High-temperature detergency is a core indicator for evaluating the ability of engine oil to inhibit the formation of sludge and carbon deposits under high-temperature and high-pressure environments, especially in high-temperature working conditions such as turbocharged and direct-injection engines. Synthetic gasoline engine oil and ester synthetic engine oil show significant advantages and disadvantages in high-temperature detergency due to differences in base oil structure and additive system.
Synthetic gasoline engine oil uses poly-α-olefin (PAO), alkyl naphthalene (AN) and other base oils. Its molecular chain is regular and has high saturation, carbon-carbon bond stability is strong, and its resistance to thermal oxidation decomposition is significantly better than ester base oil. Ester synthetic engine oil is mainly diester and polyester. The polarity of the ester group (RCOOR') makes it highly adsorbable to metal surfaces, but the ester bond is easily hydrolyzed to generate acidic substances at high temperatures, which may aggravate sludge formation. For example, under working conditions above 200°C, the ester bond breakage rate of ester engine oil can reach 15%-20%, resulting in the breakage of the base oil molecular chain, while the molecular structure of PAO base oil is stable and the breakage rate is less than 5%. This difference directly affects the stability of the detergency performance of the engine oil at high temperatures.
High-temperature oxidation is the main cause of sludge formation. Synthetic gasoline engine oil can maintain oxidation resistance for more than 50 hours at 250°C (ASTM D5483 test) by adding phenolic and amine antioxidants. Its antioxidants form a synergistic protective film with the base oil, effectively inhibiting free radical chain reactions. However, due to the catalytic oxidation of ester groups, the antioxidant life of ester engine oil is shortened by 30%-40%. Especially in an environment containing metal ions such as copper and iron, the hydrolysis of ester bonds is accelerated to generate by-products such as carboxylic acids. These acidic substances further promote sludge formation. For example, in a bench test at 220°C, the acid value of an ester engine oil increased by 2.5mgKOH/g after 100 hours, while the synthetic gasoline engine oil only increased by 0.8mgKOH/g, indicating that it has better oxidation resistance.
Ash content is a key factor affecting high-temperature detergency. The polar groups of ester engine oil are easily complexed with metal ions to form hard deposits, especially in high-temperature parts such as piston rings and turbocharger blades. Synthetic gasoline engine oil can effectively reduce deposit formation through low ash content formula (such as calcium and magnesium content <0.8%) and dispersant technology. Data shows that the amount of deposits on turbocharger blades of ester oil is 40%-60% higher than that of synthetic gasoline engine oil. Long-term use may lead to reduced turbine efficiency, while synthetic gasoline engine oil has significantly stronger deposit control ability.
The additive system of synthetic gasoline engine oil (such as detergent and dispersant) is more compatible with the base oil, which can form a dense protective film and effectively inhibit the formation of sludge. The strong polarity of ester oil may interfere with the action of additives, such as reducing the suspension ability of ashless dispersants, resulting in the aggregation of small particles of sludge. A laboratory test showed that the suspension efficiency of dispersants in ester oil for soot is only 60% of that of synthetic gasoline engine oil. This difference directly affects the cleaning performance of the oil at high temperatures.
In the NEDC cycle test, the TBN (total base number) of ester engine oil decays 20%-30% faster than that of synthetic gasoline engine oil, resulting in a decrease in acid neutralization ability and aggravated sludge formation. In the 150℃ high temperature shear stability test, the viscosity loss rate of synthetic gasoline engine oil is <10%, while that of ester engine oil can reach 15%-20%, affecting the integrity of the oil film. For example, under long-term high temperature conditions, the oil film thickness of ester engine oil decays 30% faster than that of synthetic gasoline engine oil, resulting in increased wear of engine components.
The biodegradability of ester engine oil is better than that of synthetic gasoline engine oil (such as the degradation rate of bio-ester engine oil>60%), but some high temperature performance needs to be sacrificed. Synthetic gasoline engine oil meets the National VI emission standards through a low SAPS (sulfate ash, phosphorus, sulfur) formula, while the high ash content of ester engine oil may clog the particulate filter (GPF) and requires a dedicated after-treatment system. For example, in the GPF plugging test, the pressure difference of a certain ester engine oil rises 50% faster than that of synthetic gasoline engine oil, and long-term use may cause the aftertreatment system to fail.
Synthetic gasoline engine oil is recommended for turbocharged, direct injection, and high-performance engines, especially for long-mileage, high-temperature conditions (such as racing cars and heavy-duty trucks). Its high-temperature cleanliness, anti-oxidation and durability advantages are significant, which can effectively extend the life of the engine. Ester synthetic engine oil is suitable for short-distance and low-speed conditions (such as urban commuting), or special scenarios with extremely high requirements for environmental protection (such as temporary use by racing teams), but the oil change cycle needs to be shortened to make up for the performance shortcomings. For example, in a certain fleet test, the oil change cycle of ester engine oil needs to be shortened to 5,000 kilometers, while synthetic gasoline engine oil can reach more than 10,000 kilometers.
Synthetic gasoline engine oil is superior to ester synthetic engine oil in high-temperature cleanliness, and its stability, additive compatibility and environmental protection are better balanced. Although ester engine oil has an advantage in biodegradability, its shortcoming in high-temperature performance limits its application range. In the future, with the development of low-ash ester base oil and composite additive technology, the high-temperature performance of ester engine oil is expected to improve, but in the short term, synthetic gasoline engine oil is still the first choice for high-temperature conditions. Users should choose the most suitable engine oil type according to actual needs and working conditions.
