Rotary Spark-Ignition Engine

5-08 Discuss the rotary engine.

As the two-stroke engine has shown, the fewer parts used, the better the power production and the smaller the engine can be. The rotary engine fits into the same category of using fewer parts to produce power. The rotary combustion (RC) engine has found its way into automobiles, planes, helicopters, boats, motorcycles, lawn mowers, and other applications. Displacement has varied from small air-cooled models to much larger, liquid-cooled, multirotor units (FIGURE 5-23).

FIGURE 5-23 The rotary engine was used by Mazda for its high revving capability and compact size.

The rotary engine is also called the Wankle engine because it was improved upon by Felix Wankle for automotive use in the 1940s. The RC engine was commercially released in 1964 in the NSU Wankle Spider and in 1967 with a two-rotor engine in the NSU Ro 80. Under license from NSU, Mazda successfully used the rotary engine in several vehicles from the late 1960s all the way through the RX series. The engines were in a redefining period for engine development, but they were never really a success for a number of other companies.

Basic Principles of the Rotary Engine

The rotary engine (Wankle engine) is not as common as the four-stroke or two-stroke cycle engines, but its basic principle is well accepted. The rotary engine layout is vastly different from that of a reciprocating engine. The piston engine is called a reciprocating engine because the pistons move back and forth over the same path. This reciprocating motion is converted to rotary motion at the crankshaft.

By contrast, a rotary engine does not use a piston that reciprocates; rather, it has a rotor that rotates. The rotary engine does not need to convert inefficient reciprocating motion to rotary motion, since the rotor functions as the piston in the engine. In the reciprocating engine, the piston assembly must stop at BDC and move back up to TDC, then move back down to BDC, and so on, many times a second. The continual stopping and starting of the piston assembly places a tremendous amount of pressure on the connecting rod and rod bolts.

Due to inertia, the piston tries to move out of the top of the cylinder bore and through the bottom of the oil pan. The rotary engine does not have to stop/start its “piston” as it rotates. The rotor is roughly triangular and turns inside of a housing. The housing works on a geometric principle called an epitrochoid curve. An epitrochoid curve is a circular movement around the perimeter of another circle. The rotor moves in a unique pattern to ensure that the rotor ends follow the oblong shape of the housing (FIGURE 5-24).

FIGURE 5-24 A pressure differential valve with a leak in the hydraulic braking system.

Because the rotor spins, rather than moving up and down, engine operation is relatively smooth and vibration-free. Each rotor and housing is akin to that of a three-cylinder two-stroke engine because of the rotor’s three-sided shape. The rotor has three working chambers; thus, each rotation of the rotor produces three power pulses. Low-end torque is improved to the point that (while not recommended) a four-speed transmission rotary vehicle can be driven and accelerated from a standstill in fourth gear without excessive lugging. Rotary engines are made with one, two, or even three or more rotor housings stacked side to side.

Look at the basic principles of a rotary engine. While it appears different from an ICE, the rotary engine is still an ICE. Recall the five events common to all ICEs: intake, compression, ignition, power, and exhaust.

The rotary engine’s intake cycle occurs when one face of the rotor passes the intake port and draws the air-fuel mixture into the working chamber through the inlet port (FIGURE 5-25). The turning rotor then carries it around to the spark plugs. Along the way, the volume of the working chamber decreases and compresses the mixture. The mixture ignites and combustion occurs. Expanding gases produce a power pulse, driving the rotor farther around. When the exhaust port is uncovered, exhaust occurs as the rotor sweeps burned gases out of the housing. Each face of the rotor is a separate working chamber, so three combustion events occur for each single revolution of the rotor.

FIGURE 5-25 Operation of a Rotary engine.

Basic Components of the Rotary Engine

The rotor mounts in an oval housing. The housing is made of aluminum alloy, but the curved interior surface has hard chromium plating. This surface has to put up with the wear and tear of the rotor seals sliding against it as it turns in the housing. Cooling passages machined into the housing allow coolant to flow around the outside surface of the housing to cool the engine.

There are usually two spark plugs fixed to the housing that enables combustion to occur; these are referred to as leading and trailing spark plugs. The combustion chamber design is a long trough, which can contribute to incomplete combustion due to the large surface area of the chamber and the distance the flame travels in the combustion chamber. Designers found that by installing a leading and trailing plug, a more efficient combustion process could be produced. The rotor housing also has an intake port to let air and fuel into the combustion chamber and an exhaust port to expel burned gases.

The rotor has three apexes, or points, which have seals between the rotor and the rotor housing. These seals work like piston rings in a reciprocating piston engine. Side and corner seals create a seal between the rotor and the side housing to prevent combustion gases from leaking around the rotor at the apex seals. The rotor also has oil seals on the side of the rotor that keep oil that comes from inside the rotor from finding its way into the combustion chamber.

The combustion chamber of the rotary engine is formed by hollows in the flanks of the rotor. These hollows are sometimes called bathtubs. Front and rear housings, or side housings, are bolted to each side of the rotor housing. If it is a two-rotor engine, there is an intermediate housing between the two rotors. An internal gear in each rotor meshes with a corresponding stationary gear in the front and rear housings.

When combustion occurs, the meshing of the teeth forces the rotor to walk around the stationary gear. The teeth being in mesh combines with an eccentric shaft to make the rotor follow the curved surface of the housing, and it gives the rotor planetary motion. The rotor is attached to the eccentric shaft at points called rotor journals. This eccentric shaft is like a crankshaft in a piston engine but with the journals off-center. Because the rotor is off-center, the force applied to the shaft is off-center too. The whole shaft is supported by main journals, so the final output is smooth rotary motion.

Engine Power Pulses

A single-rotor rotary engine will produce three power pulses per rotor rotation or one power pulse per eccentric shaft rotation. In a four-stroke engine, there is one power pulse per cylinder for every two crankshaft revolutions, and in a two-stroke engine, there is one power pulse per cylinder for each crankshaft revolution. Because of the ratio of the gears in the housings and rotor, the eccentric shaft makes one revolution for each power phase. That is the same as three eccentric shaft revolutions for each rotation of the rotor. So the eccentric shaft turns at three times the speed of the rotor. A standard rotary engine typically has two rotors, offset from each other, in separate chambers, so it ends up with two power pulses per revolution of the eccentric shaft.

Renesis Rotary Engine

Due to emission regulations and fuel economy requirements, Mazda updated the standard rotary engine to the Renesis engine. The operating cycle of the Renesis rotary engine is the same as a conventional, or Wankel, rotary engine, but it has some design changes that improve fuel economy under load and compliance with current emission regulations. There is a low-output version of this engine for use with automatic transmissions and a high-output version for use with manual transmissions. The low-output Renesis engine has two intake ports: primary and secondary. The ports were enlarged and moved to the side of the housing. In this position, they can open sooner, improving power and torque, extending engine efficiency over a wider range of engine speeds, and reducing port overlap, which resulted in unburned fuel exiting the exhaust port.

The intake manifold has primary, secondary, and auxiliary ducts. The primary duct has no control valve; the secondary and auxiliary ducts are controlled by butterfly valves. At low engine speeds, air flows into the engine through the primary intake ducts only, keeping the air velocity in the manifold high, which provides better air-fuel mixing. At medium engine speeds and when the engine load is high enough, the secondary intake ducts are opened by butterfly valves, reducing restriction and increasing airflow and torque. Intake manifolds on reciprocating piston engines now use a similar system, called intake manifold runner control. The rotary engine opens an extra air duct on the air cleaner at high engine speeds, allowing more air to be drawn into the engine.

The high-output engine has three intake ports: primary, secondary, and auxiliary. At engine speeds above 6,000 rpm, the auxiliary duct opens, allowing the engine to draw air in through all six ports (three intake ports per rotor), further increasing engine breathing. A butterfly valve located between each housing’s main intake duct is used at speeds above 7,000 rpm to shorten the effective length of the intake tubes so that pressure pulses force more air into the engine.

Low-output engines have two fuel injectors per rotor: primary and secondary. The primary fuel injectors operate at all times; the secondary injectors operate at engine speeds over 3,700 rpm and when the engine load demands more fuel. The high-output engine has additional primary injectors, named primary 2, which operate only at high speed and heavy load conditions. The exhaust ports in a Renesis engine have also been enlarged and moved to the side of the combustion chamber housing. In this location, the exhaust ports open later than in a conventional rotary engine, and the rotor has been machined to delay the closing point. These changes deliver a longer expansion stroke and increase thermal efficiency.