Old magazine rotary articles
07-23-2003, 12:16 AM
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Technobabble: September 1998
How the rotary engine will be saved
By Dave Coleman
In last month's Technobabble, I explained the basics of how the rotary engine works, and hinted at a new design change that could bring the rotary engine back to our shores. Without repeating the entire explanation, the vital points were as follows: The triangular rotor wobbles around in a vaguely oblong housing causing the volume trapped on each side of the rotor to expand and contract as the rotor turns. Add fuel and some spark plugs, and the expansion and contraction gives you the same combustion cycles as a piston engine. To get air and fuel in and the exhaust out, ports are cut into the engine. The intake port is on the side housing where it is opened and closed by the side of the rotor sliding over it. The exhaust port, on the other hand, is on the rotor housing, where it is always open, and the corners of the rotor sliding by only switch which combustion chamber is being exhausted.
This exhaust port arrangement, known as a peripheral exhaust port, is partially responsible for several of the rotary's shortcomings, including relatively weak low-rpm torque, high hydrocarbon emissions, poor fuel economy, and hot, loud exhaust. Lets take a look at how the peripheral exhaust port affects each of these problems, and how the side exhaust port helps solve it:
Weak Low-Rpm Torque
The peripheral exhaust port, since it's open for so long, necessitates a large amount of port overlap (time when both the intake and exhaust ports are open). This much overlap in any engine, rotary or piston, tends to make the engine run poorly at low rpm. At low rpm, the late-closing exhaust port dilutes the intake charge with exhaust gases that come back in through the open port. This is part of why rotaries traditionally have poor low-rpm torque. (At high rpm, peripheral exhaust ports-and even periperhal intake ports--work well since they stay open very long, allowing more time for the exhaust to exit without causing excessive pumping losses. The exhaust charge also has more momentum and less time to reverse direction and re-enter the combustion chamber.)
The side exhaust port, on the other hand, can be positioned to minimize or even eliminate port overlap. Mazda recently built a research engine using the standard six-port intake configuration that was used on the most recent naturally aspirated 13B rotary engines, but replaced that engine's peripheral exhaust port with a new side exhaust port. That exhaust port was positioned so there was absolutely no port overlap. This reduced the amount of exhaust gas diluting each intake charge, but more importantly, it made that dilution more predictable, allowing the engine to be tuned more precisely.
Though the side exhaust is less advantageous at high rpm, the earlier exhaust closing allows the intake ports to be repositioned and enlarged slightly, so it is possible to design the high-rpm power right back into the engine.
High Hydrocarbon Emissions
Rotary engines are notorious for their high hydrocarbon emissions (Hydrocarbons, or HCs are basically unburned, or partially burned fuel and oil). Ironically, the rotary engine was poised to replace the piston engine in the mid-'70s because it had bettter emissions characteristics than contemporary piston engines. The emissions advantage was in oxides of nitrogen (NOx), however, which are inherently low in rotary engines, and were difficult ro remove from piston engine exhaust before the three-way catalytic converter was introduced. The rotary eventually disappeared from almost every automaker's plans (except Mazda, of course) when the three-way cat was introduced because the NOx advantage was no longer there, and the HCs were so high that they were too difficult to control.
There are many reasons for the rotary's high HC emissions, many of which the side exhaust port does not help. For example, the sliding surface of the rotor housing has to be lubricated to keep the apex seals from wearing out, just like the cylinder walls of a piston engine have to be lubricated. In a piston engine, the cylinder walls can be lubricated easily by splashing oil on from below and then scraping it off with the oil control ring. The apex seals of a rotary have no below to lubricate from, since there is a combustion chamber on either side of the seal. Instead, oil has to be slowly dripped into the intake and mixed with the fuel. All of that oil eventually burns and goes out the exhaust as HCs.
The shape of the rotary's combustion chamber is also a problem. The long, narrow shape means it has a high surface area-to-volume ratio. In other words, for an engine of its size, there is an inordinately large amount of surface area in the combustion chamber. Since gases floating in the middle of a combustion chamber tend to burn more completely and consistently than those on the edges, this is a disadvantage.
Another cause for high HCs comes from fuel condensing on cool surfaces in the combustion chamber (cool, obviously, is a relative term here). Because the combustion chambers move as the rotor wobbles through it's eccentric orbit, each event in the engine's cycle (intake, compression, ignition, and exhaust) occurs in a different area of a rotary engine. In a piston engine, the cylinder walls are heated by the ignition and exhaust strokes, and then cooled by the intake stroke, so they are hot when the fuel and air enter the cylinder. In a rotary, the intake stroke's combustion chamber walls are much cooler, since all they see is air that has been cooled by the evaporation of fuel. The fuel that condenses on the walls doesn't burn completely, increasing hydrocarbon emissions.
Finally, the long, narrow shape of the combustion chamber means the flame front has a long way to travel, and sometimes it doesn't make it all the way to the ends. Partially because the motion of the rotor sweeps all the condensed fuel to the back of the combustion chamber, there tends to be a pocket of fuel-rich gases near the trailing apex seal. This pocket of unburned gas turns into a spurt of HCs as it gets pumped out the exhaust. The side exhaust port helps tremendously here, for the simple reason that it closes before the trailing end of the rotor gets there. The pocket of fuel-rich gases simply get swept around into the next intake stroke for a second chance at complete combustion. In the aforementioned research engine, moving the exhaust port to the side cut hydrocarbon emissions by 35 to 50 percent!
Poor Fuel Economy
The fuel crisis of the early '70s was a boon for most Japanese automakers, since their small, fuel-efficient cars suddenly became popular in America. Mazda was not so lucky, though, as its rotary-powered cars sucked down fuel as fast as any gas-guzzling V8. Modern fuel injection and emissions control systems have improved the rotary's fuel efficiency tremendously, but it is still at the low end of the piston engine range.
Much of the rotary's fuel-efficiency woes stem from the same problems listed earlier. The exhaust gas dilution of the intake charge, for example, makes combustion less stable since the exhaust gets in the way of all those fuel and oxygen molecules that have to find each other in order to burn. The only way to make combustion stable and consistent is to richen the fuel mixture and give the fuel molecules a better chance at finding their oxygen.
The condensation problem also affects fuel economy. The fuel deposited on the combustion chamber walls is removed from the overall mix, making the air/fuel mixture leaner. To compensate, the air/fuel ratio must be made overly rich, so that after this condensation, it's still rich enough for consistent combustion. All this richness translates to poor fuel economy.
Hot, Loud Exhaust
The peripheral exhaust port provides a straight, open hole from the combustion chamber to the exhaust. The opening and closing characteristics of the port are also very sudden. The sharp, strong pressure waves from this exhaust port add up to a very loud and universally unpleasant exhaust note. Placing a turbocharger on the exhaust quiets the rotary's voice significantly by breaking up the exhaust pulses--just as it does in a piston engine--but the side-exhaust-port rortary shines as a naturally aspirated powerplant.
In a piston engine, the exhaust valve opens more slowly, and the port shape and exhaust valve muffle the exhaust. Because the initial burst of exhaust pressure is spread out, and the shock waves are broken up by the valve, the exhaust is generally quieter and more pleasant on a piston engine. Luckily, the side exhaust port is not only slightly more convoluted than the peripheral exhaust port (the side port has one 90-degree bend), it also opens and closes more gradually, and so produces a more pleasant exhaust note.
Beyond the aesthetic advantages of the side exhaust's note, the lower noise level and reduced temperature means the exhaust can be constructed with less expensive, lighter weight materials, and the mufflers can be less restrictive. In short, the powertrain can be lighter, cheaper, and more powerful.
With all these advantages coming from one simple change, the obvious question is, why didn't someone think of this earler? In the late '60 and early '70s, when the basic dimensions and layout of the rotary engine were still being solidified, Mazda engineers realized that a peripheral intake port caused too many problems with low-rpm drivability, noise, and fuel consumption, so they moved the intake ports to the side housing. Doesn't it seem like trying a side exhaust port as well would have taken less than 30 years?
Actually, according to Racing Beat's Jim Mederer, Mazda did try the side exhaust years ago. At the time, the side exhaust's benefits were recognized, but there were problems with it getting clogged with soot. Earlier apex seal designs required more oil to prevent wear, and the oily exhaust, combined with the relatively imprecise fuel metering provided by the carburetors of the day, caused the narrower, more convoluted exhaust ports to get gunked up. The peripheral port was needed for longevity at the time.
Between then and now, apparently, everybody has been thinking "within the box", only making small adjustments to a basic design that hasn't changed since the R100 hit these shores in 1970. What prompted the rediscovery of the side exhaust is a mystery, but there is apparently a lot of excitement at Mazda now over the new engine.
The side-exhaust rotary has only appeared to the public in one car, the RX-01 show car that debuted at the Tokyo motor show a few years back. That car made 220 hp without forced induction, and could pass Japan's strict emissions standards. The engine was packaged very tightly into a purpose-built car that would have been rather expensive to build, and the engine was fitted with unnecessarily expensive features like dry-sump lubrication.
Rumors are that a new rotary sports car is on the way, and it will be designed to be much more cost effective than either the RX-01 or the third-generation RX-7. Lots of shared components with the Miata will likely be one effective cost cutter, as will the new side-exhaust rotary. Sports car lovers, keep your fingers crossed, the RX-7 may be re-born after all.
How the rotary engine will be saved
By Dave Coleman
In last month's Technobabble, I explained the basics of how the rotary engine works, and hinted at a new design change that could bring the rotary engine back to our shores. Without repeating the entire explanation, the vital points were as follows: The triangular rotor wobbles around in a vaguely oblong housing causing the volume trapped on each side of the rotor to expand and contract as the rotor turns. Add fuel and some spark plugs, and the expansion and contraction gives you the same combustion cycles as a piston engine. To get air and fuel in and the exhaust out, ports are cut into the engine. The intake port is on the side housing where it is opened and closed by the side of the rotor sliding over it. The exhaust port, on the other hand, is on the rotor housing, where it is always open, and the corners of the rotor sliding by only switch which combustion chamber is being exhausted.
This exhaust port arrangement, known as a peripheral exhaust port, is partially responsible for several of the rotary's shortcomings, including relatively weak low-rpm torque, high hydrocarbon emissions, poor fuel economy, and hot, loud exhaust. Lets take a look at how the peripheral exhaust port affects each of these problems, and how the side exhaust port helps solve it:
Weak Low-Rpm Torque
The peripheral exhaust port, since it's open for so long, necessitates a large amount of port overlap (time when both the intake and exhaust ports are open). This much overlap in any engine, rotary or piston, tends to make the engine run poorly at low rpm. At low rpm, the late-closing exhaust port dilutes the intake charge with exhaust gases that come back in through the open port. This is part of why rotaries traditionally have poor low-rpm torque. (At high rpm, peripheral exhaust ports-and even periperhal intake ports--work well since they stay open very long, allowing more time for the exhaust to exit without causing excessive pumping losses. The exhaust charge also has more momentum and less time to reverse direction and re-enter the combustion chamber.)
The side exhaust port, on the other hand, can be positioned to minimize or even eliminate port overlap. Mazda recently built a research engine using the standard six-port intake configuration that was used on the most recent naturally aspirated 13B rotary engines, but replaced that engine's peripheral exhaust port with a new side exhaust port. That exhaust port was positioned so there was absolutely no port overlap. This reduced the amount of exhaust gas diluting each intake charge, but more importantly, it made that dilution more predictable, allowing the engine to be tuned more precisely.
Though the side exhaust is less advantageous at high rpm, the earlier exhaust closing allows the intake ports to be repositioned and enlarged slightly, so it is possible to design the high-rpm power right back into the engine.
High Hydrocarbon Emissions
Rotary engines are notorious for their high hydrocarbon emissions (Hydrocarbons, or HCs are basically unburned, or partially burned fuel and oil). Ironically, the rotary engine was poised to replace the piston engine in the mid-'70s because it had bettter emissions characteristics than contemporary piston engines. The emissions advantage was in oxides of nitrogen (NOx), however, which are inherently low in rotary engines, and were difficult ro remove from piston engine exhaust before the three-way catalytic converter was introduced. The rotary eventually disappeared from almost every automaker's plans (except Mazda, of course) when the three-way cat was introduced because the NOx advantage was no longer there, and the HCs were so high that they were too difficult to control.
There are many reasons for the rotary's high HC emissions, many of which the side exhaust port does not help. For example, the sliding surface of the rotor housing has to be lubricated to keep the apex seals from wearing out, just like the cylinder walls of a piston engine have to be lubricated. In a piston engine, the cylinder walls can be lubricated easily by splashing oil on from below and then scraping it off with the oil control ring. The apex seals of a rotary have no below to lubricate from, since there is a combustion chamber on either side of the seal. Instead, oil has to be slowly dripped into the intake and mixed with the fuel. All of that oil eventually burns and goes out the exhaust as HCs.
The shape of the rotary's combustion chamber is also a problem. The long, narrow shape means it has a high surface area-to-volume ratio. In other words, for an engine of its size, there is an inordinately large amount of surface area in the combustion chamber. Since gases floating in the middle of a combustion chamber tend to burn more completely and consistently than those on the edges, this is a disadvantage.
Another cause for high HCs comes from fuel condensing on cool surfaces in the combustion chamber (cool, obviously, is a relative term here). Because the combustion chambers move as the rotor wobbles through it's eccentric orbit, each event in the engine's cycle (intake, compression, ignition, and exhaust) occurs in a different area of a rotary engine. In a piston engine, the cylinder walls are heated by the ignition and exhaust strokes, and then cooled by the intake stroke, so they are hot when the fuel and air enter the cylinder. In a rotary, the intake stroke's combustion chamber walls are much cooler, since all they see is air that has been cooled by the evaporation of fuel. The fuel that condenses on the walls doesn't burn completely, increasing hydrocarbon emissions.
Finally, the long, narrow shape of the combustion chamber means the flame front has a long way to travel, and sometimes it doesn't make it all the way to the ends. Partially because the motion of the rotor sweeps all the condensed fuel to the back of the combustion chamber, there tends to be a pocket of fuel-rich gases near the trailing apex seal. This pocket of unburned gas turns into a spurt of HCs as it gets pumped out the exhaust. The side exhaust port helps tremendously here, for the simple reason that it closes before the trailing end of the rotor gets there. The pocket of fuel-rich gases simply get swept around into the next intake stroke for a second chance at complete combustion. In the aforementioned research engine, moving the exhaust port to the side cut hydrocarbon emissions by 35 to 50 percent!
Poor Fuel Economy
The fuel crisis of the early '70s was a boon for most Japanese automakers, since their small, fuel-efficient cars suddenly became popular in America. Mazda was not so lucky, though, as its rotary-powered cars sucked down fuel as fast as any gas-guzzling V8. Modern fuel injection and emissions control systems have improved the rotary's fuel efficiency tremendously, but it is still at the low end of the piston engine range.
Much of the rotary's fuel-efficiency woes stem from the same problems listed earlier. The exhaust gas dilution of the intake charge, for example, makes combustion less stable since the exhaust gets in the way of all those fuel and oxygen molecules that have to find each other in order to burn. The only way to make combustion stable and consistent is to richen the fuel mixture and give the fuel molecules a better chance at finding their oxygen.
The condensation problem also affects fuel economy. The fuel deposited on the combustion chamber walls is removed from the overall mix, making the air/fuel mixture leaner. To compensate, the air/fuel ratio must be made overly rich, so that after this condensation, it's still rich enough for consistent combustion. All this richness translates to poor fuel economy.
Hot, Loud Exhaust
The peripheral exhaust port provides a straight, open hole from the combustion chamber to the exhaust. The opening and closing characteristics of the port are also very sudden. The sharp, strong pressure waves from this exhaust port add up to a very loud and universally unpleasant exhaust note. Placing a turbocharger on the exhaust quiets the rotary's voice significantly by breaking up the exhaust pulses--just as it does in a piston engine--but the side-exhaust-port rortary shines as a naturally aspirated powerplant.
In a piston engine, the exhaust valve opens more slowly, and the port shape and exhaust valve muffle the exhaust. Because the initial burst of exhaust pressure is spread out, and the shock waves are broken up by the valve, the exhaust is generally quieter and more pleasant on a piston engine. Luckily, the side exhaust port is not only slightly more convoluted than the peripheral exhaust port (the side port has one 90-degree bend), it also opens and closes more gradually, and so produces a more pleasant exhaust note.
Beyond the aesthetic advantages of the side exhaust's note, the lower noise level and reduced temperature means the exhaust can be constructed with less expensive, lighter weight materials, and the mufflers can be less restrictive. In short, the powertrain can be lighter, cheaper, and more powerful.
With all these advantages coming from one simple change, the obvious question is, why didn't someone think of this earler? In the late '60 and early '70s, when the basic dimensions and layout of the rotary engine were still being solidified, Mazda engineers realized that a peripheral intake port caused too many problems with low-rpm drivability, noise, and fuel consumption, so they moved the intake ports to the side housing. Doesn't it seem like trying a side exhaust port as well would have taken less than 30 years?
Actually, according to Racing Beat's Jim Mederer, Mazda did try the side exhaust years ago. At the time, the side exhaust's benefits were recognized, but there were problems with it getting clogged with soot. Earlier apex seal designs required more oil to prevent wear, and the oily exhaust, combined with the relatively imprecise fuel metering provided by the carburetors of the day, caused the narrower, more convoluted exhaust ports to get gunked up. The peripheral port was needed for longevity at the time.
Between then and now, apparently, everybody has been thinking "within the box", only making small adjustments to a basic design that hasn't changed since the R100 hit these shores in 1970. What prompted the rediscovery of the side exhaust is a mystery, but there is apparently a lot of excitement at Mazda now over the new engine.
The side-exhaust rotary has only appeared to the public in one car, the RX-01 show car that debuted at the Tokyo motor show a few years back. That car made 220 hp without forced induction, and could pass Japan's strict emissions standards. The engine was packaged very tightly into a purpose-built car that would have been rather expensive to build, and the engine was fitted with unnecessarily expensive features like dry-sump lubrication.
Rumors are that a new rotary sports car is on the way, and it will be designed to be much more cost effective than either the RX-01 or the third-generation RX-7. Lots of shared components with the Miata will likely be one effective cost cutter, as will the new side-exhaust rotary. Sports car lovers, keep your fingers crossed, the RX-7 may be re-born after all.
07-23-2003, 12:25 AM
#6
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Join Date: Jun 2002
Location: La Plata, Maryland
Posts: 9,725
I know a certain editor has a thing for rotaries 
Technobabble: August 1998
Hula hoops, Spirographs, and Rotary Engines
By Dave Coleman
In terms of technical diversity, these are very dull times in the automotive world. Sure, today's cars are cleaner, more efficient, and more powerful than ever, but they're all essentially the same under the skin--pistons, valves and cams under every hood. In the United States, every car sold today is some type of water-cooled, four-stroke piston engine. Volkswagen's VR6 is the only one of those powerplants to even get creative with layout. The rest are all some sort of in-line, or vee, with only two exceptions--Porsche and Subaru stand out as the daring duo just for being so bold as to use a horizontally opposed engine. What ever happened to Saab's 2-stroke engines, or at least their backwards-mounted engines in the old 900s? Or more realistically, what happened to the direct injection two-strokes that were supposed to hit the market a few years ago? What about Porsche's air-cooled engines? And what about the oddest oddball of all, Mazda's Wankel rotary?
I don't expect air-cooled engines to ever make a comeback, but there have been some relatively recent developments that could bring the rotary back into Mazda's lineup here in the states. An unreliable Australian source, quoting an equally questionable Japanese source, says that Mazda should be debuting a new rotary-powered sports car at the 1999 Tokyo Motor Show. The car should borrow many parts from existing Mazdas to keep costs down, and should be powered by a radically simple new naturally aspirated version of the 13B rotary that puts out about 220 hp. Price is supposed to begin in the low $25,000 range--low enough to be a sales success again, unlike the third generation RX-7. Enough with the rumors, though, what about this radically simple new engine? With one seemingly obvious design change, most of the rotary engine's vices (poor emissions, bad gas mileage, hot, loud exhaust, and poor low-rpm torque) can be cured, while simultaneously improving durability and reducing cost. It's like 1972 all over again--now the rotary can be perfect once more!
Before explaining exactly what this new engine does differently, lets review how a rotary engine actually works. A rotary is so strange in its motion that even most technically savvy people don't really understand what goes on inside of one.
You've probably seen pictures or diagrams before that show a triangle wobbling around in some strange, peanut-shaped box, but understanding the exact nature of that wobble is the key to understanding where that peanut shape comes from, and how the rotary's motion works. Remember the Spirograph? They were basically a set of cheap plastic gears with holes strategically drilled in them so you can trace, with a pen, the motion of the gears as they rotate around each other. Spirographs have inside gears and outside gears, and if you match them up right, you can make the basic mechanics of a rotary engine.
For example, take an outside gear (a circular gear with teeth on the outside) with 70 teeth--we'll call this the stationary gear, since, well... it will be stationary. Now, place the stationary gear in the middle of an inside gear (a circular gear with teeth on the inside) with 105 teeth on it--we'll call this one a phasing gear for reasons that should be clear soon. (This exercise will actually work with any number of teeth as long as the stationary gear has 2/3 as many teeth as the phasing gear). Now, keep the stationary gear stationary (Duh!), and let the phasing gear swing around it like a hula hoop around a hula hooper's waist. Are we having fun yet?
Now, place a pen in one of the pen holes of the phasing gear, and see what you get. After three hula-hoop swings, you should have something that looks like the peanut-shaped rotor housing, only narrower in the mid-section. If you use a pen hole that is farther out on the phasing gear (farther out than most Spirograph sets will allow), you will get a wider mid-section and an overall shape that more closely resembles Mazda's rotary. Now, pick three pen holes that are the same distance out on the phasing gear, and draw an equilateral triangle using these points as the corners of the triangle. Start hula-hooping again, and miracle of miracles, all three corners of the triangle follow the same peanut-shaped path! Congratulations, you have just built a rotary engine. (Amazingly, Felix Wankel came up with his rotary engine before the spirograph was invented!)
In a Mazda rotary, the stationary gear is bolted to the side housing (the closest thing a rotary has to an engine block), the phasing gear is riveted to the triangular rotor, and the three corners of the rotor trace the shape of the rotor housing, maintaining a seal at all times.
OK, so now you know that wacky Wankel wobble is actually a nice, circular Hula-hoop-like swing, but how does this make an engine? If you place seals at each apex (corner) of the rotor (triangle), and sandwich the rotor and housing (triangle and peanut) between two, flat side housings, you get a sealed chamber on each of the rotor's three sides. As the rotor rotates, these chambers expand and contract, and if you add air and fuel to any chamber that expands and contracts, you can make an engine.
As the rotor wobbles around, fuel and air go through the same intake, compression, ignition, and exhaust phases that they do in a piston engine. Follow along with the photos on this page and you'll see how it works:
In these photos, one side housing has been removed to show the internals of the engine. The stationary gear has also been removed, since its mounting base is so large it would obscure both gears. At any given time, there are three combustion chambers going through three different phases of the cycle. This is one reason people often say a two rotor engine is equivalent to a six cylinder piston engine. It isn't, but I'll get into that later.
In the first photo, chamber A is in the intake "stroke". As the rotor rotates, the intake port (the triangular hole on the back wall) is opened as the leading edge of the rotor uncovers it, and is closed when the trailing edge of the rotor covers it up. This elegantly simple method for opening and closing the ports means no valves, cams, rocker arms, pushrods, or timing chains. It also means that the shape of the intake port is directly tied to the port timing. Because of this, a little time with a grinder in the intake ports can go a long way. Porting a rotary is the equivalent of not only porting a piston engine, but also changing the cam and valves. This also means that the effective equivalent of a VTEC system can be built simply by adding an extra intake port up high on the side housing where it would work like a high-lift, long duration cam. This port could then be opened and closed as needed by a simple butterfly valve. Imagine a butterfly valve doing everything that the immensely complex VTEC system does! (Actually you don't have to imagine, the "6-port" 13B rotary had 6 intake ports instead of the normal four, two of which are long duration ports that are opened by butterfly valves.)
As the rotor swings past, it closes the intake port (picture 2, chamber A), and begins to compress the gas and fuel. At full compression, (picture 1, chamber B), the combustion chamber is extremely long and narrow, with a high surface area to volume ratio. This is an inherent weakness of the design. The long combustion chamber means the flame front has a long way to travel, which typically means high levels of unburned hydrocarbons will find their way out the exhaust. The combustion chamber isn't quite as narrow as it looks, however, as there is a dish cast into the rotor face, much like a dish in the top of a low-compression piston. Still, to ensure that enough fuel can be burned in this long combustion chamber, two spark plugs are used with their timing staggered by a few degrees. In case you were wondering, the spark plugs are recessed into holes in the rotor housing so the apex seals can slide over them.
The power stroke can be seen in photo 2, chamber B, and finally, the exhaust stroke is seen in chamber C, beginning in photo 1 and ending in photo 2. You can't see the exhaust port in these photos because it is cut into the rotor housing, not the side housing. This is known as a peripheral exhaust port, since it is cut into the periphery, rather than the side of the chamber. The peripheral exhaust port has some advantages. It's always open, since the narrow apex seal is the only part of the rotor that slides over it. At the end of the exhaust stroke, the apex seal slides past, and the port is instantly open to the next exhaust stroke. This straightforward exhaust port means the exhaust pulses are extremely strong--perfect for driving a turbo--but it also means they are extremely loud and hot. Expensive high-nickel steel and lots of mufflers must be used on a rotary's exhaust to contain the noise and heat.
Now that we have a self-propelled triangle bouncing around in a box, all we need to do is get power out from it. It's easier than it looks. The hula-hooping swing is actually a very simple motion. The center of the phasing gear simply travels around the center of the stationary gear in a perfect circle. In a Mazda rotary, the center of the phasing gear travels in a 15-mm circle, so to get power from a rotary, you just use a crankshaft with a 15-mm stroke (actually, in a rotary it is called an eccentric shaft, and the stroke is called the eccentricity, but the concept is exactly the same.) The middle of each rotor has a bearing in it that is the equivalent of a rod bearing on a piston engine, and the eccentric shaft runs through the middle of the engine, taking power from both rotors. Nothing could be simpler (except perhaps a turbine).
Is your brain sweating yet? Figuring out the weird wobble of a Wankel can take a lot out of you, so I'll let it soak in for a month. Next month I'll talk some more about that rotary motion, explode some myths about the displacement of a rotary, and explain the big new change that could make the next rotary-powered car so much better. Just a hint--the exhaust port is on the side.
Until next month, happy Wankeling.
Technobabble: August 1998
Hula hoops, Spirographs, and Rotary Engines
By Dave Coleman
In terms of technical diversity, these are very dull times in the automotive world. Sure, today's cars are cleaner, more efficient, and more powerful than ever, but they're all essentially the same under the skin--pistons, valves and cams under every hood. In the United States, every car sold today is some type of water-cooled, four-stroke piston engine. Volkswagen's VR6 is the only one of those powerplants to even get creative with layout. The rest are all some sort of in-line, or vee, with only two exceptions--Porsche and Subaru stand out as the daring duo just for being so bold as to use a horizontally opposed engine. What ever happened to Saab's 2-stroke engines, or at least their backwards-mounted engines in the old 900s? Or more realistically, what happened to the direct injection two-strokes that were supposed to hit the market a few years ago? What about Porsche's air-cooled engines? And what about the oddest oddball of all, Mazda's Wankel rotary?
I don't expect air-cooled engines to ever make a comeback, but there have been some relatively recent developments that could bring the rotary back into Mazda's lineup here in the states. An unreliable Australian source, quoting an equally questionable Japanese source, says that Mazda should be debuting a new rotary-powered sports car at the 1999 Tokyo Motor Show. The car should borrow many parts from existing Mazdas to keep costs down, and should be powered by a radically simple new naturally aspirated version of the 13B rotary that puts out about 220 hp. Price is supposed to begin in the low $25,000 range--low enough to be a sales success again, unlike the third generation RX-7. Enough with the rumors, though, what about this radically simple new engine? With one seemingly obvious design change, most of the rotary engine's vices (poor emissions, bad gas mileage, hot, loud exhaust, and poor low-rpm torque) can be cured, while simultaneously improving durability and reducing cost. It's like 1972 all over again--now the rotary can be perfect once more!
Before explaining exactly what this new engine does differently, lets review how a rotary engine actually works. A rotary is so strange in its motion that even most technically savvy people don't really understand what goes on inside of one.
You've probably seen pictures or diagrams before that show a triangle wobbling around in some strange, peanut-shaped box, but understanding the exact nature of that wobble is the key to understanding where that peanut shape comes from, and how the rotary's motion works. Remember the Spirograph? They were basically a set of cheap plastic gears with holes strategically drilled in them so you can trace, with a pen, the motion of the gears as they rotate around each other. Spirographs have inside gears and outside gears, and if you match them up right, you can make the basic mechanics of a rotary engine.
For example, take an outside gear (a circular gear with teeth on the outside) with 70 teeth--we'll call this the stationary gear, since, well... it will be stationary. Now, place the stationary gear in the middle of an inside gear (a circular gear with teeth on the inside) with 105 teeth on it--we'll call this one a phasing gear for reasons that should be clear soon. (This exercise will actually work with any number of teeth as long as the stationary gear has 2/3 as many teeth as the phasing gear). Now, keep the stationary gear stationary (Duh!), and let the phasing gear swing around it like a hula hoop around a hula hooper's waist. Are we having fun yet?
Now, place a pen in one of the pen holes of the phasing gear, and see what you get. After three hula-hoop swings, you should have something that looks like the peanut-shaped rotor housing, only narrower in the mid-section. If you use a pen hole that is farther out on the phasing gear (farther out than most Spirograph sets will allow), you will get a wider mid-section and an overall shape that more closely resembles Mazda's rotary. Now, pick three pen holes that are the same distance out on the phasing gear, and draw an equilateral triangle using these points as the corners of the triangle. Start hula-hooping again, and miracle of miracles, all three corners of the triangle follow the same peanut-shaped path! Congratulations, you have just built a rotary engine. (Amazingly, Felix Wankel came up with his rotary engine before the spirograph was invented!)
In a Mazda rotary, the stationary gear is bolted to the side housing (the closest thing a rotary has to an engine block), the phasing gear is riveted to the triangular rotor, and the three corners of the rotor trace the shape of the rotor housing, maintaining a seal at all times.
OK, so now you know that wacky Wankel wobble is actually a nice, circular Hula-hoop-like swing, but how does this make an engine? If you place seals at each apex (corner) of the rotor (triangle), and sandwich the rotor and housing (triangle and peanut) between two, flat side housings, you get a sealed chamber on each of the rotor's three sides. As the rotor rotates, these chambers expand and contract, and if you add air and fuel to any chamber that expands and contracts, you can make an engine.
As the rotor wobbles around, fuel and air go through the same intake, compression, ignition, and exhaust phases that they do in a piston engine. Follow along with the photos on this page and you'll see how it works:
In these photos, one side housing has been removed to show the internals of the engine. The stationary gear has also been removed, since its mounting base is so large it would obscure both gears. At any given time, there are three combustion chambers going through three different phases of the cycle. This is one reason people often say a two rotor engine is equivalent to a six cylinder piston engine. It isn't, but I'll get into that later.
In the first photo, chamber A is in the intake "stroke". As the rotor rotates, the intake port (the triangular hole on the back wall) is opened as the leading edge of the rotor uncovers it, and is closed when the trailing edge of the rotor covers it up. This elegantly simple method for opening and closing the ports means no valves, cams, rocker arms, pushrods, or timing chains. It also means that the shape of the intake port is directly tied to the port timing. Because of this, a little time with a grinder in the intake ports can go a long way. Porting a rotary is the equivalent of not only porting a piston engine, but also changing the cam and valves. This also means that the effective equivalent of a VTEC system can be built simply by adding an extra intake port up high on the side housing where it would work like a high-lift, long duration cam. This port could then be opened and closed as needed by a simple butterfly valve. Imagine a butterfly valve doing everything that the immensely complex VTEC system does! (Actually you don't have to imagine, the "6-port" 13B rotary had 6 intake ports instead of the normal four, two of which are long duration ports that are opened by butterfly valves.)
As the rotor swings past, it closes the intake port (picture 2, chamber A), and begins to compress the gas and fuel. At full compression, (picture 1, chamber B), the combustion chamber is extremely long and narrow, with a high surface area to volume ratio. This is an inherent weakness of the design. The long combustion chamber means the flame front has a long way to travel, which typically means high levels of unburned hydrocarbons will find their way out the exhaust. The combustion chamber isn't quite as narrow as it looks, however, as there is a dish cast into the rotor face, much like a dish in the top of a low-compression piston. Still, to ensure that enough fuel can be burned in this long combustion chamber, two spark plugs are used with their timing staggered by a few degrees. In case you were wondering, the spark plugs are recessed into holes in the rotor housing so the apex seals can slide over them.
The power stroke can be seen in photo 2, chamber B, and finally, the exhaust stroke is seen in chamber C, beginning in photo 1 and ending in photo 2. You can't see the exhaust port in these photos because it is cut into the rotor housing, not the side housing. This is known as a peripheral exhaust port, since it is cut into the periphery, rather than the side of the chamber. The peripheral exhaust port has some advantages. It's always open, since the narrow apex seal is the only part of the rotor that slides over it. At the end of the exhaust stroke, the apex seal slides past, and the port is instantly open to the next exhaust stroke. This straightforward exhaust port means the exhaust pulses are extremely strong--perfect for driving a turbo--but it also means they are extremely loud and hot. Expensive high-nickel steel and lots of mufflers must be used on a rotary's exhaust to contain the noise and heat.
Now that we have a self-propelled triangle bouncing around in a box, all we need to do is get power out from it. It's easier than it looks. The hula-hooping swing is actually a very simple motion. The center of the phasing gear simply travels around the center of the stationary gear in a perfect circle. In a Mazda rotary, the center of the phasing gear travels in a 15-mm circle, so to get power from a rotary, you just use a crankshaft with a 15-mm stroke (actually, in a rotary it is called an eccentric shaft, and the stroke is called the eccentricity, but the concept is exactly the same.) The middle of each rotor has a bearing in it that is the equivalent of a rod bearing on a piston engine, and the eccentric shaft runs through the middle of the engine, taking power from both rotors. Nothing could be simpler (except perhaps a turbine).
Is your brain sweating yet? Figuring out the weird wobble of a Wankel can take a lot out of you, so I'll let it soak in for a month. Next month I'll talk some more about that rotary motion, explode some myths about the displacement of a rotary, and explain the big new change that could make the next rotary-powered car so much better. Just a hint--the exhaust port is on the side.
Until next month, happy Wankeling.
07-23-2003, 12:31 AM
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Ultimate StreetCar Challenge 02: 2002 Laminar Concepts Viking SRX-7
Composite Birdcage
By Josh Jacquot
Photography: Josh Jacquot, Henry DeKuyper
Fink's Ultimate Street Car Challenge entry form says it all: "Weight, horsepower, weight, styling, weight, handling, weight, engineering." Fink is the president of Advanced Engineering, Inc. in Media, Penn., where lighter is, apparently, better. AEI is responsible for the extremely light, extremely powerful car on these pages.
One look at the Laminar Concepts Viking SRX-7 and it's clear its creators are serious about their mission: To build a very loosely based Lotus Seven replica capable of winning every contest of automotive performance. To do so, they use a less-is-more philosophy, which makes the late Colin Chapman look like a conservative ninny by comparison.
Let's put the SRX-7 in perspective. Fink claims it weighs 1,710 lbs and packs 450 or so horsepower. If true, those figures put the SRX-7 worlds away from virtually any other street car and only a notch or two down from a Champ car in the power-to-weight department. Even with a 20 percent BS factor, those numbers are serious enough to make Chapman soil his knickers and give this two-seater a real shot at winning every performance test in the USCC.
The SRX-7 is constructed of 4130 tubing and covered in structural composite bodywork, so it does have some similarities to the original Lotus Super Seven. However, the likeness ends there. Power comes from a remanufactured and internally stock Mazda 13B rotary, which has all the right parts to make the claimed power figure. A single-turbo conversion using a custom-built, equal-length stainless-steel turbo header feeds the Turbonetics T04B 60-1 turbocharger. An HKS wastegate, 3-inch exhaust, high-flow catalytic converter and twin Walker Dynomax mufflers complete the exhaust system.
On the cold side of the turbo there's an air-to-air intercooler, which is force-fed cool air by a large hood scoop and extensive composite ducting. The stock intake manifold is Extrude Honed for better flow and the throttle body is ported. There are two stock RX-7 fuel pumps drawing fuel from the 10-gallon Fuel Safe fuel cell. Fuel is fed through -6 lines to the 550 cc/min stock injectors and an additional 1600 cc/min secondary injector. An A'PEXi Power FC stand-alone ECU controls all engine parameters except boost, which is handled by A'PEXi's AVC-R.
The rear suspension started life as a complex multi-link, semi-trailing-arm type from a second-generation RX-7, but has been heavily modified. All rubber bushings are replaced with spherical bearings. The geometry is changed, allowing for a modified roll center, as well as adjustable camber and toe. The front suspension consists of custom-built upper and lower control arms. Damping is handled by triple-adjustable Penske shocks at all four corners. And the SRX-7 rolls on the stickiest street tire known to man-the Hoosier R3S03, measuring 245/35ZR-18 in front and a staggering 275/35ZR-18 in the rear. Brakes are equally serious. Since the SRX-7 is essentially a designed-from-the-ground-up package, Fink uses a Wilwood pedal assembly with a Tilton master cylinder. Four-piston Wilwood Superlite calipers and 11.75-inch rotors make up the front brake hardware, while stock second-generation RX-7 calipers and 10.5-inch calipers are used in the rear.
On paper, the Laminar Concepts Viking SRX-7 has all the right goods to be a serious contender for the "Ultimate" title. Weaknesses? Well, history tells us that turbo rotaries are extremely sensitive to poor tuning and heat, and then there's the unknown issues that arise with any homebrew special like the SRX-7. Fact is, history hasn't been very kind to men like Jerry Fink who have the ***** to build their own car. Anyone remember Jerry Weigert's Vector? DeLorean's DeLorean? Bricklin's Bricklin? At least Fink has physics firmly on his side. Tune in next month to see if this unique machine has what it takes.
LAMINAR CONCEPTS VIKING SRX-7
ENGINE
Engine Code: 13B
Type: Turbocharged and intercooled, two-rotor wankel
Internal Modifications: None
External Modifications: Single turbo conversion with custom stainless-steel header and Turbonetics T04B 60-1 turbocharger, HKS wastegate, 3-inch exhaust with high-flow catalytic converter and twin Dynomax mufflers, upgraded intercooler, Extrude Honed intake manifold, ported throttle body
Engine Management Modifications: A'pexi AVC-R boost controller, A'pexi Power FC ECU, two RX-7 fuel pumps, 1600 cc/min additional injector, Essex fuel pressure regulator
DRIVETRAIN
Layout: Front/mid engine, rear-wheel drive
Drivetrain Modifications: Centerforce clutch, aluminum flywheel
SUSPENSION
Front: Penske three-way adjustable coil-overs on custom upper and lower control arms
Rear: Penske three-way adjustable coil-overs on modified second-gen. RX-7 multi-link/trailing arm combo
BRAKES
Front: 11.75-in. cross-drilled rotors, Wilwood Superlite four-piston calipers, Hawk pads, braided stainless steel lines
Rear: 10.5-in cross-drilled rotors, single-piston sliding RX-7 calipers, Hawk pads, braided stainless steel lines external
EXTERNAL
Wheels: BBS RK 18x8.5-in. front, 18x10-in. rear
Tires: Hoosier R3S03, 245/35ZR-18 front, 275/35ZR-18 rear
Composite Birdcage
By Josh Jacquot
Photography: Josh Jacquot, Henry DeKuyper
Fink's Ultimate Street Car Challenge entry form says it all: "Weight, horsepower, weight, styling, weight, handling, weight, engineering." Fink is the president of Advanced Engineering, Inc. in Media, Penn., where lighter is, apparently, better. AEI is responsible for the extremely light, extremely powerful car on these pages.
One look at the Laminar Concepts Viking SRX-7 and it's clear its creators are serious about their mission: To build a very loosely based Lotus Seven replica capable of winning every contest of automotive performance. To do so, they use a less-is-more philosophy, which makes the late Colin Chapman look like a conservative ninny by comparison.
Let's put the SRX-7 in perspective. Fink claims it weighs 1,710 lbs and packs 450 or so horsepower. If true, those figures put the SRX-7 worlds away from virtually any other street car and only a notch or two down from a Champ car in the power-to-weight department. Even with a 20 percent BS factor, those numbers are serious enough to make Chapman soil his knickers and give this two-seater a real shot at winning every performance test in the USCC.
The SRX-7 is constructed of 4130 tubing and covered in structural composite bodywork, so it does have some similarities to the original Lotus Super Seven. However, the likeness ends there. Power comes from a remanufactured and internally stock Mazda 13B rotary, which has all the right parts to make the claimed power figure. A single-turbo conversion using a custom-built, equal-length stainless-steel turbo header feeds the Turbonetics T04B 60-1 turbocharger. An HKS wastegate, 3-inch exhaust, high-flow catalytic converter and twin Walker Dynomax mufflers complete the exhaust system.
On the cold side of the turbo there's an air-to-air intercooler, which is force-fed cool air by a large hood scoop and extensive composite ducting. The stock intake manifold is Extrude Honed for better flow and the throttle body is ported. There are two stock RX-7 fuel pumps drawing fuel from the 10-gallon Fuel Safe fuel cell. Fuel is fed through -6 lines to the 550 cc/min stock injectors and an additional 1600 cc/min secondary injector. An A'PEXi Power FC stand-alone ECU controls all engine parameters except boost, which is handled by A'PEXi's AVC-R.
The rear suspension started life as a complex multi-link, semi-trailing-arm type from a second-generation RX-7, but has been heavily modified. All rubber bushings are replaced with spherical bearings. The geometry is changed, allowing for a modified roll center, as well as adjustable camber and toe. The front suspension consists of custom-built upper and lower control arms. Damping is handled by triple-adjustable Penske shocks at all four corners. And the SRX-7 rolls on the stickiest street tire known to man-the Hoosier R3S03, measuring 245/35ZR-18 in front and a staggering 275/35ZR-18 in the rear. Brakes are equally serious. Since the SRX-7 is essentially a designed-from-the-ground-up package, Fink uses a Wilwood pedal assembly with a Tilton master cylinder. Four-piston Wilwood Superlite calipers and 11.75-inch rotors make up the front brake hardware, while stock second-generation RX-7 calipers and 10.5-inch calipers are used in the rear.
On paper, the Laminar Concepts Viking SRX-7 has all the right goods to be a serious contender for the "Ultimate" title. Weaknesses? Well, history tells us that turbo rotaries are extremely sensitive to poor tuning and heat, and then there's the unknown issues that arise with any homebrew special like the SRX-7. Fact is, history hasn't been very kind to men like Jerry Fink who have the ***** to build their own car. Anyone remember Jerry Weigert's Vector? DeLorean's DeLorean? Bricklin's Bricklin? At least Fink has physics firmly on his side. Tune in next month to see if this unique machine has what it takes.
LAMINAR CONCEPTS VIKING SRX-7
ENGINE
Engine Code: 13B
Type: Turbocharged and intercooled, two-rotor wankel
Internal Modifications: None
External Modifications: Single turbo conversion with custom stainless-steel header and Turbonetics T04B 60-1 turbocharger, HKS wastegate, 3-inch exhaust with high-flow catalytic converter and twin Dynomax mufflers, upgraded intercooler, Extrude Honed intake manifold, ported throttle body
Engine Management Modifications: A'pexi AVC-R boost controller, A'pexi Power FC ECU, two RX-7 fuel pumps, 1600 cc/min additional injector, Essex fuel pressure regulator
DRIVETRAIN
Layout: Front/mid engine, rear-wheel drive
Drivetrain Modifications: Centerforce clutch, aluminum flywheel
SUSPENSION
Front: Penske three-way adjustable coil-overs on custom upper and lower control arms
Rear: Penske three-way adjustable coil-overs on modified second-gen. RX-7 multi-link/trailing arm combo
BRAKES
Front: 11.75-in. cross-drilled rotors, Wilwood Superlite four-piston calipers, Hawk pads, braided stainless steel lines
Rear: 10.5-in cross-drilled rotors, single-piston sliding RX-7 calipers, Hawk pads, braided stainless steel lines external
EXTERNAL
Wheels: BBS RK 18x8.5-in. front, 18x10-in. rear
Tires: Hoosier R3S03, 245/35ZR-18 front, 275/35ZR-18 rear
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