A gasoline engine that employs unique technologies to switch between two- and four-stroke modes has proven its practical merit based on the findings from recently completed testing.
"The key outcomes of the initial engine test program were the ability to achieve mode-switching between four-stroke and two-stroke operation without misfire and to demonstrate the peak torque output from the engine when it is in two-stroke mode [230 N·m/L]," noted Richard Osborne, Chief Engineer on the 2/4SIGHT project, which was led and coordinated by Ricardo. Other partners in the project included Denso, the developer and supplier of the direct fuel injection and advanced engine control systems.
A top-entry port configuration combustion system that provides "effective two-stroke cycle scavenging without sacrificing four-stroke performance" is a 2/4SIGHT engine concept highlight, according to Osbourne. The prototype engine's valvetrain is "an electrohydraulic cam-less system featuring Moog servo control valves. (An electrohydraulic valve actuation system was used for the prototype development rig so that the project team could assess control strategies.)
The engine's air handling system is based on two-stage boosting and intercooling using a Rotrex supercharger and Honeywell turbocharger. (In the initial testbed prototype configuration, boosting was provided by external compressed air supply.) The control system uses a Denso engine control unit, an ETAS rapid prototyping system, and 12 valve control units.
Although conventional four-stroke engines can be boosted to produce more torque, there are pitfalls from the high pressures and temperatures that occur—especially at low engine speeds. A highly boosted four-stroke engine typically uses a lower compression ratio in order to operate. A two-stroke engine when operating under low-speed/high-load conditions typically produces torque from each firing stroke that is about half that of a four-stroke engine.
The ability to switch between two- and four-stroke modes on the 2/4SIGHT is demand-driven and fully automatic, according to Neville Jackson, Group Technology Director of Ricardo. In normal driving conditions and moderate acceleration, the 2/4SIGHT engine operates as a conventional four-stroke engine, while two-stroke operations would typically be used when low-speed high torque is required. "To put this into perspective: in two-stroke mode, the testbed engine has achieved over 230 N·m/L. This enables a 2.0-L switching engine to achieve over 450 N·m, similar to a 4.4-L naturally aspirated engine," noted Jackson.
Based on initial testing, the 2/4SIGHT engine extends the envelope of engine downsizing, which means it can net higher fuel economy. Ricardo carried out a vehicle drive cycle and acceleration performance simulation—via Ricardo powertrain block-set in the MSC Easy5 software package for detailed models—based on the steady-state fuel consumption and full-load performance of the 2/4SIGHT engine. The baseline vehicle in the simulation study was an 1800-kg (3970-lb) European-market-sold passenger car with a 3.5-L naturally aspirated V6 gasoline engine and five-speed automatic transmission with torque converter.
Simulation results indicate that vehicle acceleration performance, including launch from rest, can be maintained with a 2.0-L V6 2/4SIGHT gasoline engine replacing the 3.5-L baseline powerplant. Fuel savings of 27% were predicted using the New European Drive Cycle as well as vehicle CO2 emissions of the baseline going from 260 g/km to 190 g/km. The concept, in principle, is applicable to any spark-ignited engine, but is ideally suited to automotive duty cycles. "It is unlikely that 2/4SIGHT would be hybridized as both technologies are enablers of aggressive downsizing and therefore address many of the same issues," noted Jackson.
The 2/4SIGHT is one of many next-generation technologies in gasoline, diesel, and hybrid powertrain research that Ricardo is pursuing. "The most obvious next stage of development would be to implement a vehicle prototype version of the engine. Ricardo is currently in discussion with a range of partners about possible continuation of this research project," Jackson noted.