Podcast
Questions and Answers
When does the exhaust process initiate in relation to Bottom Center (BC)?
When does the exhaust process initiate in relation to Bottom Center (BC)?
- 40 to 60° before BC (correct)
- 30 to 50° after TC
- 20 to 30° after BC
- 15 to 25° before Top Center (TC)
The exhaust valve closes approximately 15° before Top Center (TC).
The exhaust valve closes approximately 15° before Top Center (TC).
False (B)
What is the main purpose of the displacement phase in the exhaust process?
What is the main purpose of the displacement phase in the exhaust process?
To scavenge the cylinder using piston movement toward Top Center (TC).
During valve overlap, both the exhaust and intake valves remain _____ for a period.
During valve overlap, both the exhaust and intake valves remain _____ for a period.
Match the following phases of the exhaust process with their descriptions:
Match the following phases of the exhaust process with their descriptions:
Flashcards
Exhaust Initiation
Exhaust Initiation
The phase where burnt gases exit the cylinder due to pressure difference, starting 40-60 degrees before the piston reaches the bottom of its stroke.
Displacement Phase
Displacement Phase
The period during the exhaust process where the piston moves upwards from the Bottom Center (BC) and pushes the remaining burnt gases out of the cylinder.
Valve Overlap
Valve Overlap
The amount of time both the exhaust and intake valves of an engine are open simultaneously.
Intake Process
Intake Process
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Backflow during Valve Overlap
Backflow during Valve Overlap
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Study Notes
Exhaust Process
- Exhaust initiation begins 40-60° before Bottom Center (BC)
- Burned cylinder gases are discharged due to pressure difference between the cylinder and exhaust system.
- Blowdown phase: Gases escape due to pressure differences up to BC.
- Displacement Phase: Piston movement toward Top Center (TC) removes remaining gas from the cylinder.
- Exhaust valve timing closes approximately 15° after TC.
Intake Process
- Valve overlap: Both intake and exhaust valves are open for a period.
- Backflow of exhaust into the cylinder, and cylinder gases into the intake occur during overlap, when the pressure ratio of intake to exhaust is less than one.
- Advantages of valve overlap: Improves volumetric efficiency at high engine speeds due to longer valve open periods.
- Intake valve timing:
- Opens 10-15° before Top Center (TC)
- Closes 50-60° after Bottom Center (BC)
- Gas flow during intake: As the piston moves past Top Center (TC), cylinder pressure drops below intake pressure, enabling fresh charge to flow into the cylinder.
Volumetric Efficiency
- Volumetric efficiency (VE): A measure of how effectively a four-stroke cycle engine functions as an air-pumping device.
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