Aircraft diesels and cheap jets may be the sexy newcomers to the aviation engine biz but the poking and prodding of the old standbys from Lycoming and Continental continues at an unprecedented pace. Research in electronic engine controls leads the way.
If theres such a thing as accepted wisdom, it seems to be that if the conventional gasoline piston engine is to survive, it will have to have sophisticated electronics, possibly including a single-lever power control wired to a full-authority digital engine control (FADEC) of the type turbine powerplants have sported for years.
The road to electronically managed engines, however, is proving bumpy and it may take some re-thinking about the way engines are operated to make these systems worth the bother and expense of installing them. As Mooney learned with its daring and stunningly (at least at the time) advanced PFM, owners love the ease of single-lever operation but not many would-be buyers are willing to pay for it. (At least they werent 10 years ago, when the Porsche-powered PFM was introduced.)
Were not sure if the PFM experience has caused the manufacturers to be gun-shy about high tech accessories for conventional engines but one thing is apparent: Three major airframers, Cessna, Piper and Mooney have tested Unisons new LASAR electronic ignition system and said no thanks.
The reason: little measurable performance gains or at least none commensurate with the cost of adding the system to new production aircraft. As are individual owners, the OEMs are seeking either substantial fuel savings or noticeable increases in power output to justify the additional cost of electronic ignition.
Unisons response, in part, has been to delve more deeply into the realm of electronic engine controls in the form of a soon-to-be available electronic mixture control for fuel-injected Lycomings using the Bendix/RSA fuel system in conjunction with the LASAR. (By soon-to-be-available, we mean within a few months and certainly before the end of the year.)
Unison debuted the LASAR ignition system at Oshkosh in 1994, making it available first for experimental aircraft and later for four-cylinder, carbureted Lycomings such as the O-320 in the Cessna 172.
In brief, LASAR is an acronym for limited authority spark advance regulator. It works in concert with modified conventional aircraft magnetos but has limited authority to electronically advance spark timing far beyond the fixed 22 to 28 degrees before top dead center used on most aircraft engines. Furthermore, it generates a higher energy spark for starting than do conventional impulse coupling or shower-of-sparks systems and will thus ignite less-than-perfect fuel/air mixtures-especially the over-rich mixtures encountered during hot starting.
Variable timing-along with sophisticated computer monitored oxygen sensing-has long been used in cars to deliver better fuel economy and/or more horsepower at certain power settings. But very few airplanes have had its benefits-the Porsche-Mooney was one, the Rotax-powered Diamond Katana is another.
But every airplane of recent manufacture coming out of Independence, Vero Beach, Kerrville and Wichita has had conventional magnetos. (Interestingly, the new Continental-powered Diamond Katana has conventional mags, not electronic ignition.)
This leads us to wonder if the major manufacturers arent hopelessly mired in the technological backwater of 1950s engines development. Or perhaps theyre simply quaking in their boots at the liability exposure that a new, relatively untried major system represents.
Were sure liability figures into the equation but to their credit, all of the major manufacturers except Raytheon jumped on the LASAR bandwagon very soon after Unison made test systems available. Given the competitiveness and limited size of the new aircraft market, we were told by spokespersons from Cessna, Piper and Mooney, that manufacturers have little choice but to at least consider every legitimate new engine technology. Thus far, the results havent been encouraging.
We invested far more time and money in a test program than I ever thought we would, said Pipers Werner Hartlieb. At the end of it, we just werent comfortable that LASAR added enough value to justify the cost. He estimates that a LASAR system would add $2000 to the price of new Warrior or Archer but that Pipers tests revealed only tiny improvements in fuel economy.
Unless someone comes along and explains that something has changed, we dont intend to do any more evaluation for now, Hartlieb added.
Cessna and Mooney report similar results. Mooney Chief Engineer Tom Bowen told us the company experimented with three LASAR systems on an IO-360-powered J-model equipped with a torque meter. The flight trials, he said, revealed small power increases and improved fuel economy in the range of 3 to 4 percent, and then only at high altitudes or low power settings.
One of the tests-which involved pre-certification test LASARs-yielded rough engine operation which smoothed out when the LASAR was shut down. Moreover, Bowen reported premature spark plug failures, evidently due to the LASARs high energy output. (Unison says its working on fixes and plans another run at Mooney.)
Cessna, which two years ago hinted that it might offer the LASAR on new Skyhawks, reported test results much like Mooneys. Cessna spokesperson Jan McIntire declined to provide details but she said the company has decided against LASAR for now.
It wasnt categorically that this didnt or wasnt going to work, she said, its just that we didnt see a return on the additional cost.
Meanwhile, Owners Buy
Disinterest from the manufacturers aside, some aircraft owners are forging ahead with electronic ignition, although the systems arent exactly flying off the suppliers shelves. Field overhaul shops tell us engine customers are buying LASAR systems by the handful, not the truckload.
We contacted some of these early LASAR buyers for their impressions of how the system works and we also spoke with Bill Benedict, at Vans Aircraft, a leading kit builder which offers the LASAR system as an option on Lycoming engines for its RV-4, -6- and -8 series aircraft.
The consensus: LASAR-equipped engines start more easily and run smoother but, in Benedicts words, performance improvements may be subtle and possibly elusive. Speed increases due to higher power are so slight as to be within the noise level of the instruments. It requires careful data collection to see them at all.
In a normally aspirated engine, improved fuel economy will be noticeable only at altitudes above 6000 to 8000 feet as the engines power output characteristically falls off and LASAR begins advancing the timing according to the performance map and look up data burned into the systems computer controller.
When combined with correct leaning, advancing the ignition timing increases power slightly while reducing fuel flow. Leaning alone reduces power output because although theres less fuel in the cylinder, the fuel/air mixture burns more slowly, thus peak cylinder pressures occur later in the power stroke.
By advancing the spark in a leaned cylinder, the combustion process begins sooner, giving the mixture a little extra time to burn. This restores the peak pressure and peak power found at the richer mixture, but at a lower fuel flow.
The downside of advanced timing is that longer combustion duration tends to raise CHTs. Indeed, some LASAR operators do notice higher CHTs; the system has CHT limiting circuitry, which backs off the ignition advance if the temperatures exceed pre-set limits.
At high power settings-full takeoff power at sea-level, for instance-theres little advantage to advanced spark; the LASAR reverts to standard 25-degree BTDC timing. Owners confirm that LASARs benefits-which are not dramatic, in any case-are most noticeable at high density altitudes, in the form of slight improvements in climb rate.
Benedicts tests at Vans Aircraft duplicated our experience in flying the LASAR 18 months ago in Unisons test-bed Cessna 172. He recorded a 9 to 10 percent reduction in fuel flow with the LASAR engaged and in one test, he eked out a 2 knot speed gain, also at lower fuel flow. (The tests were done at 11,500 and 12,500 feet.)
Other owners we contacted confirmed the easier starting and smoother running but fuel flow and economy are harder to pin down. Fuel flow? I couldnt say for sure, said Thomas Brennan, of Oil City, Pennsylvania, who recently installed a LASAR system on his Mattituck-overhauled Cherokee. He told us the engine was still in the break-in phase and hadnt been leaned aggressively.
Another LASAR buyer, Charles Adkisson of Midland, Texas, said starting his Lycoming O-360-powered Cessna 170 is wonderful. One or two blades and its running.
Like Brennan, hes unsure about improved fuel economy but has noticed higher CHTs, which he ascribes to a combination of the LASARs advanced timing and engine baffling problems. Other owners weve talked to report varying versions of the same story: LASAR yields faster starting, smoother operation but no dramatic or at least uncertain reductions in fuel flow. Speed gains, if any, are also elusive.
Nonetheless, Unison is pressing on with LASAR development and has recently obtained STC approval for Lycomings IO-540 series engines, although those systems arent expected to ship for a few months. Approvals already exist for the O-320, O-360 and IO-360 series, except for engines using the Bendix dual magneto. LASAR approvals for Continental engines are in limbo; TCM says its aggressively working on its own electronic engine controls, having bought Aerotronics Controls late last year expressly for this purpose.
Realizing that leaning is critical to squeezing improved performance out of variable timing, Unison has entered a joint development project with Precision Airmotive Corp. to develop a fully automated electronic mixture control for use in conjunction with LASAR mags.
Precision manufactures and overhauls the Bendix/RSA fuel injection system found on injected Lycoming engines. Presumably, the EMC would be available for any engines-new or overhauled-that use the RSA system.
EMC At Work
The EMCs guts are housed in a small metal box-called a valve driver module-attached to the side of the RSA servo body, as shown in the photo on page 21. The driver module is connected electrically to the LASARs control computer and it houses a small solenoid-controlled air valve that handles the leaning commands.
The EMCs fuel control represents a parallel addition to the servos leaning circuitry. In other words, the existing leaning mechanism remains in place and the red knob is fully functional. Theres no electromechanical servo, so when EMC leans, the mixture knob doesnt move.
Operationally, the pilot sets the mixture to full rich, as on a normal takeoff. Referring to the LASARs electronic performance map, the EMC maintains a full rich mixture under any conditions where the engine operating manual recommends it.
As the aircraft climbs and power falls off due to increasing altitude, the EMC smoothly and continuously leans to a setting a few percent rich of best power. Initially, says Precisions Roger Hall, the EMCs operating logic will be open loop, meaning it (and the LASAR) will respond to engine RPM, load, altitude, pressure and temperature. If untouched, it will automatically lean to the slightly-rich-of-best-power setting.
But the pilot retains the option of leaning any way he pleases, including peak or lean-of-peak EGT operation. The EMC remembers that setting and will automatically maintain the appropriate fuel/air ratio until its sensors encounter a condition where the engine manual calls for full rich.
Eventually, the system will evolve to closed loop, which would include EGT feedback to automatically lean to a particular power or economy setting. In that case, the engine runs according to the LASARs electronic map, which is derived from test cell runs and the operating manual. No pilot tweaking allowed.
We were surprised at Unisons aggressive timetable for the EMC project. They hope to have flight tests completed by mid-1998, with production models available shortly thereafter. No information on pricing is available at press time but our blue sky guess is that EMC will cost under $1000 when added at overhaul, when the RSA servo is rebuilt.
When combined with LASAR mags, then, the typical system might add $4000 to the cost of an overhaul, give or take $500. Initially, only normally aspirated engines with the RSA 5 servo will qualify for EMC.
We think Unison is heading in the right direction with the EMC system. Although the LASAR system alone undeniably yields faster starting and smoother operation, its economy and power benefits are subtle and nonexistent without methodical leaning, which explains why the OEMs havent been impressed with it as a $2000 add on.
Price matters. So does engine displacement. For many owners, spending $2500 to save perhaps .7 gallon per hour comes up short of a drop-dead good deal. Three engine shops we spoke to told us LASAR would have made a bigger splash thus far if Unison had pioneered it on Lycomings 540 series, where fuel savings of 10 percent represent $3 or $4 sliced off the hourly operating costs.
Frankly, we wish Unison, Precision and the engine manufacturers would apply a sharper pencil to the issue of fuel/air distribution in the typical injected engine, with an eye toward variable ignition timing and lean-of-peak operation in place of the tried-and-not-so true conventional wisdom found in the typical engine operating manual.
Setting aside the issue of single-lever power control, we think a $4000 investment to save 10 percent in fuel costs will be only marginally attractive for most owners. A 15 to 20 percent reduction in fuel burn-which is probably achievable with radical leaning-means such a system might have a one-year payback on a large displacement engine flown 150 hours a year. Thats an attention getter, in our view.
Precision believes that an additional benefit of automatic leaning is more stable engine temperature control which, logically, might extend engine life or at least improve the prospects of reaching TBO. We like that idea, but show us some results first.
Short term then, if youre considering LASAR, you can expect it to perform as claimed only if youre willing to lean carefully. Even then, the benefits wont be dramatic. Were looking forward to seeing if the EMC system changes that, at least for block-to-block fuel economy.
Meanwhile, if you want LASARs quick-start benefits, consider Unisons companion product, Slick Start, which offers effective ignition boosting at a fraction of LASARs cost.
-by Paul Bertorelli