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
A smaller cylinder means the piston travels less distance to complete a rotation and the engine can achieve a higher RPM. A higher rpm means more peak HP. In a performance car that’s what you want. You don’t necessarily need to low-end out out you get from larger cylinders because the car is designed to stay in a higher rpm range. So this is why performance cars like Lamborghinis, Aston martins, Ferraris etc run a lot of V10 and V12 engines. But like you said, they do cost more and have smaller more delicate components which is why you won’t see a lot of cheap cars with V10’s
Straight out one reason to move to an I6 is that that engine layout is inherently very balanced. It’s going to run smoother without the same effort put into the I4 to try to balance it.
Second, an I6 has a piston firing every 120 degrees of crankshaft rotation, while an I4 has a piston firing every 180 degrees. So you get more power strokes per full revolution. Yay!
But say you want it to rev higher to get more horsepower. There’s a mechanical limit on how fast the pistons can go up and down (piston velocity). The higher the rpm of the engine, the faster those pistons are moving. And all that mass of the pistons is being yanked back and forth, which is a lot of stress. Eventually something breaks. For our purposes, let’s say that I4 is already running at the fastest piston velocity it should.
So you have a 2.0l 4 cylinder. Each cylinder is 0.5 liters. This is determined by the diameter of the cylinder and the stroke, or how much the piston goes up and down. If you go to I6, each cylinder is .333 liters, a third smaller.
We go I6 with the same displacement and cylinder diameter as the I4, but with that one-third less displacement per cylinder. This means the stroke of each piston is a third shorter, which means it doesn’t have to go up and down as far. So we get a lower piston velocity on the stroke than we had on the I4, all else being equal. Now we can rev higher and get more horsepower without having to make everything a lot stronger to handle higher piston velocity.
Or we can reduce the cylinder diameter so that the pistons are smaller and thus lighter, and thus easier to build to withstand a higher piston velocity
Or we can reduce both, a somewhat shorter stroke with smaller and lighter pistons.
Because of vibration.
For example, the displacement of the Lycoming O-360 flat-4 airplane engine is 360cid.
And of course the displacement of the LS1 V8 car engine is famously 350cid
The O-360 could theoretically have been used in cars, but you wouldn’t like the noise/ride/etc… We wear ear-pro headsets when flying a plane, even when it has a muffler installed (not all of them do)….
The LS1 is meanwhile rather heavy for airplane use…
(Note: Ignoring the expense difference due to bureaucracy/regulation and extensive engineering expense to make an airplane engine as-light-as-possible while still holding up – there’s a reason airplanes use air cooled engines. Also a reason why an O360 costs more than an entire car for just the engine.)….
There are really good answers here. I think the answer is sometimes as simple as what the manufacturer already has on hand for engine technology.
More smaller cylinders run smoother and get better performance at high speeds. When you’re moving that fast, the time for air and fuel to enter the cylinder and exhaust to leave starts to really matter, and a smaller cylinder can do that exchange quicker. These are positives for passenger cars, so many of them will feature V6 and V8 engines instead of I4s with larger displacement.
Fewer, larger cylinders give you more torque and a simpler design, but also more noise and vibrations to deal with. These are often used for farm equpment, generators, and other industrial applications.
Basically, having more cylinders allows for smoother and more balanced power delivery, which translates to better performance and efficiency. Plus, increasing displacement beyond a certain point can lead to diminishing returns and reduced fuel economy. So, while more cylinders do add complexity, the benefits can make it worthwhile for engine designers to opt for them.
It has a lot to do with how fast fire can move, flame front and flame propagation speed. Gist for ELI5 is that the explosion/fire can only move so fast and how fast it moves actually depends one the mix of gas and air. Too lean or too rich actually slows down the propagation of flame through a cylinder.
Typical engine cylinder has the spark plug near center roof. It takes time for the flame to burn all the way to the edges of the cylinder. You want the expanding flame/gases/molecules can push down evenly on the piston not just the center part. The cylinder bore size and stroke is optimized for this.
What others have also mentioned about the walls cooling the flame as well also does absolutely play a roll in complete combustion. If the cylinder walls are it too cold, it essentially puts the flame out. Fire needs heat, fuel, oxygen.
Most interesting college class I took was combustion.
4 bangers are like sewing machines. If you go to 6 or 8 the engine smooths out dramatically.
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