TM 9--2815--247--34 0004 00--10 THEORY OF OPERATION -- CONTINUED 0004 00 1--16 MANIFOLD AIR INDUCTION AND HEATER SYSTEM -- CONTINUED Intake Manifold Heater -- Continued The manifold heater fuel inlet solenoid valve prevents fuel pumped by the fuel pump assembly from entering the air intake manifold heater unless the heater system is energized. A manifold heater fuel return check valve and sole- noid valve are located at the rear of the engine. The solenoid valve is also energized (opened) when the ignition unit and heater spark plugs are energized to permit excess fuel to be returned to the fuel tanks. The main fuel backflow valve prevents back flow of the fuel when the purge pump in the driver’s compartment is actuated. Aftercoolers  Each cylinder bank incorporates an exhaust--driven turbosupercharger aftercooler, manifold heater intake tube, and intake manifolds attached to the cylinder intake parts. The air flow through the engine begins with filtered air flow from the vehicle mounted air cleaners. The air enters the turbosupercharger where it is com- pressed for increased mass air flow, necessary for the engine power related output, which actually improves the thermal efficiency of the combustion. Compressed air from the turbosupercharger is discharged to the induction air aftercoolers where it is cooled to increase density. The air temperature is regulated by a bypass valve. The piston actuated aftercooler bypass valve responds to manifold air pressure and functions as follows in inches of mercury (hg): Full bypass (no cooling) below 40 In. Hg. absolute. Modulated flow from 40 to 57 In. Hg. absolute. No bypass (full capacity cooling) above 57 In. Hg. absolute. EXHAUST SYSTEM AND TURBOSUPERCHARGER The exhaust system consists of four manifolds, one for each group of three adjacent cylinders. The two exhaust manifolds on each cylinder bank are connected to the turbosupercharger on their respective side of the engine. Exhaust gases from the engine exhaust manifold enter the turbosuperchargers through ports in the turbine hous- ing. Pressure and heat from the exhaust gases turn the turbine wheel. The gases leave the turbine housing through the exhaust outlet and are expelled through the vehicle exhaust system. The turbine wheel and the com- pressor wheel are mounted on a common rotor shaft. When the turbine wheel spins, so does the compressor wheel. The spinning compressor wheel draws air from the vehicle air filter into the compressor cover. The air is compressed and blown out of the compressor cover through the outlet port, where it enters the intake manifold. The increased volume and density of the air that is delivered to the engine cylinders causes a corresponding in- crease in engine output power. COOLING SYSTEM NOTE The key numbers shown below in parentheses refer to Figure 1--6. Fans  The top of the engine is shrouded to house two cooling fans (40 and 46) which draw cool air from the under- side of the engine, through the cylinder fins, and discharge the hot air vertically from the top shroud. The fans are attached to adapters (39) and are mounted on shafts which are driven through a fan drive clutch assembly (38). The rear fan clutch is driven by the rear fan drive shaft (34), fan drive bevel gearshaft (33), and fan--driven gear- shaft (35). The front fan clutch is driven by the front fan drive shaft (32), fan drive bevel gearshaft (41) and another fan--driven gearshaft (35).