|
S60 & V60 '18> / XC60 '17> / S90 & V90 '16> / XC90 '15> General Forum for the SPA-platform 60- and 90-series models |
Information |
|
V90 D4 MPG ImprovementViews : 5905 Replies : 60Users Viewing This Thread : |
|
Thread Tools | Display Modes |
Aug 19th, 2019, 21:25 | #51 | |
Premier Member
Last Online: Nov 22nd, 2021 00:53
Join Date: Feb 2017
Location: West of Carmarthen
|
Quote:
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.
__________________
XC90 D5, 2017 Inscription. Dark Grey. Xenium pack. Winter Plus with HUD. Black leather interior with alloy trim inserts. Ownership ended June 2020, Happy motoring times! |
|
Aug 20th, 2019, 06:46 | #52 |
New Member
Last Online: Nov 14th, 2020 16:49
Join Date: Oct 2008
Location: Newbury
|
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 |
Aug 20th, 2019, 08:23 | #53 |
New Member
Last Online: Nov 14th, 2020 16:49
Join Date: Oct 2008
Location: Newbury
|
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. |
Aug 20th, 2019, 22:06 | #54 | |
Premier Member
Last Online: Nov 22nd, 2021 00:53
Join Date: Feb 2017
Location: West of Carmarthen
|
Quote:
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]
__________________
XC90 D5, 2017 Inscription. Dark Grey. Xenium pack. Winter Plus with HUD. Black leather interior with alloy trim inserts. Ownership ended June 2020, Happy motoring times! |
|
Aug 20th, 2019, 22:50 | #55 |
New Member
Last Online: Nov 14th, 2020 16:49
Join Date: Oct 2008
Location: Newbury
|
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 :/ |
Aug 20th, 2019, 23:36 | #56 | |
Premier Member
Last Online: Nov 22nd, 2021 00:53
Join Date: Feb 2017
Location: West of Carmarthen
|
Quote:
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.
__________________
XC90 D5, 2017 Inscription. Dark Grey. Xenium pack. Winter Plus with HUD. Black leather interior with alloy trim inserts. Ownership ended June 2020, Happy motoring times! Last edited by Quacker; Aug 20th, 2019 at 23:44. |
|
Aug 20th, 2019, 23:46 | #57 |
New Member
Last Online: Nov 14th, 2020 16:49
Join Date: Oct 2008
Location: Newbury
|
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.... |
Aug 21st, 2019, 03:17 | #58 | |
Premier Member
Last Online: Nov 22nd, 2021 00:53
Join Date: Feb 2017
Location: West of Carmarthen
|
Quote:
__________________
XC90 D5, 2017 Inscription. Dark Grey. Xenium pack. Winter Plus with HUD. Black leather interior with alloy trim inserts. Ownership ended June 2020, Happy motoring times! |
|
Aug 21st, 2019, 06:20 | #59 |
New Member
Last Online: Nov 14th, 2020 16:49
Join Date: Oct 2008
Location: Newbury
|
|
Aug 21st, 2019, 09:44 | #60 | |
Senior Member
Last Online: Feb 26th, 2024 16:44
Join Date: Jul 2015
Location: Manchester
|
Quote:
...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. |
|
Currently Active Users Viewing This Thread: 1 (0 members and 1 guests) | |
|
|