Date of Award

2017

Document Type

Open Access Master's Report

Degree Name

Master of Science in Mechanical Engineering (MS)

Administrative Home Department

Department of Mechanical Engineering-Engineering Mechanics

Advisor 1

Scott A. Miers

Committee Member 1

Jeremy J. Worm

Committee Member 2

David D. Wanless

Abstract

Knock, in spark ignition engine is the combustion caused by the autoignition of the fuel-air mixture. It is the phenomenon that limits engine performance and thermal efficiency. Knock also has an adverse effect on emissions and fuel economy. Engine designers target engines with maximum power and torque output without compromising on fuel economy. Engine downsizing is the method generally adopted. The main goal of engine downsizing is to achieve better fuel economy while increasing the power and torque output of the engine. Better fuel economy is achieved by reducing the displaced volume which in turn means a much higher brake mean effective pressure. It is common for downsized engines to have BMEP values higher than 20 bar. As a comparison, this value reduces to about 15 bar without downsizing for the same power output. To compensate for the reduced volume, boosting devices like turbochargers or superchargers are incorporated. This increased pressure leads to a higher temperature of the compressed mixture. As a result, the self-ignition temperature is attained quicker than expected which promotes the occurrence of knock. When targeting high engine outputs at lower speeds, sustained knocking events can prove to cause catastrophic engine damage. The need to understand the phenomenon of knock as completely as possible is extremely important. Elimination of knock will prove to be vital for further engine development.

The major factors affecting knock are the octane rating of the fuel, spark timing, compression ratio of the engine, the percentage of exhaust gas re-circulation employed and lambda value. This report studies the effect of changing the fuel octane rating and spark timing on intensity of knock. The report briefly introduces knock, theories of its occurrence, detection methods and control techniques. Three fuels, E10 87, E0 91 and E15 91 were tested on a spark ignited, liquid cooled, two-cylinder carbureted engine. The fuels were selected as they represent a range of octane ratings usually available for daily use. In-cylinder pressure and crankcase vibrations are the two parameters used for knock detection. Each fuel was tested for a set of three spark timings set 10 CAD apart.

With an increase in spark advance, the knocking intensity increases when all other engine operating parameters are maintained constant. From the comparison of results for E0 91 and E15 91 fuels it can be concluded that the knock intensity decreases with an increase in ethanol content when all other engine operating conditions, including fuel octane rating and spark advance, were kept unchanged. Finally, the comparison of results from E0 91 and E10 87 fuels exhibit mixed effects of rise in ethanol levels and drop in octane rating on the knock intensity. While, for lower loads, the effect of increase in octane rating dominates resulting in lower knock intensity for E0 91, for higher loads the increase in ethanol content seems to have an upper hand resulting in lower knock intensity for E10 87 fuel.

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