Control-oriented modelling of combustion phasing for a fuel-flexible spark-ignited engine with variable valve timing

Abstract

In an effort to reduce dependence on petroleum-based fuels and increase engine efficiency, fuel-flexible engines with advanced technologies, including variable valve timing, are being developed. Fuel-flexible spark-ignition engines permit the increased use of ethanol-gasoline blends. Ethanol, an alternative to petroleum-based gasoline, is a renewable fuel, which has the added advantage of improving performance in operating regions that are typically knock limited due to the higher octane rating of ethanol. Furthermore, many modern engines are also being equipped with variable valve timing, a technology that can increase engine efficiency by reducing pumping losses. Through control of valve timings, particularly the amount of positive valve overlap, the quantity of burned gas in the engine cylinder can be altered, eliminating the need for intake throttling at many operating points. However, the presence of elevated levels of in-cylinder burned gas and ethanol fuel can have a significant impact on the combustion timing, such that capturing these effects is essential if the combustion phasing is to be properly controlled.This paper outlines a physically based model capable of capturing the impact of the ethanol blend ratio, burned gas fraction, spark timing and operating conditions on combustion timing. Since efficiency is typically tied to an optimal CA50 (crank angle when 50% of fuel is burned), this model is designed to provide accurate estimates of CA50 that can be used for real-time control efforts -allowing the CA50 to be adjusted to its optimal value despite changes in ethanol blend and burned gas fraction, as well as the variations in engine thermodynamic conditions that may occur during transients. The proposed control-oriented model was extensively validated at over 500 points across the engine operating range for four blends of gasoline and ethanol. Furthermore, the model was utilized to determine the impact of ethanol blend and burned gas fraction on the CA50, as well as their impact on the optimal spark timing. This study indicated that the burned gas fraction could change the optimal spark timing by over 20°at some operating conditions and that ethanol content could further affect the optimal spark timing by up to 6°. Leveraging the model in this manner provides direct evidence that accounting for the impact of these two inputs is critical for proper spark-ignition timing control.

Premixed Combustion in Spark Ignition Engines and the Influence of Operating Variables

“…Valve timing influences combustion duration mostly through changes in the levels of dilution and in-cylinder filling; the results show that these influences are captured by the proposed models. Other effects which might results from valve timing changes, such as the changes in turbulence level, appear to be secondary and no explicit account of these has been required [46,47,60].…”

“…The exhaust manifold temperature appears in the calculations, weakening (due to the inherent slow response time of temperature sensors) the applicability of the approach to transient operation. In Hall et al [47] the degree of positive valve overlap is considered directly responsible for the charge characterization, while modulation of valve lift and, more in general, changes of valve opening profiles, which may directly affect the level of turbulence, are not investigated.…”

“…Nevertheless, these models also require some form of calibration using real measurements. Recent published work by Hall et al [47] and by Prucka et al [5] offers examples of relatively simple phenomenological flame propagation and entrainment models, suited for inclusion into fastexecution ST control algorithms, to ensure optimal phasing of the 50% MFB location (i.e. 7 or 8 CA degree ATDC) and improve engine efficiency and fuel economy.…”

“…Different sources spanning across several decades continue to indicate that the main driver for in-cylinder turbulence is engine speed and that other factors actually exert only a minor influence [12,20,21,46,47,57]. However, increasing engine speed also extends the burn process over wider CA intervals and the effect of greater turbulence is only to moderate such extension.…”

“…As discussed previously, locating the 50% MFB event at 7 or 8 CA past TDC has been correlated to achieving best fuel efficiency in SI engine combustion. The recent work by Hall et al [47] outlines a relatively simple, physics-based, quasi-dimensional flame propagation model capable of capturing the influence of variable valve timing, ethanol/gasoline blend ratio and other parameters relevant to modern SI engines, to provide estimates of the 50% MFB location and hence phase the combustion period most appropriately with respect to TDC. The model advanced by Prucka et al [5] is similarly used for predicting best ignition timing in complex SI engine architectures (featuring not only VVT technology but also a charge motion control valve), but it is different in its logic as it relies on a simplified adaptation of the turbulent flame entrainment concept originally introduced by Blizard and Keck [61].…”