Part-time All-wheel drive (AWD) operate as a front-wheel drive under normal conditions. When traction is lost for any reason, the system automatically sends a portion of the engine's power to the rear wheels. As soon as the system detects good traction, it disengages the rear wheels and shifts back to front drive. This is a less expensive system that adds less weight to the vehicle (usually a car). This makes it popular on compact vehicles. It typically gets better fuel economy than all-wheel drive vehicles. They do not have a low range or a transfer case.