| Name: ______________________________ | Date: ______________ |
Instructions
- Study how a camshaft controls the opening and closing of engine valves.
- Use the vocabulary and timing ideas to solve each task.
- Remember that a four-stroke engine completes one cycle in two crankshaft revolutions.
- Show careful reasoning when calculating valve timing.
Example: A four-stroke engine has a camshaft turning at 1,200 rpm. How fast is the crankshaft turning?
Answer: The crankshaft turns twice as fast as the camshaft: 1,200 × 2 = 2,400 rpm.
Part 1: Camshaft Vocabulary
Complete each sentence with the correct technical term.
1. The raised portion of a camshaft that pushes a follower is called a __________.
2. The camshaft controls the opening and closing of the engine's __________.
3. In a typical four-stroke engine, the camshaft rotates at __________ the speed of the crankshaft.
4. The circular part of a cam lobe that does not lift the follower is called the __________ circle.
5. The distance a valve is lifted from its seat is called valve __________.
Part 2: Inside the Valve Train
Choose the best answer for each question.
6. Why does a four-stroke camshaft rotate at half crankshaft speed?
| ○ A. The piston completes two strokes per crankshaft revolution | ○ B. The four-stroke cycle needs two crankshaft revolutions |
| ○ C. The valves open only during the exhaust stroke | ○ D. The camshaft is always smaller than the crankshaft |
7. What usually happens when a cam lobe reaches its greatest height under a follower?
| ○ A. The valve is lifted its greatest distance | ○ B. The valve is completely closed |
| ○ C. The crankshaft stops turning | ○ D. The fuel pump shuts off |
8. Which valve normally opens to let the air-fuel mixture enter a spark-ignition cylinder?
| ○ A. Intake valve | ○ B. Exhaust valve |
| ○ C. Relief valve | ○ D. Thermostat valve |
9. What is one likely result of incorrect camshaft timing?
| ○ A. The valves open at the wrong moments | ○ B. The engine oil becomes gasoline |
| ○ C. The piston becomes magnetic | ○ D. The coolant turns into air |
Part 3: Timing Truth Check
Write True or False for each statement.
10. A camshaft changes rotary motion into timed motion that operates the valves. True / False
11. In a four-stroke engine, the camshaft normally turns twice for every one turn of the crankshaft. True / False
12. Valve overlap is the short period when the intake and exhaust valves are both slightly open. True / False
13. Increasing camshaft lift always guarantees better engine performance at every speed. True / False
14. A worn cam lobe can reduce valve lift and affect engine breathing. True / False
Part 4: Cycle Timing Lab
Complete the missing crankshaft angles and engine strokes using the four-stroke sequence.
| Stroke | Crankshaft angle range | Piston movement | Main valve action |
|---|---|---|---|
| Intake | 0°–180° | Moves downward | Intake valve open |
| 180°–360° | Moves upward | ||
| Moves downward | Both valves closed | ||
| 540°–720° | Exhaust valve open |
Part 5: Engine Detective Challenges (optional challenge)
Solve each problem and include the requested unit or phrase.
16. A camshaft is turning at 1,500 rpm in a four-stroke engine. What is the crankshaft speed? Answer: __________ rpm |
17. A crankshaft turns at 4,800 rpm in a four-stroke engine. What is the camshaft speed? Answer: __________ rpm |
18. An intake valve opens at 10° before top dead center and closes at 40° after bottom dead center. For how many crankshaft degrees is it open? Answer: __________ crankshaft degrees |
19. An exhaust valve opens at 50° before bottom dead center and closes at 15° after top dead center. For how many crankshaft degrees is it open? Answer: __________ crankshaft degrees |
20. A cam lobe has a base-circle radius of 20 mm and a nose radius of 25 mm. Ignoring any rocker-arm ratio, what is the valve lift? Answer: __________ mm |
Answer Key
| 1. | cam lobe |
| 2. | valves |
| 3. | half |
| 4. | base |
| 5. | lift |
| 6. | B (The four-stroke cycle needs two crankshaft revolutions) |
| 7. | A (The valve is lifted its greatest distance) |
| 8. | A (Intake valve) |
| 9. | A (The valves open at the wrong moments) |
| 10. | True |
| 11. | False |
| 12. | True |
| 13. | False |
| 14. | True |
| 15a. | Compression · 180°–360° · Moves upward · Both valves closed |
| 15b. | Power · 360°–540° · Moves downward · Both valves closed |
| 15c. | Exhaust · 540°–720° · Moves upward · Exhaust valve open |
| 16. | 3,000 rpm |
| 17. | 2,400 rpm |
| 18. | 230 crankshaft degrees |
| 19. | 245 crankshaft degrees |
| 20. | 5 mm |