RO­TARY-EN­GINE BA­SICS

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Since its first pro­duc­tion in the late 1960s, the ba­sic de­sign prin­ci­ples of the Mazda ro­tary en­gine haven’t changed an aw­ful lot. Al­though very dif­fer­ent in de­sign from a reg­u­lar pis­ton en­gine, the ro­tary shares many sim­i­lar­i­ties — like a four-stroke com­bus­tion cy­cle — but with many fewer com­po­nents. Over these two pages, we’ll get back to real ba­sics and hope­fully shed some light on the ba­sic work­ing of a Mazda ro­tary en­gine.

De­sign con­cept

As you can see by the pic­tures, ro­tary en­gines are very sim­ple in their ma­jor en­gine com­po­nents’ make-up. Com­pared with a sim­ple four-stroke pis­ton en­gine, a ro­tary has sub­stan­tially fewer mov­ing me­chan­i­cal com­po­nents. In fact, there are only three in a two-ro­tor (10A, 12A, or 13B) en­gine — the two ro­tors them­selves and the ec­cen­tric shaft. In the pis­ton en­gine, it’s easy to see there are many, many more, in­clud­ing the pis­tons, con­nect­ing rods, crank­shaft, camshafts, valves, etc. All the ro­tary-en­gine parts move in one di­rec­tion — un­like in the pis­ton en­gine, in which com­po­nents move up and down, and round and round, all at the same time — and this is ba­si­cally where these en­gines get their un­ri­valled smooth­ness and free-revving na­ture.

In op­er­a­tion, just as in a pis­ton en­gine, the ro­tary makes its power by us­ing pres­sure that is cre­ated when the air/fuel mix­ture is com­busted. Think about it like this: in a pis­ton en­gine, com­bus­tion in the cylin­ders makes the pis­ton move up and down; be­cause the pis­tons join con­nect­ing rods, which in turn are con­nected to the crank­shaft, this upand-down mo­tion (or side-to-side in a Subaru boxer en­gine) is con­verted to a ro­ta­tional mo­tion, which is used to drive the car. In a ro­tary en­gine, the ro­tor is like the pis­ton, the ro­tor hous­ing like the cylin­ders, and the ec­cen­tric shaft like the crank­shaft. It’s very sim­i­lar, but also very dif­fer­ent.

Op­er­a­tion

Mazda ro­tary en­gines use a four-stroke com­bus­tion cy­cle, just like a reg­u­lar pis­ton en­gine, but the way it works in a ro­tary is not the same. In its most ba­sic form, the ro­tor (there are two of them in a 10A, 12A, and 13B and three in a 20B) spins around

1. IN­TAKE STROKE

The first part of the com­bus­tion cy­cle starts with the in­take, in which the en­gine draws in the mix­ture of air and fuel. When the tip of the ro­tor passes the in­take port and the port is ex­posed to the cham­ber, the cham­ber is at its small­est size, but then, when the ro­tor moves past the in­take port, the cham­ber size grows as it draws in the mix­ture. Fi­nally, as the ro­tor tip passes the in­take port, the cham­ber is fully sealed off, and the com­pres­sion stroke be­gins. in­side the ro­tor hous­ing. Each of the three peaks of the ro­tor is in con­tact with the hous­ing at all times, and this cre­ates three sep­a­rate vol­umes of gas as the ro­tor moves around the cham­ber. Each vol­ume of gas al­ter­nately ex­pands and con­tracts. On the in­take, it’s drawn into the en­gine; on

2. COM­PRES­SION STROKE

As the ro­tor con­tin­ues its path around the hous­ing, the cham­ber size gets smaller and the air/fuel mix­ture gets com­pressed as that’s all it can do — it has nowhere else to go. By the time the face of the ro­tor has made it around to the spark plugs, the vol­ume of the cham­ber is again close to its min­i­mum vol­ume, and this is when the com­bus­tion stage of the four-stroke cy­cle starts.

3. COM­BUS­TION STROKE

the com­pres­sion, it’s com­pressed; in the com­bus­tion, it ig­nites; and on the ex­haust, it’s ex­pelled. The beauty of the ro­tary en­gine is that each of the three faces of the ro­tor are al­ways work­ing on one part of the cy­cle at any given time. Let’s look at the four strokes. As the com­bus­tion cham­ber is long, Mazda uses two spark plugs for each ro­tor, as the flame would spread too slowly if there were only one. As the spark plugs ig­nite, the well-com­pressed mix­ture there is a com­bus­tion, which ex­pands the gases and forces the ro­tor to move (the same as a com­bus­tion in a pis­ton en­gine, which forces the pis­ton down­wards). This then pro­vides the mo­tive force un­til the ro­tor tip passes the ex­haust port.

4. EX­HAUST STROKE

Once the tip of the ro­tor passes the ex­haust port, those com­bus­tion gases start to flow out of the ex­haust port, which feeds into the ex­haust man­i­fold and ex­haust sys­tem. As the ro­tor con­tin­ues to move, the cham­ber vol­ume de­creases in size, again forc­ing the re­main­ing ex­haust out of the port. By this time, the tip of the ro­tor is once again pass­ing the in­take port, and the whole cy­cle starts again.

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