A direct-injected engine, such as a typical diesel engine, works by fuel being injected in an atomized spray through an injector, when the piston is near the top of the compression stroke. The high temperature and pressure due to the compression of air cause the fuel to combust, releasing the chemical energy stored in the fuel. The exact timing of starting this fuel injection into the cylinder is controlled electronically in most of today's modern engines. The timing is usually measured in units of crank angles before Top Dead Center (TDC) that the piston is at. For e.g., if the ECM/ECU initiates fuel injection when the piston is 10 degrees before TDC, the start of injection or "timing" is said to be 10 deg BTDC.

Advancing (injecting when the piston is further away from TDC) the start of injection results in higher in-cylinder pressure and higher efficiency but also results in higher Nitrous Oxide (NOx) emissions. At the other extreme, very retarded start of injection or timing causes incomplete combustion. This results in higher Particulate Matter (PM) emissions and higher smoke. Due to its impact on emissions and efficiency (fuel economy), controlling Start of injection for a given engine speed and load is a key lever in the optimization process involved in the design of diesel engines.