reciprocating engine basic (common flat four layout )

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 This is a reciprocating engine working on the four strock cycle in two crankshaft  complete turns ;

One piston will move 4time in it's barrel and only one of those move will develop torque to the propeller this will happen during 2 turns of the crankshaft

• First move during  the first 1/2 turn of the crankshaft the piston starting from against the head goes down in the cylinder sucking air through the intake valve judiciously opened at the time by the rocker arm and pushrod  . By doing so it also suck gas provided in the appropriate ratio by the carburetor or injection. The big limiting factor here is the quantity of gas-air mixture that can be sucked in : the higher you are the lesser air  and oxygen there is . There are some ways to solve this problem :  
  1. one rather short term solution : like for the pilot the idea is to  provide the engine with oxygen enriched air . This have been done on the old fighter by providing the engine with nitrous oxide (N O2 the important thing is the O2 side :it is oxygen) this permit  to use combat power for a short time.
  2. one other solution : here  the idea is to  pressurize the air to the sea level value or higher . This is done by a compressor that is geared to the crankshaft , some power is lost through the compressor but the net gain in engine power is still interesting some fighter were even fitted with a "gearbox" to permit using the high blower speed at high altitude when even the standard compressor was not enough . The compressor drawbacks are : weight ,reliability , efficiency
  3. one very common solution : the idea is to  pressurize the air above the ambient value and up to a design value if possible using a device called turbo compressor the "turbo" . It is made of 2 parts : a turbine spinned by the exhaust gas and attached to it  there is a compressor blowing air to the intake manifold . The big advantages of the turbo is it's efficiency : it uses otherwise wasted exhaust gases energy , it is also relatively lightweight and rather simple to regulate  ; The turbo drawbacks are : the possibility of overboosting the engine should the regulation fail, some cheap turbos do not even have regulation so the pilots have to keep an eye on it (work load issue)  . There is also a rather noticeable inertia (the turbo lag at least 5 second and often much more (again  the work load issue)) , the engine working with compressed air give more power output but also more heat that is not always easy to get rid of at high altitude where the cooling air is thin , and this is also hard on the pistons the heads and the cylinders , an other rather surprising drawback : at very high altitude a power reduction will let the turbine decelerate but when power will be applied again by the pilot there will not be enough compressed air to produce sufficient exhaust flow to spool up the turbine : the turbo will not restart at this altitude the pilot will  need to goes down . Also because the engine is built for working with more air than the cylinder can take the compression ratios must been kept lower than for non turbo engines  in order to avoid detonation . This fact is important because when the engine is idling the turbo does not work and it result in the effective compression ratio to be too low leading to the possibility to foul the spark plugs .
  4. nearly the same solution : like for the standard turbo the idea is to  pressurize the air up to the sea level value but not more  using a turbo . This is called turbonormalising The big advantages of the turbonormalising is that it can be fitted easily on non turbo engine  keeping the original engine compression ratio because the pressure is not supposed to goes above the sea level value ,  it can maintain this pressure up to a useful altitude for the light airplanes category . The turbonormalising drawbacks are : the same as the plain turbo but to a much lesser extent : there is only a slight inertia variable with altitude and the heat out put issue  is much easier to handle , also note that the efficiency is not as good  

• Second move during  the second 1/2 turn of the crankshaft the piston starting from down (bottom dead center)  goes up against the head in the barel compressing the gas-air mixture previously sucked in  , now the intake valve  and exhaust valves are closed
• Third move the only one developing torque this move is initiated by a spark firing the  compressed gas-air mixture , due to the heat developed this mixture greatly expand pushing down the piston during the third 1/2 turn of the crankshaft the piston starting from against the head goes down in the cylinder with both valves closed , the rotational inertia acquired during this third move will be used by all the other moves to accomplish their work
• Fourth move during the fourth 1/2 turn of the crankshaft the piston starting from down (bottom dead center)  goes up against the head in the cylinder but this time the exhaust valve opens at the beginning of the move letting the burned  the gas-air mixture escape through the exhaust valve , when the piston arrives  against the head at (the dead top center) the exhaust valve closes and the intake valve opens for the first move again .....

common facts about turbos:  They are oftenly used to provide high pressure air for the cabin pressurization and pressure to the deice boots . Also very important they provide pressure for the magnetos : the air is used as an insulating media in the ignition system if the air pressure drop with altitude the insulation will also drop to a point where a short circuit (arc) will occur failing the magnetos . In order to avoid this high altitude flying airplanes are fitted with pressurized magnetos . 

handling specials : 

• If working with the constant flow system theN O2  usage require to engage the device only at an already high power setting (high engine rpm) , if it is engaged at low rpm during one engine cycle it will have sufficient time to provide the cylinders with an excess of combustible mixture resulting in extreme high cylinder pressure ; extreme detonation ... those leading to the possibility of immediate engine catastrophic failure (possible explosion) .

• Some compressor are fitted with a clutch permitting to engage or disengage when needed ; it should be engaged only when really needed because of mechanical wear and questionable availability and cost of old fighter spare parts . 

• It is very important for the turbos to adhere to very strict engine cooling procedure before engine shut down : when some power is applied the turbine part of the turbo is immersed in a flow of exhaust gas at around 900 deg celsius this turbine spinning at around 50 000 rpm is supported by a shaft that should also be at the same temperature  but is cooled at it's other  extremity supporting  the compressor . Should the engine been recklessly stopped two thing will happen :

  • 1    first if residual power was applied the turbine may still spin fast  for a long time because the compressor is stalled  , but because oil supply as stopped with engine the turbos bearing  may wear down or even seize up after a while.        

  • 2    second possibility because there is no more  cooling on the compressor side the residual heat from the turbine side will migrate toward the compressor side along the turbo shaft : this will heat up the shaft to around 400 deg cel and result in burning  of the oil on the bearing , this will cause this oil to solidify and this deposit that may  wear down or even seize up the bearing after a while  .        A good procedure is to let the engine idling at least 2 minutes before engine stop

  • Power adjustment must be  made progressive  a lot of care must be taken not to over shoot some parameter and cowl flap must definitively be used to control head temperature

These  4 stroker engines are the most common on home built and factory made small airplanes mostly in the flat 4  arrangement (4 opposite 2by2 cylinders) or the flat six  . The radial arrangement is found sometimes on somewhat bigger airplanes they are all pretty reliable and well known .

reciprocating engine basic (common 2 stroke single cylinder layout ) 

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.This is a reciprocating engine working on the two stroke cycle in one crankshaft  complete turn ;

One piston will move 2time in it's barrel and only one of those move will develop torque to the propeller this will happen during 1turns of the crankshaft

• First move the only one developing torque this move is initiated by a spark firing the  compressed gas-air mixture , due to the heat developed this mixture greatly expand pushing down the piston during the first 1/2 turn of the crankshaft the piston starting from against the head goes down in the cylinder  compressing gas-air mixture in the crankcase , while going down the piston will uncover the exhaust port letting the high pressure escape then shortly after it will also uncover the transfers ports letting the  now fully compressed gas-air mixture   in the  crankcase  goes through the transfers ports and wash away any remaining exhaust gases in the cylinder  , the rotational inertia acquired during this first move will be used by   the other move  to accomplish it's work  
• Second move during  the second 1/2 turn of the crankshaft the piston starting from down (bottom dead center)  goes up against the head in the barel  first closing the transfers ports and the exhaust port then compressing the gas-air mixture that previously washed away the remaining exhaust gases during this time a new gas-air mixture is loaded in the crankcase there are several ways to do this
  1. one cheap solution there may be an intake port in the cylinder whose top would be just barley lower than the bottom of the exhaust port this way when the piston is up against the head the piston skirt will mask the exhaust port but will uncover the intake port letting the gas-air mixture being sucked into the crankcase , as soon as the piston goes down it will mask the intake port in order to compress the gas-air mixture in the crankcase  . This solution is the more reliant the cheaper and efficient enough
  2. one solution  giving good torque at any rpm the crankcase may be fitted with reed valves that let air and gas from carburetor or injection goes in but it will not let it goes out there is however a reliability issue : should one of the reed valves fail the engine will stop
  3. one solution  optimizing torque for one rpm setting (expensive) the idea is the same as the  cylinder intake port but is located somewhere on the side of the crank case and the port instead of been controlled by the piston skirt is fitted with a spinning disk with a hole cut in it masking the port and opening when the hole arrives in front of the port this way permit non symmetric inlet diagrams  very well optimized for one narrow rpm range the optimization can be easily changed by changing the disk there is however a reliability issue : should the disk fail (from very small foreign body ingestion )  the engine will stop

These  2 stroker engines are pretty common on ultralight airplanes mostly in the single cylinder or twin (2 cylinder) arrangement some times three or four cylinder but multi cylinder are not very easy to build because the crankcase must be airtight  . They have a low altitude power to weight  ratio that is better than a 4 stroker due to the fact that half of their cycle are torque producer in stead of 1/4 of the cycle for a 4 stroker . There is however quite some  drawbacks : the reliability  ; sensitive to spark plug fouling , possibility of engine seizure if run too hot , sensitive to air temperature , the noise and pollution (the lubricating oil is lost burned) ;

the big advantages ares power ,weight,cost (very interesting) ,size they are  very well suited for slow low ultralight .

handling specials : 

• The 2 stroker engine is extremely sensitive to spark plug fouling and oil quality it is better to stay with the trade that is working well with the engine

• Some of those engine are sensitive to big change in the weather  conditions such as during a long travel .  They are also quite sensitive to piston seizure in very hot weather

• Special care must be taken in the maintenance of the exhaust system especially the tuned ones (resonator) 

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 four stroke  movie   2.869  MB. warning this is a big file !

it show the cinematic of the valves ; pistons  ; connecting rods ; and  crankshaft during one complete cycle

ps the tooted gears may seem to spin backward !     this is due to the computer display refresh rate being somewhat a multiple of the number of tooth that passed at a given place during the time between 2 frames (stroboscopic effect)  

  two stroke  movie   1.900  MB. warning this is a big file !

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it show the cinematic of the transfers ; ports and  pistons  with connecting rods ; and  crankshaft during one complete cycle

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