Clean or Change Air Filters For Top Engine Performance
The preventive maintenance program for April usually combines the monthly, three and six-month inspections, including maintenance to renew equipment and get it in shape for the coming hot weather. We have considered the engine oil filter change as one of the most important maintenance items, but it is not the only filter requiring our attention on this inspection. Other filters are used on the crankcase breather, brake power cylinder (hydraulic brakes) or air compressor (air brakes), fuel line and engine air intake. And of them all, the most important is the engine air filter!
An air filter was a relative rarity on automotive engines in 1930, but is a vital arid essential unit on the modern internal combustion engine and must be inspected and maintained if the engine is to render satisfactory service. We are prone to think of fuel for the engine as gasoline or diesel oil. Air is also necessary for combustion. For every gallon of liquid fuel consumed by an engine, approximately 12,500 gallons, or 1,500 cubic feet, of air is required. To permit the flow of this volume of air, the air cleaner must be properly maintained to avoid a restrictive buildup or plugging of the filter element.
The basic function of an air filter is to exclude dirt in various forms from entering the engine. An air filter that is not sealed tightly at all connections against the entrance of unfiltered air, or has holes in the filtering element, permits dirt particles to pass, into the engine, cause rapid wear of cylinders, rings and bearings and contaminate the oil. Some of these dirt particles are silica, sharp and harder than steel, which easily scores cylinders and bearings.
Two basic types: Engine air filters are provided in two basic types, wet or dry. The dry-type filter is made in several designs. The simplest is a metal cylinder with vanes to provide a centrifugal action with a rapid rotation of the air vortex which centrifuges the dirt particles by discharging them to outlet louvers which bleed off some of the incoming air to exhaust the dirt particles.
The second type of dry air cleaner employs a paper filter element which permits the air to pass through, but largely filters out the dirt particles. The third type is a combination of the first two. The centrifuging action disposes of most of the dirt and the paper element collects and screens the finer particles. None of the air cleaners we have today is 100 percent efficient, but there are some that come very close.
Regardless of the dry-type filter design on your departmental vehicles, make sure the six-month inspection includes a thorough checking of the filter element. Hold it against a strong light to be sure there are no pinholes. Also, if the light pattern is not uniform, but shows dark spots or areas, replace the element. Some elements can be cleaned by using a non-sudsing household detergent and thoroughly drying. Your manufacturer’s manual should give the correct procedure.
Particles form mat: Knocking off the dust from the element does not make the element clean. Small fibrous particles floating in the air form a mat on the surface of the element that restricts the air flow.
Some filters are very efficient and the porosity-permitting or particle-limiting size is measured in microns. A micron equals one, one thousandth of a millimeter or 3.937 hundred thousandths of an inch, (.00003937 inch). Yes, this is filtering very fine. Actually, none gets to only one micron; 40 microns, about 1/20th the thickness of a human hair, is the smallest filter opening.
With this fine filtering, the pores or openings do become clogged, and as the degree of clogging increases, there is a decrease in air entering the engine. This restriction changes the air to fuel ratio and results in an enriched fuel mixture. This not only produces objectionable black smoking of the exhaust, but produces fouling of spark plugs in a gasoline engine and fouled injector nozzles in a diesel engine. The excess fuel is not all burned but dilutes the engine oil, cutting lubrication on the cylinder walls, causing sticking of piston rings and inlet valves and increasing engine bearing wear. Fuel consumption increases and what is worse, engine power decreases because the engine is starving for air.
Checking air restriction: If it is desired to check for excessive air restriction, there are three instruments that provide the information. One measuring device is a vacuum-operated switch connected to a red light. When the restriction exceeds the pre-set maximum, 20 inches of water or 1 1/2 inches of mercury for naturally aspirated engines (or 25 inches of water or 1.8 inches of mercury for engines with blowers or superchargers) the red light shows to indicate that filter maintenance check is required.
A second device is also vacuum-operated, and it indicates the degree of restriction at any time by the proportion of red color showing on a card. When the card is all red, the filter must be serviced.
The third device is a simple U tube type manometer that will indicate the height differential of two columns of water, with a scale graduated in inches for a height of 30 inches. If mercury is to be used instead of water, the height differential would be 1⅛ inches.
Any of these devices must be connected to a fitting at the filter outlet.
The wet type of air cleaner is provided in both medium and heavy duty designs. In either, it is important to check and maintain the oil level at the proper height. In the fire service, this check should be made at least once each month to prevent “pull-over” caused by periods of high humidity producing condensation in the air cleaner, raising the oil level too high. This “pull-over” or spill will cause clogging of the lower screen and filter, restricting the air supply to the engine.
The screen and filter should be thoroughly cleaned with kerosene or some other suitable solvent. The screen should show a uniform light pattern when held up to a strong light. The lower pan should be carefully examined to be sure it is really clean. It may require soaking in a solvent for several hours to remove the hardened sludge.
Clean the inlet and outlet tubes with a lint-free cloth before reassembly of the cleaner. Check all the gaskets to be sure they are providing an airtight joint. All tubing connections should be wiped clean and the clamps tightened to insure against air leaks.
Never, but never, use oil drained from the crankcase in the air filter even if it does “look” all right.