Delphi advancing Gasoline Direct-Injection Compression-Ignition engine concept; new two-stage supercharger/turbocharger boost system
At SAE World Congress next week in Detroit, Delphi Automotive will present two technical papers describing its ongoing progress with the Gasoline Direct-Injection Compression-Ignition (GDCI) engine concept. (Earlier post.)
GDCI is an advanced low-temperature combustion concept that uses compression ignition under lean to near stoichiometric fueling conditions over the complete engine operating range. Previous studies of GDCI have shown good potential for very high efficiency, low NOx, and low PM over the full speed-load range. GDCI achieves low-temperature combustion using multiple-late injection (MLI), intake boost, and cooled EGR.
GDCI combines aspects of diesel and spark-ignited engine technology. The compression ratio is high with multiple-late-injections (MLI)—similar to diesel—but commercial gasoline fuel is used that vaporizes and mixes quickly at low injection pressure, unlike diesel.
The injector is central-mounted, with a symmetrical chamber and piston bowl. The engine is operated unthrottled and diluted with excess air or EGR, depending on load.
The work being presented at this year’s World Congress describes the development of a boost system for GDCI and GDCI part-load operation.
|Two-stage supercharger/turbocharger system with two liquid charge air coolers (LCCA) to control intake air temperature (IAT) for GDCI. Hoyer et al. Click to enlarge.|
Boost system. Like other high-efficiency concepts, GDCI has low exhaust temperatures; unlike some others, however, GDCI relies heavily on fuel injection strategy to ensure proper combustion, the Delphi engineers noted. Since the injection strategies depend on the in-cylinder pressure, temperature, and composition conditions, the injection is coupled to the boost system.
For the study—the goal of which was to demonstrate a practical and efficient boost system for a low-temperature 1.8L, 4-cylinder GDCI engine—the team assumed that the injection strategy will permit proper combustion provided the boost system can deliver the correct air and EGR mixture at the correct pressure and temperature.
They used simulation tools to select the boost architecture, to develop the selected system for full-load and part-load requirements, and to evaluate transient response. They also developed a methodology for simulation-based calibration to determine the best settings for efficient operation and potential for low fuel consumption over the full speed-load map.
They found that a two-stage supercharger/turbocharger system that also has two charge air coolers could supply the needed air charge to meet the GDCI requirements for full-time operation. Other boost system architectures could not meet the GDCI performance targets; turbocharger system performance was degraded by low temperature exhaust, high EGR, and LIVC operation.
The undertook a design effort to package practically the two devices and two coolers into a vehicle. Interactions of the devices affect the performance and efficiency of the system. When the proper combination of supercharger drive ratio and turbine geometry settings are chosen, the simulated results demonstrate that this system can operate efficiently at full load. For part load operation, results show that turbine rack settings and supercharger bypass valve can be used to achieve low BSFC.
Preliminary transient simulations of the system for an aggressive acceleration from idle to full load indicates potential for reasonably fast response, they found. Real transient response will depend on the engine control system.
Overall, they concluded, simulation results indicate the GDCI engine with the proposed two-staged boost system should meet performance objectives with good transient response.
Part-load operation. In the part-load operation study, which involved both a single-cylinder and multi-cylinder engine, Delphi engineers designed a new piston to match with the injection system. Piston design requirements for GDCI are different than for diesel, they noted. The new piston supports a near quiescent chamber with low squish and low or no swirl. This is desirable to maintain stratification created and controlled by the injection process and to avoid overmixing of fuel and air.
Delphi matched the bowl shape and fuel spray for typical GDCI injection timing. This resulted in significantly reduced piston surface area with less propensity for fuel wall wetting. Low-temperature GDCI combustion performs best when injected fuel stays in the hot bulk gases for complete oxidation and minimal heat losses to piston and liner surfaces, the team noted.
With the fuel injection system and the new piston design, the GDCI combustion system demonstrated very good fuel efficiency and emissions in single-cylinder engine testing. At part-load conditions, injection pressure could be lowered significantly, while achieving low fuel consumption, NOx, PM, and combustion noise.
Heat losses and combustion losses were both very low, and contributed to indicated thermal efficiencies of approximately 47%, the Delphi team said. Losses associated with CO and HC emissions were higher than desired and are strong candidates for near term work.
Particulate emissions, measured using a TSI particle size spectrometer, were near levels for filtered room air. The Delphi team suggested that there is good potential to eliminate aftertreatment for PM and NOx species; however, an oxidation catalyst for HC and CO is expected to be needed.
Other findings of the study included:
At part-load conditions, boost pressure and EGR could be lowered with improved combustion stability. COV IMEP and variation of CA50 and were both within acceptable ranges. Tests showed that injection parameters could be used to control combustion phasing and other combustion parameters.
Good low-speed BMEP was predicted with maximum BMEP of 20 bar at 1800 to 2000 rpm. The system lends itself to aggressive down-sizing, down-speeding, and up-loading for the most efficient operation.
Vehicle simulations for various drive cycles predicted very good fuel economy for all city and highway modes. Predicted fuel economy was improved 65%, 28%, and 28% for the FTP City, Highway, and US06 cycles, respectively, for a combined FE improvement of 50%. For the NEDC, EUDC, and WLTC cycles, fuel consumption (L/100km) decreased 37%, 27%, and 33%, respectively.
On a multi-cylinder engine, preliminary parametric tests at 2000 rpm-11 bar IMEP showed ISFC and combustion noise were low at 175 g/kWh and 88 dB, respectively, and were comparable to that measured on the single-cylinder engine. NOx and smoke emissions were low and below targets, but HC emissions were above expectations. More work is needed to characterize the engine over the operating map, conduct cold starts and transient tests, and optimize systems for minimum fuel consumption at target emissions levels, the team concluded.
Hoyer, K., Sellnau, M., Sinnamon, J. and Husted, H. (2013) Boost System Development for Gasoline Direct- Injection Compression-Ignition (GDCI). SAE Int. J. Engines 6(2) doi: 10.4271/2013-01-0928
Sellnau, M., Sinnamon, J., Hoyer, K., Kim, J., Cavotta, M., and Husted, H. (2013) Part-Load Operation of Gasoline Direct-Injection Compression Ignition (GDCI) Engine. SAE 2013-01-0272 doi: 10.4271/2013-01-0272