Mazda Rx-8 Rotary History
October 25, 2008 · Print This Article
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 rotary engine will make 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.