4-stroke engines fuel is somewhat different from those of 2-strokes. They contain generally less oil and more nitro methane. The reason for the lower lubricant content (although these engines have more moving parts than 2-strokes) is that they run in quite lower engine speeds, typically up to 10.000 rpm, while the 2-strokes run at 13,000 rpm or more (depending, of course, of the engine’s displacement). Less rpms also means less friction per minute, so less heating.
Also, because the 2-stroke engines have one ignition every 2 cycles, (ie by a complete cycle of the piston), while the 4-stroke has exactly half, (ie one ignition at two complete cycles of the piston), the drop in average temperature is greater than that of 2-stroke engines even though the exhaust gases have a higher temperature.
2-stroke engine cycles
4-stroke engine cycles
Because of this phenomenon, carbon deposits accumulate on 4-stroke engines, when castor oil is used, therefore use of less oil is justified. It will not hurt to use fuel in 4-stroke engine which is designed for 2-stroke and contains more oil, but eventually it will cause a number of problems in operation such as reduced performance.
The material which is burned and provides the power of the engine is methanol. Therefore, when a portion of the methanol is replaced with oil which is not combustible, the produced power is reduced. The fact that every two full cycles of the piston a single ignition occurs, already creates a problem, because it reduces the plug’s coil wire temperature, as it cools down enough between two ignitions. Excess oil magnifies the problem because it also cools more the coil wire. For this reason the plugs that are used in 4-stroke engines are specially designed having thicker coil wire made of certain material.
Also 4 stroke engines fuels contain a higher percentage of nitro methane, which reduced fuel octane makes it easier to ignite (reducing the octane to a fuel decreases its resistance to compression).
Because 4-stroke engines develop high point temperatures during combustion (even though the average temperature is less) they favor the carbon layer formation inside the combustion chamber and exhaust, so the manufacturers, unlike the 2-stroke engines recommend the use of fuel containing synthetic oil only.
As an example SAITO recommends that: Since the four-stroke engine has high exhaust temperature and carbon is apt to accumulate when castor oil type lubricants are used, avoid using fuel containing them.