Because of this, you need to mix oil (the lubricant) with fuel before putting it in a 2 stroke engine—otherwise, the engine will seize up. In a 4 stroke engine, air and fuel (and the combustion ...

starts with the piston at the top of the cylinder (TDC) and the intake valve open to admit air and fuel. At bottom-dead-center (BDC) the inlet valve is closed and the compression stroke (b) starts.