Guy Leitch asks the question in his Attitude for Altitude column.
For years I have moaned about how crappy piston aircraft engines are. They are prohibitively expensive, yet have antiquated technology and are far less efficient and reliable than modern car engines.
The modern car now has mature technology engine management systems that prevent knocking and detonation. They burn fuel at the optimal stochiometric ratio – so you don’t have to fool around with the all-important mixture. They are well cooled and so don’t need fussy cowl flaps or have the risk of shock-cooling on the descent. I could go on ….. and on.
I used to fantasise about the ideal personal plane – an A36 Bonanza with a V8 BMW petrol engine on the front. Ever since the Rolls Royce Merlin, we have known that aero engines can be made that will turn at an efficient 5000 rpm and so have much better power to weight ratios and longevity than clunky direct-drive air-cooled ‘Lycocontasaureses’.
Water cooling works well – despite those who claim that a water cooled aero engine makes as much sense as an air cooled submarine engine. A good example is Rotax flat fours, which happily spin at 5000 plus rpm for thousands of hours.
12 years ago there was widespread panic that Avgas would be killed off, as it is the last fuel to still use super-toxic tetra-ethyl lead. And as already noted, Avgas is expensive. Why it should be so expensive remains something of a mystery. There’s very little reason for it to be more expensive than car fuel, as it does not carry road users’ tax or the huge Road Accident Fund insurance premiums. Yet it’s about 25% more expensive than petrol.
With the environmentalists hating it, and rip-off pricing, it looked like the end of Avgas was approaching fast. The obvious solution was to use engines that burned simple cheap and readily available paraffin = JetA. And the best way to do that is to use good old Rudi Diesel’s compression combustion engines. Cars and trucks have been using them successfully for more than a hundred years.
We own a 20 year old Pajero with the 165 hp 4-cylinder DiD engine. It has not given a moment’s trouble, is bullet proof and fuel efficient and not very heavy. If it was equipped with a backup for the common rail injection system to get you home in case of electrical failure, it would be ideal for an aircraft installation.
The trouble is – it turns out that it’s not that easy to adapt diesel car engines for planes.
So engines being made specifically for aircraft use are required. In 2014 the big hope was a 300 hp diesel in an SR22 Cirrus, which was first flown in Mojave California by Dick Rutan. This SR22 was powered by the Graflight V-8 diesel engine from Engineered Propulsion Systems (EPS). The company claimed the engine burns less than 11 gallons a Jet-A per hour at an economy power setting and has a TBO of 3,000 hours.
These diesel engines are still unproven, so there’s no assumption that they will be durable enough to be repeatedly overhauled. Instead, the engine builders talk about TBR – Time Between Replacement.
EPS said it hoped to start selling the Graflight diesel engine, which it called the Vision 350, immediately after FAA certification, which it expected in late 2016. Yet, here we are, nine years later, and it’s still not certified, let alone flying anywhere I can find.
Yet another big diesel hope that spluttered to a stop was Continental’s V6 310 hp JetA1 burning engine that they reckoned they could bring to market in two years. Continental now has the backing of deep Chinese pockets, so it acquired the bankrupt Austrian Thielert engines – which had delivered more than 4,000 diesel aero engines, flying in 2,600 aircraft to date.
Continental now owns the type certificate for the 350-HP Centurion 4.0 developed for aircraft use by Thielert from the Mercedes Benz OM648. It uses the same core engine, but Continental added a prop speed reduction unit (PSRU) and clutch and redesigned the fuel system to accommodate the lower lubricity of Jet-A1.
Continental produced the range of Thielert engines, called the CD 100, CD 200 and CD 300, under its own brand. (CD stands for Continental Diesel). Like other Thielert-derived engines, the 310 hp V6 is based on a Mercedes car engine – the MB OM642, a 72-degree V6, 3-litre, aluminium block 24-valve turbocharged diesel with electronic fuel injection.
Continental said it would first aim to get European Aviation Safety Agency (EASA) certification, followed by U.S. FAA and Chinese CAAC validation. Given the problems Cessna had certifying the well-proven French SMA engine in its Cessna 182 JT-A, this turned out to be wildly optimistic.
These are all 4-stroke engines. My dream has always been a 2-stroke diesel. Imagine – no electrical system and no complex and heavy valve gear. However, this dream has always been just tantalisingly out of reach.
Back in 2015, at Sun n Fun in Florida USA, I had a lengthy conversation with an earnest African American the size of a quarterback who was marketing the much-hyped DeltaHawk engine.
Like many others, I was beguiled by the DeltaHawk 2-stroke diesel. I love the wonderful simplicity of a 2-stroke diesel as it produces power twice as often as a four-stroke, has no power-sapping valves and no ignition system.
The sad thing is, DeltaHawk was launched in 1998 and in the intervening 25 years has never quite delivered on its promise. It was dangled in front of an enthusiastic aviation community for so long that I dare say most airshow visitors, myself included, tended to just write it off as just a dream. And then; hold and below! – out of the blue – the DeltaHawk got its FAA certification ticket in April this year. This is a colossal achievement, and signifies that the engine is no longer just a dream.
Technically, the DeltaHawk is a liquid-cooled, turbo-supercharged, direct-drive, V configured monoblock engine. As a diesel, it will happily burn JetA, diesel, power paraffin or even cooking oil, even used potato chip oil. Despite being a 2-stroke and mechanically injected V4, it has demonstrated basic specific fuel consumption (BSFC) of .38-.40.
In 2014 DeltaHawk installed their 180 hp engine in a Cirrus SR20. Doug Doers, DeltaHawk’s Chief Technical Officer, flew it all the way from Florida to Oshkosh. “The engine has proven itself to run very smoothly, with minimal vibration relative to the Continental it replaced,” Zucker said.
If Zucker’s claim of smoothness is to be believed, this is indeed remarkable, as DeltaHawk still relies on simple mechanical fuel injection. Without a modern common-rail fuel feed with Piezo injectors it would be expected to have the typical old fashioned explosive banging of diesel combustion. On modern electronic ‘common rail’ diesels, Piezo injectors squirt fuel into the cylinder as many as seven times in one stroke, thus extending and slowing the combustion. This improves smoothness and fuel consumption and reduces the stresses in the engine so it can be built out of aluminium, rather than cast-iron.
Traditionally diesels had to be heavily built to handle the loads of explosive diesel detonation. But the total weight of the V4 engine is 330 lbs, and this includes the exhaust, turbocharger, alternator, and starter.
DeltaHawk says a six-cylinder model may be developed to address a mid-range of power and weight requirements. With a projected weight of 465 lb, a 450 hp V-8 is not far from that holy grail of weight efficiency; 1.00 lb/hp.
Diesel engines run lean, with high compression ratios, making them more efficient than petrol engines. Diesel engines do not need priming to start, have no carburettor icing problems, no mixtures to fiddle with, and need less maintenance. The JetA used by new diesel engines is half the price of Avgas, and the engines are claimed to be cleaner, as diesel avoids lead emissions while cutting carbon monoxide and hydrocarbon output.
There are currently over 41 types of aircraft models worldwide being designed around the DeltaHawk engine. With the engine now certified, hopefully we will soon see it in production aircraft as either original equipment – or as an aftermarket STC.
So is the diesel aero-engine finally going to achieve the prominence it has always promised? I believe that, particularly in Africa, the answer is now a qualified ‘Yes’.
In many parts of the world it is already very difficult to find Avgas. Conversely, JetA and diesel are both readily available. Although there will still be Avgas refined, it will be produced in such small quantities that it will be extremely expensive.
Will we all be flying JetA diesels in the future? Only time will tell, but the signs look good.