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We went outside the box and chose a DeltaHawk Diesel for our powerplant; here's why.
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--------------------------------------------------------------------> our current engine We've been flying a Grumman-American Traveler behind a Lycoming 0-320 for nearly 8 years, and have never been satisfied with the engine. The "lycosaurus" has been around for years; it's built on proven technology and there are many, many aircraft flying behind them--and they're obsolete! The internal combustion aviation engine industry in this country (like many other industries) is stagnant. There have been no new R & D efforts by any major manufacturers for a number of years (decades?). This is due to a number of factors, but mainly the double-whammy of Tort (product liability law) and the hurdles involved with FAA certification. The FAA certification issue is especially heinous. Don't get me wrong, I think the FAA has a purpose in the aircraft design and manufacturing system. Ensuring the safety of new designs is important, but can't that goal be met without completely stagnating an entire industry? Industry experts estimate the R & D costs to get a "new" engine design through FAA certification at nearly $20 million! Then, after that huge investment in time and money, you have a product that the Government certifies as safe for use in aircraft. You've followed a Government-mandated R & D program that will ensure there are no product defects. So if some numbskull uses your product improperly or contrary to recommended procedures you're safe from product liability lawsuits, right? Because the Government has already said your product is safe, right? You've spent $20 million dollars in R & D, engineering, testing, documentation, etc. and the Government has given it's blessing -- you can't be touched legally, right? Wrong! You're still a big, fat, juicy Tort target. You must be, you just spent $20 million for product R & D! So this is what the major engine manufacturers have been dealing with for the past couple of decades. Who can blame them for not sticking their necks out? Especially since the General Aviation Revitalization Act (GARA) was enacted in 1994. GARA protects manufacturers from lawsuits for products that've been on the market for 18 years. Why would anyone produce anything new when the "current" stuff is litigation proof? Another serious issue for those that choose the status quo is fuel. 100LL Avgas is the fuel of choice for engines developed by the major manufacturers. Special additives (lead) raise the octane rating of the fuel and allow turbo-charged, air-cooled aviation engines to produce gobs of power by mitigating pre-ignition/detonation problems. It's a specialty item, can't be mixed with automotive fuels, has additives that are very toxic and harmful to the environment, and finally, has few users compared to automotive fuels and Jet A, otherwise knows as jet fuel. All of this adds up to my prediction that 100LL Avgas will likely go away. Sure, there will be an alternative available by then, but whatever they come up with won't have the octane rating of 100LL necessitating either (1) a reduction in power output, or (2) modification of existing engines to avoid pre-ignition and/or detonation while still producing the same power.
---------------------------------------------------------------------> powerplant musts Understanding the state of the aviation engine manufacturing industry (re major manufacturers) clarifies some concerns for our choice:
----------------------------------------------------------------------------------> options Since we couldn't expect anything new from the major players, we had to start thinking outside the box.
Click a sub-topic to the left, or just click:
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The smaller manufacturers are selling the same engines as the major manufacturers with minor improvements (mostly Quality Control issues), but no substantial increase in performance or reduction in price. Plus, they still burn Avgas. Although some are advertising engines that'll burn auto fuel, those engines do so at the cost of performance. The reality is you can't air-cool a turbocharged engine, or one using high-compression-ratio cylinders (either of which is necessary for high-altitude performance) unless your fuel is high-octane. In other words (said with a George Bush Sr. accent), "Not gonna do it, wouldn't be prudent."
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Some automotive conversions are attractive alternatives (especially examples like Eggenfellner or Real World Solutions). Their biggest drawback is the lack of history to generate reliability stats in aviation applications. Many automotive engines are "bulletproof" designs that are "indestructible" when used to power the vehicle for which they were built. Use that "indestructible" engine in an environment (or an application) for which it wasn't designed, and all your superlatives become question marks. In order to improve fuel efficiency, automotive engines are designed to produce less than 20% of their maximum horsepower for 80% of the time (cruising speeds). That's why your car's engine has to be revved way up before your car accelerates with any alacrity. And that's why "highway miles" are "better" if you're buying a used car. Let's say your car's engine is advertised as producing 170 horsepower (HP). That 170 HP is only produced near redline (max) RPM; generally above 4000-5000 for most of today's modern automotive engines. While cruising down the highway, your engine is turning more like 2500 - 3000 RPM, which equates to roughly 20% of maximum power (the horsepower/RPM ratio isn't linear). The problem is that aircraft propellers (for the most part) are designed to turn 2700 RPM or less. Any higher, and there's a chance the thing could come apart. Additionally, they are less efficient (less of the engine's power is returned as thrust) and louder with higher RPM. Since automotive engines produce their rated power at RPMs much higher than 2700, you must incorporate some type of speed reduction mechanism (gears, belts, whatever) to get the propeller RPM back to 2700 -- another layer of complexity; another component that can fail. Piston aviation engines produce their rated power while directly driving a fixed-pitch prop at approximately 2700 RPM -- the prop is directly connected to the engine's crankshaft and spins at the same speed as the engine. Some aircraft are fitted with a constant speed prop which incorporates a mechanism to control the engine RPM by varying the propeller pitch, but the prop is still directly connected to the engine crank. Aircraft engine duty cycles rarely call for less than 65% of max power. High-speed cruise (is there any other?) settings require 75% or greater. Takeoff usually requires 100%. How does this compare to automotive engines? Your car's engine produces 20% of max power for 80% of the time. Use it in an aviation application, and now you're asking for 75% of max power 90% of the time, and 100% for the rest. Imagine driving your car around in first gear with the engine revved to within 1000 RPM of redline. I don't know about you, but when my car's engine starts vibrating and howling I cringe! Think about doing that for about 10 minutes. Now imagine doing it for several hours! Yikes! For an excellent discussion of this topic click here. Companies like Eggenfellner and Real World Solutions claim the engines they've chosen are up to the challenge in these applications; we choose to "wait and see."
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Turbine engines provide a number of advantages in aviation use compared to piston engines:
So why doesn't everyone use turbines? Small turbines suitable for use on the kind of kit planes most folks are building aren't really available yet. Innodyn has one they're flying on an RV and claim they will start mass-producing next year (2005). But they still haven't addressed the primary drawback of turbines: fuel consumption at lower altitudes. Not only do turbines suck a lot of air, they suck a lot of fuel. Because they burn so much more fuel, they don't truly become efficient until they are at altitudes allowing higher true airspeeds giving more distance traveled for the fuel burned. Unfortunately, those altitudes also require cabin pressurization (unless you want to suck on an oxygen mask the whole flight), a luxury that equates to much higher building costs and increased weight due to the pressure capsule. At the lower altitudes where most single-engine aircraft fly turbines are less efficient than their piston brethren because of their higher thrust specific fuel consumption (click here for more than you ever wanted to know about engine performance). That means they'll have to carry and burn more fuel to get the same range, which increases operating costs and decreases utility (carrying more fuel means you're carrying less of everything else). Some additional fuel can be carried due to the reduction in engine weight, but generally there isn't enough room within the airframe to store the additional fuel anyway. Another major drawback is cost; initial acquisition costs for turbines are high! Boeing and Airbus use them because turbines are the only alternative at the altitudes necessary to make intercontinental flight viable. Also, ultimate operating costs are lower because turbines may be operated longer between inspections and overhauls. Fewer inspections = less down time = more revenue. Because no one currently markets turbines for small airplanes, I foresee major costs in acquisition, operation and maintenance. Although recommended Time Between Overhauls (TBO) should be much greater for a turbine, I predict that finding someone qualified to perform the required inspections will prove to be problematic. And when you do, they're gonna make you pay for that exclusivity! And finally, most of the airframes we're talking about are designed with piston engines in mind. Maximum airframe airspeed (VNE) prevents one from taking advantage of the power of turbines at lower altitudes, lack of pressurization prevents high-altitude operations. Maybe we'll find that Innodyn has solved the efficiency problem (they have yet to post efficiency data for their turbine engines), and someone will design an airframe around their engine. Until then, I think we'll have to look elsewhere.
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Whoever heard of putting a diesel engine on an airplane? Actually the idea's been around a long time. It wasn't until recently, however, that technological advances in metallurgy produced (relatively) light-weight alloys strong enough to handle the internal cylinder pressures. Diesel engines bring a number of advantages to the table:
Diesel engines (sometimes referred to as compression ignition or diesel cycle) use fuel with more BTUs per gallon than spark-ignition engines. Diesel engines operate at much higher cylinder pressures (promoting a more complete combustion event) than spark-ignition engines. These two facts combine to make diesel engines on the order of 20% - 40% more fuel efficient than spark-ignition engines. DeltaHawk's engine operates on a 2-stroke cycle (scroll down on this link to read about the 2-stroke diesel cycle) instead of the 4-stroke cycle designs you're probably more familiar with. DeltaHawk utilizes a supercharger to provide pressurized air on the intake ports for engine start (4-cycle engines "suck" air in as the piston travels to the bottom of the cylinder on the intake cycle; 2-cycle engines need a pressurized intake to force fresh air in and exhaust out). After start up, a turbocharger maintains intake manifold pressurization for continued engine operation. Turbocharged engines are much less affected by altitude than normally-aspirated engines. A 200hp, normally-aspirated engine can only produce approximately 75% of it's rated power at 8000 ft simply because there is less oxygen due to lower overall pressure in the atmosphere. A turbocharged engine, on the other hand, can generally make it's full-rated power up to approximately 15,000 ft--the turbocharger pressurizes the intake manifold at sea-level atmospheric pressure to a much higher altitude. Additionally, a 2-stroke cycle results in twice as many power strokes (look at question #9 in this section) per engine revolution compared to 4-stroke cycle designs. Have you ever noticed that cars with V8 engines (8 cylinders) run more smoothly than cars with 4 cylinder engines? It's because power is applied to the crankshaft at 4 points during one engine revolution (4 power strokes per engine revolution). A 4-stroke 4 cylinder only applies power to the crankshaft at two points during one engine revolution (2 power strokes per engine revolution). A 2-stroke 4 cylinder has the same number of power strokes per engine revolution as a 4-stroke 8 cylinder engine. A DeltaHawk engine will pull harder, and more smoothly, at higher altitudes allowing you to take advantage of the thinner air to generate higher true airspeeds. Eliminate all components necessary to create a spark inside the cylinder (spark plug, wires, distributor, gearing, etc.) and the parts count drops. Any 1st-year mechanical engineering student will tell you that the key to improving durability and reliability is best accomplished by prudent application of the K.I.S.S. principle (correction of this explanation thanks to James Keyworth). Fewer parts means greater simplicity; it's as simple (sorry, couldn't resist) as that! An added benefit of ignition system elimination is electromagnetic noise reduction, a major source of problems for avionics. Further, DeltaHawk's engineers decided (based partly on research of diesel engine use in over-the-road trucking equipment!) to use piston porting as opposed to intake & exhaust valves. This eliminates all the associated gears, belts and other mechanical gizmos necessary to meet valve timing requirements. No sparkplugs, no distributor, no wires, no valves, no pushrods, no cams, no timing gears or chains... K.I.S.S. indeed! Because diesel engines inject fuel directly into the cylinder, a mixture control becomes unnecessary. Controlling the fuel/air ratio (manually via a mixture knob in most single-engine aircraft) is a major source of confusion for many pilots. Poorly managed fuel/air ratios have been blamed for everything from excessive fuel burn to major engine damage. All of this is eliminated with a diesel engine. The only mixture control is the throttle! Air-cooled engines have a history of problems when rapidly cooled. You must keep the engine at a high enough power setting to prevent a rapid cool down. This can complicates descent planning in some situations. And God forbid that ATC won't let you descend until the last minute! Diesel engines produce power at lower engine speeds than gas engines (generally speaking when comparing engines of the same displacement). Just like current gas engine designs, the prop is directly connected to the DeltaHawk's crankshaft. This means that the reliability stats of automotive diesel engines are applicable. Diesel engines have been used for ground transportation for... well, forever. Data pertaining to engineering, reliability, durability, development, etc. are readily available; this is a very mature technology; the only thing new is the application. Pundits have long opined that diesel engines are too heavy to be practical for aviation applications. DeltaHawk's engineers, however, have developed clever and innovative ways to utilize light-weight alloys in critical areas resulting in an engine design whose weight compares very favorably to it's spark-ignition competition. All of this means that DeltaHawk Diesel engines will cost less to purchase, be easier to maintain, probably last longer, cost less to operate and less to overhaul when the time comes (click here to see a comparison). It's new technology, it burns diesel fuel or Jet A and it's lighter on the pocketbook. Looks like we're done here too! Click here to read about our avionics & accessories.
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