Coasting vs engine braking

Devrij

Devrij

Devrij

Soldato
Joined
23 Jul 2009
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Bath
Forgive me for this, but I thought I would venture in here for once to ask this question that's been playing on my mind.

My commute involves driving up one set of hills and then down the other side (with an epic canyon run). That means half of my 45 min journey is downhill, and in rush hour it's usually slower than or at the speed limit. Because I use a fair amount of fuel in the twisties and on the big straight at the top, I try to economise on the boring bits by putting my car in neutral and just using brakes for the second half of the journey.

My assumption being that the engine is still drawing fuel through at a higher rate when engine braking at 2-3k rpm than it is idling at 900rpm. Over the course of 20mins or more every day, do you think there is any significant saving even if it's a quid or two per week? Or is my understanding of idle jets and such flawed?
 
A modern car can use more fuel idling than it will while engine braking, so I believe your logic is flawed. Modern cars have a lot of possibilities to reduce fuel use under these circumstances over old carb systems.
 
Quite a lot of cars will have a fuel cut when engine braking - the fuel is not needed to keep the engine running since the wheels spinning keep the engine running.
 
tigerstyle said:
A modern car can use more fuel idling than it will while engine braking, so I believe your logic is flawed. Modern cars have a lot of possibilities to reduce fuel use under these circumstances over old carb systems.
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This. It's counter-productive and dangerous in a way that you're not fully in control of the car.

Pretty much all modern fuel injection systems will cut most if not all of the fuel supply when your foot is off the throttle and the car is moving, i.e. there's no load on the engine. It sort of just turns into an air brake.

This can be demonstrated nicely with a Land Rover diesel engine.. silence is golden on downhill stretches ;)
 
All you're doing is wearing your pads quicker, as above there will be little or no fuel benefit and you're in less control of the car
 
That's good to know! My working knowledge is based on working on old dirtbikes, so cars are a bit different with their clever ecu's and such.

I prefer to engine brake so that's good news to me. Is my 2002 rover "modern" enough to do this?
 
Yes it should be, just stick it in whatever gear maintains a nice enough speed downhill and use the brakes and accelerator for the odd blip here and there if needed.
 
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In that case I may just have to slow down on the fun bits if I want to save cash. But that road is the only thing that gets me out of bed, so who am I kidding :p I should probably be more concerned with reducing tyre wear at this point :D
 
Clarkey said:
Yes, any EFi car will cut fuel on overrun, it will burn out the cat otherwise.
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I'm not sure my ECU got that message; there's a lot of pops and bangs when I engine-brake, so I suspect it's still throwing fuel in. - Not a huge problem since there's a silencer where the cat used to be, but the ECU doesn't know that.

My mother-in-law refers to coasting in neutral as "angel gear" because that's the one you select if you want to join the angels. :D
 
Never understood that, can someone explain how the car is any less under control when out of gear? The way I see it, a car is fundamentally more stable without drive forces acting on one of the axles. Even if you disagree, how is it any different to driving an automatic trans, are they all dangerous?
 
You are more likely to wheel slip under breaking, and should you need you have to arse about to get some power down.

Cut off on overrun depends on the manufacturer, some still add a bit on overrun to give you that 'car can roll for ages' feeling.
 
Devrij said:
That's good to know! My working knowledge is based on working on old dirtbikes, so cars are a bit different with their clever ecu's and such.
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If those old dirtbikes has 2 stroke engines you would want to avoid engine braking altogether. It's a quick way to lunch a 2 stroker :D
 
Clarkey said:
As someone that knows nothing of 2 strokes, why is that?
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The cylinder is lubricated with oil in the fuel, obviuosly as revs increase, so does the amount of oil and fuel being delivered.

unless you are on zero throttle and engine breaking, in which case you are getting a fraction of the oil required for the revs, and so you increase ring and bore wear by a factor of................lots.
 
Clarkey said:
Never understood that, can someone explain how the car is any less under control when out of gear? The way I see it, a car is fundamentally more stable without drive forces acting on one of the axles. Even if you disagree, how is it any different to driving an automatic trans, are they all dangerous?
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Car would still be in gear in an auto, at least in a lorry any way, as you still have the exhaust brake switch and it still increases engine breaking.
 
atpbx said:
The cylinder is lubricated with oil in the fuel, obviuosly as revs increase, so does the amount of oil and fuel being delivered.

unless you are on zero throttle and engine breaking, in which case you are getting a fraction of the oil required for the revs, and so you increase ring and bore wear by a factor of................lots.
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It's not really something you ever have to worry about with a two stroke. In the situations they're used, you won't have long periods of engine breaking, so not enough time for the cylinder lubricant to wear off/run out.
 
DAnDan said:
It's not really something you ever have to worry about with a two stroke. In the situations they're used, you won't have long periods of engine breaking, so not enough time for the cylinder lubricant to wear off/run out.
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