Research on formation and growth of flame kernel in spark ignition engine

ELSEVIER

JSAE Review 16 (1095) 7-11

Research on formation and growth of flame kernel in spark ignition engine Tohru Yoshinaga ~', Toshihiko Igashira a, Hisashi Kawai a, Norihiko Nakamura b Nippon Soken Inc., 13 lwaya, Shimohasumi-cho, Nishio-shi, AichL 445 Japan b Vehicle Decelopment Center 4, Toyota Motor Co., Ltd., 1200, Mishuku, Susono-shi, Shizuoka, 410-11 Japan Received 1 August 1994

Abstract The recent developments for lean mixture and high EGR ratio in the spark ignition engine have improved fuel consumption and exhaust emission, so that partial pressure of oxygen, or fuel density in the cylinder, is reduced; thus, the energy necessary for ignition is increased [1-4]. This problem is solved either by supplying high energy in a short period or by slowly raising the amount of energy. This research uses the latter method. The formation and growing process of the flame kernel was observed under various energy discharging durations. Therefore, a stable ignition that is effected by the extended energy discharging duration was found. Consequently, this helps the growth of the flame kernel. The effective spark duration under easy-to-fire conditions was also investigated.

1. Testing ignition power supply There are two types of power supply. Full transistor inductive arc system that is generally used, and alternating continuous arc system that can control spark duration. Figure 1 is the circuit diagram of the experimental apparatus of the alternating continuous arc system. When reversing the transistor phase using the induction push-pull circuit on the primary side of the closed magnetic transformer with an air gap, high alternating voltage is generated continuously every 100 g s on the secondary side of" the transformer, according to the number of turns of the coil. Figure 2 shows the spark pattern in the experiment. Shown above is the inductive arc system and below is the alternating continuous arc system whose spark duration is variable. The arc current of the inductive arc system reduces from 40 mA gradually. The arc current of the alternating continuous arc system has a constant of 40 mA. The spark end timing is set constant to the top dead center (TDC) for both. Table 1 shows the discharge patterns of ignition energy in the experiment. Energy decreases as time passes by the inductive arc pattern; however, the alternating continuous arc system can always supply constant energy.

2. Evaluating ignitability on the engine 2. I. lgnitabili~ with lean mixture and high EGR ratio Figure 3 shows No x and fuel economy in lean mixture operation with low fuel density in mixture and Fig. 4 shows NO X and fuel economy in high EGR operation with low fuel density in mixture and low partical pressure of oxygen. When air fuel ratio was increased, in type A, misfire occurred at 17 and fuel economy deteriorated. However, in type B, mixture was able to be lean up to 19, when EGR ratio was increased, in type A, fuel economy deteriorated due to misfire at 15%, while in B type, EGR was possible up to 23%. [ Circuit ]

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Fig. 1. Alternating continuous arc system.

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cause of misfire, which is assumed to have occurred judging from the increassd HC. In the case of type B, however, misfire did not occur in the area that misfire occurred in type A, and the MBT of type B was BTDC 60 ° . Thus it may be said that BTDC 60 ° of type B is "True M B T " and BTDC 35 ° of type A is "Apparent MBT".

3. O b s e r v a t i o n o f f l a m e kernel f o r m a t i o n and g r o w t h

2.2. Discharge pattern of ignition energy 3.1. Experimental apparatus Figure 5 shows the investigation results of the minimum advance for best torque (MBT) for types A and B. The MBT of type A has before top dead center (BTDC) of 35 °. Fuel consumption deteriorates with more advancing be-

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3.2. True MBT First, the spark duration was prolonged to observe if the MBT moved to the advance side. The result is Fig. 7. The single-cylinder for observation can easily misfire under the conditions of 1000 rpm and volume efficiency ~v = 30%. The MBT of type A was BTDC 20 °. At this point in time, misfire has already occurred, and on the more advancing side, misfire increased. As the spark start timing advances, type B misfire decreases to be finally eliminated and MBT is obtained at BTDC 60 °. The reason misfire increases as the spark start timing retards in type B is that the spark is

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completed at TDC and the spark duration becomes shorter. For type B, "True M B T " is obtained in the same way as shown in the results of Fig. 5.

smooth start caused by sufficient combustion. As a result, fuel consumption was improved. However, if the spark starts very early, there is a possibility that ignition timing varies much and combustion becomes unstable. Therefore, combustion start timing was investigated from the observation results. Figure 10 shows the investigation results. In this inspection the combustion start timing was defined as the timing when the flame area grows to 10% of the area of the combustion chamber. Variance of the combustion start timing was found almost the same by the spark start at BTDC 60 ° and BTDC 20 °.

3.3. Observation of flame kernel Figure 8 shows the formation and growth of the flame kernel of type A inductive arc pattern and type B continuous arc pattern. A-a shows an example of flame kernel growth into burning and A-b shows an example of nongrowth of the flame kernel that results in misfire. In A-a, after ignition of BTDC 20 ° there are flame kernel formed at BTDC 16 ° and growth at BTDC 12 °. In A-b, the flame kernel is formed at BTDC 16 °. However, it had not enough growth at BTDC 12 ° and it disappeared at BTDC 8°. If a slight increase in ignition energy were supplied, misfire should have not occurred. In type B, the spark starts at BTDC 60 ° and continues to TDC. A flame kernel that is formed at BTDC 50 ° is carried by gas flow; however, it is closely connected to the plug gap. A constant supply of energy assists the flame kernel growth. The flame kernel formation and growth that supplies ignition energy in a short time by the inductive spark pattern may be said "oviparous" and by the alternating continuous arc pattern may be said "viviparous". Figure 9 shows the cylinder pressure trace for Fig. 8. The MBT of A-a cannot be "True M B T " due to slow combustion. The MBT of B is "'True MBT" due to the

4. Erosion of the spark plug electrode

4.1. Effective spark duration This study proves that longer spark duration in lean mixture or at high EGR ratio contributes to accelerating flame kernel formation and growth. However, under easyto-fire conditions in high load operation, less ignition energy is required. Long spark duration in high load operation will accelerate energy loss and the erosion of the spark plug electrode. First the effect of the spark end timing was checked while varying load. Figures 11 and 12 show the results. Low load operation requires spark up to TDC; however, in Single Cylinder

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high load operation the spark duration can be shorter. Only 7 mJ of converted energy is required for ignition.

4.2. Erosion of the .spark plug electrode The relation between spark duration and the erosion of the spark plug electrode was also investigated. Figure 13 shows the erosion amount of the spark plug electrode under the conditions of high load, when spark start timing is constant at BTDC 18°, and the spark end timing is varied. Longer spark duration increases spark plug electrode erosion. If the spark is ended before TDC, the erosion of the spark plug electrode has not much increased. If the spark continues after TDC, the erosion of the spark 1.0

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plug electrode is drastically accelerated. The factors are atmospheric pressure and temperature during sparking. This quick erosion is assumed to occur due to sharp rising of pressure and temperature caused by combustion that has occurred often TDC. A testing container was fabricated so that pressure and temperature are altered in order to investigate the influence on the erosion amount of the spark plug electrode. Figure 14 shows the effects of pressure and temperature. The higher the pressure and temperature rose, the more the spark plug electrode accelerated in erosion. As a result, this test realized that the spark under the conditions of high pressure and temperature remarkably accelerates spark plug electrode erosion. From the above results, the spark should end before TDC in order to minimize spark plug electrode erosion. Figure 15 shows the spark duration of type A and B in the erosion test of the spark plug electrode. The spark duration of type B is controlled. Figure 16 shows the result of the electrode erosion in pattern running as the engine conditions are changed. For comparison, type A and type ,K, high energy type with 70 mJ energy by inductive spark, were also tested. The high energy type ,K had more spark plug electrode erosion than type A. The continuous spark of type B could increase energy for better ignitability with less erosion than type A.

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T. Yoshinaga et al. /'JSAE Ret iew 16 (1995) 7-11

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5. Summary (1) The lean mixture and high EGR ratio require high energy for ignition. Under running conditions, ignitability can be improved by longer spark duration for the continuous arc system and the advance to "'True M B T " is possible, resulting in improved fuel economy. The inductive spark system with energy decrease as time passes is forced to take " A p p a r e n t M B T " , due to increased misfire, causing fuel economy deterioration. (2) The " v i v i p a r o u s " growth of the flame kernel in the continuous arc system is the key to improve ignitability; that is, energy is successively supplied to the flame kernel. However, the flame kernel growth in the inductive arc system is " o v i p a r o u s " due to energy supplied in a short time. (3) Under the conditions that require less energy for ignition, short spark duration and the spark ending before TDC are effective to minimize energy loss and the erosion of the spark plug electrode. Particularly, the spark at A T D C (high t e m p e r a t u r e / p r e s s u r e ) remarkably accelerates erosion of the electrode.

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References [1] Tanuma, T. et al., Ignition, Combustion and Exhaust Emissions of Spark Lean Mixtures in Automotive Spark Ignition Engines, SAE Paper, No. 710159 (1971). [2] Richard, R. et al.. Measuring the Effect of Spark Plug and Ignition System Design on Engine Performance, SAE Paper, No. 720007 (1972). [3] Konishi, M. et al.. Analysis of the Mechanism of Misfire and the Effects of [gniton System Modification on the Lean Misfire Limit (in Japanese), TOYOTA Technical, Vol. 27, No. 2, pp. 25-33 (1977). [4] Iguchi, S., Combustion and Current Status of Lean Burn Engine (in Japanese), SAE Symposium, No. 16, pp. 1-6 (1991). [5] Namazian, M. et al., Schlieren Visualization of the Flow and Density Fields in the Cylinder of a Spark Ignition Engine. SAE Paper, No. 800044 (1980).

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