Diesel Turbocharging System: Operation
Exhaust gases leaving the cylinders flow through the exhaust manifold directly into the turbocharger's turbine housing. The force of the gases turns the turbine wheel, which in turn spins the compressor wheel at the same speed. Turbine and compressor wheel speeds can reach up to 100, 000 RPM. See Figure and Fig 1.
The fresh air drawn in by the compressor wheel is compressed and delivered to the combustion chamber above the pistons. At idle speed, the engine operates like any other. However, with increasing load and engine speed, exhaust gases are expelled with increasing velocity. This causes the turbine wheel to turn faster, increasing boost pressure at the compressor wheel. Boost pressure is routed to the intake manifold and to individual cylinders, completing the cycle.
The aneroid compensator on top of the fuel injection pump automatically adjusts the fuel quantity injected into the cylinders depending on existing boost pressure or atmospheric pressure in the intake manifold. Therefore, the correct air-fuel relationship is maintained at all times.
Should boost pressure control valve (wastegate) fail, a pressure switch installed in the intake manifold closes an electrical circuit, energizing a switch-over valve. This valve closes the pressure line to the intake manifold and simultaneously opens the aneroid compensator to atmosphere. This reduces the fuel quantity being injected. The pressure switch only functions when intake manifold boost pressure reaches 16 psi (1.125 kg/cm2 ). When pressure drops below this figure, the pressure switch opens the electric circuit and venting of the pressure line is stopped.