Question

As the compression ratio is decreased

• A. The exhaust temperature will decrease
• B. The exhaust temperature will increase
• C. The exhaust temperature will not get affected
• D. The exhaust temperature will first increase and then decrease

Hint:

To remember this relationship:
\[ \text{Lower Compression Ratio} \rightarrow \text{Lower Efficiency} \rightarrow \text{More wasted heat} \rightarrow \text{Hotter Exhaust} \] This is why high-compression engines run more efficiently and have cooler exhaust temperatures.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The compression ratio of an internal combustion engine is the ratio of the cylinder volume at bottom dead center (BDC) to the volume at top dead center (TDC).
This ratio is a key factor in determining the engine's thermodynamic efficiency.
Key Formula or Approach:
The thermal efficiency (\(\eta\)) of an ideal gas power cycle is given by: \[ \eta = 1 - \frac{1}{r^{\gamma-1}} \] where:
- \(r\) is the compression ratio.
- \(\gamma\) is the specific heat ratio.

Step 2: Detailed Explanation:
When the compression ratio (\(r\)) is decreased, the engine's thermal efficiency (\(\eta\)) also decreases.
Lower thermal efficiency means the engine converts less of the fuel's chemical energy into useful mechanical work during expansion.
Since more energy remains in the combustion gases at the end of the power stroke, the exhaust gas temperature increases.
In other words, a lower compression ratio means the gases undergo less expansion cooling before being released, leading to hotter exhaust temperatures.

Step 3: Final Answer:
Decreasing the compression ratio causes the engine's exhaust temperature to increase.