In car engines why don’t we use only one big cylinder instead of multiple cylinders, thereby not use cam shaft anymore?

Let me introduce you to the Suzuki LS650 Savage (’88 vintage)

650ccs, single-cylinder, SOHC. Not a horrible bike, but it’s called “Thumper” for a reason: the unmistakable “thump” every other revolution as the engine fires. The top RPM is about 6000. High power, 31 HP at 5400 RPM.

Now, let’s have a peek at this bike, the Honda CB65-SC “Nighthawk.” It’s contemporary to the Suzuki, actually pre-dating it by a few years.

Four cylinders, DOHC. Top RPM, about 12,000. Top power, 72HP at 10,000 RPM. Much, MUCH more fun to ride.

When you have only one cylinder, you have a HUGE amount of rotating mass. That limits the top RPM, and the inertial load means you just can’t spin it up very fast, so it takes approximately forever to actually GET to top RPM.

The tradeoff is that you get a lot of torque, pretty quickly. But it takes a long time to get enough R’s to do anything useful with it.

That’s why tractors, trucks, stationary load engines typically have large displacement cylinders with long strokes, and racing engines have many, many more small displacement cylinders with short strokes.