PYROMETER GP 3N7012 - Caterpillar

System Operation
The 3408/3412 High Performance Marine Diesel Engine is an electronically-controlled engine. The Electronic Control Module (ECM) controls fuel rate instead of using flyweights and linkages. The electronics also replaces the mechanical fuel-air ratio control, torque control group, and various adjustment screws.The ECM uses several sensors as inputs to control the rack. The ECM decides where it wants to position the rack. It then varies the voltage to the Brushless Torque Motor (BTM) to move the rack to the desired position. The rack position sensor tells the ECM when rack is at the desired position.Electronic Controls
The electronic controller consists of two main components: the Electronic Control Module (ECM) and the Personality Module. The ECM is the computer which controls the engine. The Personality Module is the software which controls how the computer behaves. The two must be used together: neither can do anything by itself.Rack Controls
The rack mechanism on a 3408/3412 engine is moved by a servo valve which receives oil pressure from the fuel injection pump. However, the servo spool is moved by a solenoid (BTM) rather than by a linkage controlled by flyweights and springs.The ECM determines a "desired engine speed (rpm)" based on the throttle position, and customer specified parameters. The ECM tries to maintain the desired engine speed (rpm) by sensing actual engine speed using the main engine speed sensor or the backup speed sensor, then controlling the rack to achieve the desired engine speed. To move the rack, ECM adjusts the voltage to the rack solenoid (BTM) to increase or decrease rack. More voltage results in more rack. The ECM knows how far the rack moved by reading the rack position sensor. The ECM increases or decreases the voltage to the rack solenoid until it senses the rack is at the desired position. The ECM sets certain limits on rack motion. "FRC Rack" (Fuel Ratio Control Rack) is a rack limit used to reduce the amount of acceleration smoke. It works similar to a mechanical engine FARC (Fuel Air Ratio Control); when the ECM senses a higher inlet manifold (boost) pressure (more air into cylinders), it increases the FRC Rack limit, which allows more fuel into the cylinders. "Rated Rack" is a rack limit based on rated power of the engine. It is similar to the rack stops and torque springs on a mechanical engine. It provides a power output limit and torque curves for a specific engine family and rating. All of these limits