Spark advance self-optimization with knock probability threshold for lean-burn operation mode of SI engine☆
Introduction
For spark-ignition (SI) engines, lean burn is a well-known operating mode for achieving higher efficiency due to sufficient oxygen and lower heat loss from combustion chamber with lower combustion temperature [1], [2]. Challenging issue in on-board decision of spark advance is to operate at near-boundary area for achieving the high efficiency, due to the strong combustion variation and high risk of abnormal combustion event in the lean burn mode. In the literature [3], [4], lean limit is defined and it has been shown that the lean limits can be increased by increasing the in-cylinder turbulence. It has also been pointed out that hydrogen enhancement using hydrocarbon fuel reformate can extend the lean variability limit. Usually, for general engines, lean limits might be high as to or even higher if the geometry of SI engine is refined for lean-burn. At increasing the air-fuel ratio (AFR), the cyclic variability becomes stronger. The influence of engine parameters, namely engine load, speed, AFR and spark advance (SA), to the cyclic combustion variability has been investigated by Ref. [6] where it has been shown that SA affects cyclic combustion variability mostly when engine geometry is determined.
SA is usually operated at the value where the engine achieves the maximal thermal efficiency. While, in some operating conditions, this value cannot be reached due to undesired knock [9]. Infrequent knock event is acceptable, thus, in order to increase fuel economy as highly as possible, it is necessary to operate SA at the borderline under which knock probability is lower than predetermined level. It has been shown in Ref. [20], [29] that, to prevent knock, SA must be 5–8° retarded from calibrated value with maximal thermal efficiency which is too conservative. Also, even though the calibrated value with maximal thermal efficiency can be reached, real-time adjustment of the actual set point of SA is still needed according to the environment variation in practice. However, no much attention has been paid on the issue of on-board decision problem of SA for lean burn mode in the past decades.
In this paper, we propose a SA control scheme for lean-burn operation mode of SI engines which achieves SA self-optimization with knock probability threshold. The proposed control scheme combines extremum seeking loop which aims at maximizing thermal efficiency and likelihood-based knock limit control loop which starts to constraint SA when maximal thermal efficiency cannot be reached. Each loop works independently and outputs two different amount of SA adjustment. The controller adopts one of them depending on the statistic property of knock intensity. If the average of knock intensity is lower than a pre-specified value, the result from extremum seeking will be adopted. While, if the average of knock intensity is higher than the value which implies the risk of knock is higher, knock limit control starts to work to constraint the knock probability.
This paper is organized as follows: firstly, the experimental setup is introduced; In section 3, the effect of SA on combustion process is statistically analyzed. The control purpose and strategy are illustrated in section 4. Then, the experimental results and analysis are presented in section 5. Brief conclusions are given finally.
Section snippets
Experimental setup
The test engine for this study is a V-6 3.5-L commercial SI engine with the major specifications as given in Table 1. This engine is a direct-injection spark-ignition (DISI) engine equipped with dual injection system: port fuel injection (PFI) injector and direct-injection (DI) injector and besides intake and exhaust variable valve timing system (Dual VVT-i). Although the engine was originally designed for the operation under stoichiometric condition, it can operate with an air-fuel ratio of
Statistical property at lean burn mode
As preparation for designing the control strategy, analysis is elaborated to reveal the statistical property at lean burn mode. Data for analysis was obtained from five groups of experiments under constant engine speed as 1200 rpm and constant throttle valve as 12° with different air-fuel ratios: , , , and . In every group, six different SA values were operated individually for 2000 cycles.
It is well known that the combustion phase, e.g, crank angle after top dead
Problem description
As illustrated in previous part, the effect of SA on thermal efficiency and knock intensity can be formulated as follows:Here, η and represent thermal efficiency and logarithm of knock intensity respectively. , , and are stochastic variables.
Fig. 8 shows the effect of SA on thermal efficiency for several air-fuel ratios. For fixed air-fuel ratio, as SA gets larger, thermal efficiency firstly becomes larger and then decreases after
Experimental conditions
The engine was operated at steady state with engine speed as 1200 rpm and throttle valve as 12° during experiments. The cooling water control system maintained the cooling water temperature at C. Besides, AFR control is based on simple adaptive control (SAC) and feedforward map [5], [7], [8], [10], [13], [14], [15], [16], [17], [19]. Experimental validation concerned both steady and transient AFR cases. In the steady case, the AFR was fixed at 17.52 (). While, the AFR was
Conclusions
A control scheme which optimizes SA with knock probability threshold for lean-burn SI engine is proposed and experimentally validated. The following conclusions were obtained from the results of this study:
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SA is a highly effective parameter on the combustion phase, CA50, which affects thermal efficiency and knock intensity dramatically. As the results of statistical analysis indicated, the SA-CA50 causality can be roughly considered as a linear regression coupled with a random noise. On the
Acknowledgement
The authors gratefully acknowledge the support and generosity of Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP) and Innovative Combustion Technology (Funding agency: JST), without which the present study could not have been completed.
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This work was supported by Council for Science, Technology and Innovation (CSTI), Cross-ministerial Strategic Innovation Promotion Program (SIP), Innovative Combustion Technology (Funding agency: JST).