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Text The Gas Exchange The Gas Exchange process(process( 换气过程）换气过程）

Lesson 7

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A basic part of the cycle of an internalinternal combustioncombustion engineengine is the supply of fresh air and removal of exhaust gases. This is the gasgas exchangeexchange..

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ScavengingScavenging is the removal of exhaust gases by blowingblowing in fresh air.

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ChargingCharging is the fillingfilling of the engine cylinder with a supply or charge of fresh air ready for compression.

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With supercharging(supercharging( 增压增压 )) a larger mass of air is supplied to the cylinder by blowing it in undunder pressureer pressure.

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Older engines were ‘naturally anaturally aspirated(spirated( 自然吸气自然吸气 )’—taking fresh air only at atmospheric pratmospheric pressureessure.

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Increasing the densitydensity of the charge air by the use of a suitable compressorcompressor between the air intakeintake and the cylinder, the weight ofthe weight of airair inducedinduced per working stroke is increasedis increased.

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Thereby a greater weight of fuel can be burnt with a consequent consequent increase in the cylinder power output.

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This increaseThis increase in charge air density is accomplishedis accomplished on most modern diesel engine types by use ofby use of exhaust gas turbocharging,

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in whichwhich a turbineturbine wheel driven by exhaust from the engine is rigidlyrigidly coupledcoupled to a centrifugalcentrifugal type air compressor.

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Both four-stroke and two-stroke cycle engines may be pressure chargedpressure charged.

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It should be noted that supercharging is not simply a matter of adding equipment to non-superchargednon-supercharged engines.

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The supercharged engine must be designed to withstand withstand the increased pressures and thermal loads which resultswhich results.

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ItIt is essentialessential to the scavenging process that the airthe air entering the cylinder is at a higher pressure is at a higher pressure than the gasthan the gas in the exhaust manifoldmanifold.

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On two-stroke diesels an an electrically driven auxiliary electrically driven auxiliary blowerblower is usually provided because the exhaust gas driven turbo-blower cannot provide turbo-blower cannot provide enough airenough air at low engine speeds.

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High pressure supercharged engines use turbochargers inin serieseries(s( 串联的串联的 )). The pressurizedpressurized airair is usually cooled to increase the charge air density.

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The turbo-blower or turbo-charger has on opposite ends of a single shaft an exhaust gas driven turbine and an air compressor. The compressor and the turbine are sealed from each other.

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ShaftShaft

TurbineTurbine

Air SealAir Seal

CompressorCompressor

PressurizePressurizeairair

ExhaustExhaustgasgas

Fresh airFresh air Fresh airFresh air

ExhaustExhaustgasgas

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It is essential that each cylinder should be adequatelyadequately scavenged of gases before a fresh charge of air is compressed,

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otherwise this fresh air charge is contaminatedcontaminated by residual residual exhaust gasesexhaust gases from the previous cycle.

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Further, the cycle temperature will be unnecessarily high if the air charge is heated byheated by mixing with residual gases and byand by contact with hot cylinders and pistons.

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In the exhaust turbocharged engines the necessary scavenging is achieved by providing satisfactory pressure difference between the air manifoldmanifold and the exhaust manifoldmanifold.

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The air flow through the cylinder during the the exhaust/scavenge overlap exhaust/scavenge overlap periodperiod has a valuable cooling effect;

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it helps toto increase the volumetvolumetric efficiency(ric efficiency( 容积效率容积效率 )) andand toto ensure a low cycle temperature andand also toto obtain a relatively ccoolerooler exhaust imposedimposed onon the turbine blades(blades( 叶片叶片 )).

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In the four-stroke cycle engine there is an adequate overlap between the air inlet valve opening and the exhaust valve closing.

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With two-stroke cycle engines this overlap is limited by the engine design characteristics, and some slight mixing of exhaust gases and incoming air doesdoes occur.

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A number of different scavenging methods are in use in slow speed two stroke engines.

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In each the fresh air entersenters as the inlet port is opened by the downward movement of the piston andand continuescontinues until the port is closed by the upward moving piston.

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The flow pathpath of the scavenge air is decided by the engine port shape and design and the exhaust arrangementsarrangements.

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Three basic systems are in common use: the cross flow(the cross flow( 横流扫横流扫气气 )), the loop(the loop( 回流扫气回流扫气 )) and tthe uniflow(he uniflow( 直流扫气直流扫气 )).

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In cross (or transverse) scavenging the incoming air is directed upwards, pushing the exhaust gases before it. The exhaust gases then travel down and out of the exhaust ports.

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Exhaustoutlet

(a) Cross scavenging

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In loop scavenging the incoming air passes overover the piston crown then towards the cylinder head.

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The exhaust gases are forced before the air passing down and out of exhaust ports located just above the inlet ports.

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With uniflow scavenging the incoming air enters at the lower end of the cylinder and leaves at the top.

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The outlet at the top of the cylinder may be ports (in the opposopposed piston engineed piston engine 对置活塞式发对置活塞式发动机动机 ) or a large valve (in the poppet valve engine).

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Each of the systems has various advantages and disadvantages.

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Cross scavenging requires the fittingfitting of a piston skirtpiston skirt to prevent air or exhaust gas escape when the piston is at the top of the stroke.

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Loop scavenge arrangements have low temperature air and high temperature exhaust gases passing through adjacentadjacent ports, causing temperature differential problems for the liner material.

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Uniflow is the most efficient scavenging system but requires either an opposed piston arrangean opposed piston arrangementment or an exhaust valve in the cylinder head.

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All three systems have the ports angled to swirl(swirl( 使…成漩涡运使…成漩涡运动动 ) ) the incoming air and direct it in the appropriate path.

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It should be emphasized that opposed piston slow speed engines are no longer in are no longer in productionproduction and poppet valves are used in the majority of new designs.

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There are two distinctdistinct turbocharging methods in practical use, respectively termed the ‘constaconstant pressurent pressure’ and ‘pulsepulse’ systems.

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It is the constant pressure system that is now used by all slow-speed two-stroke engines.

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At one time almost all four-stroke engines operated on the pulse system,

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thoughthough constant pressure turbocharging is now becoming more common as it provides greater fuel economy while considerably simplifying the arrangement of exhaust piping.

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READING MATERIAL

A. CHARGE AIR COOLER

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The engine is equippedequipped with a charge air cooler. The insert type charge air cooler is mountedmounted(( 安装安装 )) in a welded casing (air trunk). The trunk(trunk( 扫气箱扫气箱 )) is bolted to the engine block.

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StandardStandard is a two-stage cooler mounted. The first stage, through which the charge air flows, is cooled with water from the HT cooling systemHT cooling system and cools the charge air till approx. 90-100oC.

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The second stage is supplied with water coming from the LT LT water systemwater system and cools the charge air further till the required temperature of approx. 50oC.

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By controlling the water flow through the LT cooler the charge air temperature can be kept constant at the requirement.

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Operation

The circulating coolantcoolant water should always be under positive pressurepositive pressure and should be as clean as possible.

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The operating air temperature should be maintained by controlling the coolant inletinlet temperature.

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Whilst the engine is running the air and coolant temperatures and pressures at inlet and outlet should be checked and loggedlogged periodicallyperiodically.

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Variations from normal indicate faults developing such as foulifouling(ng( 污脏污脏 ,, 堵塞堵塞 )) of the film platfilm plateses or obstruction(obstruction( 堵塞堵塞 ,, 障碍障碍 )) of the water tubes.

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Check daily, during operation of the engine, the working of the telltale(telltale( 监视装置监视装置 ,, 起监视作起监视作用的用的 )) holehole at the free end ofat the free end of the charge air receiver.

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The function of this hole is to inform the operator the presence of water in the charge air receiver. Under normal condition only compressed air should escapeescape from this hole.

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B. OPERATION WITH DEFEDEFECTIVECTIVE((有毛病有毛病 )) OR REMOV

ED AIR COOLER(S)

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If the water tubes of an air cooler are defectivedefective, cooling water can enter the cylinders. If water or water mist(mist( 雾雾 )) flows out of a telltaletelltale holehole, check whether it is cooling water or condensatecondensate((冷凝液冷凝液 )).

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If condensate, increase the charge air temperature a few degrees. If cooling water, stop the engine as soon as possible and pluplug off(g off( 塞住塞住 )) the leaking tube.

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If cooler leakage cannot be remedied by tube plugging, exchange the cooler for a sparespare.

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If no spare cooler is available, the following can be done as an emergency solution.

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Dismantle the cooler for repair and blank off(blank off( 堵住，塞住堵住，塞住 )) the opening in the charge air cooler trunk. Shut off water supply and return pipes.

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If no time is left to remove the defective cooler for repair, shut off water supply and fit air ducts.

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When no full cooling capacity of air cooler can be obtained the engine output must be limited to the range normal full load exhaust temperature allowable.

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In case the turbocharger will surgesurge(( 喘振喘振 )) before the permitted exhaust temperatures are reached, the engine load must be further reduced to avoid continuous surgingsurging.

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Operation with Defective Turbocharger

A defective turbocharger should be treated in accordance in accordance withwith instructions given in the turbocharger instruction book.

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PermissiblePermissible engine output with blocked turbocharger is approx. 20% of full load. The engine output must, however, be limited so that the normal full load exhaust temperatures are not exceeded.

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Maximum permissible exhaust gas temperature after the cylinder at continuous operation with blocked turbocharger is 500oC.

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C. SCAVENGE FIRES

Cylinder oil can collectcollect in the scavenge space of engine. UnburnedUnburned fuel and carboncarbon may also be blown intoblown into the scavenge space.

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These could be caused by suchsuch ffaultsaults asas excessive cylinder lubrication, slack(slack( 松弛的松弛的 )), wornworn or brokenbroken piston rings,

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unevenuneven cylinder liner wear, damaged air inlet filters, crackedcracked(( 破裂破裂 )) oil cooled pistons, faultfaultyy timing timing and defective injectors.

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If flames(flames( 火焰火焰 )) from combustion in the cylinder can blow past the piston into the scavengescavenge trunkingtrunking, this accumulation(accumulation( 积聚物积聚物 )) of flammaflammable mixtureble mixture could be ignitedignited, thus resulting in a scavenge fire.

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A loss of engine power will result, with high exhaust temperatures at the affectedaffected cylinders.

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The affected turbo-chargers may surge and sparks(sparks( 火花火花 )) will be seen at the scavenge drainsscavenge drains.

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The other indications of a scavenge fire are excessive black smoke, paint blistering(blistering( 起泡起泡 )) and peeling(peeling( 脱皮、剥裂脱皮、剥裂 )) from scavenge trunking.

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Once a fire is detected(detected( 发现发现 )) the engine should be slowed down, fuel (should be) shut off from the affected cylinders and cylinder lubrication (should be) increased to minimize the risk of seizureseizure((咬缸咬缸 )).

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All the scavenge drains should be closed. A small fire will quickly burn out, but where the fire persistspersists the engine must be stopped.

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A fire extinguishingfire extinguishing medium should then be injected through the fittingfitting provided in the scavenge trunking. On no On no accountaccount should the trunking be opened up.

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To avoid scavenge fires occurring the engine timing and equipment maintenance should be correctly carried outcarried out.

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The scavenge trunking should be regularly inspectedinspected and cleanedcleaned if necessaryif necessary.

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Where carbon or oil build up is found in the scavenge, its source should be detected and the fault remediedremedied.

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Scavenge drains should be regularly blown and any oil discharges (should be) investigatedinvestigated at the first opportunity.

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Charging-air pressure too low or too high

Charging-air pressure falls if:(a) Air filters are blocked( 阻塞 ) or there is too great an air pressure drop(下降 ) in the suction air duct.(b) Air loss in the air duct, caused by a damaged gasket(垫片 ).

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(c) Loss of exhaust gas in the duct between diesel engine and exhaust gas inlet housing(管、罩 ).(d) Damaged sealing rings on the blower side of the gas inlet housing.

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Charging-air pressure increases if:(a) There is incorrect combustion.(b) Drop in the mechanical efficiency(机械效率 ) of the diesel engine.

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C. Scavenging-air pressure in the charge-air receiver(扫气箱 ) drops

Possible causes for this are:(a) Turbo-charger is either fouled or defective.(b) Loss of exhaust gases before turbine.

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(c) Increasing exhaust gas back-pressure(背压 ) after turbine.(e) Intake filters(吸气滤器 ) of turbocharger or air cooler are fouled.(f) Air losses due to leaking.

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Exhaust temperature in manifold pipe(排气歧管 ) rises

Possible causes for this are:(a) The engine is thermally(热力的 ) overloaded.(b) Ports in working cylinders are fouled.

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(a) The engine does not receive enough air because of defective of fouled turbocharger or air space.(b) There is fire in a scavenging air space.