Prompted by the promise that leaded aviation fuel will be going the way of the dodo bird and by NASA-funded development of new-technology general aviation engines, no fewer than five companies are currently working on diesel engines for the light aircraft of tomorrow. In addition, two companies have lightweight, fuel-efficient turbine powerplants in development.
The diesel engines are not rehashes of the rattling, smoking, gutless diesels that have appeared from time to time in various automobiles over the years. Every one of the new designs is turbocharged. Four of the engines being developed are rated at 200 to 205 horsepower. Two of these engines are being developed overseas; two of the engines are water-cooled, and one is an aircooled radial. And at least three of the engines, the Teledyne Continental Motors CSD-283, the SMA Morane Renault MR 200, and the DeltaHawk V-4, are scheduled to appear at EAA AirVenture 2000 in Oshkosh early this month.
Based on reports from the manufacturers, these engines will be capable of lower fuel specific consumption numbers than the most efficient engines of today, will run more smoothly, have much lower engine parts counts, and be capable of longer TBOs than any avgas-fueled engines.
Both TCM and DeltaHawk are building their engines to take advantage of two-stroke diesel technology. Because it's been decades since a two-stroke diesel has been used in aviation, let's look at this technology.
One of the advantages of a two-stroke diesel engine is high horsepower output per cubic inch of cylinder displacement. The TCM CSD 283 is a 286-cubic-inch (4.7 liter) displacement engine that is slated to produce 200 horsepower. A Lycoming IO-360 avgas engine displaces 360 cu. in. (5.9 liters) to produce the same rated power.
There are two reasons for this increase in efficiency. The first reason is that the two-stroke diesel cylinders deliver a power stroke for every 360 degrees of crankshaft rotation vs. 720 degrees of crankshaft rotation for a four-stroke cylinder. The second reason is that there is more latent energy in a gallon of diesel (actually Jet-A) than in a gallon of avgas, and because of better thermal efficiency the diesel engines extract more power out of a gallon of fuel than do avgas-fueled engines.
Doubling the number of power strokes also creates a smoother-running engine because there is less time in the rotation of the engine when the mass of the propeller and crankshaft are acting as a flywheel to keep the engine rotating. In a four-stroke gas or diesel engine the power stroke for one cylinder occurs during approximately 150 degrees of crankshaft rotation, only about 20 percent of the full 720-degree cycle. This results in two 30-degree intervals during each 360 degrees of rotation when there isn't a power stroke delivered to the crankshaft—this results in the engine's rotation being driven by the inertia of the crankshaft and prop, which translates into engine vibration.
In the diesel two-strokes, the power stroke occurs during 90 to 120 degrees of rotation in a 360-degree cycle. Since all four cylinders (in these four-cylinder engines) deliver their power strokes during a 360-degree cycle, there are no periods when the crankshaft isn't pushed by a power stroke. The result is less vibration since there aren't any thrust reversals on the crankshaft.
Twice as many power strokes per revolution and more efficient combustion also result in higher torque values being delivered to the propeller per cubic inch of engine displacement. Zoche Aero-Diesels, a German company developing radial aircooled two-stroke diesel engines, claims that its engine has been refined to the point where the engine delivers 58 percent more torque per liter of displacement than does a modern TCM IO-520 engine.
Although the two-stroke diesel has a number of inherent advantages, both Morane Renault and Lycoming have selected four-stroke diesel designs.
A modern high-performance four-stroke avgas engine typically compresses the fuel/air mixture in the cylinders by a ratio of 7.5:1 to 9:1 before the ignition event that starts the power stroke. Avgas engines could be made more thermally efficient by raising the compression ratio, but turbocharged high-performance four-stroke engines are already operating at the upper limits of the ability of the available fuels (100LL) to prevent detonation.
Diesels compress the air in the cylinders by ratios of approximately 16:1 to 20:1. The fuel system then injects fuel into the cylinders under extremely high pressure (20,000-plus psi) at the proper time to start the combustion event. The heat of compression ignites the fuel. These high compression ratios produce more complete combustion and better thermal efficiency than is possible from gas engines. This reduces environmental pollution, especially carbon dioxide. Another advantage of the very complete combustion process is that exhaust gas temperatures are considerably lower than turbine inlet temperatures in turbocharged avgas engines (1,100 to 1,150 degrees Fahrenheit vs. 1,650-plus), which decreases turbocharger wear and tear.
In addition to reduced emissions, diesels are quieter because of lower prop rpm and the muffling effect of the turbocharger in the exhaust path.
All of the manufacturers cite cruise brake specific fuel consumption numbers of approximately 0.036 while a healthy four-stroke avgas engine typically can do no better than 0.045 BSFC. Estimates on one company's Web site claim that a 1,000-mile trip at 65-percent power would take 40.9 gallons of Jet-A for the diesel and 57.3 gallons of 100LL for a comparable avgas-burning engine. Since Jet-A is less expensive than leaded avgas, direct operating costs are lower per horsepower.
One of the goals specified in NASA's General Aviation Propulsion research program is to reduce the complexity of future aircraft engines. CAD (computer-assisted design) and CAM (computer-assisted manufacturing), along with special software, have provided manufacturing processes that have never been used before on piston-powered aircraft engines. TCM's CSD 283 engine is being manufactured using a monoblock process. This means that the case, cylinder walls, and cylinder heads for one-half of the engine are all one piece. Bolt the two halves together after installing the reciprocating parts and actuating gears, and the assembly is finished.
Monoblock design allows developers to create engines with the cylinder and head strength necessary for the higher compression ratios and high combustion pressures of a diesel engine, while keeping weights low and reducing the cost of manufacturing.
At present there are two manufacturers of 200- to 205-hp diesel two-stroke aircraft engines vying to get their engines certified. The 200-hp contenders are Teledyne Continental with its CSD 283 and DeltaHawk with a 90-degree V-4 cylinder configuration. Lycoming and Morane Renault are taking a slightly different tack with four-cylinder four-stroke 200-hp turbo diesels using horizontally opposed cylinders. The TCM and the DeltaHawk two-strokes will be water-cooled; the four-strokes will be aircooled. Zoche Aero-Diesels of Munich, Germany, is developing a series of aircooled two-stroke turbo diesels that utilize a radial cylinder configuration.
NASA selected Teledyne Continental in April 1997 to develop an engine in conjunction with the GAP program. The CSD 283 is a four-cylinder, horizontally opposed, turbocharged, liquid-cooled two-stroke diesel engine with 4.7 liters displacement that is rated at 200 hp. Rated power is delivered through a governed 2,200-rpm Hartzell propeller. Test cell runs to rated horsepower of a prototype engine were to be completed in July.
The CSD 283 engine has two conventional exhaust valves per cylinder. The exhaust valves quickly scavenge the exhaust gases. Low exhaust gas temperatures should lessen valve train wear. The target weight for the engine is 300 pounds, and reports are that this weight has been met during development testing.
The CSD 283 engine utilizes a turbocharger to pressurize intake air. This turbocharger is driven by an electric motor to pressurize the air for starting.
TCM has contracted Mod Works, of Punta Gorda, Florida, to install the CSD 283 on the front engine position of a Cessna Skymaster for flight testing. This airplane is scheduled to appear at Oshkosh for the EAA AirVenture 2000 airshow.
Morane Renault's MR 200 is a 5-liter (approximately 305 cu. in.) displacement, four-cylinder, turbocharged, intercooled, four-stroke, opposed-configuration, aircooled 200-hp engine. This engine has conventional tulip-type intake and exhaust valves. With development starting in 1995, this engine was first seen at Oshkosh in 1998, and certification is expected in Europe by the end of 2000, with U.S. certification expected in the summer of 2001.
Flight testing has been carried out with the engine installed in a Socata TB20 Trinidad. The reported weight is 13 pounds less than that of a Lycoming IO-360.
The engine will be in Oshkosh for EAA AirVenture 2000, and attendees will be able to hear and see the engine run. The engine will be mounted in an Embry-Riddle Aeronautical University advanced technology powerplant demonstration device. This engine will deliver rated horsepower at a governed 2,000 rpm. This speed was chosen to create a very low noise signature and enable the engine to achieve a 3,000-hour TBO.
Both 250-hp (MR 250) and 300-hp (MR 300) versions of the four-cylinder configuration are also being prepared for certification and production.
DeltaHawk Inc. is currently testing a two-stroke, turbo- and supercharged, intercooled, four-cylinder 200-hp diesel engine with the cylinders in a 90-degree V configuration. The displacement is 202 cu. in. (3.3 liters). The V cylinder configuration was chosen because it can be more compact and lighter than opposed configurations, a claim that can be borne out since a flying model of this engine weighs 60 pounds less than the Lycoming IO-360.
The EagleHawk V-4 will deliver rated horsepower at 2,700 rpm. The pressurized inlet air and the exhaust gases are piston ported. This eliminates the complexity of the valve train. This closed-loop breathing method isn't quite as efficient as the uniflo design used in the TCM engine, but the manufacturer thought that simplicity was more important than optimum efficiency.
This engine is equipped with a supercharger to pressurize the airflow into the cylinders at low rpm and for starting. After the engine starts, the boosted air will be provided by the turbocharger. If the turbocharger fails, the supercharger has enough capacity to provide 50 percent of normal engine power.
DeltaHawk has two-stroke V-configuration engines on the drawing board for 100-, 150-, 200-, 300-, and 400-hp models.
Lycoming's engine is an opposed four-cylinder, turbocharged, four-stroke
aircooled engine rated at 205 hp. This engine is being refined from an existing design originally developed in Italy and owned by Detroit Diesel. According to Randy Jensen, director of product development, Lycoming is "very actively" pursuing this project.
A four-stroke design, the TDIO-360 features intake and exhaust valves. This engine has individually removable cylinders. Work is continuing on reducing weight, lessening vibration, improving reliability, and optimizing the fuel delivery system.
The Zoche ZO O1A is a 150-hp turbo- and supercharged, intercooled, two-stroke, four-cylinder, aircooled diesel engine with the cylinders in radial configuration. Displacement is 162.6 cu. in., and the engine weighs 185 pounds. The dimensions are 21.8 inches in height, 21.8 inches in width, 25.4 inches in diameter, and 32.0 inches in length. The cylinders are piston ported, which Zoche says is at least as efficient as the uniflo design and has fewer parts. Michael Zoche has been developing his engines, which also include the ZO O2A, a 271-pound, 300-hp, eight-cylinder version, for more than 15 years.
The radial design was chosen for its ability to be effectively aircooled and 100-percent balanced at all rpm with a simple counterweight system. All four connecting rods are attached to a single crankshaft throw—this prevents any crankshaft twisting, which is hard to balance out in opposed-configuration engines. Zoche engines use a pneumatic starting system that does away with the need for a heavy-duty starter and battery system.
Michael Zoche says, "People do not realize the magnitude of our goals; our competitors think we are slow. We are confident that as soon as our products are certified and in production, they will change their minds."
Williams International Inc., with its FJX-2 entrant in NASA's General Aviation Propulsion program, has a history of building small, powerful turbofan engines. Williams' turbine engines have been used on everything from Tomahawk cruise missiles to the Cessna CJ1 and CJ2, Raytheon Premier, and Sino Swearingen SJ30-2 airplanes.
The FJX-2 engine is expected to weigh 85 pounds, develop 700 pounds of thrust, and burn 100 pounds of fuel per hour—approximately the same fuel burn as a 300-hp piston single. Williams International produced four FJX-2 engines. The EJ22 is the production version of the FJX-22. Improved manufacturing techniques and changes brought about by extensive testing have the engine on track. The engine has been selected to power the Eclipse 500 jet.
According to Sam Williams, chairman and CEO of Williams International, "The first flight of the Eclipse aircraft, powered by two 770-pound-thrust EJ22 engines, should occur in 2002."
The Agilis TF-800 is an engine that is designed to power small four- and six-place aircraft. The goals in the design are a high bypass turbofan engine with the following characteristics: low noise, low cost, high reliability, light weight, and exceptionally low fuel consumption. This engine has been selected to power the Safire S–26, which is scheduled to fly as a prototype in mid-2002, with certification following in 2003.
Links to additional information about emerging aircraft engine technology may be found on AOPA Online ( www.aopa.org/pilot/links/links0008.shtml ). Next month's "Future Flight" will examine tomorrow's cockpit displays. E-mail the author at [email protected] .