For example, why would a car manufacturer opt to to use a 2.0L in line six instead of a 2.0L in line four in a vehicle. Are the benefits of including more cylinders in an engine worth the added complexity?
In: Engineering
More cylinders means more balance in the mechanism, reducing wear and vibration. Everything’s not a V12 because more cylinders means more parts and machining steps, both of which cost money. While the parts are smaller and this also has some benefits, the smaller parts cost about the same to make.
ELI3: I want a burger. Burger place A has a really tasty, thick, double stacked burger. Burger place B has a really tasty thinner and wider burger. They are both the same amount of food.
ELI5: Bigger cylinders means bigger something else, either how wide the pistons are, or how far they move up and down. If they are wider, they are heavier. Heavier is bad when stuff moves. If they move up and down further, the rest of the engine has to be made to accomodate different movement. Those are tradeoffs that might not be acceptable whrn adding mire cylinders can provide the same displacement at the cost of being heavier.
Non ELI5: Larger diameter cylinders, or greater stroke, or both, come with tradeoffs. Adding cylinders is just scaling (mostly) the existing engine mechanics. In addition, it keeps the cylinders the same dimensions without encountering some of those aforementioned tradeoffs, while increasing displacement. Basically, engine design is all about tradeoffs and how you manage them.
Increasing displacement increase the rotating mass which decrease the maximum speed of the engine which again reduce the power. If you make the cylinder bore larger you need a bigger piston. And there is more forces on that piston so it needs to be thicker. The piston rod also needs to be thicker to handle the increased power. This puts a lot of strain on the crankshaft which also needs to be heavier to handle the weight. But even then all that acceleration of the heavy piston and piston rod is too much and you have to limit the engine speed to prevent damage to the engine. If you instead make the cylinder taller you can keep the same piston. But now it is going twice the speed which means twice the acceleration and twice the force just to move the piston up and down. This means the piston rod needs to transfer twice as much force but making it thicker also adds force to move the piston rod so you need to add even more weight. And eventually you have the same problem of too much forces and too much weight and have to limit the engine speed to keep the forces down.
And of course engine speed times torque is power. So a lower engine speed yields lower power. This is great for an engine that needs lots of torque like a truck engine or tractor engine. But when you need power to go fast you need the engine to go fast which means less displacement per cylinder.
You can use a lot of the same parts in a 6 cylinder that you did in a 4 cylinder. If I just make a bigger 4 cylinder then I need different parts across the board, which means maintaining separate inventories at smaller scales which means it costs more.
When you have a diverse offering of things (like in vehicles, restaurants, etc.) the name of the game is making everything with the least amount of unique components as possible.
Giant 4-cylinder engines can vibrate like crazy. It’s mostly a refinement issue (and you can make big 4-cylinders refined, but at a greater cost than just using a V6 or inline 6.
With V8, V10 and V12 engines it’s again mostly a refinement thing, or in the case of a lot of American V8’s tradition.
The larger an individual combustion chamber is, the slower it will completely burn. Also a more uneven, incomplete combustion. More and smaller combustion chambers are more efficient.
However, the same power output from less displacement means higher rpm and thus lighter and better lubricated components, tighter tolerances, less margin of production error.
Hence, old engines were big, rough and slow… modern engines are smaller, but more delicate.
Since no one has mentioend this yet, car manufacturers want to sell cars and trucks rather than make the most efficient car or truck.
See this truck I can barely get into… I want that one. I don’t want the four cylinder which can go three times as far on the same amount of gas.
Oh, this one has two more cylinders… it must be better because it costs more.
Signed “the vast majority of Americans”
Displacement and RPM determine the maximum power output. The explosion in the cylinder can only travel so fast. To optimize that explosion for a desired RPM a certain bore to stroke ratio and stroke length is desired. That determines your ideal cylinder size to meet your desired RPM. You would then add the number of cylinders needed to get your desired power output.
Your specific example is one that generally wouldn’t happen unless the manufacturer needs to spin the engine really fast.
0.5l per cylinder is an almost ideal volume for an Otto cycle engine in a normal vehicle. This is why most 4 cylinder engines are close to 2l total displacement and the best performing 6 cylinders engines are around 3l.
a big part of developing an engine is the design of the combustion chamber, such as how wide(bore) deep(stroke), piston speed, and compression ratio affects how it performs and behave. there are margins you can play with to try to squeeze more power but any dramatic changes in displacement will need to redesign the whole combustion chamber where as copy and pasting extra cylinders of an existing design is much more straight forward and easier. for example a lot of V-8 engines in the 80’s 90’s had to 2 ECU, each controlling 4 cylinders on either side essentially two 4 cylinder engines merged together.
so for your example of 2L 6cylinder vs 4cylinder, the combustion chamber of the 6cylinder is 0.33L ea/cylinder and the 4cylinder is 0.50L ea/cylinder. what this means for the 6cylinder is that it will generally have a smaller bore and stroke, which means it can rev to a higher RPM than the 4 cylinder with the same piston speed. higher engine RPM will generally produce more power, which some manufacturer might find worth the extra cost of developing for something like a sports car which commands a higher premium.
Good answers in the other posts. I have also read of “cylinder filling” being an issue when you want maximum efficiency.
Ferrari’s first V12 was for the 1.5L class, very tiny.
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