Roger Cracknell

Ultra Boost for Economy: Extending the Limits of Extreme Engine Downsizing

The paper discusses the concept, design and final results from the ‘Ultra Boost for Economy’ collaborative project, which was part-funded by the Technology Strategy Board, the UKs innovation agency. The project comprised industry- and academia-wide expertise to demonstrate that it is possible to reduce engine capacity by 60% and still achieve the torque curve of a modern, large-capacity naturally-aspirated engine, while encompassing the attributes necessary to employ such a concept in premium vehicles. In addition to achieving the torque curve of the Jaguar Land Rover naturally-aspirated 5.0 litre V8 engine (which included generating 25 bar BMEP at 1000 rpm), the main project target was to show that such a downsized engine could, in itself, provide a major proportion of a route towards a 35% reduction in vehicle tailpipe CO2 on the New European Drive Cycle, together with some vehicle-based modifications and the assumption of stop-start technology being used instead of hybridization. In order to do this vehicle modelling was employed to set part-load operating points representative of a target vehicle and to provide weighting factors for those points. The engine was sized by using the fuel consumption improvement targets and a series of specification steps designed to ensure that the required full-load performance and driveability could be achieved. The engine was designed in parallel with 1-D modelling which helped to combine the various technology packages of the project, including the specification of an advanced charging system and the provision of the necessary variability in the valvetrain system. An advanced intake port was designed in order to ensure the necessary flow rate and the charge motion to provide fuel mixing and help suppress knock, and was subjected to a full transient CFD analysis. A new engine management system was provided which necessarily had to be capable of controlling many functions, including a supercharger engagement clutch and full bypass system, direct injection system, port-fuel injection system, separately-switchable cam profiles for the intake and exhaust valves and wide-range fast-acting camshaft phasing devices. Testing of the engine was split into two phases. The first usied a test bed Combustion Air Handling Unit to enable development of the combustion system without the complication of a new charging system being fitted to the engine. To set boundary conditions during this part of the programme, heavy reliance was placed on the 1-D simulation. The second phase tested the full engine. The ramifications of realizing the engine design from a V8 basis in terms of residual friction versus the fuel consumption results achieved are also discussed. The final improvement in vehicle fuel economy is demonstrated using a proprietary fuel consumption code, and is presented for the New European Drive Cycle, the FTP-75 cycle and a 120 km/h (75 mph) cruise condition.

Molecular simulation of hydrogen adsorption in graphitic nanofibres

Rodriguez, Baker and co-workers (A. Chambers, C. Park, R. T. K. Baker and N. M. Rodriguez, J. Phys. Chem. B, 1998, 102, 4253; C. Park, C. D. Tan, R. Hidalgo, R. T. K. Baker and N. M. Rodriguez, Proc. 1998 US DOE Hydrogen Program Reiew, (http://www.eren.doe.gov/hydrogen/docs/25315toc.html); C. Park, P. E. Anderson, A. Chambers, C. D. Tan, R. Hidalgo and N. M. Rodriguez, J. Phys. Chem. B, 1999, 103, 10572) have reported uptake of hydrogen in graphitic nanofibres (GNFs) of 40% by weight. If these results are confirmed, then this class of material could be a suitable storage medium for hydrogen for use in fuel cell vehicles. In order to test whether these results are feasible, we report results for grand canonical Monte Carlo simulation of hydrogen adsorption in graphitic pores. A classical technique was employed but the results obtained were shown to be consistent with previous path integral Monte Carlo calculations of Wang and Johnson (Q. Wang and J. K. Johnson, J. Chem. Phys., 1999, 110, 577; Q. Wang and J. K. Johnson, J. Phys. Chem. B, 1999, 103, 277). The interaction between hydrogen and the graphitic surface was modelled initially by dispersion forces. The predicted uptake (up to 1.5%) was much lower than the Baker–Rodriguez results. The results were found to be fairly insensitive as to whether the hydrogen molecule was modelled as a Lennard-Jones sphere or a dumbbell fluid with two Lennard-Jones sites. Two models for a hypothetical potential for chemisorption were also used in the simulation. The potential was based on calculation of the interaction between atomic hydrogen and a graphitic surface. Adsorption of up to 17 wt.% was measured with the stronger model potential but there was negligible desorption at ambient pressure, making it impractical. A more plausible, though still hypothetical, potential gave loadings of up to 8 wt.% in the model system. These results are still much lower than the Baker–Rodriguez data in spite of the fact that there is no evidence to suggest that chemisorption actually occurs in a real system.

Effect of Fuel Properties on Spray Development from a Multi-Hole DISI Engine Injector

Extensive literature exists on spray development, mixing and combustion regarding engine modeling and diagnostics using single-component and model fuels. However, often the variation in data between different fuels, particularly relating to spray development and its effect on combustion, is neglected or overlooked. By injecting into a quiescent chamber, this work quantifies the differences in spray development from a multi-hole direct-injection spark-ignition engine injector for two single-component fuels (iso-octane and n-pentane), a non-fluorescing multi-component model fuel which may be used for in-cylinder Laser Induced Fluorescence experiments, and several grades of pump gasoline (with and without additives). High-speed recordings of the sprays were made for a range of fuel temperatures and gas pressures. It is shown that a fuel temperature above that of the lowest boiling point fraction of the tested fuel at the given gas pressure causes a convergence of the spray plumes. Increasing the fuel temperature increases this convergence, whilst an associated increased rate of evaporation tends to reduce the penetration of individual plumes. The convergence increases gradually with increasing fuel temperature until all plumes combine to form a single wider plume with a penetration rate greater than that of the individual plumes. When all plumes are converged to form a single plume along a central axis to all the plumes, any further increase in fuel temperature at the given gas pressure acts to increase the rate of evaporation of the fuel. At experiments up to 180 °C fuel temperature and down to 0.3 bar absolute gas pressure, none of the tested fuels were found to spontaneously vaporize; all observed spray formations being a gradual evolution. Increasing the gas pressure at any given fuel temperature, leads to an increase in the boiling temperature of all components of that fuel and, hence, diminishes these effects.

Octane appetite: The Relevance of a Lower Limit to the MON Specification in a Downsized, Highly Boosted DISI Engine

Market demand for high performance gasoline vehicles and increasingly strict government emissions regulations are driving the development of highly downsized, boosted direct injection engines. The in-cylinder temperatures and pressures of these emerging technologies tend to no longer adhere to the test conditions defining the RON and MON octane rating scales. This divergence between fuel knock rating methods and fuel performance in modern engines has previously led to the development of an engine and operating condition dependent scaling factor, K, which allows for extrapolation of RON and MON values. Downsized, boosted DISI engines have been generally shown to have negative K-values when knock limited, indicating a preference for fuels of higher sensitivity and challenging the relevance of a lower limit to the MON specification. The Ultraboost engine is an inline-4 downsized, highly boosted prototype DISI engine designed to achieve a 35% reduction in CO2 emissions whilst maintaining performance of a production V8. A series of 14 fuel formulations were tested to probe engine response to various fuel properties. This paper presents results from a 7 fuel RON and MON decorrelated matrix at four high-load engine conditions. The K-value was found to be negative at all engine conditions; fuels of higher sensitivity were found to yield improved engine performance. Furthermore, in-cylinder experimental data from high load knocking conditions with a single standard octane fuel were used to simulate the K-value; a similar trend between theory and experiment was observed.

Simulation of hydrogen adsorption in carbon nanotubes

Computer simulations are reported of hydrogen adsorption in multi-walled carbon nanotubes (MWNTs) and single-walled carbon nanotubes (SWNTs). The gas-solid interaction was modelled both as pure dispersion forces and also with a hypothetical model for chemisorption introduced in a previous paper (CRACKNELL, R., F., 2001, Phys. Chem. chem. Phys., 3, 2091). A two-centre model for hydrogen was employed and the grand canonical Monte Carlo methodology was used throughout. Uptake of hydrogen in the internal space of a carbon nanotube is predicted to be lower than in the optimal graphitic nanofibre with slitlike pores (provided the gas-solid potential is consistent). Part of the difference arises from the assumption of pore surface area used in converting the raw simulation data to gravimetric adsorption; however, the majority of the differences can be attributed to the curvature of the pore. This reduces the uptake of hydrogen (on a gravimetric basis) in spite of deepening the potential minimum inside the pore associated with dispersion forces. It is concluded that for the uptake of hydrogen in SWNTs of 5–10% reported by Heben (DILLON, A. C., JONES, K. M., BEKKEDAHL, T. A., KIANG, C. H., BETHUNE, D. S., AND HEBEN, M. J., 1997, Nature, 386, 377), gas-solid forces other than dispersion forces are required and most of the adsorption must occur in the interstices between SWNTs.

High pressure laminar burning velocity measurements and modelling of methane and n-butane

A constant volume vessel has been used in conjunction with a numerical multi-zone model to calculate laminar burning velocities of methane and n-butane in the ranges φ = 0.8–1.4, T u = 320–470 K, p u = 1–15 bar from the pressure record. This multi-zone model has been compared with the analytical model of Luijten and de Goey. For methane, the experimental data have been compared with modelling data generated using the MIXFLA program of Warnatz. The MIXFLA data have further been used to examine the form of the correlation fitted to the experimental data, confirming the form used by Clarke. For methane, good agreement was found between the current experimental data and the literature at high pressures. For n-butane, poor agreement was found with the one data set available. However, the data from this reference did not compare well with other authors for methane.

Octane Response in a Downsized, Highly Boosted Direct Injection Spark Ignition Engine

Increasingly strict government emissions regulations in combination with consumer demand for high performance vehicles is driving gasoline engine development towards highly downsized, boosted direct injection technologies. In these engines, fuel consumption is improved by reducing pumping, friction and heat losses, yet performance is maintained by operating at higher brake mean effective pressure. However, the in-cylinder conditions of these engines continue to diverge from traditional naturally aspirated technologies, and especially from the Cooperative Fuels Research engine used to define the octane rating scales. Engine concepts are thus key platforms with which to screen the influence of fundamental fuel properties on future engine performance. ‘ULTRABOOST’, a collaborative research project which is co-funded by the Technology Strategy Board (TSB), the UKs innovation agency, is a downsized, highly boosted, 2.0L in-line 4 cylinder prototype engine, designed to achieve 35% CO2 emissions reduction without compromising the performance of a 5.0L V8 naturally aspirated production engine. To probe engine response to fuel, a matrix of 14 formulations was tested at several engine conditions. This is the first in a series of fuel related papers and focuses on the engines response to the research octane number (RON). The knock limited spark advance was determined for a series of fuels with RON varying from 95 to 112; octane was shown to provide 5 or 10° crank angle advance in knock limited spark advance at 2000 and 3000 rpm, respectively. This study demonstrates that fuel octane quality continues to be important for the performance of emerging downsized engine technologies. Furthermore, the trend for continued engine downsizing will increase the potential performance benefit associated with knock resistant fuels.

Shear viscosity of linear alkanes through molecular simulations: quantitative tests for n-decane and n-hexadecane

Atomistic molecular dynamics simulations were carried out at equilibrium to calculate the shear viscosity of n-decane and n-hexadecane under ambient and high temperature–high pressure conditions. Two force fields, one using a computationally efficient united-atom (TrAPPE-UA) approach and another an all-atom (AA) approach (Tobias, Tu and Klein), were employed. Quantitative agreement with experimental data is obtained with the AA force field, whereas the UA model predicts the viscosity within 20–30% of the experiment. The intra- and inter-molecular structure of the fluid obtained from these two models is similar, pointing to the role of differences in their dynamical characteristics to the observed difference in the calculated viscosities.

Monte-Carlo Simulations of Centrifugal Gas Separation

The use of a Monte–Carlo formalism in a centrifugal gas process separation simulation provides an efficient predictor of dew-pointing as a function of the imposed radial pressure gradient. Previously, this was done by simply calculating radial pressure and then resorting to a separate equation of state routine for evaluating whether condensation will occur or not. In our model, we incorporate the potential energy associated with rotation of a gas element into the simulation along with molecular interaction terms. This enables us to predict when sufficient nucleation has occurred that condensed material forms—an important limit for stable operation of a gas centrifuge.

Octane Sensitivity in Gasoline Fuels Containing Nitro-Alkanes: A Possible Means of Controlling Combustion Phasing for HCCI

Addition of nitroalkanes to gasoline is shown to reduce the octane quality. The reduction in the Motor Octane Number (MON) is greater than the reduction in the Research Octane Number (RON). In other words addition of nitroalkanes causes an increase in octane sensitivity. The temperature of the compressed air/fuel mixture in the MON test is higher then in the RON test. Through chemical kinetic modelling, we are able to show how the temperature dependence of the reactions responsible for break-up of the nitroalkane molecule can lead to an increase in octane sensitivity. Results are presented from an Homogenous Charge Compression Ignition (HCCI) engine with a homogeneous charge in which the air intake temperature was varied. When the engine was operated on gasoline-like fuels containing nitroalkanes, it was observed that the combustion phasing was much more sensitive to the air intake temperature. This suggests a possible means of controlling combustion phasing for HCCI.