200910 Re Zoom Rotary

8/3/2019 200910 Re Zoom Rotary

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4This Feature reprinted from the September 2000 issue of Import Service magazine.





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Rotary Engines

As the rotor turns clockwise in the curved chamber the bearing in its center describes a faster

clockwise movement turning the eccentric shaft three times for each single orbit of the rotor.



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from that fat triangular rotor to thatwobbling output shaft.

In the center of the rotor are twoelements: an internal gear and aninternal bearing. The gear engages theteeth of a stationary gear bolted infixed place at the center of the engine.The eccentric output shaft mainbearings journals fit through thebearings in the middle of these fixedgears. The stationary gear has exactlytwo teeth for every three on the inside

gear of the rotor 34 for 51 in theexample in the photos. There is no greatload on these gearteeth but they serveto keep the rotor aligned so itsmovement will keep the apexes movingexactly along the epitrochoidal surfaceof the chamber.

Notice this though: As the rotor

turns clockwise in the curved chamberthe bearing in its center describes afaster clockwise movement turning theeccentric shaft three times for eachsingle orbit of the rotor. You’ve alreadynoticed one more thing I’m sure: Thatmeans that while the rotor may not beturning particularly fast in the chamber

(a third the output shaft speed) theoutput shaft is turning quite fast in bothits main and eccentric journal bearings.These bearings between eccentric andthe stationary gear bearings andbetween the rotor and the eccentric‘crank’ journals are the most critical fitsin the engine making the importance of

regular clean oil obvious.

works you understand the basics of multicylinder powerplants. Morecylinders or rotors let you smooth outthe power delivery somewhat andchange the shape of the engine to fit ina car somewhat better but eachcylinder or rotor works the same way.

The rotor a fat equilateral trianglein cross-section turns in an waistedtwo-node epitrochoidal chamber thecurved surface of which corresponds tothe path of the apex tips of the rotor as

it turns. This keeps the threecombustion spaces between the rotorand the chamber separate from oneanother at all times. As each of thethree spaces goes around the chamberonce it expands and contracts twiceallowing it to go through each of thefour engine cycles: intake compression

power and exhaust as it uncovers andthen covers the corresponding ports.Since there are three chambers perrotor there are three power strokesfrom that rotor per complete rotation.There are also three power strokes fromthe other rotor sixty degrees out of phase with the one we can see making

six power strokes for the enginecomplete.

That sounds like it ought to makefantastic torque six power strokes perrevolution just what you’d have with atwelve-cylinder engine. But it’s not. Therotors are not the output or eccentricshaft. Let’s see how the torque

developed by the combustion burn gets



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Balance shafts on the eccentriccan perfectly offset the offset weight of the rotors insuring the Wankel’ssurprising smoothness. What’s morebecause of the triple-multiplied speedof the output shaft each power strokelasts for 270 degrees of output shaftrotation 90 degrees longer than the

equivalent for a piston engine. So we

With the engine opened up for overhaul you can see the epitrochoidal chamber with the convex triangular rotor. Around the outside are passages for the engine through-bolts and the water jacket. Inside the rotor you can see the eccentric shaft bearing and the mesh between the rotor teeth and the stationary teeth. The circular seal around the center of the rotor

keeps oil inside to cool the rotor surfaces.

Rotary Engines



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during operation the volume dependingon load speed and temperature.

Instead of intake and exhaustvalves the engine merely has intake andexhaust ports the rotors uncover at theright position in their circuits almost likea two-cycle engine. Since the chamber is

growing larger while the intake port isuncovered it develops a partial vacuumthat draws the air-fuel charge in just likea piston engine. There is one unusualfeature when the rotor passes theexhaust port: The pressure drophappens right now so the acoustic pulsefrom the engine is very sharp. Even

people who like loud exhaust notes find

Overheating a rotary engine can be as catastrophic as overheating any other. Here the housing is warped from the temperature.

It doesn’t take much of a crack to lose compression. This broken apex seal meant two lobes of the rotor had no compression.

have two separate and significantsources of smoother torque delivery.

Fuel and spark work basicallythe same way as on piston enginesso we won’t go through those here.

Take note that each rotor has twospark plugs to avoid the problem that

at some angles of spark advance thechamber is divided by the narrow pointof the housing — the ‘waist’ —masking part of the mixture from flamepropagation. Because there is noequivalent of crankcase splash belowthe piston rings to lubricate the cylinderwalls the engine also includes a device

to add some oil to the rotor chamber



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If there is damage to the apex seal there is likely to be damage to the surface it rides on as well.

The system injects a small amount of oil to lubricate the apex seals. When that system breaks down as here you’ll see damage to the housing caused by the

unlubricated apex seal.

Rotary Engines



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this sudden sound pulse too much andfit their rotaries with mufflers.

One feature of the engine’s designis how the rotors themselves arecooled: by oil only. This requires aseparate oil cooler of course because asignificant portion of the engine’swaste heat goes into the rotors. An oilthermostat keeps the rotors frombecoming too cool for their job undersome circumstances because a lowtemperature would cause poor fuel

economy fuel condensation and carbondeposits (of which more later). It alsorequires more attention to the qualityof oil and the frequency of oil changesthan some carowners are disposed topractice. And since the oil cooler is socritical to the operation of the enginemake sure it never gets clogged with

leaves or road debris.

Compression Testing

The most conclusive test thereis for the mechanical condition of apiston engine is a wet and dry

compression test. Doubtless you runsuch tests regularly for customers’cars in your shop. The rotary enginerequires the same kind of compression conditions as the pistonengine but measuring it is a morecomplicated task. It’s simplest with aspecial compression tester available

from Mazda but that tester is not so

inexpensive you’d be tempted to buyone to check one engine. Other thanshipping the car off to the dealer torun that test it is possible to useyour regular compression testervfor an approximation.

First remove the Shrader valvefrom the tip of the tester so the testerwill not retain the pressure from onecompression stroke to the next.Because a single port tests three lobesof the rotor you’re not going to find

things out in the detail you’d expect ona piston engine. It doesn’t matter whichplug you remove to do the test butobviously the other one must remain in.Watch the needle on the gauge verycarefully: It should oscillate to about 75or 80 psi. for each passing lobe. It won’tgo higher because you’re effectively

seeing only the first compression strokeof each lobe — this is a compressiontest on a piston engine in which youonly get one puff per cylinder. Such atest is not useless however; there ismuch to be learned from that first puff!

If you have more than onecompression tester use the one with

the shortest or no connecting hose toeliminate the extra internal volume.Don’t use a high quality oil-dampenedgauge because you’ll never see anyneedle pulses.

If there is damage to an apex sealthis will dump the compression onboth lobes on either side of it so you’ll

see only one pulse per cycle between



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the remaining two good seals. There’snothing to be gained from a wetcompression test since there are novalves to compare to the piston ringsfor sealing. In this you’re testing side

seals and apex seals only. If you findzero compression on one of the rotorhousings check the lobes with aninspection mirror or a borescope. It ispossible someone put spark plugs withtoo long a reach into the engine. Justas spark plugs with too long a reachcan punch holes in pistons in a

conventional engine spark plugs with

too long a reach can snap off apexseals on a rotary. Similar damageresults if someone drops a nut orcarburetor check-valve ball down theintake. If you turn the engine slowly

while watching through the spark plugholes with an inspection mirror youcan see the damage. If you find onewith a missing or seriously damagedair filter that could have the sameconsequences. Then that one’s off tothe rebuilder.

Very often you will find a rotary

engine with poor compression not

Foreign objects bolts and lock washers won’t blow harmlessly through the Mazda rotary any more than they will through a piston engine.Notice the carbon buildup on the one rotor too.

Rotary Engines



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200910 Re Zoom Rotary