Diesel Car

May 2000

 

"Diesel reaches for the sky"

"Two people flying to the South of France for a fuel cost of £10 each return.
Miles McCallum tells how it's possible"

"Most piston aircraft engines are so archaic that they would make any automotive engineer choke on his pint before falling on the ground in hysterics. Their pushrod-operated valve trains, magneto ignition, updraft carburettors and complicated handling regimes are reliable, pounding away for hour after hour, but they burn copious amounts of the only leaded fuel left in the world.

Fuel is the driving force behind the next generation of reciprocation internal combustion aircraft engines. The end is clearly in sight - if a little way off yet - for gasoline-fuelled engines. On the other hand, JetA1 Avtur - kerosene, diesel and heating oil are all part of the same family - is already more widely available throughout the world, and has far better potential in terms of production from renewable resources. All the yellow fields of rapeseed that you pass provide an excellent raw material for biodiesel.

Diesels run lean all the time - this and the high compression ratios are the reason why they are so efficient - whereas large air-cooled aircraft engines have to run rich, wasting fuel for nothing more than cooling. They truly are the last dinosaurs of the petrol engine age, and long overdue for retirement to a museum.

The race is on for the next generation of light aircraft engines - up to around 4OObhp, where gas turbine engines become cost-effective. The aircraft engine heavyweights - Continental and Lycoming - have entered the fray, as has Renault, but they are concentrating on the over-2OObhp market. A plethora of smaller (and much nimbler) firms are also in the race, and leading the pack is Wilksch Airmotive. Ex-Cosworth engineer Mark Wilksch may be the frontman and the public face, but he is at pains to point out that he is part of a team. Keith Duckworth and Mike Costin were the public faces of Cosworth, but Ben Rood was an integral and essential part of the team, pioneering innovative production techniques. At Wilksch Airmotive, Martin Long displays the same talents and is responsible for detail drawing, assembly and testing. He is a perfect foil for Wilksch. There are various other people involved, such as Phil Franklin, a very experienced diesel engine engineer, but most noteworthy is Mike Costin himself. He clearly considers Wilksch to be something of a protege, and in turn is considered to be a valuable technical sounding board with an encyclopaedic knowledge of problems and solutions. "A very wide-ranging mind" is Costin's verdict of Wilksch.

The production prototype Wilksch 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 l6Obhp four is planned, although there is a possibility that it may end up as a conventional flat four, with a 24Obhp flat six under consideration as well. There is also the promise that significantly more powerful versions of each engine will also be available as development progresses, an intercooler-equipped version of the triple being capable of a reliable 16Obhp.

The basic operating principal 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' (or is it top?) is a pressure-fed wet sump set-up 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 bits 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 often precipitates 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 concedes that almost everything in the engine has been done before, but not in this particular combination. In fact, there are several innovations the wet sump being just one - that marks the engine out as a clever piece of design.

The crankshaft and con-rods look astonishingly beefy. Mammoth journals belie the relatively light weight of the crank, courtesy of hollow main and crank pins. Currently tipping the scales at 110kg, when the bugs are ironed out of the prototype and the final patterns are produced, it should be 10 per cent lighter. The piston rod connection is interesting in that rather than employing 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 while 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 that there is no possibility of oil leaking past the bores after shut-down, 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 contra-rotating balance shafts - one of which doubles up as a cam drive with twice as many power pulses per crank rotation. As a four-stroke, 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.

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, the turbo being 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 parts are a starter, alternator and glow plugs, in the interests of reliability.

Where it does diverge from the norm is a special blower fitted into the system. Employing a driven two-lobe screw turning a three-lobe screw in a figure-of-eight housing, it consumes next to no power when the turbocharger takes over. 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.

It's interesting that the performance figures quoted are quite conservative. Indeed, power itself isn't a problem - the limiting factors are piston cooling, on which a large percentage of the testing has been concentrated, and durability. From the operator's point of view, costs and maintenance requirements will be very much lower. In Europe at least - where Avtur costs about a third the price of Avgas - both total and direct operating costs will be substantially cheaper. Wilksch calculates that a ll5bhp Lycoming - as fitted to Cessna 152s - costs nearly £24 an hour, based on purchase price and fuel costs. The comparable figure for the Wilksch diesel is £9 an hour.

Wilksch is currently the front-runner in the fastest expanding sector of the market - around the 1OObhp-plus level. All of the heavy competition is at l8Obhp and above. The engine combines the best of two and four-stroke technology, and has been designed using the latest tools, including FEA (Finite Element Analysis) computer modelling - but not 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. The chance of a Wilksch finding its way into a car is unlikely - but the technology, in terms of the piston/rod, and the blower, is a fair bet. A couple of automotive engine design bureaux are experimenting with two-stroke diesel technology, the limiting barrier being gudgeon pin (and thence piston) distortion - the Wilksch solution being just the job. The blower offers better efficiency at a lower cost than current technology. It is not often that aviation piston engine technology trickles down to the automotive world - it's usually the other way round."