Piston-Engine-Lubrication-飞机活塞发动机原理

1、Piston Engines 2,Engine Lubrication,Agenda - Lubrication,Review engine air and liquid cooling and associated components Describe functions of lubricants: Describe characteristics of engine lubricants: Describe common standards and specifications for lubricants: Explain advantages and disadvantages o

2、f different lubricants Explain Precautions when intermixing oil types Describe types of lubrication systems Wet Sump Dry Sump Describe external components of reciprocating engine oil lubrication system Describe methods for internal lubrication of reciprocating engines Describe maintenance practices

3、for reciprocating engine oil lubrication systems,Review Engine Cooling Systems,Air Cooling: transfers heat of engine from to the air passing through cooling fins on cylinders directed by cowlings, baffles and baffle seals. Liquid cooling: transfers heat from engine to liquid medium (oil or engine co

4、olant) and then heat is extracted from cooling medium by passing it through a radiator (liquid to air heat exchanger),Air Cooling System,Liquid Cooling System,Liquid Cooling System,Functions of Lubricant,Friction caused by metal to metal contact Cooling extracting heat from various engine components

5、 Sealing elimination of gaps or spaces between components Cushioning absorbing of impact or pounding force Cleaning removal of contaminants Corrosion build up of oxidants (common term is rust),Friction,Any two moving engine components contacting each other will generate friction This friction is cau

6、sed by surface imperfections such as microscopic peaks and valleys By coating the two components with a thin film of engine oil, the peaks and valleys are filled in and friction is reduce Height of these imperfections and clearance between components will determine how thick or viscose the oil must

7、be.,Friction,Cooling,Conduction of heat energy from hot engine parts into oil film bathing parts will cool parts. Oil then must be collected or removed from area and cooled Heated oil is transferred to an oil air heat exchanger (oil cooler) to cool oil Fresh cooled oil applied to hot components help

8、s to control component temperature,Sealing,“Wetting” characteristic of oil allows for a thin even film of oil to spread over parts and maintain the film through a wide variety of movement. This thin film provides an excellent seal between moving parts as we see with the oil film seal inside a cylind

9、er between the moving piston rings and the static piston walls.,Cushioning,Oil characteristics that make it a good sealing tool also helps to absorb or reduce the detrimental effect of impact between components. An Example would be the layer of oil on valve train. In this case the thin layer of oil

10、acts to cushion between the valve, rocker pushrods and tappets,Cleaning,Oil also reduces wear by acting as a cleaning agent As it circulates through the engine over the various components it also removes contaminants left by combustion and wear. These contaminants are trapped in the oil and taken aw

11、ay. Most of them will be removed in the filtering process, however some will remain thus making it necessary to change the oil on a regular basis,Corrosion Protection,Metal surfaces in engine are exposed to moisture and chemicals that can cause corrosion (oxidation). A thin oil film acts as a barrie

12、r that does not allow moisture and oxygen to form corrosion If adhesion of oil is not sufficient or oil is let settle too long, film will drain off and allow for corrosion to form,Properties of Oil,Oil specification requirements must ensure that they are heavy enough to remain in place yet thin enou

13、gh to flow freely. There are several factors used to describe oil, over the next few slides we will look at these. There are several factors used to determine engine requirements. Some of these include loading, operational speeds and temperatures,Oil Properties,Viscosity and Index Specific Gravity C

14、olour Cloud and pour point Flash and fire point Carbon residue Anti-foaming quality Operating range Resistance to oxidation Non-corrosive,Viscosity,Viscosity is the liquids resistance to flow Oil that will flow slow is said to be viscous or has a high viscosity Oil that will flow easily has a low vi

15、scosity Viscosity is measured using a Saybolt Universal Viscosimeter Viscosity number determined by time it takes for a liquid heated to 210 F and passed through a metered orifice giving a Saybolt Seconds Universal (SSU) viscosity. Typical aviation oil results include 65, 80, 100 & 120 In colder tem

16、peratures the higher numbered oils wil thicken to almost a solid state Lower viscosity numbered oil is used in colder parts of the year,Viscosity Index,Used to identify an oils rate of viscosity change for a given temperature change Two baseline or reference oils chosen by the American Society of Te

17、sting and Materials are assigned a reading of Zero & 100 respectively. The lower the change in viscosity, the higher the number,Specific Gravity,Comparison of the weight of a specific volume of oil to the same volume of distilled water American Petroleum Institute (API) has a more accurate means of

18、determining gravity but is easily converted to specific gravity using a scale,Colour, Cloud & Pour,Colour measures the light reflection through the liquid, can identify the origin of the refined oil Cloud Point temperature at which the paraffin wax & other chemicals normally suspended in the oil beg

19、in to solidify and separate into crystals Pour Point minimum temperature that oil will flow or be poured,Flash & Fire Point,Flash point temperature that oil will begin to emit flammable fumes Fire Point temperature at which an oil will emit enough fumes to support combustion For a standard aviation

20、oil, fire point is at least 50 degrees above the flash point,Carbon Residue and Ash Tests,Carbon Residue Test oil is heated to evaporation point, weight of residual carbon is then compared to original oil weight to give a carbon percentage Ash Test residual carbon is burned, newer oils leave very li

21、ttle ash, but this ash is then analysed for iron & lead content to determine engine wear,Oil Grading System,Commercial Aviation Oil Grade standard numbers assigned to viscosity level include 65, 80, 100 & 120 Society of Automotive Engineers (SAE) number assigned to oil relating to its viscosity at e

22、ither 130 or 210 degrees F. SAE 30 oil is equivalent to aviation grade 65 oil (colder temperature low viscosity grade) SAE grades that are usable in winter are designated with a “W” beside the SAE number,Oil Grading System,SAE standard only equates to viscosity and mot the other essential elements u

23、sed to determine the aviation grade oil standard Military AN Standard Engine oil is also classified by an AN number Oil identification cross reference chart figure 9-2 Oil Characteristics chart figure 9-3,Oil Identification,Oil Grading,Types of Oil,Mineral Oil Original type of aviation oil, effectiv

24、e lubricant but not quite as good as newer oils Lacks characteristics of newer oils. Tends to oxidize (foam) when aerated by agitation Tends to sludge (gel) when exposed to contaminants Used for break-in period,Types of Oil,Ashless-Dispersant Oil Does not have carbon forming restrictions of mineral

25、oil Does not generate ash that will contain suspended metal particles like mineral oil Contains dispersant that will prevent slug formation by holding particles that form sludge in suspension so they can be removed in filter This prevents sludge build-up on engine parts Also contains anti-wear & ant

26、i-foam additives and thus is so effective that it is not used for break-in period,Types of Oil,Multi Viscosity Oils Allows operation in a wide temperature range SAE 10 oil will get too hot and thin in higher operating temperatures SAE 30 oil will be too thick in colder temperatures to flow adequatel

27、y during start-up Standard aviation multi-grade: 15W50,Types of Oil,Synthetic Oil Chemical blend that includes synthesized extracts from mineral, vegetable and animal oils as the stock base rather than a petroleum stock base. Maintains a constant viscosity throughout an extremely wide temperature ra

28、nge, extremely low internal friction and very resistant to thermal breakdown Does have increased slugging tendency resulting from decreased ability to disperse and suspend contaminants Tendency to soften rubber products and resins in piston engine parts Many brands are not compatible with mineral oi

29、ls and with other brands of synthetic oil Tends to blister paint wipe with petroleum solvent if spilled,Types of Oil,Extreme Pressure Lubricants Used as grease for areas requiring high load capabilities and stability such as in propeller gearing Extreme viscosity oils such as SAE 90 used for propell

30、er reduction gear cases,Oil Distribution (figure 9-4),Pressure Lubrication: primary form of lubrication where oil is sent under pressure from positive displacement oil pump through passages to components Splash Lubrication: oil is slung around inside engine after being picked up by moving parts and

31、then thrown of to various parts Spray Lubrication: used in larger engines where additional lubrication is required by the addition of spray nozzles inside the engine fed by the pressure oil system Combination System: all engines use pressure and splash, larger engines all three of the above,Oil Syst

32、em Classification,Wet Sump Oil System: standard system used on most smaller piston engines. Most elements are contained within the engine including the oil stored in the sump at the bottom of the engine Dry Sump Oil System: used on larger engines requiring more volume of oil and for engines that req

33、uire extreme aerobatic flight system (includes inverted flight). Oil is stored outside engine in a tank and thus required a scavenge pump to return oil to the oil reservoir tank from the crankcase sump with check valves to prevent draining back from the tank to the sump,Dry Sump,Lubricating System C

34、omponents,Oil Reservoir: very similar to non-pressurized hydraulic reservoir, size determined by the maximum endurance requirements of the engine. Oil is drawn from near the bottom of the tank unless it is equipped with a Hamilton Standard Full feathering propeller. In this case, a volume of oil is

35、always available for the feathering pump. See figure 9-7,Oil Tank,Lubricating System Components,Oil Pumps: constant displacement engine driven pumps (gear type or gerotor type pumps) used to deliver oil to the pressure system. In the case of a dry sump system an additional pump is used to return oil

36、 to the reservoir referred to as a scavenge pump. (ref figures 9-8 & 9-9),Lubricating System Components,Pressure Relief Valve: set into the system on the outlet side of the pump using an adjustable spring loaded ball or dome type valve assembly. Excessive oil pressure pushing opposite to the spring,

37、 opens the valve to send excessive oil back to the reservoir. This spring tension is either increased or decreased using a threaded adjuster screw or by placing or removing spacer washers under the spring cap to increase or decrease the spring force causing a change in oil pressure required to move

38、the valve off the seat and thus a change in the force required to open the valve. This will either increase or decrease the operating oil pressure. (ref. fig. 9-10) A two stage compensated oil pressure relief valve is used on some engines to have high pressure relief when oil is cold and a low press

39、ure relief once oil warms to operating temperature. (ref. fig. 9-11),Oil Pressure Relief Valve,Oil Pressure Relief Valve,Oil Pressure Relief Valve,Compensator Relief Valve,Lubricating System Components,Oil Filters Full Flow system sends all oil from the outlet side of the pump, after the relief valv

40、e though the filter (ref. fig. 9-12) By-pass or Partial Flow system sends approximately 10% of the oil through the filter then back to the reservoir each time the oil circulates through the system. (ref. fig. 9-13),Filter Systems,Lubricating System Components,Oil Filter Bypass Valve: Design standard

41、 requirements mandate full required flow even if the filter is plugged. Like the hydraulic filter system, should the filter plug, pressure differential between inlet and outlet will increase This differential will cause the valve to open, bypassing the filter. A bypass indicator may also be incorpor

42、ated to let operator know about the bypass.,Full Flow Spin-on Cartridge Filter,Types of Oil Filters,Only four approved types of filters Depth Filtration uses a one in thick matrix of fibres that requires oil to pass through the entire mat that can hold a significant amount of contaminants. Semi-Dept

43、h Filtration: most commonly used method that includes a fibre matrix firmed into a thin paper like material that is packed inside the circumference of the spin-on filter. Oil enters the filter around the outside and exits through the spin-on or element filter core.,Oil Filtration,Surface filtration is the use of a oil screen to remove contaminants (ref. fig. 9-17) Edge filtration uses a spiral wound metal strip with ridges on one flat to act as spacers for oil to pass between. As the oil passes through the space between the wraps (wound like a coil) particles are