International Journal of Engine Research

The engine lubrication system is a vital element for engine health but causes a parasitic load on the engine which increases the fuel consumption: this load can be reduced by matching the oil flow to lubricating requirements using a variable displacement oil pump. In a first stage, two variable displacement oil pumps were installed on a 2.4-L diesel engine; experiments over the New European Drive cycle showed reductions in fuel consumption of up to 3.4% and up to 5.8% over the urban phase of the cycle. A variable displacement oil pump was subsequently installed on an instrumented engine capturing over 100 metal and fluid temperatures within the engine structure. This showed that reducing oil flows resulted in lower oil temperature by up to 4 °C during cold-start New European Drive cycle but hotter cylinder liner temperatures by up to 6 °C. The higher cylinder wall temperatures caused an increase of 3% in oxides of nitrogen emissions but a reduction of 3%−5% in carbon monoxide and hydrocarbon emissions. Finally, an energy flow analysis showed that the variable displacement oil pump can reduce oil pump energy consumption by 160 kJ (32%) but that this led to a 400-kJ reduction in friction and accessory work. These findings highlight the need for a system-level rather than a component-level approach to engine lubrication design to capture key thermal interactions.

The upper load range of homogeneous charge compression ignition engines is limited by extreme noise levels. Previous studies have investigated the sources of this noise by investigating high-frequency (>4 kHz) acoustic energy typically associated with cylinder cavity resonances excited during high in-cylinder pressure rise rate combustion events. In this work, a broader frequency band (0.5–15 kHz) is investigated experimentally for a single-cylinder homogeneous charge compression ignition engine inside an acoustic sub-enclosure. It has been found that the 0.5–4-kHz frequency band being excited by the rapid in-cylinder pressure rise rate of the homogeneous charge compression ignition combustion event is dominating the acoustic signature of the engine. This indicates that the noise is generated from the structural response of the engine structure to the impulse-like load from the rapid cylinder pressure rise rather than ringing associated with the cavity resonances. A band-level analysis is used to explore the contribution of mechanical and combustion noise on the acoustic signature for various load conditions and combustion phasings. Based on this analysis, a method is proposed where a cylinder pressure rise rate threshold is established for determining the point at which the homogeneous charge compression ignition combustion noise will significantly exceed the mechanical noise level. This threshold, based on the theoretical band-level analysis, agrees with thresholds found from previous literature.

Recently, several promising biomass-derived fuels for diesel engines have been identified, produced, and tested. Diesel engine experiments confirmed very low soot and low nitrogen oxide emissions. With regard to further improvements of the combustion system, it is desirable to complement the diesel engine experiments with numerical simulations. To date, this is hindered by the lack of suitable chemical reaction mechanisms for these novel fuels. Therefore, a surrogate approach is presented here and applied in computational fluid dynamics simulations. Combustion and pollutant formation is simulated using the representative interactive flamelet model. Ignition, combustion, and pollutant formation are described in a consistent manner by inclusion of detailed reaction chemistry. Different mixtures of n-heptane, toluene, ethanol, dimethylether, ethane, and phenol are employed to describe the combustion chemistry of the biofuels. The compositions of the surrogate fuels are compiled according to hydrogen/carbon ratio, oxygen content, cetane rating, and molecular properties of the experimental fuels. Spray, injection, and evaporation properties of the experimental fuels, as obtained from spray vessel experiments, are included in the computational fluid dynamics simulations. By systematic comparison of experimental and numerical results, the surrogate methodology is validated and an improved understanding of the limitations of the current surrogates is achieved. Thus, a methodology for the fast adoption of novel fuels for simulations is proposed that can be used regardless of the availability of specific chemical reaction mechanisms.

With the advancements in engine design, the air path systems have become so complex that advanced optimization and control design methods are nowadays needed to fully exploit the potential of technologies such as flexible valve actuation. While the automotive industry is employing model-based methodologies for the engine air path system control design process, control-oriented engine air path models are currently limited in their accuracy and predictive ability and rely significantly on calibration. As the engine system complexity increases, there is considerable interest for accurate yet computationally efficient control-oriented engine system models that are able to predict the cylinder charge composition and the thermodynamic conditions with limited calibration effort. This article presents a novel model-order reduction approach for lumped-parameter modeling of pressure wave propagation dynamics in fluids, with application to full-engine simulation. Instead of approximating the geometry of the system to a simple control volume (as typically done in control-oriented engine air path models), this modeling methodology reduces the governing equations for compressible fluid flows and allows for systematically handling both complex geometries, such as sudden area changes and pipe elbows, as well as complex boundary conditions. The model-order reduction procedure projects the partial differential equations governing the air and gas flows onto a set of eigenfunctions, resulting in a set of ordinary differential equations that can be easily implemented in forward-looking simulation software. The modeling approach presented in this article is applied to characterize the wave propagation dynamics in the air path system of a single-cylinder engine, specifically predicting the crank-angle-resolved intake and outlet pressure, as well as the cylinder air charge and volumetric efficiency. The results are benchmarked against both experimental data and simulation results from a well-established one-dimensional gas dynamic model.

This article reports the application of optical tomography and chemical species tomography to the characterisation of the in-cylinder mixture preparation process in a gasoline, direct-injection, single-cylinder, motored research engine. An array of 32 near-infrared beams is transmitted in a horizontal plane across the cylinder bore near the top of the cylinder, through a circular quartz annulus. A novel approach to enable the optical alignment of the transmitting and receiving optics is utilised. The engine is operated at a stoichiometric condition at 1200 r/min, with negative valve overlap timing. Two tomographic measurement schemes (optical attenuation and chemically specific absorption) were used to acquire data on the spatial and temporal distribution of fuel throughout the engine cycle. Optimised data pre-processing methods are described for maximal beam count and data reliability. The presence of fuel during the intake stroke was detected by the optical beam attenuation due to scattering from the liquid gasoline droplets. Optical tomographic reconstruction of the spatial distribution of these droplets was achieved at an imaging rate of 7200 frames per second, revealing rapid intra-cycle spatial variations that were consistent between consecutive cycles. During the compression stroke, chemical species tomography images of fuel vapour were reconstructed from data acquired using chemically selective spectral absorption by the hydrocarbon molecules, at an imaging rate of 2400 frames per second. Later in the compression stroke, the temporal evolution of the fuel vapour distribution in the plane of observation is relatively slow and displays inhomogeneities that are consistent between consecutive cycles. This is the first report of the use of tomography to image, within individual engine cycles, the in-cylinder evolution of both fuel spray droplet distribution and fuel vapour distribution.

The objective of this study was to demonstrate a novel combustion system in which autoignition is induced by spark discharge into a pre-mixture formed during a long ignition delay time in a direct injection diesel engine with a low compression ratio. The effects of spark discharge on the autoignition process were investigated by changing the spark timing, injection timing, equivalence ratio and fuel amount. The fuel used was lauric acid methyl ester, a fatty acid methyl ester that is relatively volatile and exhibits higher ignition quality than standard diesel fuel. The results obtained with a single-cylinder common-rail direct injection diesel engine show that spark-induced compression ignition combustion was obtained with a volumetric efficiency of 34%. In this study, the autoignition timing was advanced by spark discharge and was controlled by the spark timing within 5° crank angle. We also observed the spark-induced compression ignition combustion flame in a constant volume combustion vessel using direct high-speed photography and OH radical chemiluminescence spectroscopy by means of a combination of an optical band-pass filter, an image intensifier and a charge-coupled device camera. The results showed that spark-induced compression ignition occurs not by flame propagation due to spark ignition but by autoignition of a spray pre-mixture. It was also evident that autoignition is induced by transportation of OH radicals formed at the spark plug by the spray flow.

Within the literature, there are a number of studies investigating the benefits of exhaust gas recirculation, and as a result it has become established as a promising technology for combustion control to allow engine downsizing technology to be advanced. Aside from the dilution effect of exhaust gas recirculation, some of the components, such as NO, CO, and hydrocarbons, can have significant chemical effects. The literature shows that the component within exhaust gas recirculation which has the largest chemical effect on combustion is NO, which can promote or inhibit the onset of autoignition causing reactions within the end gas at high load. The reduction in NOx gases in catalysed exhaust gas recirculation can increase the knock limit at high load, with some authors reporting up to a 5∘ crank angle improvement in combustion phasing. There is conflicting evidence on whether this translates to an improvement in fuel consumption, with one study finding a decrease of up to 2% comparing to another finding an increase of 1.5%-3.5%. Crude calculations on the emissions of a 2.0-L direct injection spark ignition engine operating at high load show that in an extreme case the reduction in the calorific value of the inlet charge due to catalysis of the recirculated gases can be up to 4.5%. Despite the potential benefits, the literature on catalysed exhaust gas recirculation is fairly limited and the evidence seems so far inconclusive as to whether this technology may have the potential to further enhance the benefits of exhaust gas recirculation. This article uses current literature to ascertain the potential benefits of catalysed exhaust gas recirculation, compare to pre-catalyst exhaust gas recirculation, and investigates its individual components in more detail to explain how chemical interactions can either promote or inhibit ignition depending on their concentration and temperature.