The model diesel engine is very close to my heart, so I’m always pleased to see a new one appear. I was eleven years old when I saw my first, this being when my very enlightened secondary school woodwork teacher started an aeromodelling club. Imagine today’s angst-ridden teachers showing such initiative, with their hand-wringing fuss over health, wealth, safety and sorcery in this litigious age! Wandering aimlessly around the playground circa 1963 I became aware of a burping, rasping noise that turned out to be a small diesel running on a test bench outside the woodwork shop. A gaggle of curious pupils crowded around the teacher as he fiddled with the tommy bar, coaxing the engine up to speed. Greatly fascinated by the sound, smell and sheer magic of that tiny engine droning away behind the opaque disc of its propeller arc, I joined the throng. Although I have no recollection of the type (probably a Mills .75 or the like), it left a real impression on me and sewed the seeds of an engineering passion that continues to this day. Gosh… thinking about that, I may have to re-evaluate my jaundiced beliefs that books, rather than teachers, are the catalysts of learning!
The all-conquering diesels of my youth were slowly usurped by glow engines as radio control developed, to the point where relatively few diesels are seen nowadays. Diesels enjoy definite advantages over glows, the most obvious being that once started they don’t stop until the fuel runs out. No starting gear is needed and diesels are incredibly frugal, using about half the volume of fuel compared to a similar-size methanol burner. Last but by no means least, diesels are quieter and swing much larger propellers than their glow cousins, capacity for capacity. If you want a real paradoxical angle on the model diesel, in the USA where the glow engine was conceived and popularised, there’s a growing interest in glow conversions with diesel replacement heads… what a funny old world we live in!
Converting a glow to diesel is simple enough. In fact with a decent compression ratio and a healthy dollop of ether, quite a few glow engines will run as oil burners in stock condition… but their performance will be mediocre. Crank loads on a diesel are higher than those imposed by glow combustion, something discovered when folks began to ‘dieselise’ the popular Cox range. Eventually people would sing: “the rod bone’s disconnected from the crank bone,” as their puny internals keeled over.
But enough nostalgia, let’s take a look at this unusual Enya four-stroke diesel; a first for a volume producer, if not a first for industry (Neil Tidy built a few Laser 75 four-stroke diesels a while back, here in the UK).
TALE OF TWO
Having broached the subject of mechanical stress, you can relax in the knowledge that Enya have engineered this engine accordingly. Side-by-side the diesel (41 4CD) and glow (41 4C) versions of this engine look very similar, yet internally they’re very different. The diesel has a cast iron piston and a steel liner; a traditional diesel arrangement, rather than a ring or ABC arrangement commonly employed in glow engines. The highly stressed drop-forged and double bushed conrod is a much stronger item than the glow version, and the crank is heavier, too. Cylinder head changes have invoked a higher compression ratio and the plug (which looks just like an Enya No.3) is a blanking slug, used to make minor changes to the compression ratio by increasing or decreasing the number of washers.
The carburettor is of the time-honoured Enya TN (two needle) type with a reduced choke area compared to the glow version, which I fully expected given the frugality of diesel engines. The list of parts specific to the 4CD is as follows:
All other parts are interchangeable with the 41 4C glow.
DIY INTERVENTION
The Enya 41 4CD is presented to the customer in a most unusual way, with the engine arriving only partly assembled. This certainly surprised both myself and the UK importers! I discussed this with Robin at Enya UK, who told me that the 41 4CD is aimed squarely at specialists and collectors (i.e. types who know their stuff). The engine arrives with the cylinder head lightly affixed for transit, minus its pushrod tubes and rocker cover (these are included in the box). Now I don’t know about you, but if I’d splashed out for one of these I would have expected it to be fully assembled upon opening the box. The head has to be affixed for initial runs anyway, so I’ll confess to being more than a little confused by Enya’s thinking. Looking at various net forums, many seem to think that the increased compression ratio precludes the fitting of the pushrod tubes employed for the glow engine, which is why they’re not factory fitted, but this is a complete red herring as the sealing ‘O’ rings take care of any slight variation there. The rocker cover proved impossible to fit, impinging on the rockers and holding the valves open; this can be cured with a thick gasket, and makes you wonder why one such isn’t shipped with the engine. Comparisons with the 41 4C glow highlighted a changed rocker arm shape, but for a company of Enya’s standing to overlook this is, frankly, quite beyond my comprehension.
IN BITS
Disassembling the engine for an internal inspection took around ten minutes. The fits were of the ‘just right’ variety, with the included tools of above average quality for ‘freebies’. There’s nothing unusual about the construction of the 41 4CD. The piston has two rebates machined into its crown for valve clearance, something I don’t recall seeing on the glow version that must be to accommodate the diesel’s increased compression ratio. Everything else looks conventional, and the cams are the same as those on the glow. The head is specific to the CD and whilst most model four-stroke manufacturers have discovered that plain aluminium work-hardens to form a perfectly effective valve seal, this one employs inserts.
The crankshaft is strengthened and predominantly solid, so it’s even stronger than it looks. The conrod is forged and bronze bushed at either end and is again stronger than the glow version. Gear-driven from the rear of the engine, twin camshafts operate two followers that impinge on the rocker arms, which in turn open the valves as they’re lifted. The valves are closed by coil springs and have a slotted face, presumably for grinding and lapping purposes, and are secured with split collets and a keeper ring as per full-size practice. The front housing is billet machined and contains twin ball race crank support, with the front bearing secured by a hefty circlip. Interestingly the latter isn’t sealed, suggesting that Enya don’t envisage any migration of lubricant.
Overall the general construction of the engine looks to be up to Enya’s usual impeccably high standards, and I would have been more than a little surprised if it wasn’t! Enya engines have to be one of aeromodelling’s best kept secrets with an excellent record of durability and longevity, and why we don’t see more of them is a bit of a puzzle. They don’t seem to be as aggressively marketed as some, almost suggesting that Enya makes them for the sheer pleasure of doing something exceedingly well, rather than aiming for volume sales. Every Enya that’s been in my possession has proved almost impossible to wear out, indeed, I have examples more than thirty years old that still run perfectly.
REVERSE PROCESS
Taking engines apart is easy. You simply loosen the fixings and pull bits off as you go. You’ll come across the odd engine that refuses to come apart without a little heat and a bit of gentle persuasion, but as I’ve already pointed out, the Enya 41 4CD fits were perfect and disassembly wasn’t difficult, so putting it back together should just be a case of reversing the process, surely? Well, not quite! This is where the unwary come unstuck, especially with a four-stroke where the gear, belt or chain-driven camshaft(s) must be meshed correctly. If the valves don’t open and close at the right time then the engine can’t work, so I’m careful to make a record of where things go by scribing a mark here and there on the bits using a soft pencil. Most four-strokes have cam and crank timing marks, and indeed I discovered two centre pop marks on the cam wheels of the 41 4CD. Following a call to Enya UK I found out that there are two tiny mating marks on the small cam drive gear to line these up with, but you need good eyes.
A four-stroke’s ‘suck, squeeze, bang, blow’ cycle of events (i.e. intake, compression, combustion, exhaust) tells you that the inlet valve must open as the piston begins its descent down the cylinder, whilst conversely, for the spent gasses to be ejected, the exhaust valve needs to be opening as the piston begins its journey upwards again. To confirm things are at least in the ball park, at Top Dead Centre (TDC) both valves should be fully closed on the compression stroke. But be aware that individual engines vary on exact timings and overlaps, depending on their state of tune. The final job is to set valve clearances and here the Enya’s instructions fell short, and no feeler gauge was supplied. Fortunately, however, my 41 4C glow did come with very comprehensive instructions and a feeler gauge, so perhaps my early example of this engine isn’t entirely representative. Clearances are .005 – .001mm, as a rough guide; enough free play in the pushrods to allow them to rotate without resistance, with the piston set at TDC on the compression stroke with a warm engine.
TOGETHER AGAIN
Back in one piece again and time to run her up, but not before very carefully turning the engine over to confirm that there was no resistance (with the plug out) to prove that the valves weren’t tangling with each other or with the piston (not good either way and a sure sign that you’ve dropped a clanger with the timing marks). Having affixed a prop and replaced the plug I flipped the engine over compression a few times to confirm that it felt right and then armed myself with fuel, props and my best flicking finger in readiness for the test bench.
As mentioned above, the instructions are poor. Apart from a sheet of printed information specific to the diesel, all you get is a list of part numbers and some fuel information, which states that the 41 4CD will operate satisfactorily on “most kinds of model diesel fuel…” It later points out, “…the fuel mix will have an affect on the best running positions of compression ratio and needle valve…” On the subject of the cylinder head gasket the following is written: “The compression ratio is adequate for normal running, but in case you want to change the compression ratio, disassemble the cylinder head, and insert or strip the gasket between cylinder head and cylinder liner.” I have to say that this doesn’t exactly fulfil Enya’s obligations for providing concise and detailed technical coverage of the finer points of successfully setting up the 41 4CD, so I’ll attempt to fill in the gaps as this review proceeds. The most glaring omission is the need to readjust the valve clearances, an essential job once you change head shims.
Enya’s recommended fuel mix is 36 / 37% kerosene (i.e. paraffin), 36 / 37% ether and 25% castor oil, with a 2 or 3% splash of Amyl Nitrate – a most excellent additive, used freely in the past but practically impossible to obtain legally today as it’s a stimulant and on the list of ‘you must be kidding’ substances! IPN seems to be the accepted alternative, but using a Model Technics or Southern Modelcraft proprietary diesel mix will be the answer for most of us.
UNTIL NEXT TIME…
That’s all for now, I fear. Next time we’ll give her a spin and supply a few facts and figures.
