Mechanical engineers work with many kinds of machines that produce, transmit, or use power. They are concerned with mechanisms and methods that convert natural energy sources into practical uses. Mechanical engineers also design tools that other engineers need. They often work as part of a team that includes scientists who develop new theories and methods, and mechanical engineering technicians who assist engineers with some of their more routine tasks. Mechanical engineers design and develop machines that produce energy, such as car engines and nuclear reactors. They also design and develop machines that use energy including air conditioners, power saws, elevators, and printing presses. Mechanical engineers work on various types of equipment and machines ranging from tiny mechanisms for delicate instruments to huge gears for bulldozers. The field of mechanical engineering is very broad, and some of the diverse job titles include air pollution control engineer, environmental systems engineer, manufacturing engineer, and automotive engineer.