Investigation of Operating Range Capability of Gasoline Fueled Compression Ignition

Nishigandha Dileep Railkar, Purdue University

Abstract

Diesel engines are extensively used in heavy duty applications owing to their higher efficiency, but they release large amount of nitrogen oxides (NOx) and particulate matter (PM) emissions. Gasoline engines, on the other hand, have lower efficiency, but discharge lower NOx and PM emissions. Strategies like Premixed Charge Compression Ignition (PCCI) and Homogeneous Charge Compression Ignition (HCCI) have a potential of increasing the engine efficiency while simultaneously reducing engine emissions. Alternate Fuel Compression Ignition (AFCI), based on the same principle as PCCI, can be enabled via direct injection of a blend of gasoline and conventional diesel fuel to achieve a premixed combustion. Use of gasoline has an inherent advantage of lower particulate matter emissions. The low reactivity of gasoline also provides ignition delay necessary for premixing, thereby reducing the need for high EGR fractions. In spite of the advantages of AFCI, there are some challenges faced in the practical implementation of AFCI. The main challenge faced is the limited operating range of premixed combustion with gasoline fuel. The lower limit is governed by misfire and the upper limit is characterized by high pressure rise rates and is used in the current study to determine the effectiveness of the combustion. The aim of the present study was to realize maximum torque with AFCI combustion using pure gasoline fuel without violating the constraints on the peak cylinder pressure and pressure rise rate. This effort was carried out using GT-Power simulation software. Calibration of a pre-existing GT-Power model was undertaken as the first step. Experimental data available from a 6.7 liter, six cylinder 2010 Cummins ISB engine, with variable valve actuation (VVA) capability was used for this calibration. This thesis provides details of the GT-Power model calibration procedure and the application of calibrated GT-Power model for range exploration. AFCI range exploration was performed by solving a constrained optimization problem for particular fueling values increased beyond the nominal to obtain the maximum attainable torque at 1200 rpm operating speed. The independent variables or inputs used for this study were EGR fraction, VGT rack position, start of injection and rail pressure. Constraints were imposed on the peak cylinder pressure (PCP) at 2600 psi and on pressure rise rate (dP/dt) at 100, 150, 200 and 300 bar/ms. Four dP/dt constraints were considered to explore the possibility of obtaining an increase in torque on relaxing the constraint. The trends of optimal inputs with increasing quantity of fuel were assessed and were found to match the expected trends. With increase in fueling, the torque increased as expected, but for higher amounts of fueling, the increase in torque was not substantial. This was due to inability to burn the entire quantity of fuel injected. The fraction of fuel combusted decreased as the total quantity of fuel increased, mainly because of lack of air available for combustion. For fueling of 70 mg/stroke, more than 97% of total fuel was combusted and the maximum torque obtained at was 360 ft-lb, while maintaining PCP and dP/dt within the desired constraints.

Degree

M.S.E.

Advisors

Shaver, Purdue University.

Subject Area

Mechanical engineering

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