Using the 2nd Law of Thermodynamics to Optimize Variable Valve Timing for Maximizing Torque in a Throttled SI Engine 1999-01-0328
Previous investigations have demonstrated that improvements in gasoline engine performance (engine efficiency, power, exhaust emissions, engine braking, and usable engine speed range) can be accomplished if the valve timing are variable. Because of the difficulties of providing a variable valve timing (VVT) mechanism with acceptable cost, durability and reliability, very few automotive engines in normal production have been equipped till now with variable valve timing. However, owing to recent technology developments associated with electromagnetic and hydraulic valve control, and owing to recent progresses in microprocessor utilization, the application of VVT in the near future is quite feasible.
In this work valve timing strategies for maximizing engine torque in terms of the intake closing and exhaust opening timings of a commercial throttled SI engine are studied. The MICE (Modeling Internal Combustion Engines) commercial computer program which simulates an actual SI cycle has been used. Overall performance characteristics such as the cycle efficiency, maximum torque, as well as the maximum amount of work available, are calculated by using the 2nd law of thermodynamics. The model has been calibrated with data obtained from a measured indicator diagram, and validated against the overall performances of the engine.
It was concluded that when both valves and spark timings are optimized, the optimal timing of each valve depends linearly (in a good approximation), on the engine speed, while the slope depends apparently weakly on the engine load. Under these operation conditions, the rated power of the engine has been increased by 24.4%, and the engine torque at the rated speed by 9.5%. The maximum torque has been increased by 33.8% from 7.4 Nm at 3,100 rev/min to 9.9 at 1,600 rev/min. The shift of the maximum torque towards a lower engine speed is attributed to the longer expansion stroke (retarding EO), thus reducing the available energy loss to exhaust, and increasing the effective compression ratio while closing earlier the inlet valve, thus increasing the available cylinder charge at IC.
The present results are summarized as working maps for the engine designer. These show the influence of the inlet and exhaust valve timing on the engine performance at the entire range of operation conditions (engine load and speed).