Abstract
Research and/or Engineering Questions/Objective
Dual MPFI technology is recently being adopted in small gasoline engines because it partially provides direct injection effects with lower cost. It has been known that fuel economy can be improved because finer atomization of fuel and open valve injection makes combustion more stable and permits more internal EGR. The objective of this study is to evaluate the gain in performance and fuel economy obtainable by using dual MPFI technology and also to find the mechanism how these could be improved.
Methodology
The optimized performance and fuel consumption of single MPFI, dual MPFI, and dual MPFI with increased compression ratio by 1.0 were compared. The injection timing, spark timing, and cam phasing were swept to find optimal EMS calibration in the given hardware. The engines were instrumented with gas analyzer and combustion analyzer to break down performance and fuel consumption into efficiencies such as thermal efficiency, pumping efficiency and combustion efficiency.
Results
In preliminary test, dual MPFI with increased CR by 1.0 improved max torque and max power 0.9% and 2.4% respectively compared with baseline single MPFI engine, but low end torque could be improved up to 7% using open valve injection which enables knock suppression by charge air cooling and prevents hydrocarbon slip during valve overlap. Even though the improvement in performance was not so great as in direct injection engine because not all injected fuel can enter the cylinder directly due to limited valve opening time and low fuel pressure, the increased performance can be shifted to better fuel economy by increasing compression ratio. The part load BSFC was improved by 1.9~3.5% whereas the theoretical thermal efficiency improved by 1.0 compression ratio is just 2.1%. The main improvement in efficiencies lied in thermal efficiency, and the burn duration and subsequent time loss were greatly reduced. 50% burnt crank angle at MBT is advanced from around 8.5 to 6.0 deg after TDC. It is believed that the better atomized fuel makes air/fuel mixture more uniform and flame propagation is accelerated. The further improvement using intake port deactivation is being tried to balance between more fast burning and pumping loss increase.
Limitations of this study
An important limitation of the current study is that the comparison with direct injection engine was not presented due to absence of direct injection variants in baseline engine.
What does the paper offer that is new in the field including in comparison to other work by the authors?
Even though the technology are already widely adopted in vehicle production, there was little existing study which analyze the effects of dual MPFI as this study.
Conclusions
The dual MPFI technology looks promising and cost-effective solution for improving small gasoline engine. The improvement in knock mitigation in WOT and combustion stability and burning speed in part load offers engine development engineers more freedom of choices between WOT performance and fuel economy.
KEYWORDS : dual MPFI, gasoline, combustion, fuel economy, engine performance