This set of Automotive Engine Auxiliary Systems Multiple Choice Questions & Answers (MCQs) focuses on “Poppet Valves”.
1. Which of the mentioned below is the advantage of a poppet valve?
a) Moment of inertia
Explanation: Simplicity of construction, self-centering, free to rotate about the stem, maintenance of sealing efficiency are all the advantages of the poppet valve.
2. Generally which valve is larger?
a) Exhaust valve
b) Inlet valve
c) Exhaust port
d) Oil port
Explanation: Generally inlet valves are larger than the exhaust valves because the speed of incoming air-fuel mixture is less than the velocity of exhaust gases which leave under pressure.
3. What is the inlet valve face angle that is generally kept?
d) 45° or 30°
Explanation: The inlet valve face angle is generally kept 45° or 30°. A smaller angle provides greater valve opening for a given lift, but poor sealing because of the reduced sealing pressure for a given spring load.
4. What is the exhaust valve face angle that is generally kept?
d) 45° or 30°
Explanation: The exhaust valve face angle is only 45°, as this increases its heat dissipation. In some cases, a further differential angle of about ½ deg to 1 deg is provided between the valve and its seating.
5. Circumferential and longitudinal direction of valve experience which stress?
a) Shear stress
b) Principal stress
c) Longitudinal cyclic stress
d) Thermal stress
Explanation: Thermal stresses in the circumferential and longitudinal directions due to the large temperature gradient from the centre of the head to its periphery and from the crown to the stem.
6. Which property of the valve resists the tensile loads and stem wear?
c) Both strength and hardness
Explanation: Exhaust valve material requires high strength and hardness to resist tensile loads and stem wear. High hot strength and hardness to combat head cupping and wear of seats.
7. The least coefficient of thermal expansion to avoid excessive thermal stress is experienced by which part of the valve?
Explanation: The Least coefficient of thermal expansion to avoid excessive thermal stress is experienced by the head part of the valve because it experiences the heat from the combustion directly.
8. To avoid corrosion, the valve is given a coating with what material?
Explanation: To avoid corrosion, the valve is given an aluminum coating. Aluminum oxide formed separates the valve stem from the cast iron seat to keep the face metal sticking.
9. What is the highest temperature experienced by the exhaust valve?
Explanation: The exhaust valve temperature in modern engines reach very high valves of the order of 750oC and sodium is present inside the hollow stem of the valve.
10. What is present inside the hollow stem of the valve?
Explanation: The exhaust valve temperature in modern engines reach very high. So, sodium is present inside the hollow stem of the valve. These valves are also used in Heavy engines.
11. Which material is generally used in valve springs?
a) Carbon steel
b) Chromium-vanadium steel
c) Nickel-chromium steel
d) Both carbon steel or chromium-vanadium steel
Explanation: The valve springs are made from high-grade spring steel wire, the materials being generally hard-drawn carbon steel or chromium-vanadium steel.
12. What is present around the valve in the engine for the cooling?
b) Ice pack
c) Water jacket
Explanation: Water jackets are present around the valve in the engine for the cooling and there is a small clearance around the stem of the valve for lubrication.
13. Why there is a small clearance around the stem of the valve?
Explanation: The small clearance around the stem of the valve for lubrication. The valves experience a high temperature since it is in direct contact with the combustion heat. Hence, lubrication is required.
Sanfoundry Global Education & Learning Series – Automotive Engine Auxiliary Systems.
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