Abstract:
The tunable resonator is coupled to the engine speed control such that the resonator is set to a different frequency range when the engine speed is changed. The frequency range is, changed to a higher frequency by closing a valve which effectively reduces the length of the resonator.

Description:
BACKGROUND OF THE INVENTION 
     Diesel engines used to drive transport refrigeration equipment produce low frequency tones at their firing frequencies and their harmonics. The refrigeration units can be required to hold the load temperature within 0.1 F.° of the set point which may be 40° F. for flowers or produce and −20° F. for ice cream. A number of these units can be parked and running at cold storage warehouses, interstate highway rest stops, etc. Because these units can be running at various loadings and because the engine speeds of the units are operator adjustable by a couple of percent, the noise outputs will be at different frequencies, but may be relatively coherent such that the various different frequency noise sources cyclically go into and out of phase. As the noise sources go from reinforcing to opposing the other noise sources, there is a perceived varying of the sound level. These tones can be sources of annoyance in the community adjacent areas where a number of units are running. 
     U.S. Pat. No. 6,009,705 discloses a noise attenuator employing a plurality of quarter wave resonator tubes and Helmholtz resonators. Each will be tuned to a separate narrow frequency range. The effectiveness of the various resonators will drop off as the frequencies of the noise sources vary from the design frequencies as the engine speed/load changes. 
     SUMMARY OF THE INVENTION 
     A transport refrigeration unit is, typically, driven by a diesel engine. As is conventional for internal combustion engines, ambient air is drawn through a filter into the cylinders of the engine. Rather than connecting one or more attenuators effective over single narrow ranges, the present invention employs an adjustable resonator in an elbowed side branch. Within the resonator is a butterfly or flapper valve that is either fully open or closed. At low engine speed, the valve is open and the sound energy from the engine enters the resonator and is reflected back on itself, 180° out of phase. The distance between the closed end of the side branch and the inlet pipe is λ 1 /4, where λ 1  is the wavelength of the tone of interest in the inlet pipe during low speed operation of the engine. At high speed, the valve is closed and the resonator is tuned to the higher engine speed, with the distance between the inlet pipe and the closed valve being λ 2 /4, where λ 2  is the wavelength of the tone of interest in the inlet pipe during high speed operation of the engine. The valve is connected to the engine speed control such that the valve is positioned in accordance with the engine speed of a two speed engine. 
     It is an object of this invention to eliminate the need for a resonator for each frequency of interest at both high and low speed operation. 
     It is another object of this invention to provide a resonator effective in two frequency ranges. These objects, and others as will become apparent hereinafter, are accomplished by the present invention. 
     Basically, the tunable resonator is coupled to the engine speed control such that the resonator is set to a different frequency range when the engine speed is changed. The frequency range is changed to a higher frequency by closing a valve which effectively reduces the length of the resonator. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the present invention, reference should now be made to the following detailed description thereof taken in conjunction with the accompanying drawings wherein: 
     FIG. 1 is a schematic representation of the present invention as used with a diesel engine driven refrigeration system; 
     FIG. 2 is a sectional view of the side branch resonator of FIG. 1 with the valve in the open position; and 
     FIG. 3 is a sectional view of the side branch resonator of FIG. 1, rotated 90° with respect to FIG.  2  and showing the valve in the closed position. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1, the numeral  100  generally designates a transport refrigeration system. Refrigeration compressor  10  is driven by a multi-speed diesel engine  20 . Compressor  10  is in a refrigeration circuit serially including condenser  12 , expansion device  14  and evaporator  16 . Refrigeration system  100  is controlled by microprocessor  30  which receives a number of inputs such as the engine speed, the sensed ambient temperature, condenser entering air temperature, zone temperature, and zone set point which are collectively labeled as zone inputs. In operation, diesel engine  20  and compressor  10  are driven through controls  32  responsive to microprocessor  30 . Specifically, diesel engine  20  may be driven through a speed control solenoid and draw ambient air into its cylinders via inlet line  22  containing filter  24 . 
     The present invention provides an elbowed side branch resonator  50  containing a butterfly or flapper valve  52  driven by actuator  40  which is coupled to the controls  32 . Although resonator  50  is illustrated as having a gradual curve, it may be a tee such that it is straight and at 90° to inlet line  22 . The specific application may dictate the shape of resonator  50 , but no sudden directional change should be present. Referring specifically to FIGS. 2 and 3, valve  52  is specifically illustrated as a flapper valve. Although valve  52  appears to be circular and to have the same nominal dimension as the interior of resonator  50 , it will have a slightly greater dimension in the direction perpendicular to the axis of rotation than along the axis of rotation. The purpose of this non-circular configuration is to ensure a positive seating since the valve  52  will be in an interference fit with the interior walls of side branch resonator  50 . The interference can be from the valve itself and/or wiper lips  52 - 1  and  52 - 2  of a suitable material such as neoprene. If necessary, or desired, valve  52  could engage a seat located in resonator  50 . Although member  52  is described as a valve, there is no fluid flow past the valve since it is in a closed line. Structurally, however, member  52  generally corresponds to a conventional butterfly or flapper valve. Actuator  40  places valve  52  in either the open or the closed position and may be any one of a number of suitable devices. Assuming that the pivot about which valve  52  rotates includes a crank external to resonator  50 , then valve  52  can be positioned by a solenoid or any mechanism that could pull the crank in response to a control signal from controls  32  such as a linear motor, a vacuum actuator, or a rotary motor with another crank. Valve  52  may also be mounted on the shaft of a stepper motor which would be rotated 90° in alternating directions to open or close valve  52 . If the actuator is only effective in one direction, valve  52  would be biased to the other position, as by a spring. 
     At low speed, valve  52  is in the open position of FIG.  2  and sound energy from diesel  20  enters the resonator  50  and is reflected back on itself, 180° out of phase. The distance between the end  50 - 1  of side branch resonator  50  and the inlet pipe  22  is λ 1 /4, where λ 1  is the wavelength of the tone of interest in the inlet pipe  22  when diesel  20  is operating at low speed. At high speed, valve  52  is in the closed position of FIG. 3 which effectively divides resonator  50  into two chambers,  50 -A and  50 -B, respectively. Chamber  50 -A is isolated from inlet pipe  22  which effectively shortens side branch resonator  50  so that resonator  50  is tuned to the higher engine speed with the distance between the inlet pipe  22  and the closed valve  52  being λ 2 /4, where λ 2  is the wavelength of the higher frequency tone of interest in the inlet pipe  22  during high speed operation. Typical frequency ranges would be 50 Hz at low speed and 73 Hz at high speed with a typical λ 1 /4 of 68 inches and an λ 2 /4 of 46 inches, respectively. 
     As described above, side branch resonator  50  can be tuned to either high or low speed operation of diesel  20  and the present invention couples the tuning of resonator  50  to the speed control of diesel  20 . Responsive to engine and zone inputs, microprocessor  30  controls the loading of compressor  10  and controls the speed of diesel  20  which is driving compressor  10  through controls  32 . The speed of diesel  20  is controlled by a speed control solenoid, or any other suitable device, responsive to microprocessor  30 . The speed control device  40  has two positions which correspond to high and low speed, respectively, of diesel  20 . Diesel  20  and resonator  50  are coupled by microprocessor  30  and controls  32  such that changing the speed of diesel  20  produces a corresponding adjustment of resonator  50  to change the tuning. 
     From the point of view of acoustics, the system does not have to be tightly sealed as long as the leakage areas are very small compared to the active areas. As an inlet silencer, as illustrated, resonator  50  is located between filter  24  and diesel  20 , thus it must be tight enough to prevent dirt leaking in and entering the engine  20  having bypassed filter  24 . Typically o-ring type sealing for the actuator structure should be sufficient. Resonator  50  and valve  52  would be made of a material, such as steel or molded plastic, so as to provide a rigid wall for sound reflection. 
     Although the present invention has been illustrated and described in terms of a two speed diesel driven refrigeration system, the teachings of the present invention apply to reducing the inlet pulsations and can be used in any application where an engine runs at a modest number of fixed speeds such as a gas driven generator. The speeds must be far enough apart to permit plural valves which do not interfere with each other. Other changes will occur to those skilled in the art. It is therefore intended that the present invention is to be limited only by the scope of the appended claims.