|Cross-sectional view of SGDI cylinder head showing the layout of the combustion system. King et al. Click to enlarge.|
Papers on the Ricardo turbocharged spray-guided gasoline direct injection (T-SGDI) combustion system and on its HyBoost research (earlier post) took awards for most “outstanding paper” at the recent FISITA 2012 World Automotive Congress in Beijing in the “future internal combustion engines” and “future powertrain” categories.
T-SGDI. Ricardo, in collaboration with the engines business of Malaysian technology and energy company PETRONAS Research Sdn Bhd., have undertaken a four-year collaborative research program to develop the next generation of spark-ignited Spray Guided Direct Injection (SGDI) gasoline engine combustion system with robustness to blended fuels such as ethanol or methanol.
In the FISITA paper, Jason King from Ricardo mainly covered the development and benefits of stratified operation at part load with both naturally aspirated and boosted engine operation.
The initial part of the T-SGDI program enabled the successful development of a next-generation stratified charge combustion system based on spray-guided fuel injection with up to five injections per cycle. The injection sequence and injection duration was varied from a minimum of 0.1ms per injection upwards.
Subsequent research work on the multi-cylinder T-SGDI research engine has demonstrated that fuel consumption benefits were significantly enhanced through boosting, with a best BSFC of 203 g/kWh being achieved at 2250 rev/min and 13 bar BMEP.
Work reported in the paper used a spray-guided direct injection combustion system jointly developed with PETRONAS. The cylinder heads featured a transverse orientation spark plug and injector layout—i.e. the axis of the spark plug and injector was perpendicular to the crankshaft axis. The piezoelectric outwardly opening injector was located between the intake valves while the spark plug was located between the exhaust valves. Both injector and spark plug were slightly tilted towards the cylinder axis.
The piston incorporated a bowl shape, optimized for lean stratified SGDI operation. The principle design feature of the bowl was to allow for longer spray penetration lengths during late injection thus avoiding liquid fuel impingement onto the piston crown. In addition, the squish flow, generated during the compression stroke was utilized to improve the formation of a compact mixture cloud in the combustion chamber centre. The main aim of the strategy was to reduce the mixture concentration gradient in the periphery of the mixture cloud.
Experimental variables included the split of injected mass between the injection events, fuel pressure, EGR quantity, valve timing and lift to control air motion and internal residual gas fraction, and boost pressure. Rapid testing methods, analysis and final optimisation of the engine was realized using in-house Design of Experiments (DoE) techniques.
For a given engine part load condition it was shown that multiple fuel injection events resulted in improved engine combustion stability when compared to a single injection event. Furthermore, combustion stability was also less sensitive to spark timing.
The angle of 50% of mass fraction burnt in the multiple injection case was closer to the thermodynamic optimum than could be achieved in the single injection case. Low engine-out NOx emissions, an important attribute of a stratified combustion system due to the cost and regeneration fuel penalty associated with a LNT, was improved through multiple injections and external EGR.
By adding boosting to stratified operation it was possible to widen the late injection operating window from 8 bar NIMEP (net indicated mean effective pressure) to 14 bar NIMEP so unthrottled running could be maintained throughout most of the engine operating range.
The higher mass flow during part load resulting from unthrottled operation also improves turbocharger response. In addition, the best BSFC engine map areas are much closer to real-world requirements than is the case with previous stratified charge solutions. Unlike a diesel engine, there is no practical smoke-limited AFR, so lambda can be instantaneously switched from lean to λ1 or richer to maximize the air utilization for maximum torque and increased enthalpy release to the turbine for enhanced run-up.
Combining advanced multiple injection strategies with boosting enabled the stratified operating range to be extended to over 12 bar BMEP, with a further significant improvement in fuel consumption to class leading levels. As the load exceeded 8 bar BMEP the MIVIS [Multiple Injection Variable Injection Separation] injection strategy was superior in fuel consumption terms to the multiple injection strategy used at the lower loads and on the singe cylinder engine to minimize NOx emissions. The extended WOT part load operating region also increased the mass flow through the engine versus a conventional throttle boosted gasoline engine, and this resulted in an improved transient response of the turbocharger, which is a key issue particularly for downsized gasoline engines where the downsizing ratio could be as high as 50%. Finally, the robustness of the SGDI combustion system to the highest loads in combination with the latest boosting and future knock mitigation technologies has been clearly demonstrated.—King et al.
HyBoost. The HyBoost concept was demonstrated by Ricardo and its research partners Controlled Power Technologies, the European Advanced Lead Acid Battery Consortium, Ford, Imperial College London, and Valeo. HyBoost is based on a 2009 Ford Focus in which a 2.0L naturally aspirated four-cylinder gasoline engine is replaced with a 1.0L three-cylinder EcoBoost engine.
In implementing this 50% downsizing by swept volume, the research team had the objective of delivering zero degradation in driveability, performance or acceleration. This was achieved through the use of a combination of technologies including a belt starter-generator to provide regenerative braking and stop/start, exhaust energy recapture through electric turbo-compounding, advanced lead-acid batteries and super-capacitors to provide energy storage, and electric supercharging to provide improved transient response and avoid the pitfalls of turbo-lag that otherwise place a practical limit on the potential for downsizing.
The resulting architecture provides a highly cost-effective, low-voltage, mild hybrid gasoline powertrain delivering similar CO2 performance to a more expensive full-hybrid, but at a cost premium of less than a diesel.
J. King, L. Schmidt, J. Stokes, J. Seabrook, F. Nor, S. Sahadan (2012) “Multiple injection and boosting benefits for improved fuel consumption on a Spray Guided Direct Injection gasoline engine” F2012-A01-041
J. King, M. Heaney, E. Bower, J. Saward, A. Fraser, G. Morris, P. Blore, Mark Criddle, Thierry Chang (2012) “HyBoost – The Development of an Intelligently Electrified Optimised Downsized Gasoline Engine Concept” F2012-B02-070