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Date of Award

2015

Document Type

Master's report

Degree Name

Master of Science in Mechanical Engineering (MS)

College, School or Department Name

Department of Mechanical Engineering-Engineering Mechanics

First Advisor

Scott A. Miers

Abstract

This MS report studies the impact of an expansion chamber and feasibility to damp intake air pressure fluctuations in order to accurately measure the air flow rate into an engine. Forced by competition and strict emission regulations, automotive manufacturers have been devoting remarkable efforts to improve fuel conversion efficiency and to reduce emissions for decades. Advanced technologies such as variable valve timing (VVT), direct injection (DI) and turbocharging significantly compensate the drawbacks of engine and improve fuel economy by optimizing spark timing and mixing method. To accurately manipulate these parameters, such as the mixing method of air and fuel and combustion characteristics, the accurate measurement of air flow rate is essential for optimization. However, air flow measurement is very challenging in an engine, especially for carbureted engines due to high sensitivity to intake pressure changes. A common and accurate method to measure air flow rate is to install a restrictive flow device in the intake stream, such as a venturi meter, and to measure the pressure difference between the inlet and the restriction. However, air flow rate is not a constant in an operating engine due to pressure waves traveling forward and backward in the intake manifold. An expansion chamber, installed in the intake system, has the ability to damp pressure waves and to reduce fluctuations of intake air through storing air to increase system inertia. The expansion chambers of different volumes change the total volume of the intake air system and affect the air flow rate and engine performance. The impact of the volume of the expansion chamber to reduce fluctuations was investigated in this report.

The research focus is to find an appropriate intake expansion chamber volume which minimizes the impact of pressure waves in the intake track and does not reduce air flow compared to an open intake system. This is accomplished by analyzing parameters such as air flow rate, brake specific fuel consumption (BSFC), lambda and fuel pressure. The research was conducted on a two-cylinder, liquid-cooled, spark-ignition engine. Nine configurations for the intake system were tested, which ranged from a completely open intake to a system with an 18.9 L intake volume. Pressure and temperature sensors were installed on the engine to obtain measurements of intake manifold, oil, fuel, coolant, carburetor, and in-cylinder data. Engine output parameters such as torque and speed were obtained from a water brake dynamometer.

Choked flow occurred at 3,000 and 3,600 RPM with the intake configuration consisting of only a venturi meter and air filter. Then two intake configurations consisting of just an air filter and no air filter were tested and the results excluded the possibility that the air filter affects the flow. To solve the choked flow problem at high engine speed, a 4.85-liter chamber was installed on the intake air system, between the venturi meter and carburetor. Choked flow was removed by expansion chamber which can stabilize air flow speed and reduce the peak value of air flow speed below the speed of sound. Different volumes of expansion chamber were tested and the impact on engine performance was analyzed.

A drop in air flow rate was observed with the expansion chamber of 3.79 liters and 4.85 liters, compared to the open intake. A restriction in intake air flow rate occurred when using the 18.95-liter chamber because the diameter was too large, causing large expansion waves. The 7.57-liter chamber had the lowest drop in air flow. Even through all the chambers did have the ability to damp the pressure waves; they had to be sized to prevent a large drop in air flow rate. Therefore the volume and shape of the expansion chamber must be carefully selected. The 7.57-liter chamber was selected as the best option based on all the intake configurations tested. Air flow was measured by this configuration and compared to the air flow calculated by the combination of wide-band lambda sensor and fuel flow meter. A large error appeared at low engine speed because of low sensitivity of pressure transducers. The errors in other conditions were within 5%, an acceptable range.

In the future, simulations of air flow dynamics and engine performance using various expansion chambers with different volumes needs to be performed to select the most appropriate volume, shape, and location of the expansion chamber.

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