to disagree with you on some issues, and can very well justify my case  Tongue  I work in public policy research - I'm paid to think hard and critically about this stuff.  If the facts change, so will my view.
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Chris Owen
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« Reply #5 on: 05 October, 2007, 02:29:59 PM »

Chris,
My aprilia possess infinite power and therefore can approach and pass the speed of light on occasions.It must do because sometimes when I drive at night, everything suddenly goes dark so I guess I must have overtaken the main beams. Wink

More seriously,it's an interesting discussion and it always amazes me that such a fundamental aspect of motoring is still open to debate.Other factors can also outweigh the diesel/petrol discussion though. Running with low tyre pressures can have a significant impact as does a roof rack or open window.The world is not an ideal one.
Anyway, and in laymans terms , I always considered that a given car would be most efficient in terms of speed and economy when it was at the revs where it develops maximum torque in top gear.Is that an approximation to the truth or just an old fashioned misunderstanding.?
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« Reply #6 on: 05 October, 2007, 03:01:50 PM »

Great answer. But surely the search for alternative fuels is driven (sorry) not by fear of  exhaustion but by possible political blackmail from major suppliers following their own ideological agendas. Any disruption of supplies from the Middle East poses an instant and real threat to the Western economy. Already we see the newly formed, thinly disguised power centre created between Russia, India and China pushing up the price of raw materials and as result, the appearance on the world stage of an alternate and powerful trading group to dilute the status of North America as the worlds dominant peace negotiator. Gaining five miles per gallon is a waste of time if our friends in the middle east change sides.
Enjoying this but have to go back to work now
« Last Edit: 05 October, 2007, 03:11:47 PM by peterbaker » Logged

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« Reply #7 on: 05 October, 2007, 08:23:18 PM »

Peter - both I think.  Peak oil and energy security are both perceived as threats - but I think the former has the greater potential.  They will both get wrapped up with each other anyway.  As oil becomes scarcer, so will the power associated with controlling supplies become greater.  We're having wars about oil already - I hardly need point out what the future may hold.... Undecided

Senor Scarpia - I think you are right, but it would take an engineer to confirm it.  If you drive a car with a fuel economy meter in it it seems to be the case that the best fuel economy is achieved at the speed at which that car is comfortably able to pull in its top gear - in sixth in my diesel that equates to around 55-60 mph.  Its probably nearer 50 in my Delta, perhaps even slightly slower than that.  its not entirely without logic that the official fuel consumption of cars used to be measured at a steady 56mph, amongst others.  An artificial situation in the real world maybe, but the speed at which most cars, in ideal conditions, are returning close to their best mpg.
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Chris Owen
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« Reply #8 on: 05 October, 2007, 08:56:02 PM »

Quote
giving off unpresidented pedestrian damaging particles


Unpresidented or unprecedented?  Do I detect a Freudian slip?

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« Reply #9 on: 06 October, 2007, 08:08:37 AM »

I say, well done.
anyway, I thought "unprecedented pedestrian damaging particles" result from wheel spinning lancias on loose gravel
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« Reply #10 on: 06 October, 2007, 12:23:51 PM »

Chris, you select the Rover V8 as an example of prime gas-guzzler. In fuel consumption terms this is generally correct, although there is an argument that a large capacity engine can use less fuel under certain circumstances ie, prolonged hill climbing, where a small city neutral vehicle will be suffering (as well as incredibly boring) in its lower gears. Vario, Daf style transmissions are particularly bad in these conditions. Also, the more 'lazy' and powerful engine when in daily give and take will far outlast a small high revving alternative, and as we know, full life costs are included in the 'green package'.
Coming back to the V8, I would like to mention the wonderful Chevrolet Corvette, good value and capable of very high speeds and at first reaction, not the car for earth huggers. However, and to show you can put forward a strong case to protect out and out fun motoring, the Corvette is powered by a V8 that first saw the light of day back in 1954. It is a bullit proof and very basic, utilising cast iron. In other words, millions are put together without incorporating modern weightsaving but costly options. I venture to say, that in the States, a V8 of this type can be produced at half the cost of a Prius option. Especially when you take into account the number of man hours needed in construction and the lack of complicated production tools dedicated to that one model. And I have not yet mentioned, replacement part cost and end of life dismantling of electric motors etc. Not for nothing did CNW Marketing Research put the American Jeep Wrangler top of their list. Finally on this point, in America, employees travel less mileage to work than their friends in Japan, share there cars and eat on site. Oh yes, if I may say, the bulk of V8 powered cars are made, sold and used in the country of manufacture. Unlike the Japanese companies who export 80% of product and therefore  need to incorporate the shipping aspect. Container ships are expensive to rent, spend a lot of time in port with power running and are the biggest polluters of the oceans, leaving rubbish wherever they go. But that is another story.   
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« Reply #11 on: 06 October, 2007, 01:36:48 PM »

Peter ,
utter and sustained nonsense of the highest order ! One presumes you are also a member of the flat earth society? Wink

After this impossibly broad series of sweeping generalisations it's difficult to know where to begin but I would be interested to hear the independent support for the assertion that Americans travel less far in their cars than the Japanese to go to work.The Japanese are probably one of the outstanding users as a nation of good coordinated public transport.The Americans are not famous in this regard , which to be fair to them is probably due to the low cost of fuel historically in their home market. That's however the only reason that their cars, which in design terms date from Conan Doyle's lost world, have been able to survive.
I hear there's an environmental study due out from a Bush family sponsered environmental think tank .It carries the snappy title "Global warming, just do it..".

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« Reply #12 on: 06 October, 2007, 02:16:34 PM »

I may be wrong about the journey times but you have to admit, the Americans put less energy into everything they do compared with the Japanese
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« Reply #13 on: 06 October, 2007, 05:33:14 PM »

Forgive me for coming late on this thread, but I have been busy and I hesitated to make a post that might be accused of being a personal attack on the originator. Peter claims nothing less than “whatever the method of propulsion, the faster a car is allowed to travel, the less pollution it creates”. What a breathtaking claim! He then goes on to use all sorts of technical terms to suggest he knows what he is writing about and even suggests that someone who challenges this is “brainwashed”. Oh dear.


Let’s look at reality:

Consider for example an Integrale poodling along at say 60 MPH on some smooth flat road. The engine will not be working very hard – all it has to do is overcome the rolling resistance in the wheel bearings, the flexing of the tyres (both of which are trivial) and most significantly, the drag effect of pushing the car through the air. Overcoming drag accounts for between 60% and 90% of the power output in these circumstances.

Now the unforgiving thing about drag is that it is related to the SQUARE of the speed. This means that for an increase in speed by a factor of two, say from 30 to 60 MPH, the drag goes up by a factor of four. Similarly, for an increase from 30 to 90 MPH, the drag increases by a factor of nine. So, the engine has to do about nine times as much work to push a car through the air at 90 MPH as it does at 30 MPH. This difference will be seen in a proportional increase in fuel consumption and hence emissions.

This explains why having a low drag coefficient is a good thing for attaining high speeds and greater efficiency. But, even with a low coefficient you can’t shake off the reality that drag is disproportionately punishing at higher speeds. If anyone doubts this, simply remember the difference between cycling into a 20 MPH head wind as opposed to cycling downwind.

Sorry mate, there is no getting away from it – whatever benefits you might think you gain from shortening your journey times by driving faster, you are going to lose out in increased fuel consumption and hence greater emissions.

Colin
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« Reply #14 on: 06 October, 2007, 08:00:39 PM »

Hi Colin,

OK OK from my moronic point of view I need to ask
how come at 16 - 25 mph in town I get 17 MPG and yet at 75 - 80 mph (not in town) I get 31.5 MPG ??

Geoff
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