This set of Automotive Engine Auxiliary Systems Multiple Choice Questions & Answers (MCQs) focuses on “Engine Components – Pistons”.
1. Which are the two materials used in a bimetallic piston?
a) Copper and steel
b) Magnesium and aluminum
c) Steel and cast iron
d) Aluminum and steel
Explanation: The bi-metal piston is made from both steel and aluminum. The skirt is formed by steel and the aluminum alloy cast inside of forms piston head and piston pin bosses. As the coefficient of thermal expansion for steel is quite small, the piston will not expand and hence smaller cold clearance can be maintained.
2. Which among the following is provided on the pistol in a two-stroke engine to increase the compression ratio?
a) Flat spot
Explanation: In a two-stroke engine a deflector is provided on the piston to increase the compression ratio and even in the scavenging process by scavenging out the exhaust gases through the outlet port with the help of the fresh charge transferred via the transfer port.
3. Which among the following is the correct formula of the compression ratio in terms of clearance volume and swept volume?
a) R= swept volume+clearance volume
b) R=swept volume-clearance volume
c) R=2×(swept volume-clearance volume)
d) R=(swept volume+clearance volume)/clearance volume
Explanation: The compression ratio indicates the extent to which the charge in the engine is compressed. This is calculated as the ratio of the volume above the piston at B.D.C to the volume above the piston at T.D.C.R=(swept volume+clearance volume)/clearance volume.
4. By adding which of the following material to aluminum alloy piston slap can be avoided?
Explanation: The material added to the piston to avoid piston slap is silicon. The other advantage of adding silicon is the piston becomes harder and thus less liable to scuffing and wear. However, higher silicon content can also make the alloy brittle which leads to piston developing cracks if the detonation or pre-ignition occurs.
5. Which among the following casting process is used in the casting of the piston?
a) Investment casting
b) Low-pressure casting
c) Sand casting
d) Gravity die casting
Explanation: Pistons are either cast or forged. Castings are made by gravity die-cast process using 5-piece inner dies, a two-piece outer and usually ahead chill which can form anycast valve pockets and bowls. Core pins are used to form pinholes.
6. Which of the following is not a method to avoid piston slap?
a) Taper piston
b) Piston with T-slots
c) Offset piston
d) Increase the size of the piston skirt
Explanation: The methods of reducing piston slap are the use of a vertical or T-slots, taper piston, cam ground piston, use of special alloys, wire wound pistons, autothermic piston, bimetallic piston and offset piston whereas an increase in the piston skirt does not affect.
7. Which is the material used to wound on a wire wound piston?
Explanation: In a wire wound piston a band of steel wire under initial tension is put between the piston pin and the oil control ring, thus restricting the expansion of the skirt which is of slip type or cam ground type. The main reason to wire-wound is to avoid piston slap.
8. Which among the following type is named when the silicon content is 12% in an aluminum alloy?
Explanation: Aluminum alloy containing silicon when increased leads to a decrease in its coefficient of expansion. Presently two alloy with silicon content 12%(eutectic) and 22% (hyper-eutectic) are being used, which have less coefficient of expansion to that of aluminum.
9. Which of the following is the main reason for a burnt piston?
a) Hot air intake
b) Leak of coolant
c) More clearance
Explanation: Burnt piston is chiefly on account of detonation or preignition. The burning due to detonation is generally at a point farthest from the spark plug where the hot end gases rapidly release their energy.
10. Which of the following part of the piston is shown in the arrow below?
c) Piston rings
d) Piston pin boss
Explanation: The arrow mark shows the piston rings which are attached to the piston. There are generally two types of piston rings compression ring and oil control rings generally there is more compression rings on a piston when compared to oil rings.
11. Which of the following is not a reason for piston failure?
a) Insufficient lubrication
b) Overloading the engine
d) Spark plug misfire
Explanation: Piston scuffing occurs due to excessive heat the piston expands and becomes tight in the cylinder. The main reasons for this are insufficient lubrication of cylinder walls, overloading the engine, detonation. Whereas spark plug misfire does not affect.
12. Which if the following is defined as the distance between the axis of the piston pin and top of the piston crown?
a) Clearance volume
b) Swept volume
c) Compression height
d) Detonation length
Explanation: The distance between the axis of the piston pin and the top of the piston crown is called compression height and determines the compression ratio for a given engine. Thus for the same engine, a piston with lesser compression height would give a lesser compression ratio and vice versa.
13. Which material is coated on the piston to increase the service life?
Explanation: Piston is generally coated with tin plating to reduce the possibility of scoring. This even helps the piston in increased the service life of the engine piston. Tin plated surface treatment is antioxidant and tarnish resistant.
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