Evaluation of Compressed Natural Gas as a Viable Alternative Fuel for Spark Ignition, FourStroke Engines in Passenger Vehicles

by

Kevin DeVos

A Project Submitted to the Graduate

Faculty of Rensselaer Polytechnic Institute

in Partial Fulfillment of the

Requirements for the degree of

MASTER OF ENGINEERING

Major Subject: MECHANICAL ENGINEERING

The original of the complete thesis is on file

In the Rensselaer Polytechnic Institute Library

Approved:

______

Professor Sudhangshu Bose, Project Adviser

Professor Ernesto Gutierrez-Miravete, Project Adviser

1 Rensselaer Polytechnic Institute

Hartford, Connecticut

December, 2014

2 © Copyright 2014

by

Kevin DeVos

All Rights Reserved

3 CONTENTS

4 LIST OF TABLES

5 LIST OF FIGURES

6 DEFINITIONS

Internal Combustion Engine An engine in which fuel is burned inside an enclosed volume and where the combusted fuel directly acts upon a piston in order to produce mechanical work.

7 ACRONYMS

The following is a list of acronyms and abbreviations that are used throughout this paper.

Acronym Definition

ICE Internal Combustion Engine

8 NOMENCLATURE

The following is a list of nomenclature used throughout this paper:

Symbol Description Unit

9 ACKNOWLEDGMENT

I would like to thank

10 ABSTRACT

This project assesses compressed natural gas (CNG) fueled vehicles and evaluates their suitability as alternatives to traditional gasoline powered passenger vehicles. First, the combustion process, specific fuel consumption, and overall efficiency of a CNG four-stroke engine is discussed and compared to a typical gasoline fourstroke engine. Environmental benefits explored, including reduced vehicle particle emissions and greenhouse gas emissions. Finally, practical concerns of using CNG as a fuel source, such as access to fueling stations and dangers of vehicular accidents, are presented.

11 1. Introduction

12 1.1 Background

With the beginning of the industrial revolution in the late 1700s, work has progressed to convert the chemical energy in various fuel sources to motion. The invention of the combustion engine, first developed in the form of a steam engine, allowed vehicles to be propelled by stored onboard fuel instead of by an external mechanism, such as being pulled by a horse. The internal combustion engine was a further improvement on this principal. It increased the efficiency and greatly decreased the required size of the engine for a given power output. These engines were generally small enough that they could be attached to wheeled vehicles and used as the basis of practical transportation. These early contraptions would quickly evolve to become the automobiles that we know today.

There are approximately 1 billion passenger vehicles in the world and about 25% of those are in the United States. The vast majority of these vehicles are spark ignition, gasoline fueled vehicles. Recently, environmental and political pressures have highlighted the need for alternative fuels that are more readily available and the combustion of which causes less environmental pollution. Compressed natural gas has emerged as a possible fuel due to its relative abundance and cleaner burning properties.

13 1.2 Otto Cycle

The Otto cycle is the basis of the four cycle spark ignition engine. Within each cylinder of a four cycle engine, a piston sliding inside the cylinder draws fuel in, ignites it, harnesses the power of the fuel as it burns, and then exhausts it from the cylinder. The four cycles correspond to a single stroke of the piston and can be described as inlet, compression, combustion, and exhaust. At the start of the inlet stroke, the piston is at the top of the cylinder. It slides down the cylinder and at the same time, a valve in the top of the cylinder opens allowing a charge of air and fuel to enter. Once the piston reaches the bottom of its travel, the valve closes. The piston then slides back up the cylinder bore, compressing the air and fuel mixture. When the piston nears the top of its stroke, the air and fuel mixture is ignited by a spark plug. The air/fuel mixture combusts which causes it to expand. The expanding gas mixture then drives the piston downward. When the combustion has been completed and the piston reaches the bottom of its travel, an exhaust valve opens. The piston moves up pushing the exhaust gases out of the cylinder. The cycle is then repeated with a new charge of air and fuel.

14 1.3 Suitability of Fuels

The cycle described above is not specific to a type of fuel. Many different materials can be used in such an engine to produce power. Due to the practicalities of producing, storing, transporting, and selling fuels, some types of fuels are favored for commercial use. The attributes of a good fuel include being inexpensive, energy dense, stable at room temperature, abundant, and clean burning.

One family of fuels stands out as having many of these properties: saturated hydrocarbons. Carbon atoms can form a total of four bonds with other atoms. Saturated hydrocarbons are molecules composed of strings of carbon atoms with all open bond sites being taken by hydrogen atoms. The simplest hydrocarbon is methane. It consists of one carbon atom bonded to four hydrogen atoms, designated CH4. More complex molecules can be formed when carbon atoms bond to each other as well as hydrogen atoms. When two carbon atoms bond to each other and then each bonds with three hydrogen atoms, ethane (C2H6) is formed.

Longer strings such as propane (C3H8), butane (C4H10), and octane (C8H18), are formed when more carbon atoms bond together. See Figure (XX) below. The gasoline that is used by most passenger vehicles is a blend of hydrocarbons usually ranging from four carbon strings (C4H10) to twelve carbon strings (C12H26).

Figure XX: Hydrocarbon strings

Energy is stored in the chemical bonds between atoms and can be released to produce work through the chemical reaction of combustion.

15 1.3.1 Chemical Properties of Natural Gas

Natural gas is, as its name implies, a mixture of volatile gases that occur naturally. Natural gas is approximately 95% methane. The remaining 5% is made up of higher level hydrocarbons. Natural gas is formed from the decomposition of plant and animal matter. Small organisms, called methanogens, break down organic matter and emit methane as a byproduct. Deposits of natural gas are found underground, commonly in proximity to other petroleum products. Natural gas is extracted by drilling down to deposits and

The extraction of

In the combustion process which is used by automobile internal combustion engines, the hydrocarbons undergo oxidation. The hydrocarbons react with oxygen forming carbon dioxide (CO2) and water (H20). Since the atmosphere is saturated with water, adding

The ratio of hydrogen atoms to carbon atoms

16 1.4 Problem Statement

This project will analyze

The viability of using CNG for personal vehicles will be explored. This will include a theoretical efficiency study, a comparison of vehicle emissions with conventional automobiles, and a discussion of pragmatic realities of operating an alternative energy vehicle.

17 1.5 Previous Work

18 2. Theory

19 2.1 Second Law of Thermodynamics

20 3. Methodology

21 3.1 Overview

CNG has a lower energy content per unit volume than gasoline, but CNG fueled engines can operate at higher compression ratios and therefore higher efficiencies. The tradeoffs involved will be contrasted.

The stoichiometric equations for combustion indicate the resulting exhaust gases that will make up the vehicle emissions. The predicted emissions will be compared to test data. Additional lifecycle greenhouse gas emission differences will be discussed.

The practical concerns to be investigated include refueling, fuel concerns during accidents, higher initial cost, and less range due to storage tank size.

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22 3.2 Assumptions asdf

23 3.3 Data Extraction

24 4. Results and Discussion

25 4.1 Problem Table

26 5. Conclusion

In conclusion,

27 6. References

28 6.1 Works Cited

[1] Moran, Michael J., and Howard N. Shapiro. Fundamentals of Engineering Thermodynamics. New York: Wiley, 2008. Print.

[2] Dominion. Nuclear Media Guide, Information on Millstone Power Station. Waterford: Dominion, 2012. Dominion, 2012. Web. 19 Aug. 2013.

[3] Kemp, Ian E. Pinch Analysis and Process Integration - A User Guide on Process Integration for the Efficient Use of Energy. 2nd ed. Oxford: Elsevier, 2007. Print.

[4] Tjoe, T. N., and Bodo Linnhoff. "Using Pinch Technology for Process Retrofit." Chemical Engineering 28 (1986): 47-60. Web.

[5] March, Linnhoff. Introduction to Pinch Technology. 1998. Targeting House Gadbrook Park, England.

[6] Singh, Kamel, and Raymond Crosbie. "Use of Pinch Analysis in Sizing and Integrating a Heat Exchanger into an Existing Exchanger Network at a Gas Processing Plant." The Journal of the Association of Professional Engineers of Trinidad and Tobago 40.2 (2011): 43-48. Print.

[7] Bi, Bao-Hong, and Chuei-Tin Chang. "Retrofitting Heat Exchanger Networks Based on Simple Pinch Analysis." Ind. Eng. Chem. Res. 49 (2010): 3967-971. Web.

[8] Pinch Analysis: For the Efficient Use of Energy, Water, and Hydrogen. N.p.: Canada, 2003. Print.

29 6.2 Additional References Consulted

Bakhtiari, Bahador, and Serge Bedard. "Retrofitting Heat Exchanger Networks Using a Modified Network Pinch Approach." Applied Thermal Engineering 51 (2012): 973- 979. Science Direct. Web. 17 Aug. 2013.

Linnhoff, B., and E. Hindmarsh. "The Pinch Design Method for Heat Exchanger Networks." Chemical Engineering Science 38.5 (1983): 745-63. Print.

Rossiter, Alan P. Using Spreadsheets for Pinch Analysis. Tech. no. 96D. N.p.: Unpublished, 2004. Print.

Zebian, Hussam, and Alexander Mitsos. "A Double-pinch Criterion for Regenerative Rankine Cycles." Energy 40.2 (2012): 258-70. Print.

30 7. Appendices

31 7.1

32