# How a Turbocharger Works A cutaway turbocharger on a running engine: exhaust spins the turbine, the compressor squeezes air, and controls change boost, intercooling, scrolls and the number of turbos By Ri Man Topic: Machines [Open the interactive Jig](https://ask.jig.com/j/O31QfHMm70rO) ## Transcript ### 1. A fan blown by its own exhaust This little engine on the workbench wears a turbocharger: two fans on one shaft, cut open so you can see inside\. Hot exhaust spins one fan\. That fan turns the other, which pushes extra air into the engine\. Drag anywhere to look around\. Scroll or pinch to move closer\. ### 2. Exhaust spins the turbine Burnt gas leaves the cylinders hot and fast\. The snail\-shaped housing, called a volute, narrows as it curls, speeding the gas up before it hits the blades of the turbine wheel\. More engine speed means more exhaust and a faster spin\. A turbo shaft can spin between 80,000 and 200,000 times a minute\. Here it is shown slowed right down\. ### 3. One shaft, two wheels The turbine and the compressor sit on the same steel shaft, so they always turn together\. The aluminium compressor wheel draws in fresh air and flings it outward into its own snail housing, squeezing it\. Exhaust and fresh air never mix\. The shaft floats on a thin film of engine oil, which also carries heat away\. ### 4. Cool air is denser air Squeezing air heats it up, and hot air is thin\. The intercooler is a radiator for the air: it cools the charge so more oxygen fits into each cylinder\. Switch it off and watch the air stay warm and the power drop\. More oxygen lets the engine burn more fuel in each stroke\. A turbo adds air, and the engine adds the matching fuel\. ### 5. The wastegate sets the limit Boost is the extra pressure above normal air\. Too much could hurt the engine, so a spring\-loaded valve called the wastegate opens a side door and lets some exhaust bypass the turbine\. A stiffer spring allows more boost and more power\. Normal air pressure is about 14\.7 psi, so about 7 psi of boost packs in roughly half as much air again\. ### 6. Waiting for boost Press the button to floor the throttle from idle\. The engine responds at once, but the wheels need a moment to spin up before the boost arrives\. That short wait is called turbo lag\. Bigger, heavier wheels take longer to spin up but can move more air at high speed\. ### 7. One scroll or two Each cylinder fires in turn, sending out puffs of exhaust\. In a single scroll housing all the puffs share one passage and can get in each other's way\. A twin scroll keeps cylinders 1 and 4 apart from 2 and 3 in two passages, so the pulses hit the wheel cleanly and boost comes sooner\. Twin scroll is not the same as twin turbo: it is one turbo with a divided housing\. ### 8. More than one turbo Engines can use two turbos\. Parallel twins split the exhaust, each smaller turbo spinning up quickly\. In a sequential setup a small turbo works alone at low speed, and a bigger one joins in at higher speed\. Sequential systems usually bring in the second turbo at around 4,000 to 5,000 engine rpm\. ### 9. Your turbo engine Now tune the whole system\. Gentle keeps boost low and easy\. Sport raises the limit with a twin scroll turbo\. Race adds two turbos and the highest boost\. Earlier steps keep their own controls if you want to fine\-tune\. The same engine can make much more power with a turbo, which is why small turbo engines are so common today\.