Turbo power reaches new heights

DN Staff

September 9, 1996

10 Min Read
Turbo power reaches new heights

Back in the 1980s, as part of a classified program called Teal Rain, a triple-turbocharged reciprocating engine demonstrat-ed sustained performance at 90,000 ft. Recently the government declassified portions of the program. Today, the engine technology is being incorporated into flight hardware on remote piloted vehicles (RPVs).

NASA officials intend to use Teal Rain's high-altitude reciprocating-engine technology to improve performance of advanced, long-endurance RPVs for the agency's Environmental Research Aircraft and Sensor Technology (ERAST) program. "The focus of the program is on scientific analysis of the atmosphere," says Dave Dent, light aircraft consultant for NASA at Dryden Flight Research Center in Edwards, CA. "Other appli-cations include mi-crowave relays in communications systems, geophysical surveys, natural disaster monitoring, forest fire detection, border and agricultural patrols, and intelligence and tactical applications.

"NASA needs powerful, lightweight engines for these applications," says Dent. Engines evaluated for the ERAST program included: the Rotax 912 and 914, Norton NR 801 and 622, General Atomics KH-800, Nelson H-63CP, Rotoway RW152, and others. Four-cycle reciprocating engines, according to Dent, have four major advantages over turbine engines: light weight, lower brake-specific fuel consumption (bsfc), cost, and lower aircraft velocities for sensor applications.

Reliability is the key. Made by Bombardier Rotax in Austria, the opposed 4-cylinder, 4-stroke Rotax 914 engine is NASA's choice for ERAST because of its availability, power-to-weight ratio, and proven dependability. "After 250 hours of testing we've experienced no mechanical problems," says Dent.

The unmodified engine develops 100 hp at 5,500 rpm and 91 lb-ft of torque at 4,800 rpm. It weighs 134.7 lbs when equipped with an electric starter, carburetors, fuel pump, air filters, oil system, and exhaust system.

Before altitude testing, engineers replaced the engine's standard dual carburetors and diaphragm fuel pump with an electronic fuel-injection (EFI) system originally developed for endurance race car engines. An engine-control unit (ECU) controls both ignition and EFI, and is an integral part of engine data acquisition. Electronic ignition replaces the engine's original magneto. This system includes two spark plugs and one coil per cylinder. A custom-designed intake manifold replaces the original, and the turbine system and custom-exhaust replace the original exhaust.

In 1992, Thermo Mechanical Systems Co., Canoga Park, CA, installed the engine at the firm's high-altitude test facility and began testing. In flight hardware, the modified engine with dual turbocharger weighs 519 lbs from the firewall forward. This figure includes prop, gearboxes, coolers, and controls.

Triple turbocharger. Three turbochargers combined in series--low, intermediate, and high pressure--were designed and fabricated by TMS. To optimize efficiency, engineers tweak design and performance parameters to eliminate surging and choking. "The individual turbochargers must be tuned and work together as a team," says Jim Harp, president of TMS. "Surging, when the compressor over-supplies air to the engine, and choking, when the engine demands more air than the compressor can supply, both de-crease efficiency."

Early turbochargers featured adjustable turbine nozzles and compressor diffuser vanes to optimize performance. The design, de-veloped in the 1980s, has been used ever since. Each turbocharger uses the same basic design scaled to size.

The turbochargers are extremely light compared to commercial technology. "We use aluminum as much as possible because weight reduction improves the overall fuel factor," says Harp.

The high-pressure turbo has a three-inch-diameter compressor wheel, the intermediate a five-inch wheel, and the low pressure wheel is 7.5 inches in diameter. Compressor wheels are machined from a billet of titanium on a five-axis machine tool. "The cost of the programming required for machining the wheel is about one-third of the total wheel cost," according to Harp.

Aluminum's strength rapidly degrades at temperatures above 350F. "We could almost use aluminum for the low-pressure compressor wheel--temperatures don't usually get that high--but we can't afford the risk of a failure," says Harp.

While the turbine wheels are constructed of Inconel(R) 713, TMS makes the compressor housings of aluminum and the turbine housings from 321 stainless steel. The three turbochargers combined weigh about 85 lbs. Low-pressure, intermediate, and high-pressure turbochargers weigh in at about 50, 25, and 10 lbs, respectively.

Despite the light weight of the turbo, its relative size compared to the small engine is startling. Harp says: "The power plant can be viewed as a gas turbine system with an internal-combustion-engine combustion chamber."

Engineers check turbocharger vibration on a separate test rig at TMS' facility prior to installation on the engine because engine vibrations overshadow vibration from the turbocharger. Hot exhaust (1,600F) from a J-33 turbo-jet combustor enters the turbine that spins the compressor. A butterfly valve on the compressor outlet controls flow from surging point to 50% efficiency. Pressure and temperature data are collected and used to develop speed curves and compressor functions.

Fast as a bullet. Normally, turbocharger/compressor wheel tips sustain speeds of about 1,800 ft/second. "That's somewhere between the speed of a pistol bullet and a rifle bullet," says Harp. In a choking condition, tip speed can ap-proach Mach 1. Compressor wheel rpm is a function of tip speed and diameter. Low-pressure, intermediate, and high-pressure turbine/compressor wheels run at 55,000, 82,000, and 135,000 rpm, respectively. The overall turbo system compression ratio exceeds 64:1.

Turbocharger compressor pressure ratios (exhaust pressure/inlet pressure) for a three-stage turbocharger will vary from 3.7 to 4.75 per stage. Average pressure ratio for each stage is about 4. "At 80,000 ft the triple turbocharger supplies 33 inches of Hg at the intake manifold. Power output is 80 horsepower at 5,000 rpm," explains Harp.

Pressurized oil (Mobil-1 automotive grade) from a high-volume pump cools the turbochargers' high-speed, long-life hardened ball bearings. The same oil is used in the engine and turbo system.

At altitudes below about 5,000 ft, the engine runs without turbocharger boost. After the aircraft passes through 5,000 ft, the turbo system's waste gate closes and directs 1,550 to 1,600F exhaust through turbine nozzles to spin the turbine wheels.

At 90,000 ft, ambient air enters the low-pressure turbocharger at -65F and 0.5 inches of Hg absolute pressure. Assuming average compression ratio of 4 per stage, outlet pressure to the coolers from the low, intermediate, and high pressure turbochargers amounts to about 2, 8, and 32 inches of Hg. Allowing two inches Hg for cooler losses, this supplies air to the intake manifold at 30 inches of Hg.

Altitude chamber. What is it like to fly at 90,000 ft? The temperature is -65F, startlingly warmer than the -77F at 65,000 ft. Pressure is about 1/2 inch of Hg, or about 1/4 psia. Large wing and propeller surfaces are necessary. Other challenges include oil foaming and cavitation, and fuel vaporization.

Capable of simulating temperatures and pressures up to 90,000 ft, TMS' high-altitude engine test facility has been operated for 15 years. Data acquisition at the facility relies on LabView software from National Instruments. This state-of-the-art system monitors 55 parameters simultaneously in real time. Used for on-line data reduction, the parameters allow calculation of engine bsfc and air flow, compressor compression ratios, and other performance factors.

"We measure torque to calculatethe horsepower. Horsepower is used with fuel flow data to calculate bsfc. The data accumulation system scans every eight seconds, and hard-copy printouts are available with the click of a mouse," explains Harp. Other data are used in an artificial neural network for high-altitude engine-performance modeling.

To complete the ready-to-fly fuel control system, the system records optimum fuel pulse width (measured in Hz) at each altitude. Personnel at TMS use the test data to program the ECU for actual flights.

Engineers can adjust fuel flow and spark advance/retard from the altitude chamber's control panel to optimize engine performance. "The engine generally runs pretty close to factory settings," says Harp.

Prior to testing, engineers mount the modified Rotax 914 engine and triple turbocharger to the chamber's engine cradle, then test and install vibration isolators of varying durameters. They wrap the exhaust manifold and triple turbocharger duct work with thermal insulation to prevent radiated heating of the chamber.

Two fixed-displacement 6,000-cfm vacuum pumps suck air out of the altitude chamber into the heat exchanger network and dual-media chillers. Compressed air from the turbochargers undergoes cooling in large shell-and-tube heat exchangers. The latter have compressed air on the tube side and 55F water on the shell side.

Into the blue. High-altitude testing of the modified Rotax 914 engine with triple turbocharger will continue into mid-1997. Earlier this year, an engine equipped with a single turbocharger developed 100 hp at 33,000 ft. A dual-turbocharged engine developed 100 hp at 54,000 ft, and 47 hp at 70,000 ft. The goal set for the triple-turbocharged engine is 80 hp at 80,000 ft and "as much as possible" at 90,000 ft.

According to NASA's Dent, NASA intends to install the TMS triple-turbocharged engine on aircraft hardware by 1997's end, and conduct actual test flights at 90,000 ft by the year 2000. To date, only Harp's three-inch turbo-charger has been flown. Test flights in an experimental General Atomics airplane called Altus reached altitudes of 19,000 ft equipped with a single TMS turbocharger. This aircraft, with 330-lb payload, is designed for high-altitude scientific analysis of the stratosphere and troposphere. Scheduled to be tested at 35,000 ft later this year, it has a goal of 45,000 ft with a single turbo. ERAST officials plan to eventually take it to 65,000 ft equipped with a TMS dual turbocharger.

High-altitude hurdles

What technological problems must engineers overcome to fly aircraft at 90,000 ft? Propeller reduction systems, propeller efficiency improvements, intercooler design, and flight hardware design are some of the challenges. At high altitudes, engineers also must overcome challenges like fuel vaporization and oil cavitation.

High-altitude aircraft must increase velocity to maintain lift. ERAST is evaluating belt and gear reduction systems to control propeller speeds at constant engine rpm. Belt reduction systems are lighter, but are unable to vary speeds. Gear reduction systems add weight and take up premium space. "A compact, lightweight, gear reduction system would be an ideal solution for the ERAST program," says Dave Dent, light- aircraft consultant for NASA at Dryden Flight Research Center in Edwards, CA. Longer propellers reduce the chance of prop tips reaching critical Mach speeds, and variable-pitch prop designs improve efficiencies.

NASA is evaluating two thermal-management designs. The first is a proven air/air type. It uses two large radiators, one for the engine coolant and another for compressed air. The disadvantage: increased surface area and increased drag. The other type, an air/liquid/air system, uses a radiator and a small heat exchanger. Compressed air is cooled in a small countercurrent air/liquid heat exchanger, while the radiator cools both engine coolant and heat-exchanger coolant.

Fuel tends to boil or vaporize at the low pressures associated with high altitude. "You can lose five or six percent of the fuel from vaporization," explains Harp. Engineers evaluate the trade-offs involved in fuel chilling and pressurization during endurance flights. The technologies, while reducing fuel vaporization, add weight to the aircraft and reduce its range.

Harp has run into high-altitude lubricating problems due to oil foaming and cavitation. A specially designed scavenging pump helped Harp eliminate oil-pump cavitation problems at altitudes to 70,000 ft. "We've eliminated the problems for now," says Harp, "but we will be ready for them if they return at higher altitudes."

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