To prevent damages, the air that reaches the engine must first be cleaned of tiny particles of dust. In turn, the air that is supposed to reach the passenger compartment is cleaned of both solid particulates and harmful gaseous substances. In both cases the air filter plays a key role.
This is the case because filtering is affected by other components, such as the filter housing filter or its location. It is important to be aware about how much work the air filter has to do in our cars. As we all now, atmospheric air is polluted by particles that float from roads and by industrial dusts. However, it might be a surprise for you to hear that the highest intensity of dustiness is located at the height of about 0.5 m, which is where there air intake holes in cars.
On average, dust content in the air can range from 0.2 to 50 mg/m3. Depending on road conditions, it might be more. If we assume that for 1 kg of fuel is required 14 kg of air (this value might vary depending on used fuel, type of injection, etc.), and the car rides 20 000 km per year, with an average fuel consumption of 8 l / 100 km, in one year up to 6.2 kg of dust can pass through the engine!
Thus, improper filtration leads to dust getting into the engine and into oil, which causes wear of piston rings, cylinder parts, turbocharger bearings, etc. Currently, air filtration systems consist of a tube with air inlet leading to the filter housing. Behind the filter there is a tube for purified air and a flow meter.
It can be determined in several ways how effective a filter is. The measure of effectiveness might be degree of separation, which refers to the percentage of separated particles. One can also determine the initial degree of separation which refers to new filtering layer. Filters are evaluated by subjecting them to standardized tests that use specific measurement methods and a separation curve is determined on their basis.
Air filters with housing
Filters of high quality in passenger cars are capable of separating 99.8% of impurities, while in commercial vehicles this value reaches up to 99.95%. Paper, polyester or fabric materials are used for filtrating materials. Sometimes it is possible to find an oil-soaked fabric that has a high dust absorption capacity of about 1000 g m3, whereas in the case of paper it is only about 200 g m3. These materials are characterized by thickness, air permeability, fiber thickness and resistance to penetration.
Air filters without housing
It is also important to optimize the placement of filtering material, thanks to which a large filtering surface is provided. Correct arrangement ensures proper support and better resistance to gluing. In order to make filter life longer, one can use fabric with a varying fiber density. Gradient design of the filter is based on the use of a tighter fabric on the side of purified air. Thus varied tightness allows for gradual and selective separation of particles. It is worth noting that gradient fibers, compared to standard cellulose-based paper equivalents, are about 50% better when we speak about blocking impurities.
Air filters are usually replaced during periodical inspection, which usually takes place after car rides 15 to 30 000 km. Still, filling of the filter can occur earlier and depends not only on operating conditions but also on quality of the filter. In cars which are used in an intense manner, the filter should be replaced every 20 000 km. If mileage is relatively small, it is recommended to replace the filter every 2-3 years as it might start to crumble after that time.
Of course, the basic task of the filter is cleaning the air independently of operating conditions. That is why it must be resistant to large temperature fluctuations, humidity or engine oil. Design of the filter is a kind of compromise between its capacity to retain impurities and its throughput. Good filter should also reduce the suction noise.
When designing a filter, there are a number of factors which are specific for particular car model. Need for air is different in spark-ignition engine, diesel engine and that supercharged with compressor. Location of the filter and the air velocity in this location are also important. In order for the air to be properly cleaned, velocity of its flow through the filter must not be greater than the critical speed specified for each material of the filter.