Development of a Gasoline Direct Injection Compression Ignition (GDCI) Engine

In previous work, Gasoline Direct Injection Compression Ignition (GDCI) has demonstrated good potential for high fuel efficiency, low NOx, and low PM over the speed-load range using RON91 gasoline. In the current work, a four-cylinder, 1.8L engine was designed and built based on extensive simulations and single-cylinder engine tests. The engine features a pent roof combustion chamber, central-mounted injector, 15:1 compression ratio, and zero swirl and squish. A new piston was developed and matched with the injection system. The fuel injection, valvetrain, and boost systems were key technology enablers. Engine dynamometer tests were conducted at idle, part-load, and full-load operating conditions. For all operating conditions, the engine was operated with partially premixed compression ignition without mode switching or diffusion controlled combustion. At idle and low load, rebreathing of hot exhaust gases provided stable combustion with NOx and PM emissions below targets of 0.2g/kWh and FSN 0.1, respectively. The coefficient of variation of IMEP was less than 3 percent and the exhaust temperature at turbocharger inlet was greater than 250 C. 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 NOx, PM, and combustion noise below targets. MAP was reduced to minimize boost parasitics. At full load operating conditions, near stoichiometric mixtures were used with up to 45 percent EGR. Maximum BMEP was about 20 bar at 3000 rpm. For all operating conditions, injection quantities and timings were used to control mixture stratificaton and combustion phasing. Transient co-simulations of the engine system were conducted to develop control strategies for boost, EGR, and intake air temperature control. Preliminary transient tests on a real engine with high rate of load increase demonstrated potential for very good control. Cold start simulations were also conducted using an intake air heating strategy. Preliminary cold start tests on a real engine at room temperature demonstrated potential for very good cold starting. More work is needed to calibrate the engine over the full operating map and to further develop the engine control system. CITATION: Sellnau, M., Foster, M., Hoyer, K., Moore, W. et al., "Development of a Gasoline Direct Injection Compression Ignition (GDCI) Engine," SAE Int. J. Engines 7(2):2014, doi:10.4271/2014-01-1300. 2014-01-1300 Published 04/01/2014 Copyright © 2014 SAE International doi:10.4271/2014-01-1300 saeeng.saejournals.org developed with significant improvements. Delphi [23, 24, 25, 26, 27] reported single-cylinder and multi-cylinder engine test results with various injectors using single, double, and triple injection strategies. Engine tests have also been performed using naphtha fuels on both modified spark-ignited engines [28] and modified diesel engines [29]. Naphtha has significantly lower octane number (RON 70 to 84) and significantly lower processing cost compared to current market gasoline. This work has shown compatibility of the PPCI combustion process with lower octane fuels in a longer term perspective. PPCI has demonstrated very good potential for very high fuel efficiency with low engine-out NOx and PM emissions using a range of gasoline-like fuels. However, towards a production solution, significant issues remain. Due to the lower exhaust enthalpy of low temperature engines using PPCI, it is difficult to produce intake boost with acceptable boost system parasitics. 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. As part of a US Department of Energy funded program, Delphi has been developing a multi-cylinder engine concept for PPCI combustion with current US market gasoline (RON91). The engine has four cylinders with a 1.8L displacement and was designed based on extensive simulations and single-cylinder engine tests. 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 preignition limits in this process enabled a higher compression ratio of 15. The engine operates “full time” [25] 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 Direction Injection Compression Ignition (GDCI) in reference to this combustion process. One program objective was to build a practical GDCI engine that achieves diesel-like fuel efficiency using current market gasoline (RON91) with engine-out NOx and PM emissions below that needed for NOx or PM aftertreatment. Table 1 lists initial targets for engine testing and development. Combustion noise level (CNL) limits are shown in Figure 1. Other program objectives include demonstration of good transient load response and room temperature cold starts. In the current work, analysis and simulation tools were used to design and fabricate a new multi-cylinder GDCI engine. Design tools were used to package the powertrain in a D-class vehicle. Engine dynamometer tests were conducted over a range of operating conditions and included preliminary calibration mapping. With this data, a competitive assessment of brake specific fuel consumption (BSFC) was performed using published data for gasoline, diesel, and hybrid vehicle engines. Finally, aggressive transients with high rate of load increase were simulated and then tested on the real engine. Cold starts at various ambient temperatures were also simulated and tested on the real engine. Table 1. Preliminary Targets for Engine Testing. Figure 1. Combustion Noise Level (CNL) Limits used for Testing. GDCI CONCEPT AND INJECTION STRATEGY The GDCI engine concept combines the best of diesel and spark-ignited engine technology. Like diesel engines, the compression ratio is high, there is no intake throttling, and the mixture is lean for improved ratio of specific heats. GDCI uses a new low-temperature combustion process for partiallypremixed compression-ignition. Multiple-late-injections of gasoline (RON91) vaporize and mix very quickly at low injection pressure typical of direct injected gasoline engines. Low combustion temperatures combined with low mixture motion and reduced chamber surface area result in reduced heat losses. A schematic of the GDCI combustion chamber concept is shown in Figure 2. The engine features a shallow pent roof combustion chamber, central-mounted injector, and 15:1 compression ratio. A quiescent, open chamber design was chosen to support injection-controlled mixture stratification. Swirl, tumble, and squish were minimized since excessive mixture motion may destroy mixture stratification created during the injection process. The piston and bowl shape were matched with the injection system and spray characteristics. The bowl is a symmetrical shape and was centered on the cylinder and injector axes. The GDCI injection strategy is central to the overall GDCI concept and is depicted in the Phi-T (equivalence ratiotemperature) diagram shown in Figure 3. The color contours in Figure 3 show simulated CO emissions concentration. The injection process involves one, two, or three injections during Sellnau et al / SAE Int. J. Engines / Volume 7, Issue 2 (July 2014)