Ford is aiming to squeeze another 25% brake thermal efficiency (BTE) out of its next-generation EcoBoost engines, using a combination of downspeeding, further downsizing, aggressive cooled EGR—and potentially using optimized stratified charge.
And engineers are hopeful of BTE improvements of up to 40% in gasoline engines without excessive incremental cost.
Eric Curtis, Technical Leader and Manager of Engine and Powertrain Systems, outlined the automaker’s overall strategies and tactics for future EcoBoost (turbocharged, gasoline-direct injection) engines April 23, at the SAE 2012 High Efficiency IC Engines Symposium in Detroit.
Curtis noted that meeting dramatically higher (54.5 mpg) U.S. fleet fuel efficiency, with more stringent exhaust emission standards, will be tough indeed. “With LEV3, North American emission standards will remain the most challenging in the world,” he told the SAE audience.
To achieve these goals, Ford engineers and research scientists are leading development of future EcoBoost engines in a project called "Advanced Gasoline Turbocharged Direct Injection (GTDI) Engine Development." The four-year program, launched in October 2010, is supported by Michigan Technological University, including studies of advanced ignition systems. It is jointly co-funded by Ford and the U.S. Department of Energy, each of which invested $15 million.
Ford’s objectives for the Advanced GTDI program are to demonstrate 25% fuel-economy improvement in a midsize sedan using a downsized, advanced gasoline turbocharged direct injection (GTDI) engine with no or limited degradation in vehicle level metrics, and to demonstrate the vehicle is capable of meeting Tier 2 Bin 2 emissions on the FTP-75 cycle.
Curtis believes a 30-40% BTE improvement in gasoline engines “is possible at moderate cost.”
The primary technology suites being explored in the 2010-14 project include: advanced dilute combustion with cooled EGR and advanced ignition; advanced lean combustion with DI and advanced ignition; advanced boosting systems that include turbocompounding; and advanced cooling and aftertreatment systems. These are estimated to enable a 5% fuel-efficiency improvement.
Friction-reduction technologies and advanced engine control strategies are expected to add a 3% overall improvement, on average. Further engine-displacement downsizing will contribute another 18% to the 25% improvement target.
Stratified charge, cooled EGR, advanced aftertreatment
In noting Ford’s success to date in applying EcoBoost technology across most of its global gasoline engine portfolio, Curtis said “the more you downsize, the more high-load operations you get,” which in turn help reduce the percent of fuel energy expended as waste heat to the coolant.
Downspeeding is a logical next step to downsizing, as it helps shift the “good” brake-specific fuel consumption (BSFC) map toward the area of higher utilization. There are many benefits to increasing load, which “reduces the percent of fuel energy expended as waste heat to the coolant,” he explained.
Curtis said Ford engineers have established an optimum bore/stroke ratio of 0.8 for its GTDI engines (compared with an average 1.2 for its naturally aspirated engines) in the quest to improve thermal efficiency. He said a 500- to 600-cm3 cylinder is Ford’s “sweet spot” thus far in production GTDI engines, avoiding the broad bores that are prone to combustion knock.
Optimized stratified charge (lean burn) with cooled EGR and advanced aftertreatment is a pathway Ford is exploring, Curtis said, reminding the audience that “lean combustion is very challenging in the context of LEV3 emissions “Even 10-15% cooled-EGR rates would give good benefit,” he said. Ford and its MTU colleagues are exploring both high- and low-pressure loop EGR systems.
With top engineers from major turbocharger suppliers in the SAE audience, Curtis discussed the boosting challenges facing the next-gen GTDI engines in achieving their fuel-efficiency and emissions targets. “We need a greater range of boost capability,” he said, noting the benefits and trade-offs of various types of turbocharger, including radial, mixed-flow, and axial-flow systems.
Variable-valve actuation studies are a key component to the Advanced GTDI program, with Ford investigating three-step phasers, electric actuation, and hydraulic types.
Amid the discussion of hardware, Curtis dropped a request to the energy industry by noting that the research-octane rating (RON) of U.S. pump gasoline “has remained constant since the early 1960s.”