Abstract:
A method of improving scavenging operation of a two-stroke internal combustion engine. The exhaust pressure of the engine is analyzed to determine if there is a pulsation frequency. Acoustic modeling is used to design an absorber. An appropriately designed side branch absorber may be attached to the exhaust manifold.

Description:
RELATED PATENT APPLICATION  
       [0001]     This application claims the benefit of U.S. Provisional Application No. 60/722,479, filed Sep. 30, 2005 and entitled “Side Branch Absorber For Exhaust Manifold of Two-Stroke Internal Combustion Engine.” 
     
    
     GOVERNMENT LICENSE RIGHTS  
       [0002]     The U.S. Government has a paid-up license in this invention and the right in certain circumstances to require the patent owner to license others on reasonable terms as provided for by the terms of Contract No. DE-FC26-02NT41646 for the United States Department of Energy. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0003]     This invention relates to control systems for internal combustion engines, and more particularly to a method for using a side branch absorber at the exhaust manifold of a two-stroke internal combustion engine to dampen resonant pulsations.  
       BACKGROUND OF THE INVENTION  
       [0004]     The gas transmission industry operates over 4,000 integral engine compressors, the majority being two-stroke, with a median age of 45 years and a median size of 2000 horsepower. These engines pump a large portion of the natural gas presently consumed in the United States. These engines are no longer produced, and with the projections for future increased demand of natural gas and the expense of replacement, it would be advantageous to modernize the existing fleet to allow for continued operation with increased efficiency and emissions compliance.  
         [0005]     Integral gas compression engines have historically exhibited poor performance and high emissions, due in part to poor engine control. The end results are misfires and partial burns that lead to increased fuel usage and exhaust emissions. Many of the slow-speed integral engines in the gas compression industry use control systems that are outdated and slow.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0006]     A more complete understanding of the present embodiments and advantages thereof may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which like reference numbers indicate like features, and wherein:  
         [0007]      FIG. 1  illustrates an example of a two-stroke internal combustion engine.  
         [0008]      FIG. 2  is a plot of exhaust back pressure and cylinder pressure as functions of crank angle degree, for the engine of  FIG. 1 .  
         [0009]      FIG. 3  is a frequency analysis of the exhaust pressure, illustrating its resonant modes.  
         [0010]      FIG. 4  illustrates an example of a side branch absorber, designed in accordance with the invention and attached to the exhaust manifold of the engine of  FIG. 1 .  
         [0011]      FIGS. 5A and 5B  illustrate the damping effect of the absorber.  
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0012]     The following description is directed to a side branch absorber attached to the exhaust manifold of a two-stroke internal combustion engine. The effect of the absorber is to dampen resonant pulsations that adversely affect scavenging during engine operation.  
         [0013]     As explained below, the absorber is similar in principle to a Helmholtz resonator. In general terms, a Helmholtz absorber is an enclosure with a port to couple a volume of airspace in the enclosure to another airspace. The resonant frequency of the absorber is controlled by the depth of the enclosed airspace in the box behind the port and the width and depth of the port.  
         [0014]     For purposes of the present invention, the absorber is designed to dampen the frequency response exhibited within an exhaust system of an engine. It is a “side branch” absorber in the sense that it is oriented perpendicular to the air flow within “exhaust runners” that carry exhaust gas from the engine cylinders to the exhaust manifold.  
         [0015]     The absorber is especially designed for stationary two-stroke internal combustion engines. Stationary engines are typically loaded by equipment such as compressors, pumps, or generators. However, without undue experimentation, significant inventive concepts of designing and using an absorber to damp pulsations within an exhaust system may be shown to be applicable to other engines.  
         [0016]      FIG. 1  illustrates an example of an engine  100 , with which the invention is used. Engine  100  is a spark-ignited lean-burn two-stroke stationary engine. An example of such an engine is an integral compressor engine running on natural gas. It has six power cylinders  101 , and uses a turbocharger  103  for scavenging. A commercially available engine of this type is the Cooper-Bessemer Legacy GMV integral engine, a product of the Cooper Cameron Corporation.  
         [0017]     Engine  100  has an exhaust manifold  102  and exhaust runners  104 , which carry exhaust from the cylinders  101  to the manifold  102 . Engine  100  operates in accordance with the two-stroke principle of using intake air to clean or “scavenge” the cylinders  101  of exhaust gases.  
         [0018]      FIG. 2  illustrates a plot of measured exhaust back pressure (EBP) of engine  100  as a function of crank angle degree. Plots are also shown of manifold intake air pressure (MAP) and of in-cylinder (combustion chamber) pressure. For EBP measurement, exhaust pressure may be measured dynamically in any of the six exhaust runners  104 . The pressures are shown in psia, and the crank angle degree is relative to top dead center (TDC)=0.  
         [0019]      FIG. 2  further indicates the timing (in crank angle degrees) of various two-stroke operations. Exhaust Port Open (EPO) occurs near 110 degrees, and Exhaust Port Closed (EPC) occurs near 260 degrees. Inlet Port Open (IPO) is near 135 degrees, and Inlet Port Closed (IPC) is near 245 degrees.  
         [0020]     As shown in  FIG. 2 , the exhaust back pressure undergoes two distinct cycles between crank angle degrees of about 150 and 216 (66 degrees). This is during the time when the inlet port is open, and thus during the scavenging cycle.  
         [0021]     The engine speed during the measurements was 330 revolutions per minutes. Thus, the frequency of the cycles may be calculated as: 
 
2 cycles/66 degrees×360 degrees/rev×330 rev/min×1 min/60 sec=60 Hz 
 
 This frequency is referred to herein as the “pulsation frequency” during scavenging. 
 
         [0022]     As illustrated, the cylinder pressure and the exhaust back pressure both exhibit the pulsation frequency. This indicates that the pulsations have a systematic effect; that is, they are indicated in both the exhaust system and in the cylinder-piston system.  
         [0023]      FIG. 3  illustrates how the dynamic exhaust pressure data may be analyzed in the frequency domain, finding the pulsation frequency to be resonant. A first mode occurs at approximately 64 Hz, and a second mode occurs at approximately 124 Hz.  
         [0024]     Using acoustic modeling techniques and appropriate processing hardware and software, the exhaust manifold  102  may be modeled and dynamically simulated. The acoustic model is then used to design an absorber to mitigate the pulsating exhaust pressure.  
         [0025]     Thus, a feature of the invention is the recognition that the pressure changes in runners  104 , exhibited as a pulsation frequency, adversely affects the scavenging operation of the cylinders  101 . It is further recognized that acoustic modeling may be used to design an appropriate absorber.  
         [0026]      FIG. 4  illustrates one example of a side branch absorber  41 , designed in accordance with the invention, and installed on an exhaust manifold  102 . As illustrated, absorber  41  comprises a chamber  41   a  and a choke tube  41   b . The choke tube  41   b  provides a port from the chamber  41   a  to the exhaust manifold  102 .  
         [0027]     In effect, absorber  41  is a volume  41   a  separated from exhaust manifold  102  by means of choke tube  41   b . In the example of this description, the design of engine  100  provides a convenient attachment location for absorber  41  at the end of exhaust manifold  102 . In other embodiments, absorber  41  could be connected at other locations on the exhaust manifold  102  or elsewhere in the exhaust system. A particular engine design may facilitate placement of an in-line absorber.  
         [0028]     Because of the nature of the exhaust gas environment in which absorber  41  is placed, it is made from a material capable of withstanding high temperatures, such as steel. The attachment of absorber  41  may be by various means, such as by means of bolts.  
         [0029]     For the example of this description, the acoustic dimensions of the chamber  41   a  are 6″ of Schedule 40 steel pipe having an inner diameter of 28″. This 28″ inner diameter is substantially the same as the diameter of the end of exhaust manifold  102 , permitting it to be easily bolted to the manifold. The acoustic dimensions of choke tube  41   b  are 4.7″ of Schedule 40 steel pipe having an inner diameter of 3″. The Schedule 40 characteristic of the pipe reflects its thickness and pressure capabilities, and the pipe is but one example of a geometry and material suitable for use with engine  100 .  
         [0030]     The physical dimensions of choke tube  41   b  are not necessarily the same as the acoustic dimensions. In this example, the physical length of choke tube  41   b  is 2.7″. The acoustic dimensions and the resulting physical dimensions are determined by the acoustic model for the particular engine and its exhaust pressure characteristics. The length and diameter of the choke tube  41   b  are critical to the damping effect, and vary depending on the frequency desired to be dampened, with the volume of chamber  41   a  being another factor.  
         [0031]      FIGS. 5A and 5B  illustrate the results of computer analysis of the effects of the side branch absorber  41  on exhaust pressure. In  FIG. 5A , pressure is shown to be reduced at a single engine speed and order. The “order” reflects a certain speed and firing speed of the engine  100 . In  FIG. 5B , pressure is shown to be reduced at a single engine speed.  
         [0032]     In other embodiments, multiple side branch absorbers having different acoustic dimensions could be attached to the exhaust manifold to dampen pulsations at different engine speeds. Also, one or more side branch absorbers could be attached to the exhaust system at locations other than directly to the exhaust manifold.