Analysis of Energy Transfer in the Ignition System for High-Speed Combustion Engines

Abstract

In order to produce reliable and reproducible ignition of lean fuel–air mixtures and highly stratified mixtures, it is necessary to ensure a high concentration of spark discharge energy and to provide a strong energy impulse for the triggering of chain processes of chemical decomposition of fuel molecules. For this reason, studies have been undertaken on the flow of electrical energy from the ignition system to the spark plug and on the formation of an electric discharge arc with a high concentration of thermal energy. The experimental results were obtained using an ignition coil energy test stand and a constant volume chamber with high-speed spark discharge recording capability. It was confirmed that increasing the charging time of the ignition coil from 0.5 ms to 5.0 ms increases the energy delivered to the coil from 9.5 mJ to 330 mJ. In the same range, the energy generated by the coil was recorded to range from 4.2 mJ to 70 mJ. The coil’s efficiency was found to decrease with increasing charging time from 45% up to 20.5%. Further energy losses were presented when the spark discharge energy was analyzed. In the paper, the results of investigations concerning electric discharge arc development have been presented, illustrated by a few exemplary photos, and discussed. The mathematical interpretation of the electrical energy flux in the ignition system resulting from the energy of the discharge arc has been conducted and illustrated by some functional independences and relationships.

1. Introduction and Motivation

The most common type of ignition of the air–fuel mixture in Otto engines is from the primary ignition source, which is an electrical discharge between the spark plug’s electrodes. A high concentration of energy characterizes this process and has been used since the beginning of SI engines’ existence. The development of ignition systems is aimed at increasing the energy generated in the electric arc mainly by modifying coils and spark plugs, which is particularly important in the combustion of lean, difficult-to-ignite mixtures.

Already, preliminary studies of publications and research work in the last ten years have shown that in the SI engines, both reducing the emission of THC, CO, and PM and reaching higher engine thermal efficiency are possible to achieve by operating on lean or intensively stratified mixtures.

In the case of hydrocarbon fuels (liquid and gaseous), the energy-efficient process requires several prerequisites: appropriate preparation of the combustible mixture and its appropriate distribution in the combustion space (e.g., charge layering), proper initiation of chain reactions of hydrocarbon particle decomposition, and initiation of oxidation processes of carbon and hydrogen particles contained in the fuel, and then controlling their course by using an appropriate supply reaction zone. In such cases, properly initiating and intensifying the combustion process requires a much higher activation energy to be delivered to the mixture during its ignition. This task requires applying a more efficient ignition system, which will deliver higher electrical discharge energy to the cylinder charge. While the authors have been working on the problems of two- or three-stage combustion in SI engines in recent years, the question of operating a spark-ignition system has become of significant interest [1,2,3,4].

The main area of the authors research interest in these studies has become the spark ignition system, which is significantly responsible for the efficiency of the combustion initiation process, especially of lean gas–air mixtures. In liquid fuel engines operating on lean mixtures, fuel is injected near the spark plug; such a procedure for gaseous fuels is much more difficult [5].

The initiation of combustion of lean mixtures with a significant excess of oxygen (air) is strongly hindered because it requires a relatively high initial energy supply (activation energy). In SI engines, this energy is supplied by an electrical discharge on the electrodes of a spark plug powered by an ignition coil, which converts the low voltage supplied from the battery into the high voltage necessary to generate a spark. Knowledge of the efficiency of the ignition coil can be useful information for the potential further development of ignition systems of this type. Knowledge about the relationship between the coil charging time and the impact of ambient conditions, such as charge pressure and temperature, on the parameters of the electrical discharge, can also provide important information about the operation of spark ignition systems and their impact on spark energy, and will allow us to select the appropriate ignition parameters. This is why it was decided to conduct an experiment in this work: to determine the efficiency of the ignition coil depending on its charging time and to examine the parameters and factors on which the energy of an electrical discharge depends.

2. Present State of Knowledge on Ignition System Concepts and Applications

Over the many years of existence of spark-ignition engines, different systems have been developed with varying degrees of complexity and efficiency. One of the earliest, because it dates back to the end of the nineteenth century, and the simplest system, was the magneto ignition; one of the first applications is attributed to Edward Butler (1862–1940), who used the so-called magneto ignition in his engine in 1888. However, more often talked about is the first practical high-tension magneto developed by Frederick Richard Simms, working with Robert Bosch. In such a system, high-voltage pulses are generated due to the rotation of the metal core in a magnetic field, and the interruption of the primary circuit induces a current in the secondary circuit, causing a flashover of a spark on the spark plug electrode. Although such a system does not require a heavy battery of electric charge and is quite reliable, it produces only small discharge energy, especially at low rotational speeds, and the phenomenon of sparking (energy dissipation) occurs, requiring the use of a capacitor in the secondary circuit [6,7]. A few years later, ignition magnetos were used on most cars for both low-voltage systems (which used secondary coils to fire the spark plugs) and high-voltage magnetos (which fired the spark plug directly, similar to induction coil ignition). Ignition coils largely replaced ignition magnetos once batteries became common in cars, since a battery-operated coil can provide a high-voltage spark even at low speeds, making starting easier.

The use of a capacitor in the system was associated with the creation of a capacitor ignition system CDI (capacitor discharge ignition). In such a system, the ignition spark is produced by the current generated by the discharge of the capacitor. When it is necessary to create a spark, high-voltage electrical energy is transferred to the primary winding of the coil, which in this system acts as a pulse transformer, not an energy storage device [8]. The electric spark produced in this way has a high intensity but is short-lived (about 0.1 ms), so it does not provide reliable ignition for the lean fuel–air mixtures used in more and more modern engines [8]. Since the charging time of the system’s electrical capacity is not dependent on the engine speed, there is no need to use ignition advance angle control. This system is, therefore, used in small high-speed combustion engines.

In contrast to systems with point ignition of combustion, surface ignition solutions have also appeared. For example, a hot-tube ignitor was a device from the early 1900th that fit onto the cylinder head of an internal combustion engine and was used to ignite the compressed fuel–air mixture by means of a flame heating part of the tube red-hot. A hot-tube ignitor consisted of a metal or porcelain tube, closed at one end and attached to the cylinder head and an adjustable burner that could be moved to position its flame at any point along the length of the tube. In early designs, ignition timing was controlled by adjusting the position of the red-hot spot on the tube—this was accomplished by moving the burner along the length of the tube. Most later styles used a fixed burner and varied tube lengths to change the ignition timing [9].

In the search for combustion systems with increased energy efficiency, attempts were made to use the combustion of lean fuel–air mixtures, but initiating combustion under conditions of fuel deficiency in the mixture requires a significant increase in the initial energy, the so-called activation energy. The concept of combustion of such mixtures in aircraft engines was worked on in the 1950s by Nikolai Semenov (Russia), and later its use in internal combustion engines was carried out by Lev Ivanovich Gussak who constructed the LAG system (Lavinia Aktivatisia Gorenia or Avalanche Activated Combustion) [10,11,12,13]; The concept created at that time was often referred to as spark-jet ignition.

Similar ignition systems have been studied in Japan (Honda CVCC, Toyota TGP, Nissan NVCC, Toyota TGP), Germany (Porsche SKS, Volkswagen PCI, PCV, Daimler-Benz TSC), and England (BLMC). In the 1980s, research was carried out in Japan on the effects of different sizes of pre-chambers and the channels connecting them to the cylinder [14]. It was then found that the ignition of lean mixtures is most effective due to composite ignition [14]. The interaction of free radicals and thermal phenomena achieved ignition. The ignited mixture (propane–air) in the pre-chamber had approximately the value of λ = 0.91, and the lean one in the cylinder had λ = 2.4.

Jet ignition uses the ignition of a rich mixture in a relatively small pre-chamber, and the burning mixture is ejected with high energy into the main combustion chamber to ignite the lean or very lean mixture in it.

Among the jet ignitions, we can distinguish JPIC (jet plume injection and combustion) from others. It is a system that was created on the basis of PJC (pulsed jet combustion) and developed by Prof. Antony Oppenheim (1915–2008) in Berkeley (USA) [15,16,17,18,19].

The pre-chamber is built around a modified spark plug in the pulsed jet combustion system. According to the PCJ idea, the fuel, or air–fuel mixture, is delivered through a hollow injector electrode to a pre-chamber at the bottom of the combustion chamber. Some of the supplied mixture also gets into the cylinder, where the rich mixture (ca. λ = 0.85) and the residual exhaust gases from the previous cycle are located. At the end of the injection time, the rich mixture in the pre-chamber is ignited by means of a spark plug, which is then injected into the cylinder in the form of a stream, causing turbulence and further ignition and producing a large number of diffuse ignition centers, which result in a higher concentration of ignition energy and a short ignition time for lean mixtures, as the flames have a shorter travel distance. This type of ignition system differs from JPIC in that it uses high pressure generated in the pre-chamber rather than high-pressure injection. The advantage of pulsed jet combustion is a faster increase in pressure and a higher maximum pressure than in the case of ignition with a classic spark plug [15,17,18].

The next stage in the development of lean-burn ignition techniques is HAJI, i.e., hydrogen-assisted jet ignition. This system uses hydrogen as fuel for the pre-chamber. The HAJI system is mounted to the spark plug hole. The HAJI module consists of a small pre-chamber, a spark plug, and a direct injector to the pre-chamber. To start the ignition, a small amount of hydrogen is injected into the pre-chamber, creating a rich mixture there. Then, a spark from the plug ignites the mixture. The burning rich mixture passes through the hole/channel into the main chamber at high velocity, where it ignites the lean mixture. The energy generated in the pre-chamber is more than two orders of magnitude higher than in the case of classic spark ignition [14].

Lean-burn combustion allows for a lower combustion temperature, resulting in lower nitrogen oxide emissions and higher engine efficiency. It was found that it was possible to burn lean mixtures with an air excess coefficient from 2 to 5 [14]. However, the low combustion temperature results in higher hydrocarbon emissions. There is also a similar combustion system referred to as HFJI (hydrogen flame jet ignition). The research determined that turbulence is more important during ignition than larger radicals, which is why hydrogen streams with greater turbulence are used in this system [14]. HAFJI enables combustion at a lower pressure and can provide a more stable process.

A swirl chamber spark plug is a type of spark plug with a small pre-chamber located inside the spark plug. During ignition, an engine equipped with this type of spark plugs has two fuel injections. The first injection forms a lean mixture (λ = 1.4–1.7) and takes place during the intake stroke. The second injection is much smaller (approx. 3% of the total fuel weight used), is directed directly at the piston crown, and takes place during the compression stroke. The movement of the piston pushes the fuel from the second injection into the pre-chamber of the spark plug, where a rich mixture is formed. Then, by electrical discharge, ignition occurs in the pre-chamber and the flame propagates into the cylinder, where the lean mixture is ignited. This technology has lower nitrogen oxide emissions, lower fuel consumption, and greater resistance to knocking under full load than a traditional spark plug.

A patent for a PCFA (premixed charge forced auto ignition) engine was filed in 2003 by General Motors [14]. In the ignition system, this type of jet of reactive air–fuel mixture passes from the pre-chamber to the main chamber, where it combines with the pre-mixed ultra-lean mixture. As a result of this process and the compression, rapid ignition and expansion begin. This ignition method is to be used only in the engine operating at low speeds, and at higher engine speeds, ignition is to take place conventionally. The advantages of this method are its higher efficiency and lower nitrogen oxide emissions.

The idea of HCJI (homogeneous combustion jet ignition) is to control the moment of ignition in HCCI engines by means of jet ignition in the pre-chamber [14]. There are two chambers attached to the cylinder, in which self-ignition takes place. The microvalves of the pre-chambers close near the beginning of the compression stroke and then, due to the action of the “pre-chamber pistons” located there, the pressure increases, and spontaneous ignition occurs. Then the microvalves open and a stream of hot gases ignites the mixture in the cylinder. The lack of an ignition spark in this technology is to eliminate uneven combustion.

A pre-chamber ignitor equipped with a pre-chamber fuel supply system was patented in 2006 [14]. There is also a groove with a spark gap in the middle of the ignition. The device also has a second external spark gap directly into the cylinder. When the engine is running at low speeds, the rich mixture is ignited in the pre-chamber, and then a stream of active radicals is introduced into the main chamber, where the leaner mixture is ignited. When the engine is running at high speeds, ignition takes place without the use of an internal spark gap and without a fuel supply system to the pre-chamber; ignition is similar to ordinary spark ignition.

Homogeneous combustion radical ignition (HCRI) is used to ignite homogeneous mixtures. This system connects the main combustion chamber through narrow channels to additional compact chambers [14]. A richer mixture is created in the small chambers, and radical ignition occurs. After entering the main chamber, the radicals initiate the combustion of a poorer homogeneous mixture.

Some other ideas exist for alternative and more effective initiation of the combustion process based on conventional ignition systems. Renault’s radio frequency ignition system (RFIS) operating with the traditional spark plug should be mentioned here. In this case, the input voltage produced by an external power source is amplified in the resonant circuit of the transformer. The best resonant frequency is 4.97 MHz. An alternating high-voltage electrostatic field causes ionization, excitation of electrons to high-energy states, and an increase in the number of free radicals. The fuel–air mixture ignites if the cylinder has the right physical and chemical conditions. This system does not require a second electrode and ignites a larger area than using standard spark plugs.

It was stated that the use of RFIS allows the duration of the spark to be varied according to the need resulting from the actual operation of the engine and that it is also more efficient in transmitting the ignition energy from the spark. The lack of limitations resulting from the distance between the spark plug’s electrodes, and the multi-channel discharge structure allows a larger area to be ignited with a spark. RFSI shortens the combustion duration, enables a higher rate of heat generation, and provides greater stability of the combustion process for loaded engines. Another observed effect of RFSI was an increase in engine efficiency by 1% to 5% and the ability to carry out the combustion process with leaner fuel–air mixtures. The use of RFSI also reduces the emission of HC and CO particles, while NOx emissions are higher than those of ignition with a standard plug, due to the higher peak temperature in the cylinder. Reducing NOx emissions with RFSI is possible by taking advantage of the ignition system’s ability to carry out the ignition process for leaner air–fuel mixtures.

3. Ignition of Lean Mixtures

Most ignition systems mentioned here are designed to initiate the combustion of lean fuel–air mixtures. A lean mixture is a fuel–air mixture in which the amount of fuel in relation to the air is less than in the stoichiometric mixture (λ > 1). Combustion of lean mixtures leads to an increase in the energy efficiency of the engine. It reduces the emission of harmful substances, especially nitrogen oxides, which results from decreasing the engine cylinder’s maximum temperature [20,21]. Excess air also promotes the oxidation of carbon monoxide and unburned hydrocarbons. It causes a reduction in the temperature gradient between the combustion products and the cylinder walls, which is equivalent to reducing heat loss to the walls. However, it should be remembered that if the mixture is too lean, the combustion process may not start, or it will be necessary to increase the ignition energy. The disadvantage of burning lean mixtures is a decrease in power per unit of engine displacement, a longer combustion time, and increased engine unevenness. It has also been shown that lean-burn engines respond much faster to even small changes in fuel composition.

The energy of spark ignition depends on the parameters characterizing the design of the spark plug used and the method of discharge on the electrodes. An important parameter resulting from the construction of the plug is the distance of the electrodes, which must be greater than the quenching distance. This is the distance of the walls of the vessel or duct below which the spread of flame is not possible due to the heat removal through these walls. The spacing should also be large enough to ensure that the mixture ignites during idling and at full engine load. It must also be small enough to ensure ignition in difficult conditions, such as cold electrodes, reduced ignition voltage, or a high compression ratio. The spacing between the electrodes depends on the type of motor in which they are used.

Some of the energy that was stored in the inductive ignition system is dissipated in the electrical system (losses), and the rest is released in the mixture. Its main form of distribution in the mixture is heat and as electromagnetic and acoustic waves. The aim is to achieve the shortest possible single discharge time. If the extinguishing distance is appropriate, the ignition energy mainly depends on the fuel–air mixture parameters (Figure 1). The energy required for spark ignition of fuel–air mixtures used in internal combustion engines is less than 1 mJ. The ignition energy increases when there is a pressure drop, when the proportion of oxygen in the air decreases, the closer the composition of the mixture is to the upper or lower ignition limit, when the flow velocity increases, when the level and scale of turbulence in the mixture medium increases, the less similar the mixture is to the stoichiometric from whence it comes, and the lower the air temperature (for t < 0 °C) [22,23].

The required ignition energy should achieve values as great as 60–80 mJ for lean and very lean mixtures. Achieving such values requires both an increase in electrical energy in the primary circuit of the ignition system and an increase in the efficiency of converting this energy into spark discharge energy delivered directly to the ignitable mixture.

4. Research Problem and Scope of Investigations

Based on the literature studies, it was concluded that the energy flux’s efficiency in the ignition system has still not been sufficiently investigated and quantitively described. Therefore, it was considered necessary to conduct appropriate investigations that would enable a quantitative assessment of the ignition system’s effectiveness, especially for its appropriate configuration for the ignition of lean gas–air mixtures.

The main aim of the work has been defined as experimental identification of energy fluxes in the ignition system beginning with the electrical energy in the ignition coil up to the determination of the electrical arc flash and its light radiation resulting from flame initiation. It was planned to determine the energetic efficiency of the ignition system by comparing the electrical discharge energy with the energy delivered to the ignition coil’s primary circuit.

The tests were carried out in a constant volume chamber with a backpressure of 0, 5, 8, 10, and 12 bar for different current values (4.0 … 11.0 A) and various times of supply to the coil, i.e., t = 1.5, 2.0, … 5.0 ms. For the determination of electrical energy, both current (I) and voltage (U) in the primary and secondary circuit of the ignition system were measured and compared with the current determined on the ground electrode of the spark plug. These data allowed us to determine the electrical energy delivered to the coil and generated electrical discharge energy.

5. Investigation Methodology

5.1. Main Concept of the Research

For the solution to the research problem formulated in Section 4, the experimental investigation method was adopted. The experimental test bed had to be designed and constructed for this task. As the research object, the classical ignition system has been applied and equipped with the necessary connection to the primary and secondary electrical circuits to constantly measure the current flux. The system has been equipped with a control system to adjust the current delivery time.

5.2. The Test Bench, Its Construction, and the Measuring Equipment Used

Research on the energy flow in the ignition system was divided into two stages. The first concerns identifying energy flow through a COP-type ignition coil excluding the spark plug, while the second covers the extent of energy flow through the spark plug to spark discharge.

The configuration of the test stand used for the first stage of the study is shown in Figure 2. This stage aimed to determine the efficiency of the ignition coil based on the input/output energy balance. The stand was equipped with an ignition system including a controller (maximum coil charging current of 11 A), an eight-channel high precision Sirius data acquisition system from DEWESoft (Trbovlje, Slovenia) with a maximum sampling rate of 200 kHz and voltages of 200 V and 1200 V (accuracy ± 0.03%), pp218 current clamps, an HSD Sequencer C711 signal trigger device, and R1 (1 MΩ) and R2 (1 kΩ) resistors. Resistors with a tolerance of 1% were used. An M10 NGK LMAR8BI-9 spark plug (Nagoya, Japan) with a side electrode with a 0.8 mm electrode gap was used for the tests. The spark plug had an iridium center electrode and a 5 kΩ value. The ignition coil originated from a modern TSI direct-injection gasoline engine.

In the second stage of testing, the previously described equipment (without resistors) was transferred into a constant volume chamber test stand (Figure 3). The equipment included an additional high-speed image recording camera, a chamber interior pressure management system, and a PP178 capacitive probe to measure the high voltage. The coil charging time and the moment of discharge were adjusted. A sequencer with a control precision of 1 ns was used to precisely synchronize the camera and ignition controller. The devices were connected to the computer receiving the measurement data.

The chamber had a fixed volume of 2.2 dm³ and 5 visors made of 30 mm thick quartz glass, allowing for observations of the processes inside it (Figure 4). The viewfinders also made it possible to record images using a camera for high-speed filming. Inside the test chamber, a spark plug was placed, which was the main subject of the research.

In order to allow pressure changes, an air pumping system was connected to the chamber. It consisted of an air compressor, a cylinder, and solenoid valves that controlled the charge exchange.

The image was recorded using the LaVison HSS5 camera (Göttingen, Germany). This camera model can record at 10,000 frames per second. The camera was equipped with a Nikkon main lens and a 700FS80-50 bandpass optical filter (Tokyo, Japan). This type of filter allows users to adjust the transmitted light in the wavelength Λ range from 660 to 740 nm.

5.3. Calculation Procedure

The energy delivered to the ignition coil by the control system during the conducted investigation is calculated based on the recorded voltage and current supplied to the primary circuit of the coil using the following formula:

$$E_{coil\_in}={\int }_0^t{V}_{prim}·I_{prim}·dt$$

where V_prim is the voltage in the primary circuit, I_prim is the current intensity in the primary circuit, and t is the coil charging time.

The parameters used for the calculations were recorded at a frequency of 200 kHz, capturing five consecutive discharges each time.

Connected in series with the ignition coil is resistor R1 with a resistance of 1 MΩ and R2 with a resistance of 1 kΩ. According to Ohm’s law, the current intensity in the measurement circuit (I = const.) is determined based on the following formula:

$$I_{R1\_R2}={\displaystyle \frac{V_{R2}}{R_2}}$$

where I_{R1_R2} is the current intensity generated by the ignition coil and V_R2 is the voltage across the measuring resistor R₂.

Subsequently, based on the current in the circuit and the varying voltage value V_R2, the energy generated by the coil was determined based on the following formula:

$$E_{coil\_out}={\int }_0^t{{(I}_{R1\_R2})}^2·(R_1+R_2)·dt$$

where t is the ignition coil discharge time.

The electrical efficiency of the ignition coil was calculated based on the energy balance equation according to the following formula:

$${\eta }_{coil}={\displaystyle \frac{E_{coil\_out}}{E_{coil\_in}}}·100\%$$

The electrical energy of the spark was determined based on the voltage at the central electrode and the current measured at the ground of the constant volume chamber, according to the following formula:

$$E_{spark}={\int }_0^t{(V}_{sec}-I_{sec}·R_{plug}{)·I}_{sec}·dt$$

where V_sec is the voltage in the secondary circuit (high voltage), I_sec is the current in the secondary circuit, R_plug is the resistance of the spark plug resistor, and t is the duration of the spark discharge. Since measuring high voltage at the central electrode of the spark plug is challenging, the voltage in the coil’s secondary circuit and the resistance of the spark plug resistor (5 kΩ) were used. Both signals were filtered using a digital low-pass filter.

Given the electrical energy of the discharge and the energy generated by the backfire coil, the efficiency of the coil-spark assembly was determined according to the following formula:

$${\eta }_{coil\_spark}={\displaystyle \frac{E_{coil\_out}}{E_{spark}}}·100\%$$

Moving on to optical signal post-processing, a flat projection of the spark was recorded on a plane parallel to the axis of the spark plug, referred to as a side view. In the first stage of image processing, the background was removed. Subsequently, the number of pixels with a grayscale value above 0 (scale 0–1023) was counted to determine the area of the electrical arc, which constitutes the first parameter for assessing the geometric discharge indicator. The second parameter is the average intensity of luminescence in the image, determined by the following formula:

$$I_{lum\_av}={\displaystyle \frac{A_{spark}}{\sum _{i=1}^{N}pi}}$$

where A_spark is the area of the electrical arc, N is the number of pixels, and pi is the value of the i-th pixel.

6. Ignition Energy Transfer

6.1. Energy Transfer from the Controller to the Ignition Coil

The analysis of energy transfer through the ignition coil was conducted based on three measured electrical parameters, exemplified in Figure 5. The first parameter is the charging current waveform of the ignition coil, with the charging time ranging from 0.5 to 5 ms, depicted in red. During the ignition coil charging, the electrical system is loaded, resulting in a voltage drop (green) proportional to the increase in current intensity. The discharge of the coil is characterized by a sudden drop in current intensity and the appearance of voltage across the measurement resistor R₂ (blue). In the experimental setup, resistor R₁ was used both as the main load of the system and to relieve the measurement resistor.

Increasing the charging time of the ignition coil results in higher peak current values towards the end of charging (Figure 6). The current waveforms for various charging times are marked with different colors. As mentioned above, in each case, the voltage drop in the power supply circuit correlates with the current waveform. The maximum charging current reached 11.0 A for a charging time of 5 ms, while the minimum was 2.8 A for a charging time of 0.5 ms. It is important to note that in the analyzed system, the minimum charging time required to achieve spark ignition at the spark plug is 1.5 ms (resulting in 4.6 A at the end of charging). Furthermore, the requirements regarding the charging time increase with higher ambient pressure around the spark plug electrodes.

Based on the electrical parameters of the primary side of the coil, the characteristic of energy delivered to the coil relative to charging time was determined (Figure 7). The values indicated are the mean value of ten repeats. The obtained curve exhibits exponential characteristics typical for coil systems. The energy values obtained are significant, reaching up to 0.33 J for the maximum charging time of the ignition coil. The energy delivered to the primary circuit far exceeds the ignition requirements, even for very difficult-to-ignite gas–air mixtures.

The charging time of the coil, and consequently the amount of delivered energy, affects the obtained output energy. Figure 8 depicts the voltage waveform across the measurement resistor. The voltage value increases with charging time, but the increments become smaller over time. The difference between the first and second points is 6.92 V, whereas between the penultimate and last points, it is only 0.44 V. This trend is associated with the decrease in coil capacitance; the cause does not lie within the current source characteristic.

Based on the voltage waveform across the measurement resistor and the energy delivered to the coil, the energy generated by the ignition coil and its efficiency in engine operation were determined as a function of the saturation time (Figure 9). As the energy delivered to the coil increases, the amount of energy generated during the ignition pulse also increases, but the resulting curves exhibit different characteristics. The reduction in the increment of generated energy during coil discharge has various causes. Limiting the amount of energy generated by the ignition coil is due, among other factors, to magnetic core saturation, which prevents an increase in magnetic flux despite increasing charging energy. Another factor is the thermal effect, which increases resistive losses in the coil windings as well as hysteresis and eddy current losses in the core of the coil.

The combined effects of magnetic saturation, thermal resistance changes, and core losses contribute to a decrease in the efficiency of the ignition coil with increasing charging time. As mentioned earlier, the charging energy up to 1 ms and an ambient pressure of 5 bar do not produce sparks at the spark plug electrodes, but this range achieves the highest efficiency, typically around 40–50%. For coil charging times between 3 to 5 ms, which are commonly used in light vehicle engines, the efficiency is lower, ranging from 20% to 35%.

6.2. Ignition Coil to Spark Plug Energy Transfer

The previous section focused on identifying the energy transfer along the power supply–ignition coil–discharge path. This section now shifts its attention to the spark discharge processes, which directly influence the combustion processes within the internal combustion engine chamber. Utilizing the configuration of a second measurement setup equipped with a constant volume chamber, electrical processes and optical signals were recorded on the secondary side of the ignition coil for various coil charging times and varying ambient pressures, which significantly impact the discharge intensity.

Figure 10 presents an exemplary waveform of high voltage generated by the ignition coil and the electrical current intensity registered on the spark plug housing (side ground electrode). During the tests, the constant volume chamber was filled with atmospheric air. It is important to note that the charging phase of the system is longer than the arc burning phase. On the ignition coil’s secondary side, two distinct electrical arc phases can be distinguished: the arc breakdown phase, characterized by a voltage peak, and the arc burning phase, where the electrical current intensity decreases from its maximum value to around 0 mA. There is significantly higher voltage on the secondary side, but the discharge current drops sharply to a few milliamps.

In Figure 11, the voltage waveform in the secondary circuit of the ignition coil is presented for various absolute pressures in a constant volume chamber. As the air density around the spark plug electrodes increases, the number of charge carriers also increases. Consequently, a higher voltage is required to initiate the spark discharge, which rises from 0.65 kV for the ambient pressure (black) to 20.34 kV for a relative pressure of 12 bar (purple). Furthermore, the discharges become progressively shorter, leading to more intense conduction and increased energy concentration within the electrical arc.

The electrical measurements on the secondary side of the ignition system and the consideration of the spark plug resistance form the basis for determining the electrical spark energy (Figure 12). It should be noted that this is not thermal energy, which is typically determined based on changes in thermodynamic parameters in a small, enclosed volume around the spark plug electrodes. As the surrounding pressure and coil charging time increase, the spark energy also increases. The increase in energy primarily results from a higher voltage at the central electrode rather than changes in the current waveform during discharge. The largest increase in spark energy due to increased ambient pressure was observed at 262.5% for the shortest coil charging time. For subsequent times, this increase was 192.6% and 128.6% for 3 ms and 4 ms, respectively. Coil charging time had the greatest impact at ambient pressure. Increasing the charging time from 2 ms to 4 ms resulted in a 162.5% increase in spark energy. Subsequently, the increase decreased to a minimum of 65.5% at 12 bar.

The relationship of the energy transfer efficiency between the secondary circuit of the coil and the spark plug electrodes is depicted in Figure 13. The maximum achieved efficiency value was 76.7% for the highest ambient pressure (12 bar) and coil charging time (4 ms). The energy transfer efficiency through the spark plug increases with increasing coil charging time. This is in contrast to energy transfer efficiency through the ignition coil, where the efficiency decreases with longer charging times. The increase in ambient pressure is the primary factor driving the increased efficiency of spark discharge. The highest efficiency increase, amounting to 30.2%, was noted for a coil charging time of 3 ms with an increase in ambient pressure from 0 to 12 bar. The average efficiency increase was 8.1% over the coil charging time range of 2 to 4 ms.

6.3. Spark Discharge Optical Results

The thermal effects of the spark discharge were visualized through optical signal recordings. Figure 14 presents a series of images depicting the electric arc over a period of 10 μs from the beginning of the discharge, for coil charging times ranging from 1.5 to 5 ms and ambient pressures in the constant volume chamber ranging from 0 to 8 bar. The images are displayed using a spectrum color scale, where black indicates areas with zero luminescence and red signifies the highest intensity (white pixels within the arc region are the most intense). The color scale correlates with the temperature of the arc, although the exact temperature was not directly determined. Additionally, white contours of the spark plug electrodes were overlaid on the images.

Each electric arc is consistently located between the electrodes, regardless of the experimental conditions. The most pronounced variation in arc visualization occurs due to changes in pressure; increased surface area and reddening of the area are clearly visible. In the absence of overpressure in the constant volume chamber, the electric arc is least intense, markedly differing from points where pressure has increased. It is important to note that the optical system was not modified during the recording of all discharges.

Based on the obtained images, the area covered by the electric arc in the plane parallel to the spark plug axis was determined, and the results were presented as a contour map using spline interpolation. The resolution of the recorded area was 128 × 80 pixels, where each pixel represents the area covered by the electric arc. For contour maps, the spectrum color scale was also applied. According to the images, with increasing ambient pressure and coil saturation time, the area of the electric arc surface increases, reaching a maximum in the upper right corner of the map (Figure 15). The difference between the smallest and largest areas of the electric arc exceeds twofold, indicating a significant influence of environmental conditions and control parameters on ignition capabilities, which are enhanced with increasing discharge geometry indicators.

The average brightness intensity of the arc area was determined based on the intensity of each pixel corresponding to the phenomenon (Figure 16). It was observed that the coil saturation time in the pressure range from 0 to 5 bar does not affect the average intensity of the phenomenon. This suggests that despite the increasing surface area, increasing energy is ultimately transferred to the combustible charge. However, within the same range, increasing the pressure in the constant volume chamber significantly increases the intensity. Above pressures of 5 bar, the intensity remains roughly constant with further pressure increases and coil charging time adjustments across the entire range.

7. Summary and Conclusions

The research presented in this article focuses on the quantitative identification of energy flow at various stages of the spark ignition system. The study analyzes a modern solution based on an individual single-pole COP ignition coil from a direct injection gasoline engine. The amount of energy delivered by the controller to the coil, the energy generated by the ignition coil, the spark’s electrical energy, and the spark discharge’s initial stage were characterized.

The test results’ analysis enabled us to draw conclusions regarding the impact of changes in the coil charging time and pressure variations on the spark plug’s electrical discharge. This study addresses an existing gap in the understanding and analysis of this phenomenon.

A nearly linear relationship was demonstrated between the energy delivered to the ignition coil and the saturation time. For a maximum coil charging time of 5 ms, the coil charging energy was 330 mJ, compared to 72 mJ of generated energy. As the charging time increases, the efficiency of energy conversion between the primary and secondary circuits of the ignition coil decreases. Therefore, controlling the charging time well below the saturation of the ignition coil becomes advantageous. It is important to avoid areas of ignition coil saturation. In an engine, the discharge frequency is high, so significant losses can be generated in general. The achieved efficiency ranges from 20% to 50%. The energy conversion efficiency from the ignition coil to the spark also increases with increasing ambient pressure and charging time, reaching 78%. Minimum values were recorded for no counter-pressure in the 38% to 48% range. Electrical data were correlated with discharge images. The increased discharge energy due to higher ambient pressure and charging time translates into an increased area covered by the electric arc. Above 5 bar of counter-pressure, there was no observed effect of charging time and further increase in counter-pressure on the average intensity of the electric arc luminescence.

During the experimental process, thermal energy from the discharge was not analyzed; however, without these data, significant losses can be assumed at each ignition system component.