Impact of Fuel Sensitivity (RON-MON) on Engine Efficiency

<div class="section abstract"><div class="htmlview paragraph">At part load and wide open throttle operation with stratified charge and lean mixture conditions the Direct Injection Spark Ignition (DISI) engine offers similar efficiency levels compared to compression ignition engines</div><div class="htmlview paragraph">The present paper reports on results of recent studies on the impact of the in-cylinder processes in DISI engines e. g. the injection, the in-cylinder flow, the mixture preparation and the ignition on the combustion, the energy conversion and the exhaust emission behavior. The analyses of the spray behavior, of the in-cylinder flow during compression as well as of the flame propagation have been carried out applying advanced optical measurement techniques. The results enable a targeted optimization of the combustion process with respect to engine efficiency and exhaust emissions. The benefits of an increase in fuel injection pressures up to 100 MPa are discussed. The investigations have been mainly performed in a single cylinder research engine featuring a spray-guided injection process with central injector location. The considered operating range includes part load conditions with both stratified and homogeneous mixture preparation as well as full load operation with homogeneous mixture preparation. The results obtained from the research work are very promising and hence a 4-cylinder production engine is modified to operate with a suitable injection system with rail pressures in the range of 50 MPa to 60 MPa.</div><div class="htmlview paragraph">Additionally, irregular combustion events in the full load regime such as knocking combustion and low speed pre-ignition have been investigated in a 4-cylinder DISI production engine. The results support the development of mitigation strategies for both engine knock and low speed pre-ignition (LSPI).</div></div>

The contribution of intermediate-temperature heat release to octane sensitivity

<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="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="htmlview paragraph"><i>The rapid trend in recent years toward higher octane fuels and high-performance engines has suggested a reappraisal of the ability of current laboratory knockrating methods to define the antiknock behavior of commercial gasolines in modern multicylinder passenger car engines. Road antiknock performance data have been obtained on 91 premium gasolines in five 1957 test cars and three cars equipped with experimental 11-to-1 compression ratio engines. In addition, 16 prototype gasolines were studied in the three high compression ratio cars.</i></div> <div class="htmlview paragraph"><i>The data were analyzed by establishing a linear relationship between the road ratings and the laboratory data to determine the relative importance of each variable in enhancing the accuracy of the prediction equation. Of the many fuel laboratory variables, combinations of Research and Motor octane number, fuel composition and volatility were investigated as predictors of road performance.</i></div> <div class="htmlview paragraph"><i>For any given combination the results were rather consistent from car to car, but varied greatly with car speed. Research octane number was found to be the best single predictor of low-speed road performance (2000 rpm), and Motor octane number the best single predictor of high-speed performance (3000 rpm and higher).</i></div> <div class="htmlview paragraph"><i>If two laboratory variables are used as predictors, a combination of Research and Motor octane numbers serves best for predicting low-speed road performance. At high speed, the combination of Motor octane number and percent aromatics was found to be the best two-variable predictor.</i></div> <div class="htmlview paragraph"><i>Low-speed correlations based on Research and Motor octane number were not improved when olefin content was included as a third predicting variable. At high speed, however, the use of a hydrocarbon-type variable in addition to Research and Motor octane numbers resulted in a significant reduction of the standard error of prediction.</i></div> <div class="htmlview paragraph"><i>The use of more than three variables as predictors did not improve the accuracy of prediction at low speed and improved the accuracy at high speeds to a degree which is not considered to be of practical value.</i></div> <div class="htmlview paragraph"><i>An increase in Research octane number was found to be of more importance than an increase in Motor octane number in improving low-speed road ratings. At high speeds, however, the contribution of Motor octane number was decidedly greater than that of Research octane number.</i></div> <div class="htmlview paragraph"><i>When Research and Motor octane numbers were held constant, an increase in aromatic content or a decrease in olefin content improved the high-speed road rating of a fuel</i></div>

<div class="section abstract"><div class="htmlview paragraph">The development of highly boosted and high compression spark-ignition engines with enhanced thermal efficiencies is primarily limited by knock and super-knock. Super-knock is an excessively high intensity knock which has been related to a developing detonation process. This study investigates the knocking tendency of different gasoline surrogate fuels with varying research octane numbers (RON), octane sensitivity (S) and composition. The <i>ξ/ɛ</i> diagram with an enclosed detonation peninsula is used to assess the knocking tendency of different fuels. The diagram plots <i>ξ</i>, the ratio of acoustic to auto-ignitive velocity, against ɛ, the ratio of the transit time of an acoustic wave through a hot spot, to the heat release time (<i>τ<sub>e</sub></i>). Constant volume simulations of auto-ignition delay times (<i>τ<sub>i</sub></i>) and excitation times (<i>τ<sub>e</sub></i>) obtained from chemical kinetic calculations, enable calculations of <i>ξ</i> and <i>ɛ</i>. Their location for different fuels and operating conditions on the <i>ξ/ɛ</i> diagram, relative to the detonation peninsula, defines their mode of reaction propagation and the severity of a detonation. It was shown that excitation times are not affected by RON and S of the fuel. However, they are strongly dependent on the mixture composition. Fuels exhibiting a strong negative temperature chemistry (NTC) region are found to enter detonation development and explosion region, and are more likely to result in super-knock events in boosted spark-ignition engines.</div></div>

The Co-Optimization of Fuels and Engines (Co-Optima) initiative from the US Department of Energy aims to co-develop fuels and engines in an effort to maximize energy efficiency and the utilization of renewable fuels. Many of these renewable fuel options have fuel chemistries that are different from those of petroleum-derived fuels. Because practical market fuels need to meet specific fuel-property requirements, a chemistry-agnostic approach to assessing the potential benefits of candidate fuels was developed using the Central Fuel Property Hypothesis (CFPH). The CFPH states that fuel properties are predictive of the performance of the fuel, regardless of the fuel's chemical composition. In order to use this hypothesis to assess the potential of fuel candidates to increase efficiency in spark-ignition (SI) engines, the individual contributions towards efficiency potential in an optimized engine must be quantified in a way that allows the individual fuel properties to be traded off for one another. This review article begins by providing an overview of the historical linkages between fuel properties and engine efficiency, including the two dominant pathways currently being used by vehicle manufacturers to reduce fuel consumption. Then, a thermodynamic-based assessment to quantify how six individual fuel properties can affect efficiency in SI engines is performed: research octane number, octane sensitivity, latent heat of vaporization, laminar flame speed, particulate matter index, and catalyst light-off temperature. The relative effects of each of these fuel properties is combined into a unified merit function that is capable of assessing the fuel property-based efficiency potential of fuels with conventional and unconventional compositions.

<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">Since ethanol is a renewable source of energy and it contributes to lower CO2 emissions, ethanol produced from biomass is expected to increase in use as an alternative fuel. It is recognized that for spark ignition (SI) engines ethanol has advantages of high octane number and high combustion speed and has a disadvantage of difficult startability at low temperature.</div> <div class="htmlview paragraph">This paper investigates the influence of ethanol fuel on SI engine performance, thermal efficiency, and emissions. The combustion characteristics under cold engine conditions are also examined.</div> <div class="htmlview paragraph">Ethanol has high anti-knock quality due to its high octane number, and high latent heat of evaporation, which decreases the compressed gas temperature during the compression stroke. In addition to the effect of latent heat of evaporation, the difference of combustion products compared with gasoline further decreases combustion temperature, thereby reducing cooling heat loss. As a result, torque and thermal efficiency increase compared to gasoline. Also, reductions in CO2, NOx, and THC are described.</div> <div class="htmlview paragraph">To improve combustion under cold conditions, methods of increasing fuel temperature or the compressed gas temperature during the compression stroke are considered. In the latter case, simulation showed valve timing to be effective. Based on simulation results, the effect of valve timing optimization on improving combustion at low temperature is investigated with an engine.</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>

Chemical kinetics of octane sensitivity in a spark-ignition engine

<div class="section abstract"><div class="htmlview paragraph">Nitric Oxide (NO) can significantly influence the autoignition reactivity and this can affect knock limits in conventional stoichiometric SI engines. Previous studies also revealed that the role of NO changes with fuel type. Fuels with high RON (Research Octane Number) and high Octane Sensitivity (S = RON - MON (Motor Octane Number)) exhibited monotonically retarding knock-limited combustion phasing (KL-CA50) with increasing NO. In contrast, for a high-RON, low-S fuel, the addition of NO initially resulted in a strongly retarded KL-CA50 but beyond the certain amount of NO, KL-CA50 advanced again. The current study focuses on same high-RON, low-S Alkylate fuel to better understand the mechanisms responsible for the reversal in the effect of NO on KL-CA50 beyond a certain amount of NO. Experiments were conducted to measure the responses of KL-CA50 and trace-autoignition CA50, the latter being indicative of CA50 at which end-gas autoignition starts to become measurable from the apparent heat-release rate. Chemical-kinetics simulations were conducted to reveal the role of NO for end-gas autoignition, with a specific focus on sequential autoignition in a thermally stratified end-gas.</div><div class="htmlview paragraph">The simulation results reveal that the magnitude of low-temperature heat release (LTHR) generally increases with NO. However, the relative importance of NO for enhancing LTHR diminishes when the LTHR inherent to a fuel’s chemistry is strong, such as at lower temperatures in a thermal boundary layer. This rendered more uniform LTHR within a hypothetical thermal boundary and led to a more sequential (<i>i.e.</i> slower) autoignition event. It was also revealed that a change in compression ratio influences the importance of intermediate-temperature heat release (ITHR) due to changes of the temperature-pressure history of the end-gas. Together with the condition where end-gas autoignition occurs more sequentially, the shorter time spent in LTHR and ITHR regime can counter the increase in autoignition reactivity at high NO levels and allow KL-CA50 to advance.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Spark ignition engines have low tailpipe criteria pollutants due to their stoichiometric operation and three-way catalysis and are highly controllable. However, one of their main drawbacks is that the compression ratio is low due to knock, which incurs an efficiency penalty. With a global push towards low-lifecycle-carbon renewable fuels, high-octane alternatives to gasoline such as ethanol are attractive options as fuels for spark ignition engines. Under premixed spark ignition operating conditions, ethanol can enable higher compression ratios than regular-grade gasoline due to its high octane number. The high cooling potential of high-ethanol content gasolines, like E85, or of ethanol-water blends, like hydrous ethanol, can be leveraged to further reduce knock and enable higher compression ratios as well as further downsizing and boosting to reduce frictional and throttling losses.</div><div class="htmlview paragraph">In this work, a split injection strategy is used, where 5-15% of the total injected fuel is injected during the compression stroke with E75 gasoline (75% ethanol, by volume) and hydrous ethanol (190 proof ethanol, 92% ethanol, 8% water, by mass). These experimental results indicate that injecting a small amount of E75 or WE92 during the middle of the compression stroke such that the fuel cannot completely homogenize with the air before the spark fires results in knock mitigation that exceeds the amount of knock mitigation expected by evaporating that amount of fuel in air in a premixed setting. This split injection strategy achieved a 1.3% relative fuel conversion efficiency benefit with E75 at 19 bar IMEPn and a 1.4% relative fuel conversion efficiency benefit at 15 bar IMEPn with WE92. Overall, these results indicate that a split injection strategy with a high cooling potential alcohol fuel can further increase a fuel’s effective octane number, enabling highly boosted, high compression ratio stoichiometric spark ignition operation.</div></div>

<div class="section abstract"><div class="htmlview paragraph">This paper explores the benefits that would be achieved if gasoline marketers produced and offered a higher-octane gasoline to the U.S. consumer market as the standard grade. By raising octane, engine knock constraints are reduced, so that new spark-ignition engines can be designed with higher compression ratios and boost levels. Consequently, engine and vehicle efficiencies are improved thus reducing fuel consumption and greenhouse gas (GHG) emissions for the light-duty vehicle (LDV) fleet over time. The main objective of this paper is to quantify the reduction in fuel consumption and GHG emissions that would result for a given increase in octane number if new vehicles designed to use this higher-octane gasoline are deployed.</div><div class="htmlview paragraph">GT-Power simulations and a literature review are used to determine the relative brake efficiency gain that is possible as compression ratio is increased. Engine-in-vehicle drive-cycle simulations are then performed in Autonomie to determine an effective, on-the-road vehicle efficiency gain. With the possible efficiency gain determined at an individual vehicle level, a fleet model is then used to calculate the aggregate benefit for the LDV fleet. Our simulations indicate that roughly 69% of all LDVs on the road by 2040 will be of this higher-octane variety that uses premium gasoline (98 or 100 RON). Meanwhile, premium gasoline is projected to account for approximately 80% of the total gasoline demand by 2040. Ultimately by redesigning vehicles to take advantage of premium gasoline, fleet fuel consumption and GHG emissions can be reduced by 4.5-6.0% over the baseline case, where no additional higher-octane vehicles are introduced.</div></div>