1. Concept of suitable filtration efficiency
At present, the filtration efficiency of filters in various countries refers to the percentage of the amount of dust removed by the air filter per unit time compared to the amount of dust entering the air filter with air. This traditional concept uses the weight method to measure filtration efficiency. Essentially, the size and quantity of particles entering the engine directly affect its lifespan. Small particles of dust entering the engine will not cause engine damage, and only when the dust particles reach a certain size will it cause engine damage.
Therefore, it is necessary to change the traditional concept of filtration efficiency and introduce a new concept, which is suitable for filtration efficiency. The so-called suitable filtration efficiency is the lowest filtration efficiency that ensures the reliable operation of the engine. The suitable filtration efficiency adopts the ratio of the diameter and quantity of dust particles filtered by the air filter to the diameter and quantity of dust particles entering the air filter with air. The appropriate filtration efficiency unifies the filtration capacity of the air filter with the degree of engine protection. The air filter can specifically control large dust particles that affect the service life of the engine, without filtering out small dust particles. This is beneficial for reducing the intake resistance of the air filter, reducing its volume, and reducing costs.

2. Engine requirements for air filters
In order to fully tap into the potential of engines, engine manufacturers have put forward many requirements for air filters, including intake resistance, filtration efficiency, service life, maintenance, size, weight, and cost. No matter how applied an air filter is, it is inevitable to have a series of compromises and concessions, but the core is filtration efficiency. Engine manufacturers often have high requirements for air filters, which filter manufacturers find difficult to meet. According to the concept of appropriate filtration efficiency, the current urgent task is to determine the requirements of different engines for the appropriate filter efficiency value of the air filter, that is, how many dust particles and quantities can provide reliable protection for the engine. 

3. Wear and particles of internal combustion engines
The wear and tear of internal combustion engines is a very complex problem. The intake system, lubrication system, and fuel system of the internal combustion engine are respectively equipped with air filters, oil filters, and fuel filters. The various dust particles and combustion products that are not filtered out by the air filter mainly cause wear on the air ring and upper part of the cylinder. The metal debris that has not been filtered out by the oil filter, the large amount of dust particles in the air entering through the crankcase vent, and the debris entering the oil pan from the combustion chamber mainly promote the wear of the oil ring and the lower part of the cylinder.