单选题To obtain maximum efficiency, two stage air compressors are usually （）.Ahorizontally mountedBnever fitted with intake filtersCfitted with intercoolersDoperated in an unloaded condition at all times
单选题
To obtain maximum efficiency, two stage air compressors are usually （）.
A
horizontally mounted
B
never fitted with intake filters
C
fitted with intercoolers
D
operated in an unloaded condition at all times
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第一组： The Wartsila 50DF operates on the lean-bum principle: the mixture of air and gas in the cylinder has more air than is needed for complete combustion. Lean combustion reduces peak temperatures and therefore NOx emissions. Efficiency is increased and higher output is reached while avoiding knocking. Combustion of the lean air-fuel mixture is initiated by injecting a small amount of LFO (pilot fuel) into the cylinder. The pilot fuel is ignited in a conventional diesel process, providing a high-energy ignition source for the main engine. To obtain the best efficiency and lowest emissions, every cylinder is individually controlled to ensure operation at the correct air-fuel ratio and with the correct amount and timing of pilot fuel rejection. Wartsila has developed a special electronic control system to cope with the demanding task of controlling the combustion in each cylinder, and to ensure optimal performance in terms of efficiency and emissions under all conditions by keeping each cylinder within the operating window. Stable and well-controlled combustion also contributes to less mechanical and thermal load on the engine components. Current stringent emission regulations demand the reduction of NOx emissions. In an internal combustion engine this means controlling peak temperature and residence time, which are the main parameters governing NOx formations. In the Wartsila 50DF engine, the air-fuel ratio is very high (typically 2.2). Since the same specific heat quantity released by combustion is used to heat up a larger mass of air, the maximum temperature and consequently NOx formation are lower. The mixture is uniform. throughout the cylinder since the fuel and air are premixed before introduction into the cylinders, which helps to avoid local NOx formation points within the cylinder. Benefiting from this unique feature, NOx emissions from the Wartsila 50DF are extremely and comply with the most stringent existing legislation.What is the combustion feature of the Wartsila 50DF?A.Lean air in the cylinderB.Excessive fuel in the cylinderC.Excessive air in the cylinderD.Spark ignition
Compressed air is supplied by air compressors to______.A.the air receiversB.the starting valvesC.the cylinder coversD.the distributors
Diesel engines obtain combustion air through turbo chargers，blowers，or ______．A.Air startersB.CarburetorsC.Natural aspirationD.Air receivers
单选题Hot gas defrosting uses the hot gas from （） to defrost the evaporator.Astarting air bottlesBdiesel enginesCboilersDrefrigeration compressors
单选题By far the greatest use is for diesel engine starting using air pressure of 25 bar or more provided by （）.Arotary compressorsBreciprocating compressorsCcentrifugal compressorsDscrew compressors
单选题Condensate must be drained periodically from the air compressor receivers to prevent （）.Asecond stage cylinder lockupBoil sump contaminationCfaulty operation of pneumatic valvesDcorrosion of air receiver baffles
单选题Condensate must be drained from the intercooler and after-coolers of an air compressor because （）.Athe cooling effect of the condensate reduces the compressors efficiencyBa danger of explosion exists whenever water is present in a compressorCwater causes erratic operation of pneumatic componentsDthe volumetric capacity of the first stage is reduced if water remains
单选题Tube leaks in the distillate cooler of a two stage flash evaporator will result in （）.Aloss of second stage vacuumBcontamination of the distillateClower feed inlet temperatureDcontamination of the second stage condenser
单选题Intercoolers installed on starting air compressors, reduce the possibility of（）Adust entering the high pressure stageBlube oil carbonizationCdischarge pulsationsDinter-stage vapor lock
单选题In the operation of a flash type evaporator equipped with air ejectors,the air and non- condensable gases are evacuated directly from the （）.Afirst stage flash chamberBsecond stage flash chamberCfirst stage after condenserDsecond stage distilling condenser
单选题Which of the following statements is true concerning the overall efficiency of air compressors?（）AHigh pressure compressors are more efficient than low pressure compressorsBMechanical efficiency divided by compressor efficiency will equal the overall efficiency of the compressor unitCTwo stage compressors are generally more efficient than single stage compressorsDThe volumetric efficiency is decreased in multistage compressors
单选题On small, low pressure, air compressors, the cylinders are usually lubricated by the （）.Asplash methodBmechanical force feed lubricatorsCdetached sump methodDinternal cooling passages in the crankshafts and connecting rods
单选题Increased air （） will raise scavenge efficiency and allow a greater mass of air to be compressed； more fuel may now be burned giving an increased power.Aspecific volumeBdensityCviscosityDcalorific value
单选题Which of the listed operating conditions would have the greatest effect on the volumetric efficiency of an operating reciprocating air compressor?（）AThe density of the air entering the compressorBThe temperature of the air entering the compressorCThe designed rotating speed of the compressorDA leaking head gasket
单选题In reference to air conditioning, when air attains the maximum amount of moisture it can hold at a specific temperature, it is said to be （）.AsuperheatedBsaturatedCcondensedDconvected
单选题In order to distribute the side pressures over a wide area of the cylinder walls and liners, which of the listed types of pistons are used in modem low pressure air compressors?（）ADifferentialBTrunkCBarrelDValve-in-head
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单选题Pump up the starting air reservoirs to their maximum pressure and （）the air reservoirs and air system.AopenBcloseCdrainDvent
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单选题A vacuum is initially established in the first and second stages of a low pressure evaporator by the use of （）Avacuum drag from the auxiliary condensate systemBindividual vacuum pumpsCa single non-condensing air ejectorDa two stage air ejector
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单选题The efficiency of turbocharge system is increased by fitting a charge air cooler （） the compressor.AinsideBbeforeCafterDagainst
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