Q. Why not use the intended aircraft engine, the IO-540?
A. Two major reasons:
1) Reliability. Unlike most people, I am not all that impressed with the reliability of aircraft engines.(especially the large high compression ones) You will usually hear people talking about a 1,400 to 2,000 hour TBO but the reality is you will probably do a "minor" or top end overhaul at around 500 hours. Ironically enough, a "minor" overhaul replaces about 75% of the moving parts in the engine! For those of you not familiar with the terminology a "minor" (or "top end") is where you remove the cylinders and have them rebuilt. My point here is that aircraft engines are really only good for a few hundred hours, not the 1400 to 2000 hour figure usually bandied about. After you pay as much as one of these engines cost, you would think you could get more than a few years wear out them before they need a lot of work but for the most part, you don't. The rotary engine is one of the most reliable engines ever built, especially if you exclude turbines.
2) Cost. In Trade A Plane, the lowest rebuilt IO-540 I have found has been $22,000. The absolute lowest price I have found was $4,500 for a yellow tagged (rebuildable) junker. The next best compromise I found was one of the engines off a crashed Twin Commander with uncertain history for $9,000. IF you go for a turbocharged IO-540 (TIO-540) the prices take a serious upward jump. ( a friend of mine just had the turbocharger rebuilt on his T-210 and it ran him $4,000. Just to REBUILD the turbocharger *only*. Ridiculous) When you do a "minor" overhaul plan on $1,000 to $1,500 per cylinder for the machine shop work (+ or - depending on the shape it is in of course) PLUS the labor to remove and replace the cylinder.
The all out cost for my 20B installation should run about $9,000 for a turbocharged engine with everything new or rebuilt. This may not sound like a huge savings off hand but when you factor in that the 20B will be lighter, much more powerful AND very low cost maintenance (*complete* rebuild including all accessories for less than the cost to rebuild a single cylinder on a IO-540!) I find the price very acceptable.
I have had some skeptics of this price write me. Here is a complete
Mazda 20B engine $3,000 (13b for $500 to $1000)
PSRU $2,000 (From Tracy Crook)
EFI/I $850 (EFI/I = Electronic Fuel Injection / Ignition)
Custom Built Turbo $1,000
Intercooler & ducting $500
Rebuild kit for 20B $200
Raw stock for intake,
exhaust and motor
$7,750 leaving me $1,250 for unexpected expenses.
If you are building a smaller plane like a standard Velocity and you don't want to fly at 20,000 feet like me, you can use a 13B and leave off the turbo and get 180 hp for $3,500 +/-.
Q. OK, if you are going to use a car engine why a rotary?
A. I looked hard at almost every alternative. I looked at V8's and six cylinders. When I started looking I really knew very little about rotary engines. A friend of mine (Russ Bassett) encouraged me to include rotaries in my search. I did not want to but he kept after me until I did. (just LOOK into it will ya Rob? Tell me what you find) Well, after I did some research I discovered that the rotary engine is nearly the perfect aircraft engine. In fact it is much more suited to airplanes than it is to cars! Here are some of the reasons it is so good
Q. Okay, what's the down side?
A. There are a couple but I think they are overblown and/or workable.
1. The number one complaint about rotaries that they have a hotter than normal exhaust. About 200 to 300 degrees hotter than a standard piston engine. This requires that some care be used in where you route the exhaust pipes and what material you use for the exhaust pipes. Standard automotive mild steel exhaust pipes wont last long. I plan to use stainless steel for the exhaust system. This material will stand the temperatures with out any problems.
2. The next problem is that the rotary needs a little bit of oil in the fuel to help lubricate the apex seals. (No, the rotary is NOT a two stroke engine. The rotary uses far less oil in the gas than does a two stroke- No blue exhaust smoke! :) ). The stock system simply injects a little bit of crankcase oil down the intake manifold. In the long run, this can cause problems because standard crankcase oil is not formulated to burn cleanly like two stroke oils are. Over time this causes carbon build ups inside the engine. These build ups will eventually cause the rotor seals to stick and result in a loss of compression. Don't get me wrong, this process takes 70,000 to 100,000 miles in the car but it is something people don't like. There are two solutions. a) remove the stock oil injection system, then watch how much fuel the lineman puts in the tank then pour in the needed amount of clean burning two stroke oil (1 oz. of oil per gallon, easy to figure). b) leave the stock oil injection system intact and use aircraft oil in the crankcase. Since our high dollar aircraft engines burn copious amounts of oil when they are in good shape, aircraft oil is formulated to burn cleanly.
Plan B is how I intend to do it. My 172 burns a quart of oil every four hours which, coincidentally is about an ounce per gallon, so I am use to putting expensive oil in the plane already. Another plus is that aircraft oil is easy to get at airports and two-stroke oil is not. (duh!)
3. The Gas Mileage is not good. In aircraft the equivalent term is BSFC. A diesel generator can run as low as .38 (very efficient!) Your modern car with EFI/I can get .42. A normal light aircraft engine runs about .45. The rotary, in a car, runs .5. This is not considered efficient!
Now my take on this is three fold. First off, Mazda does not make a sedan or mini van with a rotary engine. They put them in sports cars. High performance sports cars. Fuel efficiency was not a goal. If you compare the MPG of the RX-7 with other sports cars of the same horsepower you get about the same MPG.
Second, EPA requirements increase the fuel burn some. By removing all the tubing, ducting, restrictive exhaust system, air pump and the stock ECU that the EPA requires, some more efficiency can be gained.
Third, it is difficult to make any engine efficient over a wide rpm range. In your car, you demand engines that are efficient at any rpm between idle and redline. In a plane I only demand efficiency at cruise power. This is a much simpler task. It is also the logic behind the new "hybrid" electric cars. Hybrid cars have a samll gas engine tuned to run at one set rpm to charge the batteries of an electric car. Since the small engine only has to run at one rpm it is much more efficient at that rpm. This is a part of why diesel generators are so efficient. That's not all the reason but part of it)
Tracy Crook is running the EFI/I that he designed on his 13B equipped RV-4 and is reporting getting .45 BSFC so I think the high fuel flow problem is solvable as well. (This EFI/I is available from his company, "Real World Solutions" as well as a conversion manual plus the "Rotary Aviators Newsletter". Check out Tracy's Website at www.rotaryaviation.com.
4. The rotary is LOUD. Not only is it loud, it is an unpleasant raspy snarl. I went to the Dallas Grand Prix a few years ago. Mazda uses 20G engines with a megaphone pipe. You ALWAYS knew when the Mazda team went by! You will not run with straight pipes like most airplanes. Some sort of muffling is mandatory. Turbochargers help a lot but I will still muffle.
Q. Why is a rotary so reliable?
A. For one reason, the parts count is very low. See the photo above. Either two or three rotors, the eccentric shaft, oil and water pumps, alternator a couple of fan belts and the distributor. That's it! Less than 10 moving parts. Now you could get picky and count the rotor seals. There are 9 seals per rotor. Still less than fifty moving parts even then.
Another major reason is that the rotary is just that, the rotor travels in an almost circular pattern.
Compare this to a piston engine where the piston gets slammed down, comes to a complete stop, then reverses direction. Now repeat that 88 times per second. These are incredible battering forces on the pistons, con rods and crank. Additionally, the longer the stroke of the engine, the longer is the throw on the crank the more leverage there is on the end of the throw trying to bend that crank. Now add in the fact that some of the pistons are compressing air while some are on the power stroke. Now you are trying to bend the crank both ways at once.
Does anyone know why they invented the harmonic balancer? I am fuzzy on the year models, but GM built a V-6 and you could hear this ringing noise while driving. Drove the engineers nutty trying to figure it out. It turned out that it was the crankshaft ringing. Ringing with the vibration of torque reversals! The harmonic balancer absorbs the reversals and stops the ringing. These same reversals create a situation known as "torsional resonance" which is another story altogether.
Watch these rotaries in action:
1) Notice that the rotor never stops and reverses direction. Just a smooth eccentric orbit. 2) since one rotor encompasses all four cycles simultaneously you eliminate torque reversals. (referencing GIF on the right) Intake is on the top right. Compression is across the top. Spark takes place at left center followed by the power stroke across the bottom and the exhaust goes out the far right just below the intake. Also, since the lobe on the eccentric shaft has such a minor throw, the forces of the rotor are not massively leveraged like the piston is at the end of a long throw on a crank. A nice little side benefit of all this is that the rotary is a very smooth and vibration free engine.
The Mazda Gran Prix team uses rotaries. They only rebuild their engine once a season, not after every race like the V8 teams.
Q. Why are rotaries more powerful than piston engines?
Some of it has to do with the same things covered in reliability. The rotor is always moving forward. No energy wasting reversals. Here is a easy experiment. Take a jack handle. Hold the bent end and swing it in a circle like a cop with a nightstick. Now take the same jack handle and use it like a hammer swinging it up then down at arms end. Which way will you get tired faster? Which one gets the most motion for the least amount of effort.
Another aspect is the lack of tappet valves and springs. I once read in a hot rod magazine that as much as 30 horsepower in a V8 engine is used simply to compress the valve springs! There's 30 free horsepower for you!
Another aspect is that with a piston engine, the power stroke only occurs once per two revolutions of the crank (4 stroke engine). Again, look at the animations- even though the rotary does have all four cycles, they are occurring simultaneously! Each side of the the rotor is always at work. This makes the two rotor 13B equivalent to a 6 cylinder engine in power output or the three rotor 20B like a 9 cylinder!
Q. What needs to be done to make the rotary airworthy?
The only mod you HAVE to make is to replace the stock ECU (Engine Control Unit and some people have not even done that!). The stock ECU is a wonderful piece of work for a car but it is not suitable for aircraft use. This is mainly because the ECU requires inputs that may not be present in your plane. One example is the air conditioner switch. Also, the stock ECU will shut down the engine if it overheats. This is a neat feature that will prevent damage to your engine in a car but it may have exactly the opposite effect in a plane!
So you have two options, either use a carburetor and a points type distributor (simplest, cheapest) or replace the stock ECU with a aircraft compatible ECU (more power, better fuel efficiency). I am taking the latter route again using Tracy Crook's unit.
Now, several modifications can be done to reduce the weight of the unit. The intake manifolds are very heavy, I will weld up some replacements from lighter gauge aluminum. The stock engine is also twin turbo charged. One turbo is for low end the other for high. Low end is not needed for aircraft use so it can go. That plus all the complex ducting and switching hardware that sequenced the two turbos can all go to be replaced by a single turbo with a simple wastegate. There other little things that can go or be made lighter that are to numerous to list. If you look at the pictures on the web page in the "Unloading the Engine" section, almost anything that you can see will be gone!
Porting. Depending on your desired power output and which year model 13B engine you are using, opening up the ports and helping the engine's breathing can be very beneficial. I have it on good authority that the 20B breaths just fine as is and needs no porting modifications.
Now these three paragraphs are by no means comprehensive. There are at least another dozen topics that could be covered but these are the "biggies"
See this sites links page to find much more info on rotary engines in general and other aircraft flying with Mazda Wankels
This page will be added to as new questions come in