Horizontal twin two stroke – spark ignition
2 x 34mm
1,100 – 6,650 tested
22 x 10 to 23 x 12 tested
1:50 – PetroMax oil
4 bolt pattern
50 ml fuel gave 1 minute 50 seconds run time (test results)
Ignition module, plug spanner, instruction sheet
I had been looking forward to be running this engine for several reasons. First that it is a horizontal twin (I really like multi cylinder engines), that I had run the 70cc version some time back for a mate (he asked me to run it in and set it up) and that I had been closely examining another ROTO engine and the make-up really appealed to me. I knew I would probably have to spend a bit of time getting the engine going for the first time and, here again, for a few reasons. I have found the Roto engines to be low on compression – a factor that plays a bit part in the extremely smooth running with incredibly low vibration. This is really good for model use – no structural failures due to transmitted engine vibration – good for the longevity of the engine as it doesn’t batter itself to an early death and exhaust systems don’t develop fatigue fractures and cracks.
As it is in many facets of life, there’s no such thing as a free breakfast – you have to pay somewhere and the payment, in this case, is the difficulty of starting the engine when new with the smaller size (in the recommended range) propellers. Here we are talking a 22 x 10 wood as the lowest size propeller due to the popularity of wooden props …and that some large size composite propellers are not now so readily available. Also, in my experience at various events, a wooden prop is the most commonly used with larger engines. I know there are some really top grade GRP & CRP propellers available, but they are in some cases lighter than a wooden propeller and, they cost a whole lot more. What we need for an engine such as this – low compression, twin cylinder (extra internal drag) and new out of the box is a nicely heavy propeller to give the engine a bit of a flywheel effect for ‘get up and go’ starting. Obviously, as we will see further on, the larger propellers in the range generally solve the problem but I prefer to lightly load a new engine for its first running period – it is load that can kill an engine very quickly. I could not hand start the engine on the small propeller but a bit of a buzz with an electric starter was a different story. The engine was up and running (see bench notes further on), holding full RPM, transitioning and idling after a reasonably brief run in procedure. While it was hot I could hand start it, but when it cooled down – which it does rather rapidly, it was still a bit cantankerous. The next propeller was easier and it was easier as I went up in size which, again, I will discuss in ‘bench notes’, soon.
Roto engines are manufactured in the Czech Republic, a country noted for some extremely fine engineering work. Many examples of engines I have seen over time show some very interesting design and manufacturing techniques plus the use of ball, roller and needle bearings seem to be the order of the day for any part that moves in a circular or semi-circular motion… no plain bearings. Parts have to last and run freely so, use composite bearings (bearings built up using inter-related parts) and the need for a high drag oil content is negated. Another interesting aspect of their design is that in many cases a part that does one function can also be used for another function. An example of this I found in a Roto four stroke where the very substantial cam follower bushes were also part of the retention system for the inner rear case that had the needle bearings for the camshafts and pinion. The alignment of these bearings with those in the absolute rear section is imperative so the extra security of the threaded in bushes was a positive move.
An area that has its proponents and opposers in engine manufacturing is the method of producing a crankcase for example. An argument that has been going on probably from the time of when casting or fabricating became an option that will, in all probability, never be completely resolved. These days we don’t see many, if any, fabricated crankcases but we do see a lot that are CNC machined from a solid block or rod which is, in a way, fabricated. Fast and accurately repetitive but very expensive to set up, and the opposite school of thought is the doubt about the strength and the working rigidity. The casting brigade will argue that cast components have a natural granular flow of the metal to follow the shape in which it is cast. Castings are generally stress relieved or aged and machined where required. The outer surface is strong, distortion is extremely low or even non existent and, in the eye of some beholders, a cast engine case looks like an engine. I’m not getting in this discussion – I am just reporting that which I hear from automotive engineers – young and old. To close this discussion, the main case of Roto engines is cast very nicely (and they are very strong), the liners and finned barrels are CNC machined – touching both bases. I must admit, the appearance of the engines is pleasing in my view with their mix of technologies and the robustness they present.
ON THE BENCH
The first point to consider with an engine such as this is it has two cylinders that are totally lacking in fuel when the engine is first ready for starting. The first charge of fuel sits in the crankcase and has to be distributed out – left and right – to both cylinders as the engine fires simultaneously – both cylinders together. Also, as the engine has not at that time been run, the mating and sealing fit of the rings to the liners has not been developed to provide the strong positive pressure of the down going pistons to push the charge of fuel instantly up the bypass channels to the combustion chamber. Note well – as such, you need quite a considerable charge of fuel for that first kick. I choked the engine but I had no response after what I considered a fair try. Okay, check a plug for spark and for petrol smell. Here I came to a real problem. The worker who tightened the plugs probably carries large anvils for pleasure during his lunch break as I could not remove the plugs. Certainly not me getting weak – the supplied plug spanner wouldn’t do the job – wouldn’t do it when I extended the handle but… I did manage to bend the handle… both sides. I bent the cross bar of another plug spanner so then I resorted to big guns. Fortunately the plug spanner is hexagonal on its external shape so I fitted a very large ring spanner and really gave it the ‘strong-arm’ treatment. The plug undid with a resounding crack sound as did the other one when I attacked it. Not necessary – really don’t know why they were done like that but, be warned when you want to remove the plugs – they might be as tight as the one in the test engine. Both plugs sparked well but were dry – not a sniff of petrol so I poured a dose in each cylinder, replaced the plugs and tightened them to my specifications and they never moved or leaked during the testing. This time the engine fired up, coughed a couple of time s then it was away and running. From then on, the starting was no problem – particularly with the larger and heavier propellers as can be seen on my YouTube site – look up BRIAN WINCH.
I did find that either the engine cooled quickly internally or the internal fuel charge was evaporated as a re-start after a propeller change and writing some notes required another choke prime to start it. As the testing period progressed, a propeller change (undoing & doing up the four bolts) was not a problem as the engine had now developed a nice bounce (compression building up as the rings bedded) when I flicked it and would start one or two flicks straight away which is also an indication that the rings and liner were mating nicely together.
The optional short can mufflers would be acceptable at most fields as the noise level is quite low and not nearly as ‘thumping’ as you get from a single 500cc engine. At all times of running it was easy to note how smooth the engine was running and this is quite evident on my YouTube site. Of interest, in response to a reader’s inquiry about how long an engine will idle reliably, I let the engine run at idle at the end of the testing and I had the video camera going. Phone rang – I answered a couple of questions for the caller, went for a p… err… drink then remembered the engine was still running under the scrutiny of dear wife (never leave an engine running without supervision). Not a real happy little Vegemite as other things had to be done and she was nodding off watching the engine with hand poised over the kill switch. Anyway, watch the video and you can see how steady the engine ran while I was otherwise occupied.
Testing was over 3 days – 5, 6 and 9.8.13 with an average temperature of 15.5 degrees C and humidity hovering around 60%. Fuel was 95 RON petrol with PetroMax oil (Model Engines special) at a ratio of 50:1. As I said, I was looking forward to the running of this engine and I was not disappointed – it is a very fine product engineering wise – it runs so well and smoothly and there is no fuss about mounting, linkage or related problems – four standoffs, direct link throttle, fit a propeller and enjoy watching it pull your model with good authority.
A LOOK INSIDE & OUT
As discussed, the crankcase is a very strong aluminium alloy casting in two pieces which makes the fitting of the crankshaft possible as the connecting rods are pre-fitted before assembly. The front section is fitted with the two crankshaft ball bearings – front one sealed and four studs which are the front studs, (2 per side) for the cylinders and heads. The rear section also has four studs (rear of the cylinders) and the mounting for the rear cover. As you can see in the photos, this is a well designed and very strong case and, the black crackle finish touches off the engine type appearance.
The rear cover is CNC machined high tensile aluminium alloy that has undergone a considerable number of machining processes for the finished product. Central position is a substantial needle roller bearing which is the rear support for the crankshaft. A large cavity and aperture in the top is the fuel ingress that is controlled by the rotary disc (more on this further on). On the rear side is the mounting base for the carburettor that is retained by two studs. Next, outward sections are the very solid engine mount arms that are certainly not going to flex or distort under load. Closer in are four holes for the long caphead bolts that retain the rear cover and the two halves of the crankcase together. Remember what I mentioned earlier about some parts doing more than one job? This is one example. When the crankcases are together and tight, the cylinders will slide down over their respective studs and are then locked in place with 7mm AF (AF = Across the Flats) acorn nuts. When they are tightened up, these studs serve to secure the cylinders and further secure the two crankcase halves together.
The finned cylinders are CNC machined aluminium alloy with press fit fine grey iron (probably a Meehanite grade – popular for liners and piston rings) liners. The liners are very substantial in that the wall are quite thick and the top rim is of large dimensions – no warping or leaking. The bores are finely honed for final sizing and oil retention which is assisted by the self lubricating qualities of cast iron – the same as is used in the engine of the car you drive. Generally speaking, it is not good engineering practice to have the same metals bearing (moving, running etc.) together. Brass on brass can be a dismal failure, steel on steel (without heat treatment) is short lived and aluminium is very inclined to gall weld both components together – permanently. However, cast iron on cast iron is very acceptable and, again, the use in full size engines – motor cycle and car for instance is testament to this. Cast iron matures with age and use – the older the better – so, as this engine has cast iron piston rings sliding in the cast iron liner – you can expect many hundreds or hours of use – and, the more it is used (within reason) – the better it will become.
The pistons are machined from very high tensile aluminium alloy (almost zero expansion under running conditions) and, being a two stroke with ports in the liners, the ring is pegged in place (in case you disassemble your engine and not notice).
The connecting rods are fully machined from bar stock high tensile aluminium alloy (zero stretch) and have fine needle rollers both ends for the crankpin and gudgeon pin – a mere sniff of good lubrication is quite adequate.
The crankshaft is a super fine example of top class design and engineering. It is fabricated with three sections that are quite interesting. The front section has the forward shaft for the propeller mounting hub and the front two bearing supports. It has a small counterweight that has a hole bored (super fine tolerance) for the pressed in crankpin. The centre web section is a parallel sided, round ended section that has the front and rear crankpins as part of its make-up – all machined from one piece. The rear section has an identical counterweight to that of the front section, a hole for the crankpin, a stub shaft for the rear bearing support and a small double flat insert (pressed in) to drive the rotary disc. Simplifying the procedure here – a conrod with its needle bearing is fitted to one crankpin (on the centre web section) and the excess length of the crankpin is then pressed into the front counterweight. The rear connecting rod is similarly fitted to the rear crankpin and the rear section of the crankshaft is pressed onto that pin. This would all be done in very accurate jigs then the shaft tested for balance. A proven system over many years and still employed for multiple cylinder engines such as motor cycles and the like.
The rear disc for the fuel induction is machined from steel, fully hardened and finely ground to accurate dimensions on all surfaces. It fits on the crankshaft rear stub and is driven by the insert fitting in a slot. This is rotary disc induction – extremely reliable and guaranteed no reed failures. As the cutout in the timed disc aligns with the aperture in the rear cover, the up-going pistons void the crankcase to let a rush of atmospheric pressure in, and on its travels it takes in a load of fuel and so the beast is fed – fuel supplied to the engine with timed amounts.
All we have left is the Walbro carburettor that was adjusted for the smaller prop range as it was supplied. I made only a small adjustment when I ran the largest propellers, and as can be seen in the video, a small adjustment to the low speed needle when I fitted a smaller propeller for the visual presentation. You might note that I had no trouble connecting and adjusting the idle needle screw due to the extreme steady running of the engine.
Last out of the box is the ROTO ignition system that can be powered by a 4 to 9 Volt battery. The battery lead is supplied with a positive lock connector, the LED (red sleeve) indicates that the system is on and the blinking can be read with a tachometer for the engine RPM. If you stop the engine by cutting the fuel supply, the LED will continue blinking at half the previous engine speed for checking with a tachometer in case you forget to do it when the engine was running.
The plug caps are very positive being a non pressure slide on, push right down then tighten the 3mm grubscrew in the surround – which is also the plug washer – to secure it.
Well, I certainly enjoyed running the engine… many times. You will enjoy one in a model so let’s now see what it got up to on the test bench.