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Item condition:
19 Aug, 2011 08:34:00 BST
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Item location:
Dobiegniew, Poland


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Item specifics

New: A brand-new, unused, unopened and undamaged item in original retail packaging (where packaging is ... Read moreabout the condition
Type: Motors & Engines
 This is soviet model air plane sport strong-running diesel motor made in 1989
Never used, factory greased .
Propeller, fuel hose, mounting bolts & nuts, instructions (Russian)

The MARZ 2.5D is quite an interesting and original design in terms of the particular combination of technical features which it displays. All of these features had been used previously by others, but the MARZ did display some quite original thinking in terms of the combination of these ideas into an effective package.

The MARZ was tested by Richard Herbert for "Aeromodeller" magazine, the test report being published in the July 1994 issue. This report includes a full description of the engine, and there is no need to repeat it in detail here. The most noteworthy design features of the MARZ are:

  • The use of an unusually over-square bore-stroke relationship for a diesel, the bore being 15.5 mm and the stroke only 13.0 mm for a displacement of 2.48 cc. The concept of a significantly over-square diesel had of course been explored previously by manufacturers such as AMCO and Allen-Mercury in the UK. This set-up can be expected to promote improved high-speed running characteristics at some cost in terms of low and moderate speed torque. Less positively, it also promotes substantially higher internal stresses on the con-rod and crankweb.
  • The use of the "reverse drum valve" system pioneered in the "classic" era by such engines as the Cox RR1 and the Frog 349. In this set-up, the interior of the drum is open to the atmosphere rather than to the crankcase (as it had been with the Rythm, for example), and the induction port feeds directly from the interior of the drum into the crankcase through a very short up-draft passage in the backplate unit. In effect, a crankshaft rotary valve in reverse. This arrangement provides both a very direct induction passage and a smaller-than-usual crankcase volume. In addition, the fact that the drum valve is un-stressed by the con-rod and does not have to transmit the engine's torque to the propeller allows the designer to take as large a "bite" as necessary for the induction port to maximize efficiency without worrying about mechanical strength. The design induction period of the MARZ is a healthy 178°, and this could be increased or the port enlarged by a tuner if desired with no loss of structural strength.
  • The use of a Cox-influenced remote needle valve, with fuel being fed from an annular chamber around the venturi directly into the venturi throat via a ring of very small holes drilled around the circumference of the venturi. This creates an unencumbered venturi section and promotes excellent homogeneity of the incoming mixture, at some cost in terms of suction. Tuning for range and performance (critical for team racing) is a simple matter of changing venturis to a larger or smaller bore. The very efficient induction system which results from all of this is supplemented by a degree of sub-piston induction.
  • The use of a modified form of the multiple internal flute transfer porting system pioneered in a diesel context by Elfin and AMCO and refined by Webra with their very powerful and successful Mach 1 diesel. This system had of course also been used in the Russian-made MK-12 series based on the Mach 1 and was also featured on the Hungarian Alag diesel engines. The MARZ uses 6 transfer flutes, as do the Webra, the Alag and the MK-12, but the flutes in the MARZ are grouped together in two groups of three flutes on each side of the liner rather than being uniformly distributed around the bore as with the others. This eliminates the front and rear transfer flutes, which would be largely encumbered by the crankweb and backplate anyway and could potentially be fouled by the gudgeon pin. It also prevents any interference between the transfer ports and the induction arrangements. The downside of this set-up is the fact that the transfer flutes have to terminate below the exhaust ports, thus restricting the available transfer period, which is only 98° on the MARZ. But the total transfer port area is very large, which doubtless does much to compensate for the short transfer period. And a fully-floating gudgeon pin can be used, making for greater simplicity of manufacture.
  • The use of a metal dowel set into the crankcase at the rear which aligns with a slot milled in the exterior of the lower cylinder liner to ensure that the liner can only be installed one way. This is essential with this design since the liner is not radially symmetrical in terms of its porting arrangements and hence will only function correctly in one specific position.
  • The retention of the cylinder liner by means of a cooling jacket which is secured to the crankcase by three screws at its base rather than in the more conventional manner by screws which pass through the entire length of the jacket. This ensures that the hold-down stresses are transferred to the liner at the exhaust port ring and that there are no distortional stresses set up in the jacket which might affect the liner above the port ring. This concept had been seen previously on engines such as the original AM 25 Mk I and the home-built Sugden Special as well as on the Russian MK-16 and MK-17 diesels of 1.5 cc displacement.

Otherwise, the MARZ is a more-or-less conventional twin ball-race diesel of "racing" pattern, with three conventional sawn exhaust ports completing the gas flow arrangements. Fits and finishes are generally good, although there are a few potential flaws (to be discussed below) arising from the fact that the engines were very much "mass-production" items. The bare weight of 5.5 ounces is by no means excessive for an engine of this specification and performance, and the unit has a nicely compact appearance with a very low frontal area.

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