Especially with increased production volume of renewable fuels and optimized powertrain solutions for flexible fuel vehicles, there is a chance for combustion engines to not only remain in the market but also be sustainable for future vehicle. Future advanced engine and powertrain configurations must address emission and fuel economy requirements for worldwide applications, transition to bio-fuels, and synergies with future powertrain trends.

—Wheeler et al.

The ACCESS project focuses on:

• Coordinating multi-mode combustion events to optimize engine performance with improved thermal efficiency and reduced emissions over the entire drive-cycle operating conditions.

• Developing robust and flexible control algorithms, on the basis of a universal modular platform, to coordinate various actuators, especially during combustion mode transitions.

• Designing a powertrain system to enable the proposed advanced combustion and control strategies, which include air-fuel path, combustion system, after-treatment system, and vehicle platform.

• Implementing the proposed approach and demonstrating its associated fuel efficiency benefits on engine dynamometer and on full-scale vehicles.

Design. The project team is replacing a 3.6L high-feature V6 in the Cadillac CTS with a modified 2.0L I4 Ecotec engine. To achieve the target fuel economy improvement over the baseline V6 configuration, the team used GTDI technology for downsizing in combination with part-load lean homogeneous charge compression ignition (HCCI) operation for further fuel economy gains. A number of engine design changes and controls enhancements enable part-load HCCI capability.

Among the design changes for the engine are the use of a dual direct and port fuel injection strategy (to be explored using both single and dual fuel concepts during the project); Delphi two-step cam-switching hardware; Denso electric VCT phasers; an Eaton R410 supercharger; HPL-cooled EGR; and an increased compression ratio of 11:1.

The team redesigned the cylinder heads to accomodate cam profile switching (CPS) and electrically actuated cam phasers to enable transient operation between HCCI and spark ignition (SI) operating modes. The cylinder head was also modified for centrally-mounted direct injectors.

The intake manifold was modified for high pressure EGR feed and port fuel injection (PFI).

The two-stage boosting system (turbo- and super-chargers) was designed to accommodate part-load HCCI range extension as well as full-load performance. Pistons were redesigned to achieve the target compression ratio and to accommodate optimized injector targeting.

Combustion studies. The team compared several combustion modes—Spark Ignited (SI); Homogeneous Charge Compression Ignition (HCCI); Spark-Assisted Compression Ignition (SACI)—under various conditions (naturally aspirated, boosted, lean, and stoichiometric) to compare the methods of controlled auto-ignition on the downsized, boosted multi-cylinder engine with an advanced valvetrain system capable of operating under wide negative valve overlap (NVO) conditions.

The engine was operated under steady state conditions at a constant engine speed of 1500rpm and various loads (2.0 - 6.0 bar BMEP) to investigate the impact of extending the HCCI/SACI operating range and compare the results to traditional SI combustion modes.

HCCI operating mode was tested under lean conditions both naturally aspirated and with mechanically supercharged operation to increase the load range. SACI combustion was examined at both lean and stoichiometric operating conditions, with the use of external cooled Exhaust Gas Recirculation (EGR) for mitigation of combustion noise at high loads.

They found that the effective load range of naturally aspirated lean HCCI is limited but provides good combustion stability with significant improvements in BSFC compared to the optimized SI combustion mode. Extending the lean HCCI range via forced induction improves the lean capability at higher loads but exacerbates loss mechanisms that reduce overall brake thermal efficiency. Extending the operating range through stoichiometric SACI combustion modes resulted in BSFC improvements at the mid- and high-load points tested.

Vehicle simulation. The team used AVL CRUISE to conduct vehicle simulations using engine maps producing via 1D simulation. The CRUISE model was validated using published data for the baseline 3.6L naturally aspirated, port fuel injected V6. They found that using the 2.0L I4 GTDI with HCCI alone resulted in a 42.4% improvement in fuel economy over the baseline engine for the Metro-Highway (M-H) cycle.

When thermal management and start-stop control technologies are added, the result is an estimated 48.9% improvement in fuel economy over the baseline.

Performance in the simulations took a bit of a hit. While the baseline 3.6L V6 accelerates from 0-60 mph in 5.5 seconds, the 2.0L I4 GTDI with HCCI (both with and without the extra control technologies), takes 6.2 seconds to go from 0-60.

Base engines have been rebuilt according to the new design and are now running at AVL, Bosch and the University of Michigan.

Resources

• Wheeler, J., Polovina, D., Frasinel, V., Miersch-Wiemers, O. et al., “Design of a 4-Cylinder GTDI Engine with Part-Load HCCI Capability,” SAE Int. J. Engines 6(1)doi: 10.4271/2013-01-0287

• Polovina, D., McKenna, D., Wheeler, J., Sterniak, J. et al., “Steady-State Combustion Development of a Downsized Multi-Cylinder Engine with Range Extended HCCI/SACI Capability,” SAE Int. J. Engines 6(1) doi: 10.4271/2013-01-1655