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.
30. Looking at an illustration of a four-stroke petrol engine, which stroke is represented when the piston moves down and the intake valve is open?
In the intake stroke the piston moves down with the intake valve open to draw the fuel-air mixture into the cylinder; illustrations typically show a downward piston and open intake valve for this stroke.
31. In a two-stroke engine diagram, scavenging refers to which process?
Scavenging in two-stroke engines is the process of pushing out burned gases and drawing in a fresh charge, often shown in illustrations by gas flow arrows through ports.
32. When comparing illustrated cycles, which feature is typical of a four-stroke engine but not of a two-stroke engine?
Four-stroke engines have distinct intake and exhaust strokes controlled by valves; two-stroke designs usually use ports and combine processes, which is clear in cycle diagrams.
33. An illustration shows the piston at top dead centre with both valves closed and the spark plug firing. Which stroke is this in a four-stroke petrol engine?
At top dead centre with valves closed and spark about to fire, the mixture is fully compressed — this is the compression stroke before the power stroke in a four-stroke engine.
34. A cross-sectional drawing of a two-stroke engine shows no camshaft and no valves. Instead it shows holes in the cylinder wall. What are these holes called?
Two-stroke engines commonly use ports (openings in the cylinder wall) for intake and exhaust instead of valves; illustrations label these as transfer, intake and exhaust ports.
35. In an instructional diagram comparing power output per engine revolution, which statement describes a two-stroke engine correctly?
Two-stroke engines produce a power stroke once each crankshaft revolution, unlike four-stroke engines which do so every two revolutions; diagrams show power events more frequently for two-strokes.
36. An illustrated sequence for a four-stroke diesel engine shows air being compressed to high pressure without a spark plug. What happens at the end of this compression in diesel operation?
Diesel engines compress air to high temperature and then inject fuel which ignites spontaneously; illustrations of diesel cycles show injection rather than spark ignition.
37. When using an illustration to teach valve timing in a four-stroke engine, which part of the camshaft diagram shows when valves open and close?
Valve timing is determined by the camshaft lobes which push on tappets or rockers to open valves; diagrams show lobe shape and position to indicate opening and closing events.
38. A classroom illustration compares two engines: one labelled 'two-stroke' and the other 'four-stroke'. Which practical example is most likely to be a two-stroke engine commonly seen in Kenya?
Older lightweight motorcycles (boda boda) and handheld equipment often used two-stroke engines; illustrations of such engines typically show simpler construction and ports instead of valves.
39. In an illustrated comparison of lubrication methods, which statement matches two-stroke engine lubrication often shown in diagrams?
Many two-stroke engines mix oil with fuel for lubrication, and diagrams show the oil-fuel mixture passing through the crankcase and around the piston; four-strokes usually use separate sump oil with a pump.
40. An illustration shows the four stages labelled intake, compression, combustion, exhaust. Which stage is another name for the combustion stroke?
The combustion stroke is commonly called the power stroke because the burned gases push the piston down to produce useful power; diagrams often label it as 'power'.
41. Looking at a two-stroke cycle illustration, which diagram feature indicates that exhaust and intake occur almost together?
Two-stroke illustrations show ports uncovered by the piston at similar times, so exhaust and incoming charge overlap, which is a key feature of two-stroke scavenging.
42. When using a diagram to compare fuel efficiency, which statement is generally true about four-stroke engines compared to two-stroke engines?
Four-stroke engines tend to burn fuel more completely and have separate lubrication, making them more fuel-efficient and lower in emissions than many simple two-stroke designs; diagrams show less scavenging loss.
43. An illustration labelled 'stroke sequence' for a four-stroke engine uses arrows for piston motion. Which sequence of piston movement describes the four-stroke cycle correctly starting from intake?
Starting with the intake stroke (piston moves down), then compression (up), power (down), and exhaust (up) — the piston movement sequence is down, up, down, up.
44. A teacher uses an exploded-view illustration of an engine to show the cylinder head. Which component seen in a four-stroke head controls the opening and closing of ports?
In four-stroke engines the camshaft operates valves in the cylinder head which control intake and exhaust; exploded diagrams highlight these head components.
45. In an illustrated lab activity, students compare sound and combustion events. Which illustration feature best shows where ignition occurs in a petrol four-stroke engine?
Petrol (spark ignition) four-stroke engines ignite the mixture with a spark plug usually drawn at the cylinder head; illustrations point to the spark plug to show ignition location.
46. An illustration of a cross-section of a two-stroke shows the crankcase being used for pumping the mixture. What does this mean for the engine design compared to a four-stroke?
Many two-stroke engines use the crankcase to compress and transfer the fuel-air mixture into the cylinder; illustrations show inlet from carburettor to crankcase then up through transfer ports.
47. When studying pistons in illustrated engine diagrams, which feature indicates top dead centre (TDC)?
Top dead centre is when the piston reaches its highest position in the cylinder; diagrams label TDC to show compression or ignition points.
48. A diagram shows the sequence for a two-stroke engine. Which sentence correctly describes when exhaust gases leave the cylinder?
Two-stroke diagrams show the piston uncovering the exhaust port around bottom dead centre, allowing burnt gases to leave as fresh charge enters through transfer ports.
49. Using an illustration to teach compression ratios, which sketch feature would show a higher compression ratio in a four-stroke engine?
Compression ratio is the ratio of cylinder volume at bottom dead centre to volume at top dead centre; diagrams showing a big difference indicate a high compression ratio.
50. An illustration used in class shows a four-stroke engine with timing marks on the crankshaft and camshaft. What is the purpose of these timing marks?
Timing marks ensure the camshaft and crankshaft are correctly aligned so valve events occur at the right piston positions; diagrams show marks for setting timing during maintenance.
51. In an illustrated classroom experiment, students trace the path of gases in a two-stroke engine shown with arrows. Which arrow direction shows fresh charge entering the cylinder?
Transfer ports route the fresh mixture from the crankcase into the combustion chamber in two-stroke engines; illustrations use arrows to show this incoming charge.
52. A diagram of a four-stroke engine labels the sequence intake, compression, combustion, exhaust. Which two strokes involve the piston moving upward?
In a four-stroke cycle the piston moves upward during compression (to compress the mixture) and exhaust (to push out burned gases); intake and power are downward movements.
53. Using an illustration, how can students visually tell that a two-stroke engine has fewer moving parts than a four-stroke engine?
Two-stroke illustrations often omit camshafts and valves (using ports instead), which visually demonstrates the simpler mechanical layout with fewer moving parts.
54. An illustration compares exhaust emissions from two engines. Which typical drawing difference explains why two-stroke engines often emit more smoke?
Many two-stroke engines mix oil with fuel and have overlap in exhaust and intake (scavenging), so illustrations show unburned mix and oil being expelled, causing visible smoke.
55. Looking at a training poster of an engine cycle, which visual detail indicates the moment of maximum pressure in the cylinder for a four-stroke petrol engine?
Maximum pressure occurs near top dead centre after the spark ignites the compressed mixture; posters show this point as the peak pressure moment during the power stroke.
56. An illustrated worksheet asks why two-stroke engines are lighter for the same power. Which reason would the picture-based answer most likely show?
Two-stroke engines are often lighter because they do not need a valve train (camshafts, valves, rockers); illustrations show simpler internals leading to lower weight.
57. A classroom diagram of a four-stroke diesel engine shows fuel injection at the end of compression. Which illustrated label would show where the injector spray enters?
Diesel injectors spray fuel directly into the combustion chamber in the cylinder head area; diagrams mark the injector position near the top of the cylinder.
58. When students sketch engine cycles, which simple symbol in an illustration best represents a spark plug used in a petrol four-stroke engine?
Spark plugs are usually drawn as a small electrode gap at the top of the combustion chamber; this symbol shows where ignition will occur in petrol engines.