DaveReading
Don't suffer fools gladly (must try harder!)
- Location
- Reading, obvs
Fuel Misers
- Thread starter Drago
- Start date
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smutchin
Cat 6 Racer
- Location
- The Red Enclave
You're not the only one.
Option not mentioned is let the Wife drive..this saves loads of fuel
Not in this house.
Jody
Stubborn git
You changed the question. Given a straight, flat road with no traffic a cruise control car will use less. Given everyday driving on undulating roads with traffic and bends cruise will use more.
Hence me saying generally a cruise controlled car will use more. There are so many variables that you can't give a definitive answer.
lazybloke
Ginger biscuits and cheddar
- Location
- Leafy Surrey
Seems there's a lot of misunderstandings about coasting & overrun upthread, and their impact on fuel consumption, so here's a very tedious roundup/comparison/verdict for anyone who still gives a fig.
NB this is for a petrol car with conventional clutch and gearbox, rolling DOWN a HILL. I haven't considered flat scenarios, as raised in the posts by @gbb and @Colin_P
Scenario 1) Gear engaged, no throttle. (aka "overrun").
The engine management cuts fuel consumption to ZERO in this scenario, because the momentum of the car keeps the engine turning.
From a safety point of view, overrun is good:
- engine braking helps keep speed sensible
- brakes & power steering work normally
- the alternator provides *free* amps from the kinetic and potential energy of the car
- car is immediately responsive if you need power
On a shallow descent speed can be lost to engine braking, easily countered by a *tiny* bit of throttle.
Scenario 2) Clutch depressed or neutral is selected (aka "coasting") ** Engine ON **
The forward momentum is no longer turning the engine, so the management system HAS to supply fuel to maintain idle speed. This is a *small* waste of fuel.
- there is no engine braking (may need to use brakes to keep speed under control)
- brakes and power steering are unaffected
- alternator cannot use the kenetic and potential energy of the car, and wastes fuel to provide a trickle of amps
- Car is NOT immediately responsive if you need power - but only takes a moment to re-enage the gear
Scenario 3) Clutch depressed, or gearbox in neutral (aka coasting) ** Engine OFF **
ZERO fuel consumption but clearly unsafe.
- brakes quickly go extremely heavy when used (servo contains only a small reserve of 'vacuum')
- no engine braking to regulate speed
- power steering dies immediately
- car not at all responsive if you need power (eg emergency situations)
- alternator provides no amps
A clear loss of control. Longer braking distances. A crash risk, etc. But hey, you saved a few drops of fuel.
Verdict: Scen 3 is out. Scen 1 looks the best even though it may take a bit of fuel to keep your speed up. I have a suspicion it's physically impossible for Scen 2 to be as fuel efficient as Scen 1, but with only 3 hours sleep I'm struggling to visualize a proof. I'll leave that as an exercise for others (they might also like to consider some flat scenarios).
PS Okay, there's a crazy 4th scenario, which is basically Scen 1 but with the engine off.
Scenario 4) Rolling downhill with gear engaged, no throttle, Engine ****OFF****
This just seems to wrong on every level; I would never treat a modern car like this. But I was once young and drove old bangers. I was Professor Bunsen, and an Austin Princess was my Beaker.
I turned off the engine at the top of Reigate Hill as I passed under the footbridge. DON'T DO THIS.
- engine braking was normal
- steering was normal because the car didn't have power steering
- brakes worked fine (the engine was turning, so it still created vacuum for the servo)
- the alternator was generating power as normal
But "What about fuel consumption?" I hear you ask. In a modern car it would be zero, but this car had a carburettor brimming with 4 star, which was being pulled into the engine with every intake stroke.
I think it was when I switched the ignition back on that the petrol ignited. It was the loudest backfire I've ever heard, and broke the back-box of the exhaust. like to think there was an impressive plume of flame from the trailpipe.
NOT TO BE REPEATED.
TLDR: Drive carefully folks
NB this is for a petrol car with conventional clutch and gearbox, rolling DOWN a HILL. I haven't considered flat scenarios, as raised in the posts by @gbb and @Colin_P
Scenario 1) Gear engaged, no throttle. (aka "overrun").
The engine management cuts fuel consumption to ZERO in this scenario, because the momentum of the car keeps the engine turning.
From a safety point of view, overrun is good:
- engine braking helps keep speed sensible
- brakes & power steering work normally
- the alternator provides *free* amps from the kinetic and potential energy of the car
- car is immediately responsive if you need power
On a shallow descent speed can be lost to engine braking, easily countered by a *tiny* bit of throttle.
Scenario 2) Clutch depressed or neutral is selected (aka "coasting") ** Engine ON **
The forward momentum is no longer turning the engine, so the management system HAS to supply fuel to maintain idle speed. This is a *small* waste of fuel.
- there is no engine braking (may need to use brakes to keep speed under control)
- brakes and power steering are unaffected
- alternator cannot use the kenetic and potential energy of the car, and wastes fuel to provide a trickle of amps
- Car is NOT immediately responsive if you need power - but only takes a moment to re-enage the gear
Scenario 3) Clutch depressed, or gearbox in neutral (aka coasting) ** Engine OFF **
ZERO fuel consumption but clearly unsafe.
- brakes quickly go extremely heavy when used (servo contains only a small reserve of 'vacuum')
- no engine braking to regulate speed
- power steering dies immediately
- car not at all responsive if you need power (eg emergency situations)
- alternator provides no amps
A clear loss of control. Longer braking distances. A crash risk, etc. But hey, you saved a few drops of fuel.
Verdict: Scen 3 is out. Scen 1 looks the best even though it may take a bit of fuel to keep your speed up. I have a suspicion it's physically impossible for Scen 2 to be as fuel efficient as Scen 1, but with only 3 hours sleep I'm struggling to visualize a proof. I'll leave that as an exercise for others (they might also like to consider some flat scenarios).
PS Okay, there's a crazy 4th scenario, which is basically Scen 1 but with the engine off.
Scenario 4) Rolling downhill with gear engaged, no throttle, Engine ****OFF****
This just seems to wrong on every level; I would never treat a modern car like this. But I was once young and drove old bangers. I was Professor Bunsen, and an Austin Princess was my Beaker.
I turned off the engine at the top of Reigate Hill as I passed under the footbridge. DON'T DO THIS.
- engine braking was normal
- steering was normal because the car didn't have power steering
- brakes worked fine (the engine was turning, so it still created vacuum for the servo)
- the alternator was generating power as normal
But "What about fuel consumption?" I hear you ask. In a modern car it would be zero, but this car had a carburettor brimming with 4 star, which was being pulled into the engine with every intake stroke.
I think it was when I switched the ignition back on that the petrol ignited. It was the loudest backfire I've ever heard, and broke the back-box of the exhaust. like to think there was an impressive plume of flame from the trailpipe.
NOT TO BE REPEATED.
TLDR: Drive carefully folks
KneesUp
Guru
The fluctuations in the average MPG per tank on the car Mrs Knees and I share would suggest that this is not always the case. However, she drives 'normally' and I drive as a miser as per the title.Option not mentioned is let the Wife drive..this saves loads of fuel
Jody
Stubborn git
Reminds me of my youth.
Particularly when you take in to account the accompanying laundry bills......
Nah - they're dead economical....
DaveReading
Don't suffer fools gladly (must try harder!)
- Location
- Reading, obvs
OK, thanks for clarifying.
It wouldn't occur to me to use cruise control in those circumstances.
OTOH I drove to Scotland and back last month with no more than three or four occasions in each direction on the M6/M74 where I needed to turn it off (not counting adjusting the setting on the fly to cope with odd bits of congestion) and I was extremely pleased with the fuel economy.
nickyboy
Norven Mankey
- Location
- You want hills? We got hills
As a practical example, I wonder which driving method results on less fuel used in this case:
Downhill, followed by a similar uphill.
1) Use overrun downhill, travelling slower than if in neutral but using no fuel. Acceleration required to get car to top of uphill, thus using fuel
2) Coast car in neutral downhill. Faster than overrun but uses some fuel. Acceleration required to get car to top of hill but, due to greater speed at bottom of hill than overrun, less fuel needed to do this
Intuitively, (2) feels like it should be less..the tiny amount of fuel needed to keep the engine idling whilst coasting downhill being less than the additional fuel needed to get up the other side in overrun due to lower speed at bottom.of the hill
KneesUp
Guru
I think it's
3) Accelerate in gear downhill to a speed that will then allow you to coast up the other side without needing to accelerate uphill
Of course the relative and absolute lengths of the uphill and downholl sections, and their gradients will affect this, but in theory it's best to gain all the kinetic energy when gravity is helping rather than when it's hindering.
roadrash
cycle chatterer
- Location
- sitting on the edge of wigan pier
