Introduction:
The concentration of Unburnt Hydrocarbons (UBHC) in the SI engine is influenced by various factors, including engine load, acceleration, deceleration, and idling. Among these, the highest UBHC concentration is observed during deceleration.

Explanation:
During deceleration, the throttle is closed or partially closed, reducing the amount of air entering the engine. As a result, the air-fuel mixture becomes richer, meaning there is a higher concentration of fuel compared to the amount of air available for combustion. This rich mixture leads to incomplete combustion, resulting in higher UBHC emissions.

Factors affecting UBHC concentration:
1. Engine Load: The engine load refers to the amount of power demanded by the vehicle. During maximum load conditions, the engine operates at high RPMs and consumes more fuel. However, the air-fuel mixture is usually optimized for complete combustion, resulting in lower UBHC emissions compared to deceleration.

2. Acceleration: During acceleration, the throttle is opened to allow more air into the engine, creating a leaner air-fuel mixture. A lean mixture promotes better combustion, reducing the UBHC concentration.

3. Deceleration: During deceleration, the throttle is closed or partially closed, restricting the airflow into the engine. This causes the air-fuel mixture to become richer, leading to incomplete combustion and higher UBHC emissions.

4. Idling: Idling refers to the engine running at low RPMs without any load. During idling, the engine operates with a lean air-fuel mixture, resulting in lower UBHC emissions compared to deceleration.

Conclusion:
In summary, the highest UBHC concentration in the SI engine is observed during deceleration. This is because deceleration leads to a rich air-fuel mixture, which promotes incomplete combustion and higher UBHC emissions. Understanding the factors affecting UBHC concentration is important for optimizing engine performance and reducing emissions in SI engines.