FORM - 2
THE PATENTS ACT, 1970
( 39 of 1970 )
PROVISIONAL
SPECIFICATION
SECTION 10
TITLE
"INTERNAL COMBUSTION ENGINE WITH IMPROVED IGNITION CHARACTERISTICS"

APPLICANT
BAJAJ AUTO LIMITED, an Indian Limited Company, of Akurdi, Pune 411 035, Maharashtra, India
The following specification particularly describes the nature of the invention and the manner in which it is to be performed

INTERNAL COMBUSTION ENGINE WITH IMPROVED IGNITION CHARACTERISTICS.
Siopo: This invention relates to improvement in the ignition characteristics of an internal combustion engine working on four stroke principle and employing two spark plugs per cylinder for effective combustion of lean air fuel mixture.
This invention further relates to small displacement engines of swept cylinder volume ranging from 75 cc to 225 cc and cylinder bore diameter 45 mm to 70 mm, preferably in the range of 100 cc to 200 cc, wherein two spark plugs are used for effective combustion of air-fuel mixture, employed as prime movers for operation of 2 and 3 wheeled vehicles or other motorized vehicles.
The applicants had earlier invented An improved Internal Combustion Engine working on 4 stroke principle having 2^valve per cylinder employing two spark plugs per cylinder for efficient burning of lean air-fuel mixture. These engines have diameter of cylinder bore ranging between 45 mm to 70 mm wherein one spark plug was located in a sleeve within timing chain cavity of cylinder head. In the said Internal Combustion Engine, the two spark plugs are energized simultaneously at a predetermined instant before top dead center to burn the lean air-fuel mixture in cylinder head cavity in an efficient manner as disclosed in applicants' earlier patent application no 649/MUM/02 dated 15-7-02 (referred to as known engines in the following description).
The simultaneous energizing of both spark plugs, however, leads to uncomfortable noise levels or harshness of sound when the known engine is rapidly accelerated and at higher engine speeds although vehicles fitted with these known engines are within the statuary limits. This uncomfortable noise and harshness of sound is due to reasons explained below.
When the vehicle is cruising at a constant speed, say between 15 to 60 km/h, the known engine is able to burn the lean fuel-air mixture at a quick enough rate so as to have an efficient combustion but uncomfortable noise and harshness of sound is not produced. In this condition, the rate of combustion pressure rise is good enough to get the most efficient combustion. However, when the known engine is accelerated rapidly, the cylinder gets instantaneously filled with more amount of air-fuel mixture which gets compressed to higher pressures leading to higher rate of rise of combustion pressure. At this instant, in the known engines, the combustion with twin spark plug ignition makes the rate of combustion pressure rise increase further, leading to uncomfortable noise levels or harshness of sound. Also, the rapid rise in combustion pressure leads to rapid acceleration of the moving parts taking up the clearances between them rapidly on side during piston movement from top dead center to bottom dead center. This further leads to uncomfortable noise levels or harshness of sound. It has been observed that if the rate of rise in combustion pressure is more than a threshold value, say 3.5 bar per degree of crank rotation, it leads to uncomfortable noise levels or harshness of sound.
Also, it was observed that when the known engine runs at higher engine RPM, say greater than 6000 uncomfortable noise levels or harshness of sound is produced. In any engine, at these speeds, higher velocity of air fuel mixture creates turbulence in the engine cylinder. Also, the air fuel ratio at this condition is on the richer side to take care of the durability of the engine components. These factors, i.e. higher turbulence inside
2

the cylinder and the richer charge leads to further higher rate of combustion pressure rise in the known engines due to twin spark plugs ignition and produce uncomfortable noise levels or harshness of sound.
Objective of Invention
One of the objectives of the present invention is to mitigate the disadvantages of known
type of internal combustion engines working on 4 stroke principle employing two spark
plugs per cylinder.
A further objective of the invention is to provide an efficient internal combustion
engine working on 4 stroke principle with two spark plugs per cylinder where lean air
fuel mixture is used.
A still further objective of the invention is to provide an engine with less uncomfortable
noise levels or harshness of sound during rapid acceleration phase and at higher engine
speeds.
According to present invetion the! improved ignition system achieves the above
advantages while retaining the benefits of known. engine i.e. low emissions and fuel
consumption.
To achieve the above objectives, it has been the endeavor of the applicants to invent an internal combustion engine working on four stroke principle employing two spark plugs per cylinder, which reduces the noise level and harshness of the vehicle during acceleration phase and at higher vehicle speeds. This is achieved by providing means for example, which are capable of sensing the rate of rise of engine speed, i.e. the acceleration during predetermined intervals of time. When the acceleration exceeds a certain predetermined value, means are provided which cause retardation or delay in energizing the firing event of one of the two spark plugs. When the acceleration reaches below the predetermined value, these means restores the simultaneous energizing of both the spark plugs. In another aspect of the invention when these means sense that the engine RPM has reached a predetermined value, they prevent functioning of one of the spark plugs, thereby bringing down the uncomfortable noise levels or harshness of sound of the engine at higher speeds also.
The present invention will now be described in detail with reference to accompanying drawings, wherein;
Figure 1 illustrates Ignition timing curves of the Ignition system according to prior art.
Figure 2 illustrates Ignition timing curves of the Ignition system according to the
present invention.
Figure 3 illustrates Block diagram of a typical Ignition System according to this
invention.
Figure 4 illustrates location of two spark plugs in a typical engine.
Fig 1 illustrates ignition timing curve of engines according to the prior art wherein x-axis shows the engine speed (RPM) and Y-axis shows the ignition timing in degrees before Top Dead Center. As the engine RPM increases, the ignition timing also varies and is applicable for both spark plugs, which are energized simultaneously.
Fig 2 illustrates ignition timing curves for the engines having two spark plugs per cylinder according to the present invention wherein x-axis shows the engine speed
3

(RPM) and Y-axis shows the ignition timing. When engine speed increases rapidly, ignition timing of one of the spark plug retards by a predetermined value as indicated by D keeping ignition timing of another spark plug unchanged as indicated by C in the figure.
With reference to the figures, the 4 stroke internal combustion engine having two spark plugs per cylinder with improved ignition characteristics according to the present invention essentially comprises of a magneto rotor (1), a pick up coil (2), an ignition coil A (10), ignition coil B (11), a spark plug (15) and a spark plug (16), an intelligent CDI (7) which further comprises of a wave shaping means (3), a sensing means or a micro-controller (4), a charging coil (5), a power supply rectification means (6), an ignition capacitor charging and trigger circuit A (8), an ignition capacitor charging and trigger circuit B (9).
With reference to figures, the magneto rotor (1) is employed for generating pulses in the pick up coil (2) which are fed to the said micro-controller (4) through wave shaping means (3). The power supply to the said micro-controller (4) is provided by conventional charging coil (5) through power supply rectification means (6). Two outputs of the said micro-controller (4) are connected to the said Ignition capacitor charging and trigger circuit A (8) and Ignition capacitor charging and trigger circuit B (9). The said Ignition capacitor charging and trigger circuit A (8) is connected to the spark plug (15) through ignition coil A (10) and the said Ignition capacitor charging and trigger circuit B (9) is connected to the spark plug (16) through ignition coil B (11). The vehicle fitted with this system can also be' fitted with a throttle position sensor (12).
When the rider increases the speed of the-Vehicle by operating the throttle, the rate of increase in the engine RPM is sensed by the "said micro-controller (4) through pick up coil (2). Based on the rate of increase in engine rpm, the said micro-controller retards the ignition timing of the spark plugs (16) by a predetermined value, maintaining ignition timing of other spark plug unchanged. As the rider completes acceleration phase by maintaining constant throttle position as sensed by the said pick up coil, simultaneous ignition timing of both spark plugs is restored. When the engine RPM increases beyond certain value, the energizing of the spark plug (16) is discontinued and the engine runs only with one spark plug energized. Retarding the ignition timing of spark plug (15) during acceleration phase and discontinuation of energizing spark plug (15) could also result in the reduced noise and harshness to certain extent.
The values of the rate of increase of the engine RPM during acceleration phase, variation in ignition timing required to be made for one spark plug and which of the two to spark plugs this is to be applied, the RPM at which the cut-off of one of the spark plugs takes place depend upon the design of engine. The design parameters such as cylinder swept volume, engine maximum power and maximum torque and at which RPM these occur, maximum speed of the vehicle, transmission gear ratios, dynamic rolling radius of tyres etc. affect such decisions.
The above description relates to a system wherein the input of the engine speed has been taken from the said pulsar coil, based on which the micro-controller controls the ignition timing of one of the two spark plugs. The inputs can also be taken based on parameters such as manifold pressure, instantaneous throttle position sensor or other parameters which can indicate or lead to indication of higher rate of combustion
4

pressure. When it is required to account for the load on the engine also, it is necessary to take the input from locations such as the manifold vacuum and / or the throttle position sensor. Also the system explained above uses micro-controller to control the ignition timing, it is possible to use other means wherein the ignition timing of one of the spark plugs is varied by assessing the rate of rise in combustion pressure. Such variations in the inputs taken are well within the scope of this invention.
It will be appreciated that in the invented engine with improved combustion characteristics as described and illustrated in the drawings, it would be possible to modify and /or alter the various features thereof by person skilled in the art without deviating from the scope of the invention. The applicant intends to include those modifications and variations within the scope and the ambit of this invention.
Dated this 1st day of July 2005.

M K CHAKRABARTI
OF L S DAVAR & CO Agent for the applicant
5