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The objective of this project is to determine and implement the critical design parameters that contribute to the overall noise output for the Cummins Power Generation C500D5 enclosed generator unit which encompasses a Cummins QSX15-G8, turbocharged and charge air cooled 15 litre diesel engine coupled to a Stamford HC4D Industrial four pole, single bearing alternator. The project outcome must support cost reduction initiative whilst ensuring compliance with customer and technical functional requirements.
This report has been produced to provide a record of the activities completed for an Engineering Systems Undergraduate final year project, undertaken as part of a degree course at the University of Greenwich for the period between 9th October 2009 and 23rd April 2010.
The objective of this report is to xxxx
Background and Context
Cummins Power Generation is a world leader in the design and manufacture of power generation equipment providing the customer the choice of an open or sound attenuated version for all models. Cummins Power Generation is a Global Company selling its equipment into all major markets in the world, to ensure its products are meeting specifications that have wide international acceptance the company must comply with certain standards and legislations. In particular work carried out on this project must comply with ISO 8528-10 1998 and British Standard EN 3744 1995 also referring to Directive 2000/14/EC.
The current Cummins C500D5 Electrical Generator product achieves an overall sound power level (SWL) of 101 dB Lwa. In accordance to the mentioned regulations the Cummins C550D5 Electrical Generator product must meet noise compliance before it can be sold into major markets. The target sound power level (SWL) for this product before it can be sold into the EMEA region is 98 dB Lwa.
Scope and Objectives
The top-level objective for this project is to reduce the overall SWL of the Cummins C500D5 Electrical Generator without increasing the manufacturing costs of the product.
The first stage of this project will investigate the methods of noise reduction for the Cummins C500D5 Electrical Generator and analyze the current design. The second stage of the project will determine the critical design parameters for the current design that aid in reducing the overall SWL of the product whilst supporting cost reduction initiative. The third stage of this project will be to implement the optimized parameters to the design reducing the overall noise level output to meet the require target SWL of 98 dB Lwa as stipulated by the Directive 2000/14/EC.
Sound is what the human ear hears; noise is simply unwanted sound. Sound is produced by vibrating objects and reaches the listener's ear as pressure waves in the air or other media. Sound is technically a variation in pressure in the region adjacent to the ear. When the amount of sound becomes uncomfortable or annoying, it means that the variations in air pressure near the ear have reached too high amplitude.
The human ear has such a wide dynamic range that the decibel (dB) scale was devised to express sound levels. The dB scale is logarithmic because the ratio between the softest sound the ear can detect and the loudest sound it can experience without damage is roughly a million to one or 1:106. By using a base-10 logarithmic scale, the whole range of human hearing can be described by a more convenient number that ranges from 0 dB (threshold of normal hearing) to 140 dB (the threshold of pain).
There are two dB scales: A and L.
- The dB(L) unit is a linear scale that treats all audible frequencies as having equal value. However, the human ear does not experience all sound frequencies as equally loud. The ear is particularly sensitive to frequencies in the range of 1,000 to 4,000 Hertz (cycles per second), and not as sensitive to sounds in the lower or higher frequencies.
- Therefore, the "A-weighting filter," which is an approximation of loudness, is used to correct the sound pressure levels to more accurately reflect what the human ear perceives. This frequency-weighting results in the dB(A) scale, which was adopted by OSHA in 1972 as the official regulated sound level descriptor.
The Need to Reduce Noise
Portable generators are used very commonly in shops, offices and homes today in order to supply power during power shutdowns. These generators emit very high levels of no ise, in addition to noxious air emissions. The noise may be generated by aerodynamic effects or due to forces that result from combustion process or may result from mechanical excitation by rotating or reciprocating engine components . Noise levels due to the operation of a 2 KVA capacity portable generator, at different distances inside and outside a laboratory, have been assessed .
Types of Generator Noise
The term airborne noise can be defined as sound waves that are being carried by the atmosphere or sound that travels into the surrounding air.
Airborne noise is apparent in generator applications and is limited to the areas of the noise source. Example of noise sources etc
Structure borne Noise
The term structure borne noise can be defined as the mechanical vibration in a structure which turn into sound waves become audible noise. Structure borne noise is only of a concern when radiation occurs and until such time is inaudible.
Structure borne noise is apparent in generator applications by the excitation of vibration in its supporting structure and enclosure, this vibration excites in its turn the walls and panels of the enclosure resulting in radiation or a so-called secondary sound.
Generator Noise Sources
Generator set noise is produced by four major sources.
- Prime Mover (Engine)
- Cooling Package (Cooling Fan)
- Structural Enclosure
3.1 Sources of noiseThe most dominant noise source to be considered is the generators prime mover.
The Cummins C500D5 Electrical Generators prime mover is a Cummins QSX15-G8 turbocharged and charged air-cooled 6 cylinder 4 stoke cycle diesel engine, which contributes heavily to the overall sound output of the product. The estimated free field sound pressure level of the engine at rated load and 7.5m is 89.5 dB(A) excluding the exhaust. The engine noise is mainly generated from four main sources.
These sources are:
- Engine Airflow Interactions
- Engine Combustion Noise
- Engine Exhaust Noise
- Mechanical noise.
Engine airflow noise is primarily generated by tones at the fundamental firing frequency and harmonics which is caused by air flowing through the cylinders of the engine at a certain frequency which is dictated by the opening and closing of the valves. The engine airflow noise is also caused by the interactions with flow passage changes within the engine and by the rotating compressor and turbine vanes of the turbocharger. The engine fan also contributes as a noise source due the additional air flow created by the air moving over the fan blades as the fan operates.
Engine combustion noise is caused due to the rate of change in gas pressure as engine combustion occurs. The QSX15 is a four-stroke cycle engine meaning that combustion occurs by four stokes of the piston. On the first downward stroke of the piston diesel fuel and air is drawn into the cylinder. The piston then strokes in the upward direction and compresses the fuel-air mixture, which is then ignited. During ignition of the fuel-air mixture the exhaust gas expands forcing the piston back down for the third stroke. The fourth stroke moves the piston back in the upward direction and evacuates the used exhaust gasses from the cylinder. This completes the four-stroke cycle and is then repeated.
When a force is applied quickly as during combustion, vibrations will be set up in the structure of the engine. The amplitude of the vibration will depend on the time of force application and on the natural periods of structure vibration.
Engine exhaust noise also contributes to the overall noise level and is caused by combustion in the cylinders of the engine The engine exhaust noise for the QSX15-G8 measured at 1 meter horizontally from the centreline of the exhaust pipe outlet at 45 degrees is 123 dB(A). The exhaust noise is dominated by the engine tones at the firing frequency and its harmonics. The harmonics for the Cummins QSX15-G8 is determined by the stroke cycles of the engine. The firing frequency for QSX15-G8 is one for every two revolutions of the engine per cylinder and can be calculated by multiplying the number of cylinders by half the rotation rate.
The QSX15-G8, 6 cylinder engine for generator application operates at a running speed of 1500 RPM, which gives a 1st order tone of 25 Hz, this can usually be referred to as the movement of the crankshaft. The exhaust firing frequency (3rd order) thus gives a tone of 75Hz. This indicates that the noise of the QSX15-G8 will be predominantly at the lower part of the noise spectrum, which is typically harder to control than noise at higher frequencies.
There can be many sources of mechanical noise from a diesel engine, important sources of noise may be the vibrations created by the impact of the pistons, valve-gear parts, bearings, gears and gear teeth. The engine structure vibrates in response to oscillatory gas pressure forces and inertial forces of the piston, connecting rod and crank mechanism that all in turn generate noise. In the aircraft industry engines use large clearances for pistons and bearings to combat high noise levels where as in the automotive industry small bearing clearances are most common to reduce noise levels.
Engine cooling fan noise can be referred to as the sound of air being moved at high speed across the engine and through the cooling package which is a function of fan tip speed, static pressure, fan power, air flow, number of blades and fan diameter. Unfortunately there are no specific methods to calculate the expected noise level emitted by a fan for a given operating condition at this present time, but generally a typical fan used for generator application will have a sound pressure level ranging from 100 dB(A) to 105 dB(A) measured at one meter.
The QSX15-G8 has a single 8 bladed fan, which is used to cool the generator cooling system. The aerodynamic noise, which is the dominant noise source of a cooling fan, is typically proportional to the blade tip velocity raised to the 5th power1. Sharp, harmonic peaks in the Blade Passing Frequencies (BPF) characterize the noise.
The fan Blade Pass Frequency (BPF) noise level intensity varies with the number of fan blades and by the rotation speed of the fan and can be expressed as:
BPF = n t / 60
BPF = Blade Pass Frequency (Hz)
n = rotation velocity (rpm)
t = number of blades
The QSX15-G8 has an 8 bladed fan, which due to the fan pulley ratio of 0.75:1 rotates at a speed of 1125 rpm. The blade pass frequency can be calculated as:
BPF = (1125 rpm) x (8 no of Blades) / (60 s/minute)
= 150 Hz
In a given system, the work done by each blade is inversely proportional to the number of blades. Increasing the number of blades reduces the loading on each blade and thus a lower pitch angle may be set which in turn will result in a lower noise level.
When the fan is bolted to the engine it sits centralized within a metal shroud facing the cooling system. The fan shroud is one of the most important contributors to fan noise. Fan noise can be greatly reduced by modifying the shape of the shroud.
Fig. 3 compares the noise spectrum of a same fan, mounted on different shrouds. In one of the shrouds, the fan in mounted exactly in the center of the rectangle, while the same fan is position to a side in another. Different BPF patterns result for the shrouds. For the centered fan, the 2nd BPF is dominant, while for the off centered fan; the 1st BPF is the largest.
The inflow velocity difference between left and right of the fan in the eccentric case result in the excitation of the first mode. In case of the centered fan, the inflow velocity changes along the two diagonals of the shroud, thus resulting in the 2nd mode excitation. Generally when a fan is moved off centre cooling performance decreases.
Fan Engine Gap
The fan spacer stipulates the gap between fan and engine and thus acts as an indirect source of fan noise. The fan noise in this case is usually associated with back draft of the flow ingested into the fan.
Fan Tip Clearance
A fan is usually less noisy when the clearance between the blade tips and radiator shroud is as small as possible. For generator application a large gap around the fan is required in order to ensure the blades will never come into contact with the housing in which it sits.
Fan Inlet Geometry and Pitch
The geometry of the fan can influence the static pressure and fan noise level. There are three commonly used fans in industry today all having affects on inlet geometry:
- Venturi, or Inlet Cone;
- Short shape- edged duct;
- Circular cutout, or Orifice plate.
For airfoil and increasing-arc impellers, the venturi is the most favourable when it comes to performance, efficiency and noise. The short, sharp-edged duct is the second most favourable option. It is less expensive to manufacture and requires less space than the venturi. The orifice plate is the least favourable for these impellers but is far cheaper and requires the least amount of space. The unfavourable effects of this inlet geometry are particularly noticeable when used with airfoil impellers.
The sickle profile blade performs most favourably in the venturi. It does, however, also perform exceptionally well in the orifice plate. There was less than 2% reduction on airflow when using the sickle profile in the orifice plate than in the venturi, with comparable low noise levels. The impeller's advantages were greatly reduced when mounted within the short sharp-edged duct.
The pitch of a fan blade can be adjusted to affect the noise level of the fan, but adjusting the pitch angle of the fan blade can also reduce cooling performance. Usually for generator application the pitch of the blade should be as little a possible to reduce the noise level but to improve the cooling performance of the fan the pitch should be greater.
Alternator noise - This is caused by cooling air and brush friction and ranges from approximately 80 dB(A) to 90 dB(A) at one meter.
To be added, more research required
Induction noise - This is caused by fluctuations in current in the alternator windings that give rise to mechanical noise that ranges from 80 dB(A) to 90 dB(A) at one meter.
To be added, research done
Engine exhaust - Without an exhaust silencer, this ranges from 120 dB(A) to 130 dB(A) or more and is usually reduced by a minimum of 15 dB(A) with a standard silencer.
To be added, more research required
Structural/mechanical noise - This is caused by mechanical vibration of various structural parts and components that is radiated as sound.
Investigation into methods of noise reduction
- Problem Analysis
- Problem Solution
- Problem Implementation
- Discussion of Results
- Future Work
- Project Timetable (e.g. GANTT chart)
- Appendices (if required)