Engine Researchers: 50% Gasoline-Engine Efficiency in Sight

The Delphi Gen3X gasoline direct-injection compression-ignition (GDCI) engine has demonstrated roughly 43.5% thermal efficiency, but developers say there's potential for more. (Delphi)

Speaking at the SAE High-Efficiency IC Engine Symposium preceding this week’s WCX19 conference in Detroit, the lead researcher for a long-running, Delphi Technologies-directed program to maximize the thermal efficiency of gasoline engines said the latest developments show promise for delivering a production-ready gasoline engine that approaches 50% thermal efficiency.

Gen3X engine BFSC compared to various competitive engines. (Delphi)

Mark Sellnau—who until recently leaving Delphi for employment at Aramco—directed the program to develop Delphi’s gasoline direct-injection compression-ignition (GDCI) combustion system and presented results of testing of the third-generation of the GDCI 4-cylinder engine dubbed Gen3X. Sellnau summarized the analysis in recent a SAE technical paper extensively detailing the Gen3X advances (SAE 2018-01-0901), by saying the advances applied to the Gen3X engine brought its brake thermal efficiency (BTE) to 43.5%.

Latest GDCI cylinder head includes a pressure sensor for each cylinder (blue), as well as an intake-air heating element (yellow) for cold-start enhancement. (Delphi)

But, he added, visions for the fourth generation of the engine are projected to hike its efficiency to near 48% or beyond—“Near the practical limits for a light-duty internal-combustion engine,” in a practical powertrain, he said, also confirming the Gen4X is “an engine we’re planning to build in the near future.”

Meanwhile, the existing Gen3X engine, coupled to an 8-speed automatic transmission and a 12-volt start-stop system and fitted in a midsize passenger car, demonstrated 61 mpg fuel economy in the highway cycle and 48 mpg on the city cycle. Sellnau said the Gen4X engine is expected to be capable of 68 mpg on the highway.

Attributes for the Delphi Gen3X GDCI engine. (Delphi)

Advances to cut cost, improve performance

For now, though, the improvements to the Gen3X engine further enhance the GDCI concept that has been under development in the $9.8-million U.S. Dept. of Energy-funded research program that began in 2011 and produced two prior versions of the engine. “All these engines are now obsolete,” Sellnau said flatly. “None meet the requirements for commercial light-duty engines.”

Mark Sellnau, for several years a primary researcher of Delphi's GDCI concept. (Bill Visnic)

Sellnau said numerous revisions have reduced the cost and complexity of the latest Gen3X engine and enhanced performance, not to mention durability. “I see better robustness,” he told attendees at the 2019 edition of the longstanding High-Efficiency IC Engines Symposium. “You can feel it.”

Chief among the design changes is fitment of a variable-inlet compressor (VIC), variable-nozzle turbo (VNT) turbocharger, which allowed the researchers to jettison the supercharger required for the second-generation GDCI engine, greatly reducing cost.

Equally important, the control of the complex “partially-premixed” air/fuel mixture (unlike some other gasoline compression-ignition designs such as Mazda’s SpCCI, Delphi’s GDCI does not use sparkplugs to augment auto-ignition under certain conditions) for the Gen3X engine now is delivered in two distinct operating “regions:” a low-load/cold-start regime and a separate operating phase for medium-to-high-load operation. “From a controls perspective, this is relatively simple,” Sellnau said.

Other significant new features for the Gen3X engine include a higher compression ratio of 17:1 (up from around 14.5:1) and an increased stroke-to-bore ratio (1.28). The increased stroke reduces piston surface volume, which helps to reduce heat losses.

On to a fourth generation

Operating-performance comparison of three generations of Delphi GDCI engines. (Delphi)

But despite meaningful gains in performance, efficiency and emissions reductions, Sellnau indicates the continuing research already is looking to the Gen4X engine, largely because low gasoline prices in the U.S. have driven consumers to larger vehicles, while pricing trends for diesel have made it more even more unfavorable in the wake of global scrutiny about diesel-engine emissions. And, he adds, the research indicates that the GDCI engine can markedly exceed the efficiency of today’s best spark-ignited gasoline engines and hybrid-electric vehicles, which he said also are heavier and more complex.

He claims the current Gen3X engine already evidences a satisfying, diesel-like torque curve and meets the projects noise goals, while its brake specific fuel consumption of 194 g per kWh equates to the 43.5% BTE that beats the current state of the art in production spark-ignition engines.

He envisions new development in thermal-barrier coatings as one new advance to help the Gen4X engine improve and said “an OEM program with several vehicles is planned” for Gen4X development.