|Operating range of 2-stroke CAI fueled with gasoline, E10 and E85. Zhang et al. (2013a) Click to enlarge.|
Researchers at Brunel University in the UK, led by Professor Hua Zhao, Head of Mechanical Engineering and Director, Centre for Advanced Powertrain and Fuels (CAPF), are investigating optimizing the performance of controlled autoignition (CAI) combustion in a four-valve camless gasoline direct injection engine running in a two-stroke cycle. Most recently, this has entailed an exploration of boosting strategies, as described in a new paper published in the International Journal of Engine Research, as well as an exploration of the effects of ethanol blends.
Controlled autoignition (e.g., homogeneous charge compression ignition, HCCI) combustion processes offer the promise of simultaneously reducing fuel consumption and NOx emissions. Accordingly, the processes have been extensively researched over the last decade and adopted on prototype gasoline engines (e.g., GM’s ongoing work, earlier post).
However, CAI combustion is usually restricted to part-load operation conditions because of its combustion mechanism and lack of effective means of control. Extending its operational range to high-load and low-load boundaries—an enabling factor for CAI to meet demand of production vehicle driving cycles—is a key challenge to full implementation of CAI combustion.
Furthermore, CAI should be compatible with different fuels, and be able to switch back to conventional spark ignition operation when necessary.
Many approaches have been attempted to achieve CAI operation on the four-stroke spark ignition engines, such as intake air heating, high compression ratio, dual fuel, recycling the exhaust gas, and so on. Amongst these approaches, internally recycling the exhaust gas, such as residual gas trapping and exhaust gas rebreathing, have proved most effective in achieving CAI combustion and demonstrated their potential to be incorporated in the production gasoline engine.
One of the challenges facing the CAI combustion operation is its narrow operating load range, limited at high load by the violent combustion and at very low load by the misfire. In order to extend the load range of CAI combustion for automotive applications, a systematic research has been carried out by the authors on such combustion process in a single cylinder engine that is capable of both 4-stroke and 2-stroke operations through flexible variable valve actuation.
As part of a consortium over the last several years, the authors worked with industrial partners in developing 2/4-stroke switchable engine technologies through engine downsizing, from which the current research on 2-stroke CAI via poppet valve operation have been initiated and performed. In order to overcome the high HC and CO emissions and durability issues associated with the conventional crankcase scavenged 2-stroke engines, the 2-stroke poppet valve engine has been developed using the same engine architecture as the current 4-stroke engine.
In addition, direct fuel injection is applied to avoid short-circuiting fuel during he scavenging process. Controlled Auto Ignition is initialized by residual gases trapped in the cylinder through incomplete scavenging, which is inherent to the 2-stroke operation. A fully variable valve train is mounted on the cylinder head so that varied amount of residual gas fraction can be obtained through controlling scavenging process to achieve CAI combustion process at different engine operating conditions.—Zhang et al. (2013a)
The roots of the 2-stroke CAI work reach back more than ten years to a three-year project funded by the UK’s EPSRC to investigate the development of a gasoline direct injection with Controlled Auto Ignition; Ford and Jaguar were partners in that project.
At the same time, Brunel was partnering in a Ricardo-led project to develop the 2/4SIGHT concept—a direct-injection gasoline combustion system in which the design of intake and exhaust ports, combined with appropriate changes in fuel injection, ignition and valve timing, enable operation both in two-stroke and four-stroke modes. (Earlier post, earlier post.)
In 2008, Brunel, in collaboration with three other universities, received a further £495,401 (US$813,000) grant from EPSRC for a project to fundamental study of a novel poppet valve 2-stroke Auto-ignition Combustion Engine (2-ACE).
In their single-cylinder, direct-injection camless engine, the Brunel engineers use an electrohydraulic valvetrain system to enable independent control of valve timings and lifts. The fuel is injected by a Denso double-slit GDI injector mounted under the intake port.
In the boosting study report in IJER, the Brunel team performed extensive engine experiments to determine the optimum boosting for minimum fuel consumption in the single-cylinder gasoline direct injection camless engine operating in the two-stroke cycle.
To minimize the air short-circuiting rate, the intake and exhaust valve timings were adjusted. Lean boost was applied to the engine operation, which was found to extend the range of controlled autoignition combustion, result in higher combustion and thermal efficiencies and significantly lower carbon monoxide and hydrocarbon emissions.
By means of the cycle-resolved in-cylinder measurements and heat release analysis, the improvement in combustion and thermal efficiencies was attributed to the improved in-cylinder mixture, optimized autoignition and combustion phases.—Zhang et al.(2014)
At FISITA 2014 in Maastricht in June, the Brunel team will present a paper on the effects of ethanol blends on 2-stroke CAI. They will report achieving a wider range of CAI operation at engine speeds from 800 to 3000rpm and with engine load varying from idle to about 7.8 bar IMEP on the 2-stroke mode.
|Indicated fuel conversion efficiency in 2-stroke CAI operation at 2000 rpm. Zhang et al. (2013a) Click to enlarge.|
Their work has shown that 2-stroke CAI combustion operation can be achieved over a wide range of engine speed and load conditions, including idle operations that could not be achieved with 4-stroke operations.
The presence of ethanol allows CAI combustion to be extended to high load conditions. In the case of E85, the maximum IMPE of 8.4 bar was obtained an 800 rpm, significantly higher than the 4-stroke equivalent. Further improvement in the high load range at higher engine speeds could be achieved with a faster camless system or mechanical camshafts, they suggested.
CO, uHC and NOx emissions are reduced significantly by injecting ethanol blended fuels. E85 has a greater effect on the emission reduction than E15.
Both combustion efficiency and thermodynamic efficiency are improved by the presence of ethanol because of the optimum combustion phasing and lower heat loss. E85 improved indicated fuel consumption buy more than 5% at 2,000 rpm.
Yan Zhang, Hua Zhao (2014) “Optimization of boosting strategy for controlled auto-ignition combustion in a four-valve camless gasoline direct injection engine running in two-stroke cycle,” International Journal of Engine Research doi: 10.1177/1468087413519991
Yan Zhang, Hua Zhao, Mohammed Ojapah, Alasdair Cairns (2013a) "CAI combustion of gasoline and its mixture with ethanol in a 2-stroke poppet valve DI gasoline engine," Fuel, Volume 109, Pages 661-668 doi: 10.1016/j.fuel.2013.03.002
Zhang, Y., Zhao, H., Ojapah, M., and Cairns, A. (2013b) "2-Stroke CAI Operation on a Poppet Valve DI Engine Fuelled with Gasoline and its Blends with Ethanol," SAE Technical Paper 2013-01-1674 doi: 10.4271/2013-01-1674
Zhang, Y., Ojapah, M., Cairns, A., and Zhao, H. (2012) “2-Stroke CAI Combustion Operation in a GDI Engine with Poppet Valves,” SAE Technical Paper 2012–01–1118 doi: 10.4271/2012–01–1118