Basic information of Wankel Engine

1. WANKEL ROTARY COMBUSTION ENGINE
1
1. Introduction:
The Wankel engine is a type of internal combustion engine using an eccentric rotary design to
convert pressure into rotating motion. In contrast to the more common reciprocating piston designs, the
Wankel engine delivers advantages of simplicity, smoothness, compactness, high revolutions per minute,
and a high power-to-weight ratio primarily due to the fact that it produces three power pulses per rotor
revolution compared to one per revolution in a two-stroke piston engine and one per two revolutions in
a four-stroke piston engine (although at the actual output shaft, there is only one power pulse per revolution,
since the output shaft spins three times as fast as the actual rotor does, as can be seen in the animation
below, making it roughly equivalent to a 2-stroke engine of the same displacement; this is also why the
displacement only measures one face of the rotor, since only one face is working for each outut shaft
revolution). The engine is commonly referred to as a rotary engine, although this name also applies to
other completely different designs, primarily aircraft engines with their cylinders arranged in a circular
fashion around the crankshaft. All parts rotate consistently in one direction, as opposed to the common
reciprocating piston engine, which has pistons violently changing direction. The four-stage cycle of intake,
compression, ignition, and exhaust occur each revolution at each of the three rotor tips moving inside the
oval-like epitrochoid-shaped housing, enabling the three power pulses per rotor revolution. The rotor is
similar in shape to a Reuleaux triangle with sides that are somewhat flatter.
2. Literature Review:
The advantage of purely rotary motion have for long been known and appreciated.
Consequently a number of patents, including the patent of rotary steam engine by james watt in 1769 were
taken. This was much before the advent of the reciprocating engine which is now in its prime. All the earlier
attempts to make the rotary engine were unsuccessful because though it was known that any pump or blower
can, in principle, be converted into an internal combustion engine, much depended on the skill and ability
of the designer, and the production techniques available to produce a proper sealing mechanism.
The principle that oval or elliptical rotor can be designed to maintain constant while turning
about fixed centres, and that three or more rotors can be run to enclose between them a continuously varying
volume, was utilised by Scheffel in 1952 for getting a parent for rotary engine. With suitable arrangement
of posts, ignition system and adequate compression ratio, the four volumes between the rotors could be
made to excuse a four-phase Otto Cycle.
However this design also failed due to its complexity and great problem that it created in its
manufacturing and development. And it was in 1954 that Felix Wankel. A German inventor, got a patent
for a design of four-phase rotary engine working on the Otto Cycle principle. In this design variable volume
chambers were formed between an equilateral curved sided triangular rotor and an outer rotary body with
a bore shape of two loaded epitrochoid. The combination of three chamber that varied in size as the body
revolved. Later Dr.Froede made the inner rotor orbit while it rotated, and permitted the outer body to be
kept stationary. Such a simple arrangement allowed the attachment of the intake, exhaust, ignition system
and coolant arrangement to be made to the stationary member. The result was what we call
Kreiskolbenmotor engine which is now popularly called Wankel rotary combustion engine. And then Many
manufacturers signed license agreements for development, attracted by the smoothness, quiet running, and
reliability emanating from the uncomplicated design. Amongst them were Alfa Romeo, American
Motors, Citroen, Ford, General Motors, Mazda, Mercedes-Benz, Nissan, Porsche, Rolls-Royce, Suzuki,
and Toyota. In the United States in 1959, under license from NSU, Curtiss-Wright pioneered improvements
in the basic engine design. In Britain, in the 1960s, Rolls Royce's Motor Car Division pioneered a two-

2. WANKEL ROTARY COMBUSTION ENGINE
2
stage diesel version of the Wankel engine. Present Mazda is the company uses Wankel Engine for all their
models.
3. Working:
The Wankel RC engine works on the familiar four phase cycle principle. In the Wankel engine,
the four strokes of an Otto cycle piston engine occur in the space between a three-sided symmetric rotor
and the inside of a housing. The gasoline Wankel is now developed whilst the diesel Wnakel is of very
recent origin. The term ‘phase’ has been used to avoid confusing it with the four stroke of the reciprocating
engine, through the phenomenon is exactly the same. Mainly there are two moving parts in the RC engine-
one rotor and the other eccentric or output shaft with its integrated eccentric. No connection rod is required
as the rotor rotates directly on the eccentric shaft. The output torque is transmission to the shaft through the
eccentric. Internal and external timing gears designed to maintain the phase relationship between the
rotation of the rotor and the eccentric shaft and thus control the orbit motion of the rotor. The smaller
external gear, which is c-axial with the eccentric shaft, is actually so fixed gear, which is co-axial with the
eccentric shaft, is actually so fixed to one side housing that the internal gear in the rotor moves around it.
Breathing is through ports so that valves and valve operating mechanism is not needed. As the rotor apexes
pass around the walls, the volume enclosed between the bore and the rotor flank varies in a sinusoidal
manner and executes the four phases. This entrapped volume expands when the apex passes over the intake
port, thereby, drawing in the fuel-air mixture. As the rotor turns further, this volume is isolated from the
intake port and compressed. Then it passes over a recessed spark plug and enlarges again during the
expansion phase, thereby, absorbing the power produced by combustion. Finally, when the apex passes the
exhaust and one thermodynamic cycle is completed. At the same time other chambers follow exactly the
sequence, enough with some phase difference.
Each of the three moving chambers formed by a flank of the rotor goes through the phases
during one completed revolution of the rotor. So there are three power impulses for each revolution of the
rotor. And since the eccentric or output shaft rotates at three time the speed of the rotor, there is only one
power impulse for each revolution of the output shaft of a single bank rotary engine. The suction phase
takes 900
of rotor movement and so also the other three phases, i.e. one complete thermodynamic cycle is
completed over 3600
rotation of the rotor. Since the output shaft makes three revolution for every single
rotation of the rotor, one thermodynamic phase is completed for every 2700
rotation of the output shaft.
T should be noted that through the charge is continuously rotated, each particular phase of
the cycle always occurs at a fixed part of the housing which makes supply of the fuel, air and cooling water
much easier.

3. WANKEL ROTARY COMBUSTION ENGINE
3
4. Features of The Rotary Engine:
As compared to the reciprocating engine. The rotary engine has the following basic features.
a. There are no reciprocating parts. Thus the problems of unbalance caused by the inertia of reciprocating
parts and complicated engine vibration is eliminated. In rotary vibration is very low because it is
possible to perfectly balance the engine by using the balance weights. The rotary engine does not require
a cranking mechanism, it has the advantages of smooth motion, less mechanical loss, simple
construction and compactness.
b. There is no separate intake-exhaust valve mechanism. In a reciprocating engine intake-exhaust valve
mechanism generates mechanical noise by operating and closing of the valves, and the valves
themselves obstructs air flow. Further, due to inertia of valve mechanism, the valves are unable to fully
follow the motion of the cam at high revolutions. This results in variations in effective valve timings of
the engine. In a rotary engine the rotor directly opens and closes the intake and exhaust ports,
eliminating the valve mechanism; hence, the correct timing for opening and closing can be maintained
even at high speeds.
c. The time for one stroke is 2700
in terms of the rotating angle of the output shaft, and there is one
explosion for one rotation of the output shaft. The long intake stroke means that the volumetric
efficiency becomes higher even at the high seeds range, and reduce the torque drop. Also the long
expansion stoke is advantageous form the point of torque fluctuations. In a rotary engine that has two
rotors, the expansion stroke overlaps so the torque fluctuations is as low as in the case of a six cylinder
reciprocating engine.
5. Performance of Wankel Engine:
a. Friction Loss:
The rotary engine shows lower values of friction loss even in the
high speed range because of the absence of a mass of reciprocating motion
.
b. Performance at Wide Open Throttle:
It is seen that rotary engine, in general, has a slightly lower output
at low speeds but develops high performance at high speeds. The lower
output at low speeds is attributable mainly t the grater time required for the
compression and combustion strokes (1.5 times longer than that of the
reciprocating engine) which causes some gas leakage loss. Higher output
at high speeds is attributable to the small friction loss of the engine even at
high speeds because of the absence of a reciprocating mass, and a high
charging due to longer intake and exhaust strokes, and the absence of
intake and exhaust valves causing gas flow resistance.

4. WANKEL ROTARY COMBUSTION ENGINE
4
c. Fuel Consumption Performance and Heat Balance:
In the rotary engine, the space to volume ratio of the working chamber is greater and the time
required for one cycle is 1.5 times longer than that of the reciprocating engine, causing a greater cooling
loss. The greater surface area/volume ratio of the working chamber also causes a greater loss of unburnt
gas.
The thermal efficiency is increased by raising the burning speed. For this the shape of the rotor
recess is selected to obtain stronger squish flow and a 2-spark plug system is used. The thermal
efficiency also increases as the mean effective pressure and specific fuel consumption. The mean
effective pressure is in proportion to the torque. Consequently , the operation of an engine at high torque
is advantageous to low fuel consumption in an automobile.
d. Vibration Due to Torque Fluctuation:
This gives a comparison of fluctuation in torque between the
2-stroke rotary engine and the 4 and 6-cylinder reciprocating engine.
As already stated the 2-rotor engine shows its fluctuations in torque
similar to those of the 6-cylinder reciprocating engine.
6. Exhaust Emission Characteristics of RC Engine:
The formation of emission of RC engine from various processes can be explained n the basis
of reciprocating engine experience. Flame quenching, incomplete combustion, blowby of partially burnt
and unburnt fuel are some important causes of emission from an engine.
a. Unburnt Hydro carbons:
The major cause of the unburnt hydrocarbons in the exhaust in the quenching of the flame front as
it approaches a relatively cool metal surface. Consequently a layer of unburnt or partially burnt fuel is

5. WANKEL ROTARY COMBUSTION ENGINE
5
Firmed near the surface. The thickness of the layer and hence the quality of the unburnt hydrocarbons
contains in it depends upon the air fuel ratio, temperature and the turbulence present. Another cause of
unburnt hydrocarbons is the rich mixture or poor mixing of fuel and air.
Emission characteristics of
Wankel Engine
In RC engine since the flame does not reach the end gas region of the trailing chamber which have relatively
higher surface volume ratio, a layer of rich unburnt hydrocarbons is formed on the wall. In case of
reciprocating engine the surface of the combustion chamber remains unaffected form cycle to cycle except
for the effect of mixing and scavenging due to fresh charge. However in the RC engine the story is entirely
different. Firstly the turbulence created by the pressure difference between the leading and trailing chamber
is comparatively lower than what occurs in the reciprocating engine chamber. Secondly most of the surface
of the rotary engines are scrapped of by the apex seals and side seals and sent onto exhaust. The crevics
formed outside the gas seals between the side of the rotor and the housing walls causes additional
quenching. That is why the HC concentrating in the exhaust phase suddenly becomes very high. In addition
Depicting emission distribution in combustion chamber at various shafts the effect of the air/fuel ratio on
emission characteristics as measured at constant spark timing. The HC emission of a RC engine is inevitably
higher than that of a RC engine.
7. Exhaust Emission Control System:
For controlling the exhaust emissions there are two methods being used in present, the thermal
reactor system, and the catalyst system.
a. Thermal reactor system:
This system has been mostly generally used for exhaust emission control of the rotary engine.
The characteristics of the combustion of the thermal reactor system with the rotary engine are follows:
1. Based on the exhaust emission properties of low NOx and high HC supply of the secondary air to
the thermal reactor will put the reactor into a stable purification reaction of exhaust emission by the
burning heat of HC.
2. Continues emission of exhaust gas from each exhaust port and relatively high temperature of
exhaust gas are the advantages to the oxidation reaction in the thermal reactor.
3. The thermal reactor can be installed in direct contact with the engine of such favourable construction
leading to less heat loss and better purification efficiency

6. WANKEL ROTARY COMBUSTION ENGINE
6
4. The fewer number of exhaust ports enables the thermal reactor to be designed completely.
5. The exhaust of simple construction an be easily provided with a port insert effective for reducing
heat loss of exhaust gas flowing into the thermal reactor.
6. The thermal reactor is installed on the bottom od the engine which is advantageous n preventing
heat determination inside the engine compartment.
b. Catalyst System:
The catalyst system has made possible the fuel economy improvement of the rotary engine to a
great extent in place of the thermal reactor system. It is used in engines where unleaded fuel is available.
The catalyst converter consists of pre-converter and a main converter. The pre-converter plays a part of
purification of exhaust gas. A monolith-type catalyst of small capacity is used to facilitate reaction in the
main converter. The main catalyst is incorporated with two beds, to efficiently purify HC,CO and NOx in
the exhaust gas. An air nozzle of a multi-hole type is provided between the two beds to supply secondary
air. The main converter is compact pellet converter of a container, Bed type capable of obtaining a high
purification rate.
8. Future development of C engine:
a. Fuel Injector Type Rotor Engine:
The advantage of the fuel injection system over the carburettor system is the possibility of
controlling the amount and timing of fuel supply more accurately leading to better performance, improved
fuel economy and less exhaust emission. The fuel injection system can be classified by the control method
into the mechanical control type and the electronic control type. By the injection used into the continues
injection type and the intermittent injection type; and by the injection position into the intake manifold
injection type and working chamber injection type.
b. Multi-Rotor type Rotary Engine:
i). Improvement of performance by increasing the total displacement.
ii). Reduction in torque displacement .
iii). Easier balance with inertia force and couple.
c. Stratified Charge Type Rotary Engine;
Many designs of stratified charge type rotary engines are under investigation. In general the
mixture is stratified by distributing easily ignitable rich mixture around the spark plug and lean mixture
surrounding the former so that a property lean mixture fills in the entire working chamber.
9. Advantages and Disadvantages:
Advantages:
 A far higher power to weight ratio than a piston engine
 Approximately one third of the size of a piston engine of equivalent power output
 No reciprocating parts
 Able to reach higher revolutions per minute than a piston engine
 Operating with almost no vibration
 Not prone to engine-knock
 Cheaper to mass-produce, because the engine contains fewer parts

7. WANKEL ROTARY COMBUSTION ENGINE
7
 Superior breathing, filling the combustion charge in 270 degrees of mainshaft rotation
rather than 180 degrees in a piston engine
 Supplying torque for about two thirds of the combustion cycle rather than one quarter for
a piston engine
 Wider speed range giving greater adaptability
 It can use fuels of wider octane ratings
 Does not suffer from "scale effect" to limit its size
 On some Wankel engines the sump oil remains uncontaminated by the combustion process, so
no oil changes are required. The oil in the main shaft is totally sealed from the combustion
process. The oil for Apex seals and crankcase lubrication is separate. In piston engines the
crankcase oil is contaminated by combustion blow-by through the piston rings.
Disadvantage:
 The design does not allow for controlling the lubrication of the rotors accurately and
precisely.
 Slow Combustion
 Bad fuel economy
 High emissions
10. What I Understood:
The Wankel engine is a type of internal combustion engine using an eccentric rotary design to
convert pressure into rotating motion. In contrast to the more common reciprocating piston designs, the
Wankel engine delivers advantages of simplicity, smoothness, compactness, high revolutions per minute,
and a high power-to-weight ratio. This type of engine gives lower friction losses because of absence of
reciprocating motion. It gives low vibrations compared to the reciprocating engine.
11. References:
1. Text book by M.L.Mathur and R.P.Sharma
2. Wikipedia
3. Mazda Rotary engine Wikipedia.