Ignition Systems for Gasoline Engines

Recent advancements of the gasoline engines combustion processes, e.g. a rightsizing or an alternative combustion process approach, make significantly higher demands on ignition systems than previous engine generations, especially with respect to charge dilution and gas exchange strategies. Among others, it is especially the optimization of the ignition system, which allows an extension of the current thermodynamic and combustion limits without drawbacks in engine performance. Besides the demands of the stationary operation one has to especially bear in mind the requirements originating from the transient operation of the engine.

In this presentation the potentials and limitations of ignition innovations are examined. With reference to a current series ignition system, both an optimization of the conventional spark ignition as well as alternative ignition concepts are analyzed. Both stationary and transient aspects will be covered. Furthermore realization relevant aspects and needs for the automotive implementation will be analyzed.

Upcoming legislation motivates further development of modern combustion systems and engine designs to achieve a reduction of fuel consumption and emissions. Increased power density, charge motion and dilution drives requirements of the ignition system. Bosch prepared a high performance ignition device focused on high energy output and long spark duration while providing high flexibility to select relevant performance parameters. The ignition concept furthermore is designed as a plug and play solution in order to fit to established hardware interfaces and offers high energy efficiency.

Based on this technology several engine tests were completed at global OEMs and inside Bosch. The following paper summarizes the main test results which cover extension of EGR rates, lean limit and reduction of emissions as well. Based on this results possibilities how the benefits can be used in real applications are derived and discussed.

Investigations were performed on different engines representing diverse combustion concepts especially with regard to charge motion level. The concluded tests also confirm that increased ignition performance only in combination with parameters of the combustion concept itself can lead to the desired benefits on engine level.

Besides the review of mentioned engine tests a new advanced ignition system called CEI (Controlled Electronic Ignition) is introduced. The main properties of the used high ignition device are described including a comparison to the visible market trend for ignition requirements.

Transient plasma ignition using nanosecond pulses has demonstrated the potential to enable improved fuel economy and reduced emissions by enabling lean and EGR limit extension in dilute burn engines. Existing spark ignition technology is not adequate because the energy transfer mechanisms between the spark and the fuel-air mixture are not efficient enough to guarantee stable ignition for dilute mixtures at high-load conditions. Additionally, long duration sparks and other advanced ignition solutions that require increased energy delivered accelerate spark plug electrode wear. To date, non-thermal plasma ignition with nanosecond pulses have demonstrated a lean ignition limit beyond an air/fuel ratio of 24 [1], demonstrated high-pressure ignition at densities equivalent to over 100 bar at the time of ignition [2], and demonstrated stable (COV <3 %) ignition at EGR dilution levels >20 % [3]. While low-energy nanosecond pulses have demonstrated strong performance compared to existing solutions, they currently only exist on the market in laboratory systems, rather than a production ready system in a single rugged, weather-proof, under-the-hood enclosure. Transient Plasma Systems (TPS) has recently demonstrated the potential for a retroffitable solution similar to coil-on-plug architecture that allows a direct replacement of existing ignition technology without any engine modification. The system was run on a gasoline direct injection engine at Argonne National Laboratory and demonstrated the same trends as previously observed with research grade systems, including lean and EGR limit extension and more stable ignition across a range of loads. The system was capable of delivering 30 kV pulses in bursts of up to 20 pulses at 30 kHz, and demonstrated stable combustion at an air/fuel ratio of 23.5, exhaust gas recirculation of 23 %, and ignition at 19.2 bar with COV <3 % using only 20 kV pulses.

To respond to the social requirement such as energy security and climate change, engine thermal efficiency has been improved. The use of charge dilution such as lean combustion and exhaust gas recirculation (EGR) has been considered as effective option to meet these requirements. In diluted combustion, in-cylinder flow-structures such as tumble are used to intensify in-cylinder turbulence and promote flame propagation. However, it is not clarified how it can contribute to the formation of flame kernel and promotion of the flame propagation under highly turbulent and diluted charge conditions. The purpose of this study is to achieve superior thermal efficiency with stable ignitability under high in-cylinder flow and diluted charge conditions. This paper describes the effect of flow velocity and turbulence intensity on the ignitability through analysis of the discharge channel behavior and initial stages of combustion phenomenon such as formation of flame kernel and flame propagation.

Engines with extreme lean combustion exhibit a significant potential to improve efficiency and to reduce NO_X raw emissions. The development of advanced combustion systems is based on the extensive usage of simulation tools to reduce the number of testing variants and development time. This paper presents a comprehensive simulation methodology for the layout of combustion systems. The presented models are able to predict the ignitibility limits and the cycle-to-cycle resolved combustion with a high degree of accuracy. An interlink approach of 1D gas exchange and 3D in-cylinder flow simulations is being used. The presented models are based on physical considerations and correlation approaches. The physical models are used to predict the ignition limits and to describe the early combustion. These models are validated by test bench measurements. The prediction of late combustion and cycle-to-cycle fluctuations is based on correlation approaches which includes a large number of different engine concepts. The methodology enables a standardized work flow for complex development projects such as required for extreme lean combustion systems.

This work presents a high energy multipole distribution spark ignition system that utilizes a three-pole spark igniter to create spatially distributed sparks within the igniter perimeter. The current prototype can fit in a M14 standard spark plug mounting thread, evenly distributing three spark gaps in a triangular pattern with a circumradius of 2.3 mm. The spark gaps can be individually energized by the attached ignition coils, thereby discharging in either simultaneous or alternating mode. The experimental results from both the single-cylinder engine and constant-volume combustion vessels indicate a clear trend that, on the basis of similar total spark energy levels, the three-pole ignition system can shorten the ignition delay, stabilize the combustion phasing, and extend the ignitable lean limits, compared to a conventional fine-electrode spark plug. Moreover, as found in the combustion vessel tests, the effectiveness of spark discharge enhancement using direct-capacitor discharge can be augmented through simultaneously discharging through the three poles, hence resulting in significantly shortened ignition delay.

Fast combustion by devising an ignition device for simultaneous ignition from eight positions around a conventional combustion chamber, in addition to an ignition plug arranged in the center of the combustion chamber was realized. A visualized experiment on the state of combustion by mounting this into a single-cylinder 2-stroke engine was conducted. And confirmed that combustion had been stabilized to a great extent by multi-point ignition. Subsequently, the ignition device was further applied to a 4-cylinder 4-cycle engine. Optimal ignition timing was retarded by more than 10°, and torque was also increased in the wide air-fuel ratio region, compared to the case of conventional single point ignition. The lean limit was also increased by a factor of 4 in the air-fuel ratio by this fast combustion. Further, the anti-knocking property was improved, making it possible to increase the compression ratio. By the reduction in time loss and the synergetic effect of air-fuel ratio leaning and high compression ratio caused by fast combustion, thermal efficiency achieved 44.49 %.

The most recent spark plug development comprehends an increasing usage of rare precious metal alloys like Iridium and are aiming to a lifetime extension of the device, as well as the improvement of the ignition efficiency. However, they are also contributing to the higher cost of the latest spark plugs.

These were the drivers for the present work, in which Delphi is proposing the monitoring of the spark plug wear in order to enable the vehicle to detect a predetermined critical level of degradation. This wear compromises not only the combustion but also the surrounding components. The real-time monitoring of the plugs eliminates the supplier requirement to replace the plug after - usually conservative - predetermined exchange cycles. The achieved extended lifetime leads to a reduced waste of precious metals, and is answering the environmental challenge as well.

This work is based on ignition coils that are using on-board electronics which include a microcontroller. The programmable controller adds new functions and more independence to the ignition itself through the flexible software solution. The paper explains the method for measuring the breakdown voltage by making use of the microcontroller software and peripheral hardware which control both the acquisition of that data and the transmission of consolidated information to the ECU. The result can then be displayed as service information in the car. Usage of the ignition coil integrated system allows to overcome the difficulty in directly measuring high voltage. This goal is achieved coil-internally and there is no need for an external device to measure this high voltage, which would be hard to place around the engine and definitely a more expensive option.

An ignition coil is subjected to a very dynamic loading environment during its operational service. Some of these loads expose the ignition coil to mechanical fatigue. In the process of designing a reliable coil, it is beneficial to make an accurate durability assessment of the coil design even before the prototype is built. A case study is presented here that explores into the thermal fatigue and durability assessment of the primary wire of a coil by using CAE simulation. The simulation presents the effects of the coil assembly loads and operating service loads upon the fatigue and durability of the wire. The simulation prediction is validated by the lab test. A validated simulation would help in quickly identifying the critical design parameters and exploring the design space to develop a reliable and durable ignition coil.

The conversion efficiency of secondary electrical energy into thermal energy was measured in air using an optically accessible spark calorimeter for high-voltage (28 kV peak) pulsed nanosecond discharges with secondary streamer breakdown (SSB) and similar low-temperature plasmas (LTP) without. Initial pressures were varied between 1 and 5 bar absolute, with the anode/cathode gap distances likewise varied between 1 and 5 mm. Secondary electrical energy was measured using an in-line attenuator, with the thermal energy determined from pressure-rise calorimetry measurements. The SSB probability at each initial pressure and gap distance was also recorded. The calorimetry measurements confirm that, similar to inductive spark discharges, SSB discharges promote ignition by increasing the local gas temperature. LTP discharges, on the other hand, had very little local gas heating, with electrical-to-thermal conversion efficiencies of ~1 %. Instead, the LTP was found to generate substantial O-atom populations — measured using two-photon laser-induced fluorescence near the anode where electric field strengths were strongest — that persisted for 100’s of microseconds after the discharge. The influence of 10 repetitive pulses spaced 100 µs apart was also evaluated for a fixed 5 mm electrode gap distance, with the conditional SSB probability for each pulse evaluated using an available photodiode, with the SSB probability found to have increased for each successive pulse. The influence of chemical and thermal preconditioning by the preceding LTP pulse was evaluated, with the increase in SSB occurrence attributed predominantly to mild gas heating that decreased number densities between the electrodes and hence the gas resistance for the subsequent pulse.

The comparison of spark ignition systems and corona ignition systems is performed by changing a spark plug to a corona igniter in an existing engine configuration on an Engine-in-the-Loop test bed. In this investigation visual inspection of the inflammation is done comparing visible image with spectral resolved two-dimensional information investigating the for flame initiation relevant OH^*, CH^* and \({\rm C}_{2}^{*}\) chemiluminescence. A general comparison between optical and thermodynamic analysis is performed. The observations are then used to evaluate transient cycle-to-cycle comparisons of the two ignition systems.

Ion current sensing is an established method for the analysis of engine combustion. It is based on the measurement of thermally induced electrical conductivity in the combustion chamber and is used for detection of misfire, engine knocking, the air-fuel-ratio or peak pressure position. Because the spark plug electrodes are used as sensor elements, the combustion signals cannot be determined during spark duration. Particular attention therefore is drawn to the temporal transition between spark and combustion at higher speeds. In this paper, the necessary circuitry for controlling the spark will be presented for an inductive ignition system, that will maximize the measuring window as well as minimize disturbing electrical oscillations.

The 2014 season represented a major shift in the focus of Formula One powerunit development. Prior to this date, engines were high revving naturally aspirated units where performance was primarily driven by volumetric efficiency. Thereafter, the regulations changed to a fuelflow limited formula [1]. This effectively meant that the competition was to achieve the minimum powerunit BSFC especially at 100 kg/h fuelflow and > 10500 rpm (as per FIA regulation). Turbocharging was once again permitted and engines were downsized from 2400 cm³ to 1600 cm³ swept volume. The focus on combustion efficiency meant that lean operation became very interesting. The move to a lean boosted engine changed significantly the requirements placed on the ignition system.

The 2014 regulations prohibited some advanced ignition systems and so development was concentrated on a conventional inductive coil and exposed spark gap plug. Early challenges were simply to cope with the very high sparking pressures and hence breakdown voltages. Once adequate robustness was obtained, the push was to have good ignitability and hence combustion stability for lean knock limited conditions.

Various aspects of spark plug geometry were explored at numerous points in the development with improvements in combustion performance of the engine often requiring tradeoffs with the ignition system to be made as sparking pressures tended to increase ever higher. As expected, the influence of the ignition system was primarily seen in early burn rates but correlations were identified between these and the optimum lambda for best knock limited efficiency. Along with significant effort on spark plug design, work also took place using an ignition coil emulator searching for even small improvements of performance through different current profiles and multisparking. Endoscopic imaging of early combustion was carried out on a single-cylinder version of the Formula One engine to aid understanding and complement conventional indication measurements.

This paper describes development that took place between 2011 and 2014.

The characteristics of a microwave-enhanced laser ignition of methane/air lean premixed mixture in the constant volume combustion vessel is experimentally investigated. The microwave-enhanced plasma is applied as an advanced ignition source due to the controllability of the plasma life time. The parameters of the microwave are varied for elucidating the effects of microwave-enhancement on laser ignition. Results shows that the microwave-enhanced laser ignition decrease the total time of combustion. This is because the large volume and long life time plasma decrease the time of initial development of the ignition kernel. In addition, the total energy of microwave enhanced laser ignition can be reduced effectively by applying the duty ratio of microwave.

Lean combustion and downsizing are two concepts to increase fuel efficiency and to reduce emissions of engines. However, the requirements on the ignition system increase consequently due to increased flow velocity and pressure at the time of ignition. In this context, the application of miniaturized passively q-switched laser spark plugs with pulse train operation provides an alternative to the conventional spark plug. To exploit the full potential of the pulse trains, ignition and combustion processes induced by passively q-switched laser spark plugs are investigated in this study.

Due to stringent emission and fuel consumption requirements, several innovative ignition systems have been proposed to address the shortcomings of current spark ignition systems. Two of the most promising candidates are high-energy spark ignition systems and corona ignition systems. However, both these systems have some fundamental drawbacks. Spark ignition systems are very mature and relatively inexpensive, but the performance of these systems is limited by the small electrode gap, which results in a small ignition volume. Corona systems, on the other hand, have a much larger ignition volume, but have been slow to enter the market due to their complexity and cost.

These drawbacks are overcome by the Advanced Plasma Ignition (API) system which is described in this paper. API-corona is a very simple, reliable and inexpensive corona system that is directly compatible with most modern engines. API-spark is a high-energy spark ignition system with a ten times longer spark length that a normal spark system.

Benefits in performance and emissions can be achieved by the use of a properly designed corona ignition system, but these can only be realised in a cost effective manner if the designer is able to assess the likely impact of design changes to both the ignition system and the combustion chamber in which it operates.

Finite element modelling is able to capture the processes with a high degree of fidelity, but at the expense of high computational cost and long run times.

Methods described here allow evaluation of design changes to be achieved with reduced computational effort, allowing for a wider array of investigations to be completed. It is shown how the performance of the total system may be improved using these methods.

Investigations are presented comparing the conventional spark ignition system with a high frequency corona ignition system at a single cylinder heavy duty truck type engine with optical access to the combustion chamber through a large piston window. The focus of the thermodynamic as well as the optical studies lies on the evaluation of the ignition system with regard to an increase of the maximum EGR Rate as well as on a reduction of cyclical variations. Further considerations are the influence of specific parameters of the corona ignition system, such as ignition duration and ignition voltage on the initial flame kernel formation. For the underlying combustion concept, the corona ignition system was found to offer a high potential for a significant increase of the EGR compatibility.

In this study, the advanced corona ignition system (ACIS) was evaluated. First, the system was evaluated in a direct-injected, single cylinder optical engine and high-speed imaging of enhanced combustion luminosity was used to visualize the ignition, flame kernel formation and flame propagation. The imaging results show that the ACIS promotes simultaneous ignition of the mixture at multiple locations in the combustion chamber as opposed to ignition being limited to the spark gap channel. Therefore, ignition delay is always shorter with the ACIS than with a traditional spark plug, inductive discharge system. The ACIS is able to support stable combustion (COV of IMEP < 3 %) in a leaner homogeneous mixture than the spark plug based system. Similarly, the ACIS has shorter ignition delay and faster burn rate for stoichiometric, rich and simulated EGR diluted mixtures. Second, the ACIS system was evaluated in a 2.0L, direct-injected, turbocharged, engine. Comparison of combustion results showed that significant reduction in 0–10 burn durations can be achieved over the production ignition system, enabling higher internal dilution limits. The ACIS was able to match the performance of the production ignition system for the idle spark sweep test. The test results showed that the close proximity of the piston to the ACIS electrode can transition the streamer discharge to arcing mode, which hampers optimal performance. Overall, ACIS has the potential to perform as well or better than the production ignition system and for best combustion performance the piston design must be optimized.