Knocking in Gasoline Engines

Use of a high expansion ratio (high compression ratio) in order to increase indicated work, or the downsizing of engine displacement and the use of a turbocharger in order to increase the frequency of use of the high-efficiency operating range, have been proposed as methods of improving the thermal efficiency of spark ignition engines, and are already in commercial use. However, the knock restricts thermal efficiency in both of these methods, and it will be essential to address this issue in order to realize further improvement in engine thermal efficiency.

For researching a technology possessing a high level of general applicability, the authors used a simple method to promote local heat transfer through moderate enhancement of the gas flow at the point of knock onset, in order to lower the end-gas temperature and by this means retard auto-ignition timing. This research focused on the fact that the position of the piston ends on the exhaust side is the point of onset for the formation of complex flow fields after the mixture guided into the cylinders reaches the piston tops, and thought to guide the flow to the knock onset position by designing guides close to the piston tops.

Comparing the numerical fluid calculations of the flow velocity distribution for piston specifications between without the flow guide and with the flow guide in a cross-section through the center of the combustion chamber in the direction of the piston pins and a cross-section orthogonal to this at −90 deg. ATDC, the results for both specifications show that the bulk flow arriving at the piston top is divided into left and right sections, after which it rises, following the surface of the piston sleeve. However, in the case of the piston specification provided with the guide section, the air that flows into the guide rises along the surface of the piston sleeve, and a high flow velocity is distributed across a broad area. In addition, this flow is maintained even when the piston rises, and the flow is enhanced at the knock onset position.

Engine performance tests was carried out, and the results indicate that the use of the flow guide advances the combustion phase 2.2 deg. without increasing the discharge of unburned exhaust components, and this advance in the combustion phase increases IMEP by 17.2 kPa.

State-of-the-art large bore gas engines for power generation are traditionally designed for long-term operation at high load. They run at high power density and high engine efficiency, which is guaranteed as long as the engine boundary conditions, e.g., fuel gas quality, stay within certain limits. A growing share of gases from alternative sources such as biomass, hydrogen from power-to-gas technologies or LNG will be found in the pipeline grids of the future. This is further promoted by the harmonization process for gas qualities in Europe. The European standard EN 16726 as well as EASEE Gas Directive set rather wide limits for relative density, calorific value and methane number. For both engine manufacturers and operators, new challenges arise that have to be met by improving operating strategies.

Operating a stationary large engine with gaseous fuels – spark or direct ignited (diesel pilot) – leads to the problem of auto-ignition. The detection of knocking combustion is an important measure for the active prevention of engine damage. Different regimes during the combustion process result in a diversification of the cylinder pressure signal as well as the acceleration sensor signal. This requires different strategies for the distinct identification of knocking combustion. The methods shown are mainly based on the use of the cylinder pressure signal and focus on knock detection, detection of knock onset and evaluation of knock intensity.

If knocking combustion is detected, the engine control system has to react appropriately in order to ensure stable and safe engine operation. Here one can distinguish between approaches that adjust soft parameters of state-of-the-art engines (ignition timing or mixture quality), and approaches that need additional hardware and their functionality to be realized (variable intake valve timing or variable compression ratio). It is shown how these approaches to knock control impact engine performance.

The irregular combustion phenomenon of low speed pre-ignition (LSPI) is still an important issue in modern internal combustion engine development and research, despite numerous publications looking into different directions such as the influence of injection strategy, scavenging, engine oil and fuel properties, and solid particulate matter as a potential ignition source.

This paper gives an insight into a new approach to investigate the phenomenon of initial pre-ignition from a rather fundamental perspective, triggered by the general research objective: Which conditions in the combustion chamber are necessary for the occurrence of an initial LSPI event?

A major goal of the herein presented path is to reduce the complexity arising from the interaction of diverse processes typically taking place in a gasoline engine. Coming from the main question, which methods to adopt and which new ones to develop in order to achieve this goal, a completely new test bed with a constant volume combustion cell as the centerpiece has been developed.

While maintaining characteristic thermodynamic conditions for a modern boosted gasoline engine, the interaction of the in-cylinder flow and the detachment of liquid drops from the liner as well as a typical fuel wall interaction possibly followed by engine oil dilution are minimized for this baseline study.

At the first step this experimental approach focuses on liquids and their physical and chemical properties as a potential trigger for auto-ignition.

The investigations are meant to result in a better understanding of the thermodynamic conditions leading to LSPI, which, according to the authors, is crucial to overcome the high risk of damage, which hinders the development of downsized gasoline engines with a higher compression ratio and thus better fuel efficiency.

Downsizing as the development path chosen in recent years allows for a significant efficiency improvement of current gasoline engine concepts. However, with power densities beyond 20 bar BMEP the likelyhood of combustion irregularities is increasing.

State-of-the-art is the detection of these abnormal combustion phenomena by indication measurement technology. However, this measuring method reaches its limits when the point of origin of abnormal combustion events must be detected. The use of optical measuring systems makes it possible to generate the necessary location information.

In this study the use of optical diagnostic tools for the detection and evaluation of auto ignitions in charged direct-injection gasoline engines is evaluated. The application of “low-speed” and “high-speed” camera systems by means of combustion chamber endoscopy as well as fiber-optic measuring methods with optical access via the spark plug is presented. In addition, the use of infrared measurement technologies is discussed to enable a thermography of the combustion chamber surfaces.

It is explained how these analysis tools can be used for the detection of auto ignitions. Furthermore, the application limits of the measurement methods are discussed as well as the specific strengths of the individual methods, e.g. Low-Speed and High-Speed Video Endoscopy. Finally, an assessment is provided to explain to which extent the presented optical tools are suitable to provide the required location information.

Pre-ignition caused by the spark plug was investigated with in-cylinder visible engine. As a result, two types of pre-ignition were observed; one was heat surface ignition at a high revolution speed and the other was non-heat surface ignition at a low revolution speed. As for the latter phenomenon, a presumption of the ignition location, investigation of the associated engine parameters, and comparison of the occurrence conditions for LSPI caused by red-hot particles were carried out to meticulously classify pre-ignition.

In view of growing global mobility requirements, energy security and climate change remain prominent issues. Consequently, there are strong drivers for further improved efficiency of vehicle powertrains, especially for internal combustion engines, which represent today’s mainstream technology.

Low-Speed Pre-Ignition (LSPI) has become a major challenge for the light duty automotive industry. The movement to downsize engines and add turbocharging has improved performance and fuel economy, but the in-cylinder conditions created have led to this stochastic and destructive knocking behavior. Many researchers studying the LSPI phenomenon have hypothesized that it is caused, at least in part, by fluid which is ejected from the piston crevice region.

If this hypothesis is true, it is critical to understand the nature of this fluid. It is well understood that there will be lubricating oil present in the crevice from the action of the rings upon the oil film on the liner. And, at the high load conditions where LSPI occurs, it may be assumed that some fuel will be present as well. However, neither of these fluids will auto-ignite in the manner seen during LSPI events. The hypothesis requires that some characteristics of the crevice fluid are altered from the original fluids.

To investigate this aspect of LSPI, a rapid-acting sampling valve was developed to permit sampling of the fluid in the piston crevice while avoiding sampling of cylinder contents when the piston crevice was not at the sampling location. The samples were analyzed with a two-dimensional gas chromatograph-mass spectrometer which allowed detailed speciation of the samples.

Sampling was performed with the engine running on fuels and lubricants which either increased or reduced LSPI occurrence. There were clear differences in the chemical content of the samples between these cases which indicate that some chemical reactions must take place in the crevice that impact the LSPI behavior of the engine.

Combustion in a spark ignition engine operated at high speed and load is investigated numerically with regard to knock behavior. The study focuses on the concurrent impact of spark timing and exhaust gas recirculation (EGR) on the severity of knock. Specifically, the possibility of knock reduction through the lowering of nitrogen oxide (NO) content in the rest-gas is examined. Simulations are carried out using a stochastic reactor model of engine in-cylinder processes along with a quasi-dimensional turbulent flame propagation model and multicomponent gas-phase chemistry as gasoline surrogate. The knock-limited conditions are detected using the detonation diagram. By lowering the NO content in the external EGR the end-gas auto-ignition is suppressed. This prevents a transition to knocking combustion and enables advancing of spark timing that yields better combustion phasing. As a result, fuel economy is improved and the potential benefits of cleaning the EGR are indicated.

Engine knock limits the efficiency of turbocharged SI engines at high loads. The occurrence of this phenomenon can be inhibited by deploying recirculation of cooled exhaust gas (EGR) at full load. However, the development of full load EGR combustion systems cannot be per-formed in the 0D/1D engine simulation, as no meaningful models for the reliable prediction of the knock limit under the influence of EGR exist.

Measurements of ignition delay times in a shock tube and a rapid compression machine under the influence of exhaust gas have been carried out. The addition of 25% EGR prolonged the ignition delay time up to 100%. A detailed reaction mechanism for gasoline surrogates was defined and validated against the measurement results. Furthermore, the effects of EGR on combustion, knock behavior and emissions were investigated on a single-cylinder re-search engine. The center of combustion could be advanced by up to 9° CA with the addition of EGR leading to a four percenter higher indicated efficiency. The influence of catalytically treated exhaust gas was examined as well. At high EGR rates of 25% catalytically treated exhaust gas allowed a 2° CA earlier center of combustion. Furthermore, the influence of nitric oxide on the knocking tendency was investigated. It has been found that a total cylinder NO concentration of about 100 ppm leads to the highest knocking tendency. At NO concentrations below 40 ppm NO the knocking tendency was decreased. Higher concentration than 100 ppm of NO in the cylinder decreased knocking tendency as well.

The pressure trace analysis of the measured single working cycles shows that the pre-reaction state of the unburned mixture at knock onset calculated with commonly used knock models decreases with rising EGR rate and engine speed, although by definition it must be constant at the time of auto-ignition. Consequently, reaction kinetics simulations at in-cylinder conditions proved that, under specific boundary conditions, the auto-ignition of the unburnt mixture resulting in knock happens in two stages. In this case, low-temperature ignition occurs in the unburnt mixture while the combustion is taking place. This phenomenon significantly influences the ignition delay of the mixture, which severely impairs the prediction capabilities of commonly used knock models.

Based on these findings, a new knock modeling approach capable of predicting the low-temperature ignition occurrence as well as reproducing its influence on the mixture’s auto-ignition was developed. The results from 3D-CFD simulations accompanying the model development supported all model assumptions made. The developed knock model was successfully validated against measurement data at various boundary conditions, such as different inlet temperatures and mixture compositions as well as EGR rate and engine speed variations. It can predict the knock limit very accurately with errors in center of combustion below 2° CA and thus contributes to an efficient development process of full load EGR combustion systems in the 0D/1D engine simulation.

Fuel impingement on the cylinder wall causes problems in both lubrication and combustion due to interaction of liquid fuel and engine oil. To analyze this issue with temporal and spatial resolution, a laser-induced fluorescence (LIF) system for simultaneous kHz-rate imaging of fuel and oil films on the cylinder wall is presented. Experiments were performed in a research engine with optical access along the entire stroke of the cylinder in fired and motored engine operation. Besides fuel wall wetting, fuel transport across the piston ring pack and the impact on the lubricant conditions in the piston group can be seen in the images.

The paper describes a method to identify and analyze pre-ignition (PI) events appearing in engine and vehicle RDE drive cycle tests or similar transient and stationary test schedules. A specific requirement in such long duration drive cycles is the large amount of data to be recorded for each combustion cycle and the data processing during and after a test procedure. Identification of PI events is accomplished with measurement of ignition delay time, i.e. spark discharge timing and the instant when flame growth is detected. PI events show up with negative ignition delay. How this simple concept is implemented in a combustion measurement procedure with many thousand cycles is explained in some detail. Classification of PI events and root cause analysis is then supported with evaluation of the optical signals of the tests. Specific evaluation procedures to accomplish such task have been described in earlier publications.

Optical imaging and sensor techniques provide detailed analysis of mixture formation, ignition and combustion processes. The evaluation of image sequences and sensor data obtains quantified measured values to characterize engine conditions.

In-cylinder infrared absorption measurement techniques are applied to record Lambda values, residual gas content (water and CO₂ concentration) and gas temperature with high time resolution. Especially the temporal analysis of the Lambda value evolution within engine cycles is a criterion for the quality of mixture formation and ignitability. This reveals quantified indices for the combustion stability.

Advanced processing of a series of highly time resolved images from an engine’s cycle generates quantified data about spray pattern formation, flame propagation and soot temperature. Images of the early flame evolution are analyzed using a dynamic thresholding algorithm and reveal the flame propagation.

Since the invention of spark ignition engines, knocking has been the limiting factor in the development of new engines with increased power density and improved efficiency. The mechanisms that lead to knocking combustion have been investigated in numerous research studies that have produced, to some extent, controversial theories. Even though it is very certain that knocking involves a detonation caused by self-ignition processes in the end-gas region, there is a great interest in growing the understanding of knocking phenomena and the associated multitude of combustion irregularities. This article gives an overview of the contemporary state of knowledge of knocking and other irregular combustion phenomena. This article discusses the kind of detection and analysis methods to investigate irregular combustion in SI engines. Additionally, knowledge about irregular combustion is still not totally understood and the areas in which further research and development are necessary are discussed. For the purpose of describing and evaluating knocking and other combustion irregularities, not only experimental investigations will be employed, but also theoretical considerations.

Demand for more efficient gasoline vehicles has driven the development of downsized, engines, which benefit from higher octane.

Features on modern SI engines such, direct injection, inter-cooling in boosted engines, cooled EGR and Millerisation lead to a much lower temperature for a given pressure in a real engine as compared to the test conditions in the CFR engine used to define the Research Octane Number (RON) and Motor Octane Number (MON) octane rating scales.

Because the end-gas in modern engines experiences a different pressure/temperature history during knocking cycles, as compared to the CFR engine, there is a growing body of evidence to suggest that for a given RON, it may actually be beneficial to have a high octane sensitivity (RON-MON) or in other words a lower MON.

To explore this further, tests have been conducted in a single cylinder DISI engine over the whole speed load map using three different compression ratios, and fuels with two different levels of RON but with three octane sensitivity levels ranging from 5 to 15. These results have been further interpreted by reference to chemical kinetic models for gasoline autoignition, which can be used to rationalise how the influence of sensitivity varies over the speed/load map.

The Model Based Knock Detection (MBKD) is an innovative method to identify knocking combustions and pre-ignitions in gasoline engines. The MBKD was a joint development by the specialized calibration team and the function development team for knock detection and control at Robert Bosch GmbH. The overall objective of the MBKD is to increase quality and to reduce the effort required for the parameterization of the individual methods that serve to identify abnormal combustion phenomena.

The first series version of the MBKD is currently calibrated in more than ten series-production projects. In comparison to knock detection methods based on signal filtering in the time domain, it increases the detection quality by reducing the susceptibility for noise interferences. Methods from the field of Artificial Intelligence, Pattern Recognition, statistics, and optimized digital signal processing are combined to an improved approach for detecting knocking combustions and pre-ignitions. Results and experiences from various series calibration projects show the high generalizability and robustness against electrical and mechanical interferences of the new knock detection method. This leads to reduced knock control interventions, less misdetections and therefore to a reduced Real Driving Emissions (RDE) fuel consumption and an optimized drivability.

The MBKD method and software has shown its suitability for mass production due to the usage in various projects with different ECUs and engine variants (number of cylinders and displacement) with high customer satisfaction.

In a current development the first series version of the MBKD functionality is extended by reconstructing the physical knock and pre-ignition pressure characteristics based on signals of structure borne noise sensors. This allows us to predict sensor signal values, which cannot be detected directly or only at high costs in series-production vehicles.

The main benefit is a more specific control behavior in any engine operation point due to a better estimation of the damage potential of single abnormal combustions.

The contribution of the paper is to propose a knock controller based on the estimation of the distribution quantile of the knock intensity measures. Despite the quantile estimation randomness, the corrections undertaken by the controller are moderated by the level of confidence in the quantile estimation. This strategy offers a fast and stable control ensuring a better compromise between the engine efficiency and the prevention of knock phenomenon. The design of the controller stabilizes the engine torque and is suited for engine transient operations.

To meet increasingly stringent emission legislations, improving the combustion efficiency becomes essential in the development of modern gasoline engines. This paper presents a comprehensive overview of available methods to improve the spark timing for maximum break torque under use of the in-cylinder pressure signal. The suitability of a series-compatible pressure sensor for the use of ignition angle control is summarized and reviewed. Finally, the results of spark timing control in a test vehicle are discussed on base of chassis dyno measurements.

Development on gasoline engines for 2025 and beyond is driven both by enhanced electrification as well as by cost optimized measures for improved engine efficiency. Besides Miller combustion process, cooled external exhaust gas recirculation (EGR) and variable compression ratio, water injection represents one of the relevant technologies. Water injection potential within gasoline engines will be evaluated within this paper.

Basically, the cooling effect of evaporating the injected water can mitigate knocking. This will result in earlier combustion phasing, in less fuel enrichment for component protection and thereby in higher thermodynamic efficiency. Water injection will be systematically investigated on a 1.6 l turbocharged engine for three different concepts: Intake manifold/plenum injection, inlet port injection and the direct injection of a premixed fuel/water emulsion. Complexity and water efficiency are compared with each other for all systems.

Regarding the application strategy, both an efficiency as well as a performance approach are considered. For the efficiency variant, fuel saving potentials will be optimized by increasing the geometric compression ratio. For the performance variant, the increase of stoichiometric power at constant compression ratio/part load consumption will be in focus of the studies.

Understanding the effects of water injection on the working process of the combustion engine is assisted by 3-D-CFD modelling of water distribution to cylinders, mixture formation, evaporation and thermal efficiency. This will be supported by in-situ video records about the water-to-air mixture formation as well as for the injection and evaporation during inlet stroke.

Based on all generated knowledge, the potential of water injection for future combustion engines will be analyzed and pros and cons for the different injection concepts are evaluated. An outlook will be provided.

Water injection into internal combustion engines has long been known as a powerful tool to increase either maximum power and/or decrease emissions. Up to now, product costs and application effort have prevented a broad serial introduction of this technology. Competition with electric drive technology and tighter emission legislation make this feature more attractive for serial application in gasoline engines. Therefore, the different benefits of water injection are evaluated in this paper. The direct injection of water into the combustion chamber is realized through the use of an emulsion with a variable water-fuel ratio. Gasoline water direct injection (GWDI) results are contrasted with data obtained by water injection into the intake pipe after the intercooler. Furthermore, investigations regarding knock limited combustion and mitigation of fuel enrichment were performed on two engines from different OEMs, both with turbocharging and direct injection. The study is completed by a detailed testing of the impact of water injection on stochastic (low speed) pre-ignitions (SPI).

Given the need to further reduce CO₂ s from passenger cars powered by internal combustion engines and in the light of future tighter exhaust emission regulations, further improvements are necessary to enhance the efficiency and emission behavior of gasoline engines. One major constraint encountered in the gasoline engine is knocking combustion. Its negative effects on efficiency and emission behavior can be counteracted by reducing knock tendency.

One approach to reducing knocking tendency lies in injecting water. This is capable of extracting heat from the cylinder charge and, on this basis, reduce final compression temperature. It also increases charge mass in the cylinder and ultimately improves efficiency as a result of the lower tendency to knock.

With a view to implementing water injection, attention initially turns to the thermodynamic aspects of water injection in the gasoline engine and to comparing and evaluating the water manifold and direct water injection concepts. With the aim of conducting fundamental studies on a test engine with direct water injection, discourse then presents the design-related aspects involved in the implementation of such. The engine tests conducted focus on identifying the main water injection parameters influencing the gasoline engine’s working cycle. Selected test results provide the basis for illustrating them and presenting the key underlying mechanisms. The potential for increasing the compression ratio can be determined in relation to the effect of water injection on reducing knock. The resultant higher level of efficiency makes it possible to reduce the emission of CO₂.

Finally, consideration is given to the challenges involved in injecting water, with further concept approaches being presented for water injection.

The use of external Exhaust Gas Recirculation (EGR) is a common technology available for the reduction of nitrogen oxides (NOx) emitted by internal combustion engines. With regard to gasoline engines, the addition of EGR at higher loads reduces knock tendency and improves fuel economy by reducing the necessity for fuel enrichment. To further maximize these benefits, the recirculated exhaust gases are cooled down that improves engine efficiency by enabling an advanced center of combustion (MFB50). Hereby gasoline engines can be operated at EGR rates up to 20%, which is enabling stoichiometric operation in the entire engine map. On the other hand, cooled EGR is leading to well-known low-temperature issues such as fouling, corrosion and condensation. In response to that challenge, in this work the use of cooled LP- and HP-EGR is analyzed by engine testing for different engine intake temperatures. For dedicated tests a higher compression ratio for improved fuel economy and a coated Gasoline Particulate Filter (CleanEGR^TM) for cleaning the EGR gas is investigated as well. As a conclusion, external cooled and cleaned EGR is measure to meet future RDE requirements (stoichiometric operation in entire map) by achieving improvements in engine efficiency and engine out emissions (PN, NOx) simultaneously.