Air filter maintenance on agricultural machinery

Correct maintenance of the air filter significantly extends the engine's useful life and periodic changes are essential for the proper functioning of the engine and consequent reduction in fuel consumption.

Three factors are necessary for good combustion: fuel, oxygen and heat. The fuel must have quality in accordance with the standards of the National Petroleum Agency (ANP). Oxygen is supplied by air and heat is needed to speed up the chemical reactions of combustion.

In Brazil, tractors are powered by four-stroke diesel engines. In the first stroke of the engine called intake, the piston leaves top dead center (position where the piston reverses direction) and goes to bottom dead center, creating a depression inside the cylinder. With the intake valve open, air is drawn into the cylinder.

When the air goes into the cylinder, it carries with it dust particles that damage the engine's moving parts. These particles adhere to the cylinder wall, pass through the rings and go into the crankcase. In addition to having an abrasive effect and prematurely wearing out piston liners and rings, these particles also contaminate the lubricating oil.

Lubricating oil is the element that most affects the useful life of the engine, in addition to interfering with its proper functioning. When the oil is contaminated by dust particles (silica), these affect its characteristics such as viscosity and the action of promoting internal cleaning of the engine, favoring the formation of sludge which compromises good lubrication and consequently increases the wear of moving parts and the fuel consumption.

The role of the air filter is precisely to remove these particles and provide the engine with clean air. There are several models of air filter, but the most used currently is the dry element.

OLD FILTERS

Older tractors still use the air filter called “oil bath”. In this type of air filter, there is a compartment to store the oil and a steel wool filter element. When the air is sucked into the cylinder, it first passes through a cyclone called a pre-filter where coarse dirt such as sand and straw is eliminated due to the action of centrifugal force. Then, the air forces the oil to soak into the steel wool through which the air circulates and the dust is trapped due to the adhesion of the oil.

For this type of air filter, daily cleaning is recommended in very dusty conditions. When cleaning, it is necessary to change the oil (it is not recommended to use burnt oil) and wash the steel wool with some solvent. This type of filter element fell into disuse due to its low efficiency (it does not adequately remove dust) in addition to requiring a lot of labor to clean.

PAPER FILTERS

With the evolution of tractors, oil bath air filtration was replaced by the dry element. The dry element filter consists of an accordion filter paper cylinder and an internal felt filter. The porosity of the filter paper retains dust particles more efficiently and is much easier to clean. Only the paper filter can be cleaned, the internal felt filter needs to be replaced after a certain period of use.

Tractors that use an air filter with a dry element normally have an indicator to show that the filter is saturated and it is time to clean it. The recommendation is to clean the filter only when the indicator shows that it is time to clean.

Some people, out of excess of zeal, clean the filter daily, but this is not recommended. The efficiency of the filter element increases with a little dirt. In other words: the filter should only be cleaned when the indicator shows it.

To clean the filter, with a dry element, it is recommended to use compressed air. The air jet must be directed from the inside out and at a certain distance to prevent the paper from tearing. After cleaning, it was recommended to go to a dark place and place a lamp inside the filter element to observe possible fissures and cracks in the paper. If these cracks are observed, the element must be replaced. But this test does not reveal microscopic flaws that could allow fine dust to pass through and consequently compromise the useful life and proper functioning of the engine.

EXCHANGE AT THE RIGHT TIME

Currently, all tractors leave the factory equipped with an air filter with a dry element and the recommendation is that the element be changed when the indicator shows it. A test that should be carried out periodically is to place a piece of board or cardboard at the inlet of the filter mouth and slightly accelerate the engine. If the filter indicator does not activate, this indicates that the indicator activation system is not working correctly.

In turbocharged engines, in addition to the suction provided by the cylinder, there is also a compressor that helps air enter the combustion chamber. In this case, after the air passes through the filtration system, the turbo compressor (which is a fan) increases the pressure when the air is directed into the cylinder.

There is a misconception that the gas that comes out through the discharge tube returns to be burned again. What happens is the following: after burning the fuel, in the fourth engine stroke (exhaust) the burnt gases are eliminated through the exhaust pipe. These gases have kinetic energy (speed and pressure) that move a turbine connected to a compressor that drives clean air into the cylinder, in other words: the burned gases only move the turbine but do not return to the combustion chamber.

As this turbine works at high speeds, what is very important when starting in the morning (cold engine) is to wait approximately 15 seconds before starting the engine so that the lubricating oil can act on the moving parts of the turbine. .

Another item that equips some Diesel engines, mainly in trucks but is also used in high-power tractors, is the intercooler or turbocooler. This component cools the air before it enters the cylinder.

The air, before entering the cylinder, passes through the air filter, the turbine and then a cold radiator. When passing through the radiator, the air cools and consequently reduces its volume, that is: the quantity (mass) of air that enters the cylinder is greater due to the fact that it is cold, therefore the fuel burns more completely, increasing engine power.

In conclusion, the air cleaning and filtration system, as well as the air passage restriction indicator, must be in perfect working order so that the engine can work properly for a long period.

Roberto da Cunha Mello, IAC

Article published in issue 157 of Cultivar Máquinas.