Increasing Torque by Dynamic Supercharging

In Fig. 10-91, the measurements of air expenditure in the slow-running engine are compared with the data from a fast-running engine. The fast-running engine was operated with and without an air stroke valve at a constant stochiometric air-to-fuel ratio of λ = 1 and at the same ignition point to enable a direct comparison of the influence of the air stroke valve on engine operation. Two operating points of 1500 and 2200 min^-1 were investigated. The results show that the air expenditure measurements of the slow-running engine can be transferred very easily to a fast-running engine.

It is notable that the torque at λ = 1 manifests a greater rise in comparison to the air expenditure. This indicates that the dual cycles of the air stroke valve with the resulting high air speed increase the charging movement in the combustion chamber and thereby positively influence the mixture.

Fig. 10-91. Increase in air expenditure and torque in slow-running and fast-running engines

A confirmation of this is provided by analyzing the combustion characteristic of the engine cycles with and without an air stroke valve as shown in Fig. 10-91 with n = 1500 min^-1. Because of the tendency of the engine to knock, a late ignition is necessary when operating without an air stroke valve. This shifts the 50% conversion point away from the time of optimum efficiency that lies at 6°-8° after TDC ignition. When an air stroke valve is used, the mixture in the cylinder bums up 20% faster.

The result is that the 50% conversion is very close to the optimum efficiency despite late ignition, and the state changes are, hence, closer to a constant volume cycle. The indicated mean effective pressure rises at the given speed from the air stroke valve by ∆p_mi = 9.8%. We note that the air expenditure does not completely reach that measured when the engine is running slowly. This is primarily because of the fact that the air stroke valve-control times could not be fully optimized due to the insufficient robustness of the drive in a fast-running engine.