Rotary spark-ignition engine & components :
Basic principles of the rotary engine
Basic components of the rotary engine
Rotary engine cycle
Rotary/piston engine comparison
Rotary engine power pulses
Renesis rotary engine
Basic principles of the rotary engine
The rotary engine is not as common as the 4-stroke or 2-stroke engine but its basic principle is well accepted. It operates very differently from a reciprocating engine.
A piston-engine is called reciprocating because the pistons move back and forth over the same path. This reciprocating motion changes into rotary motion at the crankshaft.
A rotary engine has a rotor, not a piston, and it’s called rotary because the rotor has a planetary motion. It does not reciprocate.
The rotor is roughly triangular in shape and it turns inside a housing which is a particular geometric shape called an epitrochoid curve.
Because it spins, rather than moves up and down, engine operation is said to be very smooth and vibration free.
Let’s look at basic principles of the rotary engine.
The rotary engine does look different but it is still an internal combustion engine, so let’s find the 5 key events common to all internal combustion engines.
Intake occurs when air-fuel mixture enters the working chamber at the inlet port.
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 is ignited and combustion occurs. Expanding gases produce a power pulse, driving the rotor onward.
When the exhaust port is uncovered, exhaust occurs as the rotor sweeps burned gases out of the housing.
Which brings it all back to the beginning, ready to run a new cycle.
Basic components of the rotary engine
The rotor is mounted in an oval-shaped housing. The housing is made of aluminium alloy, but the curved surface has hard chromium plating. This surface has to put up with the wear and tear of the rotor sliding against it as it turns in the housing.
There are usually 2 spark plugs fixed to the housing which enable combustion to occur.
The rotor housing also has an exhaust port to expel burnt gases. It has cooling passages for water to circulate.
The rotor has three apexes which have seals to seal between the rotor and the rotor housing. This prevents premature wear as the apex seals and housing are in continuous contact.
Side seals seal between the rotor and side housing.
The combustion chamber 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 housing and rear housings.
When combustion occurs, the meshing of the teeth forces the rotor to walk around the stationary gear. This combines with the 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 an eccentric shaft at points called rotor journals. This shaft is like a crankshaft in a piston engine but with the journals off-centre.
Because the rotor is off-centre, the force applied to the shaft is off-centre too. The whole shaft is supported by main journals, so the final output is smooth rotary motion.
Rotary engine cycle
The inlet port is uncovered and fuel-air mixture enters the working chamber. The lobe of the rotor covers the inlet port, and the rotor moves the mixture around the chamber.
The rotor turns. The working chamber becomes smaller, compressing the mixture.
It is smallest at the top of the compression phase. And that is when the air-fuel mixture is ignited. Hot, expanding gases apply force to the rotor and produce a power pulse which turns the rotor. This power impulse is also called Expansion.
The apex of the rotor uncovers the exhaust port, and exhaust gases are pushed out of the chamber.
This completes 1 cycle. In rotary engines, the stages are called phases. So one engine cycle has 4 phases.
Rotary/piston engine comparison
How does the output of 1 rotor compare with the output of a piston engine?
Look again at the working chamber. How many of these chambers are there in the housing?
There are 3 - 3 working chambers between the rotor and housing, and 3 lobes on the rotor. This means that for each complete rotation of the rotor, there are 3 power pulses. Let’s see how this occurs.
Start near the end of an intake phase for working chamber A. Apex 1 is compressing the fresh fuel-air mixture as it approaches the plugs. At the same time, chamber B has ignition and combustion, and the start of a power phase. Chamber C is full of exhaust gases being pushed out of the open exhaust port.
The mixture in chamber A keeps being compressed, getting ready for ignition. Chamber B has uncovered the exhaust port and is starting to push out its exhaust gases. Chamber C has moved on for more fuel-air mixture.
Chamber A ignites. The combustion produces a power phase for A. Chamber B keeps pushing out exhaust gases. Chamber C‘s inlet port is about to be closed after receiving more mixture.
Chamber A moves on to push out exhaust gases, B gets more fuel-air mixture, and C approaches ignition.
A finishes discharging exhaust gases and moves on for more fuel-air mixture, B is at ignition, and C is pushing out its exhaust gases. Which is where we started.
The 3 chambers continu
Rotary engine power pulses
Each chamber has 1 power pulse. So each rotor revolution produces 3 power pulses, which is similar to the output from a single cylinder 2 - stroke engine.
What if 2 rotors are bolted together?
Then their output is similar to that of a twin-cylinder 2 - stroke engine.
Because of the ratio of the gears in the housings and rotor, the eccentric shaft makes 1 revolution for each power phase. That’s the same as 3 revolutions for each rotation of the rotor. So the eccentric shaft turns at 3 times the speed of the rotor.
Renesis rotary engine
The Renesis engine is a further development of the "Wankel" rotary engine. The operating cycle is the same as the conventional rotary engine, with improvements to it's design which has the result of improving fuel economy when under load.
They are designed with either a low output (154 kw version) for use with automatic transmissions or a high output (184 kw version) for use with manual transmissions.
The fundamental design is the same, however some changes have been made so the engine can comply with current emission regulations.
The exhaust ports have been moved from the periphery of the combustion chamber to the side of the housing. They have also been enlarged. Their location delays their opening, when compared to the wankel engine, providing a longer expansion stroke, increasing thermal efficiency. In addition the rotor is machined so as to delay the closing point.
The intake ports have also been enlarged and moved to the side of the housing The low output engine has 2 intake ports, primary and secondary. The high output engine has 3 intake ports, primary, secondary and auxiliary.
The ports are enlarged when compared to the wankel engine. Their position allows them to open sooner improving power and torque and extending engine efficiency over a wider engine speed range.
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 providing better air fuel mixing.
At medium engine speeds, and when engine load is high enough, the secondary intake ducts are opened by butterfly valves, reducing restriction and increasing airflow and torque.
At high engine speeds, an extra air duct opens on the air cleaner allowing extra air to be drawn into the engine.
On the high output engine at engine speeds above 6000 rpm the auxiliary duct opens allowing the engine to draw air in through all 6 ports, further increasing engine breathing.
A butterfly valve located between each housing's main intake duct is used at speeds above 7000 rpm to shorten the effective length of the intake tubes so as to use pressure pulses to force more air into the engine.
Low output engines have 2 fuel injectors per rotor, primary and secondary. The primary fuel injectors operate at all times, the secondary injectors operate at engine speeds over 3700 rpm and when the engine load demands more fuel.
The high output engine has additional primary injectors, named primary 2, which only operate at very high speed and load conditions.
