Better MPG while decelerating or on idle?

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mk1_salami

mk1_salami

Hi all,

I've been having a bit of a discussion with a friend about when you get the best MPG from a petrol car. I argued that the best MPG is obtained whilst decelerating (ie - on overrun), while he said you get better MPG while on idle (dipping the clutch) because while on overrun the engine is still using more fuel than on idle.

Who is right and why?
 
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You won't travel many miles to any gallon on either the over run or with your clutch dipped, will you? ;)
 
mk1_salami said:
Hi all,

I've been having a bit of a discussion with a friend about when you get the best MPG from a petrol car. I argued that the best MPG is obtained whilst decelerating (ie - on overrun), while he said you get better MPG while on idle (dipping the clutch) because while on overrun the engine is still using more fuel than on idle.

Who is right and why?
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On a modern FI car the ECU will cut the fuel while the engine is decelerating, allowing you to use the compression to slow the car. If you drop into neutral or coast clutch-in, the engine will tick over which uses fuel. I would say you are right. Others will say different, don't worry about it. :)
 
mk1_salami said:
while he said you get better MPG while on idle (dipping the clutch) because while on overrun the engine is still using more fuel than on idle.
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Wrong, on a newer fuel injected car if you are doing say 4,000rpm and take your foot off the loud pedal the car will cut the fuel to the injectors and use the engines Momentum / turning force of the wheels to keep the engine spinning over, once it drops below a certain rpm it will start up the injectors again.

Whilst on idle you are using fuel, so you can't use less fuel than no fuel!
 
Firestar_3x said:
Wrong, on a newer fuel injected car if you are doing say 4,000rpm and take your foot off the loud pedal the car will cut the fuel to the injectors and use the engines Momentum / turning force of the wheels to keep the engine spinning over, once it drops below a certain rpm it will start up the injectors again.

Whilst on idle you are using fuel, so you can't use less fuel than no fuel!
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OI get yer own reply
 
It really uses no fuel ? I swear it should stall or cause big engine resistance

Anyway you dont need to dip the clutch, 20 years ago it might made a difference but our car shows 85mpg anytime I stop pressing the accelerator pedal, at any speed
 
Firestar_3x said:
I was typing that before you even posted, so it was, oh the joys of having a digital AFR Gauge in the car :cool:
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I believe you ;) Silversurfer, your big engine resistance. What do you suppose engine braking is? Should the engine stop moving totally simply because there's no fuel being burned? There's still a cycle occurring, just without the expansion bit.
 
Yea but ever try pushing a car left in gear, its pretty impossible ( because the air is still being compressed 10:1?) . As opposed to a car left to idle in gear, it usually moves forward slowly
 
Coasting in neutral will be more fuel efficient than decelerating. If you are at 5000rpm and then lift off the accelerator you will drop revs but whilst you are dropping your reves you are still reving higher than idle. The car will slow down untill you stall if you don't downshift. In neutral the car is at its slowest tickover and will be most fuel efficient. The engine isn't powering anything at this point and so is easy to see why it will get higher mpg

However it is affected by driver style. If you coast but then slow down so much you have to accelerate hard to gain speed obviously it is going to be less efficient.

However leaving the car in neutral whilst moving is not advised by advanced drivers as it leaves you without any drive.
 
No the car can be doing 6000 rpm and still get 85mpg or better just the same as if it was idling in neutral, I just dont understand how it could be no fuel at all without a lot of drag
 
Ollie's Gadgets said:
Coasting in neutral will be more fuel efficient than decelerating. If you are at 5000rpm and then lift off the accelerator you will drop revs but whilst you are dropping your reves you are still reving higher than idle. The car will slow down untill you stall if you don't downshift. In neutral the car is at its slowest tickover and will be most fuel efficient. The engine isn't powering anything at this point and so is easy to see why it will get higher mpg

However it is affected by driver style. If you coast but then slow down so much you have to accelerate hard to gain speed obviously it is going to be less efficient.

However leaving the car in neutral whilst moving is not advised by advanced drivers as it leaves you without any drive.
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All of this post apart from the last sentence is comedy. Understand this - when you lift off the ECU cuts the fuel. The engine braking is provided by the compression until you apply more throttle, then the fuel is reintroduced. Tickover uses fuel. Deceleration uses no fuel. See wikipedia for definition of 'no fuel' as frankly I can't be arsed. This thread is the forum equivalent of a well-tagged brick wall.
 
silversurfer said:
No the car can be doing 6000 rpm and still get 85mpg or better just the same as if it was idling in neutral, I just dont understand how it could be no fuel at all without a lot of drag
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There is a lot of drag, it's called engine braking. You try doing 70 mph, drop the car into second gear and drop the clutch. That'll demonstrate how much drag the engine can produce.
 
Scuzi said:
There is a lot of drag, it's called engine braking. You try doing 70 mph, drop the car into second gear and drop the clutch. That'll demonstrate how much drag the engine can produce.
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70 in second and dropping the clutch, thats a different type of engine breaking
 
A modern FI car on overrun uses zero fuel...

If you dip the clutch then it uses fuel trying to keep the engine running...

If you have an AFR meter on your car then it will go to infinate on car overrun, if you have an MPG meter it will read 9999.99 mpg on overun..

The Ecu completely shuts the fuel off when it is not required to maintain the rotation of the engine...

:cool:
 
Ollie's Gadgets said:
Coasting in neutral will be more fuel efficient than decelerating. If you are at 5000rpm and then lift off the accelerator you will drop revs but whilst you are dropping your reves you are still reving higher than idle. The car will slow down untill you stall if you don't downshift. In neutral the car is at its slowest tickover and will be most fuel efficient. The engine isn't powering anything at this point and so is easy to see why it will get higher mpg
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Wrong wrong wrong wrong wrong...

Coasting in neutral or with clutch dipped uses fuel, decelerating in gear uses no fuel as the injectors shut of..

Yes you are right with a carb'd vehicle, but you are not understanding how a Fuel injection vehicle works...

:eek:
 
Just remembered the Scotland-and-back on one tank of fuel episode on Top Gear a while back - Clarkson drove a new Audi (A8 V8 diesel?) and just made it. He made a comment on the fact that modern fuel injected cars use no fuel when coasting in gear (i.e. approaching a roundabout).
 
sampo said:
Just remembered the Scotland-and-back on one tank of fuel episode on Top Gear a while back - Clarkson drove a new Audi (A8 V8 diesel?) and just made it. He made a comment on the fact that modern fuel injected cars use no fuel when coasting in gear (i.e. approaching a roundabout).
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i remember.
the segment where he drove around 1000 miles and just made it back to the forecourt he started from.

i also believe in santa and the tooth fairy by the way.
 
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