WO2011023852A1

WO2011023852A1 - Ignition control of spark ignited reciprocating combustion engine

Info

Publication number: WO2011023852A1
Application number: PCT/FI2010/050665
Authority: WO
Inventors: Lennart Haraldson; Lennart Andersson
Original assignee: Wärtsilä Finland Oy
Priority date: 2009-08-28
Filing date: 2010-08-25
Publication date: 2011-03-03
Also published as: FI20095889A0; EP2470778A1; FI123042B; FI20095889A

Abstract

The present invention relates to a method of a reciprocating combustion engine with at least two cylinders. Primary voltage is taken from a voltage supply and transformed to secondary voltage suitable for ignition purposes. The invention is based on an adaptive control method and means of a combustion engine ignition system. The spark plugs with the highest secondary voltage demand are recognised by continuously measuring the actual secondary voltage demand. The ignition timing is advanced (503) for individual cylinders before the secondary voltage demand reaches a level where misfire can occur.

Description

Ignition control of spark ignited reciprocating combustion engine

Technical field

The present invention relates to spark ignited reciprocating combustion engines fuelled by natural gas or gasoline.

Background

Ignition systems include ignition coils which comprise primary windings and secondary windings and by which voltage transformation is implemented. The secondary voltage is distributed to spark plugs, which provides sparks across their gaps. The sparks ignite mixtures of air and fuel in cylinders. The mixtures burn and cause pressure in the cylinders to force pistons to move and thus the burnings produce mechanical power.

The timing of the spark is important. The duration in crank angle degrees that the fuel takes to burn will increase somewhat with engine speed. This means that the faster the engine runs the earlier the spark has to occur. The timing of the spark is called ignition timing. By retarding the ignition timing maximum cylinder pressures will be reduced, which can reduce some pollution formation and eliminate knocking if that is a problem. However, the retarded ignition timing will also reduce thermal efficiency of the engine and thereby increase fuel consumption. The voltage demands of the spark plugs tend to increase, when the spark plugs are aging due to wear of the electrodes. In multi cylinder combustion engines, the spark plugs tend to have different voltage demands. In the prior art the cylinder with the highest secondary voltage demand determines the time between changes of spark plugs in order to prevent unsparking of the plugs, which may resultin engine malfunction. The US patent publication 4 018 202 presents an adaptive system for digitally controlling engine performance. The drawback in the ignition system of US 4 018 202 is the fact that the misfire can occur resulting in engine malfunction, if the engine is driven after the voltage demand of at least one spark plug has reached a level where the misfire can occur. The cylinder with the highest secondary voltage demand determines the time between changes of spark plugs in order to prevent unsparking resulting in engine malfunction.

The US patent publication 5 623 209 presents a method for diagnosing a condition of a capacitive discharge ignition system. US 5 623 209 presents also means for diagnosing individual spark plugs.

Short description of the invention

The object of the invention is to increase the time between changes of the spark plugs. The objective will be achieved as presented in the independent claims. Preferred embodiments of the invention are presented in the dependent claims. The invention is based on an adaptive control method. The spark plugs with the highest secondary voltage demand are recognised by continuously measuring the actual secondary voltage demand.

The ignition timing is advanced for individual cylinders before the secondary voltage demand reaches a level where unsparking can occur. By using the invention significant benefits can be achieved. Advancing the ignition timing reduces the secondary voltage demand due to that the pressure in the cylinder decreases as ignition timing is advanced. The effect of the actual pressure in the combustion chamber at the time of the spark is significant for the secondary voltage demand. For example, in spark ignited natural gas engines the secondary voltage demand can be approx. 1000V lower for every crank angle degree the ignition timing is advanced at full load. This gives additional opportunity to prolong the time between the changes of the spark plugs, which is an important factor in many applications. Short description of drawings

The invention is presented with reference to the following figures:

Figure 1 is a graph illustrating the relation between a diagnostic number and the voltage demand of the spark plug; Figure 2 is a graph illustrating an embodiment according to the invention wherein the diagnostic number and thresholds is presented;

Figure 3 is a graph illustrating a second embodiment according to the invention wherein the diagnostic number and thresholds are presented;

Figure 4 is a flow chart of an ignition control method according to an embodiment of the invention; and

Figure 5 is a flow chart of an ignition control method according to a second embodiment of the invention.

Detailed description of drawings

In the graph of figure 1 diagnostic curve 11 illustrates a spark diagnostic value i.e. diagnostic number. The spark diagnostic value, referred as the diagnostic number in figure 2, is a number, which correlates with voltage demand at the spark plug and can be calculated at every firing of each cylinder. The spark diagnostic value can, for example, be obtained in a following way:

- detecting the voltage across the storage capacitor - time is measured between two thresholds, the first marking the start of discharge of the storage capacitor and the second marking the end of the discharge of the storage capacitor

- the spark diagnostic value, which represents a condition of a spark plug circuit, such as an high secondary spark voltage demand of the spark plug, can be determined based on this measured time. The diagnostic curve 11 is presented for giving an idea about the relation between the spark diagnostic value, that can be dimensionless, and the voltage required obtaining the spark. Although there is the relation between the spark diagnostic value and the voltage, the spark diagnostic value can be judged also by using experimental spark diagnostic value data. For example, when it has been experienced that the normal plug starts to misfire when the spark diagnostic value is approximately 160, you also know that, the exchange of the plug would be best when the spark diagnostic value reaches that value. This data can be readily stored and used when later described offset thresholds are determined.

Following values can be determined from the spark diagnostic value. The coefficient of variation of the spark diagnostic value is referred here as COV. COV indicates the relative stability of voltage demand on real-time basis. The average value of the spark diagnostic value for the cylinder is referred here as CAVG. The average value of the spark diagnostic values for all cylinders is referred here as EAVG.

Figure 2 is a graph illustrating an embodiment according the invention wherein the diagnostic number and thresholds are presented. Reference number 21 denotes CAVG, the average value of the spark diagnostic value for the cylinder. Reference number 22 denotes an offset threshold value, which activates the adjusting of the ignition timing. This provides execution of advancing the ignition in a restricted manner to occur when the average of the spark diagnostic value 21 of the spark plug exceeds the offset threshold 22. This has an advantage in that older spark plugs can be used due to the pressure reduction achieved by the advancing.

Reference number 23 denotes an indication threshold, which can also be used as an initial condition to provide a spark change notification to a user. It is noted, that additional indication thresholds for similar purposes can be used. The values of these thresholds 22 and 23 can be different from engine to engine. Thresholds can be predetermined and changed based upon historical data, because of the CAVG number differs from engine to engine and from application to application. However, it is possible to identify that the plugs start to misfire above a certain CAVG number.

Figure 3 discloses embodiment, where CAVG is compared to EAVG value, in order to determine time to advance of the ignition. EAVG, engine average CAVG value, can be calculated by adding all the individual CAVG values and divide it by the quantity of the cylinders. When an individual cylinder(s) gives a CAVG significantly lower or a lot higher than the EAVG, the user can be alarmed. The adjusting of the ignition timing can be executed for the spark plug whose average of the spark diagnostic value, CAVG (21 in figure 2) exceeds the average of the engine spark diagnostic value 31 , EAVG of the spark plugs by a predetermined deviation amount. This predetermined deviation amount can be set, for example 2 - 10% from EAVG, The advancing of the ignition is done after the deviation offset threshold

32 value is exceeded. This can also be indicated after the notification threshold

33 is exceeded. Therefore the lower pressure level at the ignition moment is achieved, and thus a lower secondary voltage demand can be used. This deviation offset threshold 32 can be predetermined based upon data obtained from experience with a normal set of spark plugs running under normal conditions.

It is noted that embodiment of figure 3 can be combined with embodiment of figure 2. This requires different procedures for separate offset thresholds. Offset threshold 32 for deviation from engine average CAVG value and offset threshold 22 for high CAVG value can be used.

This has an advantage in that the deviation between the spark plugs can be compensated starting from offset threshold 32. Use of the offset threshold 22 provides possibility to prevent misfire even when spark plug lifetime is approaching to lifetime level of 100%.

Another alternative to combine the embodiment of figure 2 with the embodiment of figure 3 is to change the deviation amount in relation to the CAVG values. The deviation amount accepted is higher when EAVG values are low and lower when EAVG values increase. This can be obtained with several offset threshold

32 values or by scaling the deviation offset threshold 32, such that lower deviation amount initiates the advancing of the ignition when spark plugs are aging. It is noted that, values for deviation amount accepted may vary. The value for deviation amount may vary, for example, in a way that it decreases when CAVG number increases.

Figure 4 is a flow chart of an ignition control method according to the invention. The method is for a combustion engine including at least two spark plugs, where primary voltage is taken from a voltage supply and transformed to secondary voltage suitable for ignition purposes. In the method of figure 4 the following steps are implemented:

501 ) Determining a spark diagnostic value, indicative to secondary voltage demand, separately for each spark plug during the ignition system is in operation. 502) Comparing spark diagnostic values to predetermined thresholds in order to determine if the secondary voltage demand for any of the spark plugs is approaching the level where misfire can occur, which may result in engine malfunction.

503) Adjusting the ignition timing, of the spark plug, whose spark diagnostic value exceeds a offset threshold, by advancing the ignition timing of the spark plug to obtain an engine angular position where combustion chamber pressure is lower than at pre-existing engine angular position. The steps 501 and 502 are implemented to find out if the voltage demand of any of the spark plugs of the system is approaching the level where misfire can occur and for noticing the spark plug(s) which is causing the risk.

Advancing the ignition timing is thus executed to the spark plug(s) before the secondary voltage demand has approached the level where misfire can occur and which may result in engine malfunction.

The flow chart of figure 5 differs from figure 4 in that the knocking level of the cylinder is taken into consideration when executing the adjusting of the ignition timing 503. By advancing the timing for the spark and at the same time keeping the load/boost pressure constant the specific cylinder will move closer to knocking. This is solved by increasing the air/fuel ratio for the specific cylinder simultaneously when the ignition timing for the same cylinder is advanced. This has an effect in that the knock margin can be controlled and kept at a constant level. Control system is ordered for a reduced exhaust gas temperature for the specific cylinder and by that will the air/fuel ratio increase for the cylinder by feeding less fuel (gas).

By feeding one cylinder with less fuel (gas), the load for the specific cylinder is reduced and this action will to some extent increase the load on the other cylinders on the engine (when keeping the engine output constant), but this effect is rather insignificant in engines with several cylinders, for example 16-20 cylinders. The embodiment of figure 5 discloses two additional steps:

504) Comparing knock value to knocking limit threshold

505) Controlling Air/Fuel ratio. Having described certain embodiments of the invention, it will now become apparent to one of skill in the art that other embodiments incorporating the concepts of the invention may be used. Therefore, the invention should not be limited to certain embodiments, but rather should be limited only by the scope of the following claims.

Claims

1. An ignition control method of a reciprocating combustion engine with at least two cylinders, in which engine ignition energy is provided to spark plugs for generating sparks suitable for firing in combustion chambers, the method comprising steps of:

- generating a secondary voltage to the spark plugs from a primary voltage supply, - determining (501 ) a spark diagnostic value (11 , 21 ) indicative of a secondary voltage demand separately for each spark plug during the engine is in operation, characterized in that the method further comprises a step of:

- adjusting ignition timing (503) of the spark plug, whose spark diagnostic value (11 , 21 ) exceeds a offset threshold (22, 32), by advancing the ignition timing of the spark plug to engine angular position where a combustion chamber pressure is lower.

2. A method of claim 1 , wherein the adjusting the ignition timing (503) is executed to the spark plug with the highest spark diagnostic value (11 , 21 , 31 ).

3. A method of claim 1 or 2, wherein the method further comprising a sub-step of:

- executing the adjusting the ignition timing (503) in a restricted manner only when average of the spark diagnostic value (21 ) of the spark plug exceeds the offset threshold (22).

4. A method of claim 1 or 2, wherein the method further comprising a sub-step of: - executing the adjusting the ignition tinning (503) in a restricted manner only when average of the engine spark diagnostic values (31 ) of the spark plugs exceeds the offset threshold (32).

5. A method of claim 4, wherein the method further comprises a sub-step of: - executing the adjusting the ignition timing (503) in the spark plug whose average of the spark diagnostic value (22) exceeds an average of engine spark diagnostic value (31 ) of spark plugs by a predetermined deviation amount.

6. A method of any previous claims, wherein the method further comprising a step of: - controlling air/fuel ratio (504) delivered to the cylinder, in such a way that the amount of fuel feeding of the cylinder, which ignition timing is advanced, is reduced in order to prevent knocking of the cylinder.

7. A method of any previous claim, wherein the method further comprising a step of: - indicating occurrence of the ignition timing advancing if the spark diagnostic value (11 , 21 ) exceeds a notification threshold (23, 33).