EFFECTS OF DUAL MPFI ON PERFORMANCE AND FUEL CONSUMPTION IN A SMALL GASOLINE ENGINE

摘要

Dual Multi-Port Fuel Injection (MPFI) technology has recently being adopted in small gasoline engines because it partially provides the benefits of direct injection with lower cost. The objective of this study is to evaluate the gain in performance and fuel economy enabled by dual MPFI technology and also to understand the mechanisms behind these improvements. In this study, the optimized performance and fuel consumption of engines with single MPFI, dual MPFI, and dual MPFI with a 1 ratio increase in Compression Ratio (CR) were compared. The injection timing, spark timing, and cam phasing were swept to find optimal calibration settings for the given hardware. It was found that with dual MPFI the fuel is so well atomized that it can be injected earlier in the intake process. This so-called Open Valve Injection (OVI), is known to provide partial direct injection effects such as charge cooling, knock mitigation and combustion stability improvements to a certain extent. The engines were instrumented with an exhaust emissions gas analyzer and a combustion pressure analyzer to break down performance and fuel consumption into efficiencies such as fuel conversion efficiency, combustion efficiency, pumping efficiency, and friction efficiency. Relatively low octane fuel (91 RON) was used to provide severe conditions for assessing the potential for increasing CR due to knock improvements. It was shown that dual MPFI improved maximum torque, maximum power, and low-end torque by 3.4%, 0.1%, 2.8%, respectively, compared with the baseline engine. When CR was increased by 1.0, maximum torque was improved by 2.8%, maximum power by 1.8%, and low-end torque by 0% over the base engine. Additional studies showed that performance could be improved up to 7.1% at 1200 rpm WOT condition when fuel is injected after valve overlap to prevent fuel bypass to the exhaust. While the improvement in performance was less than in direct injection engines since not all of injected fuel can enter the cylinder directly due to limited valve opening time, the increased performance can be traded off for better fuel economy by increasing CR. The part load Brake Specific Fuel Consumption (BSFC) was improved by 2.4～3.3% whereas the estimated thermal efficiency improved from an increase of 1.0 CR is just 2.0%. The additional improvement in efficiencies came from gains in fuel conversion efficiency due to shortened burn duration in the later phases of combustion.