Why are Turbo 4cyl engines Fuel efficient and V6/V8 Turbo’d engines not equivalent or better?


Why are Turbo 4cyl engines Fuel efficient and V6/V8 Turbo’d engines not equivalent or better?

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They are likely similar (if not better) as far as HP per Litre of gas used. The only issue is that you don’t regularly need the full power they can produce.

If you were trying to move cross-country, it’s more fuel efficient to use one semi truck and not 50 civics. But if all you need is to move 2 people, a semi truck is no longer as efficient (as you’re not needing its full potential).

V6/V8 have two/four more cylinders that need to be moved.
So by default you need more energy. It’s only when you use their fully power that they become more efficient.

It mostly comes down to volume of fuel needed to reach the ideal fuel to oxygen ratio in the displacement of each cylinder. More volume means more fuel. Attempts have been made to only use 4 cylinders on an 8 cylinder engine under certain circumstances, but this still requires all of the extra spinning mass of the pistons, pins, rods, counterweights, and the resistances in the valvetrain.

In addition, a low cost unified air intake manifold must be sized for the maximum flow potential of the full engine displacement. The large volume in this area reduces air velocity at lower engine output speeds. The lower velocity air reduces mixing in the cylinder at lower output power. This results in poor fuel economy while cruising, which is the primary state most automotive engines operate. Peak efficiency involves optimizing flow in these low output situations. Most 4 cylinder engines require fewer compromises when it comes to air flow into and out of each cylinder

If you compare a naturally aspirated V6/V8 to a blower V6/V8 if tuned for efficiency then the blown versions would be more efficient than the NA version. But V6/V8 are usually only blown for big performance gains which takes extra fuel.

You can look at a blown V8 like two inline-4 connected together at the crank. For each mile driven by the V8 it’s basically using the same gas as two 4cyl engines (not 100% accurate, but it’s a good analogy) because the V8 is more mass being moved in each revolution of the crank and each cylinder gets the same amount of fuel.

Because turbocharging an engine doesn’t change the fact that a 6 or 8 cylinder engine has more cylinders than a 4 cylinder engine. Since each cylinder uses a certain amount of fuel, more cylinders means less fuel efficient.

There have been attempts to solve that issue by inventing engines that have 8 cylinders but can run on 4 cylinders normally, only activating additional cylinders if needed for more power.