Abstract
Modern Diesel engines for passenger car application are required to achieve extremely low pollutant emissions at low fuel consumption, while featuring high specific power. An effective optimization parameter between these conflicting targets is the compression ratio. In fact, since almost 10 years, the compression ratio in light-duty diesel engines has undergone a significant decrease from about 20:1 of first generation DI common rail engines to about 16:1 of newest engines. However, the low compression ratio has significantly impaired the engine capability to ignite fuel in cold conditions just relying on mixture compression. Nevertheless, in order to successfully develop low compression ratio Diesel engines for production, robust cold startability, low misfiring rates and smooth cold idling operation have to be guaranteed. As a consequence, the importance of the glowing system is growing quickly, since it is the chief combustion chamber component able to provide the required fuel ignition quality in the above-mentioned conditions.
Within this framework, GM Powertrain Europe and the West Saxon University of Zwickau have started a research project aimed at detailed understanding of the impact that glow plug system has on the startability of a 2.0L 4-cyl in-line last-generation common rail engine featuring compression ratio of 15.5. The primary goal of the activity was to identify the most important design parameters of the glow plug system that are able to maximize fuel ignition capability, i.e. the glow plug rod design, its position and orientation with respect to the fuel spray, and its operating temperatures.
The numerical investigation integrated 1-D simulation of the overall engine and 3-D CFD of the combustion chamber, and was coupled with Design for Six Sigma approach (DFSS). DFSS, given the high number of design parameters and operating conditions involved, allowed a significant reduction of the number of simulations as well as the possibility to use a finer computational grid. This latter proved to be fundamental in order to model and monitor properly the formation of the first kernel of ignited fuel and track the evolution of its properties. Results are extensively discussed in the paper. Main outcomes show that glow plug diameter and protrusion from fireplate as well as operating temperature are the most important parameters to be considered in the design of the system.
Keywords: diesel, combustion, cold startability, glow plug, Design for Six Sigma