Ford 2011 6.7L Power Stroke® Diesel Engine Combustion System Development 2011-01-0415

A new diesel engine, called the 6.7L Power Stroke® V-8 Turbo Diesel, and code named "Scorpion," was designed and developed by Ford Motor Company for the full-size pickup truck and light commercial vehicle markets. The combustion system includes the piston bowl, swirl level, number of nozzle holes, fuel spray angle, nozzle tip protrusion, nozzle hydraulic flow, and nozzle-hole taper. While all of these parameters could be explored through extensive hardware testing, 3-D CFD studies were utilized to quickly screen two bowl concepts and assess their sensitivities to a few of the other parameters. The two most promising bowl concepts were built into single-cylinder engines for optimization of the rest of the combustion system parameters.

1-D CFD models were used to set boundary conditions at intake valve closure for 3-D CFD which was used for the closed-cycle portion of the simulation. The critical C100 operating condition for the dynamometer-certified engine was chosen to evaluate how well each combination of parameters utilized the limited available oxygen in the cylinder. A fractional-factorial Designed Experiment (DoE) was developed that assessed three shape parameters in each bowl concept as well as two swirl levels, and three fuel spray angles. Two bowl shapes were selected based on a multi-objective optimization which sought to simultaneously minimize fuel consumption and NOx and soot emissions. The DoE also allowed an assessment of sensitivities to variations in the other parameters which could be important for robustness considering manufacturing variation. Analysis of the mixing structures from the 3-D CFD proved beneficial to help explain some of the trends.

Each piston bowl was built into a single-cylinder engine block. Two cylinder heads, each with a different swirl level, were also prepared. A suite of injectors was built to vary the number of injector holes, fuel spray angle, hydraulic flow, and nozzle-hole taper. For each injector, copper washers of varying thickness were used to fine tune injector targeting. For each hardware combination, EGR sweeps were performed at four different operating conditions (C100, A100, A25, and 1500 rpm/3 bar BMEP) to allow a complete assessment of emissions and fuel economy performance. The two higher load points are critical for dynamometer-certified emissions compliance. The two lower load points are critical to typical driving fuel economy and chassis-certified emissions compliance. Analysis of the EGR sweeps proved useful for selecting all of the combustion system parameters that now form the heart of the 2011 6.7L Power Stroke® V-8 Turbo Diesel.

Glow plugs were chosen as the primary cold-starting aid. CFD studies were performed to find an optimum location that could enhance cold-start ability, cylinder head strength, and glow plug durability. The recommended location and several depths shorter and longer were tested in multi-cylinder engines to verify function and provide data for determination of the final glow plug location. The production location, outside of the valve bridge area, maximizes interaction of vapor-phase fuel with the glow plug while minimizing impingement of liquid phase fuel.