Fire Engine
The pumping apparatus: carries a pump, a water tank and hose. The most common piece of fire apparatus and the backbone of most departments.
An engine, also called a pumper, exists to move water. It carries a pump, an onboard tank, hose and the tools for interior fire attack, and it is what most departments send first to most calls.
Because engines respond to much more than fires, they accumulate hours and wear a specialized unit does not, which makes them the apparatus a fleet shop sees most.
Pump capacity and tank size govern the specification, and both drive the size and weight of the finished vehicle. A department that increases either is buying a bigger, heavier truck that may not fit the station bay it has to live in, which is a constraint discovered late more often than it should be.
Annual pump testing is a maintenance obligation of its own and belongs in the PM program rather than being treated as a fire department matter.