Split injection Miller cycle with boost decreases knock tendency and increases torque in downsized engine
Researchers at Tianjin University in China report that combing split injection to the Miller cycle with boost pressure can effectively decrease the knock tendency and increase the engine torque in a downsized spark-ignition engine. Their paper is published in the journal Fuel.
The Miller cycle is generally achieved through early intake valve closure (EIVC) during the intake stroke and late intake valve closure (LIVC) during the compression stroke, thereby reducing the effective compression ratio (CR). A low effective CR can reduce the end-gas temperature and, consequently, the Miller cycle can suppress knock. The Miller cycle also has been shown to improve fuel economy at partial loads, owing to a more widely open throttle (WOT) and lower pumping loss.
WIth an expansion stroke longer than the compression stroke, the Miller cycle can also achieve a higher efficiency than the Otto cycle upon which it is based. The Miller cycle can also reduce NOx by decreasing the charge quantity and temperature at the end of the compression stroke.
However, the cycle results in higher HC and CO emissions due to the low in-cylinder temperature. In addition, the backflow results in decreased torque at low speeds.
The current development of downsized spark ignition (SI) engines with boost pressure is a proven way of increasing the thermal efficiency and reducing CO2 emissions. However, engine knocking, which is a serious hindrance in achieving a high thermal efficiency, is likely to occur. In this context, many experimental and numerical investigations have been conducted to suppress the knock occurrence by adopting various methods such as exhaust gas recirculation (EGR), low compression ratio (CR), transforming the combustion chamber shape, alternative fuels, and delayed ignition timing (IT)
Knock is suppressed when using these methods, however, the thermodynamic efficiency and fuel economy are degraded. In previous studies, the Miller cycle has shown a potential to reduce the knock tendency and improve fuel economy in a boosted downsized SI engine. Moreover, the Miller cycle is not difficult to achieve; hence, it has become the focal point of research.
… The objective of this work is better comprehension of the effect of Miller with boost and split injection on knock suppression and engine performance. In particular, the impacts of these different techniques on torque, percentage of knock cycles, CA50, indicated thermal efficiency, and COV in terms of the Miller cycle with boost pressure and the combined method are comprehensively investigated in the present experiment using a single-cylinder engine.—Wei et al.
The researchers used a Ricardo E6 4-stroke SI engine in the study, with a swept volume of 0.5 L and a compression ratio of 10:1. To achieve split injection, they used a piezoelectric injector. The first SOI was fixed at 240 CAD BTDC and the secondary SOI timings at 60 CAD BTDC, 100 CAD BTDC, 140 CAD BTDC, and 180 CAD BTDC. The fuel quantity ratio of the first injection and secondary injection was set as 4:1.
Broadly, they found that:
The Miller cycle with LIVC and EIVC can efficiently suppress knock. Because the intake valve closure timing is retarded in LIVC, the Miller cycle can obtain a better knock resistance. As for EIVC, the knock tendency also decreases as the intake valve closure timing is advanced. In addition, although increasing the Miller degree can further suppress the knock, the engine power is significantly decreased. Without boost pressure, the torques of the engine in EIVC and LIVC are lower than those without the Miller cycle, owing to the low charge quantity. Compared to the baseline case, the knock cycle of LIVC50 is zero; however, the torque decreased by 19.6%. Therefore, by only using the Miller cycle, the anti-knock capability and engine power are compromised.
High boost pressure can compensate for the decreased intake charge of the Miller cycle. Using a 1.2 bar boost pressure, both EIVC and LIVC could achieve a better engine efficiency and suppress knocking.
LIVC was better than EIVC in terms of knock resistance and indicated efficiency, primarily owing to the lower in-cylinder temperature of LIVC50. Compared with EIVC60, the torque of LIVC50 is higher by approximately 3%, and its knock percentage is lower by 22%.
The Miller cycle with boost pressure and a split injection strategy has the capability to suppress knock and improve engine torque. The best secondary start of injection timing for the Miller cycle with boost pressure and split injection is 100 CAD BTDC. Compared with the baseline case, its torque is higher by more than 10% and its knock percentage is lower by more than 35%.
Haiqiao Wei, Aifang Shao, Jianxiong Hua, Lei Zhou, Dengquan Feng (2018) “Effects of applying a Miller cycle with split injection on engine performance and knock resistance in a downsized gasoline engine,” Fuel, Volume 214, Pages 98-107 doi: 10.1016/j.fuel.2017.11.006