Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the operating principles of two-stroke and four-stroke engines.
• Compare the advantages and disadvantages of each engine type.
• Identify the key components and their functions.
• Evaluate the environmental impact of each engine.

Lesson summary

Engines are the heart of much of the technology we rely on every day, from the bakkies that transport goods to the generators that provide electricity during load shedding. Understanding how two-stroke and four-stroke engines work is fundamental to understanding how these machines operate, their advantages and disadvantages, and their impact on the environment. Many South African mechanics, engineers, and entrepreneurs build their careers on this knowledge. A solid grasp of these principles can open doors to apprenticeships, technical college studies, and even starting your own repair business.

Lesson notes

2.1 Four-Stroke Engine Principles The four-stroke engine completes its operating cycle in four distinct strokes of the piston: intake, compression, combustion (power), and exhaust.

Intake Stroke: The piston moves downwards, creating a vacuum inside the cylinder. The intake valve opens, allowing a mixture of air and fuel (or just air in a diesel engine) to be drawn into the cylinder.

Compression Stroke: The piston moves upwards, compressing the air-fuel mixture. Both intake and exhaust valves are closed. Compressing the mixture increases its temperature and pressure, making it easier to ignite.

Combustion (Power)

Stroke: At or 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 hot compressed air, causing it to ignite spontaneously. The rapid expansion of the burning gases forces the piston downwards, delivering power to the crankshaft.

Exhaust Stroke: The piston moves upwards, pushing the spent exhaust gases out of the cylinder through the open exhaust valve.

Key Components of a Four-Stroke Engine: Cylinder: The chamber in which the piston moves.

Piston: A cylindrical component that moves up and down within the cylinder.

Connecting Rod: Connects the piston to the crankshaft.

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

Valves (Intake and Exhaust): Control the flow of air-fuel mixture into and exhaust gases out of the cylinder.

Spark Plug (Petrol Engine): Provides the spark to ignite the air-fuel mixture.

Fuel Injector (Diesel Engine): Injects fuel directly into the cylinder.

Camshaft: Operates the valves at the correct times. 2.2 Two-Stroke Engine Principles The two-stroke engine completes its operating cycle in only two strokes of the piston: compression/intake and combustion/exhaust. This is achieved by combining functions and using ports in the cylinder wall instead of some valves.

Compression/Intake Stroke: As the piston moves upwards, it compresses the air-fuel mixture (which has already entered the crankcase) and creates a vacuum in the crankcase. As the piston nears the top of its stroke, a transfer port is uncovered, allowing the pre-compressed air-fuel mixture from the crankcase to rush into the cylinder. At the same time, the rising piston uncovers the exhaust port, allowing some of the burnt gases to escape.

Combustion/Exhaust Stroke: As the piston moves downwards, the spark plug ignites the air-fuel mixture. The expanding gases force the piston down, delivering power. As the piston nears the bottom of its stroke, it uncovers the exhaust port, allowing the remaining exhaust gases to escape. Simultaneously, it also uncovers the transfer port, allowing a fresh charge of air-fuel mixture to enter the cylinder, pushing out the remaining exhaust gases (a process called scavenging). This also pressurizes the crankcase for the next cycle.

Key Components of a Two-Stroke Engine: Cylinder: The chamber in which the piston moves.

Piston: A cylindrical component that moves up and down within the cylinder.

Connecting Rod: Connects the piston to the crankshaft.

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

Spark Plug: Provides the spark to ignite the air-fuel mixture.

Ports (Intake/Transfer and Exhaust): Openings in the cylinder wall that control the flow of air-fuel mixture and exhaust gases. The piston itself acts as a valve, opening and closing these ports as it moves.

Crankcase: Sealed lower part of the engine, used to pre-compress the air-fuel mixture.

Carburetor/Fuel Injector: Provides the air-fuel mixture. Often, two-stroke engines use a carburetor to mix the fuel and air. 2.3 Comparing Two-Stroke and Four-Stroke Engines | Feature | Two-Stroke Engine | Four-Stroke Engine | | ---------------- | ------------------------------------------------- | ------------------------------------------------ | | Cycle Completion | Two strokes of the piston | Four strokes of the piston | | Power Output | Generally higher power-to-weight ratio | Generally lower power-to-weight ratio | | Efficiency | Lower fuel efficiency | Higher fuel efficiency | | Emissions | Higher emissions (more unburnt fuel) | Lower emissions (better combustion) | | Complexity | Simpler design (fewer moving parts) | More complex design (more moving parts) | | Lubrication | Oil mixed with fuel (total-loss lubrication) | Separate lubrication system | | Applications | Chainsaws, lawnmowers, some motorcycles (decreasing) | Cars, trucks, generators, most motorcycles |