Coasting vs engine braking

[Devrij]

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?

 

[Devrij]

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?

 

[Devrij]

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 I should probably be more concerned with reducing tyre wear at this point

 

[Devrij]

Lots of interesting info here. I'll definitely be listening for this cutoff at around 1500rpm to see if I can tell if it's working.

Although with regards to safety, I recognise that being in gear allows you to accelerate away from danger, if I'm engine braking in 5th at 40mph (to keep the engine quiet) I need to change gear if I want to go anywhere quickly. Also, braking without the inertial force of the engine will allow me to stop faster (although you can always apply the clutch to avoid that problem).

 

[Devrij]

No, you will tend to stop quicker with less pad wear with the car in gear (for most of the rev range) due to engine braking. As above, if you're at low revs e.g. <1500rpm only then will the engine be fed a bit of fuel which would counteract braking. Besides, you are less likely to lock your wheels under hard braking.

It depends what rpm the engine is at I guess, and in this situation where you'd be at low rpm anyway you may well be right. I was trying to point out that since your brakes can slow you down faster than your engine rpm would drop, the momentum of the flywheel will counteract your braking slightly. Marginal, I know, and only a concern at higher speeds and emergency stops. I would always clutch in when performing an emergency stop for that reason.

There's a lot of down hill travel on my commute home,

Through experimentation I've discovered I'm better off using engine braking as it cuts the fuel down to about 1200 rpm, but below that I think it idles and uses fuel, so sometimes I need to down shift to bring the revs up an fool it. It's a bit of a balancing act but i think I've got used to it.
There's also a long very slightly downhill straight, with a 30mph limit so i pop it in 5th and put the cruise control to 30 and that save quite a few mpg.

This is what I'm now doing. I can't really tell when the cutoff is, so I'm adjusting my method to go 4th down the 30 zone rather than 5th, although this involves blipping the throttle every now and then so may be counter-productive

 

[Devrij]

Interesting point, I guess that would vary depending on the engine/flywheel weight then. Most cars I suspect would drop from e.g. 4000rpm to 1000rpm (if you clutched) within a few seconds - whether you would brake quicker than that would depend on your brakes and your speed (4000 to 1000rpm in 6th might equal slowing from 100 to 25, i.e. probably not slowing quicker w/ braking, vs 4000-1000rpm in 2nd which might be 40mph to 10mph).

Exactly. It was more relevant when I rode a bike because you have more stopping power/weight, and my big slow V twin didn't drop revs very quickly, but I think it still applies in some (but not all of course) situations in a car. I think a lot of my car habits are carried over from my biking days (learnt to ride before I learnt to drive), so perhaps some of it doesn't take the clever engines in cars into account.