The TU family of small inline-four piston engines by PSA Peugeot Citroën were introduced in 1986 and used in the Peugeot and Citroën range of cars. It was first installed in the Citroën AX in October 1986, replacing the X family, although it shared many components with its predecessor. The TU was available in either petrol or a naturally aspirated diesel variant, the latter called TUD.

The TU engine was distantly related to the older X-Type engine – sharing a similar overhead camshaft architecture, but the key differences are the belt driven camshaft (the X is chain driven), and that the TU is mounted in a conventional upright position with a separate, end-on mounted transmission and unequal length drive shafts. The X engine, by comparison, had an integral transmission mounted on the side of the crankcase (giving rise to its popular nickname the "suitcase engine"), sharing a common oil supply and was mounted almost lying flat on its side within the car.

After the engine debuted in the Citroën AX in 1986, it quickly began replacing the X engine in most of its remaining applications – the Peugeot 205, Citroën BX and Citroën C15 had all transitioned by 1988 whilst it replaced the Simca "Poissy" engine in the Peugeot 309 in 1991. It was also used in the following cars: Citroën: AX, Saxo, C2, C3, C4, BX, ZX, Xsara, Nemo and Berlingo. Peugeot: 106, 206, 207, 306, 307, 405, Bipper, Partner and Hoggar, the Iranian Peugeot 405 and Peugeot Pars as well as the IKCO Runna.

The TUD engine was only used in 11 cars of which 6 were non-PSA models: the Citroën AX, Citroën Saxo, Citroën Xsara; Peugeot 106, Rover Metro/100-series, Nissan Micra, Maruti Suzuki Zen D/Di and Maruti Suzuki Esteem D/Di and IKCO Samand^{[citation needed]}, and the Tata Indigo 1.4 TD. The Tata's is a smaller version of the TUD engine, based on the 1.5D.

PSA has now stopped production of original TU engines, although the closely related EC engine family is still in production for emerging markets such as China and Russia and available in 1.6, 1.8 and 2.0 litre versions.^{[citation needed]}

The IKCO EF engines, jointly developed by Iran Khodro and F.E.V GmbH of Germany, are closely related to the TU engines.

The TU9 was the entry-level version, used in a variety of cars including the Citroën AX, Saxo, Peugeot 205 and 106. It had a displacement of 1.0 L (954 cc), with a bore and a stroke of 70 mm × 62 mm (2.76 in × 2.44 in). Power was initially 45 PS (33 kW; 44 hp), but it was increased to 50 PS (37 kW; 49 hp) in 1992, with the adoption of central fuel injection and a catalytic converter. Production was stopped in the Citroën Saxo and Peugeot 106 with the introduction of Euro III in 2001. Early versions of this engine suffered premature piston failure which were of a special fuel-saving low-friction design. Symptoms were piston slap, especially with a cold engine, excessive oil consumption and exhaust smoke. PSA repaired the affected vehicles under warranty. This involved fitting a revised piston design and replacement liners.

The TU1 has a displacement of 1.1 L (1,124 cc), with a bore and a stroke of 72 mm × 69 mm (2.83 in × 2.72 in). Power was initially 55 PS (40 kW; 54 hp), but it was increased to 60 PS (44 kW; 59 hp) in 1992, with the adoption of central fuel injection and a catalytic converter. The introduction of Euro III led to the adoption of multi point injection, but power remained the same (although there was a small torque increase). This engine was the entry-level option in the Citroën C2 and C3 and Peugeot 206.