I guess what about having extra pistons makes mpg worse vs. 1.8L, 2.5L, 3.5L, etc.?
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A V6 engine has a lot more moving parts than a V4 engine. This means there is more friction etc. At some point, the same capacity in the V4 will be more fuel efficient than a V6. Also more parts means more costs to make.
An *inline* 4-cylinder is mechanically less complex, with one bank of cylinders, one cylinder head and valvetrain. A V6 is a way of getting six cylinders in a compact space. Inline 6 is long.
V4 is less common in road cars.
Here’s one of many results I found when Google searching for “inline 4 vs v6” https://www.quora.com/If-they-have-the-exact-same-HP-and-torque-what-are-the-advantages-disadvantages-of-a-V6-vs-I4-Turbo
The move from V6 naturally aspirated (i.e. no turbo or supercharger) forced-induction (effectively, turbocharged) smaller inline 4 is https://en.wikipedia.org/wiki/Engine_downsizing
https://youtu.be/9aO2vC_iMTI gives what seems to be a more thorough discussion
you are actually asking 2 questions here. a turbo forces air into the engine and allows it to burn more fuel and make more power. by itself that is no more efficient than any other engine consuming the same amount of fuel.
engines with less cylinders are more efficient because they have fewer moving parts causing friction losses and smaller cylinders burn their fuel more completely. 4 cylinders in cars are always inline, but you could make a v6 engine with smaller cylinders. this would not help because going v configuration adds complexity and more parts, so less efficient.
There are practical limitations on parts’ sizes, in order to be durable enough, heat resistant enough, ability to be worked on, etc. that make fewer larger cylinders more practical than more tiny ones.
Short version: it would be inconvenient for little gain.
While we can get into a lot of details about power curves and such the ELI5 super-simplified version is: smaller displacement, less power. Not cylinder count, displacement.
So a smaller V6 introduces a number of manufacturing complications, more moving parts, etc… compared to an I4 of similar displacement or slightly smaller with forced induction (the turbo) to make up the lack. The extra moving parts in a v6, minimum cylinder wall thickness, etc… also take up space you might prefer to spend on displacement so at a certain point you get more displacement in the same overall space with fewer cylinders.
V6s are worth it when you want to get into a certain displacement within a certain form factor but if you’re downsizing that overall size then an I4, even turbo charged, can get similar results more easily from a consumer point of view.
V-configuration engines have more parts, more friction, thus create more inefficiencies than an equivalently sized inline engine.
Most 4 cylinder engines are inline. AFAIK only Ford built and mass marketed a V4 in Europe – and Saab used some of them as well to replace their two strokes, being a short engine. To answer your question, you can make a V6 as small as you want, it’s still more expensive than an inline 4. Think two cylinder heads instead of one. The 4 will also be more flexible from a manufacturing standpoint as you can fit it in various models, from subcompact to full size, have it non turbo, single turbo or biturbo, hybrid, Atkinson ycle, whatever.
The question is “what is the simplest and most efficient way to move as much fuel as possible through an engine?”
* simplest is fewer moving parts
* most efficient means extract as much energy as possible from the fuel
* as much fuel as possible means larger displacement
This tends to lead us to inline 4 cylinder engines with computer controlled of the valves, timing, fuel mix, fuel/air boost and all of that.
There are lots of other considerations, as well: manufacturing cost, availability of materials, maintenance costs, reliability, and so on.
Wow most responses are getting far too deep into this. It’s really as simple as a turbo isn’t always on. Every engine must use fuel in proportion to how much air is pulled into the engine. Less air going into the engine the less fuel is used.
Turbos only push more air if you accelerate hard enough to spool it up so for most people they are only using a fraction of the available power. A turbo engine pushed hard constantly will not be more efficient than an equivalent hp larger displacement engine
Small displacement v6/8/10/12s exist but are designed to be small to allow much higher rpm’s to meet a desired power output. Higher rpm’s means more air and again you are forced to put enough fuel in there basically at all times to not damage the engine. Engines designed to be happy at high rpm’s are basically undrivable at low rpm’s due to a drastic loss of torque.
Example of 3 different cars/4 different engine setups I’ve owned
2.4l NA i4 205hp 7k redline averaged 22-24 mpg
2.0l NA i4 237hp 9k redline averaged like 19 mpg
2.0l SC i4 362whp 9k redline (same engine) 12-15
2.0l TC i4 306hp 7k redline 23-24 mpg driving like a madman but lately I’ve been good and I’m at about 29-30
Superchargers are basically always on so they don’t have the efficiency boost of a turbo
Volumetric efficiency. In a naturally aspirated engine the amount of fuel air mixture entering the engine is determined by the diameter of the cylinder and stroke of the crank shaft. Combined with the compression ratio you can factor out what will be the most efficient. (Not most powerful) that is set, the only way to get more efficient is to reduce parasitic drag. Meaning less rotating mass.
Forced induction, like a turbo, can achieve higher compression ratios and with less cylinders comes less rotating mass. End result is the same power output in the target rpm range.
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