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Turbocharger center helps advance natural gas compression. (Optimizing Large-Bore Engines).

Thousands of reciprocating engines are used in the U.S. to drive compressors that move natural gas throughout the country. With few exceptions these engines, built during a time when emissions were not a national concern, will need to have their performance improved to reduce emissions and to increase fuel economy.

Rather than invest billions of dollars to replace these engines, the U. S. natural gas pipeline industry has invested in research facilities to identify retrofit strategies and technologies. One such laboratory is the National Gas Machinery Laboratory (NGML), an institute of the College of Engineering at Kansas State University (K-State).

Central to this laboratory is the Turbocharger Test & Research Facility (TTRF), which was built primarily by funding provided through Pipeline Research Council International, Gas Technology Institute, individual pipeline companies, the State of Kansas, and two aftermarket turbocharger companies. This test stand provides researchers with extensive, and otherwise not available, turbocharger operating data.

In conjunction with industry representatives, NGML researchers identified two major unaddressed issues that confront engine system designers and operators regarding engine air flow management: 1) how to identify which turbocharger compressor, diffuser, turbine blade set, and nozzle ring will work on which engine; and 2) how to determine the extent to which an engine utilizes air to scavenge exhaust products out of the cylinder.

Since that time, NGML researchers have developed the methodology to optimally match a turbocharger to an engine, and are now working to identify the parameters and methods of measurement needed to determine and impact the scavenging efficiency in two-stroke cycle engines. This article explains the underlying importance of turbocharger matching, examines how scavenging efficiency impacts performance, and identifies key components that affect air flow through a large-bore two stroke cycle engine.

Turbocharger/ Engine Matching

The generic turbocharged engine system, illustrated in Figure 1, shows the turbocharger compressor, the turbocharger turbine, and the engine. The compressor compresses air to force the air into the engine cylinder. The turbine extracts energy from the engine exhaust, and matches the energy needed to power the compressor. Complicating this relationship are the compressor and turbine efficiencies, which are not constants but rather vary with operating conditions. Figure 2 illustrates a typical centrifugal compressor map.

[FIGURES 1-2 OMITTED]

The pressure ratio across the compressor is plotted on the vertical axis and the air flow rate through the compressor is plotted on the horizontal axis. The compressor map contains lines of constant speed and islands of constant efficiency. In order to determine the efficiency of this particular compressor, two of the following are needed: the air flow rate through the compressor, the pressure ratio, and/or the compressor speed.

Another key aspect of the compressor map is the surge line, which is shown on the left side of the compressor map. This surge line separates the regions where the compressor can operate, and where it cannot. If a compressor operates near the surge line, a perturbation in the system may be enough to throw the compressor into surge. When surge occurs, the air flow reverses, and flows backward through the compressor. The flow then reverses again, and the process repeats. As one might imagine, an engine does not operate very well under this condition and disastrous effects are a serious possibility, including catastrophic failure.

The next step in the turbocharger matching process is to couple the now determined compressor and turbine interaction operational constraints with the engine's ability to utilize the air that the turbocharger compressor produces. To accomplish this, one must understand the concept of an engine air flow load line. The engine air flow load line, or simply the load line, represents the resistance to flow thai is created by the engine cylinder. The load line provides the functional relationship between the air flow rate through the engine and the pressure difference across the engine. Figure 3 graphically illustrates all engine load line.

[FIGURE 3 OMITTED]

The vertical axis represents the pressure ratio across the engine cylinder, i.e., air manifold pressure divided by exhaust manifold pressure, and the horizontal axis represents the air flow rate through the engine. As expected, as the pressure ratio increases, the air flow rate through the engine also increases. The engine load line, the compressor map, and the turbine map provide all the needed information to match a turbocharger with an engine.

An NGML short course for industry professionals covers the turbocharger/engine process in more detail. During the course, participants have access to data collected from the Turbocharger Test and Research Facility, shown in Figure 4. Because of its unique open-loop design, the test cell has been used to successfully match turbochargers to engines. This facility can handle nearly any turbocharger used in the natural gas transmission industry, including Clark and Cooper turbochargers, to create turbocharger compressors and turbine maps.

[FIGURE 4 OMITTED]

Scavenging Efficiency

Once a designer and/or system operator has determined that a turbocharger matches an engine, the next consideration is to determine to what extent the engines and turbocharger system can efficiently remove all the exhaust products from the engine cylinder. This is the concept of scavenging efficiency.

According to Heywood (1981), scavenging efficiency is the ratio between the mass of air that is delivered to and retained in the cylinder to the mass of charge trapped inside the cylinder. Note that the charge may contain exhaust products from the previous cycle. In the case where absolutely all the exhaust products are removed, or scavenging efficiency is 100 percent. In most cases, however, scavenging efficiencies near 75 percent are more common.

The scavenging efficiency is important from the standpoint of emission. A commonly used ratio is that for every 1 percent increase in scavenging efficiency one can expect a 10 percent decrease in NOX production. Hence, the ability to fully understand and control scavenging efficiency in an engine remains critical with today's low pollutant requirements. Researchers are just beginning to identify the multitude of parameters on which scavenging efficiency depends. Once all the parameters are identified, technologies can be developed to 1) measure these parameters in the field, and 2) make real time operating adjustments to the engine system.

Components That Affect Engine Air Flow

NGML researchers continue to focus their efforts on the ability to fully understand and measure scavenging, while work continues to develop modified engine port geometries and other low-cost components to control scavenging in two-stroke cycle engines. One primary barrier under investigation is the inability to accurately represent the flow rate through engine ports.

Figure 5 shows air flow through a typical port. As the flow rate becomes relatively high, the main portion of the flow stream separates from the sides of the ports. This separation area contains a recirculation flow regime that acts as a barrier to flow through the port. The effective to flow area, AE, is smaller than the geometric area of the port, AT. Since the effective flow area is reduced, so is the actual air flow rate through the port. The challenge, then, is to quantify the impact of the reduced effective area on the air flow rate through the engine ports via the development of a parameter referred to as a discharge flow coefficient. The discharge flow coefficient is a number less than one that represents the ratio between the effective flow area and the true geometric flow area.

[FIGURE 5 OMITTED]

Discharge coefficients are determined from experimental data. Literature reveals that the currently-available discharge coefficients are derived from ports that are a fraction of the size of the ports used in large-bore two-stroke cycle engines. Unfortunately, discharge coefficients for the ports in large bore engines have not been experimentally determined and reported in the literature.

To determine sets of experimentally determined flow discharge coefficients, NGML researchers designed and built a port flow facility that can accommodate engine cylinders used in the natural gas transmission industry. The port flow facility was designed to develop flow discharge coefficients for a wide variety of intake and exhaust port geometries, and over a complete range of scavenging styles. Complete details of the design process are provided by Chapman et al. (2001).

Figure 6 schematically shows the port facility. The cylinder liner is positioned within the facility and the air supply duct is attached to the top of the cylinder liner. In this particular schematic, the facility is set up to test flow through the cylinder liner and out one or mole exhaust ports. The air flow rate through the supply duct is measured using an ASME certified and reversible flow venturi. The centrifugal compressor on the left side of the diagram provides the air to the cylinder. The final key component is the piston positioner located below the cylinder liner. The piston positioner precisely positions the piston within the liner. The positioning system is accurate to within 0.001 inches and provides the capability to finely adjust how much a particular port is covered by the piston. All temperature, pressure, and flow data are recorded and processed by the data acquisition system.

[FIGURE 6 OMITTED]

The discharge coefficient is determined from the measured data by using the definition of the discharge coefficient and the expression for ideal compressible flow through a restriction. The resulting equation is:

[MATHEMATICAL EXPRESSION NOT REPRODUCIBLE IN ASCII]

The reference area, Aref, theoretically represents geometrical flow area of the port.

At present, the flow discharge coefficients for the exhaust ports in a Clark HBA cylinder have been measured at the NGML flow facility. Preliminary results are shown in Figure 7. The horizontal axis shows the percent of the port that is uncovered by the piston, and the vertical axis shows the flow discharge coefficient. The discharge coefficient leaches a minimum when the port is about 40 percent open, and then increases when the port is completely open and nearly closed. These results clearly demonstrate how port is design impacts engine scavenging with associated implications for engine performance. Port modifications and/or complete redesigns will achieve improved scavenging efficiencies and engine performance to values greater than those measured today.

[FIGURE 7 OMITTED]

REFERENCES

Chapman, K.S., T.L. Brentano, D. Malicke, and J. Brown, "Design and Construction of a Large Bore Engine Flow Bench to Experimentally Determine Port Discharge Coefficients for Better Prediction of Airflow," ASME Fall Technical Conference Proceedings, Argonne National Laboratory, Argonne, IL, September 23-26, 2001.

Heywood, J.B., Internal Combustion Engine Fundamentals, McGraw-Hill Higher Education, New York, 1981.

ACKNOWLEDGEMENTS

The port flow facility and TTRF at the NGML are financially and technically supported by the Pipeline Research Council International Compressor Station Technical Committee (CSTC) and the Gas Technology Institute (GTI). The author acknowledges Exline, Inc. and EL Paso Corp. for contributions of cylinder liners, and Greg Beshouri, president, AETC for technical assistance, The review of this article and technical comments provided by Charles E. French, program manager, compression and measurement GTI. are greatly appreciated.

K. S. Chapman is professor of mechanical and nuclear engineering and director of the National Gas Machinery Laboratory at Kansas State University.
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Article Details
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Title Annotation:Kansas State University. College of Engineering. National Gas Machinery Laboratory
Author:Chapman, K.S.
Publication:Pipeline & Gas Journal
Geographic Code:1USA
Date:Oct 1, 2002
Words:1832
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