99 RX-7 Engine Updates
By Jack Yamaguchui, SAE Asian Editor
From SAE Automotive Engineering International, September 1999
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Mazda'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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