V90 D4 MPG Improvement

[Quacker]

Originally Posted by AdamP

Sorry for dragging up an old thread but I was looking for mpg comparisons and stumbled across it.

Coasting uses less fuel under some circumstances and more for others. Engine frictional losses increase with the (almost) square of engine speed, therefore you lose more kinetic energy to engine 'braking' at 1600rpm than you input in fuel to idle the engine at 800rpm, even at 40% thermal efficiency. Therefore if you are maintaining a speed it is more energy efficient to coast. However if you are trying to slow down it is obviously not efficient to continue to add energy (fuel) so engine braking is more efficient. My experience is that the Volvo system is pretty good at deciding when to coast and when not to, as for whether is makes a significant difference to mpg I couldn't say!

Except that zero fuel is injected at the over-run. Zero. You assume that there is. And in top gear there is very little actual engine braking.
Coasting will always use fuel to maintain an idle speed. Certainly in a current Volvo. There are some American cars that shut off the engine completely during coasting, using the electric system to continue to power the steering and brakes while the engine is switched off while travelling. This is specifically to avoid idle speed fuel consumption, which is the same reason stop-start is fitted.

Do these things save any significant fuel? No. None of them are likely to be cost effective, but that is not their point.

[AdamP]

No fuel is injected on overun, the engine is turned by the kinetic energy of the vehicle. The engine doesn't suddenly turn friction free because its using no fuel! It doesn't matter what is turning the engine, it uses more energy to turn it at 1600rpm than it does to turn it at 800rpm. I've also ignored the fact that when there is torque through the gearbox during overun 'engine braking' there is increased frictional losses too.

Essentially it uses more fuel to engine brake and then accelerate to regain the lost kinetic energy than it does the coast on idle. Physics

[AdamP]

Take a couple of scenarios....

1) 70mph on the motorway, you reach an incline where the torque requirement to maintain your speed is zero. Does it use more fuel with the engine turning at 800rpm or 1600rpm?

2) You you lift off the accelerator approaching a roundabout and gently apply the brakes. The torque requirement is greater than the engine can provide through engine braking. Does it use more fuel to provide maximum negative engine torque at 1600rpm or zero torque at 800rpm?

Your explanation covers the 2nd scenario but ignores the 1st. Hence it is sometimes better and sometimes worse, scenario dependent.

[Quacker]

Originally Posted by AdamP

No fuel is injected on overun, the engine is turned by the kinetic energy of the vehicle. The engine doesn't suddenly turn friction free because its using no fuel! It doesn't matter what is turning the engine, it uses more energy to turn it at 1600rpm than it does to turn it at 800rpm. I've also ignored the fact that when there is torque through the gearbox during overun 'engine braking' there is increased frictional losses too.

Essentially it uses more fuel to engine brake and then accelerate to regain the lost kinetic energy than it does the coast on idle. Physics

If it is on the over-run, or coasting, it is running down hill or slowing anyhow and probably will be braked to further slow the vehicle as often or more than just accelerating just from coasting. The mass of the vehicle keeps the car rolling along in top for ages on even a slight downhill in top whether coasting or on the over-run. It uses NO fuel on the over-run but uses the normal fuel to idle [which is mitigated usually by the stop-start system when not rolling, so must be somewhat significant]. Friction and pumping losses on the over-run are insignificant. You ignore the potential energy and acceleration of gravity and you also ignore the fuel and power required when coasting to charge batteries, run the aircon and power the steering, none of which are assisted by gravity when coasting with the engine at idle.


The higher the gear, the less engine braking there is, and since coasting is only programmed to happed at over 40mph, when switched off the over-run at those speeds is invariably in 7th or 8th gear where engine braking is almost ineffective and revs will indeed be very low relative to speed. Even with large commercial engines of 6 litres and above, there is a massive fuel saving on the over-run where fuel consumption on the monitors falls to zero.

Just done a search for some references for my absolutely correct information and come up with this video. I hope this has been educational to you to help you understand why you are quite wrong.

[YOUTUBE]https://youtu.be/GOc-0l3lfrY[/YOUTUBE]

[AdamP]

If frictional and pumping losses are insignificant on overun, the fuel used be to overcome the even smaller losses on idle must be even less significant!

Posting videos to youtube doesn't change the laws of physics unfortunately, or my job would be significantly easier :/

[Quacker]

Originally Posted by AdamP

If frictional and pumping losses are insignificant on overun, the fuel used be to overcome the even smaller losses on idle must be even less significant!

Posting videos to youtube doesn't change the laws of physics unfortunately, or my job would be significantly easier :/

So you continue to deny the science.
Gravity provides kinetic energy which overcomes pumping losses and power to ancillaries. Air con alone takes 4 to 5 hp intermittently. A bit less for alternator usually. All powered by gravity, not fuel, when on over-run but by the engine when coasting.
In the video it was assumed, correctly, that idling a 2 litre car uses about 0.36 litres of fuel per hour or 6 millilitres per minute to overcome pumping and friction losses and run at idle. Ten hours of idling would use a minimum of 3.6 litres, which is what the stop start is there to avoid. That's without taking account of the power/energy/fuel used to power the ancillaries brakes and steering etc. All of which are otherwise powered by gravity acting on the mass of the car pushing it down even a slight gradient in a high gear. Luckily the video even explains the effect of gearing, so I don't need to so much.

[AdamP]

What was the source of the gravitational potential and kinetic energy in the first place though? Fuel is the only energy input. Unless you only ever drive downhill of course...

On a more serious note, there's a raft of published SAE papers available on the topic of fuel economy / emissions reduction that cover coasting strategies in some detail (inc. when they are advantageous and when they aren't). They make for interesting reading, although maybe not as entertaining as internet videos....

[Quacker]

Originally Posted by AdamP

What was the source of the gravitational potential and kinetic energy in the first place though? Fuel is the only energy input. Unless you only ever drive downhill of course...

On a more serious note, there's a raft of published SAE papers available on the topic of fuel economy / emissions reduction that cover coasting strategies in some detail (inc. when they are advantageous and when they aren't). They make for interesting reading, although maybe not as entertaining as internet videos....

Going up the hill, rather obviously. Same fuel used however the downhill is driven. Rather obviously.

[AdamP]

Originally Posted by Quacker

Going up the hill, rather obviously. Same fuel used however the downhill is driven. Rather obviously.

A quick reminder on the conservation of energy wouldn't go amiss here, start with the basics! I'm sure there will be something on youtube.

[Fursty Ferret]

Just done a search for some references for my absolutely correct information and come up with this video. I hope this has been educational to you to help you understand why you are quite wrong.

So on one side we have Quacker and his many years working as a mechanic...

...and on the other side we have Volvo (indeed, pretty much the entire car industry), several engineers, and a physicist. Your choice.

Personal experience: on a road with gentle rolling hills the fuel consumption is markedly reduced with coasting enabled. If it’s obvious that I’m going to have to stop (ie a roundabout at the end of a gentle descent), I’ll tap the brakes at the beginning to have engine braking the whole way down. Otherwise the car gets to decide.