What makes a dynamometer a "four-quadrant" dyno?

Question

I've got the idea of measuring torque and speed, but what the heck are quadrants...and how does this dyno operate in four of them?

Answer 1

If you consider shaft speed the X component and torque the Y component, 4-quadrant operation allows for application of load torque against the direction of rotation (normal loading) or in the same direction (overrunning), and of course provides such variability for either direction of rotation. These types of test find application in testing machines designed for an unsteady load environment, e.g., driving in uphill-downhill situations, picking up and lowering freight, etc.