They found that at all loads, GDCI was remarkably clean, with the potential for no aftertreatment for NO_x and particulate emissions. The team expects to be able to deliver further improvements in fuel consumption through planned development work.

Background. In 2010, The US Department of Energy (DOE) selected Delphi, along with partners Hyundai America Technical Center, Inc (HATCI); Wisconsin Engine Research Consultants (WERC); and the University of Wisconsin-Madison (UW) for a $7.48-million grant to develop and to demonstrate a new ultra fuel efficient vehicle (UFEV) vehicle concept. (Earlier post.) A key strategy for achieving the project goals was the further development of a new low-temperature combustion system: gasoline direct-injection compression-ignition (GDCI).

General project targets were to deliver diesel-like or better fuel economy using E10 gasoline (RON91), and to achieve low engine-out NO_x and PM using low temperature combustion, the intention being to avoid the cost of aftertreatment for NO_x and PM, in addition to meeting more stringent standards.

Throughout the world, many efforts are being made to improve the thermal efficiency of internal combustion engines. One relatively new approach is gasoline partially-premixed compression ignition (PPCI) that was introduced by Kalghatgi and first tested by Johansson. A high octane fuel was injected late on the compression stroke of a boosted diesel engine operating with high EGR. The injection process was complete prior to the start of combustion enabling partial mixing of the fuel and air prior to heat release. Very low fuel consumption, NO_x, and PM emissions were measured. This early work established that gasoline-like fuels with high resistance to autoignition are preferred for PPCI.

… PPCI has demonstrated very good potential for very high fuel efficiency with low engine-out NO_xand PM emissions using a range of gasoline-like fuels. However, towards a production solution, significant issues remain. Due to the lower exhaust enthalpy of lower temperature engines using PPCI, it is difficult to produce intake boost with acceptable boost system parasitis. A practical powertrain system with robust PPCI combustion is needed, including injection, valvetrain, boost, and exhaust subsystems. The engine must also meet vehicle packaging requirements under hood and satisfy cold start and transient response requirements.

… Delphi has been developing a multi-cylinder engine concept for PPCI combustion with the current US market gasoline (RON91).… A multiple-late-injection (MLI) strategy with GDI-like injection pressures was selected without use of a premixed charge. The absence of classic knock and pre-ignition limits in this process enabled a higher compression ratio of 15. The engine operates “full time” over the entire operating map with partially premixed compression ignition. No combustion mode switching, diffusion controlled combustion, or spark plugs were used. Delphi uses the term Gasoline Direct Injection Compression Ignition (GDCI) in reference to this combustion process.

—Sellnau et al. 2014-01-1300

Delphi says that GDCI combines the best of diesel and spark-ignited engine technology. Click to enlarge.

Current study. In this phase of the project, the engineers designed and fabricated a new multi-cylinder GDI engine, and used design tools to package the powertrain in a D-class vehicle. The conducted engine dynamometer tests over a range of operating conditions, and included preliminary calibration mapping.

They used this data for a competitive assessment of BSFC against published data for gasoline, diesel and hybrid engines.

They also simulated aggressive transients with a high rate of increased load as well as cold starts, and then tested all on the real engine.

GDCI engine. The GDCI engine features a shallow pent room combustion chamber, central-mounted injector, and 15:1 compression ratio. The quiescent, open-chamber design supports injection-controlled mixture stratification; swirl, tumble and squish were minimized to prevent destruction of the stratification created during injection. The piston and bowl—a symmetrical shape centered on the cylinder and injector axes—are matched to the injection system and spray characteristics.

The injection strategy is central to the success of the concept, and involves one, two or three injections during the compression stroke. At start of combustion (SOC), the stratified fuel-air mixture achieves stable combustion and controlled heat release.

The absence of classic combustion knock and preignition for GDCI means that a GDCI engine can be operated on RON91 gasoline at high compression ratio for high efficiency. For this reason, GDCI is a good candidate for aggressive downsizing, downspeeding, and boosting, which is a proven strategy for high vehicle-level fuel economy. These were two main consideration in base engine design.

—Sellnau et al.

Full-time GDCI combustion is achieved using exhaust rebreathing at low loads and cooled EGR at medium- to high-loads. Rebreathing is implemented using the exhaust valvetrain system, which provides a secondary exhaust lift event during the intake stroke. The team noted that this is an effective method to recuperate heat from hot exhaust gases in order to raise mixture temperatures—i.e., the heat promotes autoignition at low loads when boost pressure is zero or low.

Rebreathing also keeps the oxidation catalyst warm over a wide range of low loads without special maintenance heating.

A Delphi electric camshaft phaser actuates exhaust valvetrain and controls the secondary valve lift with very fast response.

The specially developed boost system (reported at the SAE events in 2013), features a turbocharger, supercharger, two intercoolers, and a low-pressure loop EGR system. The aftertreatment system consists only of an oxidation catalyst; tailpipe NO_x and PM are to be handled by the GDCI combustion. The OxiCat is located at turbocharger outlet for heat conservation.

Schematic of boost system and OxiCat for aftertreatment. Click to enlarge.

Results. Engine dyno tests were run at idle, part-load, and full-load. For all operating conditions, the GDCI engine was operated with PPCI without mode switching or diffusion controlled combustion. Injection quantities and timings were used to control mixture stratification and combustion phasing. They found:

• At idle and low load, rebreathing of hot exhaust gases provided stable combustion with NO_x and PM emissions below targets of 0.2g/kWh and FSN 0.1, respectively. The coefficient of variation of IMEP was less than 3%. BSFC of 280 g/kWh was measured at 2000 rpm-2bar BMEP.

• At medium-to-higher loads, rebreathing was not used and cooled EGR provided NO_x, PM, and combustion noise below targets.

• At full load operating conditions, near stoichiometric mixtures were used with up to 45% EGR. Maximum BMEP was about 20 bar at 3000 rpm.

• Good BSFC at low speed and loads, and good minimum BSFC at higher load (~213 g/kWh).

• Engine-out NO_x was below 0.2 g/kWh at every point on the map—unique GDCI characteristic, they said. NO_x decreases with increasing load, a helpful attribute for down-speeding.

• Smoke (FSN) achieved program targets.

For the competitive assessment, they matched the GDCI’s data against:

• BMW-PSA 1.6L Valvetronic 2013
• Daimler 1.6L Strat. Camtronic 2012
• Toyota 1.8L 3rd Gen Hybrid Atkinson-EGR
• Honda 2.0L Hybrid Atkinson VTEC 2013
• VW 2.0L Jetta Diesel T2B5 2009
• Honda 2.2L i-DTEC Diesel 2012

They found:

• For the best efficiency point on the map, the GDCI engine matched the hybrid vehicle engines; was about 5% higher than the two diesels; and was significantly beter than the spark-ignited engines.

• For the low speed/load range, BSFC for GDCI is comparable to the hybrid engines; better than most of the diesels; and significantly beter than the SI data.

• At the 2000 rpm, 2 bar BMEP world test point, BSFC for the GDCI was 280 g/kWh (clutch on), and 267 g/kWh (clutch off): lower than all the gasoline and diesel engines.

• For the medium-to-high speed/load range, BSFC for GDCI was within 1 to 5% of diesel, and significantly beter than SI. The hybrid engines did not produce enough torque at these loads for comparison, and were labelled “off map.”

Despite the good results, the team emphasized that the results are not optimized; further improvements can come from further development of the combustion system, improving firing friction, and refined calibrations.

Resources

• A, M., Foster, M., Hoyer, K., Moore, W. et al. (2014) “Development of a Gasoline Direct Injection Compression Ignition (GDCI) Engine,” SAE Int. J. Engines 7(2) doi: 10.4271/2014-01-1300