Optical Investigation of the Effects of Ethanol/Gasoline Blends on Spark-Assisted HCCI

Abstract

Spark assist (SA) has been demonstrated to extend the operating limits of homogeneous charge compression ignition (HCCI) modes o f engine operation. This experimental inves tigation focuses on the effects of 100% indolene and 70% indolene/30% ethanol blends on the ignition and combustion properties during SA HCCI operation. The spark assist effects are compared to baseline HCCI operation for each blend by varying spark timing at different fuel!air equivalence ratios ranging from < 1 ' > = 04-0.5. High speed imaging is used to understand connections between spark initiated flame propagation and heat release rates. Ethanol generally improves engine performance with higher net indicated mean effective pressure (IMEPn) and higher stability compared to 100% indolene. SA advances phasing within a range of ~J crank angle degrees (CAD) at lower engine speeds (700 rpm) and ~77 CAD at higher engine speeds (1200 rpm). SA does not affect heat release rates until immediately (within ~5 CAD) prior to auto-ignition. Unlike pre vious SA HCCI studies of indolene fuel in the same engine, flames were not observed for all SA conditions.In trod u ctio n The homogeneous charge compression ignition regime can achieve higher thermal efficiencies and lower NOx emissions compared to conventional gasoline engines and lower particulate matter (PM) emissions compared to diesel engines. Thus, HCCI provides a combination of the advantages of spark ignition opera tion (particularly lower regulated emissions) and the advantages of diesel operation with high compression ratios and high thermo dynamic efficiencies [1-3].HCCI engines have some unique challenges, primarily stem ming from the dependence on chemical kinetics for controlling the ignition and combustion rather than a traditional control strat egy such as spark ignition or fuel injection timing. The load range over which HCCI operation is viable is small compared to the urban driving cycle. At the low load limit, the low energy content of the charge can lead to misfire, while at the high load limit, heat release rates are so fast that the engine can be damaged. One of the promising strategies, which has been proposed to extend these limits and improve the viability of HCCI, is spark assist [4][5][6][7], Spark assist allows a portion of the fuel/air charge to be consumed by flame propagation prior to auto-ignition, and generally spark assist advances ignition compared to a baseline HCCI operating condition [7]. The timing of auto-ignition is dependent in part on the properties of the fuel. In this work, the effects of a fuel blend of ethanol and a reference grade gasoline (indolene) on the SA HCCI performance were of interest. Ethanol can be produced from renewable resources, and ethanol has been considered an al ternative fuel to gasoline for internal combustion engines. The beneficial effects of ethanol on air quality and on reducing health costs associated with greenhouse gas emissions have been high lighted by Hill et al. [8] and Al-Farayedhi et al. [9], Moreover, ethanol blends have...

An applicable approach to mitigate pressure rise rate in an HCCI engine with negative valve overlap

“…In other words, thermodynamic conditions and chemical composition of the combustible mixture at intake valve closing (IVC) define the timing and duration of subsequent combustion process. Some additional controllability after IVC can, however, be introduced by means of direct injection (DI) of fuel, providing mixture stratification [11,12], or by socalled spark assist technology [9,13].…”