Why does a 2.0 liter engine use less fuel than a 4.0 liter engine?
October 30, 2009 in Other - Cars & Transportation
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Nathan L asked:
Let’s assume that there are 2 choices of a diesel engine to put into a truck. One is a 4 liter engine and the other is a 2 liter engine. Both are commonly available industrial engine from Cummins, John Deere, Isuzu, Kubota, Deutz, VW, etc. Both have fuel curves that are exactly .34 pounds of fuel per horsepower-hour at 1800 RPMs. Why is there an advantage in the fuel efficiency of the smaller displacement engine, even though they are rated the same to push, say, a 5000 pound pickup? (This is true, even if we correct for the added weight of the larger engine). Something does not make sense here. If both are cruising at 65 MPH and the vehicles weigh the same, shouldn’t they use exactly the same amount of fuel per mile?
I realize it has more displacement. However, a 5000 lb truck moving at a constant speed requires the same horsepower to keep it moving. Say it uses 80 HP…..80 times .34 should be about 27 lbs of fuel per hour regardless of the displacement of the engine. Why in actual practice, does the 2.0 liter engine get better mileage?
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It has 2X the displacement. So it’s burning more air, and more air means more fuel is burned. Easy enough.
Hey, Gerald! Which weighs more? A pound of nails or a pound of feathers? *laughter*
Well…. a 2.0 liter engine has a smaller bore for the pistons than a 4.0. Bigger engine=more gas needed to move the pistons=more horsepower. A smaller engine can not push the same HP as a larger engine at the same RPM. Torque is going to be a HUGE issue with the smaller engine as well. They may consume the amount of fuel per HP, but the larger engine is going to crank out more HP.
“Both have fuel curves that are exactly .34 pounds of fuel per horsepower-hour at 1800 RPMs”
4ltr engine will develop a greater horse power at the given rpm than the 2ltr unit. I’ll leave you to do the maths.
You gave a lot of factors/information in your question. However I feel it has to be the weight difference. A 2 liter engine must be 2 liters lighter than a 4 liter engine. That would be like saying a 6ft 180lb woman weighs less than a 5ft 180 lb man.