7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke and compression stroke. How do the compression stroke and power stroke affect the pressure? 7.3 FIGURE 7.3 below shows a cylinder with the piston at TDC and the same cylinder with the piston at BDC. The following information is applicable: Volume (V1) of the space when the piston is at TDC = 39 cm³ Space between TDC and BDC (V2) = 330 ml Distance between TDC and BDC is 65 mm Answer the questions that follow. 7.3.1 Which term is used to describe the volume V1? 7.3.2 Which term is used to describe the volume V2? 7.3.3 Calculate the total cylinder volume in cm³. 7.3.4 Calculate the bore diameter in mm. 7.3.5 Calculate the compression ratio of this cylinder. 7.4 State TWO methods to lower the compression ratio in an internal combustion engine. 7.5 The following data refers to a two-cylinder two-stroke petrol engine: Engine speed during test: 6 500 r/min Cylinder displacement (volume): 580.7 cm³ Mean effective pressure: 450 kPa Brake power: 48 kW @ 6 500 r/min Determine the following by means of calculations: 7.5.1 Torque 7.5.2 Indicated power in kW 7.5.3 Mechanical efficiency

NSC Mechanical Technology Automotive Automotive calculations and applications: 7.1 Define the following terms r

7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke and compression stroke - NSC Mechanical Technology Automotive - Question 7 - 2021 - Paper 1

Question 7

7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke ... show full transcript

Worked Solution & Example Answer:7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke and compression stroke - NSC Mechanical Technology Automotive - Question 7 - 2021 - Paper 1

Step 1

7.1.1 Brake power

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Answer

Brake power is the usable power or actual power developed at the flywheel or at the drive wheels. It is referred to as the power output of the engine during normal operation.

Step 2

7.1.2 Torque

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Answer

Torque is the twisting effort or force applied to a shaft or wheel. It can be defined as the product of the force and the distance from the point of force application to the axis of rotation.

Step 3

7.2 The indicated power diagram represents the power stroke and compression stroke. How do the compression stroke and power stroke affect the pressure?

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The compression stroke results in a pressure rise, as the piston compresses the air-fuel mixture within the cylinder. Conversely, the power stroke sees a pressure drop as the burning gases expand and push the piston down.

Step 4

7.3.1 Which term is used to describe the volume V1?

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Answer

The term used to describe volume V1 is the 'Clearance volume'.

Step 5

7.3.2 Which term is used to describe the volume V2?

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Answer

The term used to describe volume V2 is the 'Swept volume'.

Step 6

7.3.3 Calculate the total cylinder volume in cm³.

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Answer

To find the total cylinder volume, use the formula:

ext{Total cylinder volume} = V_1 + V_2

Given that V1 = 39 cm³ and V2 = 330 cm³, the total cylinder volume equals:

ext{Total cylinder volume} = 39 + 330 = 369 ext{ cm}³

Step 7

7.3.4 Calculate the bore diameter in mm.

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To calculate the bore diameter, we first find the swept volume (SV):

SV = 330 ext{ cm}³

Using the equation for the swept volume:

SV = rac{ ext{π} D^2}{4} imes L

We can rearrange to find D:

D^2 = rac{4 imes SV}{π imes L}

Assuming the stroke length (L) is 65 mm:

D^2 = rac{4 imes 330}{π imes 65}

Calculating gives us:

D = 80.4 ext{ mm}$$

Step 8

7.3.5 Calculate the compression ratio of this cylinder.

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The compression ratio (CR) can be calculated using:

CR = rac{ ext{Total cylinder volume}}{ ext{Clearance volume}} = rac{369}{39} = 9.46

Thus, the compression ratio is approximately 9.5:1.

Step 9

7.4 State TWO methods to lower the compression ratio in an internal combustion engine.

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Fit a thicker gasket between the cylinder block and cylinder head.
Use pistons with a different shape that have larger volume.

Step 10

7.5.1 Torque

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The torque (T) can be calculated using the formula:

T = rac{BP}{2 ext{πN}} imes 60

Substituting the values:

T = rac{48 000}{2 imes ext{π} imes 6 500} imes 60 = 70.52 ext{ Nm}$$

Step 11

7.5.2 Indicated power in kW

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Indicated power (IP) can be calculated using:

IP = ext{L} imes ext{A} imes N

Where: L = cylinder volume, A = area, and N = engine speed. Using the values provided and applying the necessary conversions gives:

ext{IP} = 66.62 ext{ kW}$$

Step 12

7.5.3 Mechanical efficiency

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The mechanical efficiency (η) can be defined as:

η = rac{BP}{IP} imes 100

Substituting the given values yields:

η = rac{48}{56.62} imes 100 = 84.78 ext{%}$$

7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke and compression stroke - NSC Mechanical Technology Automotive - Question 7 - 2021 - Paper 1

Worked Solution & Example Answer:7.1 Define the following terms regarding an internal combustion engine: 7.1.1 Brake power 7.1.2 Torque 7.2 The indicated power diagram represents the power stroke and compression stroke - NSC Mechanical Technology Automotive - Question 7 - 2021 - Paper 1

7.1.1 Brake power

7.1.2 Torque

7.2 The indicated power diagram represents the power stroke and compression stroke. How do the compression stroke and power stroke affect the pressure?

7.3.1 Which term is used to describe the volume V1?

7.3.2 Which term is used to describe the volume V2?

7.3.3 Calculate the total cylinder volume in cm³.

7.3.4 Calculate the bore diameter in mm.

7.3.5 Calculate the compression ratio of this cylinder.

7.4 State TWO methods to lower the compression ratio in an internal combustion engine.

7.5.1 Torque

7.5.2 Indicated power in kW

7.5.3 Mechanical efficiency

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