7.1 Define the term torque - NSC Mechanical Technology Automotive - Question 7 - 2018 - Paper 1

The clearance volume of an engine is the volume of the space above the crown of the piston at Top Dead Center (TDC). It plays a crucial role in the engine's compression ratio.

1. Remove shims between the cylinder block and cylinder head.
2. Machine metal from the cylinder head.

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

SV = rac{ ext{Bore}^2}{4} imes ext{Stroke} Substituting the values:

SV = rac{(75 ext{ mm})^2}{4} imes 80 ext{ mm} = 353.43 ext{ cm}^3

The clearance volume (CV) can be calculated using the compression ratio (CR) formula:

CR = rac{SV + CV}{CV} Rearranging gives:

CV = rac{SV}{CR - 1} Substituting values gives:

CV = rac{353.43}{8.5 - 1} = 47.12 ext{ cm}^3

When the engine is rebored to 78 mm, the new swept volume (SV) is:

SV = rac{(78 ext{ mm})^2}{4} imes 80 ext{ mm} = 382.27 ext{ cm}^3 Assuming the clearance volume remains unchanged at 47.12 cm³, the new compression ratio is:

CR = rac{SV + CV}{CV} = rac{382.27 + 47.12}{47.12} = 9.11

Indicated Power (IP) can be calculated using the formula:

$IP = P imes L imes A imes N imes n$ Where:

• $P = 1400 ext{ kPa} = 1400 imes 10^3 ext{ Pa}$
• $L = 110 ext{ mm} = 0.11 ext{ m}$
• A = rac{(100 ext{ mm})^2}{4} = 0.00785 ext{ m}^2
• N = rac{3600}{60 imes 2} = 30 ext{ s}^{-1}
• $n = 4 ext{ cylinders}$

Substituting these values:

$IP = (1400 imes 10^3) imes (0.11) imes (7.85 imes 10^{-3}) imes (30) imes (4) = 145.068 ext{ kW}$

Brake Power (BP) is calculated using the formula:

$BP = 2 imes F imes r$ Where:

• $F = 75 ext{ kg} imes 9.81 ext{ m/s}^2 = 735.75 ext{ N}$
• $r = 0.45 ext{ m}$ Thus, $BP = 2 imes 735.75 imes 0.45 = 662.68 ext{ W}$

Mechanical Efficiency (ME) can be calculated using:

ME = rac{BP}{IP} imes 100 Substituting values: ME = rac{127.23}{145.07} imes 100 = 87.70 ext{ % }