Today's Pilot

November 2003

 

"Aero-diesel Update"

"Which is better, diesel or petrol? Roger Miller provides fuel for thought."

"At first glance, it would appear that the diesel aero-engine holds almost all the aces. Firstly, let's consider the two types of fuel. For the purposes of this essay, we will assume that the aero-engine of the future will be designed to run on either diesel or Jet A-1, although it is important to understand that although a jet will cheerfully run on diesel (or in fact almost any smelly, oily liquid) the diesel engine can be a lot pickier, although it is nowhere near as fussy as a petrol engine. Indeed, a high-compression petrol-fuelled aero-engine is most particular in its diet, and will only run satisfactorily if fed with 100LL, and preferably 130 Octane Avgas with extra lead!

Another advantage is that not only will a diesel engine return much better specific fuel consumption than a petrol engine of comparable horsepower, but diesel fuel is also easier, and therefore cheaper, to refine. There are economies of scale as well. The market for both Jet-A-1 and diesel is enormous, while the market for 100LL Avgas is (by comparison) tiny.

The diesel engine is considerably easier to operate as the entire ignition system (and all its inherent problems) is removed from the equation. Indeed, it seems likely that most aero-diesels will be designed from the outset to be operated by a single power lever - the prop rpm will be interconnected with the throttle and obviously there are no mixture or carb heat controls.

Furthermore, the diesel engine is also very efficient. Indeed, the large two-stroke marine diesel remains the most efficient internal combustion engine yet produced, with some examples recording specific fuel consumptions of almost 0.25lb/hp per hour! Part of the reason for such excellent fuel economy is that diesel engines always runs 'lean'. This makes them very efficient, unlike an air-cooled petrol engine, which is invariably running 'rich' as fuel is actually used to cool the cylinders! How wasteful is that?

Finally, diesel or jet fuel is a lot safer, as it is much less flammable than petrol. Not only does it evaporate more slowly than petrol, but its boiling point is actually higher than that of water! Another advantage is that there's a lot less CO2 in diesel exhaust, which means that if the heater malfunctions it can't poison you.

"Alright then Roger" I hear you mumble... "if the diesel engine is demonstrably superior to the petrol engine, then why is every light aircraft at my local airfield powered by Avgas?" Good question, and the answer is simple - weight or, to be more precise, power to weight ratio.

From the 180lb, 12hp engine fitted to the Wright Flyer to the Rolls-Royce Trent, which weighs 13,186lb and produces a staggering 95,500lb of thrust, aircraft designers have always understood how important the power-to-weight ratio of the engine is. Crucially, for very many years, the diesel engine always fell at this hurdle, for whenever aircraft designers even considered installing a diesel in their latest light aircraft, a quick look at the engine's empty weight soon saw them on the phone to either Lycoming or Continental!

However, a number of engine manufacturers - such as TAE, WAM, Continental, SMA, Zoche, Delta Hawk and Diesel Air - are all currently working on diesel aero-engines. These have clearly been designed as replacements for petrol-fuelled aero-engines, and some even have power-to-weight ratios comparable with their petrol-fuelled cousins. For example Diesel Air's Dair 100 has been designed to replace the Continental O-200, while the WAM-120 is aimed at the O-200/O-235/IO-240 market. Thielert's TAE Centurion 1.7 has been designed as a replacement for the Lycoming O-320, while SMA clearly views its SR305 as a replacement for the O-540.

So, is a diesel aero-engine inherently superior to a petrol-fuelled one? Well, as you'd expect, the answer is 'yes', 'no' and 'maybe', and the reason for such an unsatisfactory answer is that there are a considerable number of different parameters that need to be considered! For example, when comparing the merits of using either petrol or diesel, it is not just the weight of the engine that must be considered. Equally important is how far will the same airframe fly on a similar weight of Jet A-1, as opposed to petrol. Note here that we are interested in the weight of the fuel, not the volume, as Jet A-1 is approximately 12% heavier than Avgas. We must also consider other parameters, such as the TBO of the engine and the amount of torque that it can produce.

For example a typical light aero-engine that was in production for many years is the Lycoming O-235-L2C. This normally-aspirated, direct-drive, air-cooled flat-four has a cubic capacity of 233in3 (3.85 litres) and produces its 115 horses at 2,700rpm, while consuming around 5 Imperial gallons (22 litres) an hour in the process. Realistic consumption at cruise power is about 4.5gph, and it weighs around 218lbs dry. This gives it a power-to-weight ratio of 1.9lbs per horsepower.

The modern, petrol-fuelled European alternative in this power range is the Rotax 914. This turbocharged, geared, liquid-cooled flat-four has a cubic capacity of 74in3 (1.2 litres) and produces 115hp at 5,800rpm, although maximum power is limited to 5 minutes. Dry weight is given as 137lb, although I suspect that this doesn't include the radiator, which is clearly an integral part of the powerplant. Consequently, I am reluctant to calculate its power-to weight ratio. Fuel consumption at max power can be as high as 6gph, although this plummets to around 4gph at cruise.

Now let's consider a diesel such as the WAM 120. This two-stroke, turbo-charged, direct-drive, liquid-cooled, inverted, in-line triple weighs about 220lb (including radiator and intercooler), which gives it a power-to-weight ratio of around 1.8lb/hp.

However, and as discussed earlier, when deciding what engine to install in our next project it is not just the weight of the engine that we must consider, but also the weight of the fuel. Indeed, it is absolutely fundamental when comparing powerplants to include the weight of the fuel in any calculations, as well as the range, endurance, and perhaps most importantly, the aircraft's operational radius. Furthermore, and to make any comparison (which, generally, are odious) even remotely valid, it is imperative that the engines are only compared when fitted to an identical airframe.

Luckily for me (and this article!) Wilksch Air Motive has recently begun testing a Thorp T-211 powered by a WAM 120, alongside another Thorp T-211 that is still powered by its original Continental O-200. As the Thorp has a fuel capacity of around 17.5 Imperial gallons (80 litres) the maximum weight of fuel that can be uplifted is 144lbs of Jet A-1, or 128lbs of Avgas. Consequently, and in order to produce a fair test result, we will assume that due to excessive pie consumption by the test pilots, both aircraft can only carry 120lbs of fuel. This gives the petrol-fuelled version 16.5 gallons, and 14.6 gallons to the diesel fuelled one. And, this is where things get interesting.

While cruising at 85 knots IAS, the Thorp fitted with the O-200 burns around 4.4gph, but the WAM-powered Thorp is actually consuming less than 3gph at the same speed. Now, even those readers whose grasp of arithmetic is best described as tenuous should surely be able to appreciate that the diesel-fuelled aircraft has considerably better range and endurance than its petrol-powered cousin. However, due to the wider availability of Jet A-1, the fact that the diesel Thorp's operational radius is also far superior is of less significance than it might be.

In summary, although earlier diesel-fuelled aero-engines were fundamentally unsatisfactory, primarily because of their poor power-to-weight ratio, it is clear that the new breed of aero-diesel is a far more satisfactory proposition.

So, is the future diesel (or more likely, Jet A-1) powered? The answer is almost certainly 'yes', although the question 'when?' remains as murky as old engine oil! That 100LL is on its way out (at least in Europe) is irrefutable, but it still remains to be seen if it will be replaced by a lead-free alternative or whether the petrol engine will be made redundant by its diesel counterpart.

 

Engine Update

Currently there are a considerable number of different aero-diesels in development. These include engines by TAE, WAM, Dair, DeltaHawk, Continental, Zoche and SMA. None of these are normally aspirated (most are turbocharged, although the Dair uses a supercharger) and include a bewildering array of configurations. These inlcude 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 and liquid-cooled two-stroke in-line inverted triples!

Of these engines, some are beginning to have a significant impact on GA, while others seem doomed to linger in a state of perpetual development. For example, although Teledyne Continental Motors received a three-year contract worth $9 million in 1996 as part of the General Aviation Propulsion (GAP) programme its engine, a supercharged liquid-cooled two-stroke diesel of 200hp, has yet to reach certification.

However, the SMA SR305 has been certified. This is an air-cooled turbocharged two-stroke 230hp flat-four that obtained European certification in 2001. It is now flying in several different aircraft, including Maule M-7s and Cessna 182s.

Another successful European diesel is the Thielert TAE 1.7 Centurion. This liquid-cooled turbocharged 135hp in-line four has been chosen to power (amongst others) the innovative Diamond TwinStar. OMF Aircraft of Germany, manufacturer of the Symphony 160, also recently flew a Centurion-powered aircraft. Thielert is planning a V8 version of the Centurion with a view to supplying engines for the enormous 300-350hp retrofit market.

UK-based Wilksch Air Motive has been busy as well, and is working on a turbocharged, liquid-cooled inverted in-line four of 160hp to complement its 3-cylinder 120hp engine. WAM has chosen to produce engines for the non-certificated (homebuilt) market, and as this market is currently experiencing tremendous growth, I imagine that this will prove to be a prudent decision.

Another British-designed aero-diesel is the 100hp DAIR-100. This liquid-cooled two-stroke flat-twin is particularly interesting as it is based on the opposed piston principle. Basically, there are two pistons in each cylinder, with the combustion chamber formed between the crowns of the pistons. This eliminates the need for a cylinder head, camshaft and associated valves, and creates an engine fitted with two half-length crankshafts linked by a gear train, which drives a centrally-mounted propeller. As the cylinders are ported at each end with one ring of ports opened by the air piston supplying the fresh air charge, and the other opened by the exhaust piston and connected to the exhaust system, the gas flow through the cylinder is in one direction and is more efficient. Junkers used this system in its early 'Jumo' diesel aero-engines.

Zoche Aero-diesel of Germany has been developing an air-cooled radial two-stroke diesel for many years now. It recently announced that it expects to obtain FAA and JAA certification in 2004.

The US has been much slower to embrace diesel aero-engines, although DeltaHawk of Wisconsin has flown its 160hp V4 diesel engine in a Velocity RG."