Wankel Engine - Design - Engineering - Fuel Economy and Emissions

Fuel Economy and Emissions

Just as the shape of the Wankel combustion chamber is resistant to preignition and will run on lower-octane rating gasoline than a comparable piston engine, it also leads to relatively incomplete combustion of the air-fuel charge, with a larger amount of unburned hydrocarbons released into the exhaust. The exhaust is, however, relatively low in NOx emissions, as combustion temperatures are lower than in other engines, and also because of some inherent Exhaust Gas Recirculation -EGR- in early engines; the higher the combustion temperature is, the higher the NOx emissions are (sir Harry Ricardo proved in the 20's that for every 1% increase of the proportion of exhaust gas in the admission mix, there's a 45°F reduction in flame temperature); this allowed Mazda to meet the United States Clean Air Act of 1970 in 1973 with a simple and inexpensive 'thermal reactor' (an enlarged open chamber in the exhaust manifold) by paradoxically enriching the air-fuel ratio to the point where the unburned hydrocarbons (HC) in the exhaust would support complete combustion in the thermal reactor; while piston-engine cars required expensive catalytic converters to deal with both unburned hydrocarbons and NOx emissions. This raised fuel consumption, however (already a weak point for the Wankel engine), at the same time that the oil crisis of 1973 raised the price of gasoline. Mazda was able to improve the fuel efficiency of the thermal reactor system by 40% by the time of introduction of the RX-7 in 1978, but eventually shifted to the catalytic converter system. According to the Curtiss-Wright research, the extreme that controls the amount of unburned HC in the exhaust is the rotor surface temperature, higher temperatures producing less HC. They showed also that the rotor can be widened, keeping the rest of engine's architecture, thus increasing displacement and power output. Quenching is the dominant source of HC at high speeds, and leakage at low speeds. Automobile Wankel rotary engines are high speed engines, however, it was shown that an early opening of the intake port, and long intake ducts can provide the required amount of torque at low RPM, and thus elasticity. The shape and positioning of rotor recess-combustion chamber- influences emissions and fuel economy, the MDR being chosen as a compromise, but which shape of the combustion recess gives better results in terms of fuel economy and exhaust emissions varies depending on the number and placement of plugs per chamber of the individual engine.

In Mazda's RX-8 with the Renesis engine, fuel economy is now within normal limits while passing California State emissions requirements, including California's Low Emissions Vehicle (LEV) standards. The exhaust ports, which in earlier Mazda rotaries were located in the rotor housings, were moved to the sides of the combustion chamber, the earlier problem of ash buildup in the engine, and thermal distortion problems side intake and exhaust ports had, was solved by adding an scraper seal in the rotor sides, and by some ceramic-made added parts in the engine. This approach allowed Mazda to eliminate overlap between intake and exhaust port openings, while simultaneously increasing exhaust port area. The side port trapped the unburned fuel in the chamber, decreased the oil consumption, and improved the combustion stability in the low-speed and light load range. The HC emissions from the side exhaust port Wankel engine are 35–50% less than those from the peripheral exhaust port Wankel engine, although Peripheral Ported RCEs have a better MEP, specially at high rpm and with a rectangular shaped intake port (SAE paper 288A).