Here's why:
* Efficiency: Four-stroke engines are generally more efficient than two-stroke engines, especially at lower speeds and under heavy loads. This is because they have a dedicated power stroke, where combustion happens and work is done, and a separate exhaust stroke to expel the spent gases. This allows for better fuel combustion and higher thermal efficiency.
* Emissions: Four-stroke engines generally produce lower emissions than two-stroke engines, particularly in terms of unburnt hydrocarbons and particulate matter. This is due to their more controlled combustion process and better scavenging of exhaust gases.
* Lubrication: Four-stroke engines have a separate lubrication system that delivers oil directly to the moving parts, whereas two-stroke engines rely on oil mixed with fuel. This leads to less oil consumption and cleaner combustion in four-stroke engines.
* Power Output: While two-stroke engines can provide higher power output per unit displacement, they generally have lower torque than four-stroke engines. This makes four-stroke engines more suitable for heavy-duty applications where sustained power and torque are crucial.
Exceptions:
There are some exceptions to this rule, such as:
* Small, high-speed engines: Two-stroke engines are still common in smaller, high-speed applications like motorcycles and some marine engines.
* Large, low-speed engines: Some large, low-speed engines used in specific applications like marine propulsion might use a two-stroke cycle. However, these are relatively rare compared to four-stroke engines.
In summary: While two-stroke engines have advantages in certain situations, their drawbacks in terms of efficiency, emissions, and lubrication make four-stroke engines the preferred choice for the majority of very large engines.
