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Old 06-01-2017, 02:37 PM   #1
FlowMK6
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A stupid question

Hi everyone, as the title said, this is probably a stupid question.

My question is : Why do our car use fuel when braking ?

I noticed that it does use more fuel when the gear is in neutral. Maybe it doesn't use fuel and it's just an issue with the display...

Thanks you for your help and your time,
Kind regards.
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Old 06-02-2017, 01:40 AM   #2
MLue1
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Quote:
Originally Posted by FlowMK6 View Post
Hi everyone, as the title said, this is probably a stupid question.

My question is : Why do our car use fuel when braking ?

I noticed that it does use more fuel when the gear is in neutral. Maybe it doesn't use fuel and it's just an issue with the display...

Thanks you for your help and your time,
Kind regards.
Assuming that your MFD is set to look at fuel consumption as you drive, in Canada the MFD switches to Litres/hour from km/h.
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Old 06-02-2017, 04:02 AM   #3
FlowMK6
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Originally Posted by MLue1 View Post
Assuming that your MFD is set to look at fuel consumption as you drive, in Canada the MFD switches to Litres/hour from km/h.
MFD stands for multi function display ? And yes, it's set to display the fuel consumption as I drive, it does show litres/hour when the car is not moving.

But when I drive, it shows litres/100km. I noticed that while being engage in a gear, when I'm braking and not giving gas, my fuel consumption is about 2l/100km, but before braking it was at 0l/100km. I don't give any gas, so why my fuel consumption goes from 0l/100km to 2l/100km just by applying the brake pedal ?

Sometimes it reach about 3.5l/100km while braking, I'm not looking for fuel economy, I'm just trying to understand why the engine have to use gas when applying the brake. The brake system is hydraulic, it doesn't need work from the engine, so it's not supposed to use fuel. I would understand the gas usage if I would be in neutral the engine would have to work to stay idle, but when I'm engage, the energy of the transmission should be enough to keep the engine running and to avoid using gas.

Is it because the engine have to keep the vacuum pump running since the brake booster depends of the vacuum produced by this pump ? What if I don't have a vacuum pump anymore ?

Thanks again for your help, just trying to figure out something useless...
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Old 06-02-2017, 04:02 PM   #4
MLue1
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Lol, just trying to figure out something useless... . I don't have any definitive answers on speculation, it may have something to do with keeping the engine running, or a left over from the "rev hang" syndrome.
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