Safe Speed Forums

I wrote this on a questionaire for the low emmission zone consultation when it asked abut global warming; I'm much more concerned about local emmissions and their effects on the people in town rather than any global effect, but then look after the pennys and the ....

Yes, really.

Measurement has done over a long term period, I am up and down the motorway most days (a typical day will see 4 hours motorway driving, some days up to 12) and was done by comparing the trip counter to the amount of petrol I put in, and then resetting it ready to do the same. It's mainly an out of interest thing, I haven't kept logs, but even if I did you could easily argue that I'm lying about the speed I've done.

The car has an automatic gearbox and I believe it remained in 4th (ie. top gear) throughout the duration of both trips.

I have come up with two possible explanations for this behaviour.

1) At 80mph the car is doing approx 3000RPM, at this speed the primary turbo can actually have an effect, recycling energy lost in the exhaust stroke to aid getting air in to the intake stroke.

2) The torque converter may not lock at 60mph

I'd like to do some experimentation at 100mph as the opportunity doesn't arise that often (I am mainly UK based) at this speed (in 4th) the car does around 4.5K RPM and the secondary turbo is brought into play, this one is larger and more efficient, but requires more to get it going in the first place. Also the secondary turbo does not have a starter cat in it's downpipe whereas the primary one does, which will likely improve fuel consumption a bit too.

In all honesty I have no idea why, it could just be that the engine map is weird, that my car has an unusually low drag co-efficient and/or that the engine is just generally more efficient above 3000RPM.

A couple of point to add:

1. Removing the catalyst (a "de-cat") by itself won't do anything for CO2 emissions. It's true that a cat car runs a bit richer than a non-cat car because the cat needs a richer mixture to work properly. Unfortunately, if you just take the cat "brick" out of the exhaust eithout telling the engine management system, it will still carry on shoving in the same amount of fuel. (...not that I'm bothered but it's also illegal on most cars equipped with cats anyway)!

2. The whole concept of an "average car" is cobblers. In much the same way as people who go and buy a 120g/km CO2 car think that for every km they drive they're emitting 120g of CO2!

3. I've never come across a car that does better MPG at 80 than 60. Lum's is the first I've heard of. I go to Glasgow once a week up the M74 in a diesel MPV. I am satisfied with the accuracy of it's trip computer. Over a period of ten trips, I checked the average consumption for the motorway part of the journey (best part of 100 miles) and found that at a constant 70 it was about 7% better than at a constant 80. I've never tried it at 60 over that kind of distance but I have no reason to believe that it wouldn't be more economical still. In fact, the bst economy I've ever had out of it was a 70 mile run along the A75 (mostly NSL) following my (very timid) sister-in-law at a (very frustrating!) 40-50MPH. The car has 6 gears and I was in 4th-5th most of the time (never 6th). It gave a staggering 47MPG (compared with about 36MPG at 80).

For absolute maximum economy, I ought to leave it at home and walk - but the practicalities of having to earn a living mean that this ain't going to happen!

So, Weepj, I agree that making traffic travel at 60 on FREE FLOWING motorways would probably bring significant overall CO2 reductions but I very much doubt that the traffic would be free flowing if everyone was limited to 60!

i tend to travel around 60-65 on the motorways nowadays, and the fuel economy of my car has increased by around 8% as a result. the difference in journey times is relatively negligible.

What about the energy wasted in forcing the exhaust gasses through the cat in the first place, without that obstruction you don't need to burn quite so much fuel to achieve the same effect (ie. moving the car) in the first place?
Also it's not illegal to remove the cat, it's just a lot harder to meet emissions, and not meeting emissions is illegal. If you can meet emissions without a cat then you're sorted.

And yeah, I'm a bit puzzled about my results too, but then the whole 55mph = optimal speed thing has been widely discredited, so I suspect the most efficient speed will be different for any given car, so why can't it be 80 in mine?

But then - at 60 , hitting a hill would mean more throttle , perhaps even a change from 5th to 4th - no chance of looking ahead -seeing a hill coming up and lightly adding more speed to compensate - something the theorists don't allow for ,but then most of them have only held a licence for weeks .
I have just returned from a trip up north (approx 1800 miles) , and on each trip I try to see how much fuel I use - this time travelling a good bit faster than last time ,my consumption is only up by about 50 miles on a last trip figure of approx 500 .Time gained was about 2 hours - so time vs fuel was a worthwhile gain .

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lets bring sanity back to speed limits.
Drivers are like donkeys -they respond best to a carrot, not a stick .Road safety experts are like Asses - best kept covered up ,or sat on

But the point I was making is that people think that cats reduce CO2, because they are told that. Even in towns a cat car will produce less CO2. Never mind the HC/CO/NO, which are amply provided by the local bus company and J BLOGGS haulier.

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The world runs on oil, period. No other substance can compete when it comes to energy density, flexibility, ease of handling, ease of transportation. If oil didn’t exist we would have to invent it.”

56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing

In pretty much all modern cars I don't believe that the cat brick imposes a significant restriction - they just use it as an extra silencer, so removing it would need an additional silencer fitting to get the noise back within limits. In some cases (one manufacturer I worked for in particular) we used to get significant demand for empty cat cans. We tried this setup on the dyno and founf that the power output was slightly WORSE! We put this down to the gas entering the empty can and slowing down, then having to accelerate up again on leaving the can. We were, however, using metal substrate catalysts which impose a lot less resistance than the older ceramic types but I think most performance cars these days have them.

I'm pretty certain it IS illegal (in the UK anyway) to remove the cat on a car originally obliged to have been fitted with one (and possibly on ANY cat car). It's in the Construction and Use Regs somewhere. I think I dug it out and quoted it in a thread with Paul 1966 once but I can't remember which one and am a bit stuck for time now.

I absolutely agree that each car will have its own most efficient speed but as the resistive forces for ANY car go up with the SQUARE of the speed, that has a disproportionately large effect at motorway speeds. Petrol cars are much less efficient than diesels at part throttle so it's truer (I think) to say that with a petrol car, there will be plenty of speeds that are BELOW peak fuel economy but I think it is less true of diesel cars. Everything I have seen on the trip computer on my MPV leads me to believe that a constant 30MPH (even if that means 3rd gear all the time) would give even better consumption than a constant 40!

I saw a car this morning, driving at well under the max speed limit (probably no more than 10/15mph) get a very nasty scrape as it passed over a speed hump.

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www.thatsnews.org.uk / www.thatsnews.blogspot.com / http://thatsmotoring.blogspot.com/

There are very specific regulations governing speed humps. However - many installations breach these. If you find ones that are out of specification, the authority MUST put the matter right.

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I won't slave for beggar's pay,
likewise gold and jewels,
but I would slave to learn the way
to sink your ship of fools

No chance.
There are some on a "rat-run" in Rushden, very sharp edges lined with paving blocks and about 150mm in height. The amount of exhaust systems lying on the road tells the story.

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The world runs on oil, period. No other substance can compete when it comes to energy density, flexibility, ease of handling, ease of transportation. If oil didn’t exist we would have to invent it.”

56 years after it was decided it was needed, the Bedford Bypass is nearing completion. The last single carriageway length of it.We have the most photogenic mayor though, always being photographed doing nothing

There was a case about 15 years ago they tried out some large rounded speed humps in the new forest. A camper van turned over kiling a lad in the bunk above the cab. I believe the speed limit at that time on that road was 60 but it may have been 40.

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Speed limit sign radio interview. TV Snap Unhappy
“It has never been the rule in this country – I hope it never will be - that suspected criminal offences must automatically be the subject of prosecution” He added that there should be a prosecution: “wherever it appears that the offence or the circumstances of its commission is or are of such a character that a prosecution in respect thereof is required in the public interest”
This approach has been endorsed by Attorney General ever since 1951. CPS Code

I don't care anymore. My Tomcat is being prepped as a weekend/trackday car and my bike swivels neatly through the 'Satan's Arse Cheeks' dotted about our roads, gleefully farting out 45g km^-1 of CO2. Who cares? Methane from the rectums of farting vegetarian cyclists does 25 times more damage than the equivalent volume of CO2 and the rotting material in the rain forests accounts for nearly 2% of the methane in our atmosphere, whilst they only account for about 1% of the oxygen. Burn them all.
The oceans are the lungs of the planet, via phytoplankton and climate change is being recorded on all the bodies with atmospheres in the Solar System.

So send the Sun my bill, would you please? Me and my DragStar are too busy enjoying ourselves.

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Aquila



Licat volare si super tergum aquila volat...

Probably ten years ago I read of a couple of absolute maximum economy runs to demonstrate the efficiency of modern diesels - Audi A6 etc - and these were definitely at 38-40mph; but I also remember someone doing a 60mph run down through France and getting 800 miles out of a tank.

So there is definitely an optimum point below which you end up either straining more in a higher gear, or using a lower gear which is less efficient.

Obviously if you're driving a brick-like MPV then 80mph is pretty fast to be shoving it through the air; for a more aerodynamic car and with a more powerful engine I can see that it could be possible for 80mph in sixth gear to be more 'in the zone' than 70mph, and not too compromised by the aerodynamics.

The fuel economy in my Volvo V70 D5 Geartronic is 28mpg in term-time, doing local runs and commuting to work (11 miles Surbiton to Hammersmith, half dual-carriageway and half stop-start traffic); 32mpg in the school hols, doing the same speeds when able to, but not stop-starting in the same way; and 38/40mpg on a motorway run.

On a flat road at steady speeds the instant readout is something like 70-80mpg at 50mph, 60-70mpg at 60mph, 50-60 mpg at 70mph, 40-50mpg at 80mph and... no, I won't go any further.

Ian

i suspect there may be more than 4 times as many motorists as there are farting vegetarian cyclists

ah but add to that the farting vegetarian prius drivers, farting veggie pedestrians and farting veggie dont-go-anywhere to avoid pollution and there is a huge amount