US3905342A

US3905342A - Engine valve means and porting

Info

Publication number: US3905342A
Application number: US416213A
Authority: US
Inventors: Eyvind Boyesen
Original assignee: Performance Industries Inc
Current assignee: Performance Industries Inc
Priority date: 1973-01-10
Filing date: 1974-01-03
Publication date: 1975-09-16
Anticipated expiration: 1992-09-16
Also published as: JPS4992435A

Abstract

1. AN IGNITION TIMING CONTROL SYSTEM COMPRISING, IN COMBINATION AN AUTOMOTIVE VEHICLE INTERNAL COMBUSTION ENGINE HAVING AN INTAKE SYSTEM INCLUDING A CARBURETOR FOR MIXING FUEL WITH INTAKE AIR TO PRODUCE A COMBUSTIBLE MIXTURE, AND IGNITION PLUGS TO IGNITE SAID COMBUSTIBLE MIXTURE, AN IGNITION DISTRIBUTOR FOR SUPPLYING HIGH VOLTAGE CURRENT TO SAID IGNITION PLUGS OF SAID ENGINE IN ACCORDANCE WITH A PREDETERMINED SEQUENCE, A VACUUM ADVANCE DEVICE RESPONSIVE TO ENGINE INTAKE VACUUM PRODUCED IN SAID INTAKE SYSTEM DOWNSTREAM OF SAID CARBURETOR FOR ADVANCING THE IGNITION TIMING OF SAID IGNITION DISTRIBUTOR, AND VALVE MEANS CONNECTED IN COMMIUNICATION WITH SAID ENGINE INTAKE SYSTEM BOTH UPSTREAM AND DOWNSTREAM OF SAID CARBURETOR AND WITH SAID VACUUM ADVANCE DEVICE, SAID VALVE MEANS BEING ARRANGED TO CONTROL A VACUUM TO BE INTRODUCED INTO SAID VACUUM ADVANCE DEVICE IN ACCORDANCE WITH THE OPERATING MODES OF SAID ENGINE IN TERMS OF AN ABSOLUTE LEVEL IS INTRODUCED INTO SAID THE VACUUM AT THE ABSOLUTE LEVEL IS INTRODUCED INTO SAID VACUUM ADVANCE DEVICE DURING ACCELERATION OF SAID VEHICLE TO EFFECT A VACUUM ADVANCE IN SAID IGNITION DISTRIBUTOR AND SUCH THAT THE VACUUM AT SUCH AN ABSOLUTE LEVEL IS AT LEAST PARTIALLY REDUCED AND INTRODUCED INTO SAID VACUUM ADVANCE DEVICE DURING CRUISING OPERATION OF SAID VEHICLE AND DURING DECELERATION THEREOF TO EFFECT A DESIRED REDUCTION IN THE VACUUM ADVANCE.

Description

United States Patent [:91

Kobayashi, et al.

[ Sept. 16, 1975 IGNITION TIMING CONTROL SYSTEM Inventors: Nohuyuki Kobayashi; Masahiko Nakada; Masanori Hanaoka, all of Aichi-ken, Japan [73] Assignee: Toyota Jidosha Kogyo Kahushiki Kaisha, Aichi-ken, Japan [22] Filed: Jan. 3,1974

[2]] Appl. No.: 430,402

Primary Examiner- Charles M yhre Assistant Examiner-Tony Argenbright A ttorney- Toren, McGeady and Stanger [57] ABSTRACT and an exhaust gas recirculation system. The ignition timing control system comprises valve means in communication with the engine intake system upstream and downstream of the carburetor and with the vacuum advance device for controlling a vacuum, to be introduced into the latter, in accordance with the engine operating modes in terms of an absolute level of the intake vacuum. During acceleration of the automobile, the vacuum at the absolute level is introduced into the vacuum advance device to effect such a vacuum advance in the ignition distributor as to allow the exhaust gas recirculation system to minimize the nitrogen oxide content in the engine exhaust gases while minimizing adverse effects upon the operating characteristics of the automobile. During cruising operation of the automobile and when it is being decelerated, the vacuum at such an absolute level is at least partially reduced and introduced into the vacuum advance device to effect a reduction in the vacuum advance such as to allow the exhaust gas recirculation system to minimize the nitrogen content while again minimizing adverse effects upon driving characteristics. In the valve means, temperature responsive means is provided, which is reponsive to the warm-up condition of the engine for restricting operation of a valve mechanism of the valve means such that the valve mechanism is forced to introduce therethrough the intake vacuum into the vacuum advance device, until engine warm-up is attained, and such that the valve mechanism is released from the forced introducing action to restore its normal condition depending upon the absolute level of the intake vacuum, after the engine warm-up condition is An ignition timing control system is provided for use attained. with an automotive internal combustion engine having a carburetor, a distributor, a vacuum advance device 5 Claims,2 Drawing Figures I x i 1 LI [1 [I I] I 1 l .I

I L 0' A, 32 3| Mmngwsw 1 5 ms 3.905342 SHEET 1 0f 2 FIG. Ia FIG 2 w d E NOx c 5 HC g e a 02 E if, a L b g 2 INTAKE MANIFOLD VACUUM FIG. lb

NOx

(ADVANCE) Y (RETARD) IGNITION mama FIG. 3 39 4O '2 2? PATFMTWSE P 3.905.342

slain 2 BF 2 FIG. 4

IGNITION TIMING CONTROL SYSTEM BACKGROUND OF THE INVENTION The present invention relates to ignition timing control systems for automotive internal combustion engines, and. more particularly, to an improved ignition timing control system, by which the ignition timing is controlled to minimize nitrogen oxide content in the engine exhaust gases without adversely affecting the operating characteristics of the automobile when used with an exhaust gas recirculation system.

Some retardation in ignition timing has been found remarkably effective in reducing the noxious content in engine exhaust gases of automotive internal combustion engines. Such retardation will cause, on one hand, substantial increase in the engine exhaust gas temperature, which in turn will lead to reduction in the unburned content of elements such as hydrocarbons (I-IC). Retardation will also cause substantial decrease in the combustion temperature, which will likewise lead to reduction in the recombined content of elements such as nitrogen oxides (NO In one conventional ignition timing control system utilizing the above principle, a differential between intake manifold vacuum and atmospheric pressure is employed to control a vacuum advance device which is operative to advance ignition timings of an ignition distributor.

On the other hand, dilution of the combustible mixture with a portion of the engine exhaust gases has also been found to decrease the combustion temperature and accordingly effective in reducing the NO, content. Thus, an exhaust gas recirculation system has been proposed, by which a portion of the engine exhaust gases are recirculated between the intake system and the exhaust system in accordance with the engine operating modes.

Ignition timing control devices and exhaust gas recirculation systems have been found so critical in their ignition retarding and mixture diluting operations, that they will necessarily invite decrease in the engine output and/or deterioration in the operating characteristics of the automobile. Thus, prior art techniques have not succeeded in solving the problem of reducing the noxious content, especially the N emission for all the running conditions of the automobile.

It is therefore an object of the present invention to provide an improved ignition timing control system .for use with an automotive internal combustion engine with a view to eliminating the above drawbacks.

Another object of the present invention is to provide an improved ignition timing control system of the above type, by which the ignition timing is controlled as to minimize the NO, content in the engine exhaust gases without adversely affecting the operating characteristics of the automobile when used with an exhaust gas recirculation system.

SUMMARY OF THE INVENTION Briefly, the present invention may be described as an ignition timing control system for an automotive vehicle internal combustion engine having an intake system including a carburetor for mixing fuel with intake air to produce a combustible mixture, an ignition distributor supplying high tension current to the ignition plugs of the engine in accordance with a predetermined sequence, a vacuum advance device responsive to engine intake vacuum produced in said intake system downstream of said carburetor for advancing the ignition timing of said ignition distributor, and an exhaust gas recirculation system in communication with both said intake system and the exhaust system of the internal combustion engine for recirculating therebetween a portion of the exhaust gases in accordance with engine operating modes whereby the nitrogen oxide content of the exhaust gases is reduced. The invention is particularly characterized by the improvement which comprises valve means connected in communication between the intake system and the vacuum advance device to apply to said vacuum advance device a vacuum which controls operation thereof in accordance with the operating condition of the engine. The valve means are connected to the intake system at points both upstream and downstream of the carburetor. Retention means in the valve means operate to sense the engine operating temperature and to retain the valve means in a condition wherein only the vacuum pressure from the connected point downstream of the carburetor is applied to the vacuum advance device until the temperature of the engine reaches a predetermined level where an engine warmed-up condition is achieved. Subsequently, control means responsive to the vacuum pressure existing at the connected point downstream of the carburetor control operation of the valve means to apply to the vacuum advance device a control vacuum which selectively combines the vacuum pressure derived from the point downstream of the carburetor with atmospheric pressure derived from the point upstream of the carburetor to balance the timing advance in accordance with the pressure sensed at the connected point downstream of the carburetor. In this manner, the invention operates to cause the exhaust gas recirculation system to minimize the nitrogen oxide content of the exhaust gases while also minimizing the adverse effects produced upon the driving characteristics of the automobile.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects and advantages of the present invention will be apparent from the following detailed description of the preferred embodiments thereof taken in conjunction with the accompanying drawings, in which:

FIGS. 1(a) and 1(b) are graphical presentations of 'the noxious content of the engine exhaust gases taken Referring first to FIG. 1(a), in which the N01 emission presence in the exhaust gas recirculation is represented against the ignition timing, when the engine is operating in the cruising range, using the amount of recirculation as a parameter, the BC emission as well as the automotive operating characteristics are shown taken against the ignition timing. In cases where the amount of recirculation is percent, percent and percent, the NO, emission is seen to decrease with increase in the ignition retardation, as shown by curves a a and a respectively. For all recirculation conditions, on the other hand, it is also found that vehicle driving characteristics become deteriorated when the ignition retardation exceeds a level illustrated by a characteristic curve b. More specifically, the automobile starts vibrations at a level of the ignition retardation represented longitudinally of the characteristic curve b. In view of the curves a a and a and b, it can be deduced that the minimum amount of NO, emission presence in the exhaust gas recirculation is represented by a value a in the range of ignition timing of acceptable operating characteristics, namely, in the lefthand side of the curve b. and that this minimum amount a is obtainable even when the ignition retardation is augmented to as large a value as a value x. As shown by a curve c, on the other hand, the HC emission is decreased uniformly with increase in the ignition retardation. Thus, with a view toward reducing the HC emission, it is preferable to retard the ignition timing as much as possible. In order to be compatible with the reduction in the NO, emission resulting from the exhaust gas recirculation, however, it can be deduced that setting the ignition timing at the level x will satisfy requirements to a satisfactory extent during cruising operation of the automobile.

Turning now to FIG. 1(b), the NO, emission presence in the exhaust emission is also plotted against the ignition timing, when the vehicle is accelerating, using the amount of recirculation as a parameter. As seen from curves at to a corresponding, respectively, to the cases where the amount of recirculation is 0 percent, 3 percent, 6 percent and 9 percent, the NO r emission will also be decreased with increase in the ignition retardation, with the concurrent abrupt decrease in engine output torque.

Thus, engine output torque may be used as a parameter so as to represent the NO, emission against the ignition timing. Curves d, and d are intended to represent relationships between the NO emission and the ignition timing, respectively, in the cases where the output torques during acceleration have the values of IO Kg-m, 9 Kg'm and 8 Kg-m. The value at which the minimum NO,r emission is obtained for the different output torques is seen to fall at a point y. which is shifted from the value x to an advance side of the ignition timing. In other words, the minimum N0 emission for the same output torque can be obtained at the more advanced point y during the accelerating condition of the automobile. It has also been experimentally revealed that a con ventional ignition vacuum advance device can, if properly adjusted, effect ignition advance to the value y whereas without the vacuum advance device the obtainable ignition timing would be located at the value x or at a slightly retarded value than the value x.

In summary, it may be said that in order to obtain the minimum NO, emission presence in the exhaust recirculation without adversely affecting vehicle operating characteristics, the ignition timing should be vacuum-advanced during acceleration,but the vacuum advance of the ignition timing should be terminated for cruising operation or it should be limited to such a degree determined by taking into consideration the deterioration of the vehicle operating characteristics.

An improved ignition timing control system according to the above concept will now be described in conjunction with FIGS. 3 and 4 to 6. Referring first to FIG. 3, a carburetor I having a customary venturi 2, are both mounted in an intake system of an automotive internal combustion engine formed with an atmospheric pressure outlet 3 upstream of the venturi 2 and with an intake vacuum outlet 5 downstream of a throttle valve 4. These two outlets 3 and 5 are connected to a change-over valve 20 according to the present invention by way of air conduits 6 and 7, respectively. This change-over valve 20 is in turn connected to a vacuum advance device 10 of a distributor 9 by way of an air conduit 8. With these connection arrangements, the change-over valve 20 can introduce selectively atmospheric pressure, the intake manifold pressure or an intermediate pressure into the vacuum advance device 10 in accordance with the engine operating modes in terms of an absolute level of the intake vacuum, as will be hereinafter detailed.

The change-over valve 20 is formed with

inlets

21 and 22, which are connected to the air conduits 6 and 7, respectively, and with an outlet 23 which is connected to the air conduit 8. In the change-over valve 20, two

coextensive chambers

24 and 25 are formed, which are in communication with the two

inlets

21 and 22, respectively. Between the

chambers

24 and 25 there is formed a passage 26 which in turn is in communication with the outlet 23. The change-over valve 20 may preferably be arranged in an upright manner, as shown. Above the chamber 24, there is formed a passage 27, through which a working medium, for example, engine cooling water, engine lubricating oil or hot air for a choking operation can pass in order to provide therein a temperature medium representative of temperature variations in engine operating conditions. A shaft 28 is vertically movably inserted in

chambers

24 and 25, passing through

passages

26 and 27. In the chamber 25, shaft 28 is connected by means of a fastening member 29 to a diaphragm member 31, on the back of which a spring 30 is mounted for biasing the shaft 28 in the downward direction. At the opposite side of the chamber 25, the diaphragm 31 forms an atmospheric chamber 33 which is vented to the atmosphere through an air vent 32. The shaft 28 is formed with a vacuum valve member 34 between the chamber 25 and the passage 26 so as to provide and block communication therebetween. The shaft 28 is also formed with a stopper 35 between the chamber 24 and the passage 26. This stopper 35 is arranged such that it can contact with the lower surface of an atmospheric valve member 37, on the back of which a spring 36 is mounted for biasing the valve 37 in the downward direction and and which is operable to provide and block communication between the chamber 24 and the passage 26. It should be noted here that the two

valve members

34 and 37 have synchronized relative positions such that when the former provides communication between the chamber 25 and the passage 26 the latter blocks communication between the chamber 24 and the passage 26, and vice versa.

The shaft 28 has its upper extending end 28 movable into and out of the passage 27 which is sealed off from the chamber 24 by means ofa sealing member 38. In this passage 27 is mounted a temperature responsive body 39 which is expandable and contractable in accordance with the ambient temperature. To this temperature responsive body 39 is connected a cam 40 which is slidably accommodated in the passage 27. This cam 40 has a cut-away portion 40' at such a position as to face the extending end 28' of the shaft 28. Before warm-up of the engine is completed, the working medium, e.g., the engine cooling water, is at a relatively low temperature, so that the temperature responsive body 39 is in a contracted condition with the result that the cam 40 is pulled in the lefthand direction to place its wider portion over the extending end 28' of the shaft 28. Thus the extending end 28' is prevented from moving into the passage 27, so that the shaft 28 can be said to assume a first position in which it cannot move upwardly. After the engine warm-up condition is attained, the temperature responsive body 39 is expanded sufficiently to move the cam 40 rightwardly. Then, the extending end 28 of the shaft 28 faces the cut-away portion 40 of the cam 40, so that the shaft 28 is released from the retaining action of the wider portion of the cam 40 to enable its upward movement into the passage 27. In this instance, the shaft 28 can be said to assume a second position.

Thus, when the engine is being warmed up, the shaft 28 is restricted by the cam 40 from upward movement and is held at the first position. The valve member 34 allows communication between the chamber 25 and the passage 26, but the valve member 37 is spaced from the stopper 35 to shut off communication between the chamber 24 and the passage 26. As a result, only the intake vacuum from the carburetor l is introduced into the vacuum advance device via the air conduit 8 to thereby carry out a proper amount of vacuum advance in the dir ributor 9.

After the engine warm-up has been completed, the movement of the cam 40 will release the shaft 28 which will thus be capable of moving upwardly. During the accelerating phase of engine operation, the large effective area of the throttle valve 4 will establish a relatively smaller, i.e., a more positive relative pressure, so that the vacuum in the chamber 25 exerted on the diaphragm 31 is not so sufficient to enable the diaphragm 31 to flex upwardly against the biasing action of the spring 30. However, the shaft 28 cannot hold at the first position to provide the same degree of communication between the chamber 25 and the passage 26 around the valve member 34, thus permitting introduction of the intake vacuum into the vacuum advance device 10 to a somewhat lesser degree. Thus, a slight but proper amount of vacuum advance is effected in the distributor 9.

When, on the other hand, the automobile is running in the cruising condition or in the decelerating condition, an intake vacuum of considerably large level is built up in the intake system downstream of the carburetor, that is, a relatively more negative pressure is developed. Then, the diaphragm 31 is flexed upwardly by the pressure differential between the atmospheric pressure in the chamber 33 and the greater intake vacuum introduced into the chamber 25. As a result, the shaft 28 is also lifted to have the valve member 34 block communication between the chamber 25 and the passage 26, whereas the chamber 24 restores communication with the passage 26 with the aid of the valve member 37 which is brought into contact with the stopper 35. As a result. the atmospheric pressure in the intake system upstream of the carburetor 1 is introduced into the vacuum advance device 10, so that the distributor 9 is free from any vacuum advancement.

Reverting to FIG. 2, the vacuum advance angle during the engine warm-up is shown at a curve e, which is plotted against the intake vacuum. This curve e will be self-explanatory from the discussion set forth hereinabove.

In FIGS. 4, 5 and 6, other embodiments of the changeover valve 20 are shown, in which like reference numerals will indicate like elements or counterparts of the embodiment as shown in FIG. 3.

The difference of the embodiment of FIG. 4 from that of FIG. 3 resides in that a bypass conduit 42 having an orifice 41 is interposed between the chamber 25 and the passage 26, and in that an additional passage 24' having an orifice 43 is provided which extends from the inlet 21 to the chamber 24. With these constructional differences, therefore, the intake vacuum will pass, during acceleration of the automobile, not only through the passage 26 but also through the bypass passage 42, and will be introduced into the vacuum advance device 10. Conversely, during cruising operation, the vacuum in the chamber 25 is supplied to the vacuum advance device 10 via the bypass passage 42 even in the absence of communication between the chamber 25 and the passage 26 due to the blocking action of the valve member 34. Thus, the absolute level of the vacuum to be introduced into the vacuum advance device 10 under the latter condition is reduced to an intermediate value between those of the intake vacuum and the atmospheric pressure. As a result, the distributor 9 can be vacuum-advanced to some extent, in the case where considerable intake vacuum is developed, as illustrated by the broken linefin FIG. 2. In this instance, it should be appreciated that the orifice 41 serves to prevent excessive reduction of the vacuum established in the chamber 25 whereas the orifice 43 serves to prevent excessive reduction of the vacuum in the passage 26.

The embodiment of FIG. 5 is slightly modified from that of FIG. 4, such that the valve member 34 for providing and blocking communication between the chamber 25 and the passage 26 is eliminated and replaced by a sealing member 44, which serves to block the communication therebetween at all times. since the former is always in communication with the outlet 23 via the bypass passage 42. Other con structional arrangements and operational features are quite similar to those of the embodiment of FIG. 4, and as such repeated explanation thereof will be omitted.

In the embodiment of FIG. 6, a diaphragm member 46 is interposed between the

chambers

24 and 25, and a spring 45 is mounted on the back of the diaphragm member 46 for imparting a biasing spring action thereto in the upward direction. A passage 47 is provided which is in communication with the chamber 25 by way of the bypass passage 42. This passage 47 is formed with an orifice 48, through which it is in communication with the upper portion of the chamber 24 only. Between the chamber 24 and the passage 47 there is interposed a valve member 49 which is operative to establish and cut off communication therebetween. A spring 51 is interposed between this valve member 49 and a fastening member 56, which is operative to connect the diaphragm member 46 with a shaft 50, for applying an upward biasing force to the valve member 49. This shaft 50 is formed with a stopper 52 which can contact the valve member 49. The shaft 50 is further formed with an engagement member 54 which is positioned in a passage 53. in this passage 53 is mounted a cam 55 which is connected to the temperature responsive body 39. This cam 55 is formed with a stepped portion 55' which is engageable with the engagement member 54.

With this arrangement, before engine warm-up has been completed, the engagement member 54 is pulled leftward by the temperature responsive body 39, so that it is brought into engagement with the stepped portion 55 of the cam 55. As a result, the shaft 50 is restricted from moving downwardly, that is, just opposite to the direction of the previous embodiments. At this stage, the valve member 49 continues blocking communication between the chamber 24 and the passage 47, so that the vacuum prevailing in the chamber 25 will be introduced into the vacuum advance device via the bypass passage 42.

After engine warm-up has been completed, the engagement member 54 is disengaged from the stepped portion 55 to make it possible for the shaft 50 to move downwardly. When, in this situation, the automobile is experiencing an accelerating operation, then the intake vacuum has not achieved so sufficiently large a level as to flex the diaphragm member 46. Therefore, the shaft 50 is held at the upper position, so that the intake vacuum is also introduced into the vacuum advance device 10. When, on the contrary, the automobile is in the cruising condition or in the decelerating condition, then the sufficiently developed intake vacuum will flex the diaphragm member downwardly against the biasing action of the spring 45 to thereby lower the shaft 50. Thus, its stopper 52 is brought into contact with the valve member 49 to form a passageway therearound between the chamber 24 and the passage 47. As a result, the atmospheric pressure in the chamber 24 is introduced into the vacuum advance device 10 in addition to the intake vacuum via the bypass passage 42. In like manner as in the embodiments of FIGS. 4 and S, the absolute level of the vacuum to be supplied to the vacuum advance device 10 is reduced to have an intermediate value between the intake vacuum and the atmospheric pressure. A vacuum advance of some degree is, therefore, effected in this embodiment. Here, the

orifices

41 and 48 are also operative to prevent excessive reduction in the vacuum in the chamber and in the atmospheric pressure in the carburetor 1.

As is apparent from the foregoing description, before the engine warm-up has been completed. the engine is vacuum-advanced according to the scheme of the present invention irrespective of the absolute level of the intake vacuum. The instability in the automotive operating characteristics under this condition can be prevented from being further deteriorated or adversely affected. By making the best use of the intake vacuum which varies in dependence upon the running condition of the automobile, the engine is vacuum-advanced in the accelerating condition to properly advance the ignition timings. 1n the cruising condition, on the contrary, the engine is not vacuumadvanced or at most slightly vacuum-advanced, so that the ignition timings are retarded from a value which might otherwise be attained by the vacuum advance device. it should, therefore, be appreciated as an advantage of the present invention that the N0 emission presence in the exhaust gas recirculation can be minimized for all the running conditions to such an extent as to be free from adverse effects upon vehicle operating characteristics, or, in other words, that efficiency in cleansing the engine exhaust gases is considerably improved with the possible deterioration in the operative characteristics due to the cleansing operation being maintained at a minimum level. it should also be appreciated that the ignition timing retardation during the cruising operation will lead not only to material reduction in the NO emission due to the exhaust gas recirculation, as has been discussed, but also to additional reduction in the NO, emission due to decrease in the combustion temperature and also to reduction in the HC emission due to increase in temperature in the engine exhaust gases. in the decelerating mode of engine operations, moreover. the abrupt increase in the absolute level of the intake vacuum will terminate the vacuum advance in accordance with the present invention. Therefore, it should also be appreciated that the present invention is also applicable during engine deceleration such that the reduction in the driving torque will strengthen the obtainable braking action on the engine.

Moreover, the embodiments of FIGS. 4 to 6 are considered superior to that of FIG. 3 in that they can provide better operating characteristics in cruising operation because vacuum advance of some degree is effected even in that phase of operation. The embodiment of FIG. 5 is considered superior to that of FIG. 4 in that its construction and operation can be more simplified because in the former the two

valve members

34 and 37 need not be synchronized. The embodiment of FIG. 6 can be simplified still further than that of FlG. 5, since the single valve member 49 can perform the required change-over action.

While specific embodiments of the invention have been shown and described in detail to illustrate the application of the inventive principles, it will be understood that the invention may be embodied otherwise without departing from such principles.

What is claimed is:

1. An ignition timing control system comprising, in combination an automotive vehicle internal combustion engine having an intake system including a carburetor for mixing fuel with intake air to produce a combustible mixture and ignition plugs to ignite said combustible mixture, an ignition distributor for supplying high voltage current to said ignition plugs of said engine in accordance with a predetermined sequence, a vacuum advance device responsive to engine intake vacuum produced in said intake system downstream of said carburetor for advancing the ignition timing of said ignition distributor, and valve means connected in communication with said engine intake system both upstream and downstream of said carburetor and with said vacuum advance device, said valve means being arranged to control a vacuum to be introduced into said vacuum advance device in accordance with the operating modes of said engine in terms of an absolute level of said intake vacuum such that the vacuum at the absolute level is introduced into said vacuum advance device during acceleration of said vehicle to effect a vacuum advance in said ignition distributor and such that the vacuum at such an absolute level is at least partially reduced and introduced into said vacuum advance device during cruising operation of said vehicle and during deceleration thereof to effect a desired reduction in the vacuum advance.

2. A system according to claim 1 wherein said valve means includes a valve mechanism movable between a first and a second position, said valve mechanism operating to apply said intake vacuum into said vacuum advance device when in its first position and to introduce atmospheric pressure prevailing in said intake system upstream of said carburetor into said vacuum advance device when in said second position, a diaphragm member responsive to said intake vacuum for moving said valve mechanism into 10 warm-up condition of said engine for holding said valve mechanism at said second position irrespective of the operation of said vehicle until warm-up of said engine is attained, and for allowing said valve mechanism to move in response to operation of said vehicle after engine warm-up has been attained, and a bypass passage providing communication between said intake system downstream of said carburetor and said vacuum advance device for introducing therethrough intake vacuum into said vacuum advance device such 0 that the absolute level of the vacuum to be introduced said first position during acceleration of said vehicle and for moving said valve mechanism into said second position during cruising operation of said vehicle and when said vehicle is being decelerated, and temperature responsive means responsive to the warmed-up condition of said engine for holding said valve mechanism at said first position irrespective of the operation of said vehicle until said engine has achieved a warmed-up condition and for allowing said valve mechanism to operate in response to movement of i said diaphragm member after said engine has attained a warmed-up condition.

3. A system according to claim 2 wherein said valve means further includes a bypass passage providing communication between said air intake system downstream of said carburetor and said vacuum advance device for introducing therethrough the intake vacuum into said vacuum advance device, such that the absolute level of the vacuum introduced into said vacuum advance device during deceleration or cruising operation of said vehicle may be reduced to an intermediate level between said intake vacuum and atmospheric pressure.

4. A system according to claim 1 wherein said valve means includes a valve mechanism movable between a first and a second position, said valve mechanism when in said first position being operative to effect introduction into said vacuum advance device into said vacuum advance device during deceleration and during cruising operation may be reduced to an intermediate level between the level of the intake vacuum and of atmospheric pressure.

5. An ignition timing control system comprising, in combination an automotive vehicle internal combustion engine having an intake system including a carburetor for mixing fuel with intake air to produce a combustible mixture, an ignition distributor for supplying ignition current to said engine in a predetermined sequence, a vacuum advance device responsive to vacuum pressure for controlling the ignition timing of said ignition distributor, valve means connected in communication between said intake system and said vacuum advance device to apply to said vacuum advance device a vacuum controlling operation thereof in accordance with the operating condition of said engine, said valve means 40 being connected to said intake system at points both carburetor for controlling operation of said valve means to apply to said vacuum advance device a control vacuum which selectively combines the vacuum pressure derived from said connected point downstream of said carburetor with atmospheric pressure derived from said connected point upstream of said carburetor to effect a desired vacuum advance in said ignition distributor.