Next comes a need for some thought. If we ignite the fuel-air mixture now, with just atmospheric pressure in the cylinder (14.7 pounds per square inch at sea level), combustion will heat and raise the pressure in the cylinder to 7 x 14.7 = 103 pounds per square inch (psi). While that's a useful pressure, we can easily get a lot more. Here's how: Instead of igniting the fuel-air mixture with the piston at the bottom of its stroke, we now drive the piston toward the cylinder head, compressing the fuel-air mixture in the cylinder to a much higher pressure, say, 150 psi. It takes power to do this, but it's an investment because when we ignite our trapped fuel-air mixture compressed to 150 psi instead of at 14.7 psi, we now get 7 x 150 = 1,050 psi. That gives us some really serious force against the piston. It is to achieve this much higher combustion pressure that we compress the fuel-air mixture before we ignite it. This is the compression stroke, which, with both valves closed, begins with the piston at its bottom position and the cylinder full of fresh fuel-air mixture, and ends with the piston rapidly nearing the cylinder head.