7.1 Define the following terms: 7.1.1 Power 7.1.2 Compression ratio 7.2 The bore of an engine is 70 mm and the stroke is 75 mm. The compression ratio is 9.5 : 1. Determine the following by means of calculations: 7.2.1 The swept volume in cm³ 7.2.2 The original clearance volume in cm³ 7.2.3 The compression ratio of this engine was increased to 10 : 1 by boring the cylinder. The clearance volume and the stroke length remained unchanged. What is the new diameter of the cylinder? Give the answer in mm. 7.3 The following data was recorded during a test carried out on a four-cylinder four-stroke petrol engine. Brake arm length: 420 mm Spring balance reading: 25 kg Crankshaft revolutions: 2 000 r/min Mean effective pressure: 900 kPa Cylinder bore: 84 mm Stroke length: 86 mm Determine the following by means of calculations: 7.3.1 Torque 7.3.2 Indicated power in kW 7.3.3 Brake power in kW 7.3.4 Mechanical efficiency.

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7.1 Define the following terms: 7.1.1 Power 7.1.2 Compression ratio 7.2 The bore of an engine is 70 mm and the stroke is 75 mm - English General - NSC Mechanical Technology: Automotive - Question 7 - 2021 - Paper 1

Question 7

7.1-Define-the-following-terms:--7.1.1-Power-7.1.2-Compression-ratio--7.2-The-bore-of-an-engine-is-70-mm-and-the-stroke-is-75-mm-English General-NSC Mechanical Technology: Automotive-Question 7-2021-Paper 1.png

7.1 Define the following terms: 7.1.1 Power 7.1.2 Compression ratio 7.2 The bore of an engine is 70 mm and the stroke is 75 mm. The compression ratio is 9.5 : 1. D... show full transcript

Worked Solution & Example Answer:7.1 Define the following terms: 7.1.1 Power 7.1.2 Compression ratio 7.2 The bore of an engine is 70 mm and the stroke is 75 mm - English General - NSC Mechanical Technology: Automotive - Question 7 - 2021 - Paper 1

Step 1

Define Power

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Answer

Power is defined as the rate at which work is done or energy is transferred. It can be calculated as:

$P = \frac{W}{t}$

where $W$ is the work done and $t$ is the time taken.

Step 2

Define Compression Ratio

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The compression ratio is the ratio of the total volume of the cylinder when the piston is at bottom dead center (BDC) to the volume in the cylinder when the piston is at top dead center (TDC). It is given by:

$CR = \frac{V_{total}}{V_{cylinder}}$

Step 3

Calculate the Swept Volume in cm³

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Answer

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

$SV = \frac{\pi D^2}{4} \times L$

where:

$D = 70$ mm
$L = 75$ mm

Converting to cm: $SV = \frac{\pi (7)^2}{4} \times 7.5 \approx 288.63 \text{ cm}^3$

Step 4

Calculate the Original Clearance Volume in cm³

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Answer

The original clearance volume (CV) can be determined from the compression ratio:

$CV = \frac{SV}{CR - 1}$

where:

$CR = 9.5$
$SV \approx 288.63$ cm³

Thus, $CV = \frac{288.63}{9.5 - 1} \approx 33.96 \text{ cm}^3$

Step 5

Calculate the New Diameter of the Cylinder

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The new compression ratio is 10:1, and the clearance volume remains unchanged. Rearranging the formula for the new diameter:

$CR = \frac{SV + CV}{CV}$ Substituting values: $10 = \frac{SV + 33.96}{33.96}$ This gives: $SV = 33.96 \times (10 - 1) \approx 305.64 \text{ cm}^3$ Rearranging for diameter: $D = \sqrt{\frac{SV \times 4}{\pi \times L}}$ Substituting: $D = \sqrt{\frac{305.64 \times 4}{\pi \times 7.5}} \approx 72.03 ext{ mm}$

Step 6

Calculate Torque

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Answer

Torque can be calculated from the force and the radius:

$Torque = Force \times Radius$ Here:

Force from the spring balance reading is 25 kg, which equals: $Force = 25 \times 9.81 = 245.25 ext{ N}$
Radius is 420 mm or 0.42 m: $Torque = 245.25 \times 0.42 = 103.15 ext{ N.m}$ .

Step 7

Calculate Indicated Power in kW

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Answer

Indicated Power (IP) can be calculated as:

$IP = \frac{MAP \times A \times N \times 60}{1000}$

MAP = 900 kPa = 900,000 Pa
Area $A$ of the cylinder: $A = \frac{\pi D^2}{4} \Rightarrow A = \frac{\pi (0.084)^2}{4} \approx 0.00554 \text{ m}^2$
N (number of power strokes) per minute: $N = \frac{2000}{2} = 1000 ext{ power strokes/min}$ Finally, substituting: $IP = \frac{900000 \times 0.00554 \times 1000}{1000} \approx 28.60 ext{ kW}$

Step 8

Calculate Brake Power in kW

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Answer

Brake Power (BP) can be calculated using the formula:

$BP = 2 \pi NT$

Substituting values:

N = 2000/60 = 33.33 power strokes/sec
T = 105 N.m Thus, $BP = 2 \times \pi \times 33.33 \times 105 \approx 22.05 ext{ kW}$

Step 9

Calculate Mechanical Efficiency

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Answer

Mechanical efficiency can be calculated as:

$\text{Mechanical Efficiency} = \frac{BP}{IP} \times 100$ Thus, $\text{Mechanical Efficiency} = \frac{22.05}{28.60} \times 100 \approx 77.05 \%$

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