The collapse of engine partner Thielert put the brakes on Diamond Aircraft’s ambitions for a diesel-powered twin, but they are back with the next generation model boasting Austro AE300 engines. John Absolon finds out that it is a very different beast to its avgas-powered sister.
When Diamond Aircraft introduced diesel engines on the DA42 Twin Star in 2005, many in the aviation industry questioned why.
Initially, diesel would provide much lower operating costs when it came to fuel consumption, with increasing fuel costs around the world let alone the reducing availability of gasoline aviation fuels. With lower consumption came increased range for the same weight of fuel that was uplifted. Burning jet fuel (AVTUR/Jet-A1) rather AVGAS 100LL also gave cost benefits with AVTUR being around 30-40 cents per litre cheaper than AVGAS.
Another benefit of diesel power is that it reduces the emissions associated with leaded and even low leaded aviation fuels. This has been an issue particularly with a number of environmental groups in the US and Europe.
A bit of history
The German aircraft industry has had a long history in exploring or using diesel technology in various aircraft since the 1930s, although mainly airships.
US engine manufacturer Packard developed an air cooled radial diesel for use in the R101 Airship in the late 1920s and the Soviets experimented with diesel during WWII in the Petlyakov Pe-8 heavy bomber, but these were later replaced by conventional radial engines.
Even the French played around with diesel aircraft during the 1930s and not to out-done, the Brits tried a diesel version of the Rolls-Royce Condor powering the Hawker Horsley Bomber.
All of these proved less than reliable or efficient for their time mainly because of the weight of the engine compared to single or multiple row radial and inline or V formation engines powered by conventional high octane aviation fuel.
A number of European manufacturers have re-engined some general aviation designs to satisfy the growing trend in Europe for better economy and lower emissions and noise levels.
The original model of the Twin Star–now known as the DA42-TDI–was powered by a four-cylinder Thielert engine derived from a Daimler Benz diesel automotive unit, and it has even flown across the North Atlantic non-stop at an average fuel flow of 21.7 ltr/hr on its way to Oshkosh.
This engine has, however, had a chequered history, being plagued by gearbox and reliability problems. The original manufacturer doesn’t really exist anymore. It did prove the viability of powering modern general aviation aircraft with diesel engines running on jet fuel and led to the certification of the Austro AE300-powered variant in June 2010.
The Austro engine, which is built by a consortium backed directly by Diamond Aircraft, Daimler Benz and Bosch, builds on a similar Mercedes block with numerous improvements. As the CEO of Diamond Aircraft is also on the board of Austro Engine Gmbh, Diamond has a little more control over the development and capabilities of the Austro engine for installation in Diamond aircraft.
Now with a cast iron engine block, a significant difference is that the Mercedes Benz engine used in the new DA42-NG remains in its original proven design, as opposed to the Thielert engine which was heavily modified internally.
The injection system, which was developed by Bosch and supplied directly, is the modern Type III design instead of the earlier Type I, and the clutch has been done away with, replaced by a fluid torsion dampener coupling. This was another problem with the initial DA42-TDI Thielert diesel powerplant.
Installation of the Austro diesel is extremely neat with the block being slightly canted to the right to fit the repositioned turbo into the cowling. Mounted behind the engine and exhausting out through the grill on the top of the engine nacelle is the large air cooled intercooler. Having the exhaust grill over the point of maximum low pressure of the wing aids in the flow of intercooler air.
This intercooler is fed with forward airflow via a large carbon fibre duct from an intake on the left of each cowling.
The lower intake on the cowling, looking not unlike a turbo-prop engine intake, is actually the radiator intake, the combined coolant and oil heat exchanger being mounted almost horizontally under the whole engine installation.
A small grilled air outlet is also found behind the turbo-charger on the right side of each cowling. This allows the excess heat from around the turbo to be vented overboard. Once the cowls are on, the whole installation–although looking just a little bulkier than the conventional Lycoming installation–presents a modern almost turbo-prop look and with its rounded front, apart from the many intakes, probably reduces some profile drag compared to the conventional flat six cylinder layout with its large air cooling intakes.
Large air cooled engine intakes are like drag buckets. That’s why, for example, aircraft that have been streamlined to improve cruise performance (or race performance) have much smaller air intakes to reduce drag, but unfortunately rely heavily on high forward speed to provide adequate cooling. They don’t tolerate holding on the ground much or flying slow!
The cast iron engine block, unlike the earlier alloy 1.7 or 2.0 litre Thielert engine, is a Daimler Benz 1900cc four-cylinder unit almost the same as the engine found in numerous Mercedes Benz cars and commercial vehicles. It even has a belt drive at the rear driving the various engine ancillaries like alternator and coolant pump.
Where the clutch plate or automatic transmission would mount is an Austro-designed, fluid-coupled gearbox that reduces engine RPM by close to 1:1.7. Coupled to the front of this is a MT constant speed three-blade propeller.
The Austro produces 168 HP from the turbo-intercooled four cylinders whilst running on JET A1 in Australia or JET A in the US. The only difference between the two fuels is that JET A1, which is found in most parts of the world, has an anti- freeze agent FSII added to it. These are the only two currently primary fuels approved for the DA42-NG.
Austro has now approved the use of a number of other fuels including and not limited to JP8 a US Military specified jet fuel and EN590 Diesel. The use of diesel is good news for operators in parts of the world like Russia and Africa where diesel is probably more plentiful.
The engine is somewhat heavier at 185 kg than the Lycoming 360 installation in the DA42-L360 that was reviewed in Australian Flying [Nov/Dec 2011], but at least it has the advantage that, unlike the L360 model, no extra weight is required in the form of nose ballast when back seat passengers are carried to maintain the C of G within limits. This extra weight carried obviously is included in any payload that can be carried.
Let the testing begin
The Austro diesel engine is a modern diesel engine using the latest in injection technology from renowned injection specialist Bosch. This system uses a common-rail injection system with computer-controlled injection that actually fires multiple mini-bursts on each power stroke to optimize the efficient combustion of the diesel. The whole operation is controlled via twin Electronic Engine Control Units (EECU) on each engine.
This EECU system monitors the engine parameters and provides Full Authority Digital Engine Control (FADEC) of both propeller pitch and engine RPM.
With two EECUs per engine, there is redundancy built in should one fail. Each EECU alternately is selected on each engine start. That is; on one engine start EECU A might be used and next start EECU B will be used.
This ensures that if the same EECU is continually used each flight, and should a failure occur and the other EECU be automatically selected, it maybe found to be inoperative. Alternate cycling ensures that each EECU is used to the same extent.
Having FADEC also reduces pilot workload. This reduction in workload begins right from engine start where the EECU will not only decide on which unit has initial responsibility for the engine, but also compute whether the glow plugs are required for engine start.
No complicated priming is required; you just have to select the ENG MASTER ON and then select the ignition key to START for that engine. If the EECU recognizes the need for the glow plugs it will automatically switch them on and a message L or R ENG GLOW ON will display on the EIS. When the messages disappear, engine start can proceed. The IGNITION switch/key is neatly located on the lower panel between the two engine master switches with a spring loaded position of START beside each engine master switch.
On the test flight, only a short click to start position just like starting a car was all that was required to bring the engine to life as it was already warm.
At the time of starting the left engine, Richard Tomlin, Chief Pilot, Hawker Pacific, and I were sitting there with the forward canopy part open and neither of us had our headsets on and with the left engine now at idle, I could not believe how quiet the engine was, and conversation between the two of us was extremely easy. This is also considering that the engine has no muffler but only a spark arrester after it exhausts the turbo and then straight down to exit the cowling at the bottom forward edge.
The starboard engine was started just as easily: ENG MASTER ON, ignition key towards START and the engine was at idle.
Once the engine temperatures had stabilized and the Garmin aligned, we were ready to taxy out to the runway.
This is where the next benefit of FADEC comes in. In the engine run-up bay, we parked the brakes and with both power levers at IDLE, I held in the two ECU TEST buttons to the far left of the of the instrument panel and as a number of EECU FAIL A and B messages displayed on the EFIS the engines ran up and back above 1900 RPM while the props exercised and, after both A and B systems were self-checked and the messages extinguished, we were ready to launch. Only the other normal airframe related checks of fuel pumps, hatches and harnesses and flight controls needed to be checked.
The whole exercise was so much quicker than the Piper Arrow that had entered the run up bay just before us.
On take off it was just like flying a jet as far as engine handling was concerned.
With only a power lever for engine control, there are fewer hands flying all over the quadrant adjusting manifold pressure and RPM and monitoring engine instruments like older conventional aircraft. It’s either a go or no-go lever really.
Even the engine indications (EIS) on the right hand Multi Function Display (MFD) are calibrated in percentage torque and RPM. You really only control RPM with the power lever and the FADEC looks after the engine torque to supply the required RPM for the altitude.
Returning to the DA42 airframe itself, it is effectively the same airframe as the Lycoming powered variant right up to the engine firewall. It has the same internal cabin layout as the L360 model with only minor switch and instrumentation layout differences associated with the Austro diesel installation.
The one major change that I noticed, and didn’t really like, was that with the reduction of engine controls to just two power levers, Diamond made the levers smaller than the Lycoming model and mounted them lower on an almost flat centre console instead of in a raised quadrant like most other twins.
With these short levers and my average Aussie size, it seemed like a bit more of reach down to the levers as they were now situated almost down beside your upper leg instead of above it.
The Garmin G1000 installation was slightly different also with the inclusion of a Garmin GFC700 autopilot system and yaw damper instead of the King system in the DA42-L360.
The yaw damper can also be used when hand-flying the aircraft as it will keep the skid ball centered for those “lazy feet” jet pilots like myself. However, the yaw damper is automatically engaged when the autopilot is on. The autopilot and Garmin G1000 system is certified for RNP5 operations. (Required Navigation Performance 5 nm)
The overall finish of this Austrian-built aircraft I felt was marginally better than the DA42-L360 that I had previously evaluated, which was built in Canada. This was most obvious in the fitment of the nose locker doors and the rear seat canopy.
The NG model also included small vortex generators mounted on the top of the leading edge between the fuselage and each engine nacelle.
In the cruise
Handling of the DA42-NG differed minimally from the Lycoming variant. Rotation commenced at 80 KIAS and initial climb out at 85 KIAS until clear of any obstructions, and then best climb performance occured at 90 KIAS.
With the power reduced to 2100 RPM and 92% torque, climb rate settled to well over 1000 fpm. In fact, in a standard Australian atmosphere of around ISA +10°C, the DA42-NG will still deliver a climb rate of better than 1050 fpm at sea level at MTOW (1900kgs) and 90 KIAS or about 10.8% gradient.
On the test flight in the DA42-NG, I was able to explore the engine handling a little more closely as I had already flown the L360 model of the aircraft. The diesel handling adds no more complexity to the operation and in fact makes things simpler: there’s no separate engine controls for throttle, prop and mixture to adjust.
With a single lever control of engine RPM the FADEC system looks after the engine torque and keeps this all within limits. There is nothing else that can be controlled by the pilot.
In the unlikely event that an engine should malfunction, the engine shutdown checklist calls for the ENGINE MASTER switch to be selected OFF, obviously after confirming that you have the correct one in your hand before selecting, and then turning it OFF. The engine system will automatically feather the propeller and switch off the EECUs on the engine.
If a restart is considered, the checklist states that it should be carried out between 125 KIAS and 145 KIAS. Below this speed the engine will not windmill and above it, it may overspeed as it comes out of feather.
When we tested it during our flight, we selected the ENGINE MASTER switch ON and the propeller electric pump drove the blades out of feather and the engine began to rotate. After a couple of jerky rotations, it eventually fired.
While the engine was secured, we explored the engine-out handling. I found it much the same as the L360 version and with two of us on board and close to half fuel load at 3500 feet, we were able to achieve close to 2-300 fpm climb rate.
At sea level in our Aussie atmosphere, the Aircraft Flight Manual (AFM) shows a single engine rate of climb of 160 fpm at maximum takeoff weight of 1900kgs.
I know when we later took some photos of the DA42-NG airborne, it had no trouble keeping pace in level flight with the C172 even with one feathered.
One previous problem inherent in the Thielert powered version was that if the EECU detected a problem, then it would shut the engine down. With the Austro engine having twin EECUs, then with one failing, the other will automatically take control. In the unlikely event that this should also fail, then the unit will reduce the power on that engine to 80%.
Another problem of the DA42-TDI was with the gearbox overheating in some climatic and operating conditions, this has been overcome with a larger gearbox and associated oil cooler.
During the flight I also took the opportunity to explore the latest additions to the G1000 display. This mainly consisted of the addition of side elevation display at the bottom of Multi Function Display (MFD).
This neatly shows a lateral depiction of the aircraft and its flight path at an altitude and the corresponding terrain currently below you and what’s coming up ahead.
The map display above this not only shows the usual map and airspace but also includes a yellow box outlining this lateral situation display area with the aircraft symbol at the base of the box. The box will slew around as aircraft track changes: a handy feature warning of any rising terrain that maybe ahead at that altitude when IFR.
In the cruise during my evaluation flight at 3,500 feet and 2,000 RPM, which indicated 80% torque, the Garmin G1000 EIS showed a fuel flow of 7.2 gph (27 lph) for just over 140 KIAS and 153 KTAS.
During the recent demonstration tour that the DA42-NG participated in around Australia and across the Tasman, Stephen Pembro, Diamond Aircraft Sales Manager at Hawker Pacific tells me they averaged around 39 litres per hour total fuel flow for both engines. This was from around 60% cruise and 150 KTAS at optimum altitudes below 10,000 feet.
When you consider that at around 40 cents per litre saving over 100LL AVGAS, this alone is a $12 saving over a Lycoming powered DA42-L360. Diamond says that the DA42-NG is around 30-40% more fuel efficient than a Piper PA-44 Seminole.
Tale of the tape
Currently there are some 155 DA42-NGs including a number of DA42-MPP multi platform purpose aircraft flying all over the world and 85 DA40-NG with the Austro AE300 powerplant.
AE300 engines have apparently amassed a total of 125,000 flight hours world- wide. During this time there has only been one recorded engine shut-down which was caused by a maintenance error and not engine design or mechanical failure.
The AE300 currently has a TBO of 1200 hours on both the engine and gearbox but they are expecting this to increase to 1500 hours by the end of 2012. Austro has a progressive plan to gradually increase the TBO of the AE300 to 2,000 hours and they expect this to be achieved by the end of 2013 to mid 2014.
Peter Lietz from Austro Engine GmbH tells me that Austro are planning to expand the production of general aviation diesel engines with the AE500 engine, a 6 cylinder 280 hp unit and the AE440 producing 330 kW and keeping within a limit of 250 kg.
Diamond also has a very comprehensive maintenance package that is included with each aircraft that helps to reduce costs considering this slightly lower TBO compared to conventional Lycoming installations.
Diamond announced last year that they were planning to update the DA42-NG into the V1 version. The DA42-NG V1 has up to 40 kgs of weight loss in structural improvements and even from new lighter floor coverings. For those owners of the older Thielert powered versions, Diamond Aircraft is planning a swap-in installation for the AE300.
As well as weight reductions, Diamond has also completely redesigned the engine cowlings and wing fairings to reduce drag thereby generating close to 8 knots improvements in cruise speed. A redesign of the fin and rudder has not only resulted in a reduction in drag but a reduction of 5 knots in the Minimum Control Speed Airborne (VMCA.)
A further 3 knots in cruise speed has also been gained by the Diamond Aircraft and Austro partnership working with propeller manufacturer MT in designing a new high tech propeller with curved blades, increased diameter and new materials to deliver more efficiency from the torque of the AE300 engine.
These improvements all add up to an aircraft that can climb at 1800 fpm at MTOW after less than a thousand foot take-off ground roll, increased single-engine climb rate to 280 fpm at sea level and cruise at 197 KTAS at Maximum Continuous Power (MCP) at 10,000 feet.
In all, I found the DA42-NG to be a great aircraft to hand-fly with excellent visibility and an engine system that is not only simple to operate but also economical. And as 100LL AVGAS becomes harder and more expensive to get, the idea of diesel power from jet fuel may become as popular as the diesel-powered modern sedans on our roads today. How many servos have you seen install more diesel pumps to cater for the economy-conscious?
More details regarding the DA42-NG and the entire Diamond range can be found by contacting Stephen Pembro at Hawker Pacific
www.hawkerpacific.com or visiting www.diamondaircraft.com.
John Absolon would like to thank again Stephen Pembro, Richard Tomlin and Alan Begg from Hawker Pacific for their help in preparing this review.
Diamond Aircraft DA42-NG
Average Empty Weight: 1430 kg
Max Take Off Weight: 1900 kg
Takeoff Distance over 50 ft (ISA @ SL): 733 m
Landing Distance over 50 ft: 570 m
Cruise at MCP (92% Load, 14,000 ft): 177 KTAS
Cruise at 60% Load 14,000ft: 150 KTAS
Fuel Flow @ 60% Load: 39.4 lph (10.4 gph)
Type: Four-Cylinder, four-stroke turbo diesel aircraft engine
Dry Weight: 185 kg
Max. Power: 168 HP (125 kW)
Cruise Power: 155 HP (114 kW)
Cruise Torque: 512 Nm @ 2100 rpm
Max. Certified Altitude: 18,000 feet
Max Altitude for max RPM: 11,500 feet
Fuel: Jet A1 or JET A, JP8 (US mil spec), diesel (EN590)