Lesson Notes By Weeks and Term v5 - Grade 11

Lesson Video

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Performance objectives

• Describe the strokes of two-stroke and four-stroke engines.
• Compare the operating principles of both engine types.
• Identify key engine components and their functions.
• Explain Top Dead Center (TDC) and Bottom Dead Center (BDC).
• Draw simplified diagrams of each engine's operation.

Lesson summary

Welcome to the fascinating world of internal combustion engines! This week, we'll dive into the fundamental principles that govern how engines work, focusing on two-stroke and four-stroke engine designs. Understanding these principles is crucial, not just for excelling in Mechanical Technology, but also for comprehending how much of the machinery around us – from generators that power our communities during load shedding to the bakkies used on farms – operates. With load shedding being a reality in South Africa, knowing how generators (often powered by these engines) function becomes incredibly relevant.

Lesson notes

2.1 The Four-Stroke Engine: A Cycle of Efficiency The four-stroke engine, commonly found in cars, trucks, and larger generators, operates on a cycle of four distinct strokes or movements of the piston within the cylinder: Intake Stroke: The piston moves downwards, increasing the volume in 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. Think of it like sucking liquid into a syringe.

Compression Stroke: The intake valve closes, and the piston moves upwards, reducing the volume in the cylinder. This compresses the air-fuel mixture (or just air in a diesel engine). Compression is crucial because it increases the temperature and pressure of the mixture, making it easier to ignite. The higher the compression ratio, the more efficient the engine tends to be (up to a point).

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 directly into the hot compressed air, causing it to self-ignite. The rapid expansion of the burning gases forces the piston downwards, generating power. This is the stroke that does the 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. Think of it like emptying the syringe. 2.2 The Two-Stroke Engine: Simplicity and Power-to-Weight The two-stroke engine, often found in smaller applications like lawnmowers, chainsaws, and some motorcycles (though increasingly less common due to emissions), completes its cycle in just two strokes: Compression/Intake Stroke: The piston moves upwards, compressing the air-fuel mixture above the piston. Simultaneously, a partial vacuum is created below the piston in the crankcase. As the piston nears the top of the stroke, it uncovers the transfer port and the exhaust port. Fresh air-fuel mixture from the crankcase is transferred to the cylinder via the transfer port, pushing out some of the remaining exhaust gases through the exhaust port (scavenging).

Power/Exhaust Stroke: The spark plug ignites the compressed air-fuel mixture. The expanding gases force the piston downwards, generating power. As the piston moves down, it covers the transfer and exhaust ports. As the piston approaches the bottom of its stroke, it begins to compress the air-fuel mixture in the crankcase. This pre-compression is vital for the next cycle. 2.3 Key Components Cylinder: The central chamber where 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 linear motion of the piston into rotary motion.

Valves (Four-Stroke): Control the intake and exhaust of gases.

Ports (Two-Stroke): Openings in the cylinder wall that allow for intake, exhaust, and transfer of gases.

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

Fuel Injector: Sprays fuel into the cylinder (direct injection) or intake manifold (port injection). 2.4 Top Dead Center (TDC) and Bottom Dead Center (BDC)

Top Dead Center (TDC): The point where the piston is at the highest point of its travel within the cylinder. It's the position where the volume in the cylinder is at its minimum.

Bottom Dead Center (BDC): The point where the piston is at the lowest point of its travel within the cylinder. It's the position where the volume in the cylinder is at its maximum. Understanding TDC and BDC is crucial for timing ignition and valve events correctly. 2.5 Advantages and Disadvantages | Feature | Four-Stroke Engine | Two-Stroke Engine | | ---------------- | ------------------------------- | ---------------------------------- | | Power Delivery | Smoother, more controlled | More power per engine size | | Fuel Efficiency | Generally more fuel-efficient | Less fuel-efficient | | Emissions | Lower emissions | Higher emissions | | Complexity | More complex | Simpler design | | Maintenance | Lower maintenance | Higher maintenance | | Applications | Cars, trucks, generators | Chainsaws, lawnmowers (decreasingly)| 2.6 Worked Examples Example 1: Calculating Compression Ratio A four-stroke engine has a cylinder volume of 500 cm³ when the piston is at BDC and 50 cm³ when the piston is at TD

C. Calculate the compression ratio.

Solution: Compression Ratio = (Volume at BDC) / (Volume at TDC) = 500 cm³ / 50 cm³ = 10:1

Commentary: A compression ratio of 10:1 means the air-fuel mixture is compressed to one-tenth of its original volume.

Example 2: Understanding Valve Timing (Four-Stroke) The intake valve on a four-stroke engine opens 10 degrees Before Top Dead Center (BTDC) and closes 40 degrees After Bottom Dead Center (ABDC). Explain what this means.