Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the complete four-stroke engine cycle.
• Describe the complete two-stroke engine cycle.
• Compare the advantages and disadvantages of each engine type.
• Calculate theoretical engine efficiency from a compression ratio.
• Identify key engine components on a diagram.

Lesson summary

The internal combustion engine, whether two-stroke or four-stroke, is the heart of countless machines that power our modern world. From the bakkies that transport goods across our vast country to the generators that provide electricity during load shedding, understanding how these engines work is crucial for any Mechanical Technology student. This week, we'll delve into the fundamental principles of both two-stroke and four-stroke engines, comparing their cycles, advantages, and disadvantages. This knowledge isn't just theoretical; it's essential for diagnosing, maintaining, and potentially even designing engine systems in the future.

Lesson notes

2. 1.

Four-Stroke Engine Cycle: The four-stroke engine completes its power cycle in four strokes of the piston – hence the name.

These strokes are: Intake Stroke: The piston moves down, creating a vacuum in the cylinder. The intake valve opens, and the fuel-air mixture (or just air in a diesel engine) is drawn into the cylinder. Think of it like breathing in.

Compression Stroke: The intake valve closes, and the piston moves upwards, compressing the fuel-air mixture. This increases the pressure and temperature of the mixture, making it easier to ignite. The compression ratio (the ratio of the cylinder volume at the bottom of the stroke to the cylinder volume at the top of the stroke) is a crucial factor in engine efficiency.

Power Stroke: At (or slightly before) the top of the compression stroke, the spark plug ignites the compressed fuel-air mixture (in a petrol engine) or the fuel is injected into the hot compressed air (in a diesel engine), causing a rapid expansion of gases. This forces the piston downwards, providing the power to turn the crankshaft. This is where the engine does its work.

Exhaust Stroke: The exhaust valve opens, and the piston moves upwards, pushing the burnt gases out of the cylinder and into the exhaust system. The cycle then repeats. 2.

2. Two-Stroke Engine Cycle: The two-stroke engine completes its power cycle in only two strokes of the piston. This is achieved by combining some of the functions of the four-stroke engine.

Stroke 1 (Compression & Transfer/Exhaust): As the piston moves upwards, it compresses the fuel-air mixture in the cylinder (like in the four-stroke). Simultaneously, as the piston rises, it uncovers the exhaust port, allowing burnt gases to escape. Near the top of the stroke, the rising piston also uncovers the transfer port, allowing a fresh charge of fuel-air mixture to enter the cylinder from the crankcase.

Stroke 2 (Power & Intake/Scavenging): The spark plug ignites the compressed mixture (petrol engine), forcing the piston downwards (power stroke). As the piston moves downwards, it closes the transfer and exhaust ports. The downward movement of the piston also pressurizes the crankcase, which is essential for the next cycle. The scavenging process involves the incoming fresh charge of air-fuel mixture helping to push out the remaining exhaust gases. 2.

3. Differences and Comparisons: | Feature | Four-Stroke Engine | Two-Stroke Engine | |-----------------|-------------------------------------------------------|--------------------------------------------------------| | Strokes per Cycle| Four | Two | | Power Strokes | One per two revolutions of the crankshaft | One per revolution of the crankshaft | | Valves | Typically valves controlled by camshafts | Typically ports in the cylinder wall | | Lubrication | Separate lubrication system (oil sump, oil pump) | Oil mixed with fuel | | Efficiency | Generally more efficient | Generally less efficient | | Emissions | Generally lower emissions | Generally higher emissions | | Complexity | More complex | Simpler mechanically | | Applications | Cars, trucks, generators, larger equipment | Motorcycles, lawnmowers, chainsaws, small equipment | 2.

4. Theoretical Efficiency: The theoretical efficiency of an engine is related to its compression ratio (r). The higher the compression ratio, the higher the theoretical efficiency. The formula for theoretical efficiency (η) is: η = 1 - (1 / r^(γ-1))

Where: η = Efficiency (expressed as a decimal) r = Compression ratio γ = Specific heat ratio (approximately 1.4 for air)

Example: Calculate the theoretical efficiency of an engine with a compression ratio of 8:1. η = 1 - (1 / 8^(1.4-1)) η = 1 - (1 / 8^0.4) η = 1 - (1 / 2.297) η = 1 - 0.435 η = 0.565 Therefore, the theoretical efficiency is 56.5%. 2.

5. Key Engine Components: Both two-stroke and four-stroke engines share common components, but their arrangement and function might differ: Cylinder: The space where combustion takes place.

Piston: A moving component within the cylinder that converts pressure into motion.

Connecting Rod: Connects the piston to the crankshaft.

Crankshaft: Converts the linear motion of the piston into rotary motion.

Spark Plug: Ignites the fuel-air mixture (petrol engines).

Inlet Valve (Four-Stroke): Controls the entry of the fuel-air mixture into the cylinder.

Exhaust Valve (Four-Stroke): Controls the exit of exhaust gases from the cylinder.

Transfer Ports (Two-Stroke): Allow the fuel-air mixture to enter the cylinder.

Exhaust Port (Two-Stroke): Allows exhaust gases to exit the cylinder.

Carburettor/Fuel Injector: Mixes fuel and air in the correct proportions. Guided Practice (With Solutions)

Question 1: Describe the sequence of events in one complete cycle of a four-stroke petrol engine.

Solution: Intake: Piston moves down, intake valve opens, fuel-air mixture enters.

Compression: Intake valve closes, piston moves up, fuel-air mixture is compressed.

Power: Spark plug ignites the mixture, piston is forced down.