Knowledge gained from development of an all-new engine for unmanned aerial vehicles (UAVs) has prompted Ricardo engineers to investigate technology spin-offs for automotive and other applications.
The recently unveiled Wolverine 3 engine program was driven by aggressive military-spec requirements for mass, acoustic profile, fuel consumption, power density, and package efficiency.
“We think range-extenders [ICEs for electric vehicles] are a further application for some of what we’ve learned,” Stephen Cakebread, Ricardo’s Project Director for Unmanned Systems, told AEI during a recent technical briefing. Lightweight auxiliary power units (APUs) are another potential spin-off, he noted.
The Wolverine 3 program is aimed at small UAVs weighing 20-50 lb (9-23 kg). It is named for the 3.1-hp (2.3-kW) rated output of the 88-cm3 air-cooled, horizontally opposed twin-cylinder power unit developed at Ricardo’s Michigan facility.
A family of small hybrid-electrics
The direct-injected two-stroke is the first in a new family of lightweight engines the company is developing specifically for UAVs—small to midsized aircraft used in a growing variety of roles, particularly surveillance. The family includes two- and four-stroke engines using spark- and compression-ignition combustion systems, depending on displacement and intended application.
The engines are actually compact hybrid-electric propulsion units, with their ICEs teamed up with electric generators mounted on the front of the crankcase.
The Wolverine 3 features a 0.5-kW (0.66-hp) integrated motor/generator adapted by Ricardo engineers from a medical-robotics application. It provides supplemental power and also enables the aircraft to operate quietly in all-electric mode for low-altitude operations.
The engine-family architectures are scalable, and all are designed to operate on the U.S. military’s standard JP8 fuel. The heavy-fuel requirement drove Ricardo’s development of a proprietary combustion system, although the engine can be calibrated to run on gasoline.
“The low volatility of JP8 established the combustion chamber geometry, compression ratio, piston profiles, ring design, injection pressure, and some internal materials applications—none of which we can discuss in detail,” said Cakebread. He did reveal, however, that the cylinder bore diameter of the small “boxer” twin measures 40 mm (1.57 in).
“In our initial discussions with potential customers and government agencies, everybody told us ‘you can’t build a heavy-fuel engine that small,’” noted Kent Niederhofer, Ricardo Inc.’s President.
Added Cakebread: “For this project we were going for low mass, good power density, and keeping it simple and reliable. Compression ignition would be a favored approach because of the fuel economy benefits.”
Ricardo’s Wolverine 4, a 300-cm3 horizontally opposed, liquid-cooled, compression-ignition four-stroke rated at 25 hp (18.6 kW), is under development for a larger, higher performance UAV.
Engine internal details
Ricardo is working with Unmanned Aerial Systems (UAS), a Las Vegas-based manufacturer of the Nightwind family of UAVs. The Wolverine 3 will be installed in a Nightwind 2 aircraft and is scheduled for its inaugural test flight this summer in Nevada, said UAS CEO Sandy Mangold. The Nightwind 2 is a blended-wing design with a 2-m (6.5-ft) wingspan.
The prototype engine currently running dynamometer testing at Ricardo’s Van Buren Township facility near Detroit was developed from concept in less than six months. The piston-port design breathes through reed-valve induction. To save cost and speed development, Cakebread’s team chose off-the-shelf injectors from a small diesel and a mechanical fuel pump; both are modified for reduced mass for the Wolverine 3.
A pressed-up steel crankshaft spins on rolling-element bearings inside the two-piece cast aluminum alloy crankcase—“all very conventional at first glance but with numerous unique Ricardo details,” said Niederhofer. The pistons, which feature a proprietary crown geometry, are fitted to one-piece connecting rods (also with rolling-element big-end bearings) and ride in Nikasil-coated bores.
The Wolverine’s Magneti Marelli ECU is from a motocross bike; it controls ignition to one conventional spark plug per cylinder. A lighter, purpose-built ECU is being developed for production. A mechanical oil pump pressurizes an oil tank mounted on top of the crankcase, providing the fuel-lubricant mix. Besides the injected pre-mix, the engine also employs total-loss oiling in some locations.
Cakebread’s team is developing a tuned expansion-chamber exhaust system that packages completely within the tight confines of the Nightwind 2’s molded carbon-composite body. The exhaust system design, not yet finalized as of early June, leverages Ricardo’ extensive experience in acoustics simulation, computational fluid dynamics, and NVH attenuation. Exhaust scavenging performance is “optimized" for the application.
The Wolverine 3’s height, depth, and width measure 6.9 x 7.5 x 10.5 in (175 x 190.5 x 267 mm), respectively. The complete engine weighs 7 lb (3.2 kg).
The development program “was really about using good engineering practice and a lot of heavy analysis-led design,” Cakebread noted. The team assembled for the program numbered about 30 people but not all were dedicated to Wolverine 100% of the time, Cakebread explained.
The basic concept was based on an earlier Ricardo backpack-engine program. When it was approved, the team investigated more than 20 potential production designs, including some super-lightweight concepts using magnesium construction.
Other layouts, including a parallel twin, were discussed. The designers settled on an oversquare "boxer" arrangement because of its smooth-running characteristics, even cylinder cooling, and its flat, compact package. The team also studied use of a “power valve” mechanical port induction system as pioneered by Yamaha, but it was ultimately rejected as too complex.
Extended altitude capability
The Nightwind 2—the Ricardo-engined version is coded NW-2R—has its engine in a pusher-type configuration. The engine spins the prop directly (without gear reduction) at 6000 crankshaft rpm.
To date, UAV power (including that of the original Nightwind 2) has come from a variety of off-the-shelf sources including scale-model R/C (hobby) engines and small industrial two-strokes such as those used in weed wackers. None are optimized for UAV duty cycles and are compromised in range and performance, Mangold noted.
Niederhofer said the Wolverine 3 will enable the Nightwind 2 to operate at ceilings up to 20,000 ft (6100 m)—double the altitude of the current aircraft. Maximum altitude capability in silent (all-electric) operation is extended to 6000 ft (1829 m), a six-fold increase. And operating duration is extended from 30 minutes to several hours, depending on fuel tank capacity.
Ricardo engineers are excited about the program’s prospects in what they believe is a new market for the company.