The engine controller can throttle down the fuel-powered engines. When throttled down, they output their usual torque, but at a fraction of the speed. The benefit of this is that they consume fuel more slowly; slower speeds have greater efficiencies than faster ones. Modes are changed by right-clicking the ECU with a screwdriver. The ECU can also accept a redstone input, which will shut the engine down entirely. 

Available Modes: 

Shutdown: 0.00% Speed, 0x Fuel Efficiency

Stanby: 6.25% Speed, 4x Fuel Efficiency

Low: 25.00% Speed 2x Fuel Efficiency

Medium: 50.00% Speed, 1x Fuel Efficiency

Full: 100.00% Speed, 1x Fuel Efficiency