Question about coasting ...
Discussion
Chaps,
This is the first time I have posted on the "Engines" forum. Pardon me if I am covering stuff that is particularly elementary, or has been done before.
This is a question that I have pondered on many an occasion. (And in writing it down, I may give myself the answer.) Bear with me ...
Imagine the situation whereby you are doing 70mph, on the motorway, no traffic jams and you are coming to a slight downhill stretch. You are in 5th gear. If you put the car in neutral, you will probably coast down hill at around 50mph, slowing down gently from the 70 that you are currently doing.
You take your foot off the throttle, and the car starts slowing down. Now you put your foot back on the throttle, but only a little. Enough to maintain the speed.
What is happening in the engine? You obviously aren't throwing as much fuel in it as you normally would, but it is taking in the normal amount of air. Therefore, presumably you are running it lean? Won't this potentially be causing pinking, and damaging the engine?
Or have I got my wires well and truly crossed somewhere?
Oli.
This is the first time I have posted on the "Engines" forum. Pardon me if I am covering stuff that is particularly elementary, or has been done before.
This is a question that I have pondered on many an occasion. (And in writing it down, I may give myself the answer.) Bear with me ...
Imagine the situation whereby you are doing 70mph, on the motorway, no traffic jams and you are coming to a slight downhill stretch. You are in 5th gear. If you put the car in neutral, you will probably coast down hill at around 50mph, slowing down gently from the 70 that you are currently doing.
You take your foot off the throttle, and the car starts slowing down. Now you put your foot back on the throttle, but only a little. Enough to maintain the speed.
What is happening in the engine? You obviously aren't throwing as much fuel in it as you normally would, but it is taking in the normal amount of air. Therefore, presumably you are running it lean? Won't this potentially be causing pinking, and damaging the engine?
Or have I got my wires well and truly crossed somewhere?
Oli.
Foot off accelerator = fuel pump switched off and zero fuel used (except on pretty old engines).
Foot off accelerator and on clutch = fuel pump comes on again and engine maintains itself at tickover - bad idea due to the difficulty of matching revs to road speed when you re-engage.
Foot on the gas just enough to maintain momentum = most fuel-efficient way to drive and the condition for which most engines are optimised, ie the leanest mixture that will avoid detonation.
Serious oversimplification...
Foot off accelerator and on clutch = fuel pump comes on again and engine maintains itself at tickover - bad idea due to the difficulty of matching revs to road speed when you re-engage.
Foot on the gas just enough to maintain momentum = most fuel-efficient way to drive and the condition for which most engines are optimised, ie the leanest mixture that will avoid detonation.
Serious oversimplification...
zcacogp said:
What is happening in the engine? You obviously aren't throwing as much fuel in it as you normally would, but it is taking in the normal amount of air. Therefore, presumably you are running it lean? Won't this potentially be causing pinking, and damaging the engine?
If it's a diesel it will still pull in the full amount of air and run very lean - that's what diesels do under part throttle. If its a petrol engine the throttle will be almost closed, only a very small amount of air will get part the throttle into the cylinders - the fuel injection (or carb, on older cars) will inject a correspondingly small amount of fuel to keep the air/fuel ratio constant ish.
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