Just been reading how to drive economically and it mentions coasting uses more petrol. It said that the epu? Shuts off fuel if there's no foot on the pedal, but surely it has to put something in to prevent a vacuum? Is it true that your wasting your time coasting a modern car? Or is it bull cos they don't want people coasting?
Coasting modern cars
- Thread starter Coaster
- Start date
Sponsored Links
I reckon they are covering their arses - if car engine stalled while coasting , steering/brakes would get somewhat heavy 
. Recently been a passenger in a Golf hire car with auto box -1 year old - poxy box was playing tunes as we rolled down the hills over the Forest . Got in the 20 year old BMW 5 series diesel that I was to drive back to the coast for my M8 - beautiful - 4 speeds and hardly felt them change . That`s the wonders of modern " eco" cars for you .
. Recently been a passenger in a Golf hire car with auto box -1 year old - poxy box was playing tunes as we rolled down the hills over the Forest . Got in the 20 year old BMW 5 series diesel that I was to drive back to the coast for my M8 - beautiful - 4 speeds and hardly felt them change . That`s the wonders of modern " eco" cars for you .Most fuel injection engine wil, I understand, cut the fuel to the engine when coasting. Obviously in a low gear there is a lot of throtalling loss if you do this, but in a suitably high gear at anything other than motorway speeds it would say its proberbly negligable either way.
However, more the point, I wouldnt be aiming to coast any car other than short distances at low speeds. Having had a car with a sticky idle control valve I can confirm its quite alarming to have the engine stop at 25mph half way round a dual carageway roundabout and not something I would want to try again.
Daniel
However, more the point, I wouldnt be aiming to coast any car other than short distances at low speeds. Having had a car with a sticky idle control valve I can confirm its quite alarming to have the engine stop at 25mph half way round a dual carageway roundabout and not something I would want to try again.
Daniel
Don't know about petrol engines but diesels shut off the fuel if being 'over driven' my 406 shows 999.9mpg going down a local hill in gear, if you knock it out of gear it increases to 330 mpg
It depends though how steep the hill is, steep enough to need no throttle at all and it uses no fuel but on a gradual slope you are still using some throttle to overcome the braking effect of the engine, in which case it would use less coasting - as long as the engine doesn't stop of course.
Peter
It depends though how steep the hill is, steep enough to need no throttle at all and it uses no fuel but on a gradual slope you are still using some throttle to overcome the braking effect of the engine, in which case it would use less coasting - as long as the engine doesn't stop of course.
Peter
Sponsored Links
It's true, pretty much all modern injected cars (petrol or diesel) have an over-run fuel cut-off feature and yes, it's controlled by the ECU. By "modern", I'm talking about stuff that has generally been manufactured in the last 20 years.
On over-run, all petrol engines (and possibly some diesels these days?) generate a ferocious vacuum in the inlet manifold and cylinders. They'll do that with or without the over-run fuel cut-off. It's just because there's a throttle plate blocking the air off. It does them no harm.
You are GENERALLY wasting your time coasting in a modern car, BUT...
here's the problem.
If you're doing (say) 50 along a road with a slight downward slope and you lift off the throttle in the highest gear you've got, your car will use zero fuel, but the drag of the gearbox driving the engine (engine braking) will slow it down somewhat. If you're doing the same speed on the same road and you kick it into neutral, your engine will burn whatever fuel it would burn when idling (instead of zero fuel), BUT, there will be no engine braking, so you'll go further.
As for which is going to work out the most economical, that's a tough one! It will vary with every set of conditions. If the traffic is slowing down at least as fast as the car would have done under engine braking anyway, then in-gear is definitely the way to go as you will use no fuel. If you're going down a hill and want to maintain a constant speed, then coasting MIGHT be the way to go. As Peter N says, if you have a trip computer that gives istantaneous consumption readings, you have the luxury of trying it both ways and seeing. My personal experience is that it is generally best to decelerate in the highest gear you can (unless you need the engine braking). It's pretty rare that coasting is the better bet.
Obviously, as part of the responsible motoring "never-run-with-scissors" obligation, I could never advise anyone to coast in neutral!
On over-run, all petrol engines (and possibly some diesels these days?) generate a ferocious vacuum in the inlet manifold and cylinders. They'll do that with or without the over-run fuel cut-off. It's just because there's a throttle plate blocking the air off. It does them no harm.
You are GENERALLY wasting your time coasting in a modern car, BUT...
here's the problem.
If you're doing (say) 50 along a road with a slight downward slope and you lift off the throttle in the highest gear you've got, your car will use zero fuel, but the drag of the gearbox driving the engine (engine braking) will slow it down somewhat. If you're doing the same speed on the same road and you kick it into neutral, your engine will burn whatever fuel it would burn when idling (instead of zero fuel), BUT, there will be no engine braking, so you'll go further.
As for which is going to work out the most economical, that's a tough one! It will vary with every set of conditions. If the traffic is slowing down at least as fast as the car would have done under engine braking anyway, then in-gear is definitely the way to go as you will use no fuel. If you're going down a hill and want to maintain a constant speed, then coasting MIGHT be the way to go. As Peter N says, if you have a trip computer that gives istantaneous consumption readings, you have the luxury of trying it both ways and seeing. My personal experience is that it is generally best to decelerate in the highest gear you can (unless you need the engine braking). It's pretty rare that coasting is the better bet.
Obviously, as part of the responsible motoring "never-run-with-scissors" obligation, I could never advise anyone to coast in neutral!
was just a thought when I was going down a long straight gentle gradient..
I was in 5th,. cruising about 40 and the rev counter was over 1.5k (approx figures) , dipped the clutch and the rev's dropped obviously, and kept at the same speed. I'm aware of the dangers of coasting (login name purely co-incidental btw lol) I was just perplexed with the thought that if the car engine is getting so much less petrol in it from the ECU, there'd be a vacuum, and if ther'es a vaccum in the inlet manifold, the engine would be effectivley braking the car, same as some braking systems in lorries that use delayed exhaust expulsion from the cylinders.
forgive my basic mechanical understanding..lol
I was in 5th,. cruising about 40 and the rev counter was over 1.5k (approx figures) , dipped the clutch and the rev's dropped obviously, and kept at the same speed. I'm aware of the dangers of coasting (login name purely co-incidental btw lol) I was just perplexed with the thought that if the car engine is getting so much less petrol in it from the ECU, there'd be a vacuum, and if ther'es a vaccum in the inlet manifold, the engine would be effectivley braking the car, same as some braking systems in lorries that use delayed exhaust expulsion from the cylinders.
forgive my basic mechanical understanding..lol
That doesn't apply to diesels though because they have no throttle so have max air intake all the time, there are losses that cause deceleration though because the engine is still having to compress the air going in even when there is no fuel.
peter
peter
Almost all petrol engines are controled by use of a throtal, a restriction in the inlet, to ****** the engine and maintain the required revs. The air fuel mixture is the kept at a constant level for near optimal burn. You wont pull anything like a total vacuum, but the will significant negative presure on the inlet.
Conversely, almost all diesals have no throtal, and speed is controlled by the rate of fueling and the burn non-homogenious. The compression ratio is however a little higher.
Hence on the overrun with a petrol the looses are mainly throtalling looses with some pumping looses, on a diesal they are mainly pumping loses (compressing and re-expanding the gases on each rotation) with some inlet losses.
However, the outcome is about the same...
An 'exhuast retarder' as used from braking lorries is indeed basically a throtall on the exhaust like the throtal on the inlet of a petrol.
Daniel
Conversely, almost all diesals have no throtal, and speed is controlled by the rate of fueling and the burn non-homogenious. The compression ratio is however a little higher.
Hence on the overrun with a petrol the looses are mainly throtalling looses with some pumping looses, on a diesal they are mainly pumping loses (compressing and re-expanding the gases on each rotation) with some inlet losses.
However, the outcome is about the same...
An 'exhuast retarder' as used from braking lorries is indeed basically a throtall on the exhaust like the throtal on the inlet of a petrol.
Daniel
I'm pretty certain my last Peugeot diesel had a butterfly in the inlet manifold. I was quite surprised, but I think they are starting to use throttle plates in them, similar to a petrol engine (no idea why though)!
If you lift off at high revs in a low gear, you get pretty close to a complete vacuum in a petrol engine - certainly around 95% of one!
Getting back to Coaster's question, most of the volume that goes into your engine (petrol or deisel) is air. Shutting off the fuel supply won't make that much of a difference to the level of vacuum when you lift off.
The exhaust retarders on big diesels are just a flap that blocks off the exhaust pipe - turning the engine into a big compressor to increase the level of engine braking available. As Dhutch says, just increasing the pumping losses. They only work because the engine doesn't have a throttle, so it's still getting it's charge of air on the inlet side. If you tried it on a petrol car, there wouldn't be much air in the cylinders for them to compress when the throttle was shut.
If you lift off at high revs in a low gear, you get pretty close to a complete vacuum in a petrol engine - certainly around 95% of one!
Getting back to Coaster's question, most of the volume that goes into your engine (petrol or deisel) is air. Shutting off the fuel supply won't make that much of a difference to the level of vacuum when you lift off.
The exhaust retarders on big diesels are just a flap that blocks off the exhaust pipe - turning the engine into a big compressor to increase the level of engine braking available. As Dhutch says, just increasing the pumping losses. They only work because the engine doesn't have a throttle, so it's still getting it's charge of air on the inlet side. If you tried it on a petrol car, there wouldn't be much air in the cylinders for them to compress when the throttle was shut.
I know that the Perkins 'Prima' engine in the Sherpa had a butterfly valve in the intake this was to provide vacuum for the brakes, I hade made up a similar system previous to this for Perkins 4/108 diesel conversions in cars.
Some of the common rail diesels use 'swirl flaps' in the inlet manifold which sometimes fall of and wreck the engine but they do not create a vacuum they are designed to improve combustion.
I haven't personally come across a Peugeot/Citroen engine with a butterfly unless it has been fitted to supply vacuum, perhaps in early vehicles, most have a vacuum pump or in the case of hydropnumatic Citroens they use the main hydraulic system for braking.
Peter
Some of the common rail diesels use 'swirl flaps' in the inlet manifold which sometimes fall of and wreck the engine but they do not create a vacuum they are designed to improve combustion.
I haven't personally come across a Peugeot/Citroen engine with a butterfly unless it has been fitted to supply vacuum, perhaps in early vehicles, most have a vacuum pump or in the case of hydropnumatic Citroens they use the main hydraulic system for braking.
Peter
Not so much loss as you may think - most of the energy expended on compressing the air on the compression stroke is regained on the 'power' stroke as the compressed air charge in the cylinder expands again, even if there's no fuel being burnt.
Hadn't thought of that. I put the 'coasting' theory to the test while we were away last week, going down a hill that was just steep enough to keep up the momentum out of gear, 314 mpg, to maintain the same speed in gear 114 mpg.
Peter
Peter
Sponsored Links
