The cause of cyclic variation in SI engine

Many researchers have investigated the cause of cyclic variation with a focus on the ignition and combustion process in SI engine.

In both experimental and simulation studies, the early phase of in-cylinder combustion has been shown to be a dominant factor in overall combustion duration.

Early studies on CCV

In experimental studies, the cause of cycle-to-cycle variation (CCV) has been studied since the 1960s .

Bates observed the flame development process in each cycle of a single-cylinder SI engine and reported that the larger size of the initial flame kernel resulted in a faster burn in the cycle.

Aleiferis et al. investigated the effects of released spark energy and initial flame kernel growth on the CCV of combustion.

In the study, a high correlation was reported between the flame size at a certain timing after ignition and the crank angle of 5% mass fraction burned.

They mentioned that the “quality” of a cycle was determined within the short time of the spark discharge.

Recent studies on CVV

Recently, simulation studies with consideration for in-cylinder turbulence have been conducted .

Although the simulation technique is still in need of improvement, the cause of CCV is numerically discussed.

Enaux et al. presented multi-cycle large eddy simulation (LES) of flow and combustion in a single cylinder engine setup.

They reported that LES was able to reproduce the experimental findings such as CCV of the maximum in-cylinder pressure.

The simulation results showed that velocity fluctuations at the spark plug induced variations of the early flame kernel growth and of the overall combustion duration.

Ghaderi Masouleh et al. numerically investigated the CCV of lean methane combustion (φ = 0.59) in the SI engine assembly using LES.

They reported that early variations of thermodynamics and fluid dynamic conditions near spark region were the major origins of the cyclic variations and large spark kernels were less prone to cycle-to-cycle variations than small kernels.

Summary the reason of CVV

However, it should be noted that the estimation of the wall temperatures in the LES simulation should be improved to investigate the CCV caused by the temperature fluctuations.

After all, the above studies both in experiments and simulations suggest us that the suppression of the CCV in the initial phase of combustion is important for overall combustion phase and would result in the stable operation under extremely lean conditions.

Reference:

[1] S. Tsuboi, S. Miyokawa, M. Matsuda, T. Yokomori, N. Iida, Influence of spark discharge characteristics on ignition and combustion process and the lean operation limit in a spark ignition engine, Appl. Energy. (2019) 617–632. doi:10.1016/j.apenergy.2019.05.036.