The New Old Aero Engine!

ULPower takes a different approach to air-cooled, horizontally-opposed, direct-drive engines.

By Steve Ells
Photos: Steve Ells and courtesy of ULPower

If small airplane pilots were to get so frustrated with today’s  engines that they “rioted” for new engines, they might well be chanting: “What do we want?” “Easier starting!” “What do we want?” “Better power-to-weight ratios!” “What do we want?” “Full FADEC!” “What do we want?” “Multi-fuel engines!” “What do we want?” “Easy maintenance!”

ULPower is gaining a foothold in the Experimental fleet because they  deliver these wants. The company builds a total of eight different  engines with power ratings from 97 to 200 horsepower. These are  clean-sheet, air-cooled, direct-drive engines that were designed and  built using 21st century technologies. All of them weigh less than  existing engines of the same power, and they automatically adjust the  spark timing and fuel/air mixture to optimize performance. They are so  adaptable that they fit almost any application, and they incorporate  subtle, but proven, features gleaned from decades of developing  automotive racing engines.

Each design includes five crankshaft main bearings, a large roller-type  thrust bearing, positive-pressure oil lubrication to the cylinder valve  operating mechanisms, solid-valve lifters with diamond-coated faces, and  other user- and maintenance-friendly features.

Take Your Pick

The line of ULPower engines includes 4- and 6-cylinder models. Every  engine is delivered to the buyer with an exhaust system, an incorporated  alternator with rectifier and voltage regulator, electric fuel pump and  regulator, and a starter. The 4-cylinder engines start with the UL260 line. These include the  UL260i (97 hp) and the UL260iS (107 hp). Displacement is 2592 cc (158.17  cu in), and the installed weight is reported at 159 pounds. The higher-power 4-cylinder engines are the UL350 line. These include  the UL350i (118 hp) and the UL350iS (130 hp). Displacement is 3503 cc  (213.77 cu in). Installed weight is 172 pounds. The 6-cylinder engines start with the UL390 line. These include the  UL390i (140 hp) and the UL390iS (160 hp). The displacement for these  engines is 3888 cc (237.2 cu in). Installed weight is 220 pounds. The bigger-displacement 6-cylinder engines are the UL520 line. These  include the UL520i (180 hp) and the UL520iS (200 hp). The 520 line has  5254 cc (320.6 cu in) displacement and an installed weight of 238  pounds.

All ULPower engines have a 105.6 cc (4.1 inch) bore. The 260 and 390  have a stroke of 74 mm (2.9 in); the 350 and 520 have a stroke of 100mm  (3.9 in). All ULPower engines are direct-drive, wet-sump engines, just like a  Lycoming or Continental. But unlike the traditional engines, ULPower  cylinder heads are not screwed onto the cylinder barrels to form a  one-piece cylinder/head. ULPower heads are bolted to the barrels. This  feature makes valve and head work much easier and less intrusive to the  engine since the ring-to-barrel seal is maintained throughout the  process. Another advantage is that the torque values for the cylinder  case halves are not relaxed during head work, which maintains main  bearing crush and fit. Another difference between traditional engines and ULPower engines is  cylinder head and upper cylinder valve train component lubrication.  Each ULPower upper cylinder is constantly lubricated through an external  oil line. This flow not only lubricates the valves and valve guides,  but also cools the heads. Pressure oil also squirts at the underside of  each piston and cylinder wall.

The Squish

All ULPower engines are multi-fuel engines. They are designed to run  on mogas, including mogas blended with up to 15% ethanol, as well as on  100LL leaded avgas. The “i” models use mogas with an anti-knock index  (AKI) of 91, while the “iS” models require an AKI of at least 93. When asked how ULPower is able to safely use mogas in its engines  with compression ratios of 9:1, Lionel D’Hondt, the designer of the  engine, explained that it is possible due to the “squish” designed into  the cylinder head/combustion chamber. As the piston nears top dead  center (TDC) on the compression stroke, a portion of the fuel/air  mixture is “squished” between a portion of the piston and the head.  Squish design utilizes a very narrow gap of less than 1 mm between a  portion of the piston and the cylinder head. Squish creates turbulence,  causing a more homogeneous fuel/air mixture that results in a  better-controlled and faster combustion event.

Pre-Takeoff Full-Power Check

The fuel injection system utilized with ULPower engines is a closed loop  system. Fuel flows from the tank(s) to a pressure pump—a second pump is  available as an option—and then to the two fuel injector blocks mounted  on each side of the inlet collector atop the engine. The fuel pressure  at the blocks is regulated to maintain 43 psi (3 bar) above the existing  atmospheric pressure. Changes in atmospheric pressure automatically  result in changes in the fuel/air mixture. Since maintaining this  pressure is critical for safe engine operation, the ULPower operating  manual requires a full-power engine run of at least 5 seconds prior to  takeoff to determine if the fuel pump(s) is/are functioning correctly.

Cool Runnings

The redline (maximum) cylinder head temperature (CHT) for ULPower  engines is surprisingly low at 356° F (180° C), especially for pilots  and technicians familiar with the 460° F (238° C) and 500° F (260° C)  CHT red lines for Continental and Lycoming engines, respectively.  Maximum continuous CHT is 320° F (160° C). There are three reasons for these surprisingly low numbers. The first  is the sheer number and large size of the cylinder head cooling fins,  especially in the exhaust port area of each cylinder. Each ULPower engine is also shipped with a left and right cylinder  head ram air box. The design of these boxes is not left up to the  airframe manufacturers; ULPower designs them. In addition to supplying  these boxes, part of the post-installation testing and evaluation  procedures consists of measuring the air pressure differential across  the cylinders and across the oil cooler (if installed). When testing  cylinder cooling effectiveness, one end of a simple water tube manometer  is open to the pressure of the ram air above the cylinders, while the  other end is open to air pressure below the cylinders near the cooling  air exit from the cowling. A differential of at least 20 mm (0.78 in) at  120 km/hr (64.7 kts/hr) TAS is required across both the cylinders and  the oil cooler. The third reason, although it’s one not touted by ULPower, is the  transfer of cylinder head heat due to the continuous flow of lubricating  oil flowing to the valve mechanism in the head and being sprayed on the  bottom of the aluminum pistons. That’s the reason it’s critical to  ensure there’s a sufficient pressure drop across each oil cooler  installation.

Mounting Options

Each ULPower engine is shipped with a straight (not Dynafocal)  four-lug mounting plate bolted to the back of the engine. There’s also  an option to add a two-lug mount at the front of the engine to utilize a  cradle mount system. Engines are shipped with a standard-length 55 mm  (2.166 in) prop flange, but since the flange can be removed without  splitting the case—try that on your Lycoming or Continental engine—three  optional lengths can be used if desired: 35 mm (1.38 in), 90 mm (3.54  in), and 110 mm (4.33 in). Propellers are available from companies such  as Sensenich, WhirlWind, Catto, Prince and Airmaster. Due to the large roller-type thrust bearing, all ULPower engines can  be installed in tractor (prop in front) or pusher (prop in back)  configurations. All that’s required to adapt the engine for pusher use  is to rotate the inlet air collector, the exhaust system, and ram air  collector boxes 180° to accommodate the change. ULPower also offers a  fan and ducting kit to convert any of its engines to helicopter use.

Manuals and Tools

The ULPower web site  provides a full complement of installation, operation, parts, and  maintenance manuals, as well as service bulletins on every engine. All  of these files are available for download in pdf format. In addition to  the manuals, all of the engine service bulletins are also on the web  site pages. The only part of the engine that is not fully illustrated in an  exploded view is the engine control unit (ECU), the ignition coils, and  the starter motor. Every part of the engine, as well as every part of  the alternator, is shown in the parts manual. U.S. owners will need to buy a set of metric-sized internal wrenching  sockets, as well as a conversion chart for converting European torque  specifications to U.S. torque specs, since all specs in the manuals are  in Newton/meters. For those that want to get started right away, it’s  helpful to know that 1.35581794833 N-m equals 1 foot-lb. For example, the torque specification for the four engine-mount bolts  that secure the 350 series engines to the mount is written as 25Nm.  Conversion to 18.4 foot-pounds is easy by using any of the online  conversion sites. There are other metric notations such as thread sizes (M12 x 1.5) and  wire sizes (2.5 mm), but these are relatively easy to convert.

Real-World Experience

Since the ULPower engine’s first introduction in 2002, over 550  engines have been shipped from the factory in Belgium. In 2009 the  Twister Aerobatics Team (, a two-airplane  aerobatic team flying Silence SA1100 Twister single-seat airplanes,  selected ULPower engines when it needed more power to expand its  aerobatic routine. Peter Wells of Twister worked with ULPower to develop  an aerobatic version of its 107-hp UL260iS engine, the UL260iSA.  Working in conjunction with the ULPower design team to track engine  performance, these engines have now been flying for over 900 hours.  According to a spokesman from the factory, a ULPower engine in France  has logged over 1,100 hours. 

Robert Helms is general manager of ULPower North America. 

As a result of its work with the Twister Duo and Just Aircraft, it is  now possible to order any ULPower engine in an aerobatic configuration. ULPower engines are just beginning to gain traction in the U.S. According to Robert Helms, general manager of ULPower North America in Lake Ozark, Missouri, approximately 150 ULPower engines have been shipped to users here. One of the first companies in the U.S. to embrace ULPower was Zenith Aircraft  in Mexico, Missouri. Zenith offers full firewall-forward installation  kits for ULPower engines for the STOL CH750, the CH750 Cruzer and the  Zenith CH650. Helms also told me that ULPower North America has agreements to  supply ULPower engines to many airplane original equipment manufacturers  (OEMs) including Arion Aircraft, Kitfox, Hatz Bantam, Bede Corp, Just  Aircraft, RANS, Sport Performance Aircraft (Panther), and others. These  companies have developed, or are in the process of developing,  firewall-forward kits for ULPower engines. The kits vary from very  complete to those that supply only a mount and cowling. Additional  packages for other kit aircraft are also being developed.