Page 332 - Compression Machinery for Oil and Gas

Drivers Chapter 7 317

Engine Starting
Starting an internal combustion engine is simple in concept: a small motor spins
the engine up to a speed where combustion is consistent enough to sustain the
rotation by itself, at which point the starter motor is disengaged. The size of the
starter depends on the total torque it must generate to turn the engine’s crank-
shaft, which makes it dependent not just on the size of the engine, but also on the
equipment that is connected to the engine during cranking. In gas compression
applications, the compressor is directly coupled to the engine and thus places
additional load on the starter during cranking. Site design considerations like
an adequately sized compressor recycle loop are important to achieving a suc-
cessful starting solution for the engine.
Temperature is also a factor in engine starting—a cold cylinder block saps
heat energy from combustion during cranking, and cold oil increases in viscos-
ity resulting in higher frictional loads at start-up. Auxiliary heaters for the
engine’s coolant and lubricating oil may be required as starting aids in a cold
weather environment.

Power, Torque, and Speed
The rated power and operating speed of an engine tend to follow from the
engine’s combustion chamber size. Because the piston’s movement in the cyl-
inder determines the volume of fuel and air taken into the engine, the combined
volume of all the cylinders becomes an important basic design detail—the
engine’s displacement. A larger-displacement engine has the ability to pull
in greater amounts of fuel, thus giving it more potential to produce power.
Larger pistons also have greater mass that must change direction at the ends
of each stroke, and the inertia forces involved in that turnaround tend to keep
operating speeds lower on larger engines.
While the exact shape of the torque-speed curve varies from engine-to-
engine, the heavy-duty gas engines used in gas compression typically advertise
constant torque across the range of speeds over which the engine may operate
continuously. This fits well with the operating behavior of reciprocating com-
pressors, which, for a given set of operating conditions, demand a fixed oper-
ating torque regardless of speed. Because the engine is optimized for operation
at the rated speed, the amount of “speed turndown” (operation at reduced
speeds) available tends to be limited, with the actual minimum operating speed
being a characteristic of each individual engine.

Durability
As with all machines, durability and ease of maintenance are prime factors in
the design of the engine. Long life adds value, both in extending the period of
time over which a given engine continues to perform well, and in limiting the