Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

This page supports the lesson note with a companion video and a short classroom-ready summary.

For class groups and homework, share this lesson page so learners also get the summary, objectives, and full lesson context.

Performance objectives

• Differentiate between two-stroke and four-stroke cycles.
• Explain the function of ports and valves.
• Compare the pros and cons of each engine type.
• Illustrate the major steps in each cycle.
• Understand compression ratio and displacement.

Lesson summary

This week, we delve deeper into the fascinating world of internal combustion engines, specifically focusing on the crucial differences and operational principles of two-stroke and four-stroke engines. Understanding these engine types is fundamental to mechanical technology as they power everything from motorcycles and lawnmowers (two-stroke) to cars and generators (four-stroke) commonly found in South Africa. The reliability and maintenance of these engines are directly related to a solid understanding of their operating cycles. Consider how essential these engines are to transportation, agriculture, and even providing backup power during load shedding – a very real issue in South Africa!

Lesson notes

2.1 Four-Stroke Engine Cycle The four-stroke engine completes its cycle in four distinct piston strokes (two revolutions of the crankshaft).

These strokes are: Intake/Induction Stroke: The piston moves downwards, creating a vacuum in the cylinder. The intake valve opens, allowing the air-fuel mixture (in petrol engines) or just air (in diesel engines) to be drawn into the cylinder. The exhaust valve remains closed. Think of this like inhaling.

Compression Stroke: Both intake and exhaust valves are closed. The piston moves upwards, compressing the air-fuel mixture (or air in a diesel engine). This compression significantly increases the temperature and pressure of the mixture, making combustion more efficient. This is like holding your breath and squeezing your chest.

Combustion/Power Stroke: Near the top of the compression stroke, the spark plug ignites the compressed air-fuel mixture in a petrol engine. In a diesel engine, fuel is injected into the highly compressed and hot air, causing spontaneous combustion. The resulting rapid expansion of gases forces the piston downwards, providing the power that drives the crankshaft. This is the 'bang' that makes the engine run.

Exhaust Stroke: The exhaust valve opens, and the piston moves upwards, pushing the burnt gases out of the cylinder and through the exhaust system. The intake valve remains closed. This is like exhaling.

Valves: Four-stroke engines rely on poppet valves (intake and exhaust) that are precisely timed by the camshaft to open and close at the correct points in the cycle. The camshaft is driven by the crankshaft, ensuring synchronization. 2.2 Two-Stroke Engine Cycle The two-stroke engine completes its cycle in only two piston strokes (one revolution of the crankshaft). This is achieved by combining some of the four-stroke events.

Upward Stroke (Compression & Exhaust): As the piston moves upwards, it compresses the air-fuel mixture in the cylinder (compression). Simultaneously, the underside of the piston creates a vacuum in the crankcase, drawing in a fresh air-fuel mixture (or just air in some designs) through the inlet port. Near the top of the stroke, the spark plug ignites the compressed mixture. As the piston approaches the top of its stroke, the exhaust port opens, allowing burned gases to escape.

Downward Stroke (Power & Intake/Transfer): The ignited air-fuel mixture expands rapidly, forcing the piston downwards (power). As the piston descends, it uncovers the transfer port, allowing the fresh air-fuel mixture from the crankcase to rush into the cylinder, scavenging the remaining exhaust gases (intake/transfer). This scavenging process relies on careful port timing and shaping. As the piston moves further down, it covers the inlet port, sealing the crankcase and preparing for the next upward stroke.

Ports: Two-stroke engines generally use ports (openings in the cylinder wall) controlled by the piston's movement instead of valves. This simplified design contributes to a higher power-to-weight ratio but often at the expense of efficiency and emissions. 2.3 Advantages and Disadvantages | Feature | Four-Stroke Engine | Two-Stroke Engine | |----------------|-------------------------------------------------------|----------------------------------------------------------| | Power Output | Generally less power per unit displacement | Generally more power per unit displacement | | Fuel Efficiency| More fuel-efficient | Less fuel-efficient | | Lubrication | Separate lubrication system (oil sump, oil pump, etc.) | Oil mixed with fuel for lubrication | | Emissions | Lower emissions | Higher emissions (due to oil burning in some designs) | | Complexity | More complex (valves, camshaft, timing system) | Simpler design (fewer moving parts) | | Maintenance | More involved maintenance | Simpler maintenance (but more frequent rebuilds) | | Applications | Cars, trucks, generators, stationary engines | Motorcycles, chainsaws, lawnmowers, outboard motors (legacy) | 2.4 Compression Ratio & Displacement Compression Ratio (CR): The ratio of the volume of the cylinder when the piston is at Bottom Dead Centre (BDC) to the volume when the piston is at Top Dead Centre (TDC). CR = (Swept Volume + Clearance Volume) / Clearance Volume. A higher compression ratio generally leads to greater thermal efficiency but can also lead to knocking (detonation) in petrol engines.

Displacement: The volume swept by the piston as it moves from BDC to TD

C. It is calculated as: Displacement = π (Bore/2)^2 * Stroke, where Bore is the cylinder diameter and Stroke is the distance the piston travels.