Don.
The Junker freepiston compressor
Your Skizzenbuch contains some fragments of descriptions of the Junker which in some cases are confusing due to misunderstandings in the sources you have been using. This is quite understandable as there are very few, if any at all, complete descriptions available of this unique piece of machinery. Just after the WW2 the compressor was a topic which was widely discussed amongst the RN and other submarine engineers as it was an advanced construction well ahead of its time, yet the beginning of the development of same started during WW1 and the first engine was shown in Leipzig 1936.
In an attempt to make a working explanation I have written the following description:
The Junker free piston compressor type 4FK-115 is an opposed piston, dieseldriven 4 stage compressor and consist of a dieselpart in in the center and two stages of compressors at the ends. The dieselengine is a single cylinder uniflow scavenged two stroke engine, having a cylinderdiameter of 115 mm and a stroke which varies normally between 218 to 225 mm. The diesel pistons are directly connected to the one- and two stage compressorpiston at the scavengingport side and a second- and third stage compressor piston at the exhaust port side.
Each piston has a yoke connected to two synchronizing gear racks driving two gearwheel with stubshaft centrally placed opposite each other on both sides of the dieselengine. The rear gearwheel stubshaft is connected to a rotary vane type cooling waterpump and by external rodconnection to a swash type of lubricator having ten supplypipes to various lubricating points, including both dieselcylinders and the four compressor cylinders.
The front gearwheel stubshaft operates a Bosch plunger fuel injection pump having its supply from the day/ setling tank in the engine room and directly connected to the cylinder fuelinjection valve.
The front stubshaft is protruding out of the compressorcasing ending in a square where a crank can be connected for moving the pistons in an outer end starting position. Around this shaft is a indicatorplate showing the position of the pistonassembly in the cylinder.
The central area surrounding the firststage compressor- and diesel enginecylinder including the scavenging ports is forming a large airbox ( light blue coloured ) which act both as a scavenging air reservoir as well as a bouncing chamber having a volume about 21 liters.
Starting procedure.
Starting is done by putting a crank on the square of the rackwheel stubshaft controlled by the synchronizing gearwheel driving the HP fuel pump. By turning the crank the pistons are forced in an outer dead end position where a hand-or pneumatic controlled catch engages and locks the piston assembly in a starting position.
By this movement the underpart of the first stage compressorpiston draws air ( light blue ) from the scavenging air inlet duct to a chamber enclosed by the underpart of the first stage compression piston and a plate with several small springloaded air supply valves ( shut at this point ) separating the scavenging air box from the enclosed chamber under the compression piston.
App. 1/3 of the first stage compressors compression stroke acts as a part air supply ( dark green) for the air box via the compressor cylinder airports through separate channels to the airbox.
The diesel engine is now in a locked starting position where the scavenging- and exhaustports are open, the scavenging air is pushing the exhaust out of the exhaust ports and the cylinder is filled with air from the scavenging air box.
Underneath the compressor is a starting air flask connected to the air valve assembly on the compressor front consisting of an airpressure maintaining valve which can be adjusted between 140 and 210 kg/cm safetyvalves cutting the fuelsupply when pressure is dangerously high and a startingvalve manually or automatic/pneumatic operated.
Via the starting valve, air at pressure of app 35 kg/cm2, from the startingflask is supplied to the various stages of the compressor cylinders. When the airpressure of the 1 stage reaches app. 3,5 kg/cm2, the catch releases and the pistons rapidly are forced towards each other, stopped by the compression, rate about 1:40, and the fuelinjection ignites. The combustion forces the pistons appart and by that the scavenging air is drawn into the the confined space under the first stage compressor piston as previously explained. App. 1/3 of the first stage compressor piston stroke supplies air via separate channels to the air box to assure sufficient scavenging. The rest of the stroke supplies air to the 2nd. stage compressor cylinder. At the dead outer end of the stroke the diesel pistons reveals the scavenging and exhaustports and air under pressure from the scavenging airbox via the scavenging ports forces the exhaust out of the exhaust ports. At this point the pistons change direction as the pistons bounce back initiated by the pressure energy of the residue airpressure kept by the adjustable outlet air pressure maintenance valve ( 140-210 kg/cm2) fitted after the fourth stage cooler, as well as some lesser bouncing chambers formed around the compressor piston rods of stage one and two, see image. they bounce back and As The pistons shuts the exhaust and scavenging ports and the diesel compression stroke starts. At the turning of the pistons, the underside of the first stage compressor piston starts acting like a compressor, forcing air through the small springloaded valves in the separating plate into the scavenging air box and fill same with fresh air. 4/5 of the scavenging air is supplied in this way ,the remaining 1/5 is supplied by the first 1/3 of the first stage compression stroke.
Prior to the end of the diesel compression stroke the fuelpump/valve is timed to end the injection at the same piston position irrelevant of the speed ( frequency). Thus any change in amount of fuel injected is done by an earlier start of the injection.
From here on the cycles are repeated and the compressor is running.
The scavenging air consumption to produce 1 kg. of HP air (205 kg/cm2) is about 1-1.25 kg of which 0,75 kg is required for combustion, the remainder is used for purging and cooling of the dieselcylinder.
The pressure fluctuation of the scavenging air varies between 0,75- and 1,25 kg/cm2.
For a proper balancing, the compressor does not deliver air before the 4th stage air pressure reaches 52 kg/cm2 controlled by the previously mentioned air pressure maintaining valve in the starting airvalve assembly.
The length of the piston stroke is not restricted, but as the Junker compressor is a so called fixed output machine, the stroke variation is limited and almost insensitive to the load. F. inst the speed increases only slightly as the counterpressure increases.
The compressor is fully balanced, has a low weight/volume output ratio, is not hampered so much under throttled condition which all makes it very suitable for submarines. However it is noisy and susceptible to bad combustion as carbon might be formed at the dieselpistons creating unbalance hence requires frequent maintenance intervals.
The cooling
The watercooling ( green ) is normally carried out by the vane type cooling waterpump driven from an eccentric at the back side synchronizing stubshaft The compressor is, with exception of the 4 th. stage, only partially waterjacketed ( green coloured). The major air cooling takes place outside at the bottom of the compressor for each stage in separate intercoolers which reduces the temperature after each stage and at the outlet of 4 th stage has an endcooler for the final cooling.
Tore
