Some modifications to the method of characteristics for the analysis of the gas exchange process in internal combustion engines are presented in this paper. The modifications are related to the calculation of the path lines and the Riemann characteristic lines at the grid points. Regarding the path lines, the algorithm for the generation and elimination of path lines has been improved, mainly for the cases in which the fluid motion passes from being null or going out of the pipe to going into the pipe. In those cases the algorithm proposed by Benson can cause some mistakes in the entropy level field of the duct. An alternative method is proposed: the duplication ofpath lines. The other modifcation proposed is related to assuming a linear interpolation for the pressure and the volume flowrate between the nearest grid points, rather than assuming a linear interpolation of the value of the Riemann characteristics. These modijcations substantially improve the results obtained in the calculation of the fluid flow in manifolds of reciprocating internal combustion engines.

Conception des tuyauteries d'echappement en vue d'optimiser le balayage dans un moteur deux temps

In this paper, the advantages of using variable valve timing systems in order to decrease pumping losses are outlined. In the first part of the paper, the reduction of pumping losses through regulation of the engine load rate, by varying the time that the intake valve remains open, is justified by reasoning on ideal diagrams. In the second part, a proposed system to vary the valve timing diagram is described. This system consists mainly of a four-bar mechanism. Finally, the advantages and disadvantages of the proposed system are considered in the real engine by means of a calculation model. The conclusion is that with this system it is possible to obtain important improvements in the effective efficiency of the engine running at partial load.

The possibility exists of adopting variable valve timings to reduce pump-losses in spark-ignition engines. With this method, engine load regulation can be achieved mainly by acting over the total time that the inlet valve remains open, and eliminating the throttle valve in an ideal case. The timing diagram (total opening valve and angular phase) should depend on the engine speed and load desired in each moment. The mechanical system for the proposed regulation system is a four-bar mechanism, which is now in a stage of final adjustment for a monocylinder engine. In order to get the valve timing, a calculation model for the inlet and exhaust processes has been developed and is presented in this article, as well as the results obtained.

One of the most interesting ways of improving the lowspeed performance of spark-ignition engines is the variation of the valve timing in order to reduce mechanical losses.