Diesel Pro/Con
In case you didnt know, diesels are not good for airplanes because they are too heavy. We are going to read a stream of articles in which researchers will be obfuscating the weight (comparable to a typical….) or fudging the power (125 HP, but it is just as good as..)
Of course, what counts is the sum of the weight of the powerplant and the fuel, and that brings us to the next point. The fuel economy of a diesel under constant load is only marginally better than that of a gasoline engine. (Remember the Continental Malibu burning 14 GPH when extreme leaned?)
If we are doing this because we have to burn Jet-A, someone should whip up a 150 HP gas turbine. The one for your Cherokee should be no bigger than a lunch box.
Oh, and find someone aggressive to do it because we cant afford Pratt & Whitney pricing. The technology for low-cost, high speed, high-temperature rotating machines has been around since turbos became commonplace in cars.
-Steve Collins
via e-mail
We refer you to Williams International, the company that proposes to build an 85-pound, 750-pound thrust jet engine for the Eclipse 500 personal jet.
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Id like to make several comments on your fine article on diesel aviation engines:
Modern diesel engines are not at all like the soot-spewing two-stroke diesels that seem to be the favorite of municipal bus system operators. With computer control of injection timing, fuel delivery and turbo boost, there is little smoke emitted from a modern (post-1996) diesel engine.
American auto manufacturers have invested little R&D on diesel engines for cars, but have made very significant strides in diesel engines for full-sized (3/4-ton and 1-ton) pick-up trucks.
The competition between U.S. automakers in diesel full-sized pickups is intense, with incredible strides being made in engine performance in the last four years.
While the very best BSFCs are indeed delivered by marine and other slow-speed (700 RPM at full load) diesels, you should note that modern Class-8 truck diesels approach BSFCs of .30 as well.
One example is the Detroit Series 60 engines. Diesels exhibit their best fuel economy when operated at their sweet spot in their RPM curve; operation at lower or higher RPMs most often will cause the engine to consume more fuel for each HP generated per unit time. Modern high-speed diesels, (operational RPMs of 1800 to 2200 RPM) often have their sweet spot around 1800 RPM; Modern OTR truck diesels (Detroit Series 60, Cat 3176, Cummins ISM) have their sweet spot between 1500 and 1600 RPM.
Note that the BSFC for engines operating on Jet-A will likely be lower than the ratings obtained for operation on number 2 diesel, owing to the lower density of Jet-A compared to diesel fuel.
Terrific TBOs: Modern OTR diesels now have TBOs of over 500,000 to 1 million miles, not 300,000. A 300,000-mile engine is no longer competitive as an OTR Class-8 truck engine. An owner of a modern diesel pick-up truck should be able to expect 300,000 miles with proper maintenance.
The probable barrier for diesel engine acceptance in GA might not be the fuel, the weight or the other end-user factors of noise, odor and so on. Most likely, it will be that A&Ps are currently not trained nor equipped to successfully overhaul diesel engines.
Diesel engines are constructed with tighter tolerances in most all aspects than gasoline engines. This is what contributes to the higher initial cost of a diesel engine and to higher maintenance costs.
The labor costs for a diesel mechanic are substantially higher than that for a gas engine mechanic, partly caused by the cost of the mechanics tooling and sub-contracting of specialized tasks to outside shops and partly caused by much higher training and expertise requirements.
The fuel injection system on a diesel engine is one of the most critical and expensive points of maintenance. Indeed, many OTR truck shops, while they might rebuild an engine, wont touch the injectors or the fuel pump.
They will send those jobs out to shops that specialize in nothing but injection pump and injector cleaning and calibration service.
In light of this, one can understand why Thielert wont authorize overhauls performed by a local A&P. Many diesel engine manufacturers now offer scheduled engine replacement programs. You can get a factory rebuilt engine for a very reasonable price with a trade-in of the same make/model of engine.
If you want to realize the full reliability and longevity of a diesel engine, you must be a fanatic for filtration. Recent experiments by mining companies have revealed how much a diesel engines TBO can be extended by ultra-filtration of the oil, intake air and fuel.
In these experiments at sites in Wyoming, engines rated for TBOs of 14,000 hours are operating well beyond 20,000 hours by insuring filtration of fuel to exclude particles larger than 3 to 5 microns from the injection pump.
Likewise, the oil is filtered with bypass filtration to trap particles larger than 8 to 10 microns.
Filtration at this level is almost unknown in gasoline engines; indeed, its not unusual for a good oil filter on a gasoline engine to allow particles as large as 30 microns to pass through the filter. Ultra-filtration regimes are not cheap to maintain but they do exhibit payoffs in longer TBOs and oil change intervals.
It is now not uncommon for modern OTR diesels to have oil change intervals of 50,000 or 100,000 miles with only filter changes and oil sampling to monitor oil composition and performance.
I believe aviation diesels are technically feasible. However, they must surmount significant economic barriers in initial engine cost and ongoing maintenance to become widely deployed. Without artificial economic barriers erected on current gasoline engines, diesels in GA will likely remain a curiosity in North America.
-Dave Stine
via e-mail
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No Parachute for Cessnas
In the July issue of The Aviation Consumer, a comment was made by reader Rae Willis about parachutes on Cirrus and Cessna aircraft. In his letter, Mr. Willis seems to be dissatisfied that Cirrus includes the Ballistic Recovery Parachute as standard equipment.
Willis goes on to state, And now Cessna is considering a ballistic chute, too! This is not the case. At this time, Cessna has no intention of utilizing a ballistic recovery system on our aircraft.
We request that you communicate with your readers that Mr. Willis statement about Cessna was inaccurate.
-Jessica Myers
Media Relations
Cessna Aircraft
Willis was referring to a recently announced aftermarket STC from BRS to equip Cessnas with parachutes. We should have clarified this.
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Tiger Yawn
With regard to your article on the new Tiger in the July issue, its just fine and dandy that Tiger LLC has dusted off this old design, just as Cessna resuscitated the Skyhawk and Skylane.
But for the industry as a whole, the fact that these companies are doing this makes me nervous. These arent the GA airplanes of the future.
-Larry Green
Bethel, Connecticut
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Corrections
Its Thielert – We would like to blame it on our spell checker but we told it what to do. In the August issue, we mispelled the name of the German aero diesel maker. Its Thielert with an e, not an a.
Avidyne numbers – In our September issue on datalink, we gave the model number of Avidynes system as the EX5000. Its actually the EX500. The EX5000 is the large-screen display used in Cirrus aircraft.
