Grade 10 power mechanics – use illustrations to explain four-stroke and two-stroke engine cycles. Quiz
1. In a four-stroke petrol engine, which stroke occurs immediately after the intake stroke?
The four-stroke cycle order is intake (air-fuel enters), compression (mixture compressed), power (combustion pushes piston), then exhaust (gases expelled). Scavenging is a two-stroke concept.
2. How many piston strokes make up one complete power cycle in a two-stroke engine?
A two-stroke engine completes a full cycle with one upward and one downward piston movement: compression up and power down, while intake and exhaust occur during these movements.
3. How many crankshaft revolutions are required to complete one full cycle in a four-stroke engine?
A four-stroke engine needs two full crankshaft revolutions because each stroke is one half-revolution; all four strokes require two revolutions to complete the cycle.
4. Which component replaces valves in many two-stroke engines to allow gas flow?
Two-stroke engines commonly use ports cut into the cylinder wall that are uncovered/covered by the piston to let gases in and out rather than separate valves.
5. When drawing an illustration of the four-stroke cycle, what label should be placed at the top dead centre (TDC) at the end of the compression stroke?
At TDC after compression the air-fuel mixture is most compressed and the spark plug fires to start the power (combustion) stroke in a petrol engine.
6. Which statement is a typical advantage of a two-stroke engine compared with a four-stroke of similar size?
Two-stroke engines produce a power stroke every crankshaft revolution and have fewer moving parts, giving higher power for their weight, though they tend to have higher emissions and worse fuel economy.
7. In a labelled diagram of a two-stroke engine, what is 'scavenging' referring to?
Scavenging is when the fresh air-fuel mix enters and forces the exhaust gases out of the cylinder, a key process in two-stroke engines often shown in illustrations.
8. Which of these is true about lubrication in many two-stroke engines commonly used in older Kenyan boda bodas and small chainsaws?
Many small two-stroke engines use a fuel-oil mixture so the oil lubricates crankshaft and piston parts; four-strokes usually have a separate oil sump and pump.
9. On an illustration of a four-stroke engine, which valve is open during the exhaust stroke?
During the exhaust stroke the exhaust valve opens so burnt gases can be pushed out of the cylinder as the piston moves up.
10. Which stroke in a four-stroke engine produces the mechanical work that turns the crankshaft?
The power stroke is when the burning fuel expands and forces the piston down; this is the stroke that delivers useful mechanical energy to the crankshaft.
11. If an illustration shows a piston at bottom dead centre (BDC) with ports open in a two-stroke engine, what is likely happening?
At BDC in many two-stroke designs the piston uncovers intake/transfer and exhaust ports so fresh mixture enters and exhaust gases exit as part of scavenging.
12. Which engine type typically has a camshaft, valves and an oil sump?
Four-stroke internal combustion engines usually use a camshaft to operate intake and exhaust valves and have a separate oil sump for lubrication; many two-strokes lack these features.
13. When teaching with illustrations about timing, what does 'TDC' stand for and why is it important?
TDC is the highest piston position; it is a reference point for ignition timing, valve timing and understanding stroke transitions in illustrations and maintenance.
14. Which illustration feature shows that a two-stroke engine fires once every revolution while a four-stroke fires once every two revolutions?
Two-stroke engines produce a power pulse each crankshaft revolution, whereas four-strokes produce one every two revolutions; illustrations showing pulses per revolution make this clear.
15. Which is a common disadvantage of two-stroke engines shown in diagrams comparing emissions?
Because intake and exhaust processes overlap in two-strokes, some fresh mixture can escape, causing higher emissions; diagrams often show this loss compared with four-strokes.
16. In an illustrated cross-section of a four-stroke engine during the intake stroke, what is the piston movement and valve state?
During intake the piston moves down toward BDC creating space and drawing the air-fuel mixture in while the intake valve is open.
17. A simple classroom illustration shows a two-stroke engine with no oil sump. Which method of lubrication would the teacher point out?
Most small two-stroke engines mix oil with fuel so the oil passes through the crankcase and lubricates parts; there's no separate oil sump as in four-strokes.
18. Which diagram label would you use to show when the exhaust valve begins to open in a four-stroke engine?
The exhaust valve opens near the end of the power stroke to allow exhaust gases to exit as the piston moves up in the exhaust stroke.
19. Which practical classroom activity helps students understand scavenging in two-stroke engines using illustrations and simple models?
A transparent model allows students to see how incoming flow displaces exhaust gases, making the scavenging process clear; the other activities are unrelated.
20. Which engine type is more likely to appear in modern Kenyan small cars and why, as shown in classroom diagrams?
Modern small cars use four-stroke engines for improved fuel efficiency and lower emissions; two-strokes are more common in older or small handheld equipment.
21. When comparing illustrated timing diagrams, which shows the correct timing relationship for a four-stroke engine?
A correct timing diagram for a four-stroke shows the four strokes in that order across two crankshaft revolutions; valve overlap may actually occur briefly between strokes.
22. Which pictorial feature distinguishes a two-stroke engine diagram from a four-stroke diagram?
Two-stroke diagrams often show ports cut into the cylinder wall that the piston covers/uncover rather than camshaft-operated valves used in typical four-stroke diagrams.
23. On an illustration showing piston movement, which position corresponds to the end of the exhaust stroke in a four-stroke engine?
The exhaust stroke ends when the piston reaches TDC after pushing out the burnt gases; the intake stroke usually follows as the piston moves down.
24. Which classroom drawing best explains why two-stroke engines tend to make more noise than four-stroke engines?
Two-strokes produce a power pulse every revolution and have overlapping gas flow that increases noise; a drawing highlighting these events helps explain the louder sound.
25. Which of the following is the correct stroke order for a four-stroke diesel engine as shown in an engine-cycle illustration?
Diesel four-strokes follow the same stroke order as petrol four-strokes; intake brings in air, compression raises temperature, fuel injection causes power, then exhaust removes gases.
26. Which labelled arrow on an illustration would show the direction of fresh mixture entering a two-stroke cylinder during scavenging?
Fresh mixture flows through the transfer port into the cylinder during scavenging; the spark plug and oil sump are not sources of fresh mixture.
27. Which practical observation in a lab supports the idea shown in diagrams that two-stroke engines are less fuel-efficient than four-strokes?
Fuel smell and smoke indicate unburnt fuel passing through, which is common in two-strokes and shows lower fuel efficiency compared with cleaner four-stroke exhausts.
28. When illustrating valve overlap on a four-stroke engine, what does valve overlap mean?
Valve overlap is the short period when both intake and exhaust valves are open to improve cylinder scavenging and performance; it's often shown on timing diagrams.
29. Which equipment is a typical real-world example of a two-stroke engine you could show Kenyan students in class?
Small chainsaws and many older two-wheeler engines (older boda bodas) use two-stroke designs, making them good local examples; modern cars and big diesels are usually four-stroke or electric.