Flyer

March1998

 

"The first to fly - the Wilksch Airmotive diesel"

Robin Reid writes:

"Wilksch Airmotive Ltd, of Milton Keynes, one of the new wave of British companies designing and developing diesel aero engines, has forged ahead by announcing the first flight (on 21 November,1997 ) of its unique uniflow two-stroke design in a classic Piper Cub aircraft.

Flying from a local airfield, the Wilksch diesel was airborne for 20 minutes, in the presence of an engineering team led by chief designer Mark Wilksch, the Australian former Cosworth engineer who established his own company in 1994. The pilot on the maiden flight was reportedly favourably impressed by the engine's low noise and vibration levels and good response to throttle changes under all conditions.

The new Wilksch diesel is intended to be the first of a family of modular power units. Supported by a DTI SMART award and with funding from a small group of private investors, Mark Wilksch laid out his design concept as a 100hp unit intended for formal certification.

After structural failures during development the decision was taken to initially restrict nominal output to 80hp in order to appeal to the relatively large homebuilt aircraft market. Early testing had resulted in piston seizure and major engine damage, solved by the adoption of strengthened cylinder liners and piston design modifications.

Although based of well proven basic technology used today in heavy diesel engines the world over, the Wilksch uniflow two-stroke diesel is unusual among existing light aircraft engines in that it is arranged as an inverted liquid cooled parallel twin two-stroke with dry sump lubrication and a balancer shaft. It's also equipped with an automotive fuel pump and injectors and a single exhaust driven turbo- charger to scavenge spent gases and boost cylinder pressure via circumferential ports opened and closed by the two piece aluminium pistons. Poppet exhaust valves in each cylinder head are operated by an underhead belt-driven camshaft.

The propeller is driven directly at engine speed from the crankshaft and the engine has now successfully completed the JAR-22 test programme; with a total of over 200 hours of dynamometer and prop stand running to date.

Early development problems included the difficulties in arranging a smooth transition from tickover at 900 rpm to full power at 2,100 rpm, requiring several revisions of the exhaust cam timing. Starting difficulties were solved by the inclusion of an auxiliary electric blower to scavenge the engine before the turbocharger was up to speed.

An unusual feature of the Wilksch engine is it's ball and socket little end bearing clamped between the upper and lower elements of the piston. This is considered by Mark to be a core design element in that it ensures good access for cooling oil to the undersurface of the piston crown. A potential disadvantage of this arrangement, however, is that the piston is free to rotate within the cylinder with which it must remain perfectly concentric under all operating conditions. This results in line contact between piston and bore and poor oil film generation with potential lubrication problems.

In the long term Wilksch accepts that ceramic coating of the pistons will be essential to maintain reliability and provide acceptable oil consumption.

The chosen configuration of the Wilksch diesel may present difficulties in adapting to existing aircraft designs intended for 'flat' horizontally opposed rather than the less common 'vertical' in-line engines. However, it is the company's intention to market the 80 hp diesel with a series of installation kits to allow retro-fitting to most popular types in use today.

Wilksch Airmotive are now seeking major investment to bring their 80hp engine to production in a purpose built factory which, it is hoped, will be able to deliver the first fully certificated engines for General Aviation use before the turn of the century."

 

Miles McCallum writes:

Wilksch refuses to say anything about his engine, other than the fact that it has flown, but Robin Reid's panel on the previous page gives a picture of the state of play. This one (and its larger progeny) is likely to succeed: Wilksch is, after all, the first to actually get one in the air. Quite an achievement."