A multi-client research-and-development consortium led by Southwest Research Institute (SwRI) to develop heavy-duty diesel engines that demonstrate 55% engine-system brake thermal efficiency (BTE) is making such steady progress towards its goals that one of the program’s lead researchers believes the BTE target may even be surpassed.
In a presentation at SAE International’s 2018 High-Efficiency IC Engine Symposium in Detroit, SwRI’s Jason Miwa said the four-year Clean High-Efficiency Diesel Engine (CHEDE) program—now in its seventh phase that runs through 2019—is using a variety of new technologies and research findings to progress to the super-efficient, low-emitting heavy-duty diesel engine the program aims to make feasible for commercialization.
“We are making very good progress towards 55—and maybe even 60% BTE,” Miwa said at the conference for advanced-engine developers.
He added that one of the consortium’s current research directions is for high-BTE “hot” combustion that focuses on efficiency at the expensive of tightly controlling engine-out oxides of nitrogen (NOx) emissions, opting to allow the engine aftertreatment system to do the job for which it is designed. That includes heat-loss management study of the engine’s “passive” heat loss that is affected by piston-bowl design and high air-fuel ratios. Active heat-loss study includes surface-to-volume changes, insulation and encouraging elevated piston temperatures.
With piston temperatures increased by approximately 300 deg. C (572 deg F), for example, the researchers saw a 28% reduction in piston heat transfer and a 6% cut in total in-cylinder heat transfer, Miwa’s figures showed—and led to a one-point improvement in BTE.
Other heat-optimization measures include selective activation of piston oil-cooling jets and a variable-speed oil pump that combine for one-third less oil-flow requirement.
“We’re not the only crazy ones,” Miwa said of the program’s study of deliberately maintaining high piston temperatures, noting that a U.S. Dept. of Energy program has pursued similar measures. He said the “biggest impact” from the high-heat efforts has been friction reduction.
Better boost, alternative engine layouts
Miwa also detailed the CHEDE program’s development of an improved boost system with advanced turbine and compressor aerodynamics that yields improved isentropic efficiency. Hand-in-hand with this work was modification to the intake manifold design and lower rates of exhaust-gas recirculation (EGR).
Meanwhile, in addition to encouraging “hot” combustion and curbing subsequent heat loss, other primary design priorities for the reach to 55% BTE include maintaining high compression ratio and fast, optimally-timed combustion, Miwa said, as well as high-BMEP operation.
He also said the program is evaluating opposed-piston layouts and variable compression ratio.
Overall, “We are going to have to do better than 250-bar peak cylinder pressures,” he said, later adding that getting to 50% or better engine-system BTE with future regulation-compliant NOx emission levels will require intensive work.
“I don’t think it’s impossible,” Miwa said, “but I think it’s going to be a challenge.” He said both diesel and dual-fuel engines employing the techniques currently being evaluated in the CHEDE program “show potential” to meet proposed 2027 greenhouse-gas regulations.Continue reading »