Development of Ignition Technology for Dilute Combustion Engines.
Improving vehicle fuel economy to achieve C[O.sub.2] emissions reduction for the global warming and energy issues is essential. Automakers have been developing technologies (e.g. direct injection, turbo charged downsizing and high compression ratio) to improve vehicle fuel economy through engine thermal efficiency enhancement. In recent years, dilute combustion such as cooled EGR and lean burn combustion are regarded as key technologies to achieve engine thermal efficiency enhancement [1,2]. However, excessive dilute combustion leads to combustion speed reduction because of lower fuel density, and limits the engine thermal efficiency improvement effect. Generally, intensifying in-cylinder turbulence is known to be an effective way to improve combustion speed [3,4, 5], as it can stabilize dilute combustion by promoting flame propagation. However, specifically focusing on ignition and initial process of combustion, stabilization of ignitability and initial combustion under high gas flow velocity and dilute air-fuel mixture conditions is a major issue. Therefore, development of ignition technology for achieving stable dilute combustion has been proposed by many researchers. For example, there is radio frequency ignition, corona ignition, micro wave ignition, laser ignition, multiple spark plug and so on [6, 7, 8, 9, 10], However, these technologies have not been put to practical use due to many issues such as energy consumption and packaging, etc. In recent years, technical approach by conventional spark ignition systems has also been actively proposed because conventional spark ignition has high energy density and high versatility [11, 12]. Although they have investigated the effect of dilute combustion limit by enhancing discharge current / energy, they have not yet been quantified sufficiently to optimize discharge specification based on the discharge principle under various engine conditions.
This paper focuses on the difference of flame kernel formation and growth by the discharge phenomenon such as discharge stretching, blow-off and short-circuiting under various discharge environments and discusses the optimal discharge specification for dilute combustion based on the discharge principle. Furthermore, the effect of ignitability improvement is evaluated by using a dilute combustion engine followed test result analysis.
THE AIM AND ISSUE OF DILUTE COMBUSTION
Enhancing engine thermal efficiency leads to improvement of vehicle fuel economy. The theoretical thermal efficiency is shown in Equation (1).
[[eta].sub.th] = 1-[([1/[eosilon]).sup.[kappa]-1] (1)
Where, [epsilon] is compression ratio, and K is specific heat ratio. Increasing these parameters enhances thermal efficiency. Dilute combustion is an effective way to achieve this. For example, cooled EGR combustion is a technology to enhance thermal efficiency by achieving suppression of knocking caused by high compression ratio and reduction of engine cooling heat loss. Lean burn combustion can also enhance thermal efficiency by increasing specific heat ratio. For these reasons, dilute combustion has been adopted widely in recent years.
However, unstable combustion fluctuation and speed due to lower fuel density inhibits expansion of dilute ratio. This has become an obstacle to enhancing thermal efficiency. Figure 1 shows relationship between excess air ratio and combustion duration, as well as combustion fluctuation and fuel consumption in lean burn combustion. Both 0-10 % and 10-90 % combustion duration are extended by increasing excess air ratio, and these deteriorate combustion fluctuation and fuel consumption. These extensions of combustion duration have each factor. The factor of 0-10 % combustion duration is unstable flame kernel formation and poor flame growth. The factor of 10-90 % combustion duration is flame propagation velocity reduction. These have become major issues to achieve thermal efficiency enhancement in dilute combustion.
The following images show flame kernel formation, flame growth and flame propagation. Figure 2 shows the process from flame kernel formation to flame propagation.
[E.sub.IN] = [E.sub.IG] - [Q.sub.SP] (2)
[mathematical expression not reproducible] (3)
Each parameter is defined as the following. [E.sub.IN] is energy supplied to the air-fuel mixture by the discharge, and is represented by the simple expression of Equation (2). [E.sub.IG] is ignition energy to be supplied from the ignition system, it is shown as Equation (3). [V.sub.2] is discharge voltage between the electrodes of the spark plug, [I.sub.2] is discharge current, and [T.sub.2] is discharge duration. [Q.sub.sp] is cooling loss energy in the spark plug.
After discharge initiation, the flame kernel begins to be formed by supplying energy to the air-fuel mixture while the discharge is stretched by the gas flow. Although some ignition energy is lost due to cooling of the spark plug, the flame kernel continues to grow as long as ignition energy is continuously supplied. When the flame kernel is sufficiently grown, it can propagate by chemical reaction. In general, minimum ignition energy for 50 % successful ignitability is significantly increased when excess air ratio increases . For that reason, ignition energy intensification and spark plug cooling loss reduction are required as ignition technology for promotion of flame formation and growth under dilute combustion is advanced. In addition, promotion of flame propagation is also essential for dilute combustion establishment. Equation (4) shows the flame propagation velocity.
Wf = Wb + [S.sub.T] (4)
[S.sub.T] [congruent to] [S.sub.L] + u' (5)
[S.sub.L] = [3O.5-54.9[([phi]-1.21).sup.2]][([T/[T.sub.0]]).sup.a][([P/[P.sub.0]]).sup.b] (6)
a = 2.18-0.8([phi]-1) (7)
b = -0.16-0.22([phi]-1) (8)
Where, Wf is flame propagation velocity, Wb is gas movement velocity and [S.sub.T] is turbulent burning velocity. Shown in Equation (5), [S.sub.L] is the laminar burning velocity and u' is turbulent intensity. In Equation (6). [T.sub.0] = 298 K, and [P.sub.0] = 101.3kPa are reference temperature and pressure. a and b are constants for a given fuel equivalent ratio [phi].
Flame propagation velocity is determined by gas movement velocity and turbulent burning velocity. Since turbulent intensity is almost the same, even when dilute ratio is increased, turbulent burning velocity is decreased by reduction of laminar burning velocity at a high dilute ratio condition as shown in Equation (6). In order to solve it, intensifying in-cylinder turbulence such as tumble and swirl has been used. Increase of the turbulent burning velocity promotes flame propagation. However, the larger the value of u'/[S.sub.L] shown in Karlovitz number is, the higher minimum ignition energy is [3, 13], The cause is increase of turbulent burning velocity fluctuation and heat loss to turbulence. Thus, the promotion of flame propagation also requires enhancing ignition energy. Furthermore, focusing on ignition environment, gas flow velocity around the spark plug is faster when turbulence intensity is increased. As shown in Figure 2. discharge is usually stretched by gas flow. This discharge stretch continuously supplies energy to wide air-fuel mixture area and achieves larger flame kernel formation. However, high gas flow velocity causes discharge blow-off and inhibits discharge stretch. As the result, it causes a drastic deterioration of flame kernel formation. For these reasons, ignition technology development is strongly required for dilute combustion.
THE ROLE OF IGNITION SYSTEM UNDER DILUTE COMBUSTION
The role of an ignition system is stable flame kernel formation in various ignition environments such as fuel density and gas flow, and for that, ignition systems must continuously supply necessary and sufficient energy to the air-fuel mixture. However, occurrence of discharge blow-off inhibits continuous energy supply. There, the state of flame kernel formation during a cycle in which discharge blow-off occurred on an engine was observed. Figure 3 shows the observation method and result. Visible window was set up on the rear of engine, and flame kernel formation process of cylinder #4 was observed by a high speed camera (Photoron FASCAM SA5). And also, discharge voltage was measured with a high voltage probe (Tektronix P6015A) from the middle of high tension cable and discharge current was measured with a current probe (Tektronix TCP303) attached directly to a high tension cable.
After initiation, discharge starts to supply energy to the air-fuel mixture while discharge stretches, and then the first blow-off occurred. In this time, a small flame kernel could be confirmed. After that, a second flame kernel was formed by re-discharge. Each flame existed apart from each other, and they were anti-inflammatory without flame propagation due to lack of energy. This flame observation result has revealed that blow-off inhibits stable flame kernel formation by separation of flame and lack of energy. Therefore, suppressing discharge blow-off is considered an important technology to achieve stable dilute combustion.
Before examining blow-off suppression method, the blow-off occurrence mechanism should be mentioned. Figure 4 shows the process of discharge blow-off.
Breakdown is caused by repeating ionization of gas molecules (a effect) and secondary electron emission ([gamma] effect) when a voltage difference is generated between spark plug electrodes. After that, discharge changes to the glow / arc discharge known as a continuous discharge. The discharge can be kept if a balance of the amount of charged particles produced and the amount released to the outside is held. Where, the amount of charged particles moved in the discharge space is defined as discharge current. In the gas flow condition, since charged particles are moved by the gas flow, discharge is stretched, and then parts of the charged particles are released outside of the discharge. At this time, the stronger the gas flow is, the more the release amount increases, and discharge blow-off occurs when the balance of the charged particle amount is lost. If any energy remains in the ignition system, re-discharge immediately occurs and the same process described above is repeated. In this way, since keeping the discharge is established by balance of charged particle amount, discharge current is strongly related to blow-off.
For examining blow-off suppression method, the relationship between discharge current and blow-off was confirmed. Figure 5 shows the result. Figure 5-(a) shows the relationship between gas flow velocity around the spark plug and blow-off discharge current. Where, blow-off discharge current is defined as the value of current at the start time that blow-off continuously occurs, an example is shown by dashed line in the discharge current waveform of Figure 5-(b). From this result, we found that blow-off occurs when discharge current is lower than blow-off discharge current, and also blow-off discharge current is higher in accordance with the gas flow velocity increase. This means blow-off occurrence is strongly dependent on the discharge current regardless of discharge channel length. From the above examination results, keeping higher discharge current than blow-off discharge current is considered the most effective way for suppressing discharge blow-off in various gas flow conditions.
On the other hand, a sudden change of discharge channel length occurs in spite of high discharge current. This phenomenon is short-circuiting of the discharge channel shown in Figure 6-(a), forming a discharge path in the middle of the discharge channel. This paper will refer to this phenomenon as short-circuiting. This short-circuiting occurs depending on the electric field strength around the discharge channel generated by a voltage difference between discharge channels. This also forms by relatively low voltage differences because there are a lot of charged particles diffused around discharge channel. The generated voltage depends on the environment conditions such as the gas flow, pressure and discharge characteristics. In addition, as shown in Figure 6-(b), when short-circuiting occurs, discharge voltage decreases by shortening of the discharge channel length. However, discharge current does not become zero unlike blow-off. This means that the short-circuiting does not interrupt discharge. In that respect, it is necessary to distinguish from blow-off. Impact of short-circuiting on flame kernel formation will be mentioned in detail in the next section.
Next, it makes mention of discharge duration. As noted in Equation (2). supply energy to the air-fuel mixture is energy minus spark plug cooling loss from ignition energy. Although long discharge duration is able to increase ignition supply energy, spark plug cooling loss should be reduced from a viewpoint of power consumption. Therefore, the relationship between cooling loss and discharge characteristics is investigated. Spark calorimeter test  was selected as the research method. This method detects thermal energy transferred from discharge to the gas as the gas pressure expands and quantifies supply energy. Figure 7-(a) shows experimental device outline and measurement condition. A visible window was set up for observing discharge channel, a gas injector for simulating in-cylinder gas flow and a low pressure transducer (Kistler Type 7261) were set up at the side of calorimeter chamber. Direction of the spark plug ground electrode was set perpendicular to the gas flow, so as not to disturb it.
Figure 7-(b) shows measurement results of spark plug cooling loss and discharge channel length using the calorimeter. Discharge channel length is defined as the distance from spark plug center axis to the tip of discharge. It was changed by gas flow velocity.
The longer the discharge channel, the more spark plug cooling loss percentage of total energy was reduced. This is due to the energy transfer efficiency improvement to the gas by increasing contact area between discharge and the gas. However, the effect was limited by saturation of the discharge channel length due to repeated short-circuiting at the length of about 3 mm in spite of increasing the gas flow velocity. From this result, extending discharge channel is considered to be an effective way for efficient energy supply to the air-fuel mixture. However, it is difficult to control extension of the discharge channel because discharge is stretched by gas flow. In low gas flow velocity conditions, discharge stretch speed is slow and the energy supply efficiency is deteriorated. In this case, it is necessary to continue energy supply for a long duration.
Based on the above, discharge current and duration should be properly controlled in accordance with ignition environment.
OPTIMAL DISCHARGE SPECIFICATION UNDER DILUTE COMBUSTION
Controllable discharge specifications are mainly discharge current and duration. Discharge voltage is an uncontrollable parameter determined by environmental factors such as pressure, temperature and gas flow. Figure 8 shows discharge current of a conventional ignition system. Discharge current is formed into a triangle by ignition coil characteristics. The following discusses optimal discharge specifications with reference to Figure 8.
Where, I2m is defined as the minimum discharge current value for suppressing blow-off and T2m is defined as the minimum discharge duration required for reliable flame kernel formation and growth. However, these parameters have various values given the ignition environment.
It is possible to achieve stable dilute combustion when discharge current including region A, formed by I2m and T2m is output. For example, in the case of discharge current that is indicated by a dashed line in Figure 8, discharge current is smaller than I2m, and the ignition system is not able to continue energy supply to the air-fuel mixture due to blow-off. In order to meet the demand of dilute combustion, discharge current needs to be enhanced as shown by the solid line in Figure 8 (conventional ignition technology). However, from the viewpoint of energy optimization and reliability in the ignition system, this discharge enhancement has two major issues. The first issue is discharge shown by region B in Figure 8. Discharge in this region is excess energy supply for flame kernel formation. Although detail will be described in the next section, it does not need to supply energy for more than necessary duration. Also, discharge in region B does not contribute to combustion due to continuous blow-off because of lower discharge current than the blow-off current level. Further, discharge energy in this region only becomes energy to accelerate ground electrode wear by repeating re-discharge (capacitive discharge). The second issue is discharge shown by region C. Discharge in region C is excessively high as discharge current for blow-off suppression. Although high energy density is a benefit of early flame kernel formation, the effect is not great. Conversely, high current density increases center electrode wear by inductive discharge. For these reasons, ideal discharge current shape should be formed to a square, illustrated by region A.
Optimal discharge current and duration under various engine conditions are revealed using a lean burn combustion engine shown Table 1 below.
First gas flow velocity around the spark plug was measured using a Laser Doppler Velocimeter to understand ignition environment. The result is shown in Figure 9.
Next, impact of discharge current value on lean burn combustion limit was confirmed. It was tested under various gas flow conditions where gas flow velocity was changed by engine speed. Discharge current was fixed to 0.5 ms during all conditions by a controllable discharge ignition system. Hereinafter, discharge current value is defined as an average value because it has control width. The test result is shown in Figure 10. Although there was no difference in lean limit regardless of discharge current value at the low gas flow velocity, higher discharge current values improved lean limit during high gas flow velocities. Focusing on blow-off discharge current, each maximum blow-off discharge current is about 40 mA, 70 mA. 100mA with a gas flow velocity of 11 m/s, 26 m/s, 44 m/s as noted in Figure 5-(a). Even if control width is considered, it is sufficiently possible to suppress blow-off by keeping discharge current of 100 mA or more at low gas flow velocity. Therefore, lean limit was not able to be improved by enhancing discharge current. In high gas flow velocity, lean limit was significantly reduced with discharge current of 100 mA because blow-off occurred during discharge duration in many cycles. This can be seen clearly from the discharge current waveform in Figure 10-(b), as discharge current is reduced to 0 mA during discharge duration (blow-off/ interruption of discharge). Figure 10-(c) shows a 50 cycle average of flame area at 2.0 ms after discharge initiation for each discharge current value at 3000 rpm (the gas flow velocity is 44 m/s). Similar to the effect of improving lean limit, flame area was larger in accordance with enhancing discharge current. This effect is due to promotion of flame kernel formation by keeping discharge without blow-off. However, the effect was saturated at 200 mA or more. The reason is because blow-off was completely suppressed with discharge currents of 200 mA or more and sufficient energy for forming flame propagation was supplied to air-fuel mixture. Thus, controlling discharge current to the optimal value according to engine condition is very effective in achieving stable dilute combustion.
Next, the effect of discharge duration under lean burn combustion was also studied. Discharge current was selected according to the ignition environment based on the above evaluation results. The result at 1200 rpm is shown in Figure 11-(a). Although lean limit was improved by the extension of discharge duration, the effect gradually showed a tendency of saturation. Growth of the flame kernel was observed using a high speed camera in order to reveal factors of this saturation. The observation results of two different discharge duration conditions are in Figure 11-(b). Time described in the bottom of figure means time from discharge initiation. Comparing flame area under both conditions, it can be seen the flame area is larger with a discharge duration of 1.5 ms than 0.6 ms. Focusing on the flame with a discharge duration of 1.5 ms, flame area is sufficiently grown by the end of the discharge, and it is possible for the initial flame to cover both spark plug electrodes. Since there is no unburned air-fuel mixture around the discharge, additional supply energy is not able to contribute to combustion. Thus, although discharge duration is an important parameter for flame kernel formation and growth, it is not desirable to make it any longer than necessary. This necessary discharge duration also changes at different engine conditions. Verification result of necessary discharge duration at various gas flow velocity is shown in Figure 11-(c). The discharge duration was shorter in accordance with an increase of gas flow velocity. We have considered the following two factors. The first factor is improvement of energy supply efficiency by fast discharge stretch, and the other factor is an increase of energy density by enhancing discharge current for suppression of blow-off. As well as discharge current, controlling discharge duration to the optimal value according to engine condition is also very effective in achieving stable dilute combustion.
Finally, the impact of short-circuiting for flame kernel formation as described briefly in the previous section is discussed. Short-circuiting often occurs when discharge is held over long duration. Although it is not able to continue supplying discharge energy to the same air-fuel mixture like blow-off, it can keep the discharge without discharge current interruption, unlike blow-off. This difference has an effect on flame kernel formation. Thus, the state of flame kernel formation at the time of short-circuiting was observed. The observation results are shown in Figure 12. After discharge initiation, some flame kernels were formed while repeating discharge stretch and short-circuiting. Also, their flame kernels were overlapped during the flame growth process, and then they grew into the sufficient initial flame. Thus, since short-circuiting is able to form the next flame kernel at a relatively close distance unlike blow-off as shown in Figure 3, it can achieve stable initial flame formation by forming multiple flame kernels due an extended discharge duration.
From these results, keeping long discharge duration without blow-off during low engine speeds and enhancing discharge current in short duration at high engine speeds are considered the most optimal ignition conditions for dilute combustion.
IGNITION SYSTEM CONFIGURATION FOR DILUTE COMBUSTION
Figure 13 shows the ignition system configuration to optimally control discharge in each dilute combustion engine condition. This system consists of not only an ignition coil but also an electronic drive unit (EDU) for discharge control. The EDU mounts several circuits such as the energy charger, discharge current control and igniter. This system enhances discharge by adding energy from the EDU to the ignition coil, keeping discharge current constant by controlling the amount of energy. The following principle describes detail operation of this ignition system. First, the EDU charges energy for enhancing discharge. This energy is constantly controlled to keep a full charge. The ignition coil charges energy to the primary coil by an ignition signal, and it applies high voltage to the spark plug using counter electromotive force due to mutual induction. The spark plug begins to discharge by breakdown between electrodes. After that, the EDU immediately outputs the energy charge in advance to the primary coil. As a result, discharge current is enhanced. In this time, discharge current is controlled to a desired value by controlling the amount of energy in secondary current feedback. Discharge duration is also controllable by energy output time from the EDU. In this way, controlling discharge current to optimal values for each engine condition can contribute to achieving stable dilute combustion.
EFFECT OF DISCHARGE CURRENT CONTROL FOR DILUTE COMBUSTION ENGINES
For confirming the effect of discharge current control, engine tests were evaluated in comparison with conventional ignition technology. These engine tests were demonstrated in various engine conditions using the lean burn engine shown in Table 1. Each ignition system specification is shown in Table 2. Also, measuring devices such as a voltage probe, current probe, and pressure transducer were set up shown by Figure 3. Discharge specifications at each engine conditions were set to optimal values in advance based on discharge environment. Table 2 describes discharge specifications at 1200 rpm, BMEP 0.5 MPa and 3000 rpm, BMEP 0.9MPa.
The result of engine tests at 1200 rpm, BMEP 0.5 MPa is shown in Figure 14. Figure 14-(a) shows coefficient of variance of indicated mean effective pressure (COV of IMEP) for excess air ratio with each ignition system. For this result, controlling discharge current to optimal values achieved a lean limit improvement over 100 mJ of a conventional energy system, and it was same effect with 450 mJ of conventional system. We can consider the reason of this result from the relationship between 0-10 % combustion duration and IMEP as shown Figure 14-(b) Generally, when 0-10 % combustion duration is extended, that is, when initial combustion speed is slow, it leads to decrease of IMEP. Figure 14-(b) clearly shows that a discharge current control system has stabilized COV of IMEP by suppressing fluctuation of 0-10 % combustion duration when compared to a 100 mJ conventional ignition system. This effect is supported by achieving blow-off suppression and flame kernel formation stabilization by controlling discharge current.
The result of engine tests at 3000 rpm, BMEP 0.9 MPa is shown in Figure 15. Similar to above condition, it was able to show lean limit improvement by stabilizing flame kernel formation. In addition, the improvement effect was more than that of 1200 rpm. To understand the reason, discharge waveforms were analyzed at each condition. A discharge waveform for the 100 mJ conventional system and discharge current control system are shown in Figure 14-(c) and Figure 15-(c). Focusing on conventional discharge waveform during relatively weak in-cylinder gas flow at 1200 rpm, discharge could be kept without immediately blow-off while repeating short-circuiting until about 1.0 ms. Therefore, the discharge current control system was not able to gain a large benefit even if it could achieve discharge during 1.5 ms of duration. On the other hand, because in-cyfinder gas flow is strong at 3000 rpm, the conventional system immediately experienced blow-off after discharge initiation, and then it repeated blow-off and re-discharge. However, the discharge current control system can achieve discharge maintenance and stretch without blow-off by enhancing discharge current. This discharge behavior difference in the conventional system is due to the environment which has led to a difference in lean limit improvement effect.
Next, the paper discusses lean limit in the discharge current control system. Figure 15-(d) shows the relationship between 0-10 % combustion duration and IMEP at the lean limit, and Figure 15-(e) shows the discharge voltage waveform for 200 cycles at the lean limit. Observing discharge voltage during the worst IMEP cycle shows a small discharge voltage rise during discharge duration. That is, discharge had not been stretched at that cycle. The cause for this is in-cylinder gas flow fluctuation between combustion cycles. During a cycle of low gas flow velocity around the spark plug, the ignition system was not able to supply sufficient energy to air-fuel mixture creating a high possibility that initial combustion speed will decrease. In this case, similar to low engine revolution, although it is possible to improve initial combustion by extending discharge duration, it becomes the cause of excessive energy supply at a cycle of high gas flow velocity. Therefore, discharge specification has to be appropriately determined in accordance with ignition environment.
Finally, energy efficiency of each system will be examined. As mentioned above, the conventional system has two wasted energy zones. In contrast, controlling discharge current to optimal values is an effective way to achieve optimal energy efficiency while eliminating wasted energy zones. The result demonstrated in this evaluation is shown in Figure 16. This figure shows lean limit for each ignition system. Comparing at the same discharge energy, it is clear that the discharge current control system achieved stable dilute combustion when compared to a conventional system. Also, it can be proved controlling discharge current to optimal values is effective in reducing spark plug electrode wear as well as power consumption because the same dilute combustion is achieved using less energy. From the above results, controlling discharge current to optimal values under each engine conditions can contribute to stable dilute combustion resulting in vehicle fuel economy improvement for C[O.sub.2] emission reduction.
In this paper, for the purpose of improving vehicle fuel economy to reduce C[O.sub.2] emission, ignition technology based on the discharge principle is studied for dilute combustion engines to create a thermal efficiency improvement technology, as obtained the following knowledge.
1. Occurrence of discharge blow-off is strongly dependent on discharge current regardless of discharge channel length. Therefore, enhancing discharge current is able to suppress blow-off even in the high gas flow velocity condition.
2. Discharge channel stretch reduces spark plug cooling loss, and also improves discharge energy efficiency.
3. Although short-circuiting discharge occurs to inhibit discharge channel stretch in spite of keeping high discharge current at the gas flow environment, it is able to grow into sufficient initial flame by overlapping of flame kernels.
4. Ideally the ignition system should control discharge current in accordance to engine condition. Extending discharge duration with lower current in low gas flow velocity, and enhancing discharge current with short duration in high gas flow velocity is the most effective approach for dilute combustion stabilization and energy savings.
In addition, by controlling discharge current to optimal values a dilute combustion limit of maximum 0.2 point of excess air ratio was achieved compared to conventional ignition technology in the engine tests. As a result, it contributed to improved vehicle fuel economy.
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Naoto Hayashi, Akimitsu Sugiura, and Yuya Abe
Toyota Motor Corp.
The authors would like to thank NIPPON SOKEN for assisting the analysis of optimal tests.
Table 1. Engine specifications Displacement 2489 cc Cylinder number 4 Compression ratio 10.0 Tumble ratio 3.5 Valvetrain system DOHC 4valve Fuel type Gasoline Injector position Port and Direct injection Boost Turbocharger Table 2. Ignition system specifications Ignition system Conventional Discharge system current control Ignition coil High energy coil High energy coil Spark plug Dual fine-wire plug Dual fine-wire plug Discharge current 120 mA 250 mA 100 mA at Ave. at 1200 rpm Triangle Triangle Square shape shape shape Discharge duration 1.2 ms 2.2 ms 1.5 ms at 1200 rpm Discharge energy 100 m J 450 mJ 220 mJ at 1200 rpm Discharge current 120 mA 250 mA 200 mA at Ave. at 3000 rpm Triangle Triangle Square shape shape shape Discharge duration 0.7 ms 1.4 ms 0.5 ms at 3000 rpm Discharge energy 100 mJ 450 mJ 240 mJ at 3000 rpm