GDCI, a low-temperature combustion (LTC) process for gasoline partially premixed compression ignition (PPCI), has been under consideration and development for about 5 years, Sellnau said, with efforts predating the 2010 DOE funding. GDCI uses a high compression ratio with multiple late injection (MLI)—similar to diesel—along with intake boost and moderate EGR for high efficiency with low NO_x and PM over the entire speed-load map.

The relatively long ignition delay and high volatility of pump gasoline combined with an advanced injection system and variable valve actuation provides controlled mixture stratification for low combustion noise.

Among the objectives of the work reported in the paper presented were 1) to determine the best injection strategies for low NO_x and PM using low-to- moderate injection pressures; and 2) to evaluate an engine concept for full-time operation over the speed-load map from idle to full load. Use of variable valve lift profiles was instrumental in enabling full-time GDCI operation, the team noted. The team developed and tested five different injectors.

Sellnau reported results derived from testing on a single-cylinder research engine. The cylinder head of the Ricardo Hydra light-duty single-cylinder engine has four-valves with double-overhead camshafts and central injection. The aluminum cylinder head is rated at 200 bar peak cylinder pressure (PCP). For all tests, intake air temperature was 50 C.

• At a low-load condition of 1500 rpm-2 bar IMEP, Delphi used a secondary-exhaust-valve-lift event to rebreathe hot exhaust gas and promote autoignition. A “BDC” (Bottom Dead Center) intake cam was also used to maximize the effective compression ratio. Even though heat losses increased somewhat due to the rebreathing, they obtained good indicated specific fuel consumption (ISFC) of about 230 g/kWh, stable combustion, and exhaust port temperatures of about 250 C.

• Example of a triple-injection process. The piston is seen rising in each frame. Sellnau et al. Click to enlarge.

  At a medium-load condition of 1500 rpm-6 bar IMEP, injector developments combined with a MLI strategy (triple) and low swirl produced the best ISFC and lowest smoke. The most advanced injector design did not require swirl to achieve very low smoke and NO_x levels. Measurements of exhaust particulate size distribution showed that very low PM emissions could be obtained with this combustion system.

• At higher loads, late intake valve closing was used to reduce cylinder pressure and temperature, and increase ignition delay. Delphi obtained a minimum ISFC of 181 g/kWh. Combustion noise, maximum pressure rise rate, and ringing intensity were in acceptable ranges, however, the correlation among these noise parameters was poor.

• For IMEP from 2 to 18 bar, engine-out NO_x and PM emissions were below targets of 0.2 g/kWh and 0.1 FSN, respectively, indicating that aftertreatment for these species may be reduced or eliminated.

• Measurements of exhaust particulate size distribution indicated very low particle count, especially for a preferred injector with low levels of in- cylinder swirl.

Overall, single-cylinder engine tests of a GDCI combustion system indicate good potential for a high- efficiency, low-emissions powertrain. Additional testing and development on a multi-cylinder engine is needed, including cold-starting and transient operation.

—Sellnau et al.

In his talk at the High Efficiency IC Symposium, Sellnau said that a modeled 1.8L GDCI engine for vehicle simulations showed large regions with fuel consumption of less than 190 g/kWh: loads of 6-20 bar, and speeds of 1800-3500 rpm. Applied in a mid-size passenger car, such an engine could potentially—without optimization or with a start-stop system, although with variation of gear ratios and shift schedules—deliver 60% improvement in city driving fuel economy, and 40% on US06, for a combined fuel economy improvement of about 51%, Sellnau said.

Resources

• Sellnau, M., Sinnamon, J., Hoyer, K. and Husted, H., Full-Time Gasoline Direct-Injection Compression Ignition (GDCI) for High Efficiency and Low NO_x and PM. SAE Int. J. Engines 5(2) doi: 10.4271/2012-01-0384.