A powertrain is the set of parts that make a car move: the engine or the electric motor, and the gearbox. It takes energy from fuel or from a battery and turns it into the turning force at the wheels. Suppliers make powertrain parts for many carmakers.
In a petrol or diesel car, the powertrain is the engine and the gearbox. The engine burns fuel to turn a shaft. The gearbox changes the speed of that shaft so that the wheels turn at the right speed. An electronic control unit tells the engine how much fuel to inject and when. In a petrol engine, it also decides when the spark plug fires. The engine must also meet India's Bharat Stage VI (BS6) limits on the harmful gases it produces.
In an electric car, the powertrain is an electric motor, a simpler gearbox and the battery that supplies power. The motors and gears are described under electric motors, bearings and gearboxes. The battery is described under battery pack and battery management system. An electric powertrain has far fewer moving parts than an engine, so it needs less servicing. This is a large change for suppliers who make engine parts.
Some cars use both, in a hybrid. A hybrid can run on its engine, on its motor or on both together, and a control unit chooses which.
For instance, a commuter in Indore presses the accelerator of an electric car by half. A sensor reads the pedal position. The control unit works out that the car should deliver a particular torque, the turning force at the wheels. It tells the motor's controller to draw that amount of power from the battery. The controller sends the right current to the motor, and the car accelerates.
The powertrain's computers are arranged by the vehicle E/E architecture. They use electronic control units that suppliers set up for each engine and motor. Engineers tune an engine's settings on a test bench, using engine calibration tools. The big suppliers of powertrain parts are Tier 1 suppliers, as described under Tier 1 and Tier 2 suppliers.