7.1 Define the term compression ratio of an engine - NSC Mechanical Technology Automotive - Question 7 - 2019 - Paper 1

The swept volume (SV) can be calculated using the formula:

The clearance volume (CV) can be derived from the compression ratio (CR): CR = rac{SV + CV}{CV} \Rightarrow CV = rac{SV}{CR - 1} Substituting the values: $CR = 8.5 \Rightarrow CV = \frac{498.76}{8.5 - 1} = 66.50 ext{ cm}^3$.

Starting with the relation: $CR = \frac{SV + CV}{CV}$ We have a CR of 9.5, which gives us: $SV = CV \times (CR - 1)$ We can use the CV previously calculated: $SV = 66.50 imes (9.5 - 1) = 66.50 imes 8.5 = 566.25 ext{ cm}^3$ Now, setting this into the formula for SV: $\frac{π}{4} \times D^2 \times L = 566.25$ Solving for the new diameter (D): $D^2 = \frac{566.25 \times 4}{π \times 90} \Rightarrow D = \sqrt{\frac{2265}{π}} \approx 89.4 ext{ mm}$.

Torque (T) can be calculated using the formula: $T = F \times r$ Where:

• F = 125 kg × 10 = 1250 N
• r = 0.3 m (300 mm) Thus, $T = 1250 imes 0.3 = 375 ext{ Nm}$.

Indicated power (IP) can be calculated using: $IP = P \times L \times A \times N \times n$ Where:

• P = 950 kPa
• L = 0.14 m (140 mm)
• A = \frac{πD^2}{4} = \frac{π(120)^2}{4} \approx 11.31 \times 10^{-3} ext{ m}^2
• N = \frac{2400}{60} = 40 ext{ rev/s} Thus,

Using the formula for brake power (BP): $BP = 2πnT$ Here:

• n = 40 rev/s
• T = 375 Nm Calculating: $BP = 2π \times 40 \times 375 = 94247.78 ext{ W}$ or 94.25 kW.

Mechanical efficiency (η) can be calculated as: