Review on the effects of dual-fuel operation, using diesel and gaseous fuels, on emissions and performance

In recent years the automotive industry has been forced to reduce the harmful and pollutant emissions emitted by direct injected diesel engines. To accomplish this difficult task various solutions have been proposed. One of these proposed solutions is the usage of gaseous fuels in addition to the use of liquid diesel. These gaseous fuels have more gasoline-like properties, such as high octane numbers, and are thereby are resistant against auto-ignition. Diesel on the other hand, has a high cetane number which makes it prone to auto-ignition. In this case the gaseous fuel is injected in the inlet manifold, and the diesel is direct injected in the cylinder at the end of the compression stroke. Thereby the diesel fuel spontaneously ignites and acts as an ignition source. The main goals for the use of a dual-fuel operation with diesel and gaseous fuels are the reduction of particulate matter (PM) and nitrogen oxides (NOx) emission. Furthermore, the application of such a dual-fuel operation can offer potential economic and efficiency advantages. Depending on the gaseous fuel used these goals can be achieved. In general, dual-fuel combustion of gaseous fuels and diesel decreases soot emissions compared with normal diesel combustion except for syngas. Furthermore, increasing load and/or gaseous fuel content leads to a further decrease in soot emissions. Both the application natural gas and liquefied petroleum gas as gaseous fuel offer the possibility to diminish nitrogen oxide emissions probably due to homogenous mixture compositions and/or decreased mixture temperatures. However, the using hydrogen or syngas in dual-fuel combustion tends to increase nitrogen oxide emissions; this might be due to the higher flame temperatures and combustion rates of these gasses. Furthermore, the emissions of unburned hydrocarbons and carbon monoxides tend to increase for all evaluated gaseous fuels with dual fuel combustion mainly due to incomplete combustion of mixture trapped in crevices. Efficiencies of the different gaseous fuels are in the same order of magnitude. Some seem to lead to slight efficiency improvements (hydrogen and LPG) while others result in a slight decrease (natural gas and syngas). However, the significant price difference of natural gas and LPG compared to diesel can offer a considerable economic advantage

Application of partially premixed combustion using low octane fuels in a heavy duty engine

No abstract

Multi-zone modelling of PCCI combustion

Early Direct Injection Premixed Charge Compression Ignition (EDI PCCI) combustion is a promising concept for the diesel combustion. Although EDI PCCI assures very low soot and NO xemission levels, the injection is uncoupled from combustion, which narrows down the operating conditions. The main purpose is to analyse the effect of mixing. A multi-zone model is presented with the use of detailed chemical models. The paper presents the effects of parameters, like number of zones and chemical model, on emissions and ignition delay. A dedicated set of experiments is also utilised to assess the quality of the model

Modeling of PCCI combustion with FGM tabulated chemistry

Premixed Charge Compression Ignition (PCCI) is a new combustion concept aiming a simultaneous reduction of oxides of nitrogen and soot emissions. Therefore the operation focuses on improved fuel–air mixing before ignition and lower maximum in-cylinder temperatures during the complete engine cycle. In the PCCI-regime, the injection and ignition events do not overlap due to the longer ignition delay timings. As such ignition is not influenced by the injection event like in the conventional operation and it is essentially governed by chemical kinetics. Numerical methods should incorporate flow and chemistry models in an accurate way. However, the computational demand for modeling these phenomena is high and the researchers work on several reduction techniques to achieve a practical computational efficiency. In this work the Flamelet Generated Manifold method is applied within the Computational Fluid Dynamics (CFD) framework to study PCCI combustion. In FGM, thermo-chemical properties are preprocessed by solving canonical systems (here, Igniting Counter-flow Diffusion Flamelets and Homogeneous Reactors) and stored in a manifold as a function of controlling variables. Since ignition control is difficult in the aforementioned combustion concept, the accurate prediction of ignition phenomena is significant. Simulations are performed with three different mesh settings, where the course grid proves to be sufficiently accurate. Later the effect of multiple pressure levels is investigated using both canonical systems and the study shows that a number of three pressure levels is sufficient to capture the ignition phasing with Homogeneous Reactors based FGM tables. Finally, the sensitivity of ignition with respect to injection timing is shown to be predicted precisely

Experimental study of fuel composition impact on PCCI combustion in a heavy-duty diesel engine

Premixed Charge Compression Ignition (PCCI) is a combustion concept that holds the promise of combining emission levels of a spark-ignition engine with the efficiency of a compression-ignition engine. In a short term scenario, PCCI would be used in the lower load operating range only, combined with conventional diesel combustion at higher loads. This scenario relies on using near standard components and conventional fuels; therefore a set of fuels is selected that only reflects short term changes in diesel fuel composition.Experiments have been conducted in one dedicated test cylinder of a modified 6-cylinder 12.6 liter heavy duty DAF engine. This test cylinder is equipped with a stand-alone fuel injection system, EGR circuit and air compressor. For the low load operating range the compression ratio has been lowered to 12:1 by means of a thicker head gasket. It is shown that emission levels and performance strongly correlate with the combustion delay (CD=CA50-SOI), independent of how this combustion delay is achieved.In a longer term scenario, both engine hardware and fuels can be adapted to overcome intrinsic PCCI challenges. At higher loads and at 15:1 compression ratio, necessary for good full load efficiency, a less reactive fuel is required to delay auto-ignition and phase combustion correctly. A number of low reactivity fuel blends have been used to investigate the desired Cetane Number for PCCI operation at different loads. For these blends too, all emission levels as well as the efficiency are shown to greatly correlate with the combustion delay. With an improved efficiency because of the higher compression ratio, the blend with an estimated CN of 25 was found to be the most flexible in being able to choose the optimum CD for the conditions and load used

Predicting auto-ignition characteristics of RCCI combustion using a multi-zone model

The objective of new combustion concepts is to meet emission standards by improving fuel air mixing prior to ignition. Since there is no overlap between injection and ignition, combustion is governed mainly by chemical kinetics and it is challenging to control the phasing of ignition. Reactivity Controlled Compression Ignition (RCCI) combustion aims to control combustion phasing by altering the fuel ratios of the high- and low octane fuel and injection timings. In this study the dual fuel blend is prepared with gasoline and diesel fuels. The applied injection timings of the diesel are very early (90 to 60° CA bTDC). In the detailed reaction mechanism, n-heptane and iso-octane represent diesel and gasoline fuel, respectively. A multi-zone model approach is implemented to perform RCCI combustion simulation. Ignition characteristics are analyzed by using CA50 as the main parameter. In the experiments for the early direct injection (DI) timing advancing the injection time results in a later ignition. Qualitatively, the trend effect of the diesel injection timing and the effect of the ratio gasoline/diesel are captured accurately by the multi-zone model

Low octane fuel composition effects on the load range capability of partially premixed combustion

To determine the influence of physical and chemical properties of fuels’ load range capacity in partially premixed combustion, seven fuels have been blended, with a fixed RON70 reactivity. Four of these fuels are blended from refinery streams, with different boiling ranges, aromatic- and bio-content. Furthermore, three ternary mixtures of Toluene, n-Heptane, Ethanol and iso-Octane are used, of which the aromatics (toluene) and oxygenate (ethanol) content are varied. The load range capacity of these fuels is determined based on their fuel efficiency, smoking tendency and its sensitivity to the fuel pressure used, nitrogen oxides emissions, and combustion efficiency and stability at low load and engine speed

Commercial Naphtha blends for partially premixed combustion

Partially Premixed Combustion has shown the potential of low emissions of nitrogen oxides (NOx) and soot with a simultaneous improvement in fuel efficiency. Several research groups have shown that a load range from idle to full load is possible, when using low-octane-number refinery streams, in the gasoline boiling range. As such refinery streams are not expected to be commercially available on the short term, the use of naphtha blends that are commercially available could provide a practical solution. The three blends used in this investigation have been tested in a single-cylinder engine for their emission and efficiency performance. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to legislated emission levels. Conventional diesel combustion benchmarks are used for reference to show possible improvements in indicated efficiency. Analysis of the heat release patterns revealed an interesting and strong correlation between the premixed fraction and the amount of soot produced. To be specific, each of the fuels showed a decrease in this fraction as either fuel pressure was lowered or load was increased, showing a transition from more premixed to mainly mixing-controlled combustion, with the corresponding soot emissions. For one blend, over the whole load range EURO VI PM levels were approached or achieved, combined with a peak gross indicated efficiency of 50% clearly indicating the potential of this concept

Butanol-diesel blends for partially premixed combustion

Partially Premixed Combustion has shown the potential of high efficiency, emissions of nitrogen oxides (NOx) and soot below future emissions regulations, and acceptable acoustic noise. Low-octane-number gasoline fuels were shown to be most suitable for this concept, with the reactivity determining the possible load range. Other researchers have used several refinery streams, which might be produced by a refinery if they were required to do so without additional investment. Some of refinery streams are, however, not expected to be commercially available on the short term. For the present investigation, n-butanol (BuOH) has been selected as a blend component in diesel, and is used from 50 – 100%. The blends then have a reactivity range similar to the refinery streams, so single-cylinder engine tests for their emission and efficiency performance can also be used to determine their applicable load range. The current paper presents a summary of the performance of such BuOH-diesel blends with respect to emissions and efficiency in the Partially Premixed Combustion regime. Besides a presentation of the sensitivity to injection strategies, dilution levels and fuel pressure, emission performance is compared to upcoming legislated emission levels. The effect of the blend ratio on load ranges is shown and conventional diesel combustion benchmarks are used to show improvements in indicated efficiency. Butanol-diesel blends are shown to be a viable approach to partially premixed combustion, with its high soot reduction potential and stable operation. EURO VI emission levels can therefore be achieved, with moderate or slightly increased fuel pressure. Combustion efficiency is shown to be very reasonable over the whole load range, similar to that of conventional diesel combustion. Combined with an improved thermal efficiency a moderate butanol-diesel blend is shown to have an average gross indicated efficiency of 50% over the whole load range

Optimizing engine efficiency by balancing dilution, heat release rate and combustion phasing

In this investigation highly diluted engine tests have been conducted to study effects of dilution, heat release rate and combustion phasing on both efficiency and emissions in the Partially Premixed Combustion regime. It has been reported that over the complete phasing range an increase of more than two percent points in indicated efficiency at high load could be expected, with an astonishing 4 to 5 percent points for the low load case. The origins of these increases were sought and found to be caused by improved heat release shapes, combustion efficiencies or heat losses or a combination of these. On the other hand, soot emissions are greatly reduced due to the excess oxygen available, but nitrogen oxide emissions are found to increase: both in concentration and power specific units