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A regular flex-fuel engine can operate on any blend of fuel between pure gasoline and E85. Flex-fuel engines have relatively low efficiency on E85 because the hardware is optimized for gasoline. If instead the engine is optimized for neat ethanol, the efficiency may be much higher, as demonstrated in this paper. The studied two-liter engine was modified with a much higher compression ratio than suitable for gasoline, two-stage turbocharging and direct injection with piezo-actuated outwards-opening injectors, a stratified combustion system and custom in-house control system. The research engine exhibited a wide-open throttle performance similar to that of a naturally aspirated v8, while offering a part-load efficiency comparable to a state-of-the-art two-liter naturally aspirated engine. NOx will be handled by a lean NOx trap. Combustion characteristics were compared between gasoline and neat ethanol.

The effects of using an SI stratified charge in combination with HCCI combustion on combustion phasing, rate of heat release and emissions were investigated in engine experiments to identify ways to extend the operational range of HCCI combustion to lower loads. In the experiments an optical single-cylinder engine equipped with a piezo electric outward-opening injector and operated with negative valve overlap (NVO) and low lift, short duration, camshaft profiles, was used to initiate HCCI combustion by increasing the exhaust gas recirculation (EGR) and thus retaining sufficient thermal energy to reach auto-ignition temperatures. Two series of experiments with full factorial designs were performed, to investigate how the tested parameters (amounts of fuel injected in pilot injections and main injections, stratification injection timing and spark-assistance) influenced the combustion.

Optical studies of combusting diesel sprays were done on three different alternative liquid fuels and compared to Swedish environmental class 1 diesel fuel (MK1). The alternative fuels were Rapeseed Oil Methyl Ester (RME), Palm Oil Methyl Ester (PME) and Fischer-Tropsch (FT) fuel. The studies were carried out in the Chalmers High Pressure High Temperature spray rig under conditions similar to those prevailing in a direct-injected diesel engine prior to injection. High speed shadowgraphs were acquired to measure the penetration of the continuous liquid phase, droplets and ligaments, and vapor penetration. Flame temperatures and relative soot concentrations were measured by emission based, line-of-sight, optical methods. A comparison between previous engine tests and spray rig experiments was conducted in order to provide a deeper explanation of the combustion phenomena in the engine tests.

The effects of using n-butanol, n-octanol, fossil Diesel, hydrotreated vegetable oil (HVO), and blends of these fuels on spray penetration, flame and soot characteristics were investigated in a high-pressure high-temperature constant volume combustion chamber designed to mimic a heavy duty Diesel engine. Backlight illumination was used to capture liquid and vapor phase spray images with a high-speed camera. The flame lift-off length (LOL) and ignition delay were determined by analyzing OH* chemiluminescence images. Laser extinction diagnostics were used to measure the spatially and temporally resolved soot volume fraction. The spray experiments were performed by injecting fuels under non-combusting (623 K) and combusting (823 K) conditions at a fixed ambient air density of 26 kg/m3. A Scania 0.19 mm single straight hole injector and Scania XPI common rail fuel supply system were used to produce injection pressures of 120 MPa and 180 MPa.

An experimental investigation was carried out in which an HSDI Common Rail Diesel engine was operated in both HCCI and conventional Diesel combustion modes, using conventional Diesel fuel in both cases. The engine used in the experiments was a single cylinder version of a modern passenger car engine with a displacement of 480 cc. In HCCI mode, the fuel was injected in multiple stages during the compression stroke, using a nozzle with a 60° included angle. To control the phasing and rate of combustion, the effective compression ratio was reduced by retarded intake valve closing. In addition, increased amounts of EGR were used. HCCI operation reduced soot and NOx emissions significantly. The use of a narrow included angle for conventional Diesel operation increased emissions significantly. The effect of a wider included angle and modifications to the piston were investigated experimentally and numerically.

In this work are presented experimental and simulated data from a one-cylinder direct injected Diesel engine fuelled with Diesel, two different biodiesel blends and pure biodiesel at one engine operating point. The modeling approach focuses on testing and rating biodiesel surrogate fuel blends by means of combustion and emission behavior. Detailed kinetic mechanisms are adopted to evaluate the fuel-blends performances under both reactor and diesel engine conditions. In the first part of the paper, the experimental engine setup is presented. Thereafter the choice of the surrogate fuel blends, consisting of n-decane, α-methyl-naphtalene and methyl-decanoate, are verified by the help of experiments from the literature. The direct injection stochastic reactor model (DI-SRM) is employed to simulate combustion and engine exhaust emissions (NOx, HC, CO and CO2), which are compared to the experimental data.

The effect of ammoniac deoxidizing agent (Urea) on the reduction of NOx produced in the Diesel engine was investigated numerically. Urea desolved in water was directly injected into the engine cylinder during the expansion stroke. The NOx deoxidizing process was described using a simplified chemical kinetic model coupled with the comprehensive kinetics of Diesel oil surrogate combustion. If the technology of DWI (Direct Water Injection) with the later injection timing is supposed to be used, the deoxidizing reactants could be delivered in a controlled amount directly into the flame plume zones, where NOx are forming. Numerical simulations for the Isotta Fraschini DI Diesel engine are carried out using the KIVA-3V code, modified to account for the “co-fuel” injection and reaction with combustion products. The results showed that the amount of NOx could be substantially reduced up to 80% with the injection timing and the fraction of Urea in the solution optimized.

Water injection is a promising technology to improve the fuel efficiency of turbocharged gasoline engines due to the possibility to suppress engine knock. Additionally, this technology is believed to enable the efficient operation of the three-way catalyst also at high-load conditions, through limiting the exhaust temperature. In this numerical study, we investigate the effect of water on the chemical and thermodynamic processes using 3D computational fluid dynamics (CFD) Reynolds-averaged Navier–Stokes (RANS) with detailed chemistry. In the first step, the influence of different amounts of water vapor on ignition delay time, laminar flame speed, and heat capacity is investigated. In the second step, the impact of water vaporization is analyzed for port and direct injection. For this purpose, the water mass flow and the injection pressure are varied.

When using dimethyl ether (DME) to fuel diesel engines at high load and speed, applying high amounts of exhaust gas recirculation (EGR) to limit NOX emissions, carbon monoxide (CO) emissions are generally high. To address this issue, the combustion and emission processes in such engines were analyzed with the three-dimensional CFD KIVA3V code. The combustion sub-mechanism (76 species and 375 reactions) was validated by comparing simulated ignition delays and flame velocities to reference data under diesel-like and atmospheric conditions, respectively. In addition, simulated and experimentally determined rate of heat release (RoHR) curves and emission data were compared for a heavy-duty single-cylinder DME engine (displaced volume, 2.02 liters) with DME-adapted piston and nozzle geometries. The simulated RoHR curves captured the main features of the experimentally measured curves, but deviated in the premixed (higher peak) and late combustion phases (too high).

Calculations using homogeneous and stochastic reactor models were performed in order to find an explanation to observed properties of NOx HCCI engines. It was found that for moderate NOx levels, N2O reactions play an important role in the NOx formation. Further, the high proportions of NO2 found in from some HCCI engines is due to high temperature inhomogeneities, poor mixing and slow overall combustion. N2O is often emitted from HCCI combustion. The levels of NOx in the exhausts are highly sensitive to temperature; however N2O has a weak negative dependence on temperature. While fuel rich operation naturally leads to high temperatures and thus high NOx levels; once the temperature effects are decoupled the fuel rich conditions themselves has a favorable effect on low-NOx engine operation.

The introduction of a physics-based zero-dimensional stochastic reactor model combined with tabulated chemistry enables the simulation-supported development of future compression-ignited engines. The stochastic reactor model mimics mixture and temperature inhomogeneities induced by turbulence, direct injection and heat transfer. Thus, it is possible to improve the prediction of NOx emissions compared to common mean-value models. To reduce the number of designs to be evaluated during the simulation-based multi-objective optimization, genetic algorithms are proven to be an effective tool. Based on an initial set of designs, the algorithm aims to evolve the designs to find the best parameters for the given constraints and objectives. The extension by response surface models improves the prediction of the best possible Pareto Front, while the time of optimization is kept low.

Operating the HCCI engine with dual fuels with a large difference in auto-ignition characteristics (octane number) is one way to control the HCCI operation. The effect of octane number on combustion, emissions and engine performance in a 6 cylinder SCANIA truck engine, fuelled with n-heptane and isooctane, and running in HCCI mode, are investigated numerically and compared with measurements taken from Olsson et al. [SAE 2000-01-2867]. To correctly simulate the HCCI engine operation, we implement a probability density function (PDF) based stochastic reactor model (including detailed chemical kinetics and accounting for inhomogeneities in composition and temperature) coupled with GT-POWER, a 1-D fluid dynamics based engine cycle simulator. Such a coupling proves to be ideal for the understanding of the combustion phenomenon as well as the gas dynamics processes intrinsic to the engine cycle.

In this study, the effects of varying the start of injection in a Free Piston Engine (FPE) have been investigated, using the KIVA-3V CFD code. In order to simulate the FPE the code has been modified by replacing the conventional crank shaft controlled piston motion by a piston motion profile calculated using a MATLAB/SIMULINK model. In this model, the piston motion is controlled by Newton's second law and the combustion process is represented by a simplified model based on ignition delay integrals and Wiebe functions. The results were tuned using predictions from the SENKIN software which are based on the detailed chemical kinetics mechanism of a Diesel oil surrogate represented by a blend of the main aliphatic (70% n-heptane) and aromatic (30% toluene) components. In order to help analyze the emission formation resulting from the HCCI/PPCI combustion modes in the engine, a special approach based on the temperature-equivalence ratio maps has been developed.

Advanced combustion concepts such as reactivity controlled compression ignition (RCCI) have been proven to be capable of fundamentally improve the conventional Diesel combustion by mitigating or avoiding the soot-NOx trade-off, while delivering comparable or better thermal efficiency. To further facilitate the development of the RCCI technology, a robust and possibly computationally efficient simulation framework is needed. While many successful studies have been published using 3D-CFD coupled with detailed combustion chemistry solvers, the maturity level of the 0D/1D based software solution offerings is relatively limited. The close interaction between physical and chemical processes challenges the development of predictive numerical tools, particularly when spatial information is not available.

In this paper an online method for automatically reducing complex chemical mechanisms for simulations of combustion phenomena has been developed. The method is based on the Quasi Steady State Assumption (QSSA). In contrast to previous reduction schemes where chemical species are selected only when they are in steady state throughout the whole process, the present method allows for species to be selected at each operating point separately generating an adaptive chemical kinetics. The method is used for calculations of a natural gas fueled engine operating under Homogenous Charge Compression Ignition (HCCI) conditions. We discuss criteria for selecting steady state species and the influence of these criteria on the results such as concentration profiles and temperature.

The formation of exothermic centers was modeled with a Stochastic Reactor Model (SRM) to investigate their impact on HCCI combustion. By varying the exhaust valve temperature, and thus assigning more realistic wall temperatures, the formation of exothermic centers and the ignition timing was shifted in time. To be able to study the exothermic centers, their formation and their distribution, Scatter plots, standard deviation plots and Probability Density Function (PDF) plots were constructed on the basis of the data the SRM calculations provided. The standard deviation for the particle temperatures was found to be an useful indicator of the degree of homogeneity within the combustion chamber, and thus of how efficient the combustion process was. It was observed that when the standard deviation of the temperature was higher, the emissions of CO and of hydrocarbons present at the end of the closed cycle were higher.

In this work, we present an unsteady flamelet progress variable approach for diesel engine CFD combustion modeling. The progress variable is based on sensible enthalpy integrated over the flamelet and describes the transient flamelet ignition process. By using an unsteady flamelet library for the progress variable, the impact of local effects, for example variations in the turbulence field, effects of wall heat transfer etc. on the autoignition chemistry can be considered on a cell level. The coupling between the unsteady flamelet library and the transport equation for total enthalpy follows the ideas of the representative interactive flamelet approach. Since the progress variable gives a direct description of the state in the flamelet, the method can be compared to having a flamelet in each computational cell in the CFD grid.

The possibilities for extending the range of engine loads in which soot and NOx emissions can be minimised by using low temperature combustion in conjunction with high levels of EGR was investigated in a series of experiments with a single cylinder research engine. The results show that very low levels of both soot and NOx emissions can be achieved at engine loads up to 50 % by reducing the compression ratio to 14 and applying high levels of EGR (up to approximately 60 %). Unfortunately, the low temperature combustion is accompanied by increases in fuel consumption and emissions of both HC and CO. However, these drawbacks can be reduced by advancing the injection timing. The research engine was a 2 litre direct injected (DI), supercharged, heavy duty, single cylinder diesel engine with a geometry based on Volvo's 12 litre engine, and the amount of EGR was increased by adjusting the exhaust back pressure while keeping the charge air pressure constant.

The objective of the study presented here was to examine the possibility of simultaneously reducing soot and nitrogen oxide (NOx) emissions from a heavy duty diesel engine, using very high levels of EGR (exhaust gas recirculation). The investigation was carried out using a 2 litre DI single cylinder diesel engine. Two different EGR strategies were examined. One entailed maintaining a constant charge air pressure with a varied exhaust back pressure in order to change the amount of EGR. In the other strategy a constant pressure difference was maintained over the engine, resulting in different equivalence ratios at similar EGR levels. EGR levels of 60 % or more significantly reduced both soot and NOx emissions at 25 % engine load with constant charge air pressure and increasing exhaust back pressure. However, combustion under these conditions was incomplete, resulting in high emissions of carbon monoxide (CO), unburned hydrocarbons (HC) and high fuel consumption.

To elucidate the processes controlling the auto-ignition timing and overall combustion duration in homogeneous charge compression ignition (HCCI) engines, the distribution of the auto-ignition sites, in both space and time, was studied. The auto-ignition locations were investigated using optical diagnosis of HCCI combustion, based on laser induced fluorescence (LIF) measurements of formaldehyde in an optical engine with fully variable valve actuation. This engine was operated in two different modes of HCCI. In the first, auto-ignition temperatures were reached by heating the inlet air, while in the second, residual mass from the previous combustion cycle was trapped using a negative valve overlap. The fuel was introduced directly into the combustion chamber in both approaches. To complement these experiments, 3-D numerical modeling of the gas exchange and compression stroke events was done for both HCCI-generating approaches.