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Rotary engine

2007-03-13 11:04
A rotary engine may go against the automotive norm, but it's actually a very simple alternative - in technical terms at least.

Rotary engines gained niche appeal and status with the Mazda RX-7 sports car, first launched in 1978. Mazda continued to prove the true potential of rotary engines by winning the Le Mans 24-Hour race in 1991with the quad-rotor 787B after years of racing, testing and development - only to see this engine type controversially written out of the rules immediately thereafter.

Essentially the rotary engine operates with the same basic principles as the common piston engine, with the four-stroke sequence of inlet, compression, combustion and exhaust strokes producing the torque (or rotational force) to facilitate power and acceleration.

However, where in the piston engine all of these strokes take place in the same place in the cylinder, its rotary alternative sees each of the four functions occurring in separate parts of the engine housing.

Simply put, a rotary engine comprises a three-sided rotor that turns on an eccentric lobe geared to a central shaft. The rotor operates in a roughly oval-shaped engine housing, with apex seals keeping the edges of the rotor in close contact with the sides of the housing to create three separate volumes of gas.

As the rotor turns, each volume expands and contracts, making it operate like a pump that sucks in air and fuel (supplied by a fuel injection system), compresses it prior to combustion (achieved with two spark plugs) and then forces the burnt gasses out.

However, unlike the piston engine that requires camshafts and valves to control the movement of the gasses on the inlet and exhaust strokes, the rotary engine simply has fixed inlet and exhaust ports in the housing, with these functions controlled by the spinning rotor.

As a result, a single-rotor engine has just two main moving parts - the rotor and the output shaft, while the piston engine has as many as 40 moving parts to do the same job. Fewer moving parts means significantly lower overall weight (as much as 50% less) as well as greater reliability.

Each of the three convex 'faces' of the rotor is involved with one of the engine cycles at all times, with a combustion stroke taking place for every revolution of the output shaft, compared to every two revolutions for a piston engine - a characteristic that results in more power in a rotary engine than compared to a piston engine of the same capacity.

For example, Mazda's current twin-rotor RX8 generates an impressive 170 kW of power from a mere 1 308 cc capacity, while the best a conventional 1,3-litre piston engine can muster is around 70 kW.

In addition, a rotary engine delivers power for longer during each revolution, making the power delivery exceptionally smooth and progressive. All of these attributes make it possible to achieve very high engine speeds, capped at a staggering 9 500 r/min on the RX8 (with the power peak at 8 200 r/min).

Torque is not a particular highlight of the rotary engine in normally aspirated guise, with the high engine speeds required (the RX8's 211 Nm peak is produced at a heady 5 500 r/min) dictating a rather demanding driving style.


Shock absorbers

2007-03-22 14:41

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