A Simple Method to Predict Knock Using Toluene, N-Heptane and Iso-Octane Blends (TPRF) as Gasoline Surrogates

<div class="htmlview paragraph">Since the advent of the spark ignition engine, the maximum engine efficiency has been knock limited. Knock is a phenomena caused by the rapid autoignition of fuel/air mixture (endgas) ahead of the flame front. The propensity of a fuel to autoignite corresponds to its autoignition chemistry at the local endgas temperature and pressure. Since a fuel blend consists of many components, its autoignition chemistry is very complex.</div> <div class="htmlview paragraph">The octane index (OI) simplifies this complex autoignition chemistry by comparing a fuel to a Primary Reference Fuel (PRF), a binary blend of iso-octane and n-heptane. As more iso-octane is added into the blend, the PRF is less likely to autoignite. The OI of a fuel is defined as the volumetric percentage of iso-octane in the PRF blend that exhibits similar knocking characteristics at the same engine conditions.</div> <div class="htmlview paragraph">Since the OI is dependent on the engine operating conditions, it is typically measured at two standard test conditions: the Research and Motor Octane Number (RON and MON) tests. These tests are intended to bracket the knock-limited operating range, and the OI is taken to be a weighted average of RON and MON: <div id="FD1" class="formula"><div class="graphic-wrapper"><img class="article-image equation block" src="2009-01-2622_fig0001.jpg" alt="No Caption Available"/></div></div>where K is the weighing factor and S is the fuel sensitivity (RON-MON). When the tests were established in 1932, the MON test matched the OI on the road, hence the average value of K was 1; however, recent studies have found that the average value of K is negative.</div> <div class="htmlview paragraph">The parameter K, which is independent of a fuel, can be used to show the relevancy of the RON and MON tests. When K = 0.5, the average of the RON and MON tests is a good indicator of a fuels antiknock performance. However, when K is negative, for a given RON, fuels with a higher sensitivity have better antiknock performance; therefore, an increase in RON or MON does not necessarily correlate to better antiknock performance.</div> <div class="htmlview paragraph">In the decades following the development of the octane number, tests were performed to determine the value of K. However, as K approached a value of 0.5, these tests ceased to be performed.</div> <div class="htmlview paragraph">The Coordinated Research Council (CRC) collected data from 1951 to 1991 to determine the required fuel octane number to avoid fleet vehicles from knocking. The tests typically included both PRFs and reference fuels representative of commercial gasoline, the latter having a non-zero sensitivity. Therefore, a value of K can be determined for each test. Using the data from 1951, 1961, 1971, 1981, and 1991, the average values of K were found for each year.</div> <div class="htmlview paragraph">The study then explores the changes in the engine operating conditions that would cause these changes. To better understand these relationships, the values of K for modern and historic engines were determined by WAVE engine models. The models found that the average value of K is positive for a historic engine, but is negative for modern engines.</div> <div class="htmlview paragraph">This study found that for domestic engines, K is now negative, having decreased from a value of 1 in 1930. This decrease in K is primarily due to better engine cooling, better engine breathing, and the usage of fuel injectors.</div>

<div class="htmlview paragraph">A five-cylinder diesel engine, converted to a single cylinder operated optical engine is run in Homogeneous Charge Compression Ignition (HCCI) mode. A blend of iso-octane and n-heptane is used as fuel.</div> <div class="htmlview paragraph">An experimental study of the horizontal boundary layer between the main combustion and the non-reacting surface of the combustion chamber is conducted as a function of speed, load, swirl and injection strategy. The combustion behaviour is monitored by chemiluminescence measurements.</div> <div class="htmlview paragraph">For all cases an interval from -10 to 16 crank angles after top dead center (CAD ATDC) in steps of one CAD are studied. One image-intensified camera observes the boundary layer up close from the side through a quartz cylinder liner while a second camera has a more global view from below to see more large scale structure of the combustion.</div> <div class="htmlview paragraph">The averaged chemiluminescence intensity from the HCCI combustion is seen to scale well with the rate of heat release. A boundary layer is defined and studied in detail between the main combustion volume and the piston crown surface as a function of crank angle. The boundary layer is found to be in the range from 2 to 4 mm for all cases by the definition used; however, the location for the measurements becomes more and more important as combustion becomes more inhomogeneous. To get accurate calculations, the level of noise must also be considered and definitions of boundary layer thickness should not be made at to low chemiluminescence intensity.</div>

<div class="htmlview paragraph">Low temperature heat release <b>(LTHR)</b> in HCCI combustion changes according to fuel chemical composition and engine test conditions. In this study 11 pure hydrocarbon components were blended into 12 different model fuels to evaluate the effects of fuel composition on LTHR heating value, LTHR CA50 (crank angle at 50% completion of LTHR), high temperature heat release <b>(HTHR)</b>, and engine performance. From the heat release analysis of the test data from a supercharged 4-cylinder engine, it was determined that the HTHR CA50 (crank angle at 50% completion of HTHR) was strongly indicative of combustion stability and maximum rate of pressure rise. Moreover, the functional dependence of HTHR CA50 on LTHR heating value and LTHR CA50 was quantified.</div> <div class="htmlview paragraph">Test fuels denoted MD05, Base, MC05 and MX05 were prepared by adding 5.2vol%, 9.3vol%, 15.0vol%, and 18.2vol% of n-hexane, respectively, to a blend of 10 pure hydrocarbons. These four fuel blends were applied to HCCI combustion tests at the same engine speed and IMEP to evaluate how the LTHR heating value and LTHR CA50 change according to the amount of n-hexane, a fuel that exhibits large LTHR. The test results showed that the heating value and volume percent of n-hexane correlated linearly over the range of n-hexane tested. However, the relation between LTHR CA50 and volume percent of n-hexane was linear only at low volume percents, and became less so as the n-hexane content increased.</div> <div class="htmlview paragraph">From the rate of heat release calculations it was apparent that n-paraffins contribute the most towards a large LTHR heating value followed by iso-paraffins. In direct correspondence, the propensity for early initiation of HTHR was clearly distinguishable by fuel group as follows:</div> <div class="htmlview paragraph"> <div id="FD1" class="formula"> <div class="graphic-wrapper"><img class="article-image equation block" src="2005-01-0138_fig0001.jpg" alt="No Caption Available"/></div> </div> </div> <div class="htmlview paragraph">Furthermore, it was found that the aromatics, except benzene, and some of the naphthenes and olefins, have a function to reduce the LTHR that would be expected of the remaining mixture counterparts. The discovery of this “LTHR inhibitor effect” is a key finding of this study.</div> <div class="htmlview paragraph">From these results, it was obvious that the amalgam of each chemical component LTHR, including inhibitor effects, determines the HTHR CA50 timing, which in turn determines engine performance. Thus, the LTHR plays a key role in HCCI combustion and the chemical component dependencies of LTHR were examined.</div> <div class="htmlview paragraph">The significance of octane number is discussed. In the case of regular gasoline (research octane number <b>(RON)</b> 90.5) and PRF 90.5, though the research octane numbers are identical, the heat release patterns differ significantly. This can be explained by the difference in fuel composition and the LTHR inhibitor effect of certain components of regular gasoline.</div>

<div class="section abstract"><div class="htmlview paragraph">Gasoline fuels are complex mixtures which consist of more than 200 different hydrocarbon species. In order to decrease the chemical and physical complexity, oxygenated surrogate components were used to enhance the fundamental understanding of partially premixed combustion (PPC). The ignition quality of a fuel is measured by octane number. There are two methods to measure the octane number: research octane number (RON) and motor octane number (MON). In this paper, RON and MON were measured for a matrix of n-heptane, isooctane, toluene, and ethanol (TERF) blends spanning a wide range of octane number between 60.6 and 97. First, regression models were created to derive RON and MON for TERF blends. The models were validated using the standard octane test for 17 TERF blends. Second, three different TERF blends with an ignition delay (ID) of 8 degrees for a specific operating condition were determined using a regression model. This was done to examine the model accuracy for ID and study fuel composition effect on combustion events and emissions.</div><div class="htmlview paragraph">The results showed a good agreement between predicted and tested RON and MON with an accuracy of ±0.6. The model also had high accuracy during extrapolation for some fuel blends. For toluene and reference fuel blend (TRF) the model was more accurate for MON than for RON, while the situation was the opposite for ethanol and reference fuel blend (ERF) i.e. the model worked better for RON than for MON during extrapolation. The ignition delay was similar for all three TERF blends despite the differences in their composition. However, high concentration of toluene resulted in higher levels of HC, NO<sub>x</sub>, and smoke emissions.</div></div>

<div class="htmlview paragraph">The Octane Index (OI) relates a fuel's knocking characteristics to a Primary Reference Fuel (PRF) that exhibits similar knocking characteristics at the same engine conditions. However, since the OI varies substantially with the engine operating conditions, it is typically measured at two standard conditions: the Research and Motor Octane Number (RON and MON) tests. These tests are intended to bracket the knock-limited operating range, and the OI is taken to be a weighted average of RON and MON:</div> <div class="htmlview paragraph"><span class="formula inline">OI = K MON + (1-K) RON</span></div> <div class="htmlview paragraph">where K is the weighing factor. When the tests were established, K was approximately 0.5. However, recent tests with modern engines have found that K is now negative, indicating that the RON and MON tests no longer bracket the knock-limited operating conditions.</div> <div class="htmlview paragraph">Experiments were performed to measure the OI of different fuels in a modern engine to better understand the role of fuel sensitivity (RON-MON) on knock limits. The experiments were conducted in a single cylinder test engine that had been fitted with a modern pent-roof head. At each test condition, the spark timing was advanced until the engine transitioned into audible knock. Then, at each spark timing, pressure, microphone, and accelerometer data were collected to verify knock onset.</div> <div class="htmlview paragraph">These experimental results show that K is negative for the engine operating conditions tested. Thus, fuels with higher sensitivities (RON-MON), but the same RON, were found to have better anti-knock performance. The results also show that the knock limited spark advance and maximum pressure of the engine increase linearly with increasing fuel sensitivity.</div> <div class="htmlview paragraph">Similar experiments were performed to study the dependency of K on spark location, compression ratio, relative air/fuel ratio, engine speed, intake air temperature, and intake air pressure. The results show that K has a strong dependence on the intake air temperature, engine speed, and intake air pressure.</div> <div class="htmlview paragraph">Recommendations are then made for modifications to the octane number tests to better bracket the knock-limited operating conditions of modern engines.</div>

<div class="section abstract"><div class="htmlview paragraph">Ethanol and other high heat of vaporization (HoV) fuels result in substantial cooling of the fresh charge, especially in direct injection (DI) engines. The effect of charge cooling combined with the inherent high chemical octane of ethanol make it a very knock resistant fuel. Currently, the knock resistance of a fuel is characterized by the Research Octane Number (RON) and the Motor Octane Number (MON). However, the RON and MON tests use carburetion for fuel metering and thus likely do not replicate the effect of charge cooling for DI engines. The operating conditions of the RON and MON tests also do not replicate the very retarded combustion phasing encountered with modern boosted DI engines operating at low-speed high-load.</div><div class="htmlview paragraph">In this study, the knock resistance of a matrix of ethanol-gasoline blends was determined in a state-of-the-art single cylinder engine equipped with three separate fuel systems: upstream, pre-vaporized fuel injection (UFI); port fuel injection (PFI); and DI. Constant inlet temperature was held downstream of the injector for UFI and upstream of the injectors for PFI and DI. For each fuel, engine inlet pressure was swept at borderline knocking conditions at constant engine speed using each of the three fuel systems. This test method characterized each fuel's knocking behavior over a wide range of conditions, including those typical of boosted DI engines.</div><div class="htmlview paragraph">Comparison of UFI and DI results allowed the chemical octane effect on knock to be separated from the evaporative charge cooling effect. These effects were found to be of comparable importance for ethanol blends.</div><div class="htmlview paragraph">An outcome of the test method was the discovery of an interaction between combustion phasing and the sensitivity of a fuel's autoignition kinetics to temperature. For a given gasoline blendstock, increasing ethanol content significantly increased knock-limited performance with combustion phasing near the thermodynamic optimum, as expected. However, due to ethanol's high sensitivity, knock-limited performance improved to a much greater extent with increasing ethanol content as combustion phasing was retarded. This effect was further enhanced by charge cooling with DI. Increasing ethanol content also significantly increased the knock-limited performance before enrichment was required to control exhaust gas temperature.</div><div class="htmlview paragraph">The RON ratings of the fuels did not fully reflect the observed knock resistance of mid-to-high level ethanol blends (E20 and higher). K, the weighting factor for MON in the Octane Index, decreased with increasing combustion phasing retard and with increasing evaporative charge cooling, and increased with increasing inlet temperature and increasing compression ratio.</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">Combustion in modern spark-ignition (SI) engines is increasingly knock-limited with the wide adoption of downsizing and turbocharging technologies. Fuel autoignition conditions are different in these engines compared to the standard Research Octane Number (RON) and Motor Octane Numbers (MON) tests. The Octane Index, OI = RON - K(RON-MON), has been proposed as a means to characterize the actual fuel anti-knock performance in modern engines. The K-factor, by definition equal to 0 and 1 for the RON and MON tests respectively, is intended to characterize the deviation of modern engine operation from these standard octane tests. Accurate knowledge of K is of central importance to the OI model; however, a single method for determining K has not been well accepted in the literature.</div><div class="htmlview paragraph">This paper first examines four different methods for determining K, using literature results from a modern SI engine operating with direct injection (DI), port fuel injection (PFI) and homogeneous, upstream fuel injection (UFI). The test fuels were ethanol-gasoline blends spanning a wide range of RON and MON, together with isooctane as a reference. The quality of the K results from some of these methods is particularly dependent on the design of the test fuel matrix, with unreliable K values resulting in some cases.</div><div class="htmlview paragraph">One of the more reliable methods is then used to examine how K varies with the intake pressure, fueling strategy, engine speed and compression ratio, with throttled conditions considered in detail. Several of the observed trends are consistent with prior studies, including K being consistently negative at higher loads for DI. In contrast to other studies, however, K is also observed to approach 0.5 at part load, throttled conditions, irrespective of whether the engine is fuelled by DI, PFI or UFI. Preliminary analysis of the autoignition chemistry for different fuelling methods then suggests plausible reasons for these results.</div></div>

Autoignition quality of gasoline fuels in partially premixed combustion in diesel engines

<div class="section abstract"><div class="htmlview paragraph">The efficiency of spark ignition (SI) engines is usually limited by the occurrence of knock, which is linked to fuel octane number. If running the engine at its optimal efficiency requires a high octane number at high load, a lower octane number can be used at low load.</div><div class="htmlview paragraph">Saudi Aramco, along with its long-term partner IFP Energies nouvelles, has been developing a synergistic fuel engine system where the engine is fed by fuel with an octane number adjusted in real time, on an as needed basis, while running at its optimal efficiency.</div><div class="htmlview paragraph">Two major steps are identified to develop this “Octane on Demand” (OOD) concept: <ul class="list disc"><li class="list-item"><div class="htmlview paragraph">First, characterize the octane requirement needed to run the engine at its optimal efficiency over the entire map.</div></li><li class="list-item"><div class="htmlview paragraph">Then, select the best dual fuel combination, including a base fuel and an octane booster to fit this concept.</div></li></ul></div><div class="htmlview paragraph">For this purpose, the behavior of different octane boosters, including ethanol, reformate and a blend of butanol isomers (SuperButol<sup>TM</sup>), are studied on a CFR engine when blended with a very low RON naphtha-based fuel (RON 71). It is shown that the fuel combination [naphtha; ethanol] offers the most promising boosting effect.</div><div class="htmlview paragraph">Dedicated tests on an up-to-date gasoline direct injection multicylinder engine reveal the opportunity to use a naphtha-based fuel of RON 71 over a significant area of the engine map. The advantage of using ethanol as an octane booster is then clearly demonstrated, linked both to its very high RON and its high latent heat of vaporization and favorable sensitivity. Finally, around two-thirds of the engine map can be run using a moderate ethanol rate within the range 0% to 40%, making this OOD concept compatible within the E10-E20 context. Finally, deeper analyses are also made to correlate the engine octane requirement with RON, MON, Octane Index, and heat of vaporization.</div></div>

<div class="section abstract"><div class="htmlview paragraph">An increase in spark-ignition engine efficiency can be gained by increasing the engine compression ratio, which requires fuels with higher knock resistance. Oxygenated fuel components, such as methanol, ethanol, isopropanol, or iso-butanol, all have a Research Octane Number (RON) higher than 100. The octane numbers (ON) of fuels are rated on the CFR F1/F2 engine by comparing the knock intensity of a sample fuel relative to that of bracketing primary reference fuels (PRF). The PRFs are a binary blend of iso-octane, which is defined to an ON of 100, and n-heptane, which represents an ON of 0. Above 100 ON, the PRF scale continues by adding diluted tetraethyl lead (TEL) to iso-octane. However, TEL is banned from use in commercial gasoline because of its toxicity. The ASTM octane number test methods have a “Fit for Use” test that validate the CFR engine’s compliance with the octane testing method by verifying the defined ON of toluene standardization fuels (TSF). The RON test method defines TSFs in the range of 65.1 RON to 113.0 RON with blends of toluene, n-heptane, and iso-octane. Since TSFs do not contain TEL, they could potentially be used as bracketing reference fuels instead of leaded PRFs beyond RON 100. In this work, multiple CFR engines performed “Fit for Use” tests per the RON test method (ASTM D2699) and the TSF ratings closely correlated to their defined RON values without the need of intake air temperature tuning. In the next step, TSFs were used as non-leaded reference fuels to rate the RON of neat methanol, ethanol, iso-propanol, iso-butanol, ethyl acetate, and diisobutylene, all of which have a RON exceeding 100. These same fuels were tested on a separate CFR engine per the official ASTM D2699 RON test method with leaded PRFs. Their TSF-based RON ratings were found to be within the variations of RON values reported in the literature and closely matched with their standard RON rating using leaded reference fuels. Therefore, octane ratings of fuels beyond RON 100 with TSFs as reference fuels proved to be one viable pathway to rate fuels >100 RON without the need for leaded reference fuels.</div></div>

<div class="section abstract"><div class="htmlview paragraph">Gasoline knock resistance is characterized by the Research and Motor Octane Number (RON and MON), which are rated on the CFR octane rating engine at naturally aspirated conditions. However, modern automotive downsized boosted spark ignition (SI) engines generally operate at higher cylinder pressures and lower temperatures relative to the RON and MON tests. Using the naturally aspirated RON and MON ratings, the octane index (OI) characterizes the knock resistance of gasolines under boosted operation by linearly extrapolating into boosted “beyond RON” conditions via RON, MON, and a linear regression K factor. Using OI solely based on naturally aspirated RON and MON tests to extrapolate into boosted conditions can lead to significant errors in predicting boosted knock resistance between gasolines due to non-linear changes in autoignition and knocking characteristics with increasing pressure conditions. A new “Supercharged Octane Number” (SON) method was developed on the CFR engine at increased intake pressures, which improved the correlation to boosted knock-limited automotive SI engine data over RON for several surrogate fuels and gasolines, including five “Co-Optima” RON 98 fuels and an E10 regular grade gasoline. Furthermore, the conventional OI was extended to a newly introduced Supercharged Octane Index (OI<sub>S</sub>) based on SON and RON, which significantly improved the correlation to fuel knock resistance measurements from modern boosted SI engine knock-limited spark advance tests. This demonstrated the first proof of concept of a SON and OI<sub>S</sub> to better characterize a fuel’s knock resistance in modern boosted SI engines.</div></div>

<div class="section abstract"><div class="htmlview paragraph">This study examines fuel auto-ignitability and shows a method for determining fuel performance for HCCI combustion by doing engine experiments.</div><div class="htmlview paragraph">Previous methods proposed for characterizing HCCI fuel performance were assessed in this study and found not able to predict required compression ratio for HCCI auto-ignition (CR<sub>AI</sub>) at a set combustion phasing. The previous indices that were studied were the Octane Index (OI), developed by Kalghatgi, and the HCCI Index, developed by Shibata and Urushihara. Fuels with the same OI or HCCI Index were seen to correspond to a wide range of compression ratios in these experiments, so a new way to describe HCCI fuel performance was sought.</div><div class="htmlview paragraph">The Lund-Chevron HCCI Number was developed, using fuel testing in a CFR engine just as for the indices for spark ignition (research octane number and motor octane number, RON and MON) and compression ignition (cetane number, CN). By running the engine in HCCI mode, the required compression ratio for achieving auto-ignition with a combustion phasing of CA50 3° after TDC was determined for various gasoline surrogate fuels prepared from blends of n-heptane, iso-octane, toluene, and ethanol. To study the effect of different operating conditions, five inlet air temperatures ranging from 50°C to 150°C were used to achieve different pressure-temperature combinations in the engine, and the compression ratio was changed accordingly to keep a constant combustion phasing, The experiments were carried out in lean operation with a constant equivalence ratio of 0.33 and with an engine speed of 600 rpm.</div><div class="htmlview paragraph">The basis of the Lund-Chevron HCCI Number is the required compression ratio of primary reference fuels (PRF), from PRF 60 to PRF 100. For each evaluated fuel, at each condition, the Lund-Chevron HCCI Number is set equal to the octane number of the PRF having the same CR<sub>AI</sub> value at the same conditions.</div></div>

<div class="htmlview paragraph">A reduced chemical kinetic mechanism for a gasoline surrogate was developed and validated in this study for CAI (Controlled Auto Ignition) combustion. The gasoline surrogate was modeled as a blend of iso-octane, n-heptane, and toluene. This reduced mechanism consisted of 44 species and 59 reactions, including main reaction paths of iso-octane, n-heptane, and toluene. The ignition delay times calculated from this mechanism showed a good agreement with previous experimental data from shock tube measurement. A rapid compression machine (RCM) was developed and used to measure the ignition delay times of gasoline and surrogate fuels in the temperature range of 890K ∼ 1000K. The RCM experimental results were also compared with the RCM simulation using the reduced mechanism. It was found that the chemical reaction started before the end of the compression process in the RCM experiment. And the ignition delay time of the suggested gasoline surrogate was similar to that of gasoline. Finally an engine experiment on CAI combustion was performed by using a single cylinder research engine. A simple, 0-D engine simulation was conducted using the reduced mechanism, which was then compared with the engine experimental data.</div>

<div class="section abstract"><div class="htmlview paragraph">Fundamental engine research is primarily conducted under steady-state conditions, in order to better describe boundary conditions which influence the studied phenomena. However, light-duty automobiles are operated, and tested, under heavily transient conditions. This mismatch between studied conditions and in-use conditions is deemed acceptable due to the fundamental knowledge gained from steady-state experiments. Nonetheless, it is useful to characterize the conditions encountered during transient operation and determine if the governing phenomena are unduly influenced by the differences between steady-state and transient operation, and further, whether transient behavior can be reasonably extrapolated from steady-state behavior.</div><div class="htmlview paragraph">The transient operation mode used in this study consists of 20 fired cycles followed by 80 motored cycles, operating on a continuous basis. The intention of the cycle is to provide a significant transient condition, namely the change from motored to knock-limited fired operation, while also maintaining a repeatable cycle that allows for the collection of statistics during quasi- steady-state operation.</div><div class="htmlview paragraph">This study investigates the effect of transient operation on Knock-Limited Combustion Phasing (KL-CA50) compared to steady-state operation. Three compositionally dissimilar matched Research Octane Number (RON) = 98 fuels are used in this study, allowing for the assessment of fuel-specific effects on differences between steady-state and transient operation.</div><div class="htmlview paragraph">This study first characterizes the 20/80 firing cycle described above, before comparing the transient KL-CA50 measurements to the steady-state KL-CA50 measurements. Analysis of both the steady-state and transient results are used to gain insights into the effects of transient operation on end-gas autoignition, relative to steady-state operation and as a function of fuel composition.</div><div class="htmlview paragraph">The results of this study indicate the significant effect that transient operation has on KL-CA50 behavior of a fuel. This is both universal, in that all fuels show responses to the differences in compression temperatures of the charge, as well as fuel specific, in that the fuel response varies based on the fuel’s sensitivity to temperature, [O2], and trace species. All fuels showed a significant load extension under transient operation, based on tolerance of higher intake pressures. However, transient operation moved operating conditions to “beyond RON” (Octane Index K < 0) conditions, which favored higher-sensitivity fuels. Based on the analysis of system time constants (e.g. cylinder head temperature dynamic response, exhaust gas temperature dynamic response), it is expected that transient operation, and the benefits for knock-limited operation, are highly influential on drive-cycle performance.</div></div>