The Otto Cycle
“How can we describe the operation of a spark ignition engine?”
Spark ignition engines (also known as Internal Combustion Engines) were the backbone of 20th-century vehicles. And since us engineers love to describe things, how can we do so in a systematic manner? Well, to start, let’s analyze how several key variables change over time. To start, let’s draw in some air into the piston/cylinder under a constant pressure. Also, let’s label this process 0-1. Then, let’s move the piston such that an adiabatic compression takes place from the bottom dead centre (maximum height) to top dead centre (minimum height) in process 1-2. Afterwards, let’s represent the ignition as a constant volume heat transfer in process 2-3. This should soon cause an adiabatic expansion back to bottom dead center in process 3-4. Then, let’s complete the cycle with a constant pressure heat rejection in process 4-1. Afterwards, let’s reject the air at a constant pressure in the final process 1-0. This is known as the Otto cycle and is one of the most powerful tools for a Mechanical Engineer.