High Pressure Gasoline Direct Injection in Spark Ignition Engines - Efficiency Optimization through Detailed Process Analyses

<div class="section abstract"><div class="htmlview paragraph">Low-Speed pre-ignition (LSPI) in modern-day, heavily downsized, boosted, and direct-injection spark ignition (SI) engines is a well-known problem. Several mechanisms contribute towards stochastic pre-ignition (SPI), the most prominent being crevice material droplet induced and deposit induced pre-ignition mechanisms. The droplet mechanism is typically dominated by the detergent additives present in the lubricant formulation; more specifically calcium and sodium-based detergent additives correlate strongly with the increased LSPI rates. Deposits flaking off the combustion chamber surfaces can also induce LSPI under certain conditions.</div><div class="htmlview paragraph">This study aimed to develop an optical method designed to investigate the nature of pre-ignition precursors. Southwest Research Institute (SwRI) utilized an optically accessible GM 2.0 L LHU engine to study the pre-ignition phenomenon and studied the nature of pre-ignition precursors using spectral information from one of the cylinders in this engine. A custom, simplified borescope was employed to gather visible light from the combustion chamber to detect the glowing pre-ignition precursors. Further, this light signal was sent to an array of four photomultiplier tube (PMT) detectors which were tuned to visible light emission wavelength bands of interest. Three of the four wavelengths (555, 623, 645 nm) were selected to be representative of calcium and CaOH while the fourth wavelength of 715 nm (red emission) was selected to be representative of glowing soot-like aggregates. A high calcium first-generation Dexos™ 1 compliant oil with moderate LSPI activity was used in this study. The analysis revealed that the precursors contained both the signals for calcium as well as the deposit wavelengths in different proportions. More specifically, it was observed that in a series of LSPI events, the first event was usually more biased towards calcium wavelengths suggesting a pre-ignition initiated by the droplet ejecting from the crevice. A relative semi-quantitative metric was developed to compare the light signals for different frequencies to further delve into the pre-ignition mechanism. Two additional fuels were tested with different particulate matter index (PMI) values to assess the impact of PMI on pre-ignition precursors. A bulk comparison of around 30,000 combustion cycles of the two additional test fuels revealed that the high PMI fuel showed more bias towards a deposit induced mechanism while the lower PMI fuel exhibited signs of both droplet and deposit induced pre-ignition precursors.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Abnormal combustion referred to as Low Speed Pre-Ignition (LSPI) may restrict low speed torque improvements in turbocharged Direct Injection (DI) - Spark Ignition (SI) Engines. Recent investigations have reported that the auto-ignition of an engine oil droplet from the piston crevice in the combustion chamber may cause unexpected and random LSPI. This study shows that engine oil formulations have significant effects on LSPI. We found that the spontaneous ignition temperature of engine oil, as determined using High-Pressure Differential Scanning Calorimetry (HP-DSC) correlates with LSPI frequency in a prototype turbocharged DI-SI engine. Based on these findings, we believe that the oxidation reaction of the oil is very important factor to the LSPI. Our test data, using a prototype engine, shows both preventative and contributory effects of base oil and metal-based engine oil additives.</div></div>

<div class="htmlview paragraph">Modelling small and large scale fluctuations of fuel distribution is of high interest for stratified direct injection spark ignition (DISI) engines. Homogeneous combustion models need to be extended or replaced in order to account for these fluctuations. They are presently neglected in most engine simulations. Effects of mean fuel/air equivalence ratio gradient have been recently included in previous homogeneous mixture approaches.</div> <div class="htmlview paragraph">To account for local fluctuations of mixture composition, the new model ECFM-Z has been developed on the basis of recent Direct Numerical Simulation results and Coherent Flame Surface modelling. The model has been implemented in a CFD code (KMB) The influence of mixture fraction is integrated in the Extended Coherent Flame Surface combustion model. The model is based on a conditional approach. Unburnt hydrocarbons produced by lean flame local extinctions are taken into account.</div> <div class="htmlview paragraph">The model ECFM-Z is applied to investigate mixture fluctuation effects in a wide spacing, DISI engine running under stratified operating conditions. A typical stratified combustion parameter, the ignition timing, is varied. Accounting for the local fluctuations in mixture composition modifies heat release estimation. Effects of partial quenching and mass burning rate modelling on the combustion efficiency are revealed.</div>

<div class="section abstract"><div class="htmlview paragraph">Due to the upcoming regulations for particulate matter (PM) emissions from GDI engines, a computational fluid dynamic (CFD) modeling study to predict soot emissions (both mass and solid particle number) from gasoline direct injection (GDI) engines was undertaken to provide insights on how and why soot emissions are formed from GDI engines. In this way, better methods may be developed to control or reduce PM emissions from GDI engines. In this paper, the influence of engine operating parameters was examined for a side-mounted fuel injector configuration in a direct-injection spark-ignition (DISI) engine. The present models are able to reasonably predict the influences of the variables of interest compared to available experimental data or literature. For a late injection strategy, effects of the fuel composition, and spray cone angle were investigated with a single-hole injector. For an early injection strategy, the effects of multi-component fuel surrogates for gasoline, SOI timings and wall temperatures were studied with a six-hole injector. The investigations confirmed the necessity to consider the multi-component fuel composition and also demonstrate how and why wall films significantly contribute to soot emissions from DISI engines.</div></div>

<div class="htmlview paragraph">A 1998 Toyota Corona passenger car with a direct injection spark ignition (DISI) engine was tested over the Federal Test Procedure (FTP) driving cycle. Speciated engine-out hydrocarbon emissions were measured. Seven fuels were used for these tests: five blended fuels and two pure hydrocarbon fuels. One of the blended fuels was CARB Phase 2 reformulated gasoline which was used as the reference fuel. The remaining four blended fuels were made from refinery components to meet specified distillation profiles. The pure hydrocarbon fuels were iso-octane and toluene - an alkane and an aromatic with essentially identical boiling points. The five blended fuels can be grouped to examine the effects of fuel volatility and MTBE. Additionally, correlations were sought between the fuel properties and the Specific Reactivity, the exhaust “toxics”, and the pass-through of unburned fuel species.</div>

A fundamental investigation into the relationship between lubricant composition and fuel ignition quality

<div class="section abstract"><div class="htmlview paragraph">The biggest challenge in developing Turbocharged Gasoline Direct Injection (TGDI) engines may be the abnormal combustion phenomenon occurring at low speeds and high loads, known as low-speed pre-ignition (LSPI). LSPI can trigger severe engine knocks with intensities much greater than those of spark knocks and thus characterized as super knocks. In this study, behavior and patterns of LSPI were investigated experimentally with a highly-boosted 1.5L TGDI engine. It was found that LSPI could occur as an isolated event, a couple of events in sequence, or a trail of events. Although occurring randomly among the engine cylinders, LSPI took place frequently when the engine was operated at low speeds and high loads in the zone where scavenging was employed for boosting engine torques at low speeds, typically < 2500 rpm. Frequencies and patterns for LSPI were found to be influenced considerably by engine operation parameters, such as excess air coefficient, engine coolant temperature, valve timing, etc. Intensities of super knocks triggered by LSPI varied with the timing for the pre-ignition. Based on findings in this study, it was proposed that the oil particles as oil loading in the blowby recirculation could be the root cause for LSPI as they cause dirtiness of the combustion chamber roof where the oil particles could be attached and then initiate LSPI in the late compression stroke. It was demonstrated that although LSPI may not be eliminated due to the nature of the triggers, TGDI engines can be operated super-knock free through controlling conditions for combustion and engine cooling. As long as the values of the maximum peak firing pressure in LSPI events and their frequencies are under the design criteria, the engine can tolerate the LSPI events with acceptable knock intensities.</div></div>

<div class="section abstract"><div class="htmlview paragraph">In-cylinder fluid dynamics enhance performance and emission characteristics in internal combustion (IC) engines. Techniques such as helical ports, valve shrouding, masking, and modifications to piston profiles or vanes in ports are employed to achieve the desired in-cylinder flows in these engines. However, due to space constraints, modifications to the cylinder head are typically minimal. The literature suggests that introducing baffles into the combustion chamber of an IC engine can enhance in-cylinder flows, air-fuel mixing, and, subsequently, stratification. Studies have indicated that the height of the baffles plays a significant role in determining the level of improvement in in-cylinder flow and air-fuel mixing. Therefore, this study employs Computational fluid dynamics (CFD) analysis to investigate the impact of baffle height on in-cylinder flow and air-fuel mixing in a four-stroke, four-valve, spray-guided gasoline direct injection (GDI) engine. The maximum allowable baffle height is limited to the minimum distance between the piston and cylinder head surface when the piston is at the top dead centre (TDC). The analysis involves varying the baffle height from 1 mm to 3 mm. Additionally, a new discretisation scheme is introduced to assess the mixing level within the combustion chamber. The results indicate a significant influence of baffle height on in-cylinder flows. Notably, a 1 mm baffle height yields a flow and mixing pattern similar to that of the base engine without baffles. As the baffle height increases, the in-cylinder flows are altered, leading to the formation of multiple vortices due to the presence of baffles. Furthermore, an increase in baffle height enhances mixture stratification, confining more fuel toward the spark plug location.</div></div>

<div class="htmlview paragraph">It is widely accepted that engine combustion is fundamentally affected by the in-cylinder charge motion. Flow field structures present at the time and location of spark ignition are known to have a controlling effect on early flame development. Therefore, improved understanding of the variation in flow field structures local to the spark plug at the time of ignition is required.</div> <div class="htmlview paragraph">This study investigates the spatial and temporal development of flow field structures within the pent roof combustion chamber of a single cylinder, direct injection spark ignition (DISI) optical engine. High speed particle image velocimetry (HSPIV) has been used to quantify the flow field leading up to and following spark ignition. HSPIV data was recorded at a rate of 5 kHz, providing a temporal resolution of 1.8 crank angle degrees (CAD) between measurement fields and a spatial resolution of 512 by 512 pixels. Velocity field measurements were acquired over a 70.2° crank angle (CA) range, from 300° CA to 370.2° CA after top dead centre (ATDC), spanning spark ignition (325° CA ATDC).</div> <div class="htmlview paragraph">Results show the spatial and temporal development of large scale bulk flow and small scale structures up to and following spark ignition. Comparison of individual cycles highlights the variation in flow field structures within the combustion chamber and the effect this has on engine performance. The levels of turbulence that exist within each cycle have been calculated as a root mean square (RMS) velocity and compared to engine performance indicated mean effective pressure (IMEP) and mass fraction burnt (MFB). In order to determine the range of spatial and temporal scales of turbulence that affect the burn rate and IMEP, frequency analysis was carried out on the HSPIV velocity field data. The results demonstrate the link between in-cylinder high frequency turbulence and engine performance, in terms of indicated mean effective pressure and burn rate.</div> <div class="htmlview paragraph">To assess the feasibility of using multiple injection strategies to control combustion, three different injection strategies were investigated. Single, double and triple injection strategies, occurring during the intake stroke to achieve a homogenous charge, were tested. Results indicate that even with early injection a higher level of turbulence intensity is generated within the combustion chamber at the time of ignition for the multiple injection strategies.</div>

<div class="htmlview paragraph">During last four years the authors were involved in the simulation and investigation of combustion and ionization in the cylinder of gasoline and natural gas Spark Ignition (SI) engine. Two-zone model of combustion and chemical non-equilibrium ionization and results of the numerous numerical investigation were presented by the authors at SAE 2002 and SAE 2003 World Congresses. This model and numerical results provided an important insight into combustion products behavior and gave the chance to find correlation between chemical and thermal ionization of combustion products and SI engine performances.</div> <div class="htmlview paragraph">In order to provide better correspondence of the numerical data to the experimental results in the wide range of operational parameters for different geometry of engines a new three-zone model was developed and successfully tested. In comparison with previous one new model includes the additional third zone, which simulates the processes in the small volume of flame kernel inside the gap between spark plug electrodes.</div> <div class="htmlview paragraph">Following processes in the flame kernel are taken into account:</div> <div class="htmlview paragraph"> <ul class="list disc"> <li class="list-item"><div class="htmlview paragraph">detailed mechanism of chemical interaction including chemical and thermal ionization;</div> </li> <li class="list-item"><div class="htmlview paragraph">heat exchange between combustion products and electrodes;</div> </li> <li class="list-item"><div class="htmlview paragraph">mass exchange between third zone and combustion products in the cylinder.</div> </li> </ul> </div> <div class="htmlview paragraph">The comparison of numerical and experimental results showed more accurate correlation between measured and calculated values of pressure and ion current. The concentration of electrons in the gap between spark plug electrodes corresponds better to the experimental data for gasoline engines in comparison with two-zone model. Three-zone model became a new generation among the models for the investigation of in-cylinder combustion products ionization and simulation of operational and ecological performances of SI engine.</div>

Low-speed pre-ignition (LSPI) remains one of the challenges of Direct Injection (DI) Spark Ignition (SI) engines due to its potential to induce a heavy knock. Several mechanisms have been identified in the literature as plausible causes for LSPI. The physical and chemical properties of lubricant oils play a role on some of these causes. The present work aims at getting an independent procedure to determine the proneness of lubricant oils to ignite. To this end, the ignition delay (ID) of different oil formulations is experimentally determined in a constant-pressure flow facility through two different optical techniques: Schlieren and OH* chemiluminescence imaging. The investigation explores the effect of base-stock formulation, oil specification quality level, different additive types content, aging, and oxidation on oil reactivity for several thermodynamic conditions. Differences in ignition delay were found among base stocks, correlating with the American Petroleum Institute (API) group classification. However, no significant differences were found among additive packages previously reported to yield different LSPI occurrences. Hence, differences in reactivity among lubricating oil formulations are not the determining factor explaining their different LSPI occurrences in an engine. Similarly, specific lubricant additive content, aging, and oxidation do not importantly modify the measured ignition delay.

<div class="section abstract"><div class="htmlview paragraph">Modern spark ignition engines can take advantage of better fuel octane quality either towards improving acceleration performance or fuel economy via an active ignition management system. Higher fuel octane allows for spark timing advance and consequently higher torque output and higher engine efficiency. Additionally, engines can be designed with higher compression ratios if a higher anti-knock quality fuel is used. Due to historical reasons, Research Octane (RON) and Motor Octane Number (MON) are the metrics used to characterize the anti-knock quality of a fuel. The test conditions used to compute RON and MON correlated well with those in older engines designed about 20 years ago. But the correlation has drifted considerably in the recent past due to advances in engine infrastructures mainly governed by stringent fuel economy and emission standards. In prior research, the octane response of modern engines seemed to correlate better with RON than MON; however, the impact of octane sensitivity (RON-MON) has not been evaluated in detail. In this study, the aforementioned relationship between engine octane appetite and octane sensitivity was studied in a single cylinder direct injection spark ignition (DISI) engine using six fuels with two levels of RON, each of which had three octane sensitivity levels ranging from 5 to 15. Experiments were conducted under three compression ratios ranging from 9.5:1 to 11.5:1. The results show that both higher RON and octane sensitivity have positive impacts on the engine thermal efficiency, with RON being more influential than octane sensitivity. It is also found that the effect of octane sensitivity was more pronounced at lower RON fuels.</div></div>

<div class="section abstract"><div class="htmlview paragraph">The study described in this paper covers the development of the Sequence IX Low Speed Pre-Ignition (LSPI) test for the new engine oil category, ILSAC GF-6. The purpose of the Sequence IX test is to evaluate a lubricant’s ability to protect against LSPI events which are prevalent when operating a highly boosted/downsized gasoline direct-injected engine. LSPI is characterized as a combustion event that starts before ignition spark, typically followed by excessive in-cylinder pressures and heavy knock, which can cause severe engine damage and failure. Industry research has shown that oil formulation can contribute to the frequency of LSPI activity. The Sequence IX test was developed using a turbocharged gasoline direct-injected 2.0 liter Ford Ecoboost engine with dual independent variable cam timing. The engine was modified with in-cylinder pressure sensors and a high-resolution crank angle encoder to characterize individual engine combustion cycles and identify potential LSPI events. Multiple designed experiments were conducted to identify the engine operating conditions that would consistently produce LSPI events. Additional testing was conducted at these engine operating conditions using multiple engine oil formulations to evaluate the ability of the test to discriminate between oils. Once the test conditions were selected and the reference engine oils were identified, a precision matrix was conducted at three separate laboratories to establish test repeatability and reproducibility. The Sequence IX LSPI test has been approved for inclusion in the ILSAC GF-6 and API SN Plus categories, and is expected to be an integral part of the testing required to qualify passenger car engine oils.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Catalytic converters, which are commonly used for after-treatment in SI engines, exhibit poor performance at lower temperatures. This is one of the main reasons that tailpipe emissions drastically increase during cold-start periods. Thermal inertia of turbocharger casing prolongs the catalyst warm-up time. Exhaust enthalpy management becomes crucial for a turbocharged direct injection spark ignition (DISI) engine during cold-start periods to quickly heat the catalyst and minimize cold-start emissions. Thermal barrier coatings (TBCs), because of their low thermal inertia, reach higher surface temperatures faster than metal walls, thereby blocking heat transfer and saving enthalpy for the catalyst. The TBCs applied on surfaces that exchange heat with exhaust gases can increase the enthalpy available for the catalyst warm-up. A system-level transient heat transfer study using experimental or high-fidelity simulation techniques to evaluate the TBC application on various surfaces would be expensive. In this work, a reduced-order system-level modeling methodology in GT-Suite was leveraged to evaluate TBCs on exhaust ports, manifold, and runners. A multi-cylinder turbocharged DISI engine was modeled in GT-Suite, with capability to model a layer of TBC on internal surfaces. The model was calibrated using measured data from steady state operating conditions due to lack of transient cold start data. Following the TBC analysis, a theoretical study to infer the effects of turbocharger casing heat loss on the catalyst warm-up was performed. The TBCs showed no tangible benefit in the catalyst light-off delay when applied on the combustion chamber walls but showed a 20-second faster catalyst light-off when an 800-micron thick TBC was applied on the exhaust flow path walls (exhaust ports, manifold and runners). The turbocharger casing/housing heat transfer was shown to have a considerable effect on the catalyst light-off delay. An additive benefit to the catalyst light-off delay was achieved by insulating the combustion chamber walls, the exhaust flow path walls, and the turbocharger casing together which was predicted to be 25 seconds faster than the baseline.</div></div>

<div class="section abstract"><div class="htmlview paragraph">With the development of downsized spark ignition (SI) engines, low-speed pre-ignition (LSPI) has been observed more frequently as an abnormal combustion phenomenon, and there is a critical need to solve this issue. It has been acknowledged that LSPI is not directly triggered by autoignition of the fuel, but by some other material with a short ignition delay time. It was previously reported that LSPI can be caused by droplets of lubricant oil intermixed with the fuel.</div><div class="htmlview paragraph">In this work, the ignition behavior of lubricant component containing fuel droplets was experimentally investigated by using a constant volume chamber (CVC) and a rapid compression and expansion machine (RCEM), which enable visualization of the combustion process in the cylinder. Various combinations of fuel compositions for the ambient fuel-air mixture and fractions of base oil/metallic additives/fuel for droplets were tested.</div><div class="htmlview paragraph">CVC results confirmed the high ignitability of lubricant component containing fuel droplets in pre-heated air, which was responsible for the early timing of LSPI. RCEM results also revealed the high ignitability of lubricant component containing fuel droplets in the premixed fuel/air mixture. Stochastic occurrence of ignition induced by fuel droplets was also observed, particularly under a condition of a small lubricant oil fraction.</div><div class="htmlview paragraph">Finally, the contribution of base oil and metallic additives to the ignitability (delay times and probabilities) of droplets was separately evaluated to understand the fundamental mechanism involved. Results confirmed that the base oil itself can promote the high ignitability of fuel droplets and that metallic additives can also additionally promote or inhibit the ignitability of droplets in the premixed fuel-air mixture.</div></div>