Caj Niels Leermakers

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.

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.

Gasoline–diesel dual fuel : effect of injection timing and fuel balance

Recently, some studies have shown high efficiencies using controlled auto-ignition by blending gasoline and diesel to a desired reactivity. This concept has been shown to give high efficiency and, because of the largely premixed charge, low emission levels. The origin of this high efficiency, however, has only partly been explained. Part of it was attributed to a lower temperature combustion, originating in lower heat losses. Another part of the gain was attributed to a faster, more Otto-like (i.e. constant volume) combustion. Since the concept was mainly demonstrated on one single test setup so far, an experimental study has been performed to reproduce these results and gain more insight into their origin. Therefore one cylinder of a heavy duty test engine has been equipped with an intake port gasoline injection system, primarily to investigate the effects of the balance between the two fuels, and the timing of the diesel injection. Besides studying trends in the dual-fuel regime, this also allows to find best points to compare with conventional diesel combustion. Results show that compared to more conventional combustion regimes, this dual-fuel concept can escape from the common NOx-smoke trade-off, reducing both to near-zero values. Although hydrocarbon emissions are somewhat increased, indicated efficiencies are significantly improved. The absolute efficiencies are not as high as reported in other work, but the increase does confirm the potential of the concept. The increase in indicated efficiency is shown to originate from a higher thermal efficiency, because short burn durations at high gasoline fractions enable for CA50 to be phased closer to TDC, without combustion occurring too much before TDC. Pressure rise rates are as low as with conventional diesel combustion, when using the same Exhaust Gas Recirculation (EGR) percentage. Although the dual fuel concept has a much higher rate of heat release, this is phased better after TDC. A dedicated set of experiments has also shown that the late-cycle diesel injection is dominant in combustion phasing and that control has to be found in this diesel injections.

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.

Emission Performance of Lignin-Derived Cyclic Oxygenates in a Heavy-Duty Diesel Engine

In earlier research, a new class of bio-fuels, so-called cyclic oxygenates, was reported to have a favorable impact on the soot-NOx trade-off experience in diesel engines. In this paper, the soot-NOx trade-off is compared for two types of cyclic oxygenates. 2-phenyl ethanol has an aromatic and cyclohexane ethanol a saturated or aliphatic ring structure. Accordingly, the research is focused on the effect of aromaticity on the aforementioned emissions trade-off. This research is relevant because, starting from lignin, a biomass component with a complex poly-aromatic structure, the production of 2-phenyl ethanol requires less hydrogen and can therefore be produced at lower cost than is the case for cyclohexane ethanol. The goal of this paper, realized by means of experiments on a modified DAF heavy-duty diesel engine, is to investigate whether or not the (potentially prohibitively) expensive hydrogenation step from 2-phenyl ethanol to cyclohexane ethanol has an added value from an emissions perspective. The results suggest that this is not the case and hydrogenation therefore does not seem like an interesting additional step in the production process.

Combustion Phasing Controllability with Dual Fuel Injection Timings

Reactivity controlled compression ignition through in-cylinder blending gasoline and diesel to a desired reactivity has previously been shown to give low emission levels and a clear simultaneous efficiency advantage. To determine the possible viability of the concept for on-road application, the control space of injection parameters with respect to combustion phasing is presented. Four injection strategies have been investigated, and for each the respective combustion phasing response is presented. Combustion efficiency is shown to be greatly affected by both the injection-timing and injection-strategy. All injection strategies are shown to break with the common soot-NOx trade-off, with both smoke and NOx emissions being near or even below upcoming legislated levels. Lastly, pressure rise rates are comparable with conventional combustion regimes with the same phasing. The pressure rise rates are effectively suppressed by the high dilution rates used.

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 NOx emission 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.

Direct Injection of Diesel-Butane Blends in a Heavy Duty Engine

Increasing fuel prices keep bringing attention to alternative, cheaper fuels. Liquefied Petroleum Gas (LPG) has been well known for decades as an alternative fuel for spark ignition (SI) passenger cars. More recently, aftermarket LPG systems were also introduced to Heavy Duty transport vehicles. These (port fuel) systems either vaporize the liquid fuel and then mix it with intake air, or inject fuel into the engines intake ports. While this concept offers significant fuel cost reductions, for aftermarket certification and large-scale original equipment manufacturer (OEM) use some concerns are present. Unburned hydrocarbons (UHC) and carbon monoxide (CO) emissions are known to be high because of premixed charge getting trapped into crevices and possibly being blown through during valve-overlap. Apart from the higher emission levels, this also limits fuel efficiency and therefore cost savings. Direct injection of LPG offers a solution to the aforementioned concerns by directing the fuel to the desired areas of the combustion chamber. To investigate the potential of this concept, diesel has been blended with liquid butane, in different ratios. These blends were directly injected into a heavy duty test engine, using a standard common rail injector. Results show that even at low loads, a butane mass fraction of one-third can be used without negatively affecting emissions. For direct injected butane, hydrocarbon emissions were found to be an order of magnitude lower than when port fuel injected butane is used. At this butane fraction, smoke emissions decrease by 50%. Because a high combustion efficiency is retained, fuel cost savings can be significant. At higher loads, an even higher butane mass fraction can be used, further enhancing the fuel cost saving potential.

Experimental study on the impact of operating conditions on PCCI combustion

In a short-term scenario, using near-standard components and conventional fuels, PCCI combustion relies on a smart choice of operating conditions. Here, the effects of operating conditions on ignition delay, available mixing time, combustion phasing and emissions are investigated. In the PCCI regime, NOx and smoke have been shown to be effi ciently reduced with elongated mixing time. For viable PCCI combustion, one would require a Combustion Delay (CD) which is long enough to bring both NOx and smoke levels down to acceptable values. For the completeness of combustion, the resulting unburned hydrocarbon and carbon monoxide emissions, as well as the associated fuel consumption; mixing time should, however, be as short as possible. Most parameters strongly correlate with combustion delay, independent of how this is achieved. Lastly, the best points experienced for a number of cases are given.