Reducing emissions?

[iCraig]

Originally Posted by AboveTheSalt

You're American . . . I hadn't realised

What's being American got to do with it?

[phykell]

Nevertheless, I think it's very useful knowing how much fuel you're using. You can be on the motorway, with your foot resting too heavily on the pedal, you're not really accelerating, but simply using less pressure doesn't drop your speed necessarily, but it can make a big difference on your fuel. It's quite an eye-opener for me TBH - I've never had a fuel computer before.

[LeperousDust]

Less pressure doesn't drop your speed? How in gods name do you work this out? Surely thats as simple as an energy in energy out thing. More fuel means faster essentially (or obviously i'd have hoped). Otherwise you're breaking a fundamental law surely

[iCraig]

I can understand physically moving the pedal off a tiny bit might make you stay at the same speed, but this is just a bit of play in the accelerator surely and tiny movements not actually influencing the engine. If you're literally reducing the amount of fuel being burned, the engine won't be performing the same? You can't just reduce one element of the equation and have the same result?

[Zirax]

Well I do try to save some fuel on long runs. I accelerate away from roundabouts slowly and stick to 70 on the motorway. I've always coasted upto lights, its something that my dad did and I always have. Never speed as I really cba scanning all the bridges, layby's etc on a 200 mile journey.

You could say this is negated a bit by zooming up slip roads and burning away from some nasty roundabouts on route. Simple reason being if you pull away calmly you get cut up by morons in the wrong lane. Burn away and then you can let them play silly buggers behind.

[phykell]

Originally Posted by LeperousDust

Less pressure doesn't drop your speed? How in gods name do you work this out? Surely thats as simple as an energy in energy out thing. More fuel means faster essentially (or obviously i'd have hoped). Otherwise you're breaking a fundamental law surely

Nope. It seems that a lot of fuel is wasted by leaving your pedal depressed too much. Perhaps it's just diesel but it seems that you can press the pedal just enough to maintain your speed or press it a little more and still not accelerate though you can see your fuel use increase. There's certainly more to acceleration and overcoming your vehicles inertia than absolute position on the throttle. That's why a human can easily beat a cruise control device for economy if they're both trying to maintain a constant speed.

Incidentally, taking your time seems to get you to your destination quicker as well - at least, that's what it feels like

[LeperousDust]

Originally Posted by phykell

Nope. It seems that a lot of fuel is wasted by leaving your pedal depressed too much. Perhaps it's just diesel but it seems that you can press the pedal just enough to maintain your speed or press it a little more and still not accelerate though you can see your fuel use increase. There's certainly more to acceleration and overcoming your vehicles inertia than absolute position on the throttle. That's why a human can easily beat a cruise control device for economy if they're both trying to maintain a constant speed.

That must be play in the accelterator, or the inertia of the engine literally pulling more fuel through the engine anyway. If you back off the pedal, you burn less fuel, if you burn less fuel, you spend less energy, if you use less energy you go slower. I can't see how you can work this any other way? Basic physical laws we all abide by (unfortunately)

Your trip computer may be wired to tell the angle of the accelerator (amongst other things) or something, so it "thinks" you're being more efficient. Or the decrease in speed is so slight you don't notice it. There are loads of reasons why you might not notice, but suffice to say i don't think you can trick your car/world/physics into saving fuel Trip computers are still not totally correct, they're all based on calculations really.

Only way to find your real MPG is to brim, measure distance driven, brim again and read at the pump what fuel you put in, then divide. I used to try this when i have money, now i try to just fill the car up as little as possible at the time, having absolutely no money to spare, which generally means i just don't use the car where ever humanly possible, whic works out pretty well for me, and consequently the environment.

Originally Posted by phykell

Incidentally, taking your time seems to get you to your destination quicker as well - at least, that's what it feels like

I definately contest that, speeding gets me home MUCH quicker from uni :P

[phykell]

Originally Posted by LeperousDust

That must be play in the accelterator, or the inertia of the engine literally pulling more fuel through the engine anyway. If you back off the pedal, you burn less fuel, if you burn less fuel, you spend less energy, if you use less energy you go slower. I can't see how you can work this any other way? Basic physical laws we all abide by (unfortunately)

It's not play in the accelerator. It's perfectly possible to be heavier on the accelerator than is required without changing speed; and conversely, it's perfectly possible to only use as much throttle as absolutely necessary - that's the challenge for maximising fuel economy. What you're talking about would be an environment with no road friction, no wind or air, and no gradient changes in the road. Unfortunately, maintaining your speed and maximising fuel economy require judicious use of the throttle according to the physical parameters of everything that can affect the vehicle's forward velocity.

Originally Posted by LeperousDust

Your trip computer may be wired to tell the angle of the accelerator (amongst other things) or something, so it "thinks" you're being more efficient. Or the decrease in speed is so slight you don't notice it. There are loads of reasons why you might not notice, but suffice to say i don't think you can trick your car/world/physics into saving fuel Trip computers are still not totally correct, they're all based on calculations really.

Yep, fair point, but what can't lie is how many miles I get out of the tank relative to how much fuel I have to add to fill it as you go on to say. Almost invariably, I use the same pump as well so I get a good idea.

Originally Posted by LeperousDust

I used to try this when i have money, now i try to just fill the car up as little as possible at the time, having absolutely no money to spare, which generally means i just don't use the car where ever humanly possible, whic works out pretty well for me, and consequently the environment.

Yep, that's the best way - avoid such journeys in the first place!

Originally Posted by LeperousDust

I definately contest that, speeding gets me home MUCH quicker from uni :P

In absolute terms, yes; but I really do feel like a journey is quicker if I take it easy and relax - nice music, etc. Speeding just makes you more aware of how much time you're taking, if that makes sense?

[Dr. Z]

My car has an instantaneous MPG reading and it reckons there is a 5mpg difference between doing 70mph with the throttle open and 70mph with the throttle closed (while maintaining the same speed). I do about 2000 motorway miles a month and so this sort of thing is something you notice!

I have thought about this in the same way as Alex has and I have come to the conclusion that its to do with the AFR. More air means it needs more fuel to maintain the "optimal" air-fuel ratio (14.7 I believe?) but as you dont need any more power it will adjust the timing advance so you end up making about the same power. Shut off the taps slightly and you have less air so you need less fuel but a change in the timing.

Not 100% sure on the timing bit but that makes sense as to how you can develop the same amount of power with a different rate of fuel flow. My MPG readings certainly tally with what phykell is experiencing

On the whole speeding thing, I think that depends on your car - mine is more than happy to tick along at 80-90mph and it doesnt feel any different (in terms of sensation of speed, road noise etc) to 70mph. The only real difference is that I am more mindful of police presence and the fact that at 90mph indicated you are generally the fastest thing on the road.

[Matblack]

I find I drive a lot slower these days, OK I'll give the car the odd poke on A roads but motorway I almost aways accelerate to 70-75mph and bung the cruise on, my car is a lot more fuel efficient at 70-75 than just 10mph faster and I used to find that if I did 90 I was coming up against traffic that didn't want to drive as fast as me and having to slow down then speed up again as I came up behind people in the fast lane. I seems to be a mixture of getting older and also when I started to go on seriously long journeys like going to Pembrokeshire, I didn't have to look out for cameras and I didn't often have to slow down to overtake, just drifted past people in the middle lane. I think cruise has made a big difference to my driving too and although not the most fuel econmic way to drive it has probably made more of an impact than anything else.

MB