Fuel Misers

[smutchin]

I'd have thought the assumption was that most drivers fluctuate their speed more than they realise, whether on the flat or hills.

Cruise control, AIUI, works by maintaining constant revs, which very few drivers will do in normal driving - especially if they're giving proper attention to everything else that is going on around them on the road.

 

[Drago]

Hokay, fitted my Hot Frog coolant pre heater. Just waiting for the connectors to arrive so I can wire it up and make a lead.

 

[Deleted member 26715]

[QUOTE 5111413, member: 9609"]if you could get close enough, within a foot, any collision speed would be minimal - I would just worry about the heavy behind you and becoming the meat in the sandwich[/QUOTE]We used to do this on mopeds in the 70's absolutely mad we would ride with less get as close as we could, we'd get mopeds that were capable of 35ish up to above 50mph, frightening really.

 

[DaveReading]

I'd have thought the assumption was that most drivers fluctuate their speed more than they realise, whether on the flat or hills.

Cruise control, AIUI, works by maintaining constant revs, which very few drivers will do in normal driving - especially if they're giving proper attention to everything else that is going on around them on the road.

I'm not quite sure which conclusion you're arguing for here - is cruise control more or less fuel-efficient? The question of fluctuating speed due to traffic conditions isn't really relevant as even with cruise control you would have it disconnected at that point.

 

[mjr]

I'm not quite sure which conclusion you're arguing for here - is cruise control more or less fuel-efficient? The question of fluctuating speed due to traffic conditions isn't really relevant as even with cruise control you would have it disconnected at that point.

Adaptive Cruise is an attempt to cope with moving traffic without disconnecting cruise control. However, as others note, it doesn't work well on roads with too many space invaders.

 

[mustang1]

Haven't modified cars in any way but I'm generally an economical driver: accelerate gently, see what's up ahead, anticipate when the traffic lights might change, things like that.

 

[mjr]

• On over run any engine, despite having the fuel cut off acts like a massive air pump along with all the friction of the that pumping action and reciprocating masses within it.
• These pumping losses will slow the car down, especially a petrol, far far quicker than if you were to simply allow it to roll.
• …

No longer true with modern petrols, is it? The ECU on mine seems to ensure it's not compressing much air unless the brake is touched. Rather, it's like the air is turning the engine. As such P&G in gear seems to give higher MPG than using neutral.

How good is it on a hybrid? I think its engine isn't connected to the drivetrain mechanically anyway but will the control unit let the wheels roll or use it to recharge?

 

[smutchin]

I'm not quite sure which conclusion you're arguing for here - is cruise control more or less fuel-efficient? The question of fluctuating speed due to traffic conditions isn't really relevant as even with cruise control you would have it disconnected at that point.

Not arguing for any conclusion - I don't know, that's why I asked the question.

And I'm not talking about speed fluctuating due to traffic conditions anyway, I'm talking about speed fluctuating because even on a flat, straight, traffic-free road, most drivers will not be able to maintain constant revs without keeping a close eye on the rev counter. Cruise control takes the responsibility for maintaining constant revs out of your hands.

Of course, cruise control [ETA: the non-adaptive kind] will maintain the set revs regardless of whether you are on the flat, going up or downhill, or drafting behind another vehicle, so your speed will fluctuate significantly even with cruise control. So my question is really whether maintaining constant revs regardless of conditions improves fuel efficiency.

 

[DaveReading]

Adaptive Cruise is an attempt to cope with moving traffic without disconnecting cruise control. However, as others note, it doesn't work well on roads with too many space invaders.

OK. So on a flat, straight, quiet road with those factors removed, which is more efficient?

 

[smutchin]

OK. So on a flat, straight, quiet road with those factors removed, which is more efficient?

Yes, that's what I'm asking.

Although I doubt such driving conditions actually exist in the UK outside TV adverts.

 

[gbb]

Regarding the pulse and glide, over run or coasting...my take on it is as follows.

If you're coasting, youre using fuel, albeit not much.
If you're in gear and slowing down, you're not using any fuel, as said before.

Its not black and white though to say coasting is less efficient. Take my former commute, 20 miles on back roads when all was quiet, plenty of junctions, hardly any traffic. If i effectively engine brake, ie foot off the accelerator, i use no fuel but will slow down much faster than if i coast. So if i coast to a junction i can do it from considerably further back, Combine both, coast from a distance then engine brake when appropriate/needed, you get the best of both worlds. Yes its moderately slower but that's no problem if you've planned your time. I'm a very relaxed driver, i'll often get overtaken...i could'nt care less so time saving isn't an issue for me.

Obviously there are a couple of things to think about....one, don't do it when people are behind you, it will infuriate them. Two, yes there is a danger in that IF your engine cut out, it'd leave you out on a limb.

 

[Colin_P]

No longer true with modern petrols, is it? The ECU on mine seems to ensure it's not compressing much air unless the brake is touched. Rather, it's like the air is turning the engine. As such P&G in gear seems to give higher MPG than using neutral.

How good is it on a hybrid? I think its engine isn't connected to the drivetrain mechanically anyway but will the control unit let the wheels roll or use it to recharge?

It is especially true on any petrol engine as they rely on a throttle to reduce / cut off airflow on over run, effectively turning the engine into a vacuum pump.

Little experiment for you...

Find a flat quiet straight road, accelerate to 30mph then;

Come off the accelerator and let it roll in gear

Do it again but

Come off the accelerator and knock the car into neutral and let it roll.


A petrol car will roll up to x10 times further out of gear than in gear. As I've mentioned above I was just a sceptical about P&G, that is until I tried it.

No idea about hybrids.

 

[Dirk]

Of course, cruise control [ETA: the non-adaptive kind] will maintain the set revs regardless of whether you are on the flat, going up or downhill, or drafting behind another vehicle, so your speed will fluctuate significantly even with cruise control.

Eh?
If your speed is fluctuating significantly, then surely the revs are as well?

 

[Jody]

OK. So on a flat, straight, quiet road with those factors removed, which is more efficient?

Cruise control

 

[smutchin]

Eh?
If your speed is fluctuating significantly, then surely the revs are as well?

Er... yes, now you mention it, that ought to be the case.

My experience of using cruise control on my car is that the speed varies quite a lot, but I expect the RPM is varying at the same time and I'm just not noticing. Doh!