Question

In a Diesel cycle, the parameter that affects efficiency in addition to compression ratio is

• A. Cut-off ratio
• B. Pressure ratio
• C. Heat rejection
• D. Mean effective pressure

Hint:

While Otto cycle efficiency depends only on the compression ratio, the Diesel cycle efficiency depends on both compression ratio and cut-off ratio.
As the cut-off ratio $r_c$ approaches $1$, the Diesel cycle efficiency approaches the Otto cycle efficiency.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
This question concerns the air-standard efficiency of the Diesel internal combustion engine cycle.
We need to identify the parameter that, along with the compression ratio, determines this efficiency.

Step 2: Key Formula or Approach:
The air-standard efficiency of a Diesel cycle ($\eta_{\text{Diesel}}$) is given by:
\[ \eta_{\text{Diesel}} = 1 - \frac{1}{r^{\gamma-1}} \left[ \frac{r_c^{\gamma} - 1}{\gamma(r_c - 1)} \right] \]
where:
$r$ is the compression ratio ($V_1 / V_2$),
$r_c$ is the cut-off ratio ($V_3 / V_2$), and
$\gamma$ is the adiabatic index (ratio of specific heats).

Step 3: Detailed Explanation:

• The Diesel cycle consists of four processes: isentropic compression, constant-pressure heat addition, isentropic expansion, and constant-volume heat rejection.

• The compression ratio $r$ measures the volume reduction during the compression stroke.

• The cut-off ratio $r_c$ represents the ratio of cylinder volumes after and before the combustion (heat addition) process.

• From the efficiency formula, we can see that for a fixed compression ratio and specific heat ratio, the efficiency decreases as the cut-off ratio increases.

• Thus, the cut-off ratio is the primary additional parameter affecting efficiency.

Step 4: Final Answer:
The cut-off ratio is the additional parameter that affects the efficiency of a Diesel cycle.