问答题 6分

Governing prime movers, particularly diesel engines, require careful consideration. The cyclic varia...

Governing prime movers, particularly diesel engines, require careful consideration. The cyclic
variations of a diesel cycle can be passed into the governor drive if it is too "stiff". That is, the cyclic
vibrations will be imposed on the internal gearing etc. inside the governor housing. To prevent this, some
form of damper should be used on the drive into the governor. The mounting point for the governor should
be rigid; a vibration mounting would soon downgrade even the best of governors. Chain or belt drives
should be avoided as far as possible, as slapping of the belt or chain produces speed variations causing
malfunction of the governor. Similarly the drive from the end of the camshaft is subject to torsional
vibrations, and governors should not be located at that point. They should be located as close as practicable
to the fuel pumps, thereby limiting the mass/inertia of operating linkage. In all cases, the governor should
be matched to the engine requirements in terms of droop, load limiting, response time etc. On large engines
there is an inertial resistance to acceleration within the masses of reciprocating and rotating elements of the
engine, so that during acceleration fuel could be released to the cylinders at a faster rate than efficient
combustion can burn it. In an attempt to compensate for this, and to maintain acceptable combustion during
acceleration, a tapping from the scavenge space may be taken and the pressure used to regulate the rate of
fuel released to the engine. This process is incorporated into the governor operating system so that the
governor releases a quantity of fuel commensurate with the increase in air pressure in the scavenge space.
(during deceleration the problem is not so great, as excess air can be tolerated far more easily than can the
incomplete combustion associated with insufficient air.)
(The following questions are based on the passage above)

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5 问答题 6分
For a certain provision cooling plant, the capacity regulator is generally controlled depending on
pressure, temperature or relative humidity. The pulse emitter is therefore either a pressostat(压力继电器),
thermostat or hygrostat. The switching difference of the control units should be rated as large as possible.
This applies in particular to control by means of the pressure, because change to the load in the
refrigeration circuit results in relatively quick pressure changes compared to temperature, which can in turn
lead to inadmissible oscillation of the regulator.

Under partial load, the suction volume and thus the gas speed can undergo marked changes depending on the
regulation stage. For this reason, the rating and routing of the suction line must be given particular attention.
In order to guarantee sufficient oil return, neither full load nor partial load operation may allow the gas speed
to fall below a minimum level: 4 m/s in horizontal pipe sections; 8 m/s in vertical pipes.

For this reason we recommend that vertical pipes bestaggered into two separate sections (as illustration).
As a result of the falling suction gas speed velocity in partial load operation, the pipe elbow B is
blocked by an accumulation of oil. This increases the gas velocity in pipe section A. In order to guarantee adequate
oil return, both cross sections must be rated so large that the velocity does not fall below the minimum level
in full and partial load operation. In plants with extensive pipe systems it is recommended at least for R22 and NH3
to install an additional oil separator. This reduces the oil circulation period and can have positive effects above
all for cold starts with relatively high oil ejection.