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.