RO­TARY-EN­GINE COM­PO­NENTS

New Zealand Classic Car - - Front Page -

The ba­sic prin­ci­ples ap­plied to all the Mazda pro­duc­tion en­gines are very sim­ple in­deed, but, for the sake of this fea­ture, we will talk about the two-ro­tor Mazda ro­tary en­gines that you have prob­a­bly heard re­ferred to — the ‘10A’, ‘12A’ and ‘13B’.

THE RO­TORS

The ro­tor it­self is a tri­an­gu­lar-shaped com­po­nent with con­vex (rounded out­wards) faces. On each of these faces, you will no­tice a dip in­wards. How deep this dip is di­rectly re­lates to the com­pres­sion ra­tio — just like the dish on the crown of a reg­u­lar pis­ton. Each ro­tor has spe­cial seals on each of its three tips that con­stantly stay in con­tact with the ro­tor-hous­ing edge as the ro­tor makes its way around the hous­ing. That means it forms sealed cham­bers for each of the four strokes of the en­gine. These in­tri­cate blade-like strips are pro­duced A ro­tary en­gine is as­sem­bled in lay­ers. The two-ro­tor en­gine has five main lay­ers that are held to­gether by a ring of long bolts. Coolant flows through pas­sage­ways sur­round­ing all the pieces. from spe­cial met­als (or other com­pos­ites, de­pend­ing on spec) and are re­ferred to as the ‘apex seals’. On the two side faces of the ro­tor, there are also other metal ring seals that seal up the ro­tor sides to ei­ther the end or cen­tre plate. In the cen­tre of the ro­tor, you will no­tice gear teeth. These fea­ture on only one side and mate di­rectly with gear teeth that are part of the end plates. When it’s all joined to­gether, this gives the ro­tor its di­rec­tion and path as it ro­tates around its hous­ing. The ro­tors them­selves are mounted on a lobe of the out­put, or ec­cen­tric, shaft.

THE RO­TOR HOUSINGS

The hous­ing is the com­po­nent that the ro­tor spins around. While it looks kind of like an oval, it’s ac­tu­ally called an ‘epitro­choid’, and its strange in­ter­nal shape is de­signed so that the three tips of the ro­tor will al­ways stay in con­tact with the wall of the cham­ber — in turn, form­ing three sealed vol­umes of gas at the same time. Each part of the hous­ing is ded­i­cated to one part of the com­bus­tion process — in­take, com­pres­sion, com­bus­tion, or ex­haust. On one side of the ro­tor hous­ing, you will see two holes for spark plugs and, on the other side, the pe­riph­eral ex­haust port through which the burned gases leave the en­gine. The thick­ness of the ro­tor housings di­rectly re­lates to the cu­bic ca­pac­ity (in im­pe­rial terms, mea­sured in cc) of the en­gine. On a 10A en­gine, each hous­ing mea­sures 491cc and there are two of them, so that to­tals 982cc — close to 1000, and that’s where the ‘10’ in ‘10A’ comes from. In a 12A en­gine, each hous­ing dis­places 573cc (1146cc) and, in a 13B, 654cc (1308cc).

THE FRONT, INTERMEDIATE, AND REAR HOUSINGS

Sand­wich­ing the ro­tor housings to­gether are the front, intermediate (cen­tre), and rear housings. These housings are also home to the in­take ports, one on each of the end plates, and two on the cen­tre one (two ro­tors to feed — one on each side). When as­sem­bled, the in­take man­i­fold di­rectly bolts up to these three com­po­nents, much as an in­let man­i­fold joins up to the cylin­der head on a pis­ton en­gine. The faces of these three housings, which point in towards the en­gine in­ter­nals, are very smooth in or­der to al­low the sides of the ro­tor to slide over as they make their way around their ro­tor housings.

THE EC­CEN­TRIC SHAFT

The ec­cen­tric, out­put, or crank­shaft (as it’s of­ten re­ferred to) has round lobes mounted ‘ec­cen­tri­cally’, mean­ing they are off­set from the cen­tre line of the shaft. Each ro­tor fits over one of these lobes, with the lobe act­ing sort of like the crank­shaft in a pis­ton en­gine. As the ro­tor fol­lows its path around the hous­ing, it pushes on the lobes. Since the lobes are mounted ec­cen­tric to the out­put shaft, the force that the ro­tor ap­plies to the lobes cre­ates torque in the shaft, caus­ing it to spin. The lobe that the ro­tor is at­tached to is off­set from the cen­tre line of the shaft, and gives the ro­tor the lever­age it needs to turn the out­put shaft. As the ro­tor runs around in­side the hous­ing, it pushes the lobe around in tight cir­cles, turn­ing three times for ev­ery one rev­o­lu­tion of the ro­tor. So, while the tacho shows the 9000rpm the en­gine is pulling, the ro­tor it­self is ac­tu­ally only spin­ning at 3000rpm, and it’s un­der nowhere near as much stress as a pis­ton en­gine go­ing up and down at the same rate of knots.

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