Abstract:
A noise reduction shroud is provided for use with utility engines that have attached blower housings. The shroud comprises a one-piece shell positioned above and around the blower housing located on the engine to define a space between the shroud and the engine, wherein the shell is adapted such that air entering the blower housing must first flow from a bottom side of the engine up through the space between the shroud and the engine. In another embodiment, the invention is a lawnmower or other lawn and garden equipment, comprising such a noise reduction shroud.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to utility engines, particularly noise reduction shrouds for small engines. 
       BACKGROUND OF THE INVENTION 
       [0002]    The maximum permissible sound power level of lawnmowers has been regulated in Europe since the mid-1980&#39;s. Historically, the cutting deck noise has been the dominant noise source of lawnmowers, Engine noise has not been a significant noise source. On Jan. 3, 2002, the European Noise Directive for Outdoor Power Equipment (2000/14/EC) came into force within the European Union. Lawnmower manufacturers have reduced the designed operating speed of lawnmowers to comply with the requirements of this directive. Engine speeds as low as 2500 RPM with an engine load of approximately 20% rated load are typical. This directive proposes a further reduction in the maximum permissible sound power level of approximately 2.0 dBA on Jan. 3, 2006. 
         [0003]    In the outdoor power equipment industry, the reduction of lawnmower noise is usually accomplished by reducing cutting deck speed. Reductions in lawnmower cutting deck noise have also been achieved by modifications to the cutting blades—specifically, the elimination of features of the cutting blade that creates lift of grass prior to cutting. Reductions in lawnmower noise have also been achieved by placing a seal around the perimeter of the cutting deck. Each of these lawnmower design modifications degrades the grass cutting performance of the cutting deck. The seal around the perimeter of the cutting deck adds cost to the lawnmower and presents a safety problem related to the ingestion and shredding of the seal beneath the cutting deck. The advantage of this invention is the reduction of lawnmower noise without loss of grass cutting performance or safety concerns related to ingestion by the cutting deck. This invention takes advantage of the acoustic environment specified for the European lawnmower noise test and actual use of a lawnmower-operation over turf or artificial flooring. 
         [0004]    In the European lawnmower noise test (ISO 11094), a lawnmower is operated over natural turf or over artificial flooring. Artificial flooring is a sound absorbing platform. In either case, considerable acoustic energy is absorbed by the surface beneath the lawnmower. In this test, acoustic measurements are made using microphones placed above the lawnmower. 
         [0005]    Over the past two years, lawnmower manufacturers have made significant progress reducing cutting deck noise. In some cases, cutting deck noise may have been reduced to near the level of the engine noise. In such cases, further reduction of the overall lawnmower noise level will require reducing the engine noise as well as the cutting deck noise. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The hallmark of this invention is providing an acoustic barrier between the noise sources of a utility engine and the receiver (i.e., the operator). It is also a hallmark of this invention to redirect the major noise sources of utility engine noise, at the operating conditions of the European Union lawnmower noise test, towards a sound absorbing surface. 
         [0007]    In one embodiment, the invention is a noise reduction shroud for use with utility engines that have attached blower housings, the shroud comprising: a one-piece shell positioned above and around a blower housing located on an engine to define a space between the shroud and the engine, wherein the shell is adapted such that air entering the blower housing must first flow from a bottom side of the engine up through the space between the shroud and the engine. In another embodiment, the invention is a lawnmower or other lawn and garden equipment, comprising such a noise reduction shroud, 
     
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  shows a schematic side view of a prior art engine blower housing. 
           [0009]      FIG. 2  shows the shroud redirecting the cooling system noise and the induction system noise of the engine to exit towards the bottom of the engine. 
           [0010]      FIG. 3  shows the shroud redirecting the cooling system noise and the induction system noise of the engine to exit towards the bottom of a liquid cooled engine. 
           [0011]      FIG. 4  shows the geometrical configuration for wave propagation over a layered boundary. 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
       [0012]    Utility and lawn and garden equipment include lawnmowers, chainsaws, blowers, string trimmers, generator sets, pumps, and other equipment powered by small (&lt;25 hp) gasoline and diesel engines. As used herein, the term “utility engine” means an engine typically rated under 50 horsepower and typically used to power outdoor power equipment and industrial applications. 
         [0013]    Utility engines are typically cooled by forced air. As shown in  FIG. 1 , a blower housing  11  is placed around an engine  13  such that a gap  15  exists between the blower housing  11  and the engine  13 . Cooling air  14  flows through the gap  15  between the engine  13  and the blower housing  11  (shown by arrows). The cooling air  14  enters the gap  15  through an inlet  17  in the blower housing  11  and is subsequently discharged through an outlet  19  in the blower housing  11 . Generally, for utility engines, the blower housing inlet  17  is located on the top of the blower housing  11  (as shown) but other orientations are possible. The cooling air  14  is typically forced through the gap  11  by means of a fan  21  or other similar means. The blower housing  11  is designed to direct the cooling air  14  against the engine  13  in order to provide the proper cooling. 
         [0014]    On most types of outdoor power equipment and particularly zero-turn radius lawnmowers, the operator position is above the utility engine. Therefore, the operator ear position is above the engine. On zero-turn radius lawnmowers, the operator sits directly in front of the engine with a direct and short path to the operator&#39;s ears. When the utility engine has the blower housing inlet orientated to the top of the blower housing, engine noise can propagate relatively unimpeded through the blower housing inlet directly towards the operator. As such, the operator is exposed to a high noise-level environment. As explained more fully below, the invention serves to mitigate the noise exposure of the operator by imposing an acoustic barrier between the noise source and the operator. 
         [0015]    Referring to  FIG. 2 , a noise reduction shroud  23  is attached over the engine crankcase and/or the engine blower housing  11 . Preferably, the shroud  23  is attached so as to be readily removable to allow for maintenance and repair of the underlying engine or blower housing  11 . The shroud  23  is generally made as a single piece. The shroud  23  is constructed out of any of the well-known suitable materials, such as steel, aluminum, or polymeric resins. The choice of the shroud construction material is guided by usual design considerations such as strength, impact resistance, weight, operating temperature, weather resistance, and the like. 
         [0016]    The shroud  23  covers the cooling air inlet  17  of the engine blower housing  11 . The height of shroud  23  is taller than the engine blower housing  11 . Shroud  23  is also wider than the engine blower housing  11  creating a second gap  25 . Therefore, shroud  23  does not block the flow of engine cooling air to the blower housing inlet  17 . Shroud  23  redirects the cooling air to the engine. Cooling air  14  flows through gap  25  to reach inlet  17  of blower housing  11 . The cooling air  14  enters the blower housing  11  through inlet  17 , is distributed by blower  21  and exits housing  11  through outlet  19 . The height and width of shroud  23  are determined by the cooling and airflow requirements of the engine. 
         [0017]    While allowing cooling air  14  to flow, shroud  23  operates as an acoustic barrier. Shroud  23  redirects the cooling system noise and the induction system noise of the engine to exit towards the bottom of the engine. This noise would normally exit towards the top of the engine out of the opening in the blower housing. 
         [0018]    In a further embodiment, this invention can be used with a liquid cooled engine as shown in  FIG. 3 . As shown in  FIG. 3 , the air flow path is the same as from air cooled engine shown in  FIG. 2 . However, heat exchanger  26  is provided to cool liquid used to cool the utility engine. Cooling air  14  enters the blower housing  11  through inlet  17  and is forced by blower  21  through heat exchanger  26 . The contact of cooling air  14  with heat exchanger  26  allows the transfer of heat energy through the cooling liquid to the cooling air  14 . 
         [0019]    By forcing the cooling system noise and induction system noise to exit from the bottom of the engine rather than the top of the engine, two acoustic benefits are realized. First the direct radiation of these noise sources to the operator ear position is avoided. Typically, the operator of a piece of outdoor power equipment is located above the engine. Therefore, there is typically a direct path for these noise sources to the operator&#39;s ears. Second, a significantly greater percentage of acoustic energy from these noise sources will be absorbed by the turf or artificial flooring. 
         [0020]    As mentioned in the previous section, the application of sound absorption to reduce noise is well known. Materials which absorb sound change the energy of motion of molecules into heat by exciting other motion, Manufacturers of outdoor power equipment have recognized the acoustic benefit of sound absorption for several decades. Manufacturers of such equipment choose the type of turf to use for acoustic testing based on the desire to have a high sound absorption coefficient. The sound absorption coefficient is defined as follows: 
         [0000]    
       
         
           
             
               ∝ 
               
                 ( 
                 f 
                 ) 
               
             
             = 
             
               Ia 
               Ii 
             
           
         
       
     
       Where: 
       [0021]    ∝(f)=absorption coefficient 
       Ia=acoustic energy absorbed by the material 
     Ii=acoustic energy incident on the material 
       [0022]      FIG. 4  shows the geometrical configuration for wave propagation over a layered boundary. The top layer is assumed to be air, which has a density ρ 0  speed of sound c 0 , and acoustic impedance ρ 0 c 0 . The middle layer is assumed to be either turf or artificial flooring, which is a porous material with its density and speed of sound being a complex quantity. In other words, the turf/artificial flooring layer has a complex acoustic impedance such that a plane wave transmitted from the air into this layer will be refracted into the layer with a phase shift, and will be attenuated as it propagates through this material. The sound absorption coefficient varies significantly with the type of turf. The sound absorption coefficient is related in part to the void-to-volume ratio. Sound absorption coefficients for turf vary from 0.5 to 0.7. Artificial flooring is required to be constructed of mineral fiber, 20 mm thick, having an airflow resistance of 11 kNs/m 4  and a density of 25 kg/m 3 . These features of artificial flooring provide a sound absorption coefficient approximately equal to natural turf. Therefore, there is significant acoustic advantage to be gained in the test setup for a European lawnmower noise test for redirecting unwanted sound waves (i.e., noise) towards the turf/artificial flooring beneath the lawnmower. 
         [0023]    As mentioned in the previous section, the application of a barrier to reduce transmitted sound is well known. Nonporous walls of mass density greater than approximately 20 kg/m 2  may be used effectively as a noise barrier. The sound reaching the receiver must diffract around the barrier. Since a majority of the sound does not diffract, the noise reaching the receiver is significantly reduced. Acoustic barriers are effective at reducing noise at the receiver position if the barrier has sufficient mass density, the barrier obstructs the line of sight between the receiver and the noise source, and the barrier has no openings that reduce the transmission loss. The utility engine noise reduction shroud  23  will achieve all these aspects of acoustic barrier design. To economically achieve the density requirement for an acoustic barrier, it may be necessary to line the inside portion of shroud  23  with an acoustic barrier material. 
         [0024]    Known acoustic barrier materials include noise-insulating panels made of self-supporting, thermoset materials such as reaction injection molded polyurethanes, and thermoplastic materials, such as highly filled ethylene vinyl acetate copolymer, polyvinyl chloride and polypropylene. 
         [0025]    This invention will also provide an acoustic benefit for operators of outdoor power equipment, particularly operators of zero-turn radius lawnmowers.