Nice people thread part 3- Nice as pie

JonnyBravo

If you keep the car in gear you will travel less far than freewheeling due to the friction of the gear/engine components. You will use the same amount of petrol as no demand is placed on the engine. Really2 is right that your onboard computer might suggest you use more fuel freewheeling as it won't take into account freewheeling for MPG stats (Audi's &VW's certainly seem to do this), but the stat's are simply wrong here.

PN is right that it certainly was always contrary to the highway code to freewheel. Presume it still is.

JonnyBravo

Davesnave wrote: »

Freewheeling, my car is turning over at about 700rpm. You are suggesting that if I engage the drive train, the fuel injection system will cut the fuel supply below that, or even to nothing.

Ah right. Well if it did cut it to nothing then things might change but AFAIAA it just cuts it to the minimum, as does freewheeling. Exhaust fumes are still given off. Combustion noises continue.

JonnyBravo

Generali wrote: »

The way I guess it works is thusly.

If you are sitting in a car at the top of a hill and need to get to the bottom then you need to use energy for 2 main purposes: propelling the car and keeping the engine running.

You have 2 sources of energy you can use: the petrol in your tank or the potential energy you have by being at the top of the hill.

If you put the car in neutral you have to use petrol to keep the engine running and you are using potential energy to propel the car. Alternatively you can keep the car in gear and use potential energy for both.

For the descent alone, the latter will use less energy however if you continue past the descent you will be travelling faster at the bottom of the hill as you've not been using potential energy to turn over the engine so will go further. Make sense?

Makes sense but I'm not sure you're right. I don't think PE can be used to run an engine. It can assist an engine by making it's work load lighter but seeing as we're talking about not placing any demand on an engine I'm not sure you're right.

tomterm8

I hate physics. This conversation sounds like Physics. Did I mention, ever since doing physics A-level as a kid, I hate physics.

neverdespairgirl

SingleSue wrote: »

So here I am, 4.25am not able to sleep with an exam this afternoon.

Best of luck with it, Sue!

Davesnave

tomterm8 wrote: »

I hate physics. This conversation sounds like Physics. Did I mention, ever since doing physics A-level as a kid, I hate physics.

You may have hated my practical science lessons at junior level then.:(

We once spent days investigating whether changing the mass of a vehicle makes it go further when rolled down a slope. This brings up all sorts of other problems, like how many results are required for a significant test, how to make it fair, how to ensure wind resistance hasn't changed, how to be certain that Fred with the stopwatch isn't ar$ing about, who has a low attention span, and so on....

I can't remember what the outcome was, but it doesn't matter now anyway, because stuff like that stopped when the National Curriculum and SATS kicked-in seriously. :rotfl:

PasturesNew

Yesterday it was overcast, cool and a little breezy. Today it's overcast, cool and windy.

Really2

Davesnave wrote: »

I'd need to get my head around the workings of this, and how it goes in reality..

Freewheeling, my car is turning over at about 700rpm. You are suggesting that if I engage the drive train, the fuel injection system will cut the fuel supply below that, or even to nothing.

I can understand that's possible, but I'm not sure I could roll as far in 5th as I can in neutral, but I'll experiment on the route down to the station, which I know really well.

100%, modern engines inject no fuel when rolling in gear.
So rolling in as higher gear permitting is the best route to save fuel.
Engine management may kick in at around >1000rpm in 1st to stop a stall though.

lemonjelly

silvercar wrote: »

Message to posters and lurkers.

We have a long journey tomorrow by car, normally we would avoid Friday motorway traffic but we have no alternative.

Essential that you all stay home glued to the telly watching the royal wedding. Even the non-royalists amongst you need to see the dress, the procession, the crowds, the balcony kiss..who is there, who is not there., the weather, the crowds.......

Thank you.

Pfft!
I went & played golf instead...:p

GDB2222

My understanding is that modern fuel injection systems cut fuel consumption to zero/nil/nada if you coast along in gear (with your foot off the accelerator). If you disengage the clutch, then some fuel needs to be used to keep the engine ticking over. So, if you are slowing down towards red traffic lights, say, it's definitely better to keep the clutch engaged. If you are coasting down a hill with the intention of free wheeling along the level, there are more factors to consider. For example, which gear are you in and how fast is the engine turning over? This affects the amount of energy lost in friction within the engine.

Edit: just seen Really's post 2 above this one - totally agree.