Publications

Abstract not provided.

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.

This study explores the use of two conventional ignition improvers, 2-ethylhexyl nitrate (EHN) and di-tert-butyl peroxide (DTBP), to enhance the autoignition of the regular gasoline in an homogeneous charge compression ignition (HCCI) engine at naturally aspirated and moderately boosted conditions (up to 180 kPa absolute) with a constant engine speed of 1200 rpm. The results showed that both EHN and DTBP are very effective for reducing the intake temperature (Tin) required for autoignition and for enhancing stability to allow a higher charge-mass fuel/air equivalence ratio (φ{symbol}m). On the other hand, the addition of these additives can also make the gasoline too reactive at some conditions, so significant exhaust gas recirculation (EGR) is required at these conditions to maintain the desired combustion phasing. Thus, there is a trade-off between improving stability and reducing the oxygen available for combustion when using ignition improvers to extend the high-load limit. Because previous works have shown that partial fuel stratification (PFS) can be applied with more reactive fuels to reduce the heat release rate to allow higher loads or more advanced combustion timing without knock, the potential of the ignition improvers to allow effective PFS was also explored over the same range of intake pressures. The effect of the additives on NOx emissions was also studied. The results showed that NOx emissions increase with increased EHN concentration but are not affected by DTBP. This work indicates that conventional ignition improvers can effectively enhance the HCCI autoignition reactivity of conventional gasoline at naturally aspirated and modestly boosted operations, offering significant benefits for HCCI engines. © 2014 SAE International.

Abstract not provided.

Abstract not provided.