Abstract
A Variable valve Timing and lift Electronic Control system (“VTEC” below) is widely employed by Honda in order to reduce engine CO2 emissions and increase power. The use of this system increases the number of hydraulic devices and makes oil circuits more complex. It was therefore decided to develop a technology for the prediction of the allocation of lubrication oil pressure and flow rate in steady states to help enable the development of optimal oil circuits in which issues such as seizing and performance decline did not occur due to insufficient oil pressure or flow rate. This method links oil film simulations using multi-body dynamics, oil channel pressure loss simulations using three-dimensional computational fluid dynamics, and measurements of pressure loss in actual devices with one-dimensional flow analyses in order to predict the allocation of lubrication oil pressure and flow rate.
It is normal in VTEC systems to switch oil circuits for control purposes, and the ability to predict this behavior was therefore necessary in order to reduce development time. This paper proposes an analytic method to help enable the prediction of phenomena occurring during oil circuit switching. The focus of the analysis discussed here was the transient response during VTEC oil circuit switching. VTEC systems simultaneously change valve lift and valve timing by switching cams, using oil pressure to insert pins between rocker arms. Lubrication oil is used as the control oil, supplied from the main gallery by means of an electronically controlled spool valve. The time necessary for increase in the pressure applied to the pins during cam switching in these systems is extremely important to engine performance, and the setting of this parameter necessitates an investment of time and man-hours.
A simulation method able to substantiate the physical basis of the mechanism of generation of pressure waves was therefore formulated. The analysis applied the method of characteristic curves to partial differential equations, representing an equation of continuity of flow and an equation of motion.
The ability to accurately predict the speed of propagation of sound waves and oil pressure rise time were important factors in moving from static to transient analyses. The initial points of focus were therefore the valve opening characteristic of the spool valve used for switching, mechanical-hydraulic coupling, and the speed of propagation of sound waves in oil. Next, taking into consideration the fact that the aeration ratio directly affects the speed of propagation of sound waves in oil, the aeration ratio was estimated from the speed of propagation. It was found that using this method, it was possible to reproduce the pressure waves generated in, and the response time of, oil circuits during VTEC switching. This result helped to enable the formulation of a base simulation model for the prediction of transient response during VTEC oil circuit switching.
Keywords: oil circuit, engine, simulation, pressure