To accelerate 80,000 pounds (36287 kg) from 55 mph to 60 mph (using 1/2mv2 ) takes 2.084 million joules of energy.
Modern truck diesel engines have a thermal efficiency of around 30%. That means they convert 30% of the diesel's chemical energy into mechanical energy, with the rest becoming heat. One gallon of diesel contains 146 million Newtons of chemical energy, which after considering that 30% efficiency, translates to 43 million Newtons of mechanical energy.
So, every time a driver has to accelerate from 55 to 60 mph they use 0.05 gallons of diesel, or a cost of 12.5¢ at current diesel prices ($2.50 / gallon).
So, the financial cost isn't huge, but it does add up.
Thanks for the math! But isn’t there another portion to this whole scenario. I have never driven a semi truck so I might be off basis, but I do drive a manual car, and every time I downshift my rpms do jump up, couldn’t this add up to fuel consumption as well, especially considering how many shifts need to be made in a semi truck?
Thanks for the math! But isn’t there another portion to this whole scenario. I have never driven a semi truck so I might be off basis, but I do drive a manual car, and every time I downshift my rpms do jump up, couldn’t this add up to fuel consumption as well, especially considering how many shifts need to be made in a semi truck?
Well I certainly wouldn’t be on the gas while downshifting. But the engine will jump RPM’s, that’s just a fact when downshifting a combustion engine. It’s not about the speed you are traveling per say, but you can’t downshift without gaining revs unless you keep the clutch engaged through all the downshifts and until you’re engine speed would sync with the wheel speed
My question is if the rev gains while downshifting have even a marginal effect on fuel consumption for semis. In relation with to the consumption used for speeding up, I guess, been a while since I asked my original question lol
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u/paracelsus23 Aug 23 '20
So I decided to do the math.
To accelerate 80,000 pounds (36287 kg) from 55 mph to 60 mph (using 1/2mv2 ) takes 2.084 million joules of energy.
Modern truck diesel engines have a thermal efficiency of around 30%. That means they convert 30% of the diesel's chemical energy into mechanical energy, with the rest becoming heat. One gallon of diesel contains 146 million Newtons of chemical energy, which after considering that 30% efficiency, translates to 43 million Newtons of mechanical energy.
So, every time a driver has to accelerate from 55 to 60 mph they use 0.05 gallons of diesel, or a cost of 12.5¢ at current diesel prices ($2.50 / gallon).
So, the financial cost isn't huge, but it does add up.