Patent Description:
Some ignition systems are equipped with a dividing wall and a spark plug. The dividing wall isolates between a main chamber and a pre-chamber in a combustion chamber of an engine. The dividing wall has formed therein a plurality of spray holes communicating the main chamber with the prechamber. The spark plug works to create an electrical spark to ignite fuel upon application of voltage across a spark gap within the pre-chamber. Such a type of spark plug is taught in the following patent literature <NUM>.

<CIT> discloses the following: This ignition system has a spark plug and an ignition control unit that controls the spark plug. When an engine is in a predetermined operating state, the ignition control unit performs post-top-dead-center ignition control in which ignition is performed at a point after top dead center of compression. The ignition system has an airflow assistance structure that enables an airflow to flow easily to a discharge gap at least at the point after top dead center of compression. The ignition system is configured so that due to the airflow assistance structure and an ignition timing, an airflow of a speed <NUM>/s or greater flows to the discharge gap in a post-top-dead-center spark period, which is a period in which a discharge spark occurs in the post-top-dead-center ignition control.

<CIT> discloses the following: A subsidiary chamber type ignition system comprises: a partition wall that partitions a combustion chamber of an engine into a subsidiary chamber and a main chamber; and an ignition plug that ignites fuel inside the subsidiary chamber. In a high load state, an ignition control unit calculates optimum ignition timing earlier by a combustion time than prescribed optimum combustion timing, and performs optimum ignition control to control ignition timing at the optimum ignition timing. In a low load state, a jetting control unit performs jetting delay prevention control to prevent a jetting time from being longer compared with the high load state to prevent the combustion time from being long, while the ignition control unit performs the above optimum ignition control.

<CIT> discloses the following: A spark plug body is provided with a chamber having a closure at its lower end provided with a central opening and gas passages surrounding the central opening, a sparking terminal insulated from the body and extending into the opening in spaced relation to the walls thereof to form a spark gap and inwardly opening valves in said passages.

<CIT> discloses the following: Disclosed is a direct-injection spark-ignition engine designed to promote catalyst activation during cold engine operation. A fuel injection timing for a fuel injection period in an compression stroke is set to allow a first fuel spray injected from a first spray hole to enter a cavity in a piston crown surface, and allow a second fuel spray to impinge against a region of the piston crown surface located closer to an injector than the cavity, so as to cause the second fuel spray having a lowered penetration force due to the impingement to be pulled toward the cavity by a negative pressure generated in the cavity as a result of passing of the first fuel spray therethrough, The direct-injection spark-ignition engine can maximally hold an injected fuel spray around a spark plug to reliably stabilize a combustion state in a combustion mode for promoting catalyst activation during cold engine operation, while enhancing combustion efficiency in a homogenous combustion mode during normal engine operation.

The above type of ignition system works to perform a before-top-dead-center ignition task to ignite fuel before the top dead center of the compression stroke, in other words, during the compression stroke in the engine in a normal operation mode. The before-top-dead-center ignition task is to elongate a spark, as created in the pre-chamber, by means of tumble or swirl occurring in the combustion chamber. The elongated spark then ignites fuel to produce a flame which, in turn, jets into the main chamber, thereby facilitating the combustion of the fuel within the combustion chamber.

When a given condition is encountered, the ignition system alternatively performs an after-top-dead-center ignition task to ignite fuel in the expansion stroke after the top dead center of the compression stroke, i.e., during the expansion stroke. Specifically, for instance, in a first idling mode of the engine operation to warm up the catalyst installed in an exhaust path of the engine, the ignition system starts the ignition of fuel as late as possible in order to enhance the efficiency in transmitting thermal energy, as generated by the combustion of fuel, to the catalyst. The ignition system, therefore, ignites the fuel after the top dead center of the compression stroke.

After the top dead center of the compression stroke, the flow of the mixture in the pre-chamber is usually reduced in strength due to the breaking of the tumble or swirl when the piston passes through the top dead center of the compression stroke. This will result in a decrease in elongation of the spark, thereby reducing the ease of ignition of the fuel, thereby increasing the length of time in which the fuel is ignited in the pre-chamber, and the flame is jetted from the pre-chamber into the main chamber, in other words, decreasing the speed of propagation of the flame to the main chamber.

This disclosure was made in view of the above problem. It is a principal object to achieve quick propagation of a flame to a main chamber in an after-top-dead-center ignition control mode.

The principal object is solved by the feature combination of the appending independent claim <NUM>. The further dependent claims are defining advantageous embodiments.

An ignition system in this disclosure comprises a dividing wall and a spark plug. The dividing wall divides a combustion chamber of an engine into a main chamber and a pre-chamber. The has formed therein at least one spray hole which communicates between the main chamber and the pre-chamber. The spark plug works to create a spark by applying voltage across a spark gap between a first electrode and a second electrode to ignite fuel. The pre-chamber has the first electrode. The dividing wall or a member which electrically conducts with the dividing wall has the second electrode.

In the following discussion, the timing when the voltage starts to be applied across the spark gap will be referred to as an ignition timing. The center of an opening of the spray hole which is located close to the pre-chamber will be referred to as a spray hole center. A region which is located <NUM> or less away from the spray hole center within the pre-chamber will be referred to as a spray hole-nearby region.

The ignition system works to execute an after-top-dead-center ignition control mode in which an ignition operation to ignite the fuel is performed after a compression stroke top dead center is reached when a given operating condition of the engine is met. In the after-top-dead-center ignition control mode, an ignition source is provided in the form of a self-growable flame kernel within the spray hole-nearby region, the spray hole, or the main chamber within a crank angle of <NUM>° after the ignition timing is reached.

This disclosure offers the following beneficial advantages. In the after-top-dead-center ignition control mode, the ignition source is provided in the spray hole-nearby region, the spray hole, or the main chamber early, e.g., within a crank angle of <NUM>° following the ignition timing. When the ignition source is provided in the spray hole-nearby region or the spray hole, it facilitates the jetting of a flame, as grown from the ignition source, from the spray hole into the main chamber. Alternatively, when the ignition source is provided in the main chamber, it will cause a flame grown from the ignition source to propagate as it is to the main chamber. It is, therefore, possible to propagate the flame quickly to the main chamber in the after-top-dead-center ignition control mode.

The present disclosure will be understood more fully from the detailed description given hereinbelow and from the accompanying drawings of the preferred embodiments of the invention.

An embodiment in this disclosure will be described below with reference to the drawings. This disclosure is, however, not limited to this embodiment, but may be modified in various ways without departing from the principle of this disclosure.

<FIG> is a cross sectional view which illustrates the engine <NUM> on which the ignition system <NUM> in this embodiment is mounted. The engine <NUM> is implemented by a four-stroke engine in which a piston completes one combustion cycle made up of a sequence of four separate strokes (i.e., <NUM>° crank angle): intake stroke, compression stroke, expansion stroke, and exhaust stroke. In the following discussion, a top dead center between the compression stroke and the expansion stroke will be referred to as a compression stroke top dead center Td. The engine <NUM> includes the cylinder <NUM> and the head <NUM> mounted on the cylinder <NUM>.

In the following discussion, a lengthwise direction of the center line X of the cylinder <NUM> illustrated in the drawings will be referred to as a vertical direction. The engine <NUM> and/or the ignition system <NUM> may be optionally oriented in various directions. For instance, the engine <NUM> and/or the ignition system <NUM> may be oriented to have the center line X which extends obliquely to the vertical direction or alternatively extends the horizontal direction.

The cylinder <NUM> has the piston <NUM> disposed therein. The piston <NUM> is connected to the crankshaft <NUM> through the link <NUM> and reciprocates vertically following rotation of the crankshaft <NUM>. Space surrounded by an upper surface of the piston <NUM>, an inner peripheral surface of the cylinder <NUM>, and a lower surface of the head <NUM> defines the combustion chamber <NUM>.

The head <NUM> has formed therein the intake path <NUM> through which air is inducted into the combustion chamber <NUM> and the exhaust path <NUM> from which gas discharged from the combustion chamber <NUM>. The intake path <NUM> has the intake valve <NUM> installed therein. The exhaust path <NUM> has the exhaust valve <NUM> installed therein. The intake valve <NUM> is driven by the intake cam <NUM>, while the exhaust valve <NUM> is driven by the exhaust cam <NUM>. The head <NUM> has the fuel injector <NUM> mounted in the intake path <NUM>. The fuel injector <NUM> works to spray fuel.

The ignition system <NUM> is equipped with the dividing wall <NUM>, the spark plug <NUM>, and the ignition controller <NUM>. The ignition controller <NUM> is implemented by a portion of an electronic control unit (ECU) and works to analyze information derived by sensors installed in the engine <NUM> to control an operation of the spark plug <NUM>. The sensors include, for example, a crank angle sensor, a knock sensor, an intake pressure sensor, an exhaust pressure sensor, an in-cylinder pressure sensor, and a catalyst temperature sensor.

<FIG> is a cross sectional view which illustrates the prechamber <NUM> and a region therearound. The spark plug <NUM> includes the first electrode <NUM> and the porcelain insulator <NUM> disposed around the outer periphery of the first electrode <NUM>. The dividing wall <NUM> is secured to a lower end portion of the porcelain insulator <NUM>. The dividing wall <NUM> defines the prechamber <NUM> and the main chamber <NUM>. The pre-chamber <NUM> is located inside the diving wall <NUM>. The main chamber <NUM> is located outside the dividing wall <NUM>. In other words, the dividing wall <NUM> isolates the combustion chamber <NUM> of the engine <NUM> into the main chamber <NUM> and the pre-chamber <NUM>. The dividing wall <NUM> has formed therein a plurality of spray holes <NUM> communicating between the main chamber <NUM> and the pre-chamber <NUM>. The dividing wall <NUM> is made from an electrically conductive material and functions as the second electrode <NUM> of the spark plug <NUM>. The spark plug <NUM> is subjected to application of voltage across the spark gap <NUM> between the first electrode <NUM> and the second electrode <NUM> to create an electric spark f which ignites the fuel.

More specifically, the spark plug <NUM> is equipped with a primary coil and a secondary coil. By applying an electrical current to the primary coil, electromagnetic energy is charged in the primary coil. Subsequently, stopping the application of current will cause the electromagnetic energy stored in the primary coil to induce voltage at the secondary coil. The induced voltage is then applied to the spark gap <NUM> to create the spark f within the spark gap <NUM>. The time when the application of current to the primary coil is stopped will, therefore, coincide with an ignition timing Ts when the voltage starts to be applied across the spark gap <NUM> to initiate ignition of fuel.

In the following discussion, one of the spray holes <NUM> which lies on the center line X of the cylinder <NUM> will also be referred to as the center spray hole 35c. The center spray hole 35c extends vertically through the thickness of the dividing wall <NUM>. The first electrode <NUM> has a lower end located just above the center spray hole 35c. In other words, the lower portion of the first electrode <NUM> protrudes greatly downward from the lower end the porcelain insulator <NUM>, so that it is located closest to the center spray hole 35c among the spray holes <NUM>. The gap between the lower end of the first electrode <NUM> and an upper periphery of the center spray hole 35c in the dividing wall <NUM> defines the spark gap <NUM>. The spray holes <NUM> other than the center spray hole 35c are arranged around the center spray hole 35c in the dividing wall <NUM>. The center spray hole 35c and the other spray holes <NUM> may be designed to be identical or different in sectional area or configuration with or from each other.

In a normal mode of operation, the ignition system <NUM> executes a before-top-dead-center ignition control mode to ignite fuel before the compression stroke top dead center Td. Alternatively, when a given operating condition of the engine <NUM>, e.g., a fast idling mode of operation to warm up a catalyst installed in the exhaust path <NUM>, is entered, the ignition system <NUM> executes an after-top dead center ignition control mode to ignite fuel after the compression stroke top dead center Td.

In the following discussion, a gas flow passing through the spark gap <NUM> will also be referred to as an in-gap gas flow. A direction from the first electrode <NUM> toward the center spray hole 35c will also be referred to as a spray hole direction d1. A direction opposite to the spray hole direction d1 will also be referred to as a spray hole opposite direction d2. In this embodiment, the spray hole direction d1 represents a downward direction. The spray hole opposite direction d2 represents an upward direction. A direction including the spray hole direction d1 as a component will also be referred to below as a spray hole direction d1-side or merely referred to as the spray hole direction d1. A direction including the spray hole opposite direction d2 as a component will also be referred to as a spray hole opposite direction d2-side or merely referred to as the spray hole direction d2.

After the after-top-dead-center ignition control mode is entered, in the ignition system <NUM>, the in-gap gas flow is changed from the spray hole opposite direction d2-side to the spray hole direction d1-side until the ignition timing Ts is reached. At the ignition timing Ts, the in-gap gas flow is, therefore, oriented to the spray hole direction d1-side, thereby causing the spark f to be elongated toward the spray hole direction d1-side.

In the following discussion, the center of an opening of the center spray hole 35c which is exposed to the pre-chamber <NUM> will be also referred to below as a spray hole center. A region located within the pre-chamber <NUM> at an interval of <NUM> or less away from the spray hole will be referred to below as a spray hole-nearby region R.

When it is required to execute the after-top-dead-center ignition control mode, the ignition system <NUM> works at an early stage within a crank angle of <NUM>° following the ignition timing Ts to place an ignition source in the form of a flame kernel large in size enough to self-grow in the spray hole-nearby region R, the center spray hole 35c, or the main chamber <NUM>. In the flowing discussion, "in the spray hole-nearby region R, the center spray hole 35c or the main chamber <NUM>" will be generally referred to as "in the spray hole-nearby region R, etc.".

In this disclosure, "the size of the flame kernel large enough to self-grow" means that the flame kernel which will spread without being extinguished by heat loss or lean mixture in the combustion chamber even when application of voltage to the spark gap <NUM> is stopped. More specifically, the size of a flame kernel large enough to self-grow refers to a diameter of about <NUM> to <NUM>.

The placement of the ignition source within the spray hole-nearby region R, etc. in the early stage is achieved by selecting a spray hole distance D that is a distance between the first electrode <NUM> and the center spray hole 35c, a prechamber volume V that is a volume of the pre-chamber <NUM>, a total spray hole area S that is the sum of sectional areas of all the spray holes <NUM> formed in the dividing wall <NUM>, and/or a discharge voltage that is a voltage applied across the spark gap <NUM>. This will be described below in detail. In the case were the spray holes <NUM> are partially constricted, so that a sectional area of each of the spray holes <NUM> is ununiform, the smallest sectional area of each of the spray holes <NUM> will be simply referred to as a sectional area of each of the spray holes <NUM>.

First, the spray hole distance D will be described. The shorter the spray hole distance D, the greater the effects of flows of gas passing through the center spray hole 35c on a flame kernel. This facilitates the growth of the flame kernel, especially, in the spray hole-nearby region R, etc., and its peripheral region. For this reason, the shorter the spray hole distance D, the easier the creation of an ignition source in the spray hole-nearby region R, etc., in the early stage.

Next, the prechamber volume V and the total spray hole area S will be described in detail. The more the prechamber volume V, the faster a flow of gas moving through the center hole 35c in the after-top-dead-center ignition control mode. This is because the larger the prechamber volume V, the more slowly the pressure in the pre-chamber <NUM> drops following a drop in pressure in the main chamber <NUM> as long as the flow rate of gas moving out of the pre-chamber <NUM> into the main chamber <NUM> is kept constant, thereby resulting in an increased difference in pressure between the pre-chamber <NUM> and the main chamber <NUM>, which increases the velocity of gas passing through the center spray hole 35c.

The smaller the total spray hole area S, the faster the velocity of gas flowing through the center spray hole 35c will be in the after-top-dead-center ignition control mode. This is because as long as the prechamber volume V remains unchanged, the smaller the total spray hole area S, the smaller the flow rate of gas moving out of the pre-chamber <NUM> into the main chamber <NUM> will be, thereby causing a drop in pressure in the pre-chamber <NUM> to become slow following a drop in pressure in the main chamber <NUM>. This results in an increased difference in pressure between the pre-chamber <NUM> and the main chamber <NUM>, which increases the velocity of gas passing through the center spray hole 35c.

The increase in velocity of gas passing through the center spray hole 35c will facilitate the elongation of the spark f to the spray hole direction d1-side. This facilitates the growth of a flame kernel in the early stage, especially, in the spray hole-nearby region R, etc. and its peripheral region. When it comes to the prechamber volume V and the total spray hole area S, the smaller a spray hole ratio (S/V) that is a ratio of the total spray hole area S to the prechamber volume V, the more easily the ignition source is created in the spray hole-nearby region R, etc. at the early stage.

Next, the discharge voltage will be described below. The higher the discharge voltage, the easier a flame kernel is to grow. Additionally, the higher the discharge voltage, the higher the stability of the flame kernel, thereby facilitating the ease with which the spark f is elongated by the gas flow into the spray hole-nearby region R, etc. or its peripheral region. Consequently, the higher the discharge voltage, the more easily the ignition source is created in the spray hole-nearby region R, etc. at the early stage.

As apparent from the above discussion, the smaller the spray hole distance D or the spray hole ratio (S/V) or the higher the discharge voltage, the more easily the ignition source is created in the spray hole-nearby region R, etc. at the early stage. Too small the spray hole distance D or the spray hole ratio (S/V) or too high the discharge voltage will, however, result in another adverse effect. In this embodiment, the early creation of the ignition source in the spray hole-nearby region R, etc. is achieved by decreasing the spray hole distance D or the spray hole ratio (S/V) and/or increasing the discharge voltage to an extent that no above adverse effect occurs.

The creation of an ignition source in the spray hole-nearby region R or the spray holes <NUM> usually causes a flame grown from the ignition source to be quickly jetted into the main chamber <NUM>. The creation of an ignition source within the main chamber <NUM> causes a flame grown from the ignition source to propagate directly into the main chamber <NUM>. Such cases achieve the quick propagation of a flame into the main chamber <NUM>.

<FIG> is a graph which represents a relation between the distance between an ignition source and the spray hole center and a combustion stability index (which will also be referred to below as a coefficient of variation). The coefficient of variation is an index indicating a degree of fuel combustion in a range of the lowest stability of combustion of fuel, i.e., a misfire to the highest stability of combustion of fuel, i.e., complete combustion of fuel. The higher value of the coefficient of variation represents the higher stability of combustion of fuel. The graph in <FIG> shows that the greater the distance between the ignition source and the spray hole center, the greater the coefficient of variation will be and that when the ignition source is located <NUM> or more away from the spray hole center, the coefficient of variation will become large at an increased rate. In view of such a relation between the coefficient of variation and the distance between the ignition source and the spray hole center, the ignition source is arranged in the spray hole-nearby region R which lies <NUM> or less away from the spray hole center.

<FIG> is a graph which represents the growth of a flame kernel and shows that this embodiment achieves faster growth of a flame kernel than in comparative examples i and ii and also that the propagation of a flame is faster in this embodiment than in the comparative examples i and ii. It is also found that when a flame kernel, like in this embodiment and the comparative example i, has grown up to a given ignition threshold, it will be enabled to grow by itself and spread, while when a flame kernel, like in the comparative example ii, has not grown to the ignition threshold, it will have difficulty growing by itself, so that the flame disappears.

<FIG> is a graph which demonstrates a change in pressure in the combustion chamber and shows that the pressure raises with advance of the crank angle before the compression stroke top dead center Td and then drops with advance of the crank angle after the compression stroke top dead center Td in each of this embodiment and the comparative examples i and ii. The graph also shows that upon ignition, the pressure will rise again in this embodiment and the comparative example i. The flame kernel grows faster in this embodiment than in the comparative example i, and the propagation of the flame is faster in this embodiment than in the comparative example i, thereby causing the pressure in the combustion chamber to elevate quickly in this embodiment. Conversely, when the flame burns out, like in the comparative example, it will cause the pressure in the combustion chamber not to rise.

<FIG> is a view which shows a flowchart of a sequence of steps in a production process for the ignition system <NUM>. The production process incudes a setting step p1 and a production step p2.

In the setting step p1, a spray hole ratio (S/V) is calculated. The dimensions of the pre-chamber <NUM> and the spray holes <NUM> are determined as a function of the spray hole ratio (S/V) and the spray hole distance D. The setting step p1 will also be described below in detail with reference to <FIG>. Next, in the production step p2, the ignition system <NUM> is produced to have the dimensions set in the setting step p1.

<FIG> is a graph which represents a relation among the spray hole distance, the spray hole ratio, and the in-gap gas flow. The horizontal axis indicates the spray hole distance D. The vertical axis indicates the spray hole ratio (S/V). The curve a represents a relation between the spray hole ratio (S/V) and the spray hole distance D when the in-gap gas flow in the spray hole direction d1 is <NUM>/s at the ignition timing in the after-top-dead-center ignition control mode. The curve a may be mathematically expressed using the following approximation formula A. <MAT> where V denotes the prechamber volume V [cc (cubic centimeter)], S denotes the total spray hole area (i.e., a total sectional area of the spray holes <NUM>) [mm^<NUM>], D denotes the spray hole distance D [mm], and "^" represents a power. Note that "^<NUM>" is a cube, and "^<NUM>" represents a square.

On the upper side above the curve a, the in-gap gas flow at the ignition timing Ts in the after-top-dead-center ignition control mode is lower than <NUM>/s in the spray hole direction d1, while on the lower side below the curve a, the in-gap gas flow at the ignition timing Ts in the after-top-dead-center ignition control mode is higher than <NUM>/s in the spray hole direction d1.

In this embodiment, in order to have the in-gap gas flow in the spray hole direction d1 which is higher than or equal to <NUM>/s at the ignition timing Ts in the after-top-dead-center ignition control mode, the value of the spray hole ratio (S/V) is selected in a region β including the curve a and the lower side below the curve a. In other words, the value of the spray hole ratio (S/ V) is determined to meet the following Eq. B which is equivalent to Eq. A in which "=" is replaced by "≤".

Eq. B sets the in-gap gas flow in the spray hole direction d1 to be higher than or equal to <NUM>/s at the ignition timing Ts in the after-top-dead-center ignition control mode.

The curve a usually changes with a change in environment. For instance, when the speed of rotation of the engine <NUM> becomes high, the quantity of intake air increases, or the engine <NUM> is implemented by a high-compression engine, the curve a will be shifted to an upper right-hand side in <FIG>. Alternatively, when the speed of rotation of the engine <NUM> becomes low, the quantity of intake air decreases, or the engine <NUM> is implemented by a low-compression engine, the curve a will be shifted to a lower left-hand side in <FIG>. In such a case, it is advisable that Eq. B be corrected as needed.

However, in the absence of the above correction, the acceptable in-gap gas flow is expected to be obtained at the ignition timing Ts in the after-top-dead-center ignition control mode during the fast idle mode of engine operation in which the speed of the engine, the quantity of intake air, and the compression ratio are normal.

However, when the spray hole ratio (S/V) is lower than <NUM>, it causes a risk that too strong a gas flow may pass through the spray holes <NUM>, so that the flame is blown away. It is, therefore, preferable that the spray hole ratio (S/V) be selected to be <NUM> or more. When the spray hole distance D is zero in Eq. B, the right side will be <NUM>. Satisfying Eq. B, therefore, requires selecting the value of the spray hole ratio (S/V) to be <NUM> or less. Consequently, the value of the spray hole ratio (S/V) is determined to meet the following Eq. C in addition to Eq. B.

More specifically, it is advisable that the diameter of each of the spray holes <NUM> be selected to be <NUM> or more in order to eliminate a risk that a flame passing through each of the spray holes <NUM> may disappear due to thermal loss thereof. It is also advisable that the prechamber volume V be selected to be <NUM>. 2cc or more in order to ensure an amount of gas jetting from the prechamber <NUM> (i.e., the quantity of heat) large enough to enhance the propagation of a flame within the main chamber <NUM>.

The spray hole distance D is preferably selected in light of blowing out of the spark f or the amount of electrical power consumed by the spark plug <NUM> as a function of the size of the spark gap <NUM> because the spray hole distance D impinges on the size of the spark gap <NUM>. The area of a cross section of the center spray hole 35c is also preferably selected in light of adverse effects thereof on the spark gap <NUM>. The spray hole ratio (S/V) is preferably determined by selecting cross-sectional areas of the spray holes <NUM> other than the center spray hole 35c.

In the setting step p1 in <FIG>, the spray hole ratio (S/V) is set in the above way. Physical parameters of the spark plug <NUM> other than the spray hole ratio (S/V) may be determined in a known manner. In the production step p2, the ignition system <NUM> are made to meet the dimensions or parameters determined in step p1 to complete the ignition system <NUM>.

The functions of the ignition system <NUM> in this embodiment will be described below.

<FIG> is a sectional view which illustrates the ignition system <NUM> in the first comparative example which is different from this embodiment in that there is no dividing wall <NUM>, but the second electrode <NUM> (i.e., ground electrode) is arranged alone. <FIG> is a sectional view which illustrates the ignition system <NUM> in the second comparative example which is different from the first comparative example in that it includes the dividing wall <NUM>, but does not have the center spray hole 35c. The first electrode <NUM> does not protrude downward more than in this embodiment, but instead the second electrode <NUM> (i.e., ground electrode), unlike this embodiment, protrude greatly from the dividing wall <NUM> toward the first electrode <NUM> (i.e., center electrode). The second comparative example is designed not to meet the above Eq. B. In the second comparative example which, as described above, does not have the center spray hole 35c, the center of gravity of the spray holes <NUM> located closest to each other lies beneath the spark gap <NUM>. The downward direction in the second comparative example will be, therefore, referred to, like this embodiment, as the spray hole direction d1, while the upward direction will be referred to as the spray hole opposite direction d2.

<FIG> is a sectional view which illustrates the ignition system <NUM> in the first mode of this embodiment. <FIG> is a sectional view which illustrates the ignition system <NUM> in the second mode of this embodiment. The spray hole ratio (S/V) or the spray hole distance D in the first mode is smaller than that in the second mode. The spray hole distance D in <FIG> is shorter than that in <FIG>, thereby causing the in-gap gas flow in the spray hole direction d1 to be greater in the second mode than in the first mode at the ignition timing Ts in the after-top-dead-center ignition control mode. Consequently, an initial ignition source is generated in the pre-chamber <NUM> and/or the center spray hole 35c in the first mode, while it is created in the pre-chamber <NUM>, the center spray hole 35c, and/or the main chamber <NUM> in the second mode.

<FIG> is a timing chart which demonstrates the development of combustion of fuel in the after-top-dead-center ignition control mode in the first and second comparative examples. In the first comparative example, the combustion of fuel proceeds from the spark phase s1, to the main chamber ignition phase s2', and then to the main chamber flame propagation phase s5. The beginning of the spark phase s1 occurs at the ignition timing Ts. The spark phase s1 is a phase in which the voltage has started to be applied across the spark gap <NUM>, but a flame kernel is not yet generated in the combustion chamber <NUM>.

The main chamber ignition phase s2' is a phase in which a flame kernel is growing to a self-growable ignition source within the main chamber. The end of the main chamber ignition phase s2' coincides with the main chamber ignition timing Tj that is the timing when an ignition source is created within the main chamber <NUM>. The main chamber flame propagation phase s5 is a phase in which the ignition source is propagating to the main chamber <NUM>. The end of the main chamber flame propagation phase s5 coincides with the combustion end timing Te when the fuel is expected to have been burned completely.

In the second comparative example and the first mode, the combustion of fuel proceeds from the spark phase s1 to the pre-chamber ignition phase s2, to the pre-chamber flame propagation phase s3, to the gas jetting phase s4, and then to the main chamber flame propagation phase s5. The prechamber ignition phase s2 is a phase in which a flame kernel is growing to a self-growable ignition source in the pre-chamber. The end of the pre-chamber ignition phase s2 coincides with the pre-chamber ignition timing Ti that is the timing when the ignition source is created in the pre-chamber <NUM>.

The pre-chamber flame propagation phase s3 is a phase in which the ignition source is propagating to the pre-chamber <NUM>. The flame jetting phase s4 is a phase in which a flame in the pre-chamber <NUM>, that is, an ignition source is jetting from each of the spray holes <NUM> into the main chamber <NUM>. The beginning of the flame jetting phase s4 coincides with the main chamber ignition timing Tj that is the timing when the ignition source is placed in the main chamber <NUM>.

In the second mode, the spark f extends from inside the pre-chamber <NUM> into the main chamber <NUM> through the center spray hole 35c, thereby causing ignition sources to be created by the spark f within the main chamber <NUM> as well as the prechamber <NUM> and the center spray hole 35c. Consequently, like in the first mode, the combustion of fuel proceeds from the spark phase s1, to the main chamber ignition phase s2', and then to the main chamber flame propagation phase s5 in parallel to a series of ignition source development from the spark phase s1, to the pre-chamber ignition phase s2, to the pre-chamber flame propagation phase s3, to the flame jetting phase s4, and then the main chamber flame propagation phase s5. This causes the main chamber ignition timing Tj to appear earlier than in the first mode.

In each of the first and second comparative examples and each of the first and second modes, the combustion end timing Te is required to be earlier than the exhaust start time To that is the time when the exhaust valve <NUM> starts to open in order to avoid emission of unburned fuel. Accordingly, in any case, the combustion end timing Te is first determined prior to the exhaust start time To. Subsequently, the ignition timing Ts is determined to have fuel completely combusted at the combustion end timing Te. In other words, the ignition timing Ts is calculated back from the combustion end timing Te. Therefore, the combustion end timings Te almost coincide with each other in the first and second comparative examples and the first and second modes, while the ignition timings Ts are different from each other.

<FIG> is a graph which demonstrates the timing chart illustrated in <FIG> where the ignition timings Ts in the first and second comparative examples and the first and second modes are altered to coincide with each other for the sake of convenience. In the second comparative example, the spark gap <NUM> is located away from the spray holes <NUM>, so that the ignition source is arranged within the spray hole-nearby region R near the end of the pre-chamber flame propagation phase s3. The end of the pre-chamber flame propagation phase s3 in the second comparative example substantially coincides with a time, a crank angle of <NUM>° or more after the ignition timing Ts. In the second comparative example, the ignition source is arranged within the spray hole-nearby region R at a time, a crank angle of <NUM>° or more after the ignition timing Ts. This causes the main chamber ignition timing Tj to retard, so that the development of combustion of fuel becomes slower than that in the first comparative example which does not include the prechamber <NUM>.

In the first mode of this embodiment, the spark gap <NUM> is located closer to the center spray hole 35c, thereby causing the time when the ignition source appears in the spray hole-nearby region R to lie in the first half of the pre-chamber flame propagation phase s3. The first half of the pre-chamber flame propagation phase s3 in the first mode lies within a crank angle of <NUM>° after the ignition timing Ts. The ignition source, therefore, appears in the spray hole-nearby region R within a crank angle of <NUM>° after the ignition timing Ts. This results in smaller delay in the main chamber ignition timing Tj than in the first comparative example, so that the development of combustion of fuel becomes faster than that in the first comparative example which does not include the pre-chamber <NUM>.

In the second mode of this embodiment, the time when the ignition source is created by the spark f within the main chamber <NUM> coincides with the main chamber ignition timing Tj corresponding to the end of the main chamber ignition phase s2'. The main chamber ignition timing Tj in the second mode lies within a crank angle of <NUM>° after the ignition timing Ts. This causes the ignition source to appear in the main chamber <NUM> within a crank angle of <NUM>° after the ignition timing Ts, so that the development of combustion of fuel becomes faster than in the first mode.

<FIG> is a graph which demonstrates transitions in combustion percentage in the first and second comparative examples, and the first mode of this embodiment. In the second comparative example, the percentage of combustion of fuel, as described above, changes more slowly than in the first comparative example which does not include the pre-chamber <NUM>, while it changes in the first mode of this embodiment faster than in the first comparative example.

<FIG> is a graph which illustrates an enlarged portion of <FIG>. <FIG> is a graph which demonstrates transitions in the in-gap gas flow in a period illustrated in <FIG> in the second comparative example and the first and second modes of this embodiment. In the second comparative example, the in-gap gas flow is still oriented in the spray hole opposite direction d2 at the ignition timing Ts in the after-top-dead-center ignition control mode. This is because the in-gap gas flow is kept oriented by inertia thereof in the spray hole opposite direction d2 in a deep portion of the pre-chamber <NUM> for a while after the compression stroke top dead center Td is passed. Subsequently, the strength of the in-gap gas flow temporarily becomes zero, after which the direction of the in-gap gas flow turns in the spray hole direction d1. The gas flow in such a period of time is, therefore, very weak, so that the spark f does not extend sufficiently, thus facing a risk of failure in igniting the fuel.

In the first and second modes of this embodiment, the in-gap gas flow, as described above, has already turned from the spray hole opposite direction d2 to the spray hole direction d1 before the ignition timing Ts in the after-top-dead-center ignition control mode. This causes a strong flow of gas to be created, for example, at <NUM>/s or more in the spray hole direction d1 at the ignition timing Ts, thereby facilitating extension of the spark f in the spray hole direction d1, which enhances the growth of a flame kernel in or around the spray hole-nearby region R. Consequently, the first and second modes of this embodiment are capable of creating the ignition source quickly within the spray hole-nearby region R.

Referring back to <FIG>, the combustion end timing Te is, as described above, required to be earlier than the exhaust start time To. It is, therefore, impossible to retard the ignition timing Ts sufficiently in the fast idling mode in which the timing of fuel combustion needs to be as late as possible in the first and second comparative examples in which a combustion time period that is an interval between the ignition timing Ts and the combustion end timing Te, thereby resulting in a difficulty in sufficiently retarding the combustion center-of-gravity Tc that is equivalent to a time when <NUM>% of fuel has been combusted, which may lead to a lack in warming the catalyst in the fast idling mode.

In contrast to the above, the first and second modes of this embodiment are capable of shortening the combustion period of time that is an interval between the ignition timing Ts and the combustion end timing Te as compared with the first and second comparative examples, thereby enabling the ignition timing Ts to be retarded sufficiently. This permits the combustion center-of-gravity Tc to be retarded greatly as compared with the first and second comparative examples, thereby ensuring the stability in warning the catalyst in the fast idling mode, which leads to a decreased duration of the fast idling mode to improve the fuel consumption in the engine <NUM> and reduce exhaust emissions from the engine <NUM>.

This embodiment offers the following beneficial advantages. In the after-top-dead-center ignition control mode, the ignition source is created in the spray hole-nearby region R, the center spray hole 35c, or the main chamber <NUM> shown in <FIG> early, e.g., within a crank angle of <NUM>° following the ignition timing Ts. When the ignition source is arranged in the spray hole-nearby region R or the center spray hole 35c, it facilitates the jetting of a flame, as grown from the ignition source, from the center spray hole 35c into the main chamber <NUM>. Alternatively, when the ignition source is provided in the main chamber <NUM>, it will cause a flame grown from the ignition source to propagate as it is in the main chamber <NUM>. It is, therefore, possible to propagate the flame quickly to or in the main chamber <NUM> in the after-top-dead-center ignition control mode.

The second mode of this embodiment offers the following beneficial advantages. In the after-top-dead-center ignition control mode in the second mode, the spark f is extended to reach the inside of the main chamber <NUM>, thereby creating the ignition source within the main chamber <NUM> at an early point within a crank angle of <NUM>° after the ignition timing. The flame, as growing from the ignition source, propagates as it is to the main chamber <NUM>. The second mode, therefore, ensures quick propagation of the flame to the main chamber <NUM>.

Additionally, the dimensions of the pre-chamber <NUM> and the spray holes <NUM> are selected to meet the above Eq. B in order to have the spray hole ratio (S/V) falling in the region β illustrated in <FIG>. The region β is, as described above, a region where the in-gap gas flow at the ignition timing Ts in the after-top-dead-center ignition control mode moves at <NUM>/s or more in the spray hole direction d1. The in-gap gas flow which moves at <NUM>/s or more in the spray hole direction d1, as already described, serves to facilitate early creation of the ignition source in the spray hole-nearby region R.

The region β depends slightly on the circumferences, but however, is expected to produce a required degree of the in-gap gas flow at the ignition timing Ts in the after-top-dead-center ignition control mode when the engine <NUM> is operated at a usual speed, with a usual quantity of intake air, and at a usual compression ratio. The quick creation of the ignition source in the spray hole-nearby region R is, therefore, achieved easily by selecting the spray hole ratio (S/V) to lie in the region β.

The second mode also has the following beneficial advantage. When the spray hole ratio (S/V) is less than <NUM>, it will, as described above, result in too strong a flow of gas passing through the spray holes <NUM>, thereby undesirably blowing away the flame. This embodiment is, as described above, configured to set the spray hole ratio (S/V) to be <NUM> or more, thus eliminating the above problem.

The second mode also has the following beneficial advantage. The in-gap gas flow, as can be seen in <FIG>, turns from the spray hole opposite direction d2 (i.e., the spray hole opposite direction d2-side) to the spray hole direction d1 (i.e., the spray hole opposite direction d1-side) until the ignition timing Ts is reached in the after-top-dead-center ignition control mode. This causes the in-gap gas flow to be oriented in the spray hole direction d1 at the ignition timing Ts in the after-top-dead-center ignition control mode, thereby ensuring the stability in extending the spark f in the spray hole direction d1. This facilitates the quick creation of an ignition source within the spray hole-nearby region R.

The above described modes of this embodiment may be modified in the following ways. In the first embodiment, the dividing wall <NUM> has a plurality of spray holes <NUM> formed therein, but may alternatively be designed to have only the single center spray 35c. The first embodiment has the first electrode <NUM> located closest to the center spray hole 35c, but however, may alternatively be designed to arrange the first electrode <NUM> closest to one(s) of the other spray holes <NUM> to extend the spark f toward it. The dividing wall <NUM> in the first embodiment also works as the second electrode <NUM> and is attached to the head <NUM> in electrical conduction therewith, but however, it may be configured to have a protrusion(s) which is in electrical conduction to the dividing wall <NUM> and serves as the second electrode <NUM>. Alternatively, the second electrode <NUM> may be made of a member which is discrete from the dividing wall <NUM> and electrically connects with the head <NUM>.

Claim 1:
An ignition system comprising:
a dividing wall (<NUM>) which divides a combustion chamber (<NUM>) of an engine (<NUM>) into a main chamber (<NUM>) and a prechamber (<NUM>) and has formed therein at least one spray hole (<NUM>) which communicates between the main chamber (<NUM>) and the pre-chamber (<NUM>); and
a spark plug (<NUM>) in which voltage is applied across a spark gap (<NUM>) between a first electrode (<NUM>) and a second electrode (<NUM>) to create an electrical spark (f) to ignite fuel, wherein
the pre-chamber (<NUM>) houses the spark plug (<NUM>) with the first electrode (<NUM>),
the dividing wall (<NUM>) or a member which is electrically connected to the dividing wall (<NUM>) has the second electrode (<NUM>),
an after-top-dead-center ignition control mode in which an ignition operation to ignite the fuel using the spark plug (<NUM>) is performed after a compression stroke top dead center (Td) is executed when a given operating condition of the engine (<NUM>) is met, and
in the after-top-dead-center ignition control mode, an ignition source which is in a form of a self-growable flame kernel is provided in a spray hole-nearby region (R) within the pre-chamber (<NUM>), the spray hole (<NUM>), or the main chamber (<NUM>) within a crank angle of <NUM>° after an ignition timing (Ts) where the ignition timing (Ts) is a timing when the voltage starts to be applied across the spark gap (<NUM>), and the spray hole-nearby region (R) is a region which is located <NUM> or less away from a spray hole center that is a center of an opening of the spray hole (<NUM>) which faces the pre-chamber (<NUM>), characterized in that
a relation of S/V ≤ -<NUM>.025D^<NUM> + <NUM>.34D^<NUM> - <NUM>.4D + <NUM> is met where V is a volume of the pre-chamber (<NUM>) in units of cubic centimeter, S is a total sectional area of the at least one spray hole (<NUM>) in the dividing wall (<NUM>) in units of mm^<NUM>, and D is a distance between the first electrode (<NUM>) and spray hole (<NUM>) in units of mm or when the dividing wall (<NUM>) has a plurality of spray holes (<NUM>) formed therein, D denotes a distance between the first electrode (<NUM>) and one (<NUM>) of the spray holes (<NUM>) which is located closest to the spark gap (<NUM>).