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Every dirt bike i know has a single cylinder engine. I myself ride a Yamaha XT660Z which has a huge 660 cc cylinder.
What are the characteristics of those engines? what are the benefits of such design?
Another interesting topic is that a lot of the engines used in dirt bikes are 2 stroke engines. What are the pros/cons of such engines compared to 4 stroke?
The main reason for the use of a 2-stroke engine is it accelerates much faster than a similarly sized 4-stroke engine. The obvious reason for this is a 2-stroke cylinder fires on every revolution of the crank shaft, while a 4-stroke does it every other revolution. Also because the 2-stroke fires on every revolution, they only need enough weight to balance the engine. A 4-stroke engine needs added weight in the crank shaft to allow it to keep running during the non-firing stroke. This applies mainly when the engine is running in the lower RPM ranges. Due to the added weight, it takes the engine longer to increase speed from the lower to higher RPM ranges.
These engines are also primarily air cooled. Having a single cylinder engine air cooled is a lot more efficient than trying to cool a two (or more) cylinder air-cooled engine. Air cooled is a simpler solution than is water cooled.
A single cylinder engine is a lot simpler than a two cylinder engine, whether 2- or 4-stroke. There are just a lot less moving parts.
There is no issue with why larger is better ... it goes back to the old saying of There is no replacement for displacement. Having more displacement, whether 2- or 4-stroke is going to provide more low end torque.
Due to the way most 2-strokes fill the cylinder with air/fuel mixture, they are inherently less efficient than a 4-stroke engine is. The volumetric efficiency of a naturally aspirated 2-stroke limits its ability to make power. A similarly sized 4-stroke can fill the cylinder a lot more completely, allowing it to make a lot more power. For the edification of those who may not know, a 2-stroke engine only has a reed valve which functions as a check valve to allow for the correct air/exhaust flow. The inlet for the air/fuel is through low in the cylinder port(s) on one side of the cylinder. These inlets are at the position where, as the piston travels down in the cylinder, these ports are uncovered to allow the air/fuel mixture to enter the cylinder. As the piston travels back up the cylinder, the port(s) are closed off, which allows for the compression of the air/fuel mixture just prior to ignition. As the piston travels down the cylinder, it gets to the exhaust port(s) first, which are also in the side of the cylinder, only opposite the intake ports. The pistons are usually specifically shaped to direct the outgoing exhaust and incoming air/fuel mixture in the correct direction so the engine will actually function (instead of say the exhaust going out the intake port). In comparison, a 4-stroke piston is relatively flat (there are exceptions). Here's a pretty good representation of the two strokes and piston shape:
With a dirt bike, having the low end torque you need to dig your way out of a tight turn, then having the ability to get up to speed quickly ... that's where it's at. A 2-stroke engine can do both of these for a bike, at the cost of efficiency in the process.
Subaru fires each cylinder 1 at a time but the Subaru is a boxer design which means that the cylinders are opposed to each other in pancake manner like Volkswagon, Porche and Corvair.... This allows them to have higher frequency vibration with less weight because they do not have to use counterbalance weights within the motor.
Less weight and higher frequency mean a longer life and less weight means better economy.
The boxer is a German design origination.
The boxer design also allows a smaller engine with less wasted space.
A single cylinder 2 stroke or 4 stroke will have higher torque because torque is direct result of the surface area being pushed .... The bigger the bore the higher the torque .... The more cylinders the smoother that torque is delivered but splitting displacement between 2 cylinders will give smoother delivery reducing torque.
When riding on dirt you effectively have a fluid coupling to the ground on pavement is not so except on drag strips..... On dry pavement the pulse delivered by any engine is prefered to be delivered in as many pulses as possible because once traction is broken is difficult to regain one big pulse is more likely to break traction of the tire to the pavement..... .... drag racing is done with horse power you only need enough torque to start the vehicle moving then it is the horse power that makes for a low time thru the traps.....
Horse power and torque can be traded through the use of gearing
If your single cylinder is one to one ratio then the tire will get a jolt of power at the same point on it every revolution however going 1:2 it will fire the jolt at the same point but only every other revolution of the tire going the other way 2:1 ratio means the tire gets 2 jolts 180 degrees apart each revolution going 4 to 1 press a jolt at every 1/4 Rev of tire best is if you can get many jolts spread evenly.
It's similar to the firing order of a Subaru, which instead of an inline 4 firing, 3, 2, 4, 1, a Subaru fires two cylinders at a time, and it's because when you're riding on dirt you want a delay between firing. It gives you a pause, and more traction. When you're riding on pavement you want 12 cylinders, but even from the start line, you don't want to burn out, you want as much acceleration as possible without losing traction.
