Multijet is Stellantis's term for its current common rail direct injection turbodiesel engine range. Most of the Fiat, Alfa Romeo, and Lancia range, as well as certain Chrysler, RAM Trucks, Jeep, and Maserati vehicles, are equipped with Multijet engines. Ownership of some Fiat Multijet designs is shared with General Motors as part of a settlement of the failed merger between the two auto conglomerates. The GM Powertrain Torino group in Turin, Italy, manages its interest in these engines. Some PSA Peugeot Citroën diesel engines are also rebadged JTD units, and vice versa. Fiat's common-rail diesel engine is also known as JTD, an initialism of Jet Turbo Diesel.

Common rail direct fuel injection is a direct fuel injection system built around a high-pressure fuel rail feeding solenoid valves, as opposed to a low-pressure fuel pump feeding unit injectors. High-pressure injection delivers power and fuel consumption benefits over earlier lower pressure fuel injection, by injecting fuel as a larger number of smaller droplets, giving a much higher ratio of surface area to volume. This provides improved vaporization from the surface of the fuel droplets, and so more efficient combining of atmospheric oxygen with vaporized fuel delivering more complete combustion.