In the operation of four-stroke diesel engines, the term squish refers to the entry of air into the combustion chamber.

The process of squish in a four-stroke diesel engine occurs during the compression stroke. It involves the movement of the piston towards the top dead center, which results in the compression of the air-fuel mixture in the combustion chamber. During this process, the air is forced into the combustion chamber and gets tightly compressed, creating a high-pressure environment.

Importance of Squish:
Squish is an essential aspect of diesel engine operation as it helps in achieving efficient combustion. It serves several purposes, including:

1. Improved Air-Fuel Mixing: The squish action helps in better mixing of air and fuel. As the piston moves towards the top dead center, it squeezes the air-fuel mixture in the combustion chamber. This compression leads to turbulence and intense mixing, ensuring a homogenous mixture for combustion.

2. Increased Combustion Efficiency: The squish process facilitates better combustion efficiency. The tightly compressed air-fuel mixture results in a higher compression ratio and increased turbulence. This, in turn, promotes more complete combustion, leading to improved fuel efficiency and reduced emissions.

3. Enhanced Flame Propagation: Squish helps in the rapid and efficient propagation of the flame front during combustion. The compressed air-fuel mixture experiences a sudden release of pressure as the fuel ignites. This rapid expansion pushes the flame front towards the piston, ensuring a thorough burn.

4. Reduction of Unburned Hydrocarbons: The squish action minimizes the presence of unburned hydrocarbons in the exhaust gases. By promoting better air-fuel mixing and combustion, it reduces the likelihood of incomplete combustion, which can result in the formation of unburned fuel particles.

Conclusion:
In summary, the term squish, in the context of four-stroke diesel engines, refers to the entry of air into the combustion chamber during the compression stroke. This process plays a crucial role in achieving efficient combustion, improved fuel efficiency, and reduced emissions. By promoting better air-fuel mixing and combustion, squish enhances the overall performance and environmental sustainability of diesel engines.