One of the more serious disadvantages of a turbocharged engine is its low dynamics in transitional states.
The possibility of a sudden increase in load here is limited by the inertia of the turbocharger rotor. The situation can be significantly improved by the use of a simple compressor bypass system at part load of the engine.
In traditional systems, all the air supplying the engine passes through the compressor, and regulation of the flow rate is done by throttling or before, or after the compressor. In a system that quickly responds to changes in load, air is supplied at the 1st partial load of the engine through the compressor bypass line.
The flow of this air is controlled by the damper T1 the damper, T2 is then closed. At higher loads, damper T2 is opened and some air flows through the compressor. At full engine load, all air is forced through the compressor, and the throttle valve cuts off the bypass line. Proper selection of the T2 throttle opening moment and the T1 and T2 throttle opening ratio allows to achieve a smooth transition in various engine operating conditions.
The research was conducted at Callaway Turbosystems Inc.. on the standard BMW four-cylinder capacity engine 1,8 dm8 with a Rotomaster Y-4 turbocharger. Compressor shunting at part load of the engine resulted in a significant increase in the rotational speed of the turbine compared to the traditional system. In this system, the limitation of the turbine rotational speed results from the exhaust gas flow and friction in the bearings. Turbulence resistances only begin to play a role at high rotational speeds, above 60 000 RPM and higher intake manifold pressures. The above factors prevent excessive running
out of the turbocharger under conditions, when the airflow to the compressor is shut off, and the rotational speed is high. Measurements of dynamic changes in engine parameters with a sharp opening of the throttle show, that thanks to the high speed of rotation of the turbine at part load of the engine, the new system responds much faster to load changes than the conventional system. For the intake manifold pressures of the order 0,07 MPa, the ability to quickly reach full load occurs already at engine speed 2700 RPM, and for pressure 0,053 MPa - from 3300 image/ /min. Road tests also showed the advantages of the new system by reducing the acceleration times of the car. An additional advantage here is that the turbine speed is kept high when shifting gears.
One disadvantage of the bypass compressor system is the danger of unstable compressor operation, because most of the time it runs at high speeds, but with low flows. The pumping phenomena that threaten in such conditions can be counteracted by bringing the T2 throttle close to the compressor as much as possible and by using additional relief valves.. The main limitation of the new layout, just like traditional layouts, however, there is a low power of the turbine at low engine speed due to the low exhaust flow, which prevents the engine from reaching full power quickly.