How Stuff Works

The ro­tary en­gine

Classic Car Weekly (UK) - - THIS WEEK - fuzz town­shend CCW’s master me­ChaniC

’An en­gine com­bin­ing high power to weight with ex­tremely smooth oper­a­tion’

Back in the 1920s, Felix Wankel de­vel­oped an idea for an en­gine that elim­i­nated pis­tons, but it wasn’t un­til the late 1950s that a work­ing pro­to­type was built.

Al­though Wankel’s first patents on his in­ven­tion dated back to 1929, it was Hanns Di­eter Pashcke’s de­sign, which used fewer mov­ing parts, that be­came the ba­sis of the ro­tary en­gine more com­monly seen to­day.

It com­prises a triangular ro­tor with out­ward curved sides and apex seals geared to an ec­cen­tric cen­tral shaft and ro­tat­ing in an epitro­choid cham­bered sta­tion­ary hous­ing. An epitro­choid is like a ra­dius-cor­nered rec­tan­gle with pinched in­wards and curved long sides.

Each ro­tor tip’s tran­sit de­scribes the epitro­choid per­fectly, pro­vid­ing a seal at each cor­ner and con­stantly chang­ing cham­ber vol­ume.

Ro­tor tip 1 passes the in­let port, sit­u­ated in the side wall of the sta­tion­ary hous­ing and as it ro­tates, be­gins to in­crease the cham­ber vol­ume be­tween it and the fol­low­ing tip 2, caus­ing the air/fuel mix­ture to be drawn in. Tip 2 passes the in­let port and closes the cham­ber. As the ro­tor pro­ceeds around the epitro­choid hous­ing, the cham­ber be­comes smaller and the air/fuel mix­ture is com­pressed. On full com­pres­sion, tip 1 passes the spark plug (or plugs), which fires and ig­nites the com­pressed mix­ture, pro­pel­ling the ro­tor around to­wards the ex­haust port, as tip 2 also then passes the plugs. The cham­ber is now in­creas­ing in size, along with the ex­pand­ing gases. Tip 1 passes the ex­haust port and as tip 2 fol­lows the shape of the epitro­choid, the cham­ber be­tween them de­creases in size, squeez­ing the gases out.

The process then re­peats, with a com­plete four-stroke (in­duc­tion, com­pres­sion, power, ex­haust) cy­cle achieved in a sin­gle ro­ta­tion of the cham­ber formed be­tween ro­tor tips 1 and 2. Two fur­ther sim­i­lar cham­bers are formed be­tween tips 2 and 3 and again be­tween tips 3 and 1. This means that there are three com­plete cy­cles in one ro­ta­tion of the ro­tor.

Most car ro­tary en­gines have two ro­tors and hous­ings, equat­ing in ef­fect to six cylin­ders fir­ing in one rev­o­lu­tion. Each ro­tor is ef­fec­tively geared to the cen­tral ec­cen­tric shaft which con­verts the ro­tor’s or­bital mo­tion into a sin­gle ro­ta­tional axis for out­put to the gear­box.

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