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DB617 wrote: ↑Tue Mar 01, 2022 00:12
I wouldn't advise coasting it. You're descending somewhere in the region of 300m and that's not a kind thing to do to any brake set.
Depends how steep it is, air resistance etc how much braking may be required, but modern engines will use negligable fuel on such a descent even when in gear. 300m isn't really very far, I've driven much much longer on alpine descents and have found engine hasn't even warmed up after spending half-an-hour or so descending from Tignes (2300m) down to Bourg St. Maurice (750m).
Alderpoint wrote: ↑Mon Feb 28, 2022 19:53
PS Do you mean
85kph minimum design speed
If so can you explain please....
I realise that could be confusing. I meant that the design speed of the scheme drops as low as 85kph in places. Ie the design speed is at MINIMUM 85kph.
I believe traditionally the term 'coasting' in DSA+IAM type standards refers specifically to clutch out, neutral driving using solely the brake for speed control. A practice that certainly was necessary for some classic vehicles but as you say, modern ICE cars are smart and shut down fuel intake, allowing the car to be revved by the wheels and giving you essentially infinite instantaneous MPG. The difference is the mechanical drag created by the engine contributes significantly to braking especially if you leave it in low gear. My Dad didn't believe coasting was dangerous until he came down Llangynidr moor in his old Civic and had total brake fade by the time he reached the bottom. Bit of an 'I told you so but I'm glad you didn't die' moment for me.
In any case, the engine shouldn't warm up on an engine braking descent. There's very little heat produced by mechanical losses compared to the heat when fuel is burnt.
DB617 wrote: ↑Tue Mar 01, 2022 11:49
I believe traditionally the term 'coasting' in DSA+IAM type standards refers specifically to clutch out, neutral driving using solely the brake for speed control. A practice that certainly was necessary for some classic vehicles but as you say, modern ICE cars are smart and shut down fuel intake, allowing the car to be revved by the wheels and giving you essentially infinite instantaneous MPG. The difference is the mechanical drag created by the engine contributes significantly to braking especially if you leave it in low gear. My Dad didn't believe coasting was dangerous until he came down Llangynidr moor in his old Civic and had total brake fade by the time he reached the bottom. Bit of an 'I told you so but I'm glad you didn't die' moment for me.
In any case, the engine shouldn't warm up on an engine braking descent. There's very little heat produced by mechanical losses compared to the heat when fuel is burnt.
Some automatic cars have a coast mode as an option. Had a VW Tiguan as a hire car about 10 years ago on a drive down to Newport. Didn't notice it for most of the journey as did the M6, M5, M50 route.
Got on the A40 and was extremely confused when it wasnt slowing down on the downhill sections when I came of the power!
Video shot this week. The main new things are the removal of any trace of the works compound, the opening of the Gateway bridge to pedestrians and the tarmacing/opening of the cycleway to Abergavenny just on the incline up to the gateway bridge, which I take enroute as a detour near the end... About 8 mins in.
DB617 wrote: ↑Tue Mar 01, 2022 11:49
In any case, the engine shouldn't warm up on an engine braking descent. There's very little heat produced by mechanical losses compared to the heat when fuel is burnt.
I remember coming down Porlock Hill on the A39 and keeping in low gear to rely less on the brakes and the coolant temperature gauge dropped - thanks to no fuel being burned and the cooling effect of the air through the radiator.
M19 wrote: ↑Mon Jun 06, 2022 23:04
I remember coming down Porlock Hill on the A39 and keeping in low gear to rely less on the brakes and the coolant temperature gauge dropped - thanks to no fuel being burned and the cooling effect of the air through the radiator.
I've long known Porlock (and a bit further along, Countisbury, down into Lynmouth), and am appalled at those ahead who make the whole descent with brake lights on. Although I'm automatic, shift it down into Hold 2 and it goes nicely down not over 30mph, about 3,500rpm. Brakes are for stopping.
Interesting the comment about coolant temperature actually dropping, which I don't see. Basic physics then, where does the potential energy go, if it doesn't go into red-hot brakes ?
WHBM wrote: ↑Mon Jun 06, 2022 23:17
Interesting the comment about coolant temperature actually dropping, which I don't see. Basic physics then, where does the potential energy go, if it doesn't go into red-hot brakes ?
Kinetic energy/frictional losses. Think about when you bump start a car it gets enormously harder to push once it's in gear, it's the same thing. It takes a lot of energy to turn over the engine and transmission
WHBM wrote: ↑Mon Jun 06, 2022 23:17
Interesting the comment about coolant temperature actually dropping, which I don't see. Basic physics then, where does the potential energy go, if it doesn't go into red-hot brakes ?
Kinetic energy/frictional losses. Think about when you bump start a car it gets enormously harder to push once it's in gear, it's the same thing. It takes a lot of energy to turn over the engine and transmission
My understanding is that there are also compression losses due to the air intake system still being operational when the injectors are closed. The air is compressed by the piston, driven by the crank from the rotation of the wheels, meaning your downhill energy is accumulated in the cylinder and dissipated through the exhaust valves?
