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| Several bowl concepts were considered for the combustion system. These bowls are all scaled to the same volume. Styron et al. Click to enlarge. |
Ford developed a new 6.7L V-8 turbo diesel, code named “Scorpion,” for the full-size pickup truck and light commercial vehicle markets. At last week’s SAE 2011 World Congress, engineers from Ford and AVL Powertrain presented a paper on the development of the combustion system that delivers a leading 400 hp (298 kW) and 800 lb-ft (1,087 N&iddot;m) of torque, up from the previous Power Stroke’s 350 hp and 650 lb-ft along with best-in-class fuel economy.
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. Joshua Styron, PhD, and his team used computer simulations to evaluate the piston bowl shapes, precise placement of the fuel spray within the combustion chamber, glow plug positioning and other factors.
While all of these parameters could be explored through extensive hardware testing, Ford used 3-D CFD studies to screen quickly two bowl concepts and assess their sensitivities to a few of the other parameters. 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. The 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.
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 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.
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| The chamfered re-entrant bowl allows fuel to interact with the glow plug twice because it directs some of the fuel upwards towards the head. Styron et al. Click to enlarge. |
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.
The final design features a chamfered re-entrant bowl, with low swirl, and an 8-hole nozzle with a medium included spray angle, a single row of holes, and low hydraulic flow. The bowl shape and all of the optimum parameters remained constant throughout multi-cylinder engine development. The complete combustion system can be experienced in the current production engine.
The production glow plug location, outside of the valve bridge area, maximizes interaction of vapor-phase fuel with the glow plug while minimizing impingement of liquid phase fuel. 3-D CFD, multi-cylinder cold-start tests, and full engine durability tests were used successfully to design, develop, and verify the new glow plug location which is significantly further from the injector than a traditional placement.
Four hundred horsepower was the target. Early in the development program, fuel economy was viewed as much less important to this customer than power and torque. But we believed we could deliver a good combination of both—400 horsepower along with fuel economy that was not only significantly better than the old 6.4, but higher than our competitors’ engines, too.
To deliver class-leading fuel economy and refinement as well as lower emissions, the combustion system had to make the best use of the limited supply of fresh air entering the engine. Combustion systems with the best air utilization extract the most heat from the fuel, produce less soot and NOx, and minimize the energy required to pump fresh air into the engine.
—Joshua Styron
Resources
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Joshua Styron, Brian Baldwin, Brien Fulton, David Ives, and Subramanian Ramanathan (2011) Ford 2011 6.7L Power Stroke Diesel Engine Combustion System Development (SAE 2011-01-0415)