Maintaining and Repairing Propane Fuel Systems on Stationary Engines

Transcription

1 Maintaining and Repairing Propane Fuel Systems on Stationary Engines

2

3 Maintaining and Repairing Propane Fuel Systems on Stationary Engines Readers of this material should consult the law of their individual jurisdiction for the codes, standards, and legal requirements applicable to them. This material merely suggests methods that the reader may find useful in implementing applicable codes, standards, and legal requirements. This material is not intended nor should it be construed to: (1) Set forth procedures that are the general custom or practice in the gas industry. (2) Establish the legal standard of care owed by propane distributors to their customers. (3) Prevent the reader from using different methods to implement applicable codes, standards, or legal requirements. This material is designed to be used as a resource only to assist expert and experienced supervisors and managers in training personnel in their organizations and does not replace federal, state, or company safety rules. The user of this material is solely responsible for the method of implementation. The Propane Education & Research Council, Frey Associates Inc., and the Alternative Fuels Research & Education Division of the Railroad Commission of Texas assume no liability for reliance on the contents of this training material. Issuance of this material is not intended to nor should it be construed as an undertaking to perform services on behalf of any party either for their protection or for the protection of third parties. All rights reserved. No part of this text may be reproduced, utilized, or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without permission in writing. Propane Education & Research Council (2008)

4

5 ABOUT THE PROGRAM Of the energy sources available to the agricultural community, propane offers a desirable combination of characteristics for agricultural applications. Propane is among the most attractive options for reducing greenhouse gas emissions. It is readily available. It represents a proven and stable energy value. Two of propane s most important uses are providing electrical power, sometimes called distributed generation, and power to operate irrigation pumps. Both of these applications utilize propane to fuel stationary engines. Keeping the propane fuel systems of these stationary engines in proper working order is a task that requires a working knowledge of the characteristics of propane as a fuel and of the components of propane engine fuel systems. This training program is intended to provide technicians with an introduction to propane engine fuel systems as they are typically configured for the following kinds of engines: Air-cooled engines, often called small engines and used in electrical generators. Liquid-cooled engines, typically used with larger electrical generators and irrigation pumps. Definitions for terms printed in either blue or red type in the text of this publication are given in the glossary section (Appendix A) at the end of the instructional guide. Blue terms are concepts or performance measures used to describe engine operation. Red terms are components of a propane engine fuel system. Acknowledgments The Propane Education & Research Council (PERC) and the National Propane Gas Association (NPGA) gratefully acknowledge the cooperation and contribution of the following individuals and organizations for providing personnel, equipment, and technical assistance. Mitch Torp and Glen Hale TGP West Inc., 3250 El Camino Real, Suite 3, Atascadero, CA (805) Rich Fisher and Dave Campbell Continental Controls Corporation, 8845 Rehco Road, San Diego, CA (858) Franz Hofmann Railroad Commission of Texas, Alternative Fuels Research & Education Division, 6506 Bolm Road, Austin, Texas (512) franz.hofmann@rrc.state.tx.us Richard Dlugosz Sherwood Valve, (888) Members of the PERC Agriculture Advisory Committee and Stationary Engine Project Subcommittee who served as subject matter experts (SMEs) and reviewers A special thank-you goes to Michelle Swertzic, formerly of the Nebraska Propane Gas Association, for assistance in developing the project and arranging for prepublication field testing of the training materials.

6

7 Table of Contents 1.0 Physical Properties of Propane and Safety Precautions to Apply Characteristics of Propane Fuel Systems for Stationary Engines Propane-Fueled Stationary Engine Emission Control Systems Propane-Fueled Engine Fuel System Maintenance and Repair 37 Appendix A: Glossary of Terms 75 Appendix B: Referenced Publications and General Information 79 Appendix C: Educational Materials 91

8

9 1.0 Physical Properties of Propane and Safety Precautions to Apply 1

10 1.0 INTRODUCTION Working safely to maintain or repair propane fuel systems on stationary engines requires service personnel to be familiar with propane s physical properties and aware of safety precautions. The objectives of this chapter are to: 1.1 Identify the physical and combustion properties of propane. 1.2 Identify hazards associated with a release of propane. 1.3 Demonstrate safety measures to apply when working with propane engine fuel systems. IDENTIFYING THE PHYSICAL AND COMBUSTION PROPERTIES OF PROPANE General Properties of Propane Propane is classified as a hazardous material. By law, a Material Safety Data Sheet (MSDS) must be available and accessible to all employees in the workplace where hazardous materials are transferred, stored, or used. The MSDS for propane is available from propane suppliers or distributors. A complete MSDS for propane can be found in Appendix B of this manual. This chapter will discuss specific information from the MSDS that relates to maintaining propane engine fuel systems. The propane stored in containers can be either a liquid or gas. To permit the storage and transportation of propane in liquid form at temperatures warmer than its boiling point ( 44 F), pressure-tight containers are used. Propane liquid stored in these containers at temperatures at or above 44 F will vaporize and expand to pressurize the vapor space inside of the container. This vapor pressure naturally forces the propane from the container to the gas utilization equipment. Propane s liquid volume and container vapor pressure varies with its temperature. On a hot summer day, container vapor pressure may approach 200 pounds per square inch; on a cold winter day, it might be as low as pounds per square inch. (See the chart on the next page.) 2

11 Physical Properties of Propane and Safety Precautions to Apply In its natural state, propane is colorless and odorless. To increase the likelihood that a propane leak can be detected, an odorant (ethyl mercaptan) is added to propane. This odorant is added to allow propane to be detected by smell long before a combustible mixture is present. Learn to recognize the odor of propane and always be sensitive to the slightest gas smell. The Propane Education & Research Council (PERC) has produced consumer safety education and warning brochures that incorporate an odorant scratch-n-sniff patch. Contact PERC or your propane supplier to obtain these brochures to test your sense of smell and verify that you can sense the presence of the odorant. See Appendix B for more information on these brochures. Be aware that under certain rare conditions, the intensity of the odorant may diminish or fade. Some people may not be able to smell the odorant. While no odorant will be completely affective as a warning agent in every circumstance, the odorant generally used in the propane industry has been recognized as an effective odorant. If for any reason you or fellow employees cannot smell odorized propane, immediately notify your supervisor. Your safety and the safety of fellow workers may depend on your ability to smell propane in the event of a leak. For additional information on the odorant, refer to the Propane MSDS in Appendix B. 3

12 1.0 Combustion Properties of Propane A propane molecule consists of three (3) carbon atoms and eight (8) hydrogen atoms. Since carbon and hydrogen are readily burned when combined with oxygen in air and an ignition source, propane is an excellent fuel. Its motor fuel properties may be better understood when it is compared to gasoline, as shown in the following table. PROPERTY GASOLINE PROPANE ENGINE FUEL CHARACTERISTICS Formula C 8H 18 C 3H 8 High carbon fuels are better conductors of electrical energy. Thus propane requires more electrical energy (spark) to ignite the fuel / air mixture. Low carbon fuels have lower CO exhaust emissions. Octane (R + M) / With octane being a measure of a fuel s resistance to knock, propane can stand higher compression pressure and more initial advanced spark timing than gasoline. Energy Density (Btu / Lb) Lower Higher (Btu / Gal) (Btu / Cu Ft) Lower Higher Specific Gravity (Vapor) Specific Gravity (Liquid) 19,000 20, ,000 Not Applicable Not Applicable 19,920 21,650 91, High hydrogen-to-carbon ratio fuels produce more heat per pound. High carbon-to-hydrogen ratio fuels have more heat energy per gallon Both fuels vapors are heavier than air (1.0) In liquid form, both gasoline and propane are lighter than water (1.0). Boiling Point 80 to 440 F 44 F Above 44 F, propane becomes a vapor in open air. Flammability Limits Stoichiometric Combustion Air : Fuel Required by Weight 1.4% to 7.6% gas-in-air 2.37% to 9.5% gas-in-air Range of flammability limits are from leanest combustible mixture to the richest combustible mixture : :1 Stoichiometric combustion is the ideal combustion process during which a fuel is completely burned. 4

13 Physical Properties of Propane and Safety Precautions to Apply IDENTIFYING THE HAZARDS PRESENTED BY A RELEASE OF PROPANE If propane liquid is released into the air, it quickly vaporizes, expanding to 270 times its original volume. Therefore, a liquid propane leak can be more hazardous than a vapor leak due to the expanding vapor cloud. Also, when liquid propane is released into the atmosphere, its rapid vaporization causes a refrigerating effect that makes everything it touches extremely cold. If it comes in contact with skin or other tissues, it will cause third-degree freeze burns. Propane is nontoxic, but will displace air if released into a confined area. Therefore, avoid inhaling propane. Propane vapor is 1.5 times heavier than air. If released into still air, it may initially concentrate in low-lying areas. However, if there is sufficient air movement, especially outside, the vapor should dissipate in the air. When the physical and combustion properties of propane are considered together, these hazards can be identified for an uncontrolled release of propane: Chemical hazards Propane is highly flammable and presents risk of fire. Although propane is not toxic, under certain conditions it can present a danger by displacing air required for breathing. Mechanical hazards Propane is stored under pressure uncontrolled release can result in flying parts or product propelled under pressure. Temperature hazards Exposure of bodily tissues to liquid propane results in a refrigerating effect, causing immediate freezing of tissues with symptoms similar to frostbite. Protecting yourself from these hazards requires the use of proper procedures and may require the use of personal protective equipment, depending on the tasks you are performing. 5

14 1.0 Department of Labor (DOL) and/or Occupational Safety and Health Administration (OSHA) regulations require that proper personal protective equipment (PPE) be worn when procedures do not eliminate hazards associated with the work being done. Your employer is required to determine what PPE is required, provide training on when and how to use it, and verify that you are using it as required. Generally, propane PPE includes special vinyl gloves resistant to the actions of propane, and eye or face protection is appropriate for transferring propane and for purging propane from pressurized storage or fuel system components. Vinyl Gloves Safety Glasses Acoustical Ear Muff and Ear Plugs for Hearing Protection Determine if a Propane Supply Tank is Used for Liquid or for Vapor Service or for Both Most stationary engines used in agricultural applications will be supplied propane from the same type of tank used to supply propane for farm or ranch building heat. In the propane industry this type of tank is often called a domestic or residential ASME tank. Such tanks are built to comply with the American Society of Mechanical Engineers Code for Pressure Vessels. Typical ASME Tank Valve and Fitting Connections A domestic ASME tank is typically used to supply propane vapor through the vapor service valve and a first-stage pressure regulator. It may also be used to supply propane liquid from the tank if a supply valve is connected to the liquid withdrawal valve opening. Tank Valves and Fittings Connected to the Tank s Vapor Space Tank Liquid Withdrawal Excess Flow Valve. If a valve is installed here, it is connected to the tank s liquid space. 6

15 Physical Properties of Propane and Safety Precautions to Apply Procedures for Controlling Propane Hazards During Purging Operations In most cases, purging propane from engine fuel systems and reducing internal component pressure to atmospheric pressure does not involve a large volume of propane. Step 1: Verify Ignition Sources Are Eliminated or Controlled. Inspect the area where the purged propane will be directed during the purging process. Be sure that propane is only released outdoors in unconfined and oepn space that contains no ignition sources. Verify that the engine is shut down and that starting controls are locked out and/ or tagged out according to company procedures. Always remove the start-run key and disconnect the negative battery cable. Step 2: Close the Fuel Supply Valve(s) on the Propane Tank. As it applies: a. Close any liquid service valve(s) that controls propane liquid flow to liquidcooled engines used to drive irrigation pumps or large electrical generators. b. Close any vapor service valve(s) that controls propane vapor flow to the engine. 7

16 1.0 Step 3: Close Any In-line Valve(s) Installed Near the Fuel System Pressure Regulator or Converter. Determine if any in-line fuel valves are located in the fuel piping system to facilitate service operations such as filter element replacement. If present, close any and all in-line fuel valves. Step 4: Outdoors, Loosen and Partially Disconnect a Union or Other Propane Supply Line Swivel Fitting. Wearing suitable personal protective equipment and working outdoors at the propane supply tank, use the correct sized wrench to loosen the fuel line connection at the closed vapor or liquid service valve(s), whichever applies. Step 5: Step 6: After the Initial Venting of Product and Reduction of Pressure, Open Any In-Line Valve(s) Closed in Step 3, Then Slowly Disconnect the Fitting to Ensure Pressure Is Relieved. Verify Entire Fuel System Pressure Is Reduced to Atmospheric Pressure. 8

17 Physical Properties of Propane and Safety Precautions to Apply 1.0 Lab Activity Demonstrate Safety Measures to Apply When Working on Propane Engine Fuel Systems Directions: Complete each task to demonstrate proper safety measures for venting and de-pressurizing a propane engine fuel system. 1. Stop the flow of propane from the supply tank to the engine fuel system. For Valve A and for Valve B shown below, place a in the box next to the correct answer. Valve A. Propane Liquid Propane Vapor Propane Liquid Propane Vapor Valve B. 2. Applying your employer s procedures, identify Personal Protective Equipment (PPE) to use when purging propane from propane fuel lines and reducing propane pressure to atmospheric pressure prior to disassembling a component in the fuel system. For PPE A, B, and C, shown below, place a in the box below each correct answer for the listed purging operation. A. B. C. Vinyl Gloves Eye Protection Hearing Protection Purging liquid propane (high pressure) Purging propane vapor (reduced pressure) 9

18 Determine the safest area to vent purged fuel gas when preparing to disassemble a propane fuel system component. On the diagram shown below, place the following lettered items in the best location on the diagram to indicate steps in purging propane from the fuel system and de-pressurizing the system. a. Valve(s) to close to stop propane flow from the supply tanks. b. Location for purging propane in a well-ventilated area away from ignition sources. c. Location to verify that propane pressure has been reduced to atmospheric pressure. 10

19 2.0 Characteristics of Propane Fuel Systems for Stationary Engines 11

20 2.0 INTRODUCTION A working knowledge of propane engine fuel systems begins with identifying the components that make up the system and how the components differ from smaller air-cooled engines to larger glycol-water mixture cooled engines. The objectives of this chapter are to: 2.1 Identify how the propane boiling process operates in a fuel supply container. 2.2 Identify the components of a propane fuel system for a small air-cooled engine. 2.3 Identify the components of a propane vapor fuel system for a large engine that is glycol-water mixture-cooled and propane is vaporized in the fuel supply tank. 2.4 Identify the components of a propane vapor fuel system for a large engine that is glycolwater mixture-cooled and propane is vaporized in a fuel system component. 2.5 Identify the characteristics of a propane fuel system for a large engine in which the propane is injected into the engine in either a vapor or liquid state. 2.6 Identify the primary codes and safety standards that apply to propane installations. IDENTIFYING HOW THE PROPANE BOILING PROCESS OPERATES IN A FUEL SUPPLY CONTAINER Boiling: The change of physical state from liquid to vapor Unlike gasoline or diesel engine fuel systems, most propane and natural gas engine fuel systems process a dry gas (vapor state) fuel and combine it proportionally with air to provide the engine s combustion mixture. This dry gas characteristic of natural gas and propane fuels is due to their relatively low boiling points at atmospheric pressure. Conversely, gasoline and diesel are handled as liquids at atmospheric pressure due to their relatively high boiling points. A material s boiling point is the temperature at atmospheric pressure required for the material to change from its liquid state to its vapor state. Following are some facts about the storage, handling, and use of propane as a fuel help in understanding propane fuel systems. Energy in the form of heat and pressure tends to reach a point of equilibrium in a sealed storage container at temperatures above a liquid s boiling point; boiling of the liquid stops after the storage container is filled with liquid and vapor and, the balance of heat and pressure forces results in the ceasing of vaporization. 12

21 Characteristics of Propane Fuel Systems for Stationary Engines If vapor is withdrawn from a propane storage container, the decrease in container pressure allows boiling to resume, converting liquid to vapor. A change of state requires energy input in the form of heat. Because heat for vaporization is transferred to the propane liquid from the air surrounding the container through the metal wall of the fuel tank, there are limits on a propane storage/supply container s vaporization capacity. Limiting factors are: a. Wetted surface area As the amount of liquid in a fuel tank decreases, heat exchanger area decreases. As heat transfer decreases, the rate and amount of liquid vaporization decreases. b. Air temperature Heat needed for vaporization is transferred from the air surrounding the fuel tank. In colder weather the rate and amount of liquid vaporization decrease compared to vapor available in hot weather. c. High air humidity and tank refrigeration As propane vaporizes, the tank surface is refrigerated. High relative humidity (water-saturated air) may result in water condensation or in colder conditions water freezing on the tank. Either condition will reduce the rate of vaporization and volume of vapor available for fuel. IDENTIFYING THE COMPONENTS OF A PROPANE FUEL SYSTEM FOR A SMALL AIR-COOLED ENGINE Standby or dedicated electrical power generators represent a widespread application for small propane-fueled stationary engines. Farm and ranch operators located in moderate climates typically find that propane vaporized in the fuel supply tank is sufficient to provide adequate volume of propane vapor to the generator engine. The principal components of a small engine supplied with propane vapor from a tank are illustrated in the diagram on the next page. Courtesy of Marathon Engine Systems 13

22 2.0 Typically, on small air-cooled engines [25 brake horsepower (bhp) and smaller], vapor fuel systems are used, where the fuel is vaporized in the fuel tank and reduced to a pressure suitable for the propane-air mixing devices. Fuel demand for these engines is usually small enough for vaporization of liquid propane to be provided by the storage/supply tank. A pressure regulator installed at the tank decreases tank vapor pressure to approximately 5 to 10 psig pressure to downstream piping. Depending on the installation and manufacturer s instructions, an optional line service pressure regulator may be installed to further reduce inlet pressure supplied to the electric lock-off valve. A fuel lock-off valve, operated by engine vacuum or electrical current from the engine ignition circuit, is a code requirement to ensure that propane flow is stopped when the engine is not operating. In a typical small engine propane fuel system, a pressure reducing valve is installed downstream of the lock-off valve and upstream of the propane-air mixer to provide propane vapor at a negative pressure. Mixing of propane vapor and air for combustion is done in the propane-air mixer in response to the negative pressure of the engine s piston intake stroke. 14

23 Characteristics of Propane Fuel Systems for Stationary Engines IDENTIFYING THE COMPONENTS OF A PROPANE VAPOR FUEL SYSTEM FOR A LARGE GLYCOL-WATER MIXTURE-COOLED ENGINE WHERE PROPANE IS VAPORIZED IN THE FUEL TANK Engines larger than 25 brake horsepower typically produce heat exceeding the cooling ability of air passing over and around the combustion cylinders. Liquid circulating through a radiator and jackets surrounding the cylinders is required to prevent lubrication breakdown and engine damage. Although a number of stationary engines used to power electrical generators in the kw output range require liquid cooling, their propane vapor requirements often do not exceed the vaporizing capacity of a 500-water-gallon-capacity propane tank (depending on location factors). For these installations, the propane fuel system diagram shown on the previous page would be suitable. For larger engines, one or more 1,000-water-gallon-capacity propane supply tanks may be required to meet engine vapor demand. The fuel system diagram shown above is typical for larger displacement irrigation pump engines. 15

24 2.0 IDENTIFYING THE COMPONENTS OF A PROPANE VAPOR FUEL SYSTEM FOR A LARGE GLYCOL-WATER MIXTURE - COOLED ENGINE WHERE PROPANE IS VAPORIZED IN A FUEL SYSTEM COMPONENT Larger engines requiring propane vapor in quantities that exceed the vaporization capacity of a typical propane storage tank require a vaporizer outside of the fuel tank. A typical propane fuel system using liquid coolant to convert propane liquid to vapor consists of: Important Propane Fuel-System Components There are requirements for LP-gas hose or metallic piping conveying propane liquid to the lock-off/fuel filter LP-Gas Hose a. Underwriters Laboratories, Inc. (UL) listed wire-braid stainless steel approved for LP-gas service. b. Must be able to withstand pressures of 5 times 35o psig working pressure (1750 psig burst pressure). c. Manufacturer name, product code, size, and pressure rating must be continuously marked on the hose cover. d. Typically #6 hose (5/16-inch nominal inside hose diameter). 16

25 Characteristics of Propane Fuel Systems for Stationary Engines Metallic Piping a. Welded schedule 40 steel pipe is approved for liquid or vapor service not exceeding supply container pressure. (Threaded schedule 40 pipe is not permitted for conveying propane liquid or vapor at container pressure). b. Threaded schedule 80 steel pipe is approved for liquid or vapor service. c. Buried metallic piping must have adequate corrosion protection. Hydrostatic Protection for Liquid Piping or Hose a. A hydrostatic relief valve must be installed in any section of LP-gas piping or hose conveying liquid propane that can be shut off at each end. b. Hydrostatic relief valves must have a pressure setting of not less than 400 psig or more than 500 psig. Vacuum Lock-Off / Fuel Filter This component serves two functions. 1. Liquid Propane Shutoff Acting as a safety device, the vacuum-operated lock-off stops the flow of liquid propane when the engine is not running. Interruption of negative pressure ( 0.2 inch water column) from the fuel-air mixer air valve closes the internal valve. 2. Fuel Filter The white cotton filter element and screen at the top of the cutaway body picture remove solids such as pipe scale from the liquid propane material that might damage regulator discs or plug valve orifices in the lock-off or downstream pressure regulators. Cutaway View of Vacuum Lock-off Filter 17

26 2.0 Converter Vaporizer/Pressure Regulator This component also serves two functions. 1. Liquid Propane Vaporizer For engines requiring propane vapor that exceeds supply tank vaporizing capacity, the converter uses engine coolant liquid to assure adequate propane vapor is supplied. A number of converter models are available. Two things must be considered in the selection of the proper model for a given engine and application. Converter for Engines up to 110 Brake Horsepower Examples: 1.5L Inline 4-Cylinder Through 4.3L V-6 Engines Engine displacement volume of all cylinders. Vapor-combustion air mixture demand throughout the engine s power range. Converter for Engines up to 350 Brake Horsepower, up To and Including 8L Engines The cutaway drawing to the right illustrates: Propane liquid (darkest blue) entering the vaporizer where it meets the primary pressure seat. At this point, propane pressure is reduced from tank pressure to approximately 1½ to 3 psi. Heat is transferred from the circulating coolant (green) through metal jacket walls into the vaporizing liquid (medium blue). 18

27 Characteristics of Propane Fuel Systems for Stationary Engines 2. Vapor Pressure Regulator After the propane passes through the primary pressure reduction stage, further flow is stopped by the second-stage pressure seat. The second-stage operating pressure is negative in response to negative pressure from the engine. Propane vapor is not supplied to the propane-air mixer under positive pressure. It is reduced to a second-stage pressure of 0.5 to 3.5 inches water column. Propane-Air Mixer In the illustration shown at right, a propane-air mixer is mated to a throttle body, making a complete propane carburetor assembly. A propane-air mixer is shown below. A cutaway view of a propane carburetor is shown at bottom right. IMPCO Model 100 Propane-Air Mixer for Engines up to 106 Brake Horsepower IMPCO Model 125 for Engines up to 126 Brake Horsepower 19

28 2.0 Propane-air mixers illustrated on the previous page operate on the principle of pressure differential. Pressure differential operates when pressures are not equal on both sides of a diaphragm, and in response, the diaphragm moves to the side with the lower pressure. An air-valve mixer uses a diaphragm or a piston with metered orifices that transfer low pressure from one side to the other. The resulting pressure differential moves the diaphragm and the attached gas valve in proportion to the amount of air entering the engine. Movement of the gas valve then allows a predetermined amount of propane vapor out of the mixer to enter the air stream. The fuel mixes with air due to the turbulence generated by the air and fuel changing direction several times as the engine intake valves open and close. Idle fuel mixture is typically adjustable by setting a needle valve screw located on the carburetor body. Propane fuel systems and components illustrated and discussed to this point are generally used on small to moderate-sized stationary engines. Clean Air Act regulations that apply to larger stationary engines and future emissions reductions will be briefly discussed in a later section of this manual. Propane fuel-system components for some stationary engine applications may utilize components such as the variable load mixer shown to the right. Mixers of this type more closely control gas-to-air ratios in response to electronic signals from an exhaust manifold oxygen sensor and electronic control module. Continental Controls Corp EGC2 Electronic Gas Carburetor for Lean Burn Engine missions Control IDENTIFYING THE CHARACTERISTICS OF A PROPANE FUEL SYSTEM FOR A LARGE ENGINE WHERE THE PROPANE IS INJECTED INTO THE ENGINE IN EITHER A VAPOR OR LIQUID STATE Propane fuel systems based on injection of propane in either a gaseous or liquid state have been the subject of research and development by a number of firms and organizations. Of the approaches developed for spark-combustion engines, the direct injection method appears to offer the possibility of lower yield of undesired emissions, increased fuel economy, and engine efficiency across the widest range of performance requirements. Vehicle engine operating requirements more closely relate to the potential benefits that propane injection seems to offer. Consequently, propane injection systems are not currently offered for stationary engine applications. Stationary spark ignition engines function well with 20

29 Characteristics of Propane Fuel Systems for Stationary Engines less expensive and complex propane fuel systems due to their operating characteristics: Continuous loading, resulting in relatively constant intake and manifold pressures. Relatively constant engine rpm, resulting in relatively constant air-fuel demand and mix ratio. No change in altitude while operating, resulting in smaller changes in combustion air density, etc. Vehicle engines, by contrast, with their constantly changing combustion processes, may obtain the highest benefit-to-cost ratios from more complex fuel-injection systems. IDENTIFYING THE PRIMARY CODES AND SAFETY STANDARDS THAT APPLY TO PROPANE INSTALLATIONS The following codes and standards should be consulted when planning a stationary engine installation or a system modification. National Fire Protection Association (NFPA), 1 Batterymarch Park, Quincy, MA NFPA 10 Installation, Maintenance, and Use of Portable Fire Extinguishers NFPA 30 Flammable and Combustible Liquids Code NFPA 37 Stationary Combustion Engines and Gas Turbines NFPA 58 Liquefied Petroleum Gas Code NFPA 70 National Electrical Code Perhaps the most important of the NFPA publications listed above, and the one most directly related to propane engine fuel systems, is NFPA 58. In addition to NFPA standards, the following information pertaining to the installation and use of standby electrical systems is available: Article X, National Building Code, available from the American Insurance Association, 85 John Street, New York, N.Y Agricultural Wiring Handbook, obtainable from the Food and Energy Council, 909 University Avenue, Columbia, MO, ASAE EP-364.2, Installation and Maintenance of Farm Standby Electric Power, available from the American Society of Agricultural Engineers, 2950 Niles Road, St. Joseph, MI

30 Lab Activity Identify the Components and Their Functions on Typical Propane Engine Fuel Systems Directions: Complete each task to demonstrate your ability to identify components of a propane engine fuel system. 1. Fill in the numbered blanks in the diagram shown below to identify major components of a propane engine fuel system that uses propane vaporized in the supply tank. 2. Fill in the numbered blanks in the diagram below to identify major components of a propane engine fuel system that uses propane vaporized in a fuel-system component. 22

31 3.0 Propane-Fueled Stationary Engine Emission Control Systems 23

32 3.0 INTRODUCTION To minimize the production of undesirable exhaust emissions and to maximize the useful work that can be obtained from an internal combustion engine, an engine emission control system may be required. Federal and state environmental regulations may apply to new or certain existing installations of stationary engines. Service personnel who are called upon to maintain or repair stationary engines should understand the functions provided by emission control system-equipped engines. The objectives of this chapter are to: 3.1 Identify the department of the U.S. government that enforces the Clean Air Act by publishing regulations that address internal combustion engine emissions. 3.2 Identify the meaning of stoichiometric combustion and the ideal mixture ranges of propane-air fuel mixtures that tend to yield the lowest quantities of carbon monoxide and oxides of nitrogen. 3.3 Identify the general operating characteristics of an electronic emission control system, and the typical components of a system. 3.4 In relation to emission control system operations, identify the meaning of open loop and closed loop. THE ROLE OF THE U.S. ENVIRONMENTAL PROTECTION AGENCY (EPA) IN REGULATING STATIONARY ENGINE EXHAUST EMISSIONS Identifying EPA s enforcement role for the Clean Air Act The U.S. Congress established and charged the Environmental Protection Agency (EPA) with responsibility to create and enforce regulations in support of the Clean Air Act. EPA s regulations are found in Title 40 of the Code of Regulations, and can be accessed on the internet via or by going to the Government Printing Office (GPO) Web site at 24

33 Propane-Fueled Stationary Engine Emission Control Systems An example of a GPO Web page with links to the EPA Clean Air regulations is shown to the right. These important regulations affect the installation, maintenance, and repair of sparkignition stationary engines: 40 CFR part 60, subpart JJJJ. 40 CFR part 63, subpart ZZZZ. 40 CFR part 1048 Control of Emissions From New, Large Nonroad Spark-Ignition Engines. EPA regulations are subject to change after publication of a Notice of Proposed Rulemaking in the Federal Register. Usually a 90-day period for public comments is required before the proposed regulations are adopted. A compliance date is set after a Final Rule Notice is published in a subsequent Federal Register. An issue of the Federal Register is published each weekday except for federal government holidays. The most recent EPA regulatory change related to propane-fuel stationary engines was published as a Final Rule in the January 18, 2008 issue. Those rule changes became effective March 18,

34 3.0 State governments whose air quality compliance program plans have been reviewed and approved by EPA also can create and enforce stationary engine air emissions regulations. The California Air Resources Board is a leading state agency whose air quality standards and enforcement actions impact stationary engine emissions and hazardous air pollution limits. Important terms and definitions that are used in EPA regulations are listed in Appendix A in the Glossary Section at the end of this manual. The following are among the terms defined in 40 CFR that should be understood by technicians servicing stationary engines regulated by EPA: Stoichiometric means the theoretical air-to-fuel ratio required for complete combustion. Rich-burn engine means any four-stroke spark-ignited engine where the manufacturer s recommended operating air/fuel ratio divided by the stoichiometric air/ fuel ratio at full load conditions is less than or equal to 1.1. Lean-burn engine means any two-stroke or four-stroke spark-ignited engine that does not meet the definition of a rich burn engine. IDENTIFYING THE IDEAL COMBUSTION AIR-TO-PROPANE RATIO FOR SPARK-IGNITED INTERNAL COMBUSTION ENGINES Identifying the meaning of the terms stoichiometric combustion, lean, and rich Stoichiometric combustion of a fuel would result in complete burning of the fuel. In the case of propane, which is made up of hydrogen and carbon, complete combustion would produce only carbon dioxide, water, and minute quantities of oxides of nitrogen, the predominant constituent of air besides oxygen. Before exhaust emissions were a concern, stationary propane and natural gas engines were designed to run with excess air. These engines ran very well with 5% to 20% excess air. 26

35 Propane-Fueled Stationary Engine Emission Control Systems Excess air ratio is referred to as Lambda (λ). Stoichiometric air-fuel ratio is 1.0 (the blue line) in the figure on page 26. Rich-burn operation is to the left of the stoichiometric point, and lean-burn operation is any ratio to the right of the stoichiometric point. (For EPA regulatory purposes, a lean-burn engine is one with a λ ratio greater than or equal to 1.1.) The air-fuel ratio would often vary with load, and as long as the engines would carry the load and didn t detonate or misfire, their owners and operators were satisfied. Typically, carbon monoxide (CO) output is highest when an engine is running rich. Hydrocarbon (HC) emissions, which represent unburned fuel, are highest when an engine is running rich and lowest at stoichiometric, but will increase again when running lean due to incomplete combustion. Oxides of nitrogen (NOx) are lowest when running rich, due to the lower percentage of air to fuel, and highest when slightly lean of stoichiometric. NOx will decrease at further lean mixtures due to reduced combustion temperatures. Carbon dioxide (CO2) is typically highest at stoichiometric and is generally considered a measure of ideal combustion. Oxygen (O2) is lowest when running rich and highest when running lean. These five exhaust gases are typically measured during combustion analysis while setting up an engine in the field or when verifying proper operation of an engine during periodic maintenance. As exhaust emissions and reducing hazardous air pollutants became increasingly important, it was discovered that these engines were running with very high NO x levels, sometimes at the peak of the NO x curve. Two strategies evolved to reduce the NO x while limiting the carbon monoxide (CO) and unburned hydrocarbons (HC). The first strategy is stoichiometric or rich-burn combustion. The second strategy is called lean-burn combustion. 1. Rich-Burn Combustion The first method, and easiest to implement, was to operate the engines at a stoichiometric fuel mixture. A stoichiometric mixture is the chemically correct fuel mixture for combustion, with near zero oxygen left over in the exhaust. This method of operation is suitable for a three-way catalytic converter. The mixture must be precisely controlled in order for the reaction in a catalytic converter to oxidize the CO to CO 2 and reduce the NO and NO 2 to N 2 and O 2 and not have undesirable products left over. a. Rich-Burn Oxygen Sensor In order to achieve the precision in the control of the mixture required for the catalyst, an O 2 sensor is placed in the exhaust before the catalytic converter. The output of the O 2 sensor is fed back to the control device to close the loop on the amount of oxygen in the exhaust. The mixture is controlled to maintain very low oxygen content, less than 0.02 percent in the exhaust, as indicated by the voltage produced by the O 2 sensor. This indicates that the combustion process is consuming nearly all of the oxygen. If higher oxygen content is indicated, the engine is running too lean. If lower oxygen content is indicated, the mixture is too rich. 27

36 3.0 The ideal switch point is determined by the design of the O 2 sensor, but typically, 0.5 volts (500mv) is ideal. Oxygen sensors are available to meet a variety of engine applications, and therefore may have different switch points, but rarely are their switch points lower than 0.45 volts (450mv) or greater than 0.5 volts. Wide-band O 2 sensors are used for specific applications where extreme fuel mixture ranges may be experienced. Where traditional automotive engine O 2 sensors produce a rough rich-lean signal output, the wide-band O 2 sensor produces a true signal showing the actual air-fuel ratio. These two types of sensors are NOT interchangeable. b. Characteristics of Rich Burn One of the benefits of engines running in a rich-burn mode with a catalytic converter is they operate with very small quantities of NO x and CO in the exhaust. At the discharge of the catalytic converter, NO x in the range of a few parts per million is achievable. A two-way exhaust catalyst converts HC and CO into CO 2 and H 2 O. A three-way exhaust catalyst is used where NO x is converted first, then HC and CO are converted into CO 2 and H 2 O. An engine using an exhaust catalyst should use electronic fuel-mixture controls to keep the catalyst operating in its optimal range for catalytic conversion. This is difficult to accomplish with a fully mechanical system, since the airfuel mixtures can vary outside of the desired range of the catalyst due to component age and wear, fuel composition variations, and ambient temperature fluctuations. In general terms, gaseous-fueled engines may run hotter when running rich rather than when running lean because no liquid fuel is evaporating and producing a cooling effect inside the combustion chamber. An engine running at stoichiometric to approximately 10% rich will produce more power. Conversely, an engine running leaner than stoichiometric will improve economy but produce less power. 2. Lean-Burn Combustion The second strategy for reducing emissions is to run the engine with as much excess air as possible. To prevent either knocking or misfiring, the combustion process must be controlled within a narrow operating window. Charge air temperatures and volume, together with air-to-fuel ratio and other operating conditions, must be constantly monitored. The microprocessor-based engine controller regulates the fuel flow, air/gas mixture, and ignition timing. Many engines running in excess lean-burn mode utilize a turbocharger to bring the engine power back to normal levels. 28

37 Propane-Fueled Stationary Engine Emission Control Systems a. Lean-Burn Oxygen Sensor The oxygen sensors used for lean-burn engines, unlike the sensors used with rich burn, indicate a very wide range of oxygen in the exhaust. These sensors are often referred to as lambda sensors, where lambda is the air-fuel ratio that the engine is running at divided by the stoichiometric air-fuel ratio. Most engines running in lean-burn mode use a wide-band O 2 sensor because the traditional O 2 sensor does not have sufficient operational resolution in lean mode. b. Benefits of Lean Burn Engines running in the lean-burn mode offer several important advantages including lower combustion temperatures, reduced emissions, and decreased fuel consumption. Identifying the ideal mixture of propane-air ratios that tend to yield the lowest exhaust quantities of carbon monoxide and oxides of nitrogen The chart illustrates that, as a propane-air mixture goes from rich to lean, the emissions that EPA regulations seek to control are reduced. Although unburned hydrocarbons (HC) initially peak up, they are reduced. NO x, CO 2, and CO all are trending down on the chart as the engine is moved to the lean-burn side. 29

38 3.0 IDENTIFYING THE GENERAL OPERATING CHARACTERISTICS OF AN ELECTRONIC EMISSION CONTROL SYSTEM Identifying the typical components used in an emissions control system and their functions Electronic controls are made up of at least three items. 1. The fuel-control valve may be a stand-alone device or incorporated into other components, including the carburetor or mixer assembly, and the vaporizer. The fuelcontrol valve may take any of these forms: a. A pulse width solenoid that modulates air-valve vacuum, which alters the air-fuel ratio by changing the vaporizer outlet pressures. b. An electrical or pneumatically actuated valve mounted in the vapor or dry gas hose between the vaporizer and the mixer body. c. An internally mounted valve in a fully self-contained fuel carburetor assembly. This device may operate by varying the size of the fuel outlet port through the use of a movable slide, a digitally controlled stepper motor that controls a variable orifice, or an adjustable internally mounted regulator. 2. The control module may be a stand-alone device or incorporated into other components, including the carburetor or mixer assembly or the vaporizer. Most modules control only air-fuel mixtures. 3. The wiring, including the fuel-control switch. A typical control module reads outputs from the following devices: Oxygen sensor the primary emission control system sensor, which is installed in the exhaust system between the engine and any catalytic converter and/or muffler. RPM reference sensor typically installed at the flywheel, front balancer, magneto, or injector pump location. Manifold air temperature sensor detects air density. Colder air is denser than warmer air and may contain more oxygen by volume. 30

39 Propane-Fueled Stationary Engine Emission Control Systems Coolant temperature sensor Measuring engine operating temperature is critical to delivering the proper air-fuel mixture. Cold engines typically require a slightly richer airfuel mixture than engines that have reached the proper operating temperature. Gaseousfuel engines are not severely affected by rich fuel mixtures when cold, since the fuel is already vaporized. IDENTIFYING THE TERMINOLOGY USED TO DESCRIBE THE OPERATING MODES OF THE EMISSION CONTROL SYSTEM Open Loop Operation Open loop operation is used in two ways related to engine emission control systems: 1. On engines not equipped with electronic microprocessor fuel-air mixture and emissions controls. 2. On engines equipped with electronic microprocessor control systems during the period when the engine is started but has not yet reached operating temperature. At such times the engine requires a richer propane-air cranking mixture, and some of the sensors are temporarily not used to monitor exhaust gas, air density, and other operating conditions. Engines typically transition from open to closed loop at an operating temperature that is pre-determined by the fuel system manufacturer. Some systems use the engine coolant temperature sensor, while others use the oxygen sensor. The average transition from open to closed loop will usually occur at around 160 F engine coolant temperature. If the O 2 sensor has reached the proper operating temperature (generally at or around 600 F), or a voltage signal has transitioned from lean to rich, crossing the center switch point, the system may have enough information to initiate the transition. Some fuel systems may include a timer to ensure that enough time has passed to allow sufficient heating of the exhaust catalyst. Closed Loop Operation In closed loop operation, engine fuel-air mixtures, cylinder charging, and spark timing are typically varied in response to exhaust gas sensor and other sensor outputs, as they are read and interpreted by one or more microprocessors (electronic computers). Simple closed loop fuel control systems may use as few as two inputs: Ignition on. O 2 sensor. 31

40 3.0 More common systems will use: Ignition on. O 2 sensor. RPM reference. More advanced systems will use: Ignition on. O 2 sensor (either a rich-lean sensor or a wide-band Lambda sensor). RPM reference. Battery feed. Manifold air pressure (MAP). Throttle position (TPS). Engine coolant temperature sensor (CTS). The most advanced fuel systems use all or most of the following: Ignition on. O 2 sensor (either a rich-lean sensor or a wide-band lambda sensor). RPM reference. Battery feed. Manifold air pressure (MAP). Throttle position (TPS, typically with drive-by-wire throttle control, integrated into the governor). Engine coolant temperature sensor (CTS). Air temperature. Fuel temperature. Fuel pressure. Mass air flow sensor (MAF). More precise control of the air-fuel mixture is possible when the processor can receive more information (inputs) resulting in more accurate outputs to manage the combustion process. Closed loop output fuel controls may be simple vacuum solenoids that pulse vacuum to the vaporizer, altering delivery pressure, which changes the final amount of fuel delivered to the engine. Other controls use an electric or vacuum-operated valve, which mounts in the pipe between the final regulator and the engine carburetor/mixer. This motor will open or close, changing the volume of propane and the vacuum signal seen between the regulator/vaporizer and the engine/mixer. 32

41 Propane-Fueled Stationary Engine Emission Control Systems Emission Control System Components Illustrated The numbered components on the picture above are: No. Component Function 1. Mixer assembly (Woodward, N-CA200) Provides specified engine fuel-air mixture 2. Vaporizer (Woodward N-H420; closely resembles the IMPCO Model L) 3. Fuel lock-off (AFC 121 or AFC 123, Woodward #N ) Converts liquid propane to vapor and reduces container pressure to low pressure (negative) Stops the flow of propane when the engine is not running 4. Fuel control solenoid Varies fuel supply to mixer in response to O 2 sensor input to the control module 5. Ignition coil Provides proper voltage to spark plugs 6. Control module Converts O 2 sensor and other inputs to fuel system operating commands 7. Engine RPM reference for the Murphy panel Monitors engine and provides engine over rpm protection; may also provide input to control module Not shown is the O 2 sensor connection. Typically, the O 2 sensor is close to the junction of the left and right bank exhaust headers, or on one exhaust manifold. Some newer engines that use an exhaust catalyst may use two or more O 2 sensors. The primary sensor determines the exhaust composition and sends information back to the fuel control module; the secondary sensor is mounted after the exhaust catalyst. This sensor monitors the efficiency of the exhaust catalyst. If the secondary sensor produces a consistent relatively flat output signal, the catalyst is working properly. If the secondary sensor produces an output signal that closely follows the primary sensor, the exhaust catalyst is not working. 33