Penyebab berubahnya kekentalan oli mesin dan resiko yang bisa ditimbulkannya

Oli mesin itu bisa berubah sendiri kekentalan atau viskositasnya selama masa pemakaian. Perubahan ini tidak sama dengan perubahan kekentalan karena suhu. Perubahan kekentalan karena suhu merupakan sesuatu yang diterima. Di oli yang baru pun kekentalannya berubah sesuai dengan suhu. Bahkan di spesifikasi juga biasanya disebutkan berapa kekentalan dinamis pada suhu 40 derajat celcius dan 100 derajat celcius. Kekentalan pada suhu 100 derajat celcius selalu lebih encer daripada pada suhu 40 derajat celcius. Berikut contohnya pada Pertamina Fastron Techno:

 

Yang menjadi perhatian adalah bagaimana kekentalan bisa berubah selama masa pakai. Ini dijelaskan rinci pada sumber berikut:
Viscosity: A lubricant’s most important characteristic

Why an Oil’s Viscosity Drops

A decrease in viscosity may also occur when non-lubricants like solvents and diesel fuel accidently get into the lubricant. If this happens, it is a good idea to change the oil.

Another way your lubricant could be losing its viscosity is through the loss or shear down of the viscosity-index (VI) improver. For example, if you are using a multi-grade SAE gear or engine oil such as a 10W-30, this oil contains an additive known as a viscosity-index improver. During use, the VI improvers can sheer down and break apart, causing the viscosity of the oil to decrease. Remember, exposure to high heat is the biggest factor in causing the sheer of the viscosity-index improver.

Multi-grade oils aren’t the only ones that can thin due to high heat. Oils operating at extreme high temperatures can begin to crack thermally. The high temperatures can sheer/crack the oil molecules into smaller molecules, which causes a decrease in viscosity. If you are having trouble with a mineral oil losing viscosity at high temperatures, look at switching to a synthetic oil for the application.

Disebutkan bahwa oli bisa berkurang kekentalannya karena:
– Pemecahan karena panas pada molekul oli. Panas yang berlebihan dapan membuat oli terurai/bereaksi karena panas, membuat oli molekulnya menjadi lebih kecil sehingga lebih encer.
– Penguraian VI improver, terutama terjadi pada oli multi grade, dimana aditif penambah viscosity index terurai/bereaksi karena panas.
– tercampur bahan bakar. Oli bisa menjadi lebih encer karena bensin atau solar masuk ke ruang mesin (bisa karena bocor dari ruang bakar).
– tercampur dengan oli yang lebih cair

Lubricant Oxidation Analysis and Control

The most important chemical reaction of a hydrocarbon, for a lubrication engineer or analyst, is oxidation. Oxidation of hydrocarbons is commonly referred to as combustion or “burning.” When one burns paper, wood, natural gas or fuel oil, for example, hydrocarbons are oxidized. In a well-designed burner, propane gas undergoes complete combustion. This means that all carbons are completely oxidized; all hydrogens bonded to carbons are replaced by oxygens, therefore producing CO2 and H2O. The reaction is illustrated in Figure 3 where five oxygen molecules (O2) completely oxidize the illustrated hydrocarbon to produce three carbon dioxide molecules and four water molecules.

Complete combustion requires relatively high temperatures, a pure fuel source and an ample supply of oxygen. Most combustion reactions found in nature, however, are not complete, and result in various other products in addition to CO2 and H2O, as illustrated above. Incomplete combustion of the hydrocarbon chain of a lubricant can produce carboxylic acid (Figure 4) and other impurities.

As previously discussed, organic acids are produced during oxidation. These acids are detectable as an increase in the TAN number which quantifies acid concentration by measuring the volume of an alkaline (potassium hydroxide) reagent that is required to neutralize the acid in the oil. The TAN test doesn’t discriminate acids generated by oxidation from those that are ingested as contaminants from the process. Also, some additives like anti-wear, extreme pressure and some rust inhibitors, are acidic. They produce a high initial TAN that can diminish as the additive is depleted.

Oxidation – The Lubricant’s Nemesis

Heat is often employed to accelerate the oxidation process because temperature has two effects on any reaction. The first effect involves activation energy. If the system does not contain enough energy to push the reaction over the threshold, nothing will happen. The second effect is related to the speed of the reaction. A reaction (oxidation) will approximately double in rate for every 10°C (18°F) increase in temperature. Which means that the oil life will be reduced by one-half for every 10°C (18°F) increase in temperature.

Efek bila oli terlalu kental atau terlalu encer:
Oil Viscosity – How It’s Measured and Reported

A significant reduction in viscosity can result in:
Loss of oil film causing excessive wear
Increased mechanical friction causing excessive energy consumption n Heat generation due to mechanical friction n Internal or external leakage
Increased sensitivity to particle con- tamination due to reduced oil film
Oil film failure at high temperatures, high loads or during start-ups or coast-downs.

Likewise, too high a viscosity can cause:
Excessive heat generation resulting in oil oxidation, sludge and varnish build-up
Gaseous cavitation due to inadequate oil flow to pumps and bearings
Lubrication starvation due to inadequate oil flow
Oil whip in journal bearings
Excess energy consumption to over- come fluid friction
Poor air detrainment or demulsibility
Poor cold-start pumpability.

Yang bisa menyebabkan perubahan viscosity adalah:

Oli bertambah kekentalannya karena:
– oksidasi. Oksidasi terjadi terutama karena panas, dimana setiap kenaikan 10 derajat celcius akan membuat kecepatan proses oksidasi meningkat 2 kali lipat. Oksidasi juga membuat oli menjadi lebih asam.
– kemasukan air. Air juga merupakan salah satu proses pembakaran. Penguapan bensin dalam tangki pun bisa menghasilkan uap air.
– pembentukan formasi karbon atau oksida yang tidak bisa larut
– pembetukan jelaga / abu
– kemasukan antifreeze (cairan coolant radiator yang mengandung glycol)
– pencampuran dengan oli yang lebih kental

 

Berkurangnya kekentalan mesin menyebabkan:
– berkurangnya lapisan pelapis oli antar logam yang memperparah keausan mesin
– meningkatnya friksi atau gesekan sehingga mengurangi tenaga dan menghasilkan panas
– meningkatkan kemungkinan terkontaminasi partikel
– kegagalan perlindungan mesin pada kondisi temperatur tinggi atau beban berat pada saat start atau lepas gas.

Bertambahnya kekentalan mesin menyebabkan:
– pemanasan mesin yang berlebihan yang menyebabkan oksidasi dan kerak
– pembentukan ruang udara karena oli tidak mampu mengalir dengan sempurna di pompa dan bearing.
– ketidakcukupan pelumasan karena oli tidak bisa mengalir dengan baik
– loncatan oli pada bearing
– pengurangan tenaga yang berlebihan karena harus mengatasi hambatan oli
– memperparah kemungkinan pelepasan udara / air yang terperangkap
– memperberat kerja pompa oli saat mesin masih dingin.

Semoga berguna.