XUD/TUD coasting/fuel arguement

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piglet
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XUD/TUD coasting/fuel arguement

Post by piglet » Thu Feb 01, 2018 10:22 pm

Please settle a debate indoors re. No.2 sons coasting habit in TUD5 106
Having trawled the interweb for wisdom I am back here. It is stunning what some individuals out there believe.
I have established what I kind of already knew in that any electronic injection system will cut fuel supply in over-run condition,
and that an old skool carburated car will still be sucking fuel in over-run even with your foot off.

But what of mechanical injection? Using fuel on over-run or not using fuel??

Personally I think coasting is a disgusting filthy habit right up there with using front fogs as DRL's
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Doggy
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Re: XUD/TUD coasting/fuel arguement

Post by Doggy » Thu Feb 01, 2018 11:46 pm

piglet wrote:
Thu Feb 01, 2018 10:22 pm
But what of mechanical injection? Using fuel on over-run or not using fuel??
Not sure. I think no fuel on overrun.
Personally I think coasting is a disgusting filthy habit right up there with using front fogs as DRL's
+1
coasting is potentially dangerous
using front fogs confirms you're an idiot
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PeterN
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Re: XUD/TUD coasting/fuel arguement

Post by PeterN » Fri Feb 02, 2018 11:55 am

A mechanically injected engine will normally continue to deliver fuel at tickover rate when the throttle is closed which on one of mine corresponded to around 300 mpg.

An electronic system shuts right down under overun conditions using virtually no fuel, so for best economy keep it in the highest gear possible when descending a long hill so you don't have to open the throttle to maintain your speed.

Peter

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Welly
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Re: XUD/TUD coasting/fuel arguement

Post by Welly » Fri Feb 02, 2018 1:26 pm

I would think an older mechanical diesel injection system would still inject 'idle' fuel like Peter says, if you lifted off at speed and did nothing else eventually the car would slow to 10mph or whatever and just keep chugging along on idle. An electronic system would shut off all fuel on the slow-down right up until it it reaches the set idle speed or a potential stalling event.

Coasting out of gear puts you at risk of not being in control should you suddenly need to take up the drive again and also coasting pretty much turns your car into a loose trolley on 4 wheels, the engine braking affect on the driven wheels plays an important roll in a cars stability, try dipping the clutch mid-corner and you'll feel the affect.

Someone mention Fog lights? :P
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piglet
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Re: XUD/TUD coasting/fuel arguement

Post by piglet » Fri Feb 02, 2018 3:15 pm

Welly wrote:
Fri Feb 02, 2018 1:26 pm
Coasting out of gear puts you at risk of not being in control should you suddenly need to take up the drive again and also coasting pretty much turns your car into a loose trolley on 4 wheels, the engine braking affect on the driven wheels plays an important roll in a cars stability, try dipping the clutch mid-corner and you'll feel the affect.
+1 with nobs on

Putting that as an aside however, coasting for the purposes of hypermiling is complex.
Using the long descent scenario there is a watershed of gradient where the hill is steep enough to maintain speed in gear with no power.
Steeper than this gradient there would be no gain with electronic injection in coasting as you are already using zero fuel.
Less than this gradient and the car will slow with engine braking so the car will need fuel to maintain velocity.
At what point what will it be cheaper to go to neutral and coast without engine braking accepting some fuel use to maintain tickover?
Engine braking is just another term for engine friction(plus compressing/heating air), and friction is energy loss.

In the case of mechanical injection then, if the pump is delivering fuel for tickover then the savings from coasting are greatar as even with your foot off the engine is delivering tickover fuel but at a higher RPM.

All fine on a long straight descent in light traffic, but not round bends please.
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