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A gasoline engine in a factory is fitted with an automatic [#permalink]
01 Apr 2004, 05:57
100% (02:11) correct
0% (00:00) wrong based on 2 sessions
3. A gasoline engine in a factory is fitted with an automatic control
mechanism. Whenever the temperature of the engine's exhaust
increases, the flow of air to the engine is boosted . When more air
is introduced into the engine , fuel flow is increased into the
engine, fuel flow is increased to keep the air / fuel mixture is
constant. As more air and fuel are combusted , the engine's speed and
exhaust temperature both increase.
Which of the following can we expect from an engine with this control
a. The engine will oscillate between medium and low speeds.
b. The engine will not run continuously; the control mechanism will
automatically bring the engine to a stop.
c. The engine will accelerate to its maximum and possible speed and
d. The engine will overheat within an hour of the engine's ignition.
e. The engine will oscillate between high and medium speeds.
I would go with D. C is close but it seems out of scope of this topic!
There is no way to infer that the engine will overheat in exactly 1 hour. It could take 5 min. as it could take 3 days. C is better because according to the process, the temp. will keep increasing and so will the machine's speed. Obviously, it will eventually reach a maximum speed; it will then no longer be able to increase its speed anymore and it will just stay there _________________
It is not very clear where the feedback mechanism is here. Only reasonable thing to be assumed here is that speed reaches its maximum limit and engine temperature can keep on increasing because flow is limited and flue mixture is limited. There is nothing mentioned about controlling temperature
Though D looks fine, I am still ticked off about the one hour time limit.
Since the argument is talking about the temperature of the exhaust and not the engine's it is safe to expect C will occur because more the fuel more the temperature. Since speed is limited and so is the air flow which in turn causes limited fluel flow. Once flue is limited the temperature of exhaust cannot increase.
Choice C is correct. The control system described in the passage is
known as a feedback loop; as increase in the exhaust temperature
results in an increase in fuel flow, which results in an increase in
exhaust temperature, which results in yet another increase in airflow
and so on . Under these conditions , we could only expect the engine
to continue to run faster and faster until it cannot increase the
airflow or the fuel flow, or both. Since it is obviously impossible
to infinetly increase in the air or fuel flows, the engine will reach
some unknown maximum speed, and maintain that speed as in choice C.
Choices A , B and E describe results which might be obtained by some
other engine control system, but not by this feedback loop. It is not
necessarily true that the engine will overheat as in choice D.
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