Closed loop control for biodiesel blends in mixing-controlled combustion

Gayatri Adi, Purdue University

Abstract

Biodiesel is a renewable diesel fuel produced from plant oils or animal fats. There are several advantages of using biodiesel including production from domestically available feedstocks, thus reducing dependence on foreign sources of petroleum, reduction in particulate matter, carbon moNO xide, and unburned hydrocarbon emissions, as well as reduce net carbon dioxide emissions. However, the lower heating value of biodiesel is approximately 12% less than that of conventional diesel which typically leads to about 10-12% higher fuel consumption and reduced maximum torque/power. Also, in some cases up to 40% increases in NOx emissions are observed compared to diesel fuel. It is possible to mitigate these negative effects of biodiesel combustion by applying closed-loop control techniques for fuel blend accommodation in diesel engines. In order to do this, it is essential to know the cause for the increases in fuel consumption and NOx emissions with biodiesel as well as have knowledge of the biodiesel blend fraction of the fuel being used. In this study a hypothesis is proposed for the observed increases in NO x emissions, which is also used in designing a control strategy for mitigation of increases in NOx emissions. This work also describes a blend estimation technique that will be used with the control strategy to achieve biodiesel blend accommodation in a diesel engine. It is important to examine the stability of the overall system when such a control technique is added to the existing engine control structure. This work presents a stability analysis to ensure global asymptotic stability of the overall system. Experimental validation of this control strategy on a 2007 6.7 liter Cummins ISB series engine at several very different operating modes shows that this fuel flexible control strategy greatly reduced or completely eliminated increases in emissions of nitrogen oxides of up to 30% while largely maintaining the torque/power capacity of the engine when operating with biodiesel. A theoretical analysis is presented to show that the control technique is robust to variation in fatty acid composition of biodiesel. A change in fatty acid composition could result from the use of different feedstocks or also from further processing done to make biodiesel more viable for colder temperatures by reducing the cloud point of the fuel. However, it is observed that for these fuels with different fatty acid compositions, the fuel oxygen fraction and the fuel energy are fairly similar. This property results in the blend accommodation technique being effective for fuels with varying fatty acid compositions. This is also validated with experimental results.

Degree

Ph.D.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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