diesel fuel consumption

[748rhydhir]

hypothetically which would use the least fuel in a diesel car a) freewheeling down a hill & using the brakes to control the speed or b) driving down the hill in gear & use engine breaking ? or would the fuel usage be the same?

[TWR]

Free wheeling down a hill, Very dangerous driving for sure, If you have ever had a brake failer you would never do that your brakes are your secondary braking system, Engine braking! are you talking about a Exhaust er brake? If yes the Exhaust er chokes the engine so you would not be using fuel.

[748rhydhir]

all hypothetical.appreciate your answer.to amplify go down a hill foot off accelerator or go down the hill in say 5th gear (not a steep hill)with your foot off the accerator.how would the fuel consumptions compare.
assume hill about a mile long & no braking or gear change needed.

[TWR]

I will just leave you with one question regards saving derv down hill coasting. have you ever had brake failer? & this is where I will leave you to think about your questions.

[748rhydhir]

it is a hypothetical question.i never freewheel down a hill.i beleive it is illegal anyway.

[bigboots]

On modern common rail diesel engine the injectors shut off when engine braking so NO fuel is being used as the momentum of the car keeps the engine turning over
If you free wheel the fuel is being used to keep the engine idleing
So hypotheticaly engine brakeing gives betyter fuel economy

[748rhydhir]

thank you very much for your concise answer.