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.