25th August 2006

Imagine, if you will, buying a brand spanking new car at a cost of C$40,000 and being told, right there in the owners manual, to check
your oil with every fill of gasoline!

And then finding out, that sure, enough a litre (quart) is needed every 500 Kilometres (300 miles)!

As a mechanical engineer, the Wankel rotary engine has always fascinated me.

But as a garage owner who is wont to be asked for opinions, I advise staying away from anything that is using the wankel engine,
as is the current C$ 40,000 Mazda RX8.

I get calls from people who need a rebuilt rotary engine. I advise them to call their local Mazda dealer, because all the engine rebuilders
I know will not touch the rotary. Mazda has an overhaul factory in the USA and I don't doubt there are racing shops that will rebuild
rotaries, but the cost is astronomical, in either case.

Since NSU originally introduced the engine as a commercial proposition in 1964, no one, it seems, has been able to really tame
the tip seals that make this engine so unique. They leak and they leak continuously. The rest of the car is quite conventional and
serviceable, but the engine is a horse of a different colour entirely.

In yet another instalment of the Mazda RX-8 customer satisfaction drama, the manufacturer will soon announce a voluntary recall
on 2004, 2005 and some 2006 model year RX-8s. Mazda has confirmed that owners will be receiving notices in the next month
or so, as the company is doing a staggered roll-out across North America. Mazda says that the repairs needed for each RX-8 will
vary based on tests performed by each dealer. Apparently each car will receive a vacuum test to check for oil leaks, and those
vehicles suffering from sealing problems will receive new Renesis side-port rotary engines.

The head of product development and quality at Mazda, says that the company understandably, does not want dealers tearing
into the engines and instead will send the defective motors to its remanufacturing plant in North Carolina.

Each vehicle will also receive a check of its starting system, as that has also been causing trouble.

Mazda claims that the problem is brought on by hot climates or the usage of synthetic oil, which I sincerely doubt.

If the oil is allowed into the combustion chamber, they say, it eventually reaches the catalytic converter and causes a malfunction
of that emissions-control device.

Wankel engines theoretically have have several major advantages over reciprocating piston designs, in addition to having
higher output for similar displacement and physical size. Wankel engines are considerably simpler and contain far fewer moving parts.
For instance, because valving is accomplished by simple ports cut into the walls of the rotor housing, they have no valves or complex
valve trains. The elimination of rotating parts not only makes a Wankel engine much lighter (typically half that of a conventional engine with equivalent power), but it also completely eliminates the reciprocating mass of a piston engine with its internal strain and inherent vibration
due to repetitious acceleration and deceleration, producing not only a smoother flow of power but also the ability to produce more power by running at higher rpm.

However, this theoretical advantage is obviously being overshadowed by reciprocating engines that can rev smoothly to 10,000 RPM,
as in the case of the Honda S2000. And Formula One engines now routinely touch 20,000 RPM when on full song.

The Wankel engine is ideal for use in aircraft and is often used in drones and stealth aircraft. But, of course, the military doesn't care
if an engine change is necessary after every flight, they have the funds to so do.

The simplicity of design and smaller size of the Wankel engine also allow for a savings in construction costs, compared to piston
engines of comparable power output, provided of-course that you don't have to recall them all and replace most of them
- goodbye cost advantage.

The shape of the Wankel combustion chamber and the turbulence induced by the moving rotor prevent localized hot
spots from forming, thereby allowing the use of fuel of very low octane number without pre ignition or detonation, a particular
advantage for Hydrogen cars. Mazda has recently placed a hydrogen-burning rotary engine in one version of its RX-8 sports car.
This feature has also led to a great deal of interest in countries where high octane gasoline is unobtainable.

But the design of the Wankel engine requires numerous sliding seals and a housing that is typically built as a sandwich of cast iron
and aluminium pieces that expand and contract by different degrees when exposed to heating and cooling cycles in use.
These elements lead to a very high incidence of loss of sealing, both between the rotor and the housing and also between the
various pieces making up the housing. Further engineering work by Mazda may have brought these problems under control,
but the company is now confronted with concerns over both hydrocarbon emissions and a rise in the cost of gasoline, the two
biggest weaknesses of the Wankel engine, even if it wasn't also an oil burner.

Just as the shape of the Wankel combustion chamber prevents preignition, it also leads to incomplete combustion of the air fuel
charge, with the remaining unburned hydrocarbons released into the exhaust. At first, while manufacturers of piston-engine cars
were turning to expensive catalytic converters to completely oxidize the unburned hydrocarbons, Mazda was able to avoid this cost
by enriching the air/fuel mixture enough to produce an exhaust stream which was rich enough in hydrocarbons to actually support
complete combustion in an enlarged open chamber in the exhaust manifold without the need for a catalytic converter, thereby
producing a clean exhaust at the cost of some extra fuel consumption.

Unfortunately for Mazda, their switch to this solution was immediately followed by a sharp rise in the cost of gasoline so that not
only added fuel cost to their design, but the basically lower fuel economy of the Wankel engine caused sales to drop alarmingly.

A related cause for unexpectedly poor fuel economy involves an inherent weakness of the Wankel rotor design when used
with conventional fuels. Some studies have indicated that at high speeds, the rate at which the volume of the combustion chamber
increases in the moments after ignition actually outpaces the expansion of the burning fuel. The result is that, at high speeds,
less useful energy is extracted from the same volume of fuel, as the exhaust has to expend time and energy "catching up" to the
rotor before it can accomplish any work. 

Unlike a piston engine, where the cylinder is cooled by the incoming charge after being heated by combustion, Wankel rotor
housings are constantly heated on one side and cooled at the other, leading to very high local temperatures and unequal
thermal expansion. This places high and probably unreasonable, demands on the materials used.

Most of these disadvantages have been solved in the Renesis engine of the RX-8. The exhaust ports, which in earlier Mazda
rotaries were located in the rotor housings, were moved to the sides of the combustion chamber.

This is an engine whose time is yet to come, if in fact, it ever comes at all. In the meantime, you may want to think twice before
buying either a new or used Mazda with a rotary engine.