Flyer

July 1999

 

The Future for light aviation?

EXCLUSIVE The engine in this 50-year-old Piper J3 Cub is the first flying prototype produced by Wilksch Airmotive. This two-stroke turbocharged aero diesel could revolutionise our flying, says Miles McCallum

You'd better believe it

When Flyer first got wind of a new diesel aircraft engine and tracked down Mark Wilksch, the Australian engineer responsible, his reaction was not exactly forthcoming: "Press, eh? No comment (not exactly what he said – Ed). Leave me alone until I call you. When I've something to say, you'll hear it."

Mike Costin, Wilksch's mentor, Europa builder and pilot and the 'Cos-' half of high performance car engine maker Cosworth, wasn't much more informative. He asked us to leave Wilksch alone – time answering queries was time not spent on development. What Wilksch needed was anonymity until the time was right.

Of course, Wilksch wasn't working on his own. At Wilksch Airmotive, Martin Long is responsible for detail drawing, assembly and testing, and there's also Phil Franklin, a very experienced diesel engine engineer.

Both Wilksch and Long worked at Cosworth with Mike Costin. Long was involved in an experimental poppet valved two-stroke engine, and Wilksch – an ex-Formula 1 design engineer – explored potential in the aircraft engine market and conducted a technical study and concept design of a 100hp three-cylinder replacement of the O-200 for Teledyne Continental.

It was to be a flat engine – crank on one side with in-line cylinders laid horizontally much like the 'flying brick' BMW motorcycle engines. Continental denied involvement in the study when word leaked out, and the project was quietly dropped. It was, however, clearly serendipitous to Wilksch: "The project could have been a bit of a fizzer – not that there was anything wrong with it in particular, but it was obviously going to become an engine designed by committee. But by then I had come to the conclusion it would be difficult to gain acceptance for a new engine running on gasoline, even with a US partner. The future very obviously lay in other directions."

Wilksch left Cosworth in late 1993, and set up Wilksch Airmotive with the assistance of DTI grants under the SMART scheme. The result is the CITEC (Compression Ignition Turbo Exhaust Charged) diesel/Avtur burning engine. The first engine was an inverted in-line twin of 80hp, mainly used for development in a test cell, but in November '97 it became the first new generation two-stroke diesel aircraft engine to fly in 50 years – in a 50-year-old airframe (a J-3 cub), no less. Test pilot Steve Gilbert reported the engine was exceptionally smooth and responsive – and started instantly, hot or cold.

The production prototype CITEC is an inverted in-line three-cylinder engine, rather like a DH Gypsy (Tiger Moth) engine in configuration, although the resemblance from then on is purely superficial. It can be extended to an in-line four (or even five) cylinder engine. A 160hp four is planned, although there is a possibility it may end up as a conventional flat-four, with a 240hp flat-six under consideration as well. There's also a strong hint that significantly more powerful versions of each engine will also be available as development progresses, and they get into intercooler equipped versions.

The basic operating principle is two-stroke, although any similarity to your lawnmower or even a microlight engine is minimal. The cylinders are force fed (more on that later) with full circumference inlet ports, but the exhaust is via multiple poppet valves. An underhead cam operates the valves at engine speed through a low maintenance valve train. With no crankcase induction, the 'bottom end' is a pressure fed wet sump like most four-strokes, removing any threat of an oily exhaust or the need for troublesome needle roller small ends.

The sump is effectively the cam cover, so the parts that suffer the highest start up stresses are permanently bathed in oil. Poor cam and follower start up lubrication is a bugbear on many aircraft engines, and is often the reason for premature overhauls.

Surprisingly, considering the number of inverted in-line engines produced over the years, this is the first wet sump equipped type. Wilksch quite happily accedes that almost everything in the engine has been done before, but not, as he points out, in this particular combination. In fact there are several innovations – the wet sump being just one – that mark the engine out as being a clever bit of design.

The crankshaft and con-rods look astonishingly beefy; truck-sized journals belie the relatively light weight of the crank, courtesy of hollow main and crank pins. The anticipated weight of the engine will be about the same as the equivalent Lycoming or Continental to begin with, but when the bugs are ironed out of the prototype and the final patterns produced, more attention will be given to making it as light as possible. It'll never be in the Rotax league, weight-wise, but then the Rotax will never be in the same class.

The piston/con rod connection is also interesting. Instead of conventional gudgeon pins, a ball end is formed on the end of the rod and captured by a spherical bearing clipped into the piston. In practice, the piston turns randomly whilst it's running. This has the effect of spreading the wear over the entire skirt of the piston, rather than a line contact perpendicular to the connecting pin. It also means there's no possibility of oil leaking past the bores after shutdown with a smoky start to follow. The amount of oil floating around the system during operation is considerably less than required to fill up three pistons.

Built into the bottom end are a pair of contrarotating balance shafts – one of which doubles up as a cam drive. With twice as many power pulses per crank rotation as a four- s t r o k e , the engine is inherently smooth anyway – a three cylinder feels like a six – but defeating the secondary out of balance forces makes it incredibly smooth, with positive consequences for pilot fatigue and airframe component life.

In an effort to improve reliability and reduce installation errors, the entire cooling system is built into the engine. The coolant radiator is fixed to the front of the engine as an integral component. A built-in crankshaft extension (another patented feature) provides the room and lends itself to turbine shaped cowls – read that as 'slim profiles and better visibility'. In all probability, the entire induction and exhaust system will also be provided as part of the package. The fewer things left to chance, the fewer problems will crop up.

The induction system is fairly conventional up to a point. The fuel injection system is standard technology, with the air side taken care of by a simple turbocharger. No wastegate is fitted, as the turbo is closely matched to the engine's needs. The higher you climb, the faster the turbo spins to maintain manifold pressure. The fuel injection pump has been developed by Wilksch from existing automotive rotary pump technology. The only electric bits are a starter, alternator and glow plugs, in the interests of reliability.

Where it does all diverge from the norm is a special blower fitted into the system. Wilksch is tight-lipped about the details until the patents are tied up, but consider it akin to a supercharger that consumes no power.

For all but starting and descent operations (throttle shut, engine being driven by the prop) the turbo will maintain enough manifold pressure to keep the fires alight, but in those specific circumstances, it does need positive pressure for reliable operation. The prototype two-cylinder and pre-production three-cylinder engines use a simple electric fan to provide positive pressure, but he is unhappy about a critical item being electrically operated.

It's interesting that performance figures quoted are quite conservative. Indeed, power itself isn't a problem – the limiting factors are piston cooling (a large percentage of the testing has been concentrated on this) and durability.

Even a Rotax 914 is capable of producing around 180bhp but it won't last very long, as someone found out last year. The initial targeted TBO (time between overhauls) is 1000 hours, although based on common experience, you are likely to be able to double that by the time you get there. The engines will be exchanged for new ones at overhaul so the factory can maintain a close eye on durability.

The specific fuel consumption is higher than some of the opposition claims, but Wilksch is unrepentant: "There are always trade-offs, and you can have very high efficiency, or you can have stone reliable starting, low vibration and long service life. We've found that instant starts at 0°C are the norm. In practice, the engine is efficient compared to the others it will replace. Our best efficiency is in the middle of the cruise rpm, compared with many existing aircraft engines where it is at peak output."

The first three cylinder engines will be flying later on this summer – one or two in Europas (look out for one at the PFA Rally), and one in a 150 Cessna airframe, in collaboration with Atlantic Aeroengineering at Coventry.

The 150/152 Cessnas are a natural market for the engine, the type being the most widely used trainer in the world, all with engines getting rather long in the tooth. They will be operated under 'B' (test) conditions, but will provide excellent comparison data and experience for eventual certification.

"There's no point in tying ourselves up with red tape – certification almost carves the design into stone and makes it difficult to implement changes," says Wilksch. "We'll gain experience in the homebuilt market – quite the most innovative section of general aviation – before we tackle certification, and we'll have something far better and more thoroughly tested than the regulations require."

Wilksch Airmotive is currently the only serious runner in the fastest expanding sector of the market – around the 100hp+ level – as all of the heavy competition is at 180bhp and above. The engine combines the best of two-and four-stroke technology, and has been designed using up to the minute tools including FEA (Finite Element Analysis) computer modelling – but not, we were fascinated to find, as far as the drawings were concerned. They are still done on paper (film, in fact) although they will eventually find their way into a 3D file by the time serious production gets under way.

If things proceed as they have so far, a great many of us are going to be a lot more interested in whether various airfields offers Avtur. I'm willing to bet that in 20 years time, it'll be universally available. Finding Avgas will be the problem.