Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Explain the cycles of two-stroke and four-stroke engines.
• Compare and contrast the differences between these engine types.
• Identify the key components and their functions.
• Calculate the compression ratio of an engine.
• Analyse the environmental impact of different engine designs.

Lesson summary

Internal Combustion Engines (ICEs) are the powerhouses behind many machines that are crucial to South African life, from bakkies hauling goods to taxis transporting people, and even generators providing electricity during load shedding. Understanding how these engines work – specifically the difference between two-stroke and four-stroke engines – is fundamental for anyone pursuing a career in mechanical fields or simply wanting to understand the technology around them. Many South African businesses rely on the efficient and reliable operation of vehicles and machinery, making this knowledge directly applicable to future job opportunities and contributing to a more skilled workforce.

Lesson notes

Internal Combustion Engines (ICEs): ICEs convert chemical energy (fuel) into mechanical energy (motion) within the engine itself.

Two main types exist: two-stroke and four-stroke engines. 2.1 Four-Stroke Engine Principles: The four-stroke engine completes its cycle in four piston strokes (two revolutions of the crankshaft).

Intake Stroke: The piston moves downwards, increasing the volume inside the cylinder. The intake valve opens, allowing a mixture of air and fuel (in petrol engines) or just air (in diesel engines) to be drawn into the cylinder. The exhaust valve is closed. Think of it like inhaling air into your lungs.

Compression Stroke: The piston moves upwards, decreasing the volume inside the cylinder. Both intake and exhaust valves are closed. The air/fuel mixture (or just air in diesel engines) is compressed, increasing its temperature and pressure. Think of squeezing an empty plastic bottle - the air inside gets warmer.

Compression Ratio: This is the ratio of the cylinder volume when the piston is at the bottom of its stroke (Bottom Dead Center, or BDC) to the volume when the piston is at the top of its stroke (Top Dead Center, or TDC).

Formula: Compression Ratio = (Swept Volume + Clearance Volume) / Clearance Volume.

Where: Swept Volume = π (bore/2)^2 * stroke. Bore is the diameter of the cylinder, and stroke is the distance the piston travels. Clearance Volume is the volume of the combustion chamber when the piston is at TD

C. Combustion (Power)

Stroke: Both intake and exhaust valves remain closed. At or near the top of the compression stroke, the compressed air/fuel mixture is ignited. In petrol engines, a spark plug ignites the mixture. In diesel engines, the high temperature of the compressed air causes the injected fuel to ignite spontaneously. The rapid combustion creates high pressure, forcing the piston downwards. This downward movement is the power stroke, which turns the crankshaft and generates mechanical work.

Exhaust Stroke: The piston moves upwards, pushing the burned gases out of the cylinder. The exhaust valve opens, allowing the exhaust gases to escape. The intake valve is closed. Think of exhaling air out of your lungs. 2.2 Two-Stroke Engine Principles: The two-stroke engine completes its cycle in only two piston strokes (one revolution of the crankshaft). It achieves this by combining functions of the four-stroke engine.

Compression and Intake Stroke: As the piston moves upwards, it compresses the air/fuel mixture (or just air in diesel versions) in the cylinder above the piston. Simultaneously, below the piston, a vacuum is created in the crankcase. This vacuum draws in a fresh charge of air/fuel mixture through a port (often a reed valve).

Power and Exhaust Stroke: Near the top of the stroke, the compressed mixture is ignited. The expanding gases force the piston downwards, providing the power stroke. As the piston approaches the bottom of its stroke, it uncovers the exhaust port, allowing exhaust gases to escape. Slightly later, the piston uncovers the transfer port, allowing the compressed fresh charge from the crankcase to flow into the cylinder, scavenging (pushing out) the remaining exhaust gases and preparing for the next cycle. This process is less efficient than the dedicated exhaust stroke of a 4-stroke. 2.3 Comparison: Two-Stroke vs. Four-Stroke Engines | Feature | Two-Stroke Engine | Four-Stroke Engine | |------------------|-----------------------------------------------------|----------------------------------------------------| | Cycle Completion | Two strokes (one crankshaft revolution) | Four strokes (two crankshaft revolutions) | | Power | Higher power-to-weight ratio (generally) | Lower power-to-weight ratio (generally) | | Efficiency | Lower fuel efficiency | Higher fuel efficiency | | Emissions | Higher emissions (often oil mixed with fuel) | Lower emissions | | Complexity | Simpler design (fewer moving parts) | More complex design (valves, camshafts, etc.) | | Maintenance | Generally simpler maintenance | Generally more complex maintenance | | Lubrication | Often relies on oil mixed with fuel | Separate lubrication system (oil sump) | | Applications | Chainsaws, motorcycles, some small engines | Cars, trucks, generators, lawnmowers, most vehicles | 2.4 Worked

Examples: Example 1: Compression Ratio Calculation A four-stroke engine has a bore of 80mm, a stroke of 75mm, and a clearance volume of 50 cm³. Calculate its compression ratio.

Solution: Calculate Swept Volume: Bore radius = bore/2 = 80mm/2 = 40mm = 4cm Swept Volume = π (bore radius)^2 stroke = π (4cm)^2 * 7.5cm = 376.99 cm³ (approximately)

Calculate Compression Ratio: Compression Ratio = (Swept Volume + Clearance Volume) / Clearance Volume Compression Ratio = (376.99 cm³ + 50 cm³) / 50 cm³ = 426.99 cm³ / 50 cm³ = 8.54:1 (approximately) Therefore, the compression ratio of the engine is approximately 8.54:1.