Motor Head #16: Jabiru -- A New Engine That's Not All That New, But Cool Nevertheless
Unless you've been around homebuilts, you've probably never heard of a Jabiru. (Sure, you may be familiar with the varieties of stork whose name comes from the Tupi-Guarani term meaning "swollen neck.") I mean the Jabiru airplane -- a tidy design that's morphed into several varieties: two- and four-seat, 51% experimental and, soon, a ready-to-fly light sport aircraft -- but, actually more to the point, the Jabiru engine. Even if you're a certified-only kinda pilot, you should know more about Jabiru because one of its engines may well be under the cowling of that hot little light-sport aircraft you just bought.
In yet another example of invention shooting out of Mother Necessity, Jabiru started as an airplane company and quickly found that the engine choices in the circa-60-hp class were limited to practically one. And then that manufacturer stopped making the engine.
So Jabiru designer Rodney Stiff began work on replacement. If you think about it, there are scores of engine configurations out there -- and that's just for pistons, more even if you're of the mind to harness the exquisiteness of Felix Wankel's (rotary) design -- and the man who pens his own airplane has the luxury of making the space outboard of the firewall anything he chooses. So he can build an engine of pretty much any configuration: inline, V, opposed, W, X ... so many possibilities.
Not So Different? ...
And yet the clean-sheet Jabiru arrived into this world in 1992 as an air-cooled, opposed-piston, flat engine with a single, central crankshaft bolted directly to the propeller, a single central camshaft that operates two valves per cylinder through pushrods and rocker arms, and many other similarities to our supposedly outdated certified engines. In fact, you could see the Jabiru as the new product of an existing engine company leveraging its knowledge of traditional designs, only making the engine smaller and lighter. Comes to that, the Jabiru's diminutive valve covers in some ways remind me of Continental's technically superior yet ill-fated Tiara series'. Well, how about that?
Jabiru's first effort was a 1.6-liter (call it 98 cubic inches) but after the first 40 were delivered, the company turned its quest for more power into a new iteration, now displacing 2.2 liters (134 cubic inches) from a 97.5-mm bore and a comparatively short 74-mm stroke, and producing 80 horsepower. This became the Jabiru 2200. Take it one step further -- as Continental and Lycoming (and Franklin and Pratt & Whitney and Wright) did -- and bolt six cylinders to the case and you have the 3300, capable of 120 hp maximum and 107 continuous. That 3.3-liter displacement is 201 cubic inches, making this a great comparison with the Continental O-200. (More on that further down.) You may also have seen Jabiru's 5100, an eight-cylinder derivative making 180 hp. The company has put that engine on the back burner as it concentrates on the smaller mills.
... Actually, Quite Different
Take the top off any Jabiru-powered airplane and prepare to be dazzled. The Jabiru glistens in beautifully CNC-machined 5000-series aluminum. The jauntily cut cylinder heads -- separate pieces from the steel-lined barrels, presumably easing valve work in the field -- expose a pair of automotive-style spark plugs, side by side. The induction and exhaust systems are neatly tucked underneath. To ease cooling concerns, each Jabiru comes with a semi-plenum cooling shroud so that the internal layout of the cowling isn't quite so critical. (Being air cooled, though, a clean inlet flow and appropriately valued exit area remain important.)
Once your gaze has wandered from the machined cylinders, they alight upon the fully CNC-machined cases. The support structure is internal, so no bumps and lumps, nor do you see the typically grotty casting quality evident on your typical Lyconental. (Yes, I realize that the surface finish of a casting has virtually no impact on its strength, but if I'm paying the equivalent of a decent sports car on an engine alone, I'd at least like it to be attractive.) If finish alone sold engines, the Jabiru would be flying off the shelves.
Look further. Notice how the cylinders are staggered side to side? Seems a lot more than usual, something you're at first willing to chalk up to an optical illusion. But the exaggerated offset is real, the result of a five-main-bearing design for the four-cylinder and a seven-main design for the six. Each crank throw is supported on both sides by a main bearing; a traditional aero engine has only a "flying web" between opposed cylinder pairs. So a four-cylinder Lycoming, for example, has but three main bearings, albeit very large ones, particularly the massive bearing between the prop flange and the first set of cylinders. Jabiru's literature calls out eight bearings on the six-cylinder engine, apparently counting the split shells on the crank nose as two. These bearing shells are, like several parts of the engine, automotive in origin. Jabiru uses a removable prop flange so that replacement of the crank nose seal -- a fairly ugly job on a traditional flat engine -- is quite easy. Nice touch. This feature, along with the solid crankshaft, means a fixed-pitch prop or an electrically adjustable one at best.
High-Speed Crankshaft, Low-Speed Pistons
Back to the crank design. In theory, more main bearings the better, but there is a slight weight penalty and a supposed internal drag penalty as well. However, this design makes sense in light of the Jabiru's high operating speeds. For the 3300, maximum power comes at a lofty 3300 rpm, while maximum-continuous is 2750 rpm. Thanks to the 74-mm stroke (2.91 inches), the piston speeds remain moderate. A Continental O-200 has a 3.88-inch stroke, for comparison. Maximum piston speed for the 3300 is 1602 feet per minute, compared to 1778 fpm for the Continental; at the Jabiru's max-continuous speed (2750 rpm), it's a modest 1335 fpm. Your IO-520 is whipping along at 1800 fpm at takeoff and 1533 fpm at a leisurely 2300 rpm cruise setting. But let's have true perspective: A Yamaha YZF-R6 sportbike, with a teeny-tiny 42.5-mm stroke (1.67 inches), wailing at its (theoretical) 17,500-rpm redline, posts a breathtaking 4880-fpm max. piston speed. You don't want to be anywhere near that sucker when it blows.
That's a long way around to suggesting that the Jabiru's operating speeds are, in many ways, conservative. The valvetrain is similarly conservative, with a single, underslung camshaft, two valves in each cylinder and simple screw-and-locknut adjustments in the heads. Jabiru recommends checking the valve lash several times early in the engine's life, but the job appears to be a simple one. Many of the valvetrain components come from Honda, while the pistons are modified Australian-market GM pieces (the modification is to add a piston pin circlip to keep the pin from riding out and scoring the bore.) Those pistons provide a modest 8.0:1 compression ratio, which would be suitable for auto fuel if only Jabiru were confident of the go-juice's quality; because it isn't, the only recommended fuel is 100LL.
In many respects, the Jabiru follows the conventions set by the Rotax 912, arguably the most popular "nonstandard" aviation engine in the last decade and a half. (Rotax says there are more than 15,000 of them in service.) A simple, crank-mounted, electrical system powers the airplane and the ignition from a permanent-magnet alternator, much like modern motorcycles do. The timing is fixed at 25 degrees. Where the Rotax uses independent coils, the Jabiru has two modified Honda coils feeding a pair of small distributors for the dual-ignition system. Similarly, while the Rotax employs motorcycle-style Bing carburetors, the Jabiru has one feeding an under-engine plenum that shares space, Lycoming-like, with the oil pan. (Still, carburetor heat is a recommended airframe addition.)
Thanks to its greater displacement -- the Jabiru six's 3.3 liters dwarfs the 912's 1201 cc (74 cubic inches) -- the Aussie engine pounds out more power than even the turbocharged Rotax 914, and does so at much lower engine speed. (Remember that the Rotax has a reduction gearbox.) Bigger and simpler, too: No liquid cooling setup to deal with, one fewer carb (and no need to synchronize), and a pilot-friendly wet-sump oiling system that obviates the need for an external tank and the traditional "burping" required of the 912 to read the oil level properly. That and the Jabiru uses traditional aviation oil, so there'll be no need to stomp out to the Mobil station down the road from the airport when you need a quart.
In many respects, the 3300 is the perfect "tweener" engine, bigger than the stoutest Rotax (for now) and still smaller, lighter, and cheaper than the lowest Lycoming or Continental. With the low-end LSAs becoming modern darlings, Jabiru could be well placed.
Take It For A Spin
As everyone knows, there are specifications and company propaganda, and then there's flying. I had the good fortune to spend a day with a Jabiru 3300 in a Jabiru airframe recently. And while my Rotax 912 time -- probably 200 hours or more, but well spread out among a variety of airframes -- isn't terribly recent, I think I can still offer a few useful observations.
Like most good engines, the Jabiru comes to life without fuss, thanks to a decent ignition system and well-considered jetting in the single Bing. Spinning a lightweight Sensenich composite prop -- carbon fiber over wood, ground adjustable -- the engine feels responsive, and moves the J230 Experimental with ease. Runup is conventional but short: Check the ignition and glance at the gauges. There's no prop control, no mixture.
(As on the Rotax 912, the Bing carburetor self-adjusts. It's a simple and largely effective system. Inside the carburetor throat is a piston [called a slide] that descends to block air flow. Its movement is controlled by a large diaphragm above that responds to air density [volume] through the carb. At high volume, like takeoff, the slide rises, allowing more air to flow. That's great, but the trick is the needle attached to the slide. This tapered needle descends into the main jet cavity. The higher the slide, the further the needle comes out of the main jet tube and the more fuel flows. At altitude and/or reduced throttle, the slide descends and the needle blocks more of the main jet tube, reducing fuel flow. My experience with the 912 is that the system works as advertised but EGTs inevitably drop at altitude, hurting fuel efficiency. In fact, Jabiru lists the 75% cruise consumption of the 3300 as 6.87 gph, which calculates out to a specific fuel consumption of 0.51 pounds per hour per horsepower. A carbureted Lycoming can do better, manually leaned, at around 0.45-0.47 pph/hp; an injected six-cylinder Continental, with balanced injectors, can do an impressive 0.385.)
Yes, But How Does It Sound?
Stand outside and witness a takeoff, and you hear a short burst of prop-tip noise and a very quiet engine note somewhere between an IO-360 Continental and a Porsche. Not exactly grumbly but not bumblebee, either.
From inside, the 3300 moves the Jabiru smartly down the runway and results in very good climb considering the power loading. In this airplane, the temps seemed very well controlled, with CHTs staying below the magic 380 degrees -- though I don't know if that number is magic for the Jabiru or not.
Cruising is as simple as setting the rpm for the desired power and watching the temps. While I don't doubt that a controllable carb (or fuel injection) would improve the 3300's fuel specifics, somehow the drop from 7 gph to, say, 6 seems hardly worth the effort. During the approach and landing, the engine is extremely well behaved, and the ability to basically ignore it and fly the airplane is wonderful. I did manage to botch the first landing -- trying out my Beech A-36 flare on a knee-height airplane -- and noticed the only quirk of the day. The engine is very responsive, which is no surprise with a lot of cubes and a single, smallish carburetor, and asking for just a bit of power can result in actually getting a lot. Whoa, boy.
The final comparison comes with the Continental O-200. The Continental is two inches longer than the six-cylinder Jabiru, nearly five inches taller, and almost nine inches wider. Continental lists the O-200's dry weight as 170 pounds, which probably doesn't include the charging or ignition systems. The Jabiru, including an exhaust system, all electrics, and the carburetor, is said to weigh 178. (A basic 912 is some 50 pounds lighter.)
In all, the more I look at the Jabiru, the more impressed I become. Now to scam another ride in that Jabiru airplane, under the guise of continued research of course ...
Got motors on your mind? Check out the rest of Marc's columns.