Publications

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Low-temperature gasoline combustion (LTGC) engines can provide high efficiencies and extremely low NOx and particulate emissions, but controlling the combustion timing remains a challenge. This paper explores the potential of Partial Fuel Stratification (PFS) to provide fast control of CA50 in an LTGC engine. Two different compression ratios are used (CR=16:1 and 14:1) that provide high efficiencies and are compatible with mixed-mode SI-LTGC engines. The fuel used is a research grade E10 gasoline (RON 92, MON 85) representative of a regular-grade market gasoline found in the United States. The fuel was supplied with a gasoline-type direct injector (GDI) mounted centrally in the cylinder. To create the PFS, the GDI injector was pulsed twice each engine cycle. First, an injection early in the intake stroke delivered the majority of the fuel (70 - 80%), establishing the minimum equivalence ratio in the charge. Then, a second injection supplied the remainder of the fuel (20 - 30%) at a variable timing during the compression stroke, from 200° to 330°CA (0°CA = TDC-intake, 360°CA = TDC-compression) to provide controlled stratification. For both CRs, second DI timing sweeps were performed for a range of intake pressures from highly boosted to naturally aspirated conditions, allowing the CA50 control authority at each condition to be determined. By varying the late-DI timing, CA50 could be adjusted as much a 12°CA, from near the misfire limit (overly retarded CA50 with COV-IMEPg > 3%) to well beyond the acceptable knock/ringing limit (overly advanced CA50 with RI > 5 MW/m2). For different conditions, the amount of DI timing retard and CA50 advancement was limited by either engine knock, combustion instabilities, or high NOx emissions (NOx > 0.27 g/kWh). For most conditions, approximately 6-8°CA of CA50 control was possible with good stability and acceptable NOx emissions.

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Low-temperature gasoline combustion (LTGC) engines can deliver high efficiencies, with ultra-low emissions of nitrogen oxides (NOx) and particulate matter (PM). However, controlling the combustion timing and maintaining robust operation remains a challenge for LTGC engines. One promising technique to overcoming these challenges is spark assist (SA). In this work, well-controlled, fully premixed experiments are performed in a single-cylinder LTGC research engine at 1200 rpm using a cylinder head modified to accommodate a spark plug. Compression ratios (CR) of 16:1 and 14:1 were used during the experiments. Two different fuels were also tested, with properties representative of premium- and regular-grade market gasolines. SA was found to work well for both CRs and fuels. The equivalence ratio limits and the effect of intake-pressure boost on the ability of SA to compensate for a reduced Tin were studied. For the conditions studied, =0.42 was found to be most effective for SA. At lower equivalence ratios the flame propagation was too weak, whereas =0.45 was closer to the CI knock/stability limit, which resulted in a smaller range of CA50 control and Tin compensation. At =0.42, SA worked well from Pin = 1.0 to 1.6 bar, but the range of effective Tin compensation dropped progressively with boost from 21 °C at Pin = 1.0 bar to the equivalent of 12 °C at Pin = 1.6 bar. The amount of control authority using SA was demonstrated by varying the spark timing, advancing CA50 to the onset of strong knocking and then retarding CA50 to near misfire. SA provided good control, however the CA50 control range decreased from 7.2° CA at Pin = 1.0 bar to 4.2° CA at Pin = 1.6 bar. For all intake pressures at these well-mixed conditions, NOx emissions for SA were less than for compression ignition only, and all were below the US-2010 Heavy Duty limit.

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Previous work has shown that conventional diesel ignition improvers, 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), can be used to enhance the autoignition of a regular-grade E10 gasoline in a well premixed low-temperature gasoline combustion (LTGC) engine, hereafter termed an HCCI engine, at naturally aspirated and moderately boosted conditions (up to 180 kPa absolute) with a constant engine speed of 1200 rpm and a 14:1 compression ratio. In the current work the effect of EHN on boosted HCCI combustion is further investigated with a higher compression ratio (16:1) piston and over a range of engine speeds (up to 2400 rpm). The results show that the higher compression ratio and engine speeds can make the combustion of a regular-grade E10 gasoline somewhat less stable. The addition of EHN improves the combustion stability by allowing combustion phasing to be more advanced for the same ringing intensity. The high-load limits of both the straight (unadditized) and additized fuels are determined, and the additized fuel is found to achieve a higher maximum load at all engine speeds and intake pressures tested, if it is not limited by lack of oxygen. The results reveal that the higher loads with EHN are the result of either reduced intake temperature requirements at naturally aspirated conditions or a reduction in heat release rate at higher intake pressures. Such effects are also found to increase the thermal efficiency, and a maximum indicated thermal efficiency of 50.1% is found for 0.15% EHN additized fuel at 1800 rpm and 180 kPa intake pressure. Similar to previous studies, the nitrogen in EHN increases NOx emissions, but they remain well below US-2010 standards. Higher engine speeds are found to have slightly lower NOx emissions for additized fuel at intake boosted conditions.

A detailed understanding of the various factors affecting the trends in gross-indicated thermal efficiency with changes in key operating parameters has been carried out, applied to a one-liter displacement single-cylinder boosted Low-Temperature Gasoline Combustion (LTGC) engine. This work systematically investigates how the supplied fuel energy splits into the following four energy pathways: gross-indicated thermal efficiency, combustion inefficiency, heat transfer and exhaust losses, and how this split changes with operating conditions. Additional analysis is performed to determine the influence of variations in the ratio of specific heat capacities (γ) and the effective expansion ratio, related to the combustion-phasing retard (CA50), on the energy split. Heat transfer and exhaust losses are computed using multiple standard cycle analysis techniques. The various methods are evaluated in order to validate the trends. This work focuses on explaining the trends in thermal efficiency and the various energy-loss terms for independent sweeps of fueling rate, intake temperature and engine speed. Trends in thermal efficiency can be well-explained by considering variations in combustion efficiency, CA50 retard, γ and heat transfer. By identifying the energy losses, these results provide a new understanding that can help to optimize the thermal efficiency across the load/speed range in LTGC engines. Of particular importance, a picture is provided of how the heat transfer varies with changes in engine operating conditions. For example, results indicate that CA50 and the magnitude of the acoustic oscillations (i.e. knock) are fundamental parameters affecting the heat transfer.

Low-temperature gasoline combustion (LTGC), based on the compression ignition of a premixed or partially premixed dilute charge, can provide thermal efficiencies (TE) and maximum loads comparable to those of turbo-charged diesel engines, and ultra-low NOx and particulate emissions. Intake boosting is key to achieving high loads with dilute combustion, and it also enhances the fuel's autoignition reactivity, reducing the required intake heating or hot residuals. These effects have the advantages of increasing TE and charge density, allowing greater timing retard with good stability, and making the fuel ϕ- sensitive so that partial fuel stratification (PFS) can be applied for higher loads and further TE improvements. However, at high boost the autoignition reactivity enhancement can become excessive, and substantial amounts of EGR are required to prevent overly advanced combustion. Accordingly, an experimental investigation has been conducted to determine how the tradeoff between the effects of intake boost varies with fuel-type and its impact on load range and TE. Five fuels are investigated: a conventional AKI=87 petroleum-based gasoline (E0), and blends of 10 and 20% ethanol with this gasoline to reduce its reactivity enhancement with boost (E10 and E20). A second zero-ethanol gasoline with AKI=93 (matching that of E20) was also investigated (CF-E0), and some neat ethanol data are also reported. Results show that ethanol content has little effect on LTGC autoignition reactivity for naturally aspirated operation, but it produces a large effect for boosted operation, with the reactivity enhancement with boost being reduced by an amount that correlates with ethanol content. In contrast, CFE0 showed a reactivity enhancement with boost similar to E0. Related to this autoignition enhancement, the effect of fuel-type on the increase in ITHR with boost was also investigated since it correlates with the ability to retard CA50 with good stability for higher loads without knock and to apply PFS effectively. The study showed that by adding ethanol, less EGR is required with boost, leaving more oxygen available for combustion. As a result, the high-load limit could be increased from 16.3 to 18.1 to 20.0 bar IMEPg for E0, E10, and E20, respectively, and to 17.7 bar for the high-AKI gasoline. TE vs. load curves for the various fuels at typical boosted conditions are also presented and discussed. At boosted conditions, PFS was found to be very effective for increasing the TE, with the peak TE increasing from 47.8% for premixed fueling to 48.4% with PFS, and TE improvements up to 2.8 %-units were achieved at higher loads.

Modern spark-ignition (SI) engine technologies have considerably changed in-cylinder conditions under which fuel autoignition and engine knock take place. In this paper, fundamental HCCI engine experiments are proposed as a means for characterizing the impact of these technologies on the knock propensity of different fuels. In particular, the impacts of turbocharging, direct injection (DI), and downspeeding on operation with ethanol and gasoline are investigated to demonstrate this approach. Results reported earlier for ethanol and gasoline on HCCI combustion are revisited with the new perspective of how their autoignition characteristics fit into the anti-knock requirement in modern SI engines. For example, the weak sensitivity to pressure boost demonstrated by ethanol in HCCI autoignition can be used to explain the strong knock resistance of ethanol fuels for turbocharged SI engines. Further, ethanol's high sensitivity to charge temperature makes charge cooling, which can be produced by fuel vaporization via direct injection or by piston expansion via spark-timing retard, very effective for inhibiting knock. On the other hand, gasoline autoignition shows a higher sensitivity to pressure, so only very low pressure boost can be applied before knock occurs. Gasoline also demonstrates low temperature sensitivity, so it is unable to make as effective use of the charge cooling produced by fuel vaporization or spark retard. These arguments comprehensively explain literature results on ethanol's substantially better anti-knock performance over gasoline in modern turbocharged DISI engines. Fundamental HCCI experiments such as these can thus be used as a diagnostic and predictive tool for knock-limited SI engine performance for various fuels. Examples are presented where HCCI experiments are used to identify biofuel compounds with good potential for modern SI-engine applications.

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This study systematically investigates the effects of variousengine operating parameters on the thermal efficiency of a boostedHCCI engine, and the potential of E10 to extend the high-load limitbeyond that obtained with conventional gasoline. Understanding howthese parameters can be adjusted and the trade-offs involved iscritical for optimizing engine operation and for determining thehighest efficiencies for a given engine geometry. Data wereacquired in a 0.98 liter, single-cylinder HCCI research engine witha compression-ratio of 14:1, and the engine facility was configuredto allow precise control over the relevant operating parameters.The study focuses on boosted operation with intake pressures(Pin) ≥ 2 bar, but some data for Pin< 2bar are also presented. Two fuels are considered: 1) an 87-octanegasoline, and 2) E10 (10% ethanol in this same gasoline) which hasa lower autoignition reactivity for boosted operation. This study considers several engine operating parameters,including: intake temperature, fueling rate, engine speed, fueltype, and the effect of various amounts of mixture stratificationusing three fueling methods: fully premixed, early-DI, and premixed+ late-DI (termed partial fuel stratification, PFS). The effects ofthese operating parameters on the factors affecting thermalefficiency, such as combustion phasing (CA50), amount of EGRrequired, ringing intensity, combustion efficiency, γ =cp/cv, and heat transfer are also exploredand discussed. The study showed that in general, thermal efficiencyimproves when parameters are adjusted for lower intaketemperatures, less CA50 retard, and less EGR, as long as theringing intensity is ≤ 5 MW/m2to prevent knock, andcombustion efficiency is good (i.e., ≥ about 96%). Additionally,applying a small amount of mixture stratification (using PFS orearly-DI fueling) improves efficiency by allowing more CA50 advancewhen boost levels are sufficient for these fuels to be ϕ-sensitive.E10 gives a small increase in thermal efficiency because EGRrequirements are reduced. E10 is also effective for increasing themaximum load for Pin≥ 2.4 bar, and increasing thehigh-load limit to IMEPg = 18.1 bar, with no engine knock andultra-low NOx and soot emissions, compared to IMEPg = 16.3 bar forgasoline. Overall, this study showed that the efficiencies forboosted HCCI can be increased above their already good baselinevalues. For our engine configuration, improvements of 3 - 5thermal-efficiency percentage units were achieved corresponding toa reduction in fuel consumption of 7 - 11%.

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Homogeneous charge compression ignition (HCCI) combustion with fully premixed charge is severely limited at high-load operation due to the rapid pressure-rise rates (PRR) which can lead to engine knock and potential engine damage. Recent studies have shown that two-stage ignition fuels possess a significant potential to reduce the combustion heat release rate, thus enabling higher load without knock. This study focuses on three factors, engine speed, intake temperature, and fuel composition, that can affect the pre-ignition processes of two-stage fuels and consequently affect their performance with partial fuel stratification. A model fuel consisting of 73 vol.% isooctane and 27 vol.% of n-heptane (PRF73), which was previously compared against neat isooctane to demonstrate the superior performance of two-stage fuels over single-stage fuels with partial fuel stratification, was first used to study the effects of engine speed and intake temperature. The results for PRF73 show that increasing engine speed from 1200 to 1600 rpm causes almost no change in φ-sensitivity, which is defined by the advancement of combustion phasing for an increase in equivalence ratio. Consequently, the maximum combustion pressure rise rate (PRRmax) can be reduced substantially with partial fuel stratification at this higher speed as it was at 1200 rpm. In contrast, increasing intake temperature from 60°C to 174°C eliminates the low temperature heat release of PRF73. Despite the single-stage ignition at this temperature, PRF73 still shows a weak but definitive φ-sensitivity, likely due to the relatively strong intermediate temperature heat release before hot ignition. As a result, PRR max was reduced modestly with partial fuel stratification. This PRF73 result is distinctively different from that of isooctane at the same intake temperature. To study the importance of fuel composition, PRF73 is compared with a low-octane, gasoline-like distillate fuel, termed Hydrobate, which could be readily produced from petroleum feedstocks. With the similar HCCI reactivity to PRF73, Hydrobate shows little difference in φ-sensitivity and performs similarly with partial fuel stratification compared to PRF73. This result indicates that it is the overall fuel HCCI reactivity, rather than the exact fuel composition, that determines the φ-sensitivity and the consequent performance with partial fuel stratification. © 2011 SAE International.

Ethanol and ethanol/gasoline blends are being widely considered as alternative fuels for light-duty automotive applications. At the same time, HCCI combustion has the potential to provide high efficiency and ultra-low exhaust emissions. However, the application of HCCI is typically limited to low and moderate loads because of unacceptably high heat-release rates (HRR) at higher fueling rates. This work investigates the potential of lowering the HCCI HRR at high loads by using partial fuel stratification to increase the in-cylinder thermal stratification. This strategy is based on ethanol's high heat of vaporization combined with its true single-stage ignition characteristics. Using partial fuel stratification, the strong fuel-vaporization cooling produces thermal stratification due to variations in the amount of fuel vaporization in different parts of the combustion chamber. The low sensitivity of the autoignition reactions to variations of the local fuel concentration allows the temperature variations to govern the combustion event. This results in a sequential autoignition event from leaner and hotter zones to richer and colder zones, lowering the overall combustion rate compared to operation with a uniform fuel/air mixture. The amount of partial fuel stratification was varied by adjusting the fraction of fuel injected late to produce stratification, and also by changing the timing of the late injection. The experiments show that a combination of 60-70% premixed charge and injection of 30-40 % of the fuel at 80{sup o}CA before TDC is effective for smoothing the HRR. With CA50 held fixed, this increases the burn duration by 55% and reduces the maximum pressure-rise rate by 40%. Combustion stability remains high but engine-out NO{sub x} has to be monitored carefully. For operation with strong reduction of the peak HRR, ISNO{sub x} rises to around 0.20 g/kWh for an IMEP{sub g} of 440 kPa. The single-cylinder HCCI research engine was operated naturally aspirated without EGR at 1200 rpm, and had low residual level using a CR = 14 piston.

Long chain alcohols possess major advantages over ethanol as bio-components for gasoline, including higher energy content, better engine compatibility, and less water solubility. Rapid developments in biofuel technology have made it possible to produce C{sub 4}-C{sub 5} alcohols efficiently. These higher alcohols could significantly expand the biofuel content and potentially replace ethanol in future gasoline mixtures. This study characterizes some fundamental properties of a C{sub 5} alcohol, isopentanol, as a fuel for homogeneous-charge compression-ignition (HCCI) engines. Wide ranges of engine speed, intake temperature, intake pressure, and equivalence ratio are investigated. The elementary autoignition reactions of isopentanol is investigated by analyzing product formation from laser-photolytic Cl-initiated isopentanol oxidation. Carbon-carbon bond-scission reactions in the low-temperature oxidation chemistry may provide an explanation for the intermediate-temperature heat release observed in the engine experiments. Overall, the results indicate that isopentanol has a good potential as a HCCI fuel, either in neat form or in blend with gasoline.

The characteristics of ethanol autoignition and the associated HCCI performance are examined in this work. The experiments were conducted over wide ranges of engine speed, load and intake boost pressure (P in) in a single- cylinder HCCI research engine (0.98 liters) with a CR = 14 piston. The data show that pure ethanol is a true single-stage ignition fuel. It does not exhibit low-temperature heat release (LTHR), not even for boosted operation. This makes ethanol uniquely different from conventional distillate fuels and offers several benefits: a) The intake temperature (T in) does not have to be adjusted much with changes of engine speed, load and intake boost pressure. b) High P in can be tolerated without running out of control authority because of an excessively low T in requirement. However, by maintaining true single-stage ignition characteristics, ethanol also shows a relatively low temperature-rise rate just prior to its hot ignition point. Therefore, ethanol does not tolerate as much combustion-phasing retard as fuels that exhibit LTHR and/or pronounced intermediate-temperature heat release. Since combustion retard is important for avoiding excessive pressure-rise rates, the distinct single-stage ignition characteristic of ethanol can be considered a drawback when reaching for higher loads. Nonetheless, an IMEP g of 11.3 bar was demonstrated for P in = 247 kPa. Finally, the latest ethanol chemical-kinetics mechanism from the National University of Ireland - Galway was evaluated against the experimental engine data using a multi-zone model. Overall, the mechanism performs very well over wide ranges of operating conditions. © 2010 SAE International.

This work explores how the high-load limits of HCCI are affected by fuel autoignition reactivity, EGR quality/composition, and EGR unmixedness for naturally aspirated conditions. This is done for PRF80 and PRF60. The experiments were conducted in a singlecylinder HCCI research engine (0.98 liters) with a CR = 14 piston installed. By operating at successively higher engine loads, five load-limiting factors were identified for these fuels: 1) Residual-NOx-induced run-away advancement of the combustion phasing, 2) EGR-NOx- induced run-away, 3) EGR-NOx/wall-heating induced run-away 4) EGR-induced oxygen deprivation, and 5) excessive partial-burn occurrence due to EGR unmixedness. The actual load-limiting factor is dependent on the autoignition reactivity of the fuel, the EGR quality level (where high quality refers to the absence of trace species like NO, HC and CO, i.e. simulated EGR), the level of EGR unmixedness, and the selected pressurerise rate (PRR). For a reactive fuel like PRF60, large amounts of EGR are required to control the combustion phasing. Therefore, for operation with simulated EGR, the maximum IMEP becomes limited by the available oxygen. When real EGR (with trace species) is used instead of the simulated EGR, the maximum IMEP becomes limited by EGR-NOx/wall-heating induced runaway. For the moderately reactive PRF80 operated with simulated EGR, the maximum IMEP becomes limited by residual-NOx-induced run-away. Furthermore, operation with real EGR lowers the maximum steady IMEP because of EGR-NOx-induced run-away. This is similar to PRF60. Finally, the data show that EGR/fresh-gas unmixedness can lead to a substantial reduction of the maximum stable IMEP for operation with a low PRR. This happens because the EGR unmixedness causes occasional partial-burn cycles due to excessive combustion-phasing retard for cycles that induct substantially higher-thanaverage level of EGR gases.

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The characteristics of autoignition after top-dead-center (TDC) for both single- and two-stage ignition fuels have been investigated in a homogeneous charge compression ignition (HCCI) engine. The single-stage ignition fuel was iso-octane and the two-stage ignition fuel was PRF80 (80% iso-octane and 20% n-heptane). The results show that the heat-release rate and pressure-rise rate both decrease as the combustion is retarded later into the early expansion stroke. This is an advantage for high-load HCCI operation. However, for both fuel-types, cycle-to-cycle variations of the ignition and combustion phasing increase with combustion-phasing retard. Also, the cycle-to-cycle variations are higher for iso-octane compared to PRF80. These observations can be explained by considering the magnitude of random temperature fluctuation and the temperature-rise rate just prior to thermal run-away. Furthermore, too much combustionphasing retard leads to the appearance of partial-burn or misfire cycles, but the responses of the two fuels are quite different. The different behaviors can be explained by considering the thermal and chemical state of the residual exhaust gases that are recycled from one cycle to the next. The data indicate that a partialburn cycle with iso-octane produces residuals that increase the reactivity of the following cycle. However, for the already more reactive PRF80 fuel, the partial-burn products present in the residuals do not increase the reactivity enough to overcome the retarding effect of cool residual gases.

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Thermal stratification has the potential to reduce pressure-rise rates and allow increased power output for HCCI engines. This paper systematically examines how the amount of thermal stratification of the core of the charge has to be adjusted to avoid excessive knock as the engine speed and fueling rate are increased. This is accomplished by a combination of multi-zone chemical-kinetics modeling and engine experiments, using iso-octane as the fuel. The experiments show that, for a low-residual engine configuration, the pressure traces are self-similar during changes to the engine speed when CA50 is maintained by adjusting the intake temperature. Consequently, the absolute pressure-rise rate measured as bar/ms increases proportionally with the engine speed. As a result, the knocking (ringing) intensity increases drastically with engine speed, unless counteracted by some means. This paper describes how adjustments of the thermal width of the in-cylinder charge can be used to limit the ringing intensity to 5 MW/m2 as both engine speed and fueling are increased. If the thermal width can be tailored without constraints, this enables smooth operation even for combinations of high speed, high load, and combustion phasing close to TDC. Since large alterations of the thermal width of the charge are not always possible, combustion retard is considered to reduce the requirement on the thermal stratification. The results show that combustion retard carries significant potential since it amplifies the benefit of a fixed thermal width. Therefore, the thermal stratification required for operation with an acceptable knocking intensity can be decreased substantially by the use of combustion retard. This enables combinations of high engine speed and high fueling rate even for operation with the naturally occurring thermal stratification. However, very precise control of the combustion phasing will likely be required for such operation.