Builders, Here is part 2:
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“Boosted” engines: If normalizing is trying to maintain 29.92″ of sea level pressure in the intake, a boosted engine is driving the manifold pressure above this. This is actually very common, and almost every single classic radial engine was “Boosted”, except they most often used superchargers to do this. A P&W 450 hp radial is known by its displacement “the 985″ is how many cubic inches it has. They make their rated power at 36.5″ of manifold pressure. Many other classic piston engines made their power at 45 -72” of manifold pressure. High end GA engines like the GTISO-520 makes it “low power” 375 hp rating at 40″ MAP. In the big picture it is ‘turbo normalizing’ that is the oddity.
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In the car, the turbo Corvair is a Boosted engine. The 180 HP ‘Corsa’ model made its rated power at 5,200 rpm and 45″ MAP. That is about 7 pounds of boost in car-speak. Worth noting is that the same engine made 265 foot pounds of torque was down low in the rpm band, at a setting that can be used in a direct drive engine, and the turbos we have used are far better at building torque that the car original was. A 3,000 cc engine running 40″ MAP on takeoff is burning the same amount of fuel and air as a 4,000 cc naturally aspirated engine. If that doesn’t sound dramatic, read this story to understand what kind of difference 35% more power makes on a plane’s climb performance: Pietenpol Power: 100 hp Corvair vs 65 hp Lycoming
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Draw through vs Blow through: On a draw through arrangement, the air flows through the carb, then the turbo and on to the engine. A blow through is turbo-carb (or injection) and then the engine. Draw through is characterized by simplicity. Our set ups are all draw through, as was the original Corvair car. Virtually all modern cars are fuel injected, and the ones that are turboed are blow through. The primary advantages are two things which don’t matter to planes, throttle response and emmisions. One of the hidden advantages to draw through is the fuel getting fully vaporized early radically cools the intake air and makes it more dense without the need for an intercooler. Injected engines can’t do this because the fuel arrives in the intake just ahead of the valves. In one minute at full output, a turbo engine will digest and vaporize more than a pound of fuel, this has a great cooling effect on 250 cubic feet of air.
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One of the things that always comes up when you mention draw through arrangements on planes is an alarmist pointing out that every foot of the intake has air and fuel in it under slight pressure, and this is in his mind a giant fire hazard. A dozen years ago it was mentioned on the Dragonfly builder’s list that I was working on a draw through arrangement for the Corvair. The leading ‘personality’ on that list wrote a long diatribe about what a horrible person I was, and convinced most people of this by saying that no airplane was made that way. Only one problem with his argument, it was a complete lie. The US built 300,000 planes in WWII; 160,000 of them were multi engine, and 32,000 of those had four engines. If you look at all the radials, the Allisons and the Merlins on these planes, You are looking at 750,000 engines, and virtually every one of them was a draw through arrangement being boosted by a turbo, a supercharger or both. A R-4360 engine has at least forty, (40′) feet of intake piping after the blower, and every bit of it is packed with fuel and air. There are 56 couplings in that intake system that I can think of. If draw through systems didn’t work WWII would have had a different outcome.
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For a look at some of our ground testing we did in 2004 before flight testing, get a look at this link: Testing Turbo Corvair and Rotax 912S. In some of the tests we ran the MAP all the way to 60″, which is 15 pounds of boost, or a 2,700 cc engine inhaling the same amount of air as one that is 5,400 cc (330 cubic inches) The turbo we were using was a modified Garret TO-4B with a .58AR housing, machined for a carbon seal. It worked great. Not all turbos are expensive, this one was made in the USA and it was only $545 brand new. The real cost of a full turbo system is far more, because many of the other parts like the exhaust system have to be made from very high quality materials. If you look at the price of turbos on Ebay, be aware that the market is flooded with counterfeit name brand turbos that are made in red China. A turbo counts on good materials, it often runs 1,600F on the inlet and the wheel is turning 100,000 rpm while it is working. If it breaks a blade or bearing, it feeds the metal from the compressor side right into the engine. It pays to buy the real thing, especially because the US made items are often reasonably priced.
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We learned a lot in testing. My plan was to take the regular 2004 100 hp conversion on the Skycoupe and run it with the turbo on it and get some time on it to see what parts would need to be upgraded to last on a turbo engine. First, let me say that nothing on the engine broke, but judging from the 1550F EGTs and high oil temps, the engine would have gotten ‘tired’ quickly with the stock conversion parts of 2004. When I see people talking about putting a turbo on an engine out of a junkyard that was never intended to be turboed, I can only wish them good luck, because our testing indicated that any engine running in a boost condition will need the best available internal parts and systems.
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Below is a number of things our testing indicated we needed for the engine to work as a regular, long lived power plant. After each topic is a link to a story of the part we developed to address each of the issues. These developed systems also served as stand alone options that have improved regular naturally aspirated Corvairs, but the owe their origins to conditions our testing identified 10 years ago.
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Engines in lower compression and more displacement with a quench: The 2,850 pistons were developed specifically to work with turbocharging. They also happen to work very well as a dual fuel piston suited to both 100ll and auto fuel. The 3,000 cc models we developed as a spin off. read the stories by clicking on the links:
Getting Started in 2013, Part #16, 3,000 cc Piston/cylinder kits
Getting Started in 2013, Part #14, 2,850 cc piston/rod/cyl. Kits
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Pressure retard distributor: When an engine is boosted it need less ignition advance. This is done on a Corvair car with a pressure retard in the place of the vacuum advance on the stock distributor. In the Skycoupe I made a special dual points distributor that only had 25 degrees of total advance. The long term answer was the system below. As a tech spin off it will also be useful on naturally aspirated engine at very high altitudes:
Ignition system, experimental “E/E-T”
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Ultra high grade exhaust valves: We normally use 4-N stainless valves in Corvairs, but the exhaust valves of turbo motors need to be made of exceptional materal to last to a normal TBO. This is a job for the super-alloy Inconel. Mark Petz of Falcon head fame developed these in 2008. See picture below:
Above, I hold the last word in Corvair exhaust valves. In the past year, Mark Petniunas put a tremendous amount of effort into finding a source for these valves, which are precision manufactured out of the super alloy Inconel. It has greater strength at 1,500F than 4130N steel does at room temp.
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321 stainless exhaust. Our normal exhausts are made of 304 stainless, a very high quality material, but the job is better done by 321. Very few experimental engine companies have ever used this because it is 3 times the price of 304. The link below is about our regular systems, at the bottom of this page is a photo of a 321 system I made for a 601XL test.
Stainless Steel Exhaust Systems
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high volume oil pumps: Turbo engines need more oil flow to feed the bearing in the turbo, the 5th bearing and to internally cool the engine. High volume oil pumps have been around for a long time for Corvair cars, but we developed our CNC model which has better internal alignment:
High Volume Oil Pump
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large aircraft oil coolers. Turbo engine make the oil hot, it flows through the turbo’s bearing with is one inch away from the exhaust housing which can be visibly glowing. Our Gold oil systemens serve many purposes, but they would serve a turbo engine very well and allow the use of a appropriate sized cooler:
Heavy Duty Gold Oil Systems, new cooler model.
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5th bearings: These were in development by the time we were doing our turbo testing, but we did not have one on the skycoupe. Today, virtually all Corvair flight engines use one, and I would not consider building a turbo engine without one:
Getting Started in 2013, part #1, Crankshaft process options.
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I am holding the turbocharger that Woody Harris found for our test program. Note that it has an integrated wastegate. This is a common feature on modern car turbos. However, almost no modern car turbo has the capability of being used in a drawthrough application, which is a highly desirable format for aircraft use. It took us a long time to find an expert on turbos who could properly fabricate a modern turbo, appropriately sized for our application, with a carbon seal.
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I built this exhaust System out of 321 stainless. Its future home is on Woody Harris’ 601 XL. He will be retrofitting his 2,850 cc engine with a turbocharger. This is the engine half of the exhaust system, and it was built in my jigs. Our regular exhaust systems are built out of 304 stainless, which is extremely durable and fairly resistant to heat flow. 321 is the alloy of choice for Turbo Systems, as it withstands elevated temperatures even better. Notice how the one pipe crosses underneath the engine to go over and meet with the other before heading into the Turbo. It is worth noting, however, that naturally aspirated Corvair powered 601s with 2,700 cc engines have exceeded 17,000’ and have little problem with density altitudes over 14,000 feet.
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