The rapid increase in environmental pollution and global warming has necessitated stringent actions against the use of fossils fuels, the main sources of greenhouse gases. In particular, enhancing the thermal efficiency of internal combustion engines is deemed effective in enhancing fuel consumption and reducing emissions. This has led to the development of high-performance compression ignition (CI) and spark ignition (SI) engines. For SI engines, the lean-burn operation strategy is commonly used to improve thermal combustion efficiency owing to its inherent advantages like low combustion temperature.
Recent studies revealed the possibility of extending the lean limits of SI engines to excess ratio λ 2.0 using a two-stage combustion system or high energy ignition and tumble flow enhancement. This has drawn significant research attention, inspiring research towards the realization of super-lean burn SI engines operated at λ 2.0. However, knocking is still a big challenge due to higher unburned gas temperature, oxygen concentration and in-cylinder pressure. Alternatively, the flame propagation limit decelerates the burning phase making the lean-burn conditions to increase the combustion fluctuation and misfire. Therefore, developing highly effective strategies for improving thermal efficiency at super-lean burn conditions by extending the burning phase, reducing cooling loss and suppressing knocking is urgent. Water injection is a promising approach for cooling loss reduction and knocking suppression. However, water injection to SI engine at super-lean burn conditions is sparsely studied in the literature.
To this note, researchers at Tokyo Institute of Technology: Professor Tsuyoshi Nagasawa, Mr. Yuichi Okura, Mr. Ryota Yamada, Professor Susumu Sato, Professor Hidenori Kosaka, in collaboration with Professor Takeshi Yokomori and Professor Norimasa lida from Keiyo University, proposed a thermal stratification technique to enhance the thermal efficiency of SI engines at super-lean burn conditions λ 2.0. The aim was to obtain a thermal efficiency higher than 50%. The work is currently published in the International Journal of Engine Research.
In their approach, the proposed technique, also known as Stratified Water Insulated Combustion Architecture (SWICA), involved in-cylinder injection of liquid water on the top surface of the piston. First, the concept was verified through a combination of heat flux measurement on the piston and water spray distribution visualization using an optically accessible engine. The effects of the water injection timing was investigated. Finally, the effectiveness of SWICA was verified by applying it to a super-lean burn SI engine with a compression ratio of 17.
From the results, the distribution of the injected water near the surface of the piston at both ignition timing led to the realization of thermal stratification when the water injection timing (SOIw) was set at the earlier stage of the compression stroke. Moreover, the water injection at any given SOIw reduced the heat flux on the piston surface measured at the same time as the thermal stratification. In addition, the single-cylinder engine tests at constant ignition timing demonstrated the effectiveness of water injection in suppressing the knock without significant combustion instability. In contrast, water distribution near the spark plug at SOIw = -60° ATDC resulted in unstable combustion. It decreased thermal efficiency while SOIw in the range -30 to 0° ATDC exhibited no knocking mitigation effects. Furthermore, at λ = 2.0 and IMEP = 1100kPa, the water injection-induced knock mitigation effects produced spark advancement followed by improvement in the combustion, which reduced the emission of unburnt hydrocarbons.
In summary, a thermal stratification technique based on the SWICA concept is proposed for the enhancement of the thermal efficiency of SI engines. Increasing the water amount played a critical role in shortening the combustion time and ensuring combustion stability, thus resulting in the overall combustion enhancement. The feasibility of the SWICA concept was successfully verified as it managed to achieve a higher thermal efficiency of 52.63%, with a reduction in the unburnt, cooling and exhaust losses. Generally, its performance was superior to that of no water condition. In a statement to Advances in Engineering, Professor Tsuyoshi Nagasawa said the obtained thermal efficiency is one of the highest values in the world reported ever for light-duty SI engine, and the strategy would contribute to the reduction of greenhouse gases in the transportation sector.
Nagasawa, T., Okura, Y., Yamada, R., Sato, S., Kosaka, H., Yokomori, T., & Iida, N. (2020). Thermal efficiency improvement of super-lean burn spark ignition engine by stratified water insulation on piston top surface. International Journal of Engine Research, 22(5), 1421-1439.