Genset Digest / Diesel Generator Sets / The fabric in a sound hood
Acoustic hood and intake filter fabrics for gensets
A generator set acoustic hood is built from a coated or laminated technical textile over a mineral or glass fibre core, and the intake filter is a pleated glass or synthetic medium held in a frame. Both are specified on a datasheet that states mass per unit area, weave or lamination, coating chemistry, and a set of mechanical and colour tests. Heat, oil and weather decide how long that specification survives in service.
How is a technical fabric specification read?
A specification sheet for a coated textile is read in a fixed order, and the order matters more than any single number. The first line is the substrate: woven glass, woven polyester, or a nonwoven. The second is the coating or laminate: PVC, silicone, polyurethane, or a fluoropolymer film. The third is mass per unit area, given in g/m² or oz/yd², which sets both the acoustic transmission loss and the tensile strength. The fourth is thickness, then weave and yarn count for wovens, then the coating add-on as a percentage of substrate mass.
After the construction block come the performance blocks. Mechanical values are quoted with the test method attached: tensile strength in warp and weft separately, tear strength by trapezoid or tongue method, and abrasion cycles to a defined endpoint. Thermal values are quoted as continuous service temperature and short-term peak, never as a single figure. Chemical resistance is quoted against named fluids, not against "oil" in general. A sheet that omits the test method is not a specification, it is a claim.
The same discipline applies outside engine enclosures. The trade's method for reading a textile datasheet, sampling it and inspecting it on a four-point scale is set out in the technical guide to fabric specification at denim specification and inspection, and the sequence it uses, construction first, then tests with their methods, then tolerances, transfers directly to coated hood fabrics and filter media.
Which tests prove a coated textile for an engine enclosure?
Four test families carry the weight. Tensile strength, usually by strip method on a universal testing machine, gives the load at break in warp and weft; for a hood panel it predicts resistance to handling, wind load and vibration fatigue at the fixing points. Tear strength, by trapezoid or tongue tear, gives propagation resistance once a cut or a bolt hole has started a rip; this is the value that matters at seams and fastener penetrations. Abrasion resistance, by Taber or Martindale, gives cycles to visible coating removal or substrate exposure, and it predicts what happens where the hood rubs against a frame or a lifting sling. Colour fastness, to light, to rubbing and to water, gives the change that a printed or dyed outer surface will show after exposure.
For the intake filter medium, the proving tests are different. Air permeability at a stated pressure drop, burst strength of the pleated pack, and a gravimetric efficiency or fractional efficiency figure at a stated test dust and face velocity. Filter media are also tested for stiffness, because a pleat that collapses under pulsating intake flow loses its area and its pressure drop rises.
Acoustic performance is proved separately, by sound transmission loss measured in a laboratory according to ISO 8528-10 for the enclosure as a whole, not by a fabric coupon. The fabric datasheet only supports that result; it does not replace it.
What does heat and oil do to the fabric of a sound hood?
Heat acts on the coating first. A PVC coating on a glass substrate will plasticise and soften, then embrittle as plasticiser migrates out; the visible signs are a tacky surface, then cracking along fold lines. Silicone coatings tolerate higher continuous temperature but have lower tear strength and poorer oil resistance. The substrate sets the ceiling: woven glass holds its tensile strength to a higher temperature than woven polyester, which hydrolyses and loses strength well below its melting point.
Oil acts on the coating and on the bond between coating and substrate. Hydrocarbon mist from a crankcase breather, fuel spillage during filter changes and lubricant carried in cooling air all reach the hood surface. A coating with poor oil resistance swells, then delaminates, and the panel loses both its acoustic mass and its weather seal. This is why oil resistance is quoted against a named fluid at a named temperature and duration, not as a pass or fail.
Weather adds ultraviolet radiation, ozone, rain and freeze-thaw cycling. UV degrades the polymer chain of the coating and fades pigments; the colour fastness result is the early indicator. Freeze-thaw works on water that has entered an open weave or a damaged coating edge, and the resulting ice expansion opens the damage further. A hood that is specified for continuous 80 °C service with good oil resistance and a light fastness of 6 or better will hold its acoustic and sealing function far longer than one specified on temperature alone.
What happens to intake filter media over a service life?
Glass fibre media hold their pleat shape and their efficiency better than cellulosic media under humid conditions, but they are brittle and lose burst strength if the pleat pack is handled roughly during cleaning. Synthetic media resist moisture and repeated cleaning better, but their efficiency depends on the fibre charge and the oil treatment, and that treatment is what degrades first when the intake air carries oil mist.
Loading is the normal end of life. As dust accumulates, pressure drop rises, and on a turbocharged set the restriction eventually reaches the change-out threshold. A filter that has been over-oiled or exposed to fuel vapour will show a collapsed or distorted pleat pack before the restriction limit is reached, which is a failure of the medium, not of the loading.
How are hood and filter specifications checked before acceptance?
Sampling and inspection follow the same logic as any technical textile. A roll or a cut panel is sampled at a stated frequency, and the inspection records defects by size and by count against a four-point scale, so that a length is accepted or rejected on a number rather than on an opinion. For hood panels the critical defect classes are coating voids, substrate exposure, weave distortion and edge fraying. For filter media they are pleat deformation, media holes and frame bond gaps.
Acceptance testing then confirms the datasheet on the delivered lot: tensile and tear on a cut strip, abrasion on a coated sample, colour fastness on the outer face, and, for filters, pressure drop and efficiency on a sample pack. The point of the exercise is that the hood and the filter are consumable assemblies with a defined service life, and the specification is what makes that life predictable.
What sets the service interval in practice?
The interval is set by the weakest of three clocks: thermal ageing of the coating, chemical attack by oil and fuel, and mechanical wear at fixings and pleat edges. A set that runs at high load in a hot, oily environment will reach the end of its hood and filter life sooner than the same set on standby duty in a clean, temperate location. Reading the datasheet against the actual duty, rather than against a generic rating, is what turns a specification into a maintenance plan.
Sources
ASTM D5034, breaking strength of textile fabrics
Read next
The pressure drop that ends a filter's life is covered from the air side in how a buried intake adds pressure loss.
The rest of the enclosure case is in the generator set section.