Engine scale and its prevention

The raising of the question

When the engine is running, the gas temperature inside the cylinder can reach as high as 2200 to 2800K, and the average temperature during the working cycle is also 970 to 1300K. Components that come into contact with high-temperature gases (such as cylinder heads, cylinder liners, pistons, valves, etc.) will be subjected to intense heat. The function of adding cooling water in the cooling system is to promptly dissipate the heat absorbed by the parts, so as to keep their temperatures within the normal working range, ensuring the power performance and economy of the engine and extending its service life. For most water-cooled engines, the most suitable cooling water temperature under stable working conditions should be maintained at 80 to 90 degrees Celsius.

During the operation of construction machinery, due to the limitations of the on-site environment, the replenishment of cooling water is often not timely or the added water is usually hard water containing calcium, magnesium ions, etc., which causes a large amount of scale to form in the cooling system after the engine has been working for a period of time.

To determine whether the circulating water channel of the engine cooling system is clogged with scale, simply open the radiator water filling port cap. If you find that the upper water chamber is dirty or there is scale adhering at the water filling port, and if the machine tends to boil over when going uphill, cools down slowly when going downhill or stopping, it can be diagnosed that there is a considerable amount of scale deposited in the cooling system.

Analysis of the Consequences Caused by Scale

The scale in the cooling system is generally grayish-white substances such as calcium carbonate and calcium sulfate that accumulate on the metal surface. Scale causes a layer of insulating material to coat the inner surface of the metal in the cooling system, deteriorating the heat dissipation conditions and reducing the heat transfer efficiency of the components (the heat conduction capacity of scale is almost 100 times lower than that of brass of the same thickness and about 30 times lower than that of cast iron). As a result, when the components of the cooling system are normal, it often leads to excessively high water temperature, causing insufficient cooling of the engine It can cause the following faults: a decrease in the inflation coefficient, abnormal combustion of the mixture, knocking, spontaneous combustion or deflagration, resulting in a reduction in engine power. According to the data, when the cooling water temperature rises from 90℃ to 120℃, the power decreases by 5%. When the thickness of scale on the water jacket wall reaches 2mm(accounting for 6% of the total capacity of the cooling system), the inflation coefficient decreases by 0.05 and the power drops by approximately 2kw. The thermal stress on heated parts such as the cylinder head, valves, cylinder liners, pistons and piston rings increases, causing deformation of the valve seat and burning of the valves, which leads to thermal cracking at the nose bridge of the cylinder head. This is the reason why one of the three loaders in our company has a crack in the cylinder head every half a year. The lubrication conditions of various components deteriorate, destroying the fit clearances between parts, accelerating wear or causing jamming due to thermal expansion, and damaging the components (such as common piston ring cylinder pulling, piston skirt sintering and seizing, etc.). Burn out the rubber rings of the water and oil channels on the cylinder head and the waterproof rubber rings of the cylinder liner. This causes the engine oil to be exposed to high temperatures, accelerating oxidation and deteriorating prematurely, resulting in poor lubrication and an increased risk of bearing bush burning. The waterway Narrows to the point where the passage is blocked, causing the local temperature to rise. The radiator core is damaged, the cooling intensity is reduced, and the engine working conditions deteriorate.

Analysis of the Causes of scale formation

There are two causes of scale formation in the cooling system: physical changes and chemical changes. The physical change mainly refers to the fact that as the engine temperature rises, a large amount of minerals in hard water, such as calcium sulfate, precipitate in a supersaturated state and adhere to the inner walls of the cooling system to form scale. Chemical changes mainly refer to the chemical reactions that occur between certain mineral salts and impurities in hard water as the temperature rises, forming insoluble substances that deposit on the inner walls of the cooling system. The chemical reaction that forms scale is

Ca(HCO3)2=CaCO3 ↓+CO2↑+H2O

Mg(HCO3)2=MgCO3 ↓+CO2↑+H2O

Magnesium carbonate, after being slightly soluble in water, can continue to react with water to form less soluble magnesium hydroxide.

MgCO3+H2O=Mg(OH)2 ↓+CO2↑(This reaction is rare)

4 Countermeasures for Preventing scale

4.1 Methods for preventing scale formation

The root cause of scale formation in the engine cooling system lies in water quality issues. Therefore, reducing mineral metal ions in the water source and softening the water quality are the key to preventing and alleviating scale formation.

Boiling method. After hard water is boiled, the minerals in it will deposit. Taking the clean water on top and using it as cooling water to be added to the cooling system can reduce the amount of scale formed. This method is simple, but it cannot completely soften the water.

(2) Magnetization method. Put some small pieces of magnets into the engine's cooling system. The magnetic field generated by the magnets has a magnetizing effect on the cooling water. Magnetized water can reduce the formation of scale and gradually make the existing scale disappear. The construction machinery of China State Construction Engineering Corporation uses this method and the effect is very good.

(3) The method of adding softeners. The substances used by Longyan Boiler Inspection Institute to soften boiler water are sodium bicarbonate (baking soda) or sodium hydroxide (caustic soda). The softener is added at a ratio of 0.5 to 1.5g of sodium bicarbonate per liter of hard water or 0.6 to 0.7g of sodium hydroxide per liter of water. The ionic equation is

Sodium bicarbonate method

HCO3-+H2O=H 2CO3+OH-

Ca2++2OH-=Ca(OH)2

Mg+2OH-=Mg(OH)2↓

Sodium hydroxide method

Ca2++2OH-=Ca(OH)2

Mg+2OH-=Mg(OH)2↓

(4) Use the preservative (potassium dichromate K2Cr2O 7) method. Adding potassium chromate frequently to the coolant can not only prevent water corrosion but also reduce the formation of scale. In addition, if Cummins engines stipulate the use of DCA coolant and Caterpillar engines stipulate the use of coolant with added rust inhibitors and antifreeze, both can prevent cavitation, rust, scale formation and freezing.

4.2 Methods for Cleaning Scale

(1) For an engine in use, before cleaning the scale, remove the thermostat first. Then, press water into the cooling system from the opposite direction of the normal water circulation (i.e., from the outlet pipe) until the water discharged is clean. This method is extremely simple, but it is very difficult to clean the scale thoroughly.

(2) Given the complex shape of the engine water jacket, it is generally not advisable to remove scale by mechanical means. Only chemical methods can be used for scale removal. Calcium carbonate and calcium sulfate in scale can be eliminated through chemical reactions.

① A mixed cleaning method of soda (Na2CO3) and water glass (Na2SiO3). Add 40g of soda and 10g of water glass to each liter of water to prepare the cleaning solution. Then, pour it into the cooling system. Run the engine until the water outlet temperature is above 60℃. After two hours, stop the machine and drain the cleaning solution. After the engine cools down, clean it twice with clean water, drain it completely, and then fill in cooling water. Let the engine run until the outlet water temperature reaches above 75℃. Stop the machine, drain the dirty water, and finally fill in new cooling water for use.

② Trisodium phosphate cleaning method. First, remove the thermostat. Add the pre-prepared trisodium phosphate solution to the radiator (at a ratio of 50 to 80ml of 10% trisodium phosphate solution per liter of cooling water), then start the engine and let it run for about ten minutes before shutting down and not using it. Drain the coolant after one night. Add the same amount of trisodium phosphate solution (follow the same procedure as before), repeat this several times, and finally drain the cooling system solution and rinse twice with clean water. The scale can then be removed. Its chemical reaction is

3CaCO3+2Na3PO4=Ca 3(PO4)2↓+3Na2CO3

3CaSO4+2Na3PO4=Ca 3(PO4)2↓+3Na2SO4

3Mg(OH)2+2Na3PO4 =Mg3(PO4)2↓+6NaOH

Among the above products, sodium salts are highly soluble in water, calcium phosphate is loose and easy to wash away, while magnesium phosphate is like sludge and is extremely easy to impact. This method will not cause corrosion to the cylinder block, so it is suitable for all engines and has a relatively ideal cleaning effect.

(3) Phosphoric acid cleaning method. Add 100g of phosphoric acid and 50g of chromic anhydride to each liter of water, stir and then inject it into the water tank. Let it soak for 1 hour, then start the engine and run it for 20 minutes. Drain the cleaning solution and rinse it several times with clean water.

(4) Sodium silicate cleaning method. Prepare a solution by adding 15g of sodium silicate (commonly known as water glass) and 2g of laundry detergent to each liter of water. Then add it to the cooling system and let the engine run normally for about 1 hour. Finally, drain the cleaning solution and rinse it several times with clean water.

(5) Hydrochloric acid cleaning method. Remove the thermostat, prepare a 5%-8% hydrochloric acid solution with the cooling water, then add 3g/L-4g/L corrosion inhibitor (urotopine), heat to 60℃ and add it to the cooling system. Start the engine and let it idle for about 1 hour. After draining the cleaning solution, rinse it with clean water 3-4 times. Its chemical reaction is

CaSO4+2HC1=CaCl2+H2 SO4

CaCO3+2HC1=CaC12+CO2 ↑+H2O

Mg(OH)2+2HC1=MgC12+ 2H2O (This reaction is extremely rare)

Hydrochloric acid itself has the ability to remove oil stains and can clean the cooling system very thoroughly. Adding a certain amount of corrosion inhibitor urotropine enables it to adsorb on the metal surface to form a film, protecting the metal from corrosion and decomposing rust, thus playing a role in removing rust.

(6) Sodium hydroxide cleaning method. The cleaning time can be prepared by mixing 10L of water with 750g of caustic soda and 150g of kerosene. Pour it into the water tank and let the engine run for 8 to 10 hours, then drain it completely. Add clean water and run for 20 minutes. Generally, the scale can be removed in this way. Its chemical reaction is

CaCO3+2NaOH=Ca(OH)2+Na 2CO3

CaSO4+2NaOH=Ca(OH)2+Na 2SO4

Among the above products, sodium carbonate and sodium sulfate are readily soluble in water, while calcium hydroxide is turbid but does not precipitate and can be rinsed away with clean water.

(7) Alternating cleaning method with hydrochloric acid and sodium hydroxide. First, mix 8% hydrochloric acid evenly with water and then add it to the water tank. Start the engine, keep the temperature at 70℃, and run for 40 minutes before discharging the cleaning substances. Close the water valve, add 8% caustic soda solution, restart the engine and let it run for 40 minutes before discharging all the cleaning substances. Finally, rinse with clean water until the water discharged becomes clear. Then remove the water jacket baffle of the cylinder to clear the accumulated dirt.

(8) Sodium carbonate cleaning method. Prepare a solution by adding 1kg of soda ash and 0.5kg of kerosene to 10 liters of water. Soak it in the cooling system for about 10 hours, let the engine idle for 0.5 hours until the solution boils. Then release it and rinse it 3 to 4 times with clean water.

(9) Clean with descaling agent. Add a certain amount of descaling agent according to different vehicle models, start the engine and let it run for 8 hours, then drain the solution. Rinse repeatedly with clean water until the cooling water is no longer slippery to the touch, and finally replace it with fresh water.

When using the above cleaning method, it should be noted that when removing scale from the engine, the selected solution must be suitable for the nature of the scale and must not cause corrosion to the cylinder block. Therefore, the aluminum alloy cylinder block and cylinder head cannot be cleaned with hydrochloric acid, sodium carbonate or sodium hydroxide to avoid surface corrosion. The cleaning method of trisodium phosphate, phosphoric acid and sodium silicate should be adopted. It is best to clean the scale in the water jacket and radiator separately to prevent rust or scale in the water jacket of the cylinder from blocking the radiator water pipes and cores. Therefore, when cleaning the scale without disassembling the engine, always pay attention to the changes in the water temperature gauge on the instrument dial. If the water temperature exceeds 95℃, stop the cleaning, check whether the radiator is blocked, and take some remedial measures. When a loader undergoes major overhauls, it is essential not to neglect the cleaning of scale. The disassembled engine cooling system parts should be placed in a boiling pool for high-temperature boiling or removed by spraying with a cleaning machine. Chemical methods can also be used, that is, placing the radiator in a relatively dilute sodium hydroxide solution (with a concentration of 2% to 3%). The cast iron cylinder block and cylinder head are soaked in a relatively concentrated sodium hydroxide solution (15% concentration, temperature 60-70 ℃) for one night to remove scale. Aluminum alloy cylinder blocks and cylinder heads should be cleaned in a cleaning solution of trisodium phosphate, sodium silicate or phosphoric acid. Generally, after three months of use or a cumulative operation of 500 hours, the scale in the cooling system of an engine should be cleaned once.