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99 RX-7 Engine UpdatesBy Jack Yamaguchui, SAE Asian EditorFrom SAE Automotive Engineering International, September 1999Mazda's rotary engine production is at most a few hundred units a month, a fraction of what it used to be in the heyday of the Hiroshima-based company's ambitious "rotarization" movement. The rotary engine is alive in Japan, kicking harder than ever in the face-lifted and improved RX-7 sports car.The type 13B-REW twin-rotor engine inherits the 13B design, along with the epitrochoidal dimensions and geometry, and single combustion chamber volume of 0.65 L (40 cu. in.) (x two rotors = 1.3 L, thus the "13") from the predecessor whose roots trace back to 1974. In fact, the original type 13B shares two of the three key internal dimensions with the types 10A (1967) and 12A (1969). These are the "E" which is eccentricity, the amount of offset between the eccentric shaft (equivalent to the crankshaft) centerline and the rotor centerline; and R, which is generating radius, the distance between the rotor centerline and the rotor's apex (in the Wankel rotary, each rotor has three apices). In the three engines, these are E = 15 mm (0.6 in.) and R = 104 mm (4.1 in.). The earliest production engine, the type 10A, obtained a single chamber volume of 0.5 L (30 cu. in.) with a 60 mm (2.4 in.) wide trochoid chamber (B in the rotary equation). The 12A, which powered the first-generation RX-7, had the chamber width increased to 70 mm (2.7 in.) for a single chamber capacity of 0.57 L (35 cu. in.). The 13B has an 80 mm width (3.2 in.) width, obtaining 0.67 L (41 cu. in.). Why the "B" designation?There had been an odd engine, circa 1969, a twin-rotor unit with a single chamber volume of 0.65 L (40 cu. in.) with unique epitrochoidal geometry and inner dimensions: E = 17.5 mm (0.7 in.); R = 119 mm (4.7 in.); and B = 60 mm (2.4 in.). This engine was designed for a front-wheel-drive, specialty coupe, the R130 Luce Rotary, which required a shorter powerplant. The engine was given the 13A designation, as its total displacement was 1.31 L (80 cu. in.), so the later and wider-chamber, 1.31 L (79.8 cu. in.) unit had to do with the B designation.Nobuhiro Yamamoto, responsible for the rotary engine and vehicle development, confides that there are still some minor components for the latest rotary that can be found in the corporate parts bins with the prefix 0813, which was the design code of the very first production engine 10A, circa 1967! Mazda's rotary is indeed a small engine family.The 13B-REW in the third-generation RX-7, launched in late 1991, was a completely redesigned unit, with numerous internal and external modifications and improvements. The REW suffix indicates that it is a rotary engine supercharged by twin, sequential turbochargers. It was the world's first such turbo installation. The engine produced 190 kW (255 bhp) at 6500 rpm and 294 Nm (217 lb. ft.) of torque at 5000 rpm. Two years later, power output was increased to 198 kW (265 bhp).By unwritten decree and industrial agreement, no Japanese manufacturer offers passenger car models with engine output greater than 209 kW (280 bhp). Imported marques are not a party to this domestic consensus, so for them the sky's the limit. Some members of the industry limit output of their larger displacement or more powerful powerplants. Others try to attain the limit by tweaking up their engines. Honda, for example, has made the mark, adding 0.19 L to the Acura NSX's exotic quad cam V6.Now Mazda has joined the 280-bhp club. The latest 13B-REW produces 209 kW (280 bhp) at 6500 rpm, an increase of 11 to 13 kW (15 to 18 bhp) obtained above 5000 rpm. Torque output is also improved by 20 Nm (15 lb. ft.) in the critical mid-speed range above 2500 rpm, peaking at 314 Nm (231 lb. ft.) at 5000 rpm. New turbochargers, improved apex seal lubrication, freer exhaust, enhanced cooling, and more elaborate engine management are cited as bringing these benefits, while the engine's internals are inherited from the previous version.The turbocharger, two of which are used in the sequential setup, is a new Hitachi instrument with an abradable compressor sealing and an "ultra-high-flow" turbine design. The abradable compressor housing sealing has been in U.S. markets, Toyota being the first to use it in the MR-2 sports car, some years ago. It was one of the more promising items in the menu the Mazda's rotary engine designers had prepared a few years ago to get more power out of the 13B-REW, but, according to Yamamoto, the company was in a dire financial state, and the design team could not dare ask for the expensive turbocharger. The Toyota unit had plastic layer hot-sprayed onto the housing's surface, a methodology hardly suitable for large scale production, therefore costly.Hitachi has since come up with a new built-up construction of the compressor housing. A formed plastic inner sealing element is bolted onto the main housing, and then machined. The turbocharger is assembled, and run up to 100,000 rpm, abrading the inner surface of the plastic seal, obtaining the closest clearance between the compressor and housing. The new abradable turbocharger has an 80%-plus efficiency, whereas a typical instrument's efficiency is about 75%, according to a Mazda designer. This brings down the compressed air's temperature by 10 C, or about 10%, at the exit of the compressor. Air temperature is still in the region of 110 C (230 F), which is further cooled down by the air-to-air intercooler in the RX-7 installation.The latest trend in high-performance turbo technology is a "diagonal flow" turbine. Mazda asked Hitachi to come up with several candidates of this design, hoping it would boost the engine's output. They did not quite match the rotating piston engine's unique characteristics. Back to the tube, as it were, to research basic flow dynamics. The fruit of the toil is the "ultra-high-flow" turbine, with its blade length extended and its shapeless acutely curved, enlarging gas passage and reducing flow resistance. Turbo response in the low speed zone has been greatly improved. The turbocharger adopts a smaller diameter turbine, 50 mm (1.9 in.) now versus the previous unit's 51 mm (2.0 in.), again reducing inertia mass. The new "ultra-high-flow" turbine realizes about a 10% gain in efficiency, according to Mazda's turbo engineer. The twin ultra-high-flow turbochargers supply a maximum boost of 74.7 kPa (10.8 psi) at 6500 rpm to the previous high-flow instruments' 62.7 kPa (9.1 psi) at the same rpm.The rotary's reliability under the severest conditions was well proven in Mazda's competition activities in the late 80's and early 90's, including an outright win in the Le Mans 24-hour race for sports racing cars in 1991. A road car is subjected to a different kind of stress, said a Mazda designer responsible for the engine's innards, especially when the 13B-REW's output is increased to 209 kW (280 bhp). Possible problem areas are higher combustion temperature and pressure. The former could be dealt with by the cooling system's heat dissipating capacity. The later was thought to exert extraordinary pressure on the engine's gas sealing.Apex seal lubrication has become a critical issue. In a race engine, oil supply to the rotor housing by means of injection was precisely monitored and controlled, whereas in the production unit, a larger amount is supplied, just to be on the safe side. Some of the lubricant is fed into the trochoid chamber through a metering nozzle. The previous nozzle's oil passage was 2.0 mm (0.08 in.) in diameter. Negative pressure created in the rotor chamber would cause all the oil within the nozzle to be sucked out. When the engine accelerated rapidly, oil supply could not keep up with the speed. To prevent oil starvation, the previous system supplied a larger amount of oil to be on the safe side. In the new metering nozzle, the passage diameter has been reduced to 0.08 mm (0.003 in.), halving its volume of 0.0005 L (0.03 cu. in.). A new rubber seal is also inserted to fill a gap within the nozzle body where oil used to be sidetracked. Now, there is still some oil left within the nozzle after each suction, so that the lubrication system responds to the apex seal's requirement.In the exhaust system, the front tube gauge has been reduced by 0.5 to 1.0 mm (0.02 to 0.04 in.) so that flow passage is increased while retaining the tube's outer diameter. The main silencer has also been redesigned. These changes result in about a 10% reduction - about 13kPa (1.9 psi) - in exhaust gas resistance.The third-generation RX-7 had come off Mazda's rigorous development test programs on the bench and on the demanding Global Road Circuit section of the Miyoshi Proving Ground with flying colors. Yet, there was one arduous test left undone. When the car was taken to a race track near Tokyo known for its tight turns requiring short bursts of speed followed by fierce deceleration, the pride of Mazda's rotary rocket team quickly cooked its powerplant when pushed to the limit. Subsequent investigation revealed that air temperature at the entry area had risen as high as 50 C (122 F). Fresh air for the engine's consumption was taken from the single intake that also fed to the air-to-air intercooler. On wide-open driving, air flow reversed its course from the intercooler and went straight into the engine's intake. The intercooler was acting as an inter-heater! In the updated RX-7 with a designed fascia, fresh air is taken through a separate, dedicated duct guided by a newly installed air-guide. Air temperature at the engine's intake entry area has been halved to about 25 C ( 77 F), which adds about 7 kW (10bhp) to the output.The front-end's opening areas have also been increased by factors of 2.1 for the radiator opening, 1.8 for the intercooler duct, 1.8 for the oil cooler, and 1.6 for the front brake cooling duct intake over the previous RX-7. The radiator's core depth has been increased to 27 mm (1.1 in.) from the previous one's 25 mm (1 in.). The 209 kW (280 bhp) engine had its fin-pitch changed from 1.1 mm (0.04 in.) to 1.3 mm (0.05 in.). Twin cooling fans' blades have also been increased, one from five to seven blades and the other four to five blades, while the high-speed electric motors' consumption has been changed from 160W to 120W.The central computing unit operates on the same logic; however, its data parameters have been greatly increased. The CPU centrally and precisely controls air/fuel ratio, ignition timing and boost pressure. Mazda continues to offer the 194 kW (260 bhp) (and 190 kW (255 bhp) with automatic transmission) version of the 13B-REW, on the rational that not everybody wants 209 kW (280 bhp). All external modifications are shared with the higher-power engine, except the turbos and oil metering nozzles.On the chassis side, the updated RX-7's suspension has been recalibrated. There are three settings: the RS, powered by the 209 kW (280 bhp) engine, the intermediate R, and the touring-type RB. The RS is equipped with special Bilstein mono-tube shock absorbers. Spring rates are shared by the three versions.The RX-7 may now be fitted with an adjustable-rake rear wing with five alternate angles. At the standard one-degree rake, the front lift coefficient is 0.045 and the rear 0.000. At the extreme fourth setting of 14.5 degrees, The car generates a front lift coefficient of 0.053 and a negative rear lift coefficient of -0.075, pushing the rear end firmer onto the road surface.When Kenichi Yamamoto, retired Mazda Chairman/President, began developing Felix Wankel's rotating piston engine in the early '60's, his Rotary Engine Research Department had 47 "samurais" (the number is the same as that in the group of real samurais revenging for their disgraced master in feudal Japan). Over the decades, the division grew and then shrunk to a small team, now consisting of a handful of die-hard enthusiast-designers and engineers - all ten of them.Would the faster RX-7 return to the American and European shores? Unlikely, as the car has been absent from these markets where emission standards have been tightened, and in its current state, could not hope to realistically achieve. Nevertheless, the rotary movement within the Hiroshima company, now a member of Ford Motor Co.'s global family, is showing signs of more management attention, and likely revitalization, as a symbol of Mazda's technological prowess. Already, the development of a naturally aspirated, side-intake and exhaust port version of the engine is progressing well. It will power a concept sports car to be revealed at the forthcoming Tokyo Motor Show. The rotary is very much alive.By Jack Yamaguchui, SAE Asian Editor |
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