engleski

Calculating the Requirement for Additional Intake Charge Conditioning of the Ethanol Fueled Turbocharged Engine

Homogeneous charge compression ignition (HCCI) [1, 2] is a relatively new type of combustion which has a potential to achieve high efficiency with low NOx emissions. Beside its positive aspect this type of combustion has problems that need to be solved. The main challenges of the HCCI are combustion control and low power density. As reported in [3, 4, 5] the load increase in a HCCI engine can be achieved by using higher levels of boost. Homogeneous charge compression ignition can be achieved with a large number of fuels. In recent years significant efforts have been made in order to utilize larger fractions of biofuels. One of the biofuels that attracts interest today is ethanol, as it can be blended with diesel or more commonly with gasoline, or used as a pure compound. In some previous publications [6-8] it has been shown that ethanol can be utilized with HCCI combustion. Compared to gasoline, ethanol requires slightly higher intake temperatures for achieving a stable HCCI combustion. It also shows less sensitivity of required intake temperature on excess air ratio of the mixture which could be attributed to the fact that ethanol does not have low temperature heat release (LTHR) or intermediate temperature heat release (ITHR) chemistry. Therefore, it is expected that for achieving HCCI in a moderately compressed engine one would require increased intake temperature. On the other hand if boosting is used to increase the engine load limit, it will increase the temperature of the mixture with certain proportionality between boost pressure and intake temperature. Homogeneous charge compression ignition combustion requires a diluted mixture either by excess air, or by residual gases. Hence, the HCCI combustion has significantly lower exhaust temperature compared to the combustion of conventional SI or CI engine. Lower exhaust temperature leads to lower energy supplied to the turbocharger, which could lead to lower intake pressure. It has been noticed [5] that for enabling higher intake pressure in the HCCI, the turbine has to be smaller and operate with higher pressure ratios. This increases pumping losses, especially at high engine speed. Therefore, the variable turbine geometry has been suggested. Recently, there have been several numerical studies that explored the various charging and intake charge conditioning strategies for achieving high load HCCI in gasoline engines [9, 10]. The studies have shown the effectiveness of cyclesimulation software in analyzing various changes that take place in a multi-cylinder turbocharged engine. The work presented in the paper is a part of the study that aims to explore the various operational modes and means to achieve boosted HCCI combustion in ethanol fuelled turbocharged multi-cylinder engine. The study is based on the numerical results from engine models made in the cycle-simulation software AVL BOOST. This paper presents some initial results from that study. First, the specific model is used for the calculation of the intake temperature requirements for achieving auto ignition of ethanol in specific HCCI operating conditions. After that, the second model is used for studying the charging capabilities of ethanol fuelled HCCI engine with single-entry turbocharger that has a fixed geometry and waste gate control. At the end the intake temperature requirements are compared with obtained intake conditions from which the required additional heating of the intake charge is determined.

HCCI; ethanol; cycle-simulation

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