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Rotary returns with a better, powerful hum
Mazda brings back the Wankel in its new RX-8
Friday, January 24, 2003
WHAT A WANKEL: At 247 horsepower, the RX-8's 1.3-litre, high-output rotary boasts the highest specific output per litre for a naturally aspirated engine.
While several companies have toyed with the rotary engine -- or Wankel to give it its proper name -- (General Motors contemplated powering the concept car that was to become the Corvette with a four-rotor motor and Suzuki offered the RE-5, a rotary-powered motorcycle back in the '70s), Mazda is the only one to have persisted with its development. The grind to perfect its operation has been a long and certainly difficult one, but, as used in the new RX-8 sports car, the work has been well worth the wait -- a legitimate claim to the highest specific output per litre for a naturally aspirated (non-turbocharged or supercharged) engine.
The rotary design has several key advantages over a conventional engine. Chief among its attributes is that, unlike an ordinary engine, it does not have to change reciprocating movement (up and down) into the rotary motion needed to power a vehicle. Not only is the design more efficient, it is a very smooth operator that is willing to rev to dizzying heights without sounding as though it is within one revolution of self-destructing.
It is also a mechanically simple design as there are but 10 key components. For example, there are no intake or exhaust valves (or valve springs, camshafts and timing chain for that matter), and so the manner in which the engine is fed its air/fuel diet and then gets rid of the waste can be likened to the two-stroke motor that powers many lawn mowers.
Instead of opening the intake valve mechanically, which requires work and energy, the rotor in the Wankel engine uncovers the port as it spins in its housing. From here on in, it operates as a four-stroke engine (meaning there are the usual squeeze and bang phases) until it uncovers the exhaust port. It really is a simple, if technically complex, design.
Not all has been sweetness and light with the rotary engine, however. To begin with, the apex seals (which function in the same manner as a piston ring to seal the combustion chamber) have been a perennial weak point.
Believe it or not, the original engine used a seal fashioned from horse bone (yes, as in a small part of the thighbone from a four-legged nag).
Being soft, the life expectancy was short and the time needed to replace seals was long and expensive. Later models replaced the bony seal with a carbon insert. These lasted considerably longer, but it still had a life cycle well short of the expected norm. The present rotary uses a steel insert, which, in the case of the RX-8's engine, is guaranteed for 240,000 kilometres, bringing a respectable wear cycle.
The other drawback was that the rotary engine was a dirty engine by almost any standard. Here, however, the Renesis engine marks a significant (dare I say quantum) step forward in the evolution of the breed. The engine now uses a new side exhaust port design (as opposed to its previous peripheral placement) that has twice the open area. As well as easing the exhaust's exit, it allows the intake port to be enlarged by 30%.
The new design also eliminates the overlap between the intake and exhaust ports. This means the exhaust cycle ends before the intake port is uncovered, which improves fuel economy by a staggering 20%. It also means far fewer unburned hydrocarbons are dumped into the exhaust stream, allowing the engine to meet the stringent Low Emission Vehicle II standards.
The latter is notable as the rotary engine cannot employ any of the cam phasing technology being widely used to cut emissions in conventional engines. The ports are machined into fixed locations within the engine.
When the RX-8 arrives, the Renesis rotary engine will be offered in two forms, a stock unit and a high-output (HO) version. The base motor develops 210 horsepower, while the HO ups that to a worthwhile 247 stallions. The only real difference between the engines is found in the intake plumbing.
The base engine's intake system's aim is to develop better mid-range torque, which makes sense given it will only be offered with the automatic transmission when the first RX-8s go on sale in late April. The HO engine bumps the redline from the base's 7,500 rpm to 9,000 rpm and so the plumbing is altered to emphasize horsepower, particularly at the top end.
The more noticeable difference shows up when you compare the torque figures -- although given that the rotary is best described as being akin to a very, very, short-stroke engine, the numbers are naturally low. Where the HO needs to hit 5,500 rpm to reach its peak torque of 159 pounds-feet, the base unit spits out 164 lb-ft at 5,000 rpm. A small difference granted, but in everyday driving it is enough to be termed a tangible benefit. Where the lower torque output would normally be considered a pretty big drawback with a conventional engine, it is significantly less so in the rotary. And it goes back to the simple design and the free-revving nature this allows -- although the Renesis's lighter rotors and flywheel certainly help.
Once through a minor lazy stage at the bottom of the rev range, the Renesis spins up to speed so quickly it masks the torque deficiency exceptionally well. Indeed, driving the RX-8 in the hilly terrain around Carmel, Calif., last week proved it to be remarkably tactile, pulling well enough once over 2,500 rpm.
The reason is not difficult to understand. Regardless of the method of calculating the displacement, Mazda can legitimately lay claim to the highest specific output per litre for a naturally aspirated engine. (There are two displacements -- the first is Mazda's at 1.3 litres; the other puts it at 1.96L. The rationale for the second displacement boils down to the fact a conventional engine sees the crankshaft turned twice per complete cycle. The rotary engine turns the crank three times per cycle and so many up the rotary's calculated displacement by a third.) Based on Mazda's displacement claims, it would be 190 hp per litre. Using the other -- and arguably more accurate -- method, it still generates 126 hp per litre, or four more than the Honda S2000, the acknowledged champ.
The other thing noticed is the delightfully different sound the engine makes when working. Simply, the term "rotary roar" is an oxymoron. Rather than roaring, it "hmmms" (this being a reference to the early commercials where Mazda used this metaphor to differentiate the rotary engine from its conventional rivals) its way up the rev range.
Only a discreet beeping noise warns that you are about to blast through the 9,000-rpm redline -- it really is that smooth in oper-ation. The one thing left to wish for is a turbocharged Renesis. Thankfully, Mazda is keeping suspiciously mum on this one, meaning it is more than a distinct possibility.