Naturally aspirated engine
A naturally aspirated engine, also known as a normally aspirated engine, and abbreviated to N/A or NA, is an internal combustion engine in which air intake depends solely on atmospheric pressure and does not have forced induction through a turbocharger or a supercharger.[1] Many sports cars specifically use naturally aspirated engines to avoid turbo lag.

In stroke #1, the pistons suck in (aspirate) air to the combustion chamber through the opened inlet valve.