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GTP SOCRATIVE QUIZZES (INCL. EXAM SOLUTIONS )
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GTP SOCRATIVE QUIZZES 2022-2023QUIZ 1
1. The principle of propulsion by aircraft gas turbines is the increase of the … of the air.
A. density
B. pressure
C. velocity
D. temperature
Explanation:
Air is accelerated backwards (relative to the aircraft) by the engine. Because of Newton’s
Second Law, it must have experienced a force in the backward direction. Due to Newton’s
Third Law, the aircraft then experienced a force in the forward direction.
2. When is the propulsive efficiency of a jet engine the lowest?
A. At high altitude at maximum velocity and full throttle
B. During idle descent
C. During take-off
D. When doing a test run with zero velocity on the ground
Explanation:
When doing a test run with zero velocity on the ground. Propulsive efficiency is the
percentage of engine power delivered to the aircraft (and not to the accelerated air).
𝑃 = 𝐹 ∙ 𝑣. If the aircraft stands still 𝑃 = 0, so the efficiency = 0.
3. For the calculation of the thrust of an unchoked turbojet, besides the mass flow, we only
need the … of the airflow.
A. gear
B. initial velocity
C. terminal velocity
D. velocity increase
Explanation:
𝐹 = 𝑚̇ ∙ (𝐶𝑗 − 𝐶𝑖 ) ∙ 𝐶𝑗 − 𝐶𝑖 is the velocity increase.
4. What is the most common arrangement of the pistons in a reciprocating engine of a propeller
aircraft?
A. a
B. b
C. c
D. d
→
Explanation:
Circular arrangements are very old-fashioned. For several reasons, the arrangement of two
rows opposite to each other is the best engineering solution.
,5. In a cylinder, we have gas under a free movable piston. On the position is a mass of 5 𝑘𝑔. At
some point, I remove the mass of the piston so that the piston moves up VERY slowly. What
statement is NOT correct?
A. The gas in the cylinder does work
B. The pressure of the gas decreases
C. The temperature of the gas increases
D. There is heat flow to the gas
Explanation:
Because the process is very slow, the temperature stays the same as the ambient
temperature. Let’s also check the other three. Because of the constant temperature,
∆𝑈 = 0. Because the volume increases, the work done by the gas is positive (so A is correct).
Therefore, 𝑄 must be positive (so D is correct), and there must be a heat flow to the gas. For
an isothermal process 𝑝 ∙ 𝑉 = 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡. We know 𝑉 increases, so 𝑝 decreases (so B is
correct).
6. In a petrol engine whose thermodynamics is described by the Otto-cycle, the ignition is:
isochoric
Explanation:
Petrol ignites very quickly, and the piston does not have the chance to move before the
ignition is finished. So, the volume during ignition stays the same and the process is
isochoric.
7. If we want to increase the efficiency of a reciprocating engine (without changing the
dimensions of the piston) then we can do that by:
A. compressing the gas in the piston to a smaller volume.
B. giving the gas a larger density before it flows into the piston.
C. having the gas in the piston reach a higher temperature during ignition.
D. increasing the RPM of the engine.
Explanation:
A larger density before entering the piston, having the gas reach a higher temperature, or
increasing the RPM of the engine, only increases the work and not the efficiency.
RPM = Rotations Per Minute.
8. In a reciprocating engine the compression ratio is:
𝑉
A. 𝑉1
2
𝑉2
B. 𝑉1
𝑝1
C. 𝑝2
𝑝2
D.
𝑝1
Explanation:
This is a definition.
, 9. If we increase the RPM of a reciprocating engine and do not change the fuel-air ratio, then
the work per second (1) … and the specific work (2) …
A. (1) increases (2) stays the same
B. (1) stays the same (2) stays the same
C. (1) stays the same (2) increases
D. (1) increases (2) increases
Explanation:
Specific work is work per kg of air. Imagine we double the RPM. Then twice the amount of air
flows through the cycle. One kg of air still delivers the same amount of work, so the specific
work stays the same. As a result, the total amount of work doubles.
10. A Diesel engine has a (1) … efficiency than a gasoline engine. That is partly because the
Diesel has a much (2) … compression ratio than a gasoline engine.
A. (1) higher (2) higher
B. (1) higher (2) lower
C. (1) lower (2) higher
D. (1) lower (2) lower
Explanation:
Because fuel injection only takes place after compression (avoiding auto-ignition), a Diesel
engine can have a much higher compression ratio. From the formula for the thermal
efficiency, it can be seemed that this improves the thermal efficiency.
QUIZ 2
1. The (1) … is driven by the (2) …
A. (1) compressor (2) turbine
B. (1) fan (2) compressor
C. (1) fan (2) gas flow
D. (1) gas flow (2) turbine
Explanation:
To begin with, there is a difference between airflow (just air), and gas flow (air and fuel). There
is a cyclic process: the turbine drives the compressor (and the fan), the compressor ‘drives’
(compresses) the airflow and the air (+ added fuel, together forming the gas flow) drives the
turbine.
2. A turboshaft is used by:
A. commercial aircraft
B. helicopters
C. jetfighters
D. propeller aircraft
Explanation:
Propeller aircraft use a turboprop, jetfighters a (low-BPR) turbofan and commercial aircraft
a (high-BPR) turbofan. BPR = Bypass Ratio.
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