The explanation is simple — “the dog is buried” elsewhere. We must begin with improvements. And only then there will be sense in the crossing. The fact that “Junior” characterized by unusually large friction losses. And the loss as if camouflaged, manifested during operation as the lack of torque and so is not too powerful “polutorametrovoy”. And this applies to all without exception, MK-17, regardless of the quality of the sample and the degree of its refinement. How easy or spinning the shaft in the bearings, as would be ideal neither was fitting “pair” and the connecting rod — the effect of the loss of time remains.
Source of waste is in low rigidity of certain parts and their deformation under working load. When creating the MK-17 was adopted many measures aimed at improving the strength of the parts. But the important node left compared to the MK-16 without changes. Yeah, strengthened crankcase, stronger steel foot mounts, reinforced sleeve (why?) reinforced wall. But stayed on well potjazhelevshie the weak motor crankshaft bearing and the crankshaft low rigidity.
Fig. 1. Diagram of loading and deformation of parts of the Microdrive MK-17 “Junior”.
Does the engine work in General? The General opinion of modelers — no. And now the guys who have the opportunity to compare the MK-17 and MK-16, probably put in a good model it is the “sixteenth”. But this is a conversation about the article “pair”, while we are interested in the other nodes.
So, the evil in the deformations. They lead to this. Pressure compression fuel-air mixture and its combustion at all items has a solid load. Deform (“SAG”) ball bearings, bend cheek, neck crank and main rod of the crankshaft. How large is the magnitude of the deformation (fortunately, the design of the “Junior” clearly allows it to capture, and working conditions. unlike other cases, when the magnitude of the total deflection was obtained in the static)? At the edges of the neck crank… it comes down to 0.5 mm!
Clearly define the desired value allows the formulation of the nest in the spool, which in a long running motors of 0.2 mm. Add the selected gap between the socket for the neck and the backlash of the axis of the spool — that recruited such a large number. The “witness” — to develop from a neck crank in the slot of the spool is present in almost all samples of engines and rather uniform in size. Based on these data we can conclude: the angle of the axis under a load equal to approximately 1.5°.
I must admit, the deflection is enormous. And it causes a whole chain of mechanical losses and unacceptable for a good motor phenomena. As a result of the deflection of the Ball root ball, loaded almost to the limit, preoriented, after which anything about it online, nor even health can not speak. With a leaning of the neck immediately starts sliding back the rod, choosing a solid axial gaps and clinging tightly to the spool. At the same time in connection with the transition of the point of application of forces ago the deflection of the crank of the neck is increased even more!
But it is not important. The main losses arise from the powerful pressure of the spool made of duralumin, to the working plane of the rear wall of the engine (cast alloy). Let the rod moved backward (in this case it bends and goes over the top of the head to the rear of the piston, from which he also began to work in the off-design position), it rubs against the spool, let the warped main bearing — all “drown” against the background of losses in the slide distribution node.
To get rid of the negative effects of such deformations without radical alterations of the engine is possible only due to the deviation of the axis of the sleeve back, and a solid value is of the order of 0.4—0.5 mm on the landing girdle Carter! Although this value is higher than given in the article “Sprinters ice kartodromo” that caused so many conflicting opinions, we are confident in its accuracy. Only Juniors that have so much “overwhelmed” back pocket, able to work long enough with increased (dramatically!) power, not giving signs touch the slide rod. The symptom of this is easily detected by marks on vyskubenko inner plane of the spool. Of course, the deviation of the axis of the sleeve back will cause a number of losses, but the amount is negligible in comparison with the original version.
Ending the conversation about the need for a rearrangement of the sleeve (operative technique is well described in the above article), I want to note that this enhancement in contrast to all known together with the sharp increase of power gives and measurable increase in resource of major components of the engine! Because the bearing starts to operate in a state close to the settlement, the connecting rod is always placed on the axis of the cylinder, and the spool to shift when rotating in the direction of only under its own unbalanced, he and his axis is not necessary now to retain a significant part of the pressure of combustion of the fuel mixture!
By the way, about the need to domainservice spool. Indeed, the detail of this on serial samples of motor balancing are not needed. But modified, which are able to develop a much larger circulation, better or easier “cheek”, ensuring complete balancing, or even to produce a new spool of Micarta or textolite. Even with the serial axis of a soft metal on modified engines wear site were noted. Very recommend to reduce the area of contact of the spool with the plane of wall in accordance with the recommendations of the article “Sprinters ice kartodromo” — effectiveness is reflected in the increase 300-400 rpm.
Speaking about the possibility of work converted “Junior” at high speed, it is necessary to tell and about domainservice crank mechanism. In addition to the self-evident filing of the crankshaft, it is required to greatly simplify the piston. Of course, it would be easier to do it at the casting in the enterprise by changing the inserts defining the shape of the cavity of the piston, but the piston while the serial to leave in original form impossible. Proposed in the already mentioned repeatedly article completion can be considered only primary. More serious begins with the removal of excess iron from the bottom. Its thickness on the serial engine to 2.3 mm! And given the small area free of the bottom with a large margin of safety, you can leave 0,8 mm. then from the new edge of the piston measure 8.5 mm in height and cut off the excess skirt. In this embodiment, the lower edge of the piston position at TDC exhaust opens a little window that you want to exit the engine at high revs. These works reduce the weight of the piston by 30-35%, a further relief is possible only when using a drill.
Deserves special attention system of distribution of the intake of the fuel mixture and the carburetor of the engine. Serial jet carburetor is only suitable for use in educational or single-mode devices, and feeding fuel by gravity, without pressure. Why so, will become clear if you look closely at the picture. Suffice it to mention the place of leakage: thread M4 for the connection of the housing and the rear housing wall, the M4 thread on the bearing cap nut of the needle and the housing. In addition, the set of cavities of the nozzle represents a maze partially filled with liquid fuel, which does not allow to install the stable mode on multimode models.
Fig. 2. Jet production engine:
1 — swivel nut needle, 2 — steel case, 3 — swivel fitting, 4 — casing wall 5 is a removable overlay threaded, 6 — gasket, 7 — needle.
Fig. 3. Extremely short along the length of the wall, involving the use of a modified serial nozzle.
The spool is drilled new nest, the intake window rasshiryaetsya to delay the closing of the intake mixture. The body of the nozzle is mounted using a small amount of epoxy.
Fig. 4. A new scheme of selection pressure:
1 — crankcase 2 — main bearing crankshaft, 3 — fitting-valve, 4 camshaft.
Fig. 5. Completion of the serial piston.
Fig. 6. Shortened the slide wall of the engine:
1 — modified overlay threaded, 2 — wall, 3 — spool, 4 — needle with thread M2 M2,5, 5 — pin (wire OVS Ø 0.6 mm), 6 — marking drilling position tube fixing nozzle 7, the axis of the spool, 8 — screw M2,5 fixing axis 9 — washer knurled (soldering after navertyvanija to the thread of the needle), 10 — tube nozzle (after adjusting the place to glue in the resin and sacerdote), the 11 possible locations are nests of relief.
One of the simplest methods of improvement of the walls and the nozzle was given in the same article. Two, more original and effective, offered for the first time. They are designed mainly for a power supply system with fuel under pressure. Contrary to earlier claims, “dry” crankcase running “Junior” can be found. Where it is and how to get out of it the pressure for pressurization of the tank, shown in Fig.
The last stage of improvements is a significant filing the cooling jacket of the cylinder with the sides and front. This option is designed for free air flow around the engine head air without covering. Cutting through the really unnecessary parts of the fins is possible to ease the motor and to achieve a more uniform cooling of the perimeter of the cylinder. Lower power rib shirt is better left untouched.
So the MK-17 can be considered prepared for the crossing, usually consisting in the adjustment of valve timing and other specific works. But… in most cases it’s not necessary. Modified in accordance with our recommendations engine with the standard nylon propeller for the mixture without additive enters the mode 18-18,5 thousand rpm! Immediately note the modern screws made of green plastic (nylon it is irrelevant) for such testing are not suitable and even dangerous to install on a revving engine! The result of approximate calculations it turns out that “Junior” can be removed to 0.4 HP with mixtures of 1.5% amyl nitrate. This is a real 0,15 HP serial Microdrive!
In conclusion, we have carefully acquainted with the principles of the improvements, to understand their meaning, to use when identifying potential opportunities and other engines. For example, KMD-2,5. Remember what place he are most noticeable wear. The end of the steel spool to which the work is pressed against the lower head duralumin rod. Here is the tip direction of the search. In this case, note that the design of the spool KMD is generally not intended for perception of axial loads.
Please note that almost all of its proposed operations do not require the use of machine tools and can be performed by conventional plumbing equipment. From power tools drills enough (although you can do manual), and even a drill.
VLADIS V., master of sports of the USSR, the head of the circle
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We can safely say: engine MK-17 “Junior” is familiar to all modelers. One of the most popular among beginners and are produced in large series, it encourages the modeling of thousands of boys. Under the motor created a lot of industrial kits, semi-finished products. With models equipped with “Juniors” are the first sports categories air-, ship-, avtomodelisty-students and young builders of the different copies.