Two-stroke gasoline engines are widely used in small power equipment such as lawn mowers, chainsaws, and outboard motors due to their simple structure, small size, and high power density. However, cylinder scoring is a common and harmful fault in their operation. Cylinder scoring refers to the phenomenon that the inner wall of the cylinder liner and the surface of the piston (or piston rings) are scratched, grooved, or even seized due to abnormal friction. This fault not only reduces the engine's power performance and fuel economy but also may lead to irreversible damage to key components, resulting in costly repairs. To effectively prevent and solve cylinder scoring problems, it is crucial to accurately identify and analyze their root causes. Based on the working characteristics of two-stroke gasoline engines, this article systematically explores the main factors contributing to cylinder scoring.

1. Insufficient or Poor-Quality Lubrication

Lubrication is the core guarantee for reducing friction between the piston and the cylinder wall. Unlike four-stroke engines that have an independent lubrication system, two-stroke gasoline engines typically adopt a mixed lubrication method (mixing lubricating oil with gasoline) or a separate lubrication method (supplying oil independently through an oil pump). Insufficient or poor-quality lubrication is the primary cause of cylinder scoring.

Firstly, the incorrect mixing ratio of lubricating oil and gasoline in mixed lubrication systems is a common issue. If the proportion of lubricating oil is too low (e.g., exceeding the recommended ratio of 50:1 or 100:1), the oil film formed between the piston and cylinder wall will be too thin to withstand the high temperature and pressure during engine operation. The oil film will easily break, leading to direct metal-to-metal contact and severe friction. Conversely, an excessively high oil ratio can cause carbon deposits, but it rarely directly leads to cylinder scoring compared to insufficient lubrication.

Secondly, the use of inferior lubricating oil exacerbates the risk of cylinder scoring. Two-stroke engines require lubricating oil with specific properties (such as good high-temperature resistance, detergency, and anti-wear performance). Low-quality oils may decompose, oxidize, or carbonize at high temperatures, losing their lubricating effect. Additionally, impurities in inferior oils can act as abrasives, scratching the cylinder wall and piston surface.

Thirdly, failures in separate lubrication systems can also cause insufficient lubrication. For example, blockages in the oil pump, oil lines, or oil nozzles will prevent lubricating oil from being delivered to the friction surfaces in a timely and sufficient manner. Malfunctions in the oil level sensor or oil supply control mechanism may also lead to inadequate oil supply, especially under high-load operating conditions.

2. Overheating of the Engine

Overheating is another major factor causing cylinder scoring in two-stroke gasoline engines. High temperatures will significantly reduce the viscosity of the lubricating oil, weaken the oil film's load-bearing capacity, and accelerate the oxidation and deterioration of the oil. At the same time, high temperatures will cause thermal expansion of the piston and cylinder liner. If the thermal expansion exceeds the design gap, the fit clearance between the piston and cylinder will decrease or even disappear, resulting in severe friction and cylinder scoring.

There are many reasons for engine overheating. One common cause is poor cooling system performance. Most small two-stroke engines use air cooling systems, which rely on cooling fins and air flow to dissipate heat. If the cooling fins are covered with dust, oil stains, or debris, the heat dissipation efficiency will be greatly reduced. In addition, operating the engine in a high-temperature environment (such as direct sunlight in summer) or at high load for a long time (such as full-throttle operation for an extended period) will also cause the engine temperature to rise sharply.

Another cause of overheating is incorrect ignition timing. If the ignition timing is too late, the combustion process will be delayed, and the exhaust temperature will rise significantly, leading to an increase in the overall temperature of the engine. Excessive carbon deposits in the combustion chamber can also cause local overheating, as carbon deposits are poor heat conductors and can accumulate heat, resulting in elevated temperatures in the cylinder.

3. Improper Fuel Supply and Combustion

Abnormal fuel supply and combustion processes will not only affect the engine's power output but also indirectly lead to cylinder scoring.

Firstly, the use of low-quality or incorrect gasoline is a key factor. Gasoline with a lower octane rating than required will cause detonation (knocking) during combustion. Detonation generates intense pressure waves that disrupt the oil film between the piston and cylinder wall, causing local high temperatures and pressures, which easily lead to cylinder scoring. In addition, impure gasoline containing impurities (such as dust, water, or metal particles) will bring abrasive particles into the cylinder, scratching the friction surfaces.

Secondly, insufficient fuel supply can also cause problems. If the carburetor is blocked (e.g., blocked jets or fuel passages) or the fuel pump malfunctions, the engine will experience lean combustion (insufficient fuel, excessive air). Lean combustion will cause the combustion temperature to rise sharply, leading to engine overheating and increasing the risk of cylinder scoring. On the other hand, excessive fuel supply (rich combustion) will result in incomplete combustion, leading to carbon deposits. Although carbon deposits do not directly cause cylinder scoring, excessive carbon deposits can increase piston ring sticking, reduce heat dissipation, and indirectly trigger the fault.

4. Improper Operation and Maintenance

Human factors such as improper operation and inadequate maintenance are important causes of cylinder scoring in two-stroke gasoline engines.

In terms of operation, starting the engine without proper warm-up is a common mistake. When the engine is cold, the lubricating oil has high viscosity and cannot quickly reach all friction surfaces. The fit clearance between the piston and cylinder is also in the cold state. Starting the engine at full throttle immediately will cause the piston and cylinder to undergo severe friction under insufficient lubrication and improper clearance, easily leading to cylinder scoring. Additionally, prolonged overload operation (e.g., using the engine to drive loads exceeding its rated power) will increase the mechanical load and thermal load of the engine, causing the cylinder temperature to rise and the oil film to break.

In terms of maintenance, inadequate maintenance of key components is a major risk factor. For example, worn or damaged piston rings cannot effectively scrape off excess oil on the cylinder wall, leading to oil burning and carbon deposits, and at the same time, they cannot maintain the proper oil film thickness. A damaged cylinder liner (e.g., with scratches or corrosion) will directly affect the fit with the piston, increasing friction. Failure to clean the air filter regularly will allow dust and sand to enter the cylinder, acting as abrasives and causing severe scratching of the cylinder wall and piston.

5. Manufacturing and Assembly Defects

Although less common than the above factors, manufacturing and assembly defects can also lead to cylinder scoring.

In terms of manufacturing, unqualified processing accuracy of the cylinder liner and piston is a key issue. For example, uneven surface roughness of the cylinder wall, out-of-tolerance roundness or cylindricity, and mismatched piston and cylinder liner sizes will cause uneven stress distribution and poor lubrication during operation, leading to local friction intensification and cylinder scoring. In addition, material defects (e.g., insufficient hardness of the cylinder liner or piston) will reduce the wear resistance of the components, making them prone to scratching.

In terms of assembly, incorrect assembly clearance is a critical factor. If the fit clearance between the piston and cylinder liner is too small, thermal expansion during operation will cause the two to seize, resulting in severe cylinder scoring. If the clearance is too large, the piston will swing in the cylinder, causing uneven wear and possible scratching of the cylinder wall. Additionally, improper installation of piston rings (e.g., incorrect opening position or damaged rings) will affect their sealing and oil scraping functions, leading to poor lubrication and cylinder scoring.

Conclusion

Cylinder scoring in two-stroke gasoline engines is a comprehensive fault caused by multiple factors, among which insufficient or poor-quality lubrication, engine overheating, improper fuel supply, and improper operation and maintenance are the most common. To prevent cylinder scoring, it is necessary to use qualified lubricating oil and gasoline in accordance with specifications, maintain the normal operation of the lubrication and cooling systems, perform regular maintenance (such as cleaning the air filter and checking piston rings), and adopt correct operating methods (such as proper warm-up before use and avoiding prolonged overload operation). For manufacturing and assembly defects, strict quality control during the production and assembly processes is required to ensure the accuracy and reliability of components. By comprehensively addressing these factors, the occurrence of cylinder scoring can be effectively reduced, extending the service life of the engine and improving its operating stability.