Chaz
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| posted on 21/9/08 at 12:26 PM |
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Saving Fuel (staying in gear)
I think i've done pretty well getting 45mpg from my celica through driving like a granny.
I was watching the Top Gear episode about the Audi A8 which went to Edinburgh and back on one tank. One thing Clarkson said was that "if you
stay in gear when pulling up to a roundabout or junction etc you use NO fuel, which is better than putting it into neutral which uses a LITTLE bit of
fuel to stop then engine from stalling".
Is this true?
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mackei23b
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| posted on 21/9/08 at 12:31 PM |
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I would have to say.......NOT True (OK at least for carb engines)
[Edited on 21/9/08 by mackei23b]
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djtom
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| posted on 21/9/08 at 12:41 PM |
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True, most cars use overrun fuel cut which cuts the injectors completely during deceleration. Made a reasonable difference to economy while I was
tuning my Megasquirt.
Tom
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McLannahan
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| posted on 21/9/08 at 12:51 PM |
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In my VW the onboard computer will tell you this too.
Coast down a hill out of gear and you'll get let's say 60mpg. Coast down in gear but without touching the pedal and it'll rise to
200mpg and then registers -- -- as in no fuel being used so can't make an assesment.
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blakep82
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| posted on 21/9/08 at 01:06 PM |
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yep, injectors can switch off, and will only cut in to stop the engine stalling.
though i can't say every car will be the same.
i guess thats part of why carbs are less efficient than injection
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zilspeed
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| posted on 21/9/08 at 01:16 PM |
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Mine does fuel cutoff under over-run in gear too.
As mine runs on lpg, there are solenoids which control gas flow to the front end setup. As I coast in gear up to a junction, just as I dip the clutch
I can clearly hear the solenoid clicking back open again to turn the gas back on again.
Referring back to carb engines, many were fitted with an anti dieselling valve which cuts fuel off when the ignition is killed to prevent run on.
With the later mapped ignition setups on carbed engines, it's not outwith the realms of possibility that the anti dieselling valve was used for
this too.
Anyway, my Golf with Simos ECU definitely does fuel cutoff on over run in gear.
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smart51
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| posted on 21/9/08 at 04:03 PM |
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The fuel injectors are switched back on again around 1500 RPM to stop stalling but yes, the momentum of the car is more than enough to keep the engine
turning so there's no need to use fuel.
Carburettors don't do this though.
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JoelP
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| posted on 21/9/08 at 08:10 PM |
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its a case of finding the balance, because to counter engine braking you have to stay on the throttle for longer on the approach.
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smart51
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| posted on 21/9/08 at 08:40 PM |
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quote:
Originally posted by JoelP
its a case of finding the balance, because to counter engine braking you have to stay on the throttle for longer on the approach.
Not necessarily. Most people drive at the speed limit right up until the red traffic light or obstacle then will brake moderately heavily. Instead,
lift off the accelerator a little earlier and let the car slow with the fuel injectors off in overrun then brake from a lower speed at the
obstacle.
You'll save fuel, save wear on you brakes and tyres and will be less likely to be crashed into by the person behind you who is more interested
in texting than driving.
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martyn_16v
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| posted on 22/9/08 at 01:09 PM |
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Overrun fuel cut has very little to do with why carbs are so much worse than modern fuel injection when it comes to fuel economy. It's all to do
with the fact that modern injection systems allow you to optimise the fuelling at hundreds of points in the operating range of the engine. Carbs on
the other hand have half a dozen (or less) mostly linear factors which you have to fit to the engine model as best you can, and then add a bit more
fuel 'just to be on the safe side'. The same goes for modern ignition systems, being able to optimise the ignition advance over the
operating range makes a massive difference compared to clockwork ignition.
FWIW, some of the latest US state emissions regulations allow only a few mg of unburnt fuel through the exhaust on an engine test cycle that
approximates about 11 miles of driving. That's how accurate things have become, compared to the old carbs 'ooh that's eye wateringly
rich' 
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JoelP
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| posted on 22/9/08 at 04:59 PM |
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quote:
Originally posted by smart51
quote:
Originally posted by JoelP
its a case of finding the balance, because to counter engine braking you have to stay on the throttle for longer on the approach.
Not necessarily. Most people drive at the speed limit right up until the red traffic light or obstacle then will brake moderately heavily. Instead,
lift off the accelerator a little earlier and let the car slow with the fuel injectors off in overrun then brake from a lower speed at the
obstacle.
You'll save fuel, save wear on you brakes and tyres and will be less likely to be crashed into by the person behind you who is more interested
in texting than driving.
If you're driving like that then there's not much point worrying about the little bit to be saved here!  First place to save fuel is not
using the gas more than you need to, and coming off it as soon as you can!
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