Two wheelers without deflector type piston use_________.

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JKSSB JE Mechanical 02 Nov 2021 Shift 1 Official Paper
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  1. Uniflow Scavenging
  2. Reverse Flow Scavenging
  3. Loop Scavenging
  4. Cross Scavenging

Answer (Detailed Solution Below)

Option 4 : Cross Scavenging
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Detailed Solution

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Concept:

Scavenging: Clearing exhaust gas from the cylinder and replenishing the cylinder with fresh air.
Importance of Scavenging: Scavenging affects –
  • Overall efficiency
  • Power output per unit weight
  • Fuel consumption per unit power
  • Incomplete scavenging results in poor combustion
  • Lubricating oil contamination
  • Wear piston rings and liner
  • Mean temperature of cylinder high
Comparison between Scavenging systems:
  • Cross Flow Scavenging is the gas exchange process in which the charge air passes is directed upwards, passing under the cylinder cover and down the opposite side, expelling exhaust gas through exhaust ports on that side. This system is obsolete now.
    • It is the simplest system
    • Lower efficiency
    • Rare in diesel engine, however it is used in petrol engine.
  • Loop Scavenging is the gas exchange process in which he charge air passes over the piston crown and rises to form a loop within the cylinder, expelling exhaust gases through exhaust ports cut in the same side of the liner above the\ scavenge ports.
    • The inlet air ports are angled to give rotary swirling effect to the incoming air.
    • It also required long piston skirt to prevent scavenging air leak into exhaust while the piston is at top of it’s stroke.
    • Simplicity, Reliability and cylinder cover is simplified.
    • Temperature gradient exhaust to scavenge ports is high
    • Exhaust back pressure higher with deposits
    • Possibility of intermixing air and gases
    • Piston and liner distortion and uneven piston ring wear due to ports
    • Scavenge efficiency is lower than uniflow scavenge efficiency.
  • Uniflow scavenging is the gas exchange process in which the charge air passes straight up through the length of the cylinder, forcing the exhaust gas through exhaust valve at the top of the cylinder.
    • It is used extremely wide range of engines from small to large engine (15000 kW).
    • Highest scavenge efficiency.
    • Drop in scavenge efficiency lowest with high scavenge pressure
    • Most suitable for pressure charging
    • Greater area for air and gas flow
    • Possibility of short circuiting and mixing less

Explanation:

  • Cross scavenging is used in two-wheelers without deflector type piston.
  • In this, the fresh charge is directed upward, and then down to force out the exhaust gases through the oppositely positioned exhaust port.
  • So, the correct answer is option 4.
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