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RENESIS-The Future of the Rotary Engine
Mazda is synonymous with the rotary engine. The company and its innovative product have been inseparably linked since the release of the first rotary powered 'Cosmo Sports' coupe in 1967. With the world spotlight forever shining on the progress of the rotary engine - and with the introduction of the Mazda RX-8 in 2003 - Mazda will continue to be recognized world-wide for their technical achievements and driving success with rotary technology.
Ever since the first shipment of R-100 (10A) coupes back in 1969, enthusiasts have been in love with this remarkable engine and the cars they've powered. The RX-2 (12A) achieved a ‘giant killer’ reputation at a time when the V8 was unchallenged on the road. The RX-3 (12A) followed, proving very popular both on and off the race track. A new level of refinement was created with the RX-4 (13B) and virtually no other car could match its performance and luxury package at the same price. In 1976, the Cosmo (13B) gave enthusiasts an even newer level of luxury in a coupe with performance to match its looks.
Mazda has always stayed in touch with its buyers and has been a leader in innovation. In 1978 the first generation 12A powered RX-7 took the world by storm, exceeding production expectations and causing traffic jams at the dealerships. The rotary engine had found its true home and Mazda had found the heart of the sports car lover. The Mazda RX-7 achieved immediate success with "Car of the Year" awards across the globe and major racing victories at events such as SPA and Daytona, proving just how formidable the rotary could be and giving cause to the many traditional racers changing their V8 cars for rotary power.
A new fuel injected 6 port version of the race proven 13B rotary became standard equipment for the second generation or series IV Mazda RX-7 of 1986. Moving up market to become a more civilized sports car, the 13B rotary was given a turbo option boasting 182 HP by employing a newly developed twin scroll turbocharger to take full advantage of the engine's strong exhaust pulsing. The turbo was obviously the production ‘rocket’ factory had intended it to be and was now competing against the established marquees on the world market. The Mazda RX-7 was voted "The sports car to own for the everyday driver" by much of the world's motoring press. Refining an already excellent product, the 1989 Mazda RX-7 kept Mazda ahead of the pack of ever emerging sports cars. With 200 HP on tap from the totally re-developed 13B turbo engine, Mazda RX-7 sales again surged ahead of the field.
In 1991 Mazda achieved arguably the ultimate accolade for the rotary engine, victory at the Le Mans 24 Hour. It was a first for a Japanese car manufacturer and a first for the rotary engine. The Mazda 787B prototype sports car took on the world's best for 24 grueling hours to finish all three entered cars in 1st, 6th and 8th position. Producing over 700 hp the R26B quad rotor engine was peripherally ported and incorporated steeples variable induction plus three plugs per rotor (instead of the usual two).
It is through the severity of racing in events such as these that the current generation rotary engine has evolved.
When designing the third generation Mazda RX-7, Mazda's engineers knew what they wanted - a no compromise sports car that could take on the world and win. To achieve this the world's best engine had to be made better. A 20 percent jump in power from the series V Mazda RX-7 engine to 255 HP with a weight reduction to 2789 LBS had the new series VI Mazda RX-7 establish itself as the up market leader. The world's best selling sports car had now become the best ‘value for money’ sports car. Mazda chose to race the Mazda RX-7 from 1992 until 1995 under production car rules winning the prestigious Bathurst 12 hour endurance race each time whilst toppling big buck challenges from various Porsche models and other marquees. Recently a demonstration of the Mazda RX-7's rotary power and amazingly smooth aerodynamics where shown when a three rotor 13G powered version tamed the 1000 hp under its hood to set a new class speed record of just under 400kmh on the salt flats of Utah.
Today's Rotary Engine
The 13B REW twin-rotor engine fitted to all third generation RX-7s can trace its origins to 1974 and the RX-4.
A lot has changed through the years of development. As fitted to the 1974 RX-4, the carbureted 13B produced 127 HP @6500 rpm and 128 ft-lbs of torque @4000 rpm. Later versions employed a unique sequential twin turbo system to produce 255 HP at 6500 rpm and 217 ft-lbs of torque @ 5000 rpm. Torque output was increased throughout the rev range with as much as 181 ft-lbs available at a low 2000 rpm. Maximum RPM has been raised to 8000 and the rotor's compression ratio of 9.0:1 necessitated premium grade unleaded petrol.
The engine inherited the basic 13B geometry and epitrochoidal dimensions of 654 cc for each of its two rotors. However in 13B REW configuration, many of the rotary's major mechanical and electronic systems received extensive modification. Among the many internal upgrades were a thin wall cast-iron rotor with fully machined combustion recesses to ensure uniform combustion. Apex seal slots were hardened to resist wear. Modifications have also been made to the aluminum rotor housing around the "hot spot" spark plug area for more coolant flow. The engine's induction, exhaust, cooling and lubrication were modified or redesigned when compared to the series V RX-7 engine.
Cooling and lubrication are vitally important to an engine producing the power of the 13B REW. Both rotors are kept cool by splashing them internally with oil. Internal lubrication is via an electronically controlled metering system that reduces oil consumption by 25-50 percent compared to the previous method of supply to the intake and trochoid chamber combined.
Lubrication is fed under high pressure to the eccentric shaft bearings via a high-efficiency multi port rotary pump. The heated oil is then sent through two oil-coolers (one in each of the nose vents) before being reused. A lightweight aluminum and plastic radiator, fully shrouded and slanted sharply to lower the Mazda RX-7's nose was fitted up front. Maintaining the correct temperature is a pair of three stage thermo fans and keeping the flow is a lightweight aluminum water pump.
The fire in the new 13B REW was supplied by the world's first volume-production sequential twin turbocharger system. It produced more power than a conventional twin turbo setup where both turbos boost at once, and suffered far less turbo lag. The advantage of the sequential system was the ability to utilize a small and large turbo at the same time. At low rpm the 51mm diameter turbo with its "impact" blade design spools very quickly, providing boost from as low as 1500 rpm. At a calculated point the second 57mm diameter turbo is switched on, giving full boost all the way to the 8000 rpm redline.
A major downfall of other sequential twin turbo systems is the transition from single to double operation. This is due to the second turbo not spinning fast enough when it's called on, resulting in the engine "coughing" momentarily.
Mazda solved this problem by spooling the second turbo to a pre-boost speed of around 100,000 rpm with exhaust gas circulation. When the time comes for the second turbo to cut in, a bypass valve is shut to "surge" the compressor, spinning it to over 140,000 rpm. This then assures a smooth coupling with the primary turbo. Once at the required speed, it receives a full share of exhaust flow to add its boost with the primary turbo.
The twin turbo chargers are fitted to a "dynamic pressure" cast iron exhaust manifold shaped to minimize the gap between the exhaust ports and the intake of the turbos, improving boost by as much as 35 per cent. The 13B-REW was fitted standard with an air-to-air intercooler mounted above the radiator with separate ducting.
The ECU controlled fuel injection system uses air density measurement instead of the common air flow metering to allow a smoother air flow and more precise fuel management. Coupled with the precise shape of the plenum chamber and inlet tract length, the added benefit of the increased power and throttle response is the additional improvement in fuel consumption for the city/highway cycle.
Two injectors per rotor look after the varied engine loads. The primary injector takes fuel from the side of the injector body instead of the top. The resulting reduction in fuel travel through the injector body gives a quicker and more precise engine re-start after high speed/load applications. The secondary injector is mounted upstream of the inlet tract operating during mid and high rpm. Change over to twin injector operation takes place around 2,750 rpm or lower if the load commands it.
Ignition timing of the four platinum tipped spark plugs (two per rotor) is controlled via the ECU which will automatically retard timing if detonation is detected. Lightweight high energy coils are utilized to take advantage of the rotary's unique combustion characteristics.
Ensuring the emissions are lower than required, the use of a double-skin exhaust manifold and high performance three way catalysts gives the lowest flow resistance available and lower emissions. A single muffler is used for noise reduction and the whole system weighs considerably less compared to the 3rd generation RX-7.
RENESIS - The Future of the Rotary Engine
For rotary engine enthusiasts, the next exciting phase in the great engine's history has already begun. At the Tokyo Motor Show in October 1999, Mazda unveiled the RX-Evolv, a concept vehicle which later evolved into the MAZDA RX-8 four-door, four-seat sports car unveiled in January 2001 at the North American International Auto Show (NAIAS) in Detroit. The Evolv and the MAZDA RX-8 shared many advances in common, not the least of which was the latest version of the rotary engine called "RENESIS."
The Mazda RX-8 with its RENESIS engine made its debut in 2003.
When developing the RENESIS, Mazda's engineers aimed to retain power output on a par with the turbocharged 13B-REW, the rotary engine that powers the Mazda RX-7, while offering improved fuel economy and reduced emissions.
Side Intake and Exhaust Ports
Unlike previous mass-production rotary engines, which employed side intake ports and peripheral exhaust ports, the naturally aspirated RENESIS has intake and exhaust ports in the side housings. This configuration eliminates overlap between the opening of the intake and exhaust ports, enhancing combustion efficiency. The intake ports are 30% larger and their timing has been changed to make them open sooner than in previous designs. Moreover, the exhaust ports open later, resulting in a longer power (expansion) stroke and providing radically improved heat efficiency.
At the same time, the RENESIS uses a six-port induction (6PI) design, in which each rotor employs three intake ports, and a variable intake timing mechanism. Under this system, dedicated high-speed intake ports begin to operate when the engine operates at high-rev levels. This makes it possible to use the intake's dynamic effect at high and low speeds to maximize compression efficiency.
Unlike the single peripheral port per rotor of previous designs, the RENESIS uses two exhaust ports per rotor. This produces a combined exhaust port opening area nearly twice as large and results in a substantial reduction in exhaust resistance.
The rotors have also been made lighter for better performance at high-rev levels. The rotors used in the RENESIS weigh approximately 14% less than those used in the engine that powers the Mazda RX-7, which is sold in Japan.
These enhancements provide high output rivaling the power of turbocharged rotary engines with linear power characteristics from the low- to the high-rev range.
The increased heat efficiency resulting from zero overlap between the opening of the intake and exhaust ports makes it possible for the RENESIS to run on a leaner fuel mixture than conventional rotary engines. When idling, the RENESIS consumes 40% less fuel than the latest production rotary engine.
Reciprocating piston engines generally use a richer fuel mixture under high-speed and high-load conditions to prevent knocking. In contrast, rotary engines do not require a particularly rich fuel mixture under these conditions due to their special combustion characteristics. In addition, the RENESIS achieves nearly complete combustion over the entire speed range thanks to its high compression ratio and the use of new fuel injectors designed for improved fuel atomization. These enhancements allow the RENESIS to run on a leaner fuel mixture than conventional rotary engines from the low to the high-rev range. The result is the power and performance of a sports car engine and reduced fuel consumption.
Due to their configurations, rotary engines produce less nitroxide (NOx) than reciprocating piston engines, but they also tend to produce large amounts of unburned hydrocarbons. The side exhaust layout used in the RENESIS prevents unburned hydrocarbons of the combustion chamber housing from escaping to the exhaust ports.
Instead, they are carried over and burned in the next combustion cycle, dramatically reducing emissions. In addition, air injection directed into the combustion chamber increases the efficiency of the exhaust reaction, significantly over Mazda's existing system during engine startup. Together with the double-skin exhaust manifold, the new layout makes the exhaust much hotter when it reaches the catalytic converter, speeding the converter reaction for clean emissions from the moment the engine is started.
Low Center of Gravity
With their low center of gravity rotary engines have an advantage over reciprocating piston engines. We have exploited this benefit by using a special oil pan configuration to make the engine's center of gravity even lower.
Called the "wet sump" layout, it uses a baffle (dividing panel) within the oil pan to prevent oil from collecting on one side during cornering. This makes it possible to use a shallower oil pan. The new oil pan is only about 40 mm thick, about half the thickness of conventional designs.
Reduced Oil Consumption
In a rotary engine, oil is supplied directly to the interior walls of the combustion chamber to lubricate the "apex" and "corner" seals. Engineers kept the paths which supply oil in the RENESIS as small as possible, and we have redesigned the oil supply nozzles to improve their efficiency. With these enhancements, the RENESIS consumes about half as much oil as a conventional rotary engine.
Superb Response and Sound to Thrill the Senses
The RENESIS achieves a sophisticated balance between high revs and high output, on the one hand, and fuel economy and low emissions, on the other. In addition, engineers are working to enhance the performance and to realize the high degree of reliability and durability required in a mass-production sports car. Mazda's engineers wanted to achieve high output in the range of 238 horsepower.
Unlike rotary engines equipped with peripheral exhaust ports, the side layout of the RENESIS produces clear, transparent high tones and powerful low tones. Mazda recognizes engine sound as a key element in any sports car, and engineers are working to ensure that the engine produces a satisfying roar as you depress the accelerator.
TEST DRIVE TIPS
Rotary engines by their very nature, tend to operate best at higher rpm. Start your test by shifting based on feel rather than by the numbers on the tachometer. Most likely you will find yourself both shifting and cruising at a higher rpm than usual due to the rotary's unusual smoothness at high rpm.
Such is the smoothness that Mazda has provided an audible "beep" when redline is reached to remind you to shift. Listen for it, as it is easy to reach redline sooner than you expect.