LSA engines are equal to the task
The three engines that power more than 95 percent of the airplanes vying for position in the light-sport-aircraft (LSA) market are surprisingly different in the approach each takes to producing horsepower.
The oldest design, Teledyne Continental Motor's O-200, produces 100 horsepower the old-fashioned way — using four big pistons that push on the crankshaft at relatively low revolutions per minute (rpm). The newest engine of the trio, the Jabiru 3300A, has the same combustion-chamber volume as its counterpart. The difference is that the displacement is spread between six cylinders. Because the power impulses occur six times every cycle instead of four times per cycle as in TCM's O-200, the engine is smoother.
The third engine, the Bombardier-Rotax GmbsH 912 S, steps out of the traditional air-cooled large displacement mold. The displacement of this engine is almost two and a half times less than the other two engines, yet it has no trouble producing 100 horsepower by turning the crankshaft up to 5,800 rpm at takeoff. This high crankshaft rpm is reduced by a factor of 2.43 by an integral gear reduction system, resulting in a takeoff propeller rpm of roughly 2,380. This low-propeller rpm improves propeller efficiency and lessens propeller-generated noise.
Manufacturers of LSA-eligible engines are not required to comply with the same FAA-certification process as the engines installed in today's FAA-certificated aircraft (for details, see " LSA rules review," page 136), yet only one of the three most common LSA engines is what has been dubbed a "certification light" engine. The other two engines have been around for a while and are certificated to the same standards as the engines installed in FAA-certificated aircraft such as a Piper Cub or Cessna 172. In fact, many pilots trained in the 1950s, 1960s, and 1970s logged their first hours behind one of the contenders — TCM's sturdy and reliable four-cylinder, 100-horsepower O-200. Pilots who began their training in the 1990s may have done so behind the Rotax four-cylinder, 100-horsepower 912 S (the 80-horsepower 912 F is found on LSAs as well; it was the original engine installed in Diamond's two-place DA20 Katana when it hit the U.S. market in 1995).
The Jabiru 3300A is a beautifully machined, smooth-running, air-cooled, 120-horsepower, six-cylinder engine from Australia. This engine, unlike the other two, is built to American Society for Testing and Materials (ASTM, now known as ASTM International) consensus standards. There are other engines that may end up being installed in LSA, including an inverted (cylinder heads are oriented below the crankshaft) three-cylinder, two-stroke, turbo-charged diesel engine from Great Britain, but these three lead the pack.
TCM O-200A
The O-200A engine is an FAA-certificated four-cylinder, opposed-cylinder-configuration, carbureted engine that produces 100 horsepower at 2,750 rpm. Combustion cylinder displacement is 201 cubic inches with a 4.06-inch bore and a 3.88-inch stroke. The compression ratio is 7.0-to-1. It can easily burn auto gas (with the appropriate supplemental type certificate) if 100-octane fuel is not available. This engine was first introduced in the United States on Cessna's 150, a two-place trainer, in 1959.
There's nothing exotic or unusual in the design of the O-200. It's a perfect example of the large-bore, long-stroke, pushrod-activated overhead valve technology that's the standard of most of today's air-cooled aircraft engines. Each cylinder consists of a cast aluminum head that is threaded down onto a steel cylinder barrel. Ignition spark is supplied by a pair of certificated aircraft magnetos timed to fire both spark plugs at 24 degrees before top dead center. During starting, the spark timing of the left magneto is retarded by a mechanical impulse coupling and the right magneto is grounded. This permits starting by hand-propping if the aircraft battery is dead.
The carburetor mixture is controlled by the pilot with a push-pull control. These engines are very stout, can take a lot of abuse and continue to run, and are well known to light-airplane mechanics. Time between overhauls (TBO) is 2,000 hours.
The main drawback with the O-200 is that it's heavy compared to the Rotax and the Jabiru engines — but TCM has trimmed some weight by tapering the lower cylinder-barrel cooling fins and replacing the starter with a lightweight version.
Rotax 912
People sometimes confuse the Rotax line of FAA-certificated aircraft engines with Rotax's snow mobile, jet ski, and ultralight aircraft engines that have been around for nearly four decades, but those are all two-stroke engines.
Rotax LSA engines are four-stroke engines, are FAA certificated, and include the 80-horsepower 912 F, the 100-horsepower 912 S, and the 115-horsepower 914 turbocharged models. The engines all feature four cylinders in an opposed layout, like the TCM O-200. But unlike the O-200, the 912 S crankshaft turns at 5,800 rpm at takeoff, the cylinder barrels are made of aluminum — the cylinder walls are electro-coated with a wear-resistant nickel Gilnisil treatment — and the aluminum cylinder heads are liquid cooled.
Liquid cooling requires additional plumbing, an expansion tank, a thermostat, and a radiator. There's some prejudice in the field — by both technicians and pilots — against liquid cooling, but it's proven to be an effective method of controlling cylinder head temperatures (CHTs).
If the O-200 — with its breaker-point mechanical magnetos and single manual-mixture-controlled carburetor — is the "old dependable" of LSA engine offerings, then the Rotax 912 series of engines must be the "new dependable." The Rotax engines all have dual electronic capacitive-discharge-type ignition systems and dual altitude-compensating carburetors. These features mean that the quality of the spark delivered by the ignition system isn't dependent on the engine rpm as it is in magneto systems, and that the pilot doesn't have to mess with a mixture control.
Van's Aircraft, of Aurora, Oregon — arguably the country's most successful kit airplane builder and seller — chose the 100-horsepower Rotax 912 S for its RV-12, a small LSA-like airplane on the drawing boards, because "it is the best understood, most readily available and best-backed engine from an established manufacturer that [meets] our requirements of power, reliability and weight," according to the company Web site. Given Van's success and reputation for quality, that's a ringing endorsement. TBO is 1,500 hours.
The middle road
The third of the three most common LSA engines is the Australian-built Jabiru 3300A. Jabiru started building airplanes in 1992. Soon after the company's first aircraft-engine combination was approved the engine supplier folded up its tent, leaving Jabiru with an airplane without an engine. After researching the market, the company decided to go into the engine manufacturing business. Its first engine — the 60-horsepower, four-cylinder 1600 — first shown at the EAA fly-in at Oshkosh in 1994 and the Sun 'n Fun Fly-In at Lakeland, Florida, in 1995. Today's engine line includes the four-cylinder, 85-horsepower 2200A, the six-cylinder, 120-horsepower 3300A, and the eight-cylinder, 180-horsepower 5100A.
All of the Jabiru engines are machined by computer-numerical-controlled milling machines through computer-aided manufacturing processes. Visually the engines are stunning. There aren't any rough-looking sand-cast parts such as the aluminum heads on the O-200, and since the engine is air-cooled the muss, fuss, and potential for fluid leaks that occasionally accompany liquid cooling on the Rotax engines are nonexistent.
The cylinders are machined from solid billets of 4140 chrome-moly steel and the cylinder heads and case halves from billets of solid aluminum. Unlike the O-200, the heads — which form the combustion chamber and support the intake and exhaust valves — are bolted into position on top of the cylinder barrels. Compared with the massive sand-cast head on the O-200, the head on the 3300A appears quite small.
The bayonet-style-CHT probes for the TCM O-200 are most often installed on the bottom side of each cylinder head in a threaded boss, while the CHT pickups for the Jabiru engines are the spark-plug gasket-type that can be installed under only one of the two automotive-style plugs that are located on top of each cylinder in this engine. Because the Jabiru CHT pickups are much more exposed to the ram air flow than the TCM pickups, the allowable CHT values (350 degrees Fahrenheit max for 3 minutes and 300 degrees F for maximum continuous power) for the Jabiru engines seem iceberg cold when compared with the O-200 maximum of 500 degrees F. To ensure good cooling Jabiru recently increased head-cooling fin area by 40 percent.
Jabiru recommends a top-end overhaul — which Jim McCormick, chief executive officer of Jabiru Pacific LLC, says consists of cleaning the valves and valve seats and installing a new set of piston rings — at 1,000 hours time in service. At 2,000 hours a total rebuild is recommended. McCormick cited a figure of $4,000 for the parts required for the total overhaul.
The ignition system uses magnets attached to the spinning flywheel to induce current in self-energizing coils. This system will produce a hot ignition spark without battery power once the engine is started. But the drawback is that the engine must be spun at 300 rpm for the system to self-energize and produce a spark. This precludes hand-propping in the event of a dead or low battery. The Jabiru 3300A uses 100LL fuel and has an 8.0-to-1 compression ratio. McCormick said in an interview that autogas may be used if it's at least 91-octane premium grade.
Number of power pulses versus displacement
The Rotax engine produces 100 horsepower by applying relatively small power impulses to the spinning crankshaft at more than twice the rate of either the TCM O-200A or the Jabiru 3300A. The total cylinder displacement for the Rotax engine is 82.6 cubic inches — nearly 60 percent less than the TCM O-200 and the Jabiru 3300A, which both displace 201 cubic inches.
The other major difference between these two approaches to aircraft engine design is the difference in thermal efficiency of the engines. The easiest way to increase the thermal efficiency of an engine is to increase the compression ratio. Increased thermal efficiency translates into either more power for equal fuel consumption rates or less fuel consumed for the same power output. The O-200 has a compression ratio of 7.0-to-1; the Jabiru compresses the fuel-air mixture at a ratio of 8.0-to-1. The Rotax compresses the fuel-air mixture in each cylinder more than 10 times (compression ratio of 10.5-to-1) before the ignition spark is introduced.
Fuel consumption figures bear out efficiencies of the engines. At 75-percent power the O-200 burns around 5.6 gallons per hour, which is slightly more than McCormick said the 3300A (5.0 to 5.5 gph) would use at similar power settings. Performance charts show the Rotax 912 S consuming 4.3 gallons per hour at 77-percent power. The 3300A is rated at 100 horsepower, so their 75-percent rating delivers more horsepower to the prop — at lower consumption rates — than the other two engines, which are rated at 100 horsepower.
These three engines present a very interesting insight into how light-airplane engine design and production have changed in the past 50 years. TCM's venerable O-200 shows how almost all of today's light-airplane engines were designed. The Jabiru 3300A is the next step in that school of design. It still embodies the large-bore, long-stroke, low-rpm philosophy that's worked so well for so long, but it was designed and manufactured incorporating modern manufacturing techniques. This, plus the incorporation of logical improvements — the six-cylinder 3300A has eight crankshaft main bearing bosses incorporated into the case — leads to the conclusion that it will be powering light airplanes into the middle of the twenty-first century.
The Rotax 912 S is the light-airplane engine that's leading the way for future reciprocating engines. This engine is efficient, smooth running, and, because of the water jacket around the heads and the slow propeller rpm, extremely quiet. Its success paved the way for the development of Bombardier's (now under the V Aircraft Engines label) 220-horsepower V220 and 300-horsepower V300T engines that were first described in the August 2003 issue (see " Airframe & Powerplant: Twenty-First-Century Engines," August 2003 Pilot).
Most LSA manufacturers have selected one of these three engines. All are equal to the task of providing economical, safe, dependable power for the new LSA fleet.
E-mail the author at [email protected].
LSA Powerplant School
Lockwood Aviation Supply, the leading U.S. distributor for the Rotax series of engines, has formed Aero Technical Institute for the light-sport and sport-aviation community. Lockwood Aviation Supply technicians began conducting two-day classes on Rotax engines in May. The classes are held at the Lockwood facilities at Sebring Regional Airport in Sebring, Florida. Cost for the two-day course is $395. For information, visit the Web site or call 863/655-5100.
LSA Rules Review
Here's a quick primer on LSA. In August 2004 the FAA introduced a new set of rules for this new type of airplane. These rules are American Society for Testing and Materials (ASTM, now known as ASTM International) consensus standards, and they govern both the new LSA airframes and engines. Consensus standards allow government, industry, and consumers to work together to develop standards to ensure safety without the expense of current aircraft certification rules. To illustrate the freedom of these consensus standards, the complete LSA engine ASTM document — designated F 2339-04 — consists of two and a half pages of text and a quarter-page annex related to the software standards used in electronic ignition systems.
Although the FAA's Part 33 airworthiness standards for reciprocating engines are straightforward, they look positively Machiavellian compared with the consensus standards for LSA engines. Here's the LSA materials paragraph (5.1) under the design criteria heading from F 2339-04: "The materials used in the engine must be adequate for the intended design conditions of the engine." That's it. There are other paragraphs addressing fire prevention, engine cooling, engine mounting, and so on, but they are all brief and to the point. — SWE
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