Model Engine News: October 2002

The intent of the editorial page each month is to deliver time-sensitive information relating to model engines and related topics, plus whatever random ramblings I'm tempted go climb up on my soap-box about. What I don't want to do is give write about topics that do not date and that people may want to refer to--that belongs on a web page that can be indexed in some way. Now I bet you can guess that with a lead-in like that, I'm about to break my own self-imposed rule, and you'd be right! In defense, as one of my research areas is web-based metadata using Dublin Core and derivitaves, I'll make sure that this page will probably be found and indexed by Google and other popular search utilities. But first, some real time-sensitive stuff...

The Chenery Gnome Rotary Engine

For the past several months, Model Engineer have been running a well illustrated series by Mr Rowland Lowe on building a 1/5 scale Gnome Monosauppe from Les Chenery's plans. The plans themselves are also presented as the series progresses, although in reduced size from that which you would get by sending money to Les. I pick up ME from the campus news agent here (Downunder), so I'm rather behind in the news. I just received the July 12-15 issue in late September, so the series is likely finished in the UK by now, but if you want to collect the back issues, they should still be available. As ME publish on a 2-week schedule, the series follows their normal practice of appearing every other issue, so you'll need odd numbered issues commencing with Vol 189, No. 4161.

Mr Lowe presents how he approached the fabrication of each part and while I would not think of following some of his approaches, the results (from the photographic evidence) speak for themselves -- his work looks first-rate. Apart from it being the only game in town for model engine builders, the series is of use even if you never intend to build anything like this for the fittings and tooling the author describes. In particular, he makes extensive use of a "master/slave" chuck he made for his Myford lathe. This allows him to re-position large diameter parts with a very high degree of concentricity. The design appeared in Model Engineer in the early 90's (actual issues referenced; include refs??). This, coupled with a traveling bush-steady described in the second part of the series, allow him to produce some fine tollerance parts with comparitive ease. Both are well worth considering, regardless of what you are building.

Some people think that just because I maintain this site, I actually know about engines. Sometimes, I think so too, which usually means I'm about to be brought back to earth in a big way--which happened yet again this past month. This time it was a question from a guy who had acquired a Saito SA-80T which is a horizontally opposed, twin cylinder 4-stroke glow engine. Apparently, the engine was received with no documentation and had previously been disassembled and needed re-timing. Apart from seeing pictures of the engine, I knew nothing about it and proceeded to display my ignorance, making grand statements. You know, the sort that are generally prefixed with the word "Obviously...", going on say it was "..a simultaneous firing, opposed twin and timing would be straight forward--just treat it as two singles...". Idiot (me). Well after shooting my mouth off, I chanced across a review of the Saito FA-90T (a 15cc variant of the 13cc FA-80T) and discovered just how wrong I was in nearly every respect

First off, the engine is not a simultaneous firing twin, but an alternate firing engine with a single throw crankshaft and a "fork and blade" conrod arrangement. In effect, this makes the engine a two cylinder radial. The advantage to Saito was an easy to manufacture single-throw crankshaft and a relatively simple conrod arrangement that permits both cylinders to be in the same plane. Conventional "flat" twins with a two-throw crank require split big-ends on the connecting rods and staggered cylinders (although Ross managed to bring them back in-line by having the conrods displaced from the cylinder center lines).

However the arrangement, while cheap to manufacture, has some down sides. In his review of the engine in "Radio Modeller" of May 1995, Peter Chinn states:

"[as a consequence the] power strokes are now unevenly spaced, since the firing intervals are alternately, at 180 degrees and 540 degrees of crankshaft rotation instead of the regular 360 degree intervals produced by a two throw crank."

I think Mr Chinn is making the situation overly complex. The engine is still a conventional 4-stroke, so a complete cycle is 720 degrees, and since 540 - 180 = 360, we end up with one power stroke per revolution. The down side is that the reciprocating mass is unbalanced and the crankcase volume remains constant. The latter facet causes a problem in that blow-by gasses will accrete in the case as there is no pumping action taking place. As this would cause corrosive by-products to form in the case, The FA-90 designers fit an eccentric drum type 4-vane compressor into the backplate cavity to scavenge the crankcase gasses. It draws air from a nipple on the backplate and discharges the blow-by products through a nipple on the crankcase nose just behind the front main bearing.

Also, the designer wanted the inlet pipes to be on top of the engine and the exhausts on the bottom. But to economise on castings, he wanted to re-use parts from the Saito FA-45 (or FA-40 for the 80T) and this means that the cam cover parts are identical. As these castings stagger the cam followers fore-aft, this means that the the gear/cam assemblies are "handed" and cannot be transposed as the cam profiles are different for inlet and exhaust. The inlet cam is asymetric with lower "lift" than the symetric exhaust cam. Which is which? Viewing the engine from the rear with the carburators uppermost, the right hand cylinder has its inlet cam at the front, with exhaust behind. The left cylinder is the opposite: inlet at the rear and exhaust at the front. Careful inspection should disclose which cam/gear assembly goes to which side and which cylinder goes to which side.

To "time" the engine, position the crankshaft with the crankpin at Top Dead Center (TDC) for the right hand cylinder. There will be a timing mark on the the gear of each cam/gear assembly. Position the right hand cam/gear so that the timing mark is closest to the crankshaft -- ie mark "down" in relation to the "up" crank pin. The other gear is set opposite -- ie mark "up" in relation to the "down" crankpin. Now put everything else back together and set the tappet clearence to 0.1mm (0.004") when the valve push rod is at its lowest point. Nothing else requires special attention, assuming you are familiar with engine assembly techniques.

There have been two versions of the engine. The Mark I has a separate head casting attached by 4 screws, the heads of which are hidden under the rocker covers. The Mark II has one piece cylinder assemblies. You can quickly tell them apart by examining the fins below the exhaust/inlet pipes: on the Mark II, all fins are full depth while on the Mark I, radial protrusions for the head attach screws are visible in the fin gaps.

All in all, an interresting engine design and I'm glad to have had the question asked that prompted me to research it, proving yet again, that there's always something new to learn!

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