TITLE: What do we mean by the "load" on an engine?
QUESTION [0 upvotes]: In engines, we often talk about the load being something that acts against the rotating of the crankshaft. 
However, what does this mean in real life? In a car engine, for instance, what would the load refer to, while driving along a road?

REPLY [1 votes]: Cruising at constant speed, the load is the power (or sometimes the torque, depending on context) that the engine needs to output to overcome the air resistance and bearing friction to the car's constant velocity motion. These forces add up to a torque opposing the crankshaft's constant angular speed motion.
When the car is accelerating, there is a nett force on it, and this nett force must be supplied by the engine further to the resistance forces above. From the engine's, accelerated frame of reference there is a further torque opposing the crankshaft's motion that the engine has to supply that arises from the inertial "weight" of the car: the inertial force backwards on the car arising from the car's inertial resistance $m\,a$ to its acceleration.
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