Identifying the Incorrect Statement about SI and CI Engines

The question asks us to identify the statement that is incorrect regarding Spark Ignition (SI) engines and Compression Ignition (CI) engines. Let's examine each statement:

• Statement 1: CI engine works on the otto cycle.

  This statement is incorrect. CI engines, commonly known as diesel engines, primarily work on the Diesel cycle. In the Diesel cycle, heat addition occurs at constant pressure. SI engines, or petrol engines, primarily work on the Otto cycle, where heat addition occurs at constant volume.

• Statement 2: The self-ignition temperature of the fuel of SI engine is comparatively higher than that of CI engine.

  This statement is correct. Fuel used in SI engines (petrol) requires a relatively high self-ignition temperature to prevent premature ignition (knocking) due to compression. Ignition is initiated by a spark. Fuel used in CI engines (diesel) requires a relatively low self-ignition temperature because ignition is caused by the heat generated from compressing the air-fuel mixture.

• Statement 3: Compression ratio of SI engine is comparatively lower than that of CI engine.

  This statement is correct. SI engines typically have lower compression ratios (around 8:1 to 10:1) to avoid knocking. CI engines have much higher compression ratios (around 16:1 to 22:1) because the high compression ratio is necessary to raise the air temperature sufficiently to ignite the fuel upon injection.

• Statement 4: Thermal efficiency of CI engine is comparatively lower than that of SI engine for same compression ratio and same heat addition.

  This statement is correct under the specified conditions. While CI engines typically have higher thermal efficiency in practice due to their higher operating compression ratios, the ideal Diesel cycle (representing CI) is less efficient than the ideal Otto cycle (representing SI) when compared at the same compression ratio and same heat input, provided the cut-off ratio in the Diesel cycle is greater than 1.

Based on the analysis of each statement, the only incorrect statement is the first one, which claims that a CI engine works on the Otto cycle.