Improving fuel economy of bigger and smaller vehicles

[Justme]

Does it? Why? If you strongly believe that's the case, can you explain why the following is incorrect?






"It uses more fuel" is what people are told but show me a real world test. Or, at least, attempt to explain with some logic/physics

I think your own words do that best.

"a seat of the pants guess at best!"


Most modern high (especially diesel) efficiency cars have very little over run engine braking.

Your going down hill so any engine braking effect is more than covered by the effect of gravity.

The little braking effect there is is helping you maintain control (speed & handling).

You started at the top if the hill at max legal speed so there is no speed increase gain to be had.
You would keep that max speed with or without coasting.
With coasting you will also need to use the brakes.

 

[Justme]

Really,
Some people

Perhaps you need to re read the highway code section 5 item 122.


122
Coasting. This term describes a vehicle travelling in neutral or with the clutch pressed down. It can reduce driver control because

• engine braking is eliminated
• vehicle speed downhill will increase quickly
• increased use of the footbrake can reduce its effectiveness
• steering response will be affected, particularly on bends and corners
• it may be more difficult to select the appropriate gear when needed.

 

[Gareth J]

I think your own words do that best.

"a seat of the pants guess at best!"


Most modern high (especially diesel) efficiency cars have very little over run engine braking.

Your going down hill so any engine braking effect is more than covered by the effect of gravity.

The little braking effect there is is helping you maintain control (speed & handling).

You started at the top if the hill at max legal speed so there is no speed increase gain to be had.
You would keep that max speed with or without coasting.
With coasting you will also need to use the brakes.

I didn't put in your max legal speed limit caveat you have. I don't disagree with you regarding safety but that's not relevant for a discussion about whether or not something is possible or not. With that in mind, your implication you'll need to use the brakes is not relevant either. Work on the basis it's presented to you.

Now, try to explain why you think that there is never a time when, hypothetically, there is never a scenario where engine braking saps more energy than coasting at idle, please.

As I suggested, a real world experiment, or an explanation using logic/physics would be great.

 

[ajlelectronics]

You were a learner driver,
Need I so more

Yes, I was a new driver and the lesson learned was how dangerous it is to coast downhill, especially on a slippery surface. I am grateful for that expensive lesson in common sense.

 

[Wozzaaah]

Regardless of perceived fuel savings it is dangerous to coast as you have no engine braking and if you need to slow you’ll have to rely solely on brakes. If you’re on a decline then you can overheat them and suffer brake fade.

You also have no acceleration if you suddenly need it.

 

[Gareth J]

This may help some of the nay sayers;

Or read here; these guys take mpg and datalogging VERY seriousl;

Hypermiling Techniques 101: Coasting - Hypermiler.co.uk

Hypermiling Techniques 101: Coasting - Hypermiler.co.uk

www.hypermiler.co.uk

Again, I don't advocate doing this, for all the obvious safety reasons mentioned. But there's no point denying it can, absolutely be more fuel efficient.

 

[Bornfree]

It all depends on what your going to do at the bottom of the hill. If you're going to stop it's more efficient to keep it in gear. But if you're going up again and you can gain some speed on the descent ,coasting is more efficient.

 

[keithski122]

Slipstream lorries on the motorway.

 

[Mr Fro]

Slipstream lorries on the motorway.

But what if you want to get to your destination before time stops?

 

[Wallace]

I had over 50mpg urban out of a knackered R reg Fiesta with the Spanish Kent engine by switching off in traffic, coasting with the engine off and using engine braking when possible. Started so easily and idled well. Normal driving I would be lucky to reach 30mpg in it.

 

[Justme]

I didn't put in your max legal speed limit caveat you have. I don't disagree with you regarding safety but that's not relevant for a discussion about whether or not something is possible or not. With that in mind, your implication you'll need to use the brakes is not relevant either. Work on the basis it's presented to you.

Now, try to explain why you think that there is never a time when, hypothetically, there is never a scenario where engine braking saps more energy than coasting at idle, please.

As I suggested, a real world experiment, or an explanation using logic/physics would be great.

You can always stack the deck with conditions so you win.

We need to deal with the actual real world.

IE driving at the speed limit & needing brakes to stay there is not using engine braking.

Hell why not go the whole hog & turn the engine off if the fuel used to start it is less than you can save whilst its off.

 

[Justme]

It all depends on what your going to do at the bottom of the hill. If you're going to stop it's more efficient to keep it in gear. But if you're going up again and you can gain some speed on the descent ,coasting is more efficient.

Thats the issue.

Unless you were driving like a granny you have no speed to gain whilst going down hill.

 

[Justme]

This may help some of the nay sayers;

Or read here; these guys take mpg and datalogging VERY seriousl;

Hypermiling Techniques 101: Coasting - Hypermiler.co.uk

Hypermiling Techniques 101: Coasting - Hypermiler.co.uk

www.hypermiler.co.uk

Again, I don't advocate doing this, for all the obvious safety reasons mentioned. But there's no point denying it can, absolutely be more fuel efficient.

Relies on a speed increase which in real world driving its not actually going to happen.

 

[optima21]

Thats the issue.

Unless you were driving like a granny you have no speed to gain whilst going down hill.

depends on the road, one round here I can coast downhill but have to brake for the speed cameras (and they are 50mph limits), then put it into top gear at the bottom of the hill.

 

[Gareth J]

You can always stack the deck with conditions so you win.

We need to deal with the actual real world.

IE driving at the speed limit & needing brakes to stay there is not using engine braking.

Hell why not go the whole hog & turn the engine off if the fuel used to start it is less than you can save whilst its off.

No,I'm NOT stacking the deck with conditions. I'm asking simply, is it, in theory, possible to use less fuel utilising coating as a technique, or not? If not, how are the scenarios above explained?

Good point regarding switching the ignition off. Some of the hypermilers linked to above do this. Madness. But it obviously works.

Thats the issue.

Unless you were driving like a granny you have no speed to gain whilst going down hill.

Yup, conditions do exist where it's possible to provide a net benefit.

Relies on a speed increase which in real world driving its not actually going to happen.

Tell that to the hypermilers.

 

[Wallace]

Good point regarding switching the ignition off. Some of the hypermilers linked to above do this. Madness. But it obviously works.

I considered wiring in an ignition kill button so as not to use the ignition switch which also houses then steering lock. I had a hotwired Mk1 Polo drop the steering lock in place on a roundabout, that was fun.

Driving the Fiesta like that added an element of fun to the dreary commute. I spent some time thinking of simple ways to add a small electric motor into the drivetrain for stop start traffic.

 

[redlig]

I think your own words do that best.

"a seat of the pants guess at best!"


Most modern high (especially diesel) efficiency cars have very little over run engine braking.

Your going down hill so any engine braking effect is more than covered by the effect of gravity.

The little braking effect there is is helping you maintain control (speed & handling).

You started at the top if the hill at max legal speed so there is no speed increase gain to be had.
You would keep that max speed with or without coasting.
With coasting you will also need to use the brakes.

All depending on the gradient .
Of course if you want to save fuel while coasting down hill in neutral you could always turn the engine off but in certain circumstances that can make steering a bit of a hit and miss affair.

B****r! Wallace beat me to it.

 

[8ob]

Slipstream lorries on the motorway.

Makes a huge difference if you are in another truck.

Bob

 

[Wildatlanticwelder]

Vaccum performance , gradually increase trottle input as rpm rises, should wait until half way to max rpm before giving it wide open trottle. Makes a big difference on carb cars, but any petrol with a trottle flap 99% of them it should make a difference.
I reckon the car will accelerate as fast or faster, pull more smoothly and save juice. Old toyotas used to have an economy light which would flick green to red dependant on the vaccum in the inlet.

 

[julianthegypsy]

Many years ago, I had a Saab 96. You could pull a lever and activate a one way clutch in the gearbox, which meant no engine braking. You could coast down hill, in gear with the engine idling and just hit the throttle when you wanted power. It was *claimed* that it was safer on ice. I was told later that it was to stop the original two stroke engines from seizing due to lack of lube on the overrun. I preferred having engine braking, but my dad had one previously a few years before and always drove in freewheel. I did fit an aftermarket CD ignition on mine and consistently got 39mpg. Considering I was in my early twenties and didn't hang about, and it was a heavy beast, I don't quite understand why cars don't do 100mpg with ease these days.