Today's Pilot

July 2001

 

"Aero-Diesels"

"Ah, the seemingly endless saga of the diesel aero-engine. For years now we've all been teased with the prospect of a simple, yet powerful and reliable, diesel aero-engine. Such a device (we were told) would have an enormous TBO (possibly 50% longer than comparable petrol-fuelled engines). With a single power lever and no ignition system, these engines would be easier to operate and also very reliable. Furthermore, these wonderful engines would operate on Jet A-1, which is currently around 35 pence a litre as opposed to Avgas, which currently sells for about 94 pence per litre. Jet A-1 is also far more widely available. So where were all these marvellous motors? Well, the answer was actually very simple - their excess weight ….ded to preclude their use. Pound for pound and horsepower for horsepower, the petrol-fuelled engine was always going to win. However, for a variety of reasons, not the least of which is the imminent demise of 100LL Avgas, several diesel aero-engines are now being tested, and Today's Pilot looked at some of the new designs at the AERO show at Friedrichshafen last month. There were a surprisingly large number of diesels at the show, using a wide variety of different configurations. Liquid-cooled two-stroke flat twins, air-cooled two-stroke twin row radials, liquid-cooled two stroke flat fours, air-cooled four stroke flat fours, liquid-cooled in-line fours -even a liquid-cooled Wankel! It seemed that the hour of the diesel aero-engine had finally come.

One of the most interesting engines, as well as one of the few which had actually flown, was the TAE 125. This is a 125hp liquid-cooled, turbo-charged four-cylinder in-line engine adapted from an existing car motor. Frank Thielert, the engine's designer, told Today's Pilot that it had already run for over 5,000 hours on the bench and had been flight-tested in two aircraft - a Valentin Taifun and a Piper PA-28. Although more than 50 hours had been flown on the Taifun, the Piper had only logged 12 hours before the show, though Frank said he was hoping to have 1,500 hours flown on it by the end of the year. This engine, which has been designed to compete with the ubiquitous Lycoming 0-320, certainly looked very impressive on paper, Frank claimed that as the TAE 125 only burned 15 litres per hour of diesel, as opposed to the 33 litres per hour of Avgas burnt by an 0-320 at similar power settings, the direct operating costs would be reduced by 77%. Furthermore, the TBO would be increased from 2,000 hours for the Lycoming to 3,000 hours for the TAE. Other advantages would be single power-lever operation, no loss of power with altitude below 12,000ft and Full Authority Digital Engine Control. It was also interesting to note that the TAE engine was fractionally lighter than an 0-320. However, the reduction gearbox, which is necessary because of the TAE's relatively high maximum engine speed of 3,800rpm, adds an extra layer of complexity that the direct-drive Lycoming does not have.

An engine which has been generating a lot of interest, but which was not at Friedrichshafen, was the Wilksch Air Motive WAM-120. This very compact engine is a l2Ohp inverted in-line three-cylinder two-stroke, using WAM's CITEC concept. This engine uses a compact exhaust manifold, putting the turbocharger very close to the exhaust ports for efficient exhaust energy recovery. Liquid-cooled and turbocharged, the WAM-120 has been specifically designed for the light (2-4 seat) end of the market and has elicited a considerable amount of interest from kitplane builders. I spoke with Mark Wilksch during Aerofair, and Mark was very upbeat about the future for diesel aero engines. Indeed, he already has plans for a four cylinder 160hp engine. One of the features I particularly liked about the WAM-120 is that, once it has started, its operation is purely mechanical. Another plus is that the inverted configuration confers good clearance between the propeller and the ground. Wilksch Airmotive's designers have also gone to considerable trouble to produce a complete, integrated package. The WAM-120 will be shipped for £10,000 with complete oil and cooling systems all included. Installation uses WAM's "Parafocal" rear mount system and Mark said that this "should be a doddle on most tractor installations". The company is also working with a partner on a LongEZ installation which reverts to airframe mounted cooling. WAM has set up a deposit trust fund and has now taken orders from seven countries. Production is booked out well into 2002, and Today's Pilot will be testing the WAM powered Europa later this year.

Other diesel engines seen at AERO were the l00hp Diesel Air DAIR-100, the 3OOhp Zoche radial and the twin-rotor Wankel from KRM. It was interesting to note that Morane­-Renault SMA 5R305 obtained European certification just before the show. This is an air-cooled turbocharged four stroke flat-four, which produces 230hp at 2,2OOrpm. With a projected TBO of 3,000 hours, this will clearly be another engine to watch. Indeed, Cirrus Design Corporation announced during the show that Cirrus is working on an aircraft which will be powered by this engine. Known as the Cirrus SR21tdi, this aircraft is initially being developed exclusively for Cirrus's European customers, and is likely to become Cirrus's flagship product in Europe.

So is the diesel aero-engine finally going to achieve the prominence it has always promised? I believe that, particularly in Europe and probably globally (with the possible exception of America), the answer is 'yes'. Leaded petrol is definitely on the way out - indeed, in some parts of the world it is already very difficult to find Avgas l00LL. Conversely, Jet A-1 and diesel fuel are both readily available. Although there will certainly still be some Avgas refined, it will be produced in such small quantities that it will undoubtedly be extremely expensive. Will we all be flying diesels in the future? Only time will tell, but the signs look good."