Questions set 66

2 Stroke Engines

1. How can a two-stroke engine identify by?
a) Piston size
b) Absence of valves
c) Cooling system
d) Lubrication system
Answer: b
Explanation: The two-stroke engine is identified by the absence of valves. Almost all two-stroke engines do not have valves. This makes the two-stroke engine easy to produce and maintain.

2. Which of the following is not an advantage of a two-stroke engine?
a) Not valves
b) Lighter flywheel
c) More uniform torque
d) No lubrication system
Answer: d
Explanation: The two-stroke engines do not have valves. Due to which it makes them easy to manufacture. Also, the two-stroke engine has a lighter flywheel. That is the reason the two-stroke engine produces the more uniform torque.

3. . Which of the following is the most perfect method of scavenging?
a) Reverse flow scavenging
b) Loop scavenging
c) Uniform scavenging
d) Cross scavenging
Answer: c
Explanation: Uniform scavenging is the most perfect method of scavenging. In uniform scavenging, both fresh charge and burnt gases move in the same direction.

4. Which method of scavenging is used in two-wheelers without deflector type piston?
a) Cross scavenging
b) Reverse flow scavenging
c) Uniform scavenging
d) Loop scavenging
Answer: a
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.

5. How much power will two-stroke engine of the same size as the four-stroke engine develop at the same speed?
a) Four times the power
b) Twice the power
c) Half the power
d) Same power
Answer: b
Explanation: The two-stroke engine produces twice the power produced by the four-stroke engine. The power stroke happens in every one revolution in a two-stroke engine.

6. What is the delivery ratio of the crankcase scavenged engine?
a) Greater than 1
b) Less than 1
c) Equal to 1
d) Greater than 2
Answer: c
Explanation: The delivery ratio is the ratio of the air delivered to the reference volume or ratio of the mass of fresh air delivered to the reference mass. R_del = V_del / V _ref or R_del = M_del/M_ref. The delivery ratio usually varies between 1.2 and 1.5 except for crankcase scavenged engines it is less than 1.

7. What is the excess air factor in the case of a two-stroke engine if the delivery ratio is 1.7?
a) 1.0
b) 0.8
c) 0.7
d) 0.9
Answer: c
Explanation: Delivery ratio = R_del = 1.7, excess sir factor = λ. The relation between the R_del and λ is λ = R_del – 1 = 1.7 – 1 = 0.7. The delivery ratio is the ratio of the air delivered to the reference volume or ratio of the mass of fresh air delivered to the reference mass. R_del = V_del / V_ref or R_del = M_del / M_ref.

8. What is the scavenging efficiency for the perfect mixing if the delivery ratio is 1.5?
a) 80.73%
b) 81.73%
c) 82.73%
d) 84.73%
Answer: b
Explanation: Delivery ratio = R = 1.7 and scavenging efficiency = η_sc. The relation between the R and η_sc is η_sc = 1 – e^-R = 1 – e^-1.7 = 1 – 0.1826 = 0.8173 = 81.73%. In perfect mixing, the incoming fresh charge mixes completely and instantaneously with the cylinder contents. A portion which is mixed gets passed out at a rate equal to that of charge entering the cylinder.

9. For two-stroke SI engine, the scavenging efficiency is 0.5, cylinder volume of 1200 cc and the scavenging density is 1.3 kg/m³. What is the mass flow rate of air per cycle?
a) 0.00072 kg/cycle
b) 0.00076 kg/cycle
c) 0.00074 kg/cycle
d) 0.00084 kg/cycle
Answer: a
Explanation: Scavenging density = ρ_sc = 1.3 kg/m³, total cylinder volume = V_tot = 1200cc, mass flow rate of air = ṁ a and the scavenging efficiency = η_sc = 0.5, η_sc = ṁ_a / (V_tot * ρ_sc) = ṁ a = V_tot * ρ_sc * η_sc = 0.0012 * 1.3 * 0.5 = 0.00072 kg/cycle.

10. The two-stroke engine is easy to manufacture.
a) True
b) False
Answer: a
Explanation: Since the two-stroke engine has no valves and valve actuating mechanism, it is simple in making. This makes it easy to manufacture. Also, the flywheel in the two-stroke engine is lighter.