does coasting save petrol

Tobster86

Hadrian wrote: »

Also NEVER turn your engine off to coast, the steering will lock.

Not always; but a valid point. A better way to put it would be 'make sure you know how your steering wheel lock works before contemplating coasting with the engine off'.

Most steering wheel locks are a mechanical mechanism in the ignition barrel which is actuated by removing the key, and won't come on otherwise.

tbourner

Ooh look:





Fuel overrun cutoff from the late 60s.

wolvoman

Tobster86 wrote: »

The answer IS a very simple NO IT DOES NOT.

With Wig's assumption of 'having to accelerate sooner'; if your engine had that much internal drag, it would take a lot more fuel to sustain it at idle, so the model is flawed anyway. He's quoted made up numbers, but hasn't described an accurate and realistic situation where out of gear coasting saves fuel (because such a situation does not scientifically exist).

He is also describing the ability to govern fuelling to quantities less than required to sustain idle as a special feature because he read it in a manual for a Vauxhall.

Post 50 wrongly assumes that the accelerator is a fuel quanitity adjuster which it is not. Fuel quantity from an EFI system comes from where all the sensor readings are on the ECU's map, including accelerator position; and on mechanical multipoint injection systems, from the accelerator & govenor positions, plus a few other tweaking devices.


Here's a simple set of models.
Imagine an arbitrary car on a downhill slope wanting to sustain a specific speed. There are three possibilities:

-The vehicle would speed up either in neutral or in top gear. In this situation it is necessary to brake and/or select a lower gear to sustain the desired speed. In this situation, being in gear safely controls the speed and uses less fuel than coasting.

-The vehicle would speed up in neutral but slow down in top gear. In this situation, some accelerator would be required in gear but fuel quanitity would still be less than to sustain idle revs, and again provides more safe control.

-The vehicle would slow down in either neutral or top gear. In this case, being in neutral is illogical.

Picture this scenario:

You are driving up a slight hill toward a set of lights at say 30mph in 3rd gear. The lights turn red and you are left with a decision to make on how to come to a stop at the lights.

Throwing the car into 2nd will slow the car to near stationary some way before the lights.
Leaving it in 3rd will also mean the car will not make it to the lights without some extra throttle.
Putting it in neutral will allow gravity to slow the car just enough to reach zero mph right on the white line.

Which is most fuel efficient?

tbourner

wolvoman wrote: »

Picture this scenario:

You are driving up a slight hill toward a set of lights at say 30mph in 3rd gear. The lights turn red and you are left with a decision to make on how to come to a stop at the lights.

Throwing the car into 2nd will slow the car to near stationary some way before the lights.
Leaving it in 3rd will also mean the car will not make it to the lights without some extra throttle.
Putting it in neutral will allow gravity to slow the car just enough to reach zero mph right on the white line.

Which is most fuel efficient?

Either of the in-gear scenarios. The best one will be determined by the optimum rev range designed into the engine with the lowest Brake Specific Fuel Consumption (BSFC) (hey look I've learnt a new term during the course of this discussion). You'd obviously need to supplement the engine with some throttle on an uphill slope to reach the lights, I'd imagine 3rd gear would be best until the engine labours and then change down.

Wig

wolvoman wrote: »

Picture this scenario:
Which is most fuel efficient?

The answer is, no-one knows, but my guess would be neutral.

Wongsky

Wig wrote: »

The answer is, no-one knows, but my guess would be neutral.

Thing is, things to try and avoid or minimise when driving with fuel economy as the goal, are situations when petrol is being burnt but not actually contributing to actually turning the wheels, and braking - where kinetic energy is being radiated off as it's converted to heat (conveniently ignoring any high tech cars that use regenerative braking).

With that in mind, I really don't get the people clinging so strongly to the coasting out-of-gear thing. Fuel cut off on overrun has been going on for a long time, and why would you imagine that would be? In addition, you've only got to understand how modern (ish mainly since electronic management and torque convertors that lock-up) torque convertor autos react to lifting off the throttle whilst moving, to get more of a handle on what's going to be most fuel efficient in the main.

Too many seem to be trying to generalise from very specific or individual scenarios which are dubious at best, anyways. If ever there was a generalised answer, it would be that during driving, it's much more likely to be fuel efficient to leverage overrun fuel cut off whilst driving, rather than burn petrol that's just being wasted, and not actually contributing to the kinetic energy of the car.

tbourner

Agree with Wong.

At idle the engine produces maybe 5hp, which is completely wasted doing nothing.
On overrun the engine produces 0hp.
So when slowing you can use throttle up to 5hp, which would be a reducing throttle position as the revs drop, and still be better off.

Personally I think that in any situation if you were neck and neck with a coasting car, you'd be well below 5hp until the moment you both stopped.

mattyprice4004

tbourner wrote: »

I stand corrected!
I used to do it in my Supra, the torque meant I could get into 4th with no throttle on a level surface! That was when I was going into town twice a day and spending lots of time in traffic. It had a new clutch when the thrust bearing went but that was pretty standard on them when running higher power.
On the diesel Honda I do have a DMF, so probably a good thing I never go into town any more, very rarely get stuck in traffic nowadays and don't remember ever using that 'technique' on this car (only had it a year). I do tend to change gear early though and sit at 1100rpm or thereabouts, although I'm very careful about the amount of throttle I use at those revs, change down if I need to accelerate.

It's funny because my current car doesn't have a DMF, but in my last car I got into the habit of not setting off at idle. Still don't set off at idle, even now!

It's odd how the DMF in that car (Rover 75 diesel) didn't seem to mind all the extra power it had (remapped, fuel rail pressure increase with a tuning box, boost increased to 22psi, straight through exhaust with no silencers save the rear, and decat... running around 185bhp up from the standard 129) and I drove it pretty hard, yet there are stories of the same car suffering DMF failure as low as 50k (although rare). You'd have thought 320lb/ft of torque would do more damage than idle starts.

It seems it really is the idle to setting off vibration that kills them, I was always careful when setting off and did over 40k with the power as above with no ill effects.

Tobster86

wolvoman wrote: »

Picture this scenario:

You are driving up a slight hill toward a set of lights at say 30mph in 3rd gear. The lights turn red and you are left with a decision to make on how to come to a stop at the lights.

Throwing the car into 2nd will slow the car to near stationary some way before the lights.
Leaving it in 3rd will also mean the car will not make it to the lights without some extra throttle.
Putting it in neutral will allow gravity to slow the car just enough to reach zero mph right on the white line.

Which is most fuel efficient?

That's a good, challenging situation to consider and thank you for it.

I would argue for staying in third with enough initially before having to slow to a stop.

Reasoning:
Looking at conservation of energy in this scenario, the vehicle has an amount of kinetic energy (a function of mass and velocity) and needs to gain an amount of gravitational potential energy (a function of mass and altitude).

If coasting would only just achieve a stop in the correct place, and assuming other resistances to motion are negligable, the required potential energy is equal to the required kinetic energy.

In the coasting scenario, burning the fuel is serving no purpose to transfers of energy.

In the in-gear scenario, fuel is burnt to overcome engine braking.

Idling itself is, effectively, burning just enough fuel to overcome engine braking for a set RPM. So, in the absence of a specified 'optimal' engine speed (i.e. the non-zero engine speed which requires the least amount of fuel to sustain), let's assume the amounts are the same.

The in-gear situation takes less time and therefore less fuel, because 30mph is sustained for longer. In the coasting situation, speed reduces at a uniform rate.

This is purely a suggestion. This model has the following flaws:
-Lack of consideration for other resistances i.e. air and tyre resistance, which are a function of speed, and the average speed is higher in my suggestion since the time is less.
-Assumption that engine losses are not functional with engine speed, and lack of data (in-neutral engine speed vs fuel consumption) that would yield an optimum. Although I believe it's safe to assume that in the operating speeds we're referring to (in my vehicles, 3rd gear in 30mph yields ~2000rpm for the diesel and ~2500rpm for the petrol); that there is negligable difference considering overall engine efficiency.

So in this situation, fuel consumption is still pretty speculative, however the in-gear scenario provides the additional benefits of better control and shorter time.

EazyDuz

Well i only filled my tank half way, so i should expect around 200 miles or so from my normal driving behaviour, but im currently trying the neutral method.
Going by the fuel dial it seems to be using roughly the same ammount as normal driving, but on next pay day i will brim it for a proper test.
if i get a significant number over 400 miles before fuel light is flashing, ill conclude that coasting is more efficient than engine braking.

Words do little to prove anything.