The Control Of Pumping Loss And The Effect On Combustion Process Of A Gasoline Engine With Fully Variable Valve Actuators

The conventional SI engine with throttle-driven load control suffers reduced thermal efficiency and fuel economy under part load mainly due to its high pumping loss and friction loss. Variable valve actuation technology is one of the approaches capable of improving the thermal efficiency and fuel economy under part load by the means of throttleless load control.On conventional SI engines, load is controlled by throttle, and intake pressure is reduced to regulate the entrained mass at part load since valve opening duration is invariable, resulting in high pumping loss. In order to control the pumping loss, it is necessary to increase the in-cylinder pressure and at the same time shorten the valve opening duration. In this thesis, a gasoline engine with fully variable valve actuators is developed, the control of pumping loss is studied by using three control modes, including valve lift, valve phase and internal EGR, and the effects of these control modes on the combustion process and the engine performance are analyzed.The effects of intake valve lift and timing on inlet flow are studied. Also, the intake airflow waveform measured under the valve lift control mode is analyzed and compared with the throttle valve control output, based on which, the algorithm of intake mass flow measurement of the hot film flow meter is improved and a intake mass calculation model is developed.The intake valve lift adjustment is employed in the research to reduce the pumping loss and improve the fuel economy. Results show that pumping loss can be reduced by 10~30% by using the intake valve lift load control, and a 3~12% of ISFC benefit can be obtained. Also, FMEP can be reduced at the medium and low speeds, and meanwhile the BSFC can obtain a 5.7-22% of reduction. Test results at idle operation condition show that by using valve lift control, ISFC, unburned HC and CO can be reduced by 16%, 17% and 60% respectively, with comparative amount of NOx, indicating a improved performance at idling.The effects of the intake valve opening timings on PMEP and combustion process are also studied. Results show that pumping loss increases when the intake valve opening is delayed. To optimize the fuel economy, early intake valve opening and closing should be used.From the engine tests, it is found that compare to throttle control, the combustion slows down, combustion duration extends and the combustion deteriorates when the valve lift control is used, which restricts the improvement of the fuel economy. The PIV measurement of in-cylinder flow structures at valve lift control mode is carried out and it is noted through comparison with the throttle valve control test results that the high-frequency fluctuating velocity at the valve lift control mode is the major source of the long combustion duration.The exhaust valve lift and phase are used to control the internal EGR, and the effects of EGR on pumping loss and combustion process are analyzed. Results show that pumping loss can be reduced by 10~25% by using the internal EGR load control, but meanwhile the combustion process deteriorates and the combustion duration becomes longer. It is also found that at low EGR rates, fuel economy is subtly improved; as EGR increases, NOx and HC emission levels lower down but at a slight sacrifice of ISFC.