在底特律举行的SAE 2017 高效内燃发动机研讨会（SAE 2017 High-Efficiency IC Engines Symposium）上，德尔福公司预测，如果发动机采用公司的第三代汽油直喷压燃（GDCI）系统，可将热效率提升至42%左右，超过目前任何量产汽油发动机的水平（40%）。

德尔福先进传动系统工程经理Mark Sellnau表示，目前公司的第三代GDCI燃烧系统正在研发之中，未来采用这种系统的发动机的热效率将比当下的常规火花点火式直喷汽油发动机高22%，比2L的柴油发动机高11%。他说，德尔福已经克服了GDCI燃烧系统早期面临的很多挑战，公司相信这种技术可以让低温燃烧系统的排放达到美国环保署Tier 3 Bin 30水平。

具体来说，德尔福第二代GDCI系统的重大升级，包括“wetless”燃烧技术、更快的冷启动运行速度，以及优化的低温尾气处理系统（可在大约4s内转换90%的一氧化碳）。

Sellnau表示，第三代系统的压缩比已从之前的15：1提升至16：1，活塞冲程更长，上止点间隙也更大，可以保证德尔福的“wetless”运行目标，确保燃料在接触气缸或燃烧室内壁表面时能够充分汽化。系统先进燃料喷射器的工作压强为350 bar (5076 psi)，可以实现“三次喷射”。Sellnau表示，这里所说的第三次喷射“就是GDCI燃烧系统与HCCI（均质压燃）系统拉开差距的地方。”

德尔福的GDCI研发受到美国能源部一项为期4年、总规模达980万美元项目的资助，项目合作伙伴包括橡树岭国家实验室（Oak Ridge National Laboratory）、Umicore，及威斯康星大学麦迪逊分校。

除了最新研发的低温催化剂技术外，该系统还将采用多种减排措施，包括进气口加热装置、汽油微粒过滤器及尿素喷射系统等。Sellnau相信，凭借上述这些措施，再加上第三代系统的先进燃烧设计，采用GDCI燃烧系统的发动机一定可以达到Tier 3 Bin 30规定的要求，从而在很大程度上保证了这种系统能够实现量产。

此外，Sellanu承诺，最新GDCI系统对汽油没有特殊要求，目前常用汽油的辛烷值水平就没有问题。他说，“我们要打入市场，就必须能够直接使用市场中现行的油品。”

Sellanu说，项目团队从2017年2月份就开始为第三代GDCI系统收集测试结果，“未来，我们将看到一款非常高效的发动机一点点丰满起来，并最终加入我们的汽车项目。”不过，Sellanu并未预测该系统具体将于何时登陆量产发动机。

Delphi projects that engines employing its coming third-generation gasoline direct-injection compression-ignition (GDCI) combustion system will attain a thermal efficiency of 42%, the company said at the 2017 SAE High-Efficiency IC Engines Symposium in Detroit. That thermal efficiency would top any current production-vehicle gasoline engine, the most efficient of which are claimed to have peak thermal efficiency of about 40%.

Mark Sellnau, engineering manager, Delphi Advanced Powertrain, said a gasoline engine using the third-generation GDCI system now under development is projected to have approximately 22% better thermal efficiency than a current conventional spark-ignited gasoline engine with direct fuel injection and 11% better than a 2L diesel engine. He said many of the initial engineering challenges for GDCI have been overcome and that the company believes it can make the low-temperature combustion system emissions-compliant at the EPA Tier 3, bin 30 level.

Major advances over Delphi’s second-generation GDCI system include “wetless” combustion, quicker cold-start operation and an optimized low-temperature exhaust aftertreatment that achieves roughly 90% carbon monoxide conversion in about 4s.

Sellnau said the third-generation system has increased the compression ratio to 16:1 (from 15:1) and its new longer stroke and increased top-dead-center piston clearance enable the wetless operation that sees fuel completely vaporized before it contacts cylinder or combustion-chamber surfaces. Advanced fuel injectors operate at 350 bar (5076 psi) and provide three injection events. The third injection, Sellnau said, is “what differentiates (GDCI) from HCCI (homogenous-charge compression ignition).”

Much of Delphi’s GDCI research is being conducted under the auspices of a four-year, $9.8-million program funded by the U.S. Department of Energy; Delphi’s partners in the project include Oak Ridge National Laboratory, Umicore and the University of Wisconsin-Madison.

The system will require a variety of emissions-reduction measures that, in addition to the newly-formulated low-temperature catalyst, include an intake-air heater, gasoline particulate filter and urea injection. But Sellnau was confident those measures, coupled with the combustion-related design advances of the third-generation system, will enable a production engine to be Tier 3, bin 30 compliant—a vital achievement to advance the system for production-vehicle readiness.

Moreover, he promised the latest GDCI will operate with gasoline at currently-available octane. “We really need to get to market with commercial gasoline,” he said.

Sellanu said the project team began gathering test results for the third-generation GDCI system in February 2017 and added, “I think we’re going to see a very efficient engine evolving and going into our vehicle program.” He did not, however, provide a projection on when the system might be fitted to a production-vehicle engine.

Author: Bill Visnic
Source: SAE Automotive Engineering Magazine