SPRING 7G6945 - Caterpillar

A rapid reduction in the engine's load can cause a rapid increase in the engine speed. The rapid reduction in the engine's load causes the turbocharger to produce boost pressures that are greater than the requirement for the engine operation.Turbocharger surge - In response to the excess pressure in the air inlet system, the flow of air across the turbocharger compressor wheel is reversed. This reversal of the air flow is called a turbocharger surge. During a turbocharger surge, the turbocharger overspeeds temporarily. This places a greater axial load on the thrust bearing and higher than normal torque on the shaft. The turbocharger surge can result in severe damage to the turbocharger.The compressor bypass group reduces the opportunity for a turbocharger surge.
Illustration 1 g01776416
(1) Throttle actuator
(2) Compressor bypass actuator
(3) Fuel metering valve The ECM responds to the input from the power sensor or from engine speed by energizing compressor bypass actuator (2). This causes the compressor bypass actuator to open.The compressor bypass actuator is connected before throttle actuator (1). Tubing is connected from the compressor bypass actuator to the inlet for the turbocharger compressor: one set of tubing is connected to the inlet for the right turbocharger and the other set of tubing is connected to the inlet for the left turbocharger. When the compressor bypass actuator is opened, some of the compressed air/fuel mixture is diverted through the tubing away from the throttle. This reduces the back pressure that is caused by excessive pressure in the air inlet system.Additionally, the ECM sends two command signals. A command to throttle actuator (1) opens the throttle in order to reduce the resistance that contributes to back pressure. A command to the fuel metering valve (3) reduces the fuel supply in order to reduce the engine speed and the turbocharger's speed.The combined effect of these actions reduces the opportunity for a turbocharger surge.The ECM determines a time period that is required for opening of the compressor bypass actuator. After the time period has expired, the ECM de-energizes the compressor bypass actuator and the valve closes. Normal operation resumes.The compressor bypass group is also operated during engine shutdown. The compressor bypass actuator is activated when the output for the fuel control relay is OFF and the engine rpm is greater than 100. The ECM activates the output for the compressor bypass actuator until the engine rpm is reduced to zero. This reduces the opportunity for a turbocharger surge during engine shutdown. Additionally, any air/fuel mixture that may have been trapped in the tubing is purged.