International Journal of Engine Research

The demand for transport energy is increasing, but this increase is heavily skewed toward heavier fuels such as diesel and jet fuel while the demand for gasoline might decrease. As spark-ignition engines develop to become more efficient, abnormal combustion such as knock and preignition will become more likely. High antiknock quality fuels, those with high research octane number and preferably low motor octane number, will enable future spark-ignition engines to reach their full potential. Higher fuel antiknock quality is also likely to mitigate “superknock” resulting from preignition—an abnormal combustion problem in turbocharged spark-ignition engines. In many parts of the world, fuel antiknock specifications are set on the assumption that higher motor octane number contributes to increased knock resistance. Specifications for fuel antiknock quality have a great impact on fuels manufacture and will need to be revised as this mismatch between existing specifications and engine requirements widens. The primary challenge for diesel engines is to reduce emissions of soot and NO_x while maintaining high efficiency, and this becomes much easier if such engines are run on fuels of extremely low cetane. Significant development is needed before such engines can be seen on practical vehicles. In the long term, compression ignition engines are likely to use fuels with research octane number in the range of 70–85 (cetane number<∼30) but with no strict requirements on volatility. Such fuels would require less processing in the refinery than today’s fuels. Such an engine/fuels system will be at least as efficient as today’s diesel engine but could be significantly cheaper and also open a path to mitigate the imbalance in demand growth between heavy and light fuels that is expected to arise otherwise. The review concludes with a possible long-term fuel scenario.

To improve the fuel efficiency of automobile internal combustion engines, the friction reduction of each engine moving part is important. Recently, carbon films have been attracting much interest as surface-coating materials due to its excellent properties such as low friction and good wear resistance. In this study, the low-friction mechanisms of carbon films were analyzed using molecular dynamics simulations and density functional theory calculations. Molecular dynamics simulation results showed that the termination of OH groups on the surface of the diamond substantially reduced the friction coefficient from 0.07 to 0.01. This reduction was achieved because termination of OH groups weakened the covalent interaction between Fe and C atoms, which was indicated by density functional theory calculations. Additionally, based on the concept of terminating the OH groups on the surface of diamond-like carbon films, we carried out the reciprocating friction experiment between hydrogen-free diamond-like carbon surface and glycerin, which contains large number of OH groups. The friction coefficient of the glycerin was 0.028, much lower than that of the base oil, which was 0.075. The experiments confirmed that OH groups on the surface of hydrogen-free diamond-like carbon films greatly improved the friction properties of its films.

The results of full-scale regeneration tests with a unit silicon carbide diesel particulate filter installed on a single-cylinder diesel engine are presented. Two sets of transient regeneration experiments, one for each fuel used, with high level of exhaust gas flow rate (normalized per filter volume) and markedly different levels of soot loading were performed. The test fuels were conventional diesel (denoted as B0) and B20 biodiesel blend. The effect of the biodiesel on filter regeneration was further investigated by means of infrared thermography. The main objective was to investigate the catalytic soot ignition and regeneration propagation when the engine is fueled by biodiesel blends. Special focus was on the temperature distribution across the filter channels and the evolution of the regeneration at the central and peripheral part of the filter. The results indicated a faster evolution of the regeneration with lower overall filter wall temperatures with favorable effects on the filter durability, when the engine is fueled by B20.

In-cylinder strategies to reduce soot emissions have demonstrated the potential to lessen the burden on, and likely the size and cost of, exhaust aftertreatment systems for diesel engines. One in-cylinder strategy for soot abatement is the use of close-coupled post injections. These short injections closely following the end of the main injection can alter soot-formation and/or oxidation characteristics enough to significantly reduce engine-out soot. Despite the large body of literature on post injections for soot reduction, a clear consensus has not yet been achieved regarding either the detailed mechanisms that affect the soot reduction, or even the sensitivity of the post-injection efficacy to several important engine operating parameters. We report that post injections reduce soot at a range of close-coupled post-injection durations, intake-oxygen levels, and loads in an optical, heavy-duty diesel research engine. Maximum soot reductions by post injections at the loads and conditions tested range from 40% at 21% intake oxygen (by volume) to 62% at 12.6% intake oxygen. From a more fundamental fluid-mechanical perspective, adding a post injection to a constant main-injection for conditions with low dilution (21% and 18% intake oxygen) decreases soot relative to the original main injection, even though the load is increased by the post injection. High-speed visualization of natural combustion luminosity and laser-induced incandescence of soot suggest that as the post-injection duration increases and the post injection becomes more effective at reducing soot, it interacts more strongly with soot remaining from the main injection.

In-cylinder swirl flow is well known to influence emissions behaviour in diesel engine combustion. The post-oxidation part of the combustion stands for typically 25 - 45 % of total Heat Release and is of paramount importance for engine out particulate matter (PM) emissions, especially during an engine transient, at low λ. To investigate the link between in-cylinder flow and engine out emissions, single-cylinder and optical engine measurements were performed. Injection pressure, swirl and tumble were varied, and emission data, together with high speed photography of in-cylinder flow field during the injection- and the post-oxidation events, were measured. Particle image velocimetry (PIV) software was used to evaluate the flame luminescence images and to calculate the flow field in the cylinder, crank angle resolved during combustion. The glowing soot structures from the combustion were used as tracers. Single cylinder tests with an active valve train were used, which gave a controlled variation in swirl number, 0.4 to 6.7, and tumble number, 0.5 to 4.0. The main findings is that the injection pressure strongly affects the flow field in the cylinder, both before and during the post-oxidation period. Correlations between measured engine out soot emissions and changes in in-cylinder flow has been found and was coupled to the changes in swirl and injection pressure. The observed swirl vortex in the post-oxidation period deviates strongly from solid body rotation. The unsymmetrical rotation was found to be a function of injection pressure. This deviation is concluded to affect the soot oxidation in form of increased turbulence during the post-oxidation period.

This study discusses a novel approach toward homogeneous charge compression ignition operation in the 5 - 10 bar net indicated mean effective pressure range. This approach is based on the combination of boosting and variable valve actuation to maximize engine efficiency. Compression ratio plays a key role and determines low-temperature combustion feasibility in modern gasoline compression ignition concepts. In order to explore the interactions between compression ratio, boosting system and variable valve actuation, multi-cylinder engine models were utilized which employed the University of Michigan combustion model. Valve strategies featured switching from low-lift negative valve overlap to high-lift positive valve overlap, and the switching point was found to be dependent on compression ratio. A recent study by the authors suggested that heating the charge from external compression is more efficient than heating by residual gas retention strategies. Use of non-cooled intake air allows for valve events and combustion phasing that promote turbocharger performance and alleviate the backpressure problems often associated with low temperature combustion engines. Elevated intake pressure and reduced pumping work allow for improvements in efficiency with minimal NOx formation and acceptable ringing. It was found that further efficiency benefits can be realized by increasing compression ratio. Identification of trade-offs between engine hardware and combustion mode appears to be critical for homogeneous charge compression ignition operation in the 5 -10 bar net indicated mean effective pressure range. By focusing on this operating range, the present modeling study attempts to shed some light on practical applications of light-duty gasoline compression ignition concepts.

Piezoelectric fuel injectors provide a means to lower emissions, noise, and fuel consumption in advanced internal combustion engines by enabling complex injection rate profiles—referred to as “rate shaping.” Consideration of flow rate nonlinearities, piezostack hysteresis, and hydromechanical dynamics is required to achieve precise control of rate shaping. The goal of the effort described here was to develop, and experimentally validate, a dynamic, physically based model of a piezoelectric fuel injector during rate-shaping operating conditions. The model prediction captures the rate shape and demonstrates injected fuel errors of less than 5.2%.

A spray and evaporation model has been developed and embedded in a quasi-dimensional multi-zone direct injection internal combustion engine simulation framework. The model accounts for the behavior of the spray zone and fuel evaporation including sub-models for spray breakup, improved zone velocity estimations with transient fuel injection, spray penetration and tracking of evaporated fuel components. Each sub-model deals with multi-component fuel surrogates to simulate the real fuel effects. To enhance sensitivity of the model to the fuel components, additional thermophysical properties to the traditional spray model are incorporated. The ideal gas and the ideal solution are assumed in the model to retain computational efficiency. Finally, the model was validated against experiments and compared with other numerical methods. The ability of the new model to treat a multi-component fuel provides an improvement in the simulations of modern direct injection internal combustion engine when used with alternative fuels.

In this study, biodiesel effects on transient needle motion and near-exit flow characteristics of a single-orifice high-pressure diesel injector were investigated in terms of needle-lift, needle speed, exit velocity and near-exit flow structure under various injection pressures. Ultrafast x-ray phase-contrast imaging technique was employed in this study to analyze the transient needle motion and near-exit flow characteristics. High-energy sub-nanosecond x-ray pulses have a potential to visualize the needle inside the nozzle and near-exit dense supersonic flow which speed reaches over 600 m/s. Transient needle motion and the structure and velocity of near-exit supersonic flows can be obtained by fabricated analysis of the x-ray images regardless of fuel, injection condition and type of injector.

High bulk modulus and viscosity of biodiesel normally slow down the needle movement and decrease the flow performance. During opening-transient, sharp increase and following overshoot in needle speed and exit velocity were observed with a concurrent increase in spray width. The biodiesel showed a slower increase in needle speed, exit velocity and spray width but a higher degree of velocity overshoot during opening-transient. At steady-state, the biodiesel showed less turbulent flow structure and smaller spray width than diesel. During closing-transient, an abrupt increase in needle speed and decrease in exit velocity were observed with a concurrent increase in spray width. The biodiesel showed the retarded start of closing-transient and longer total injection duration under same energizing pulse duration. The difference between biodiesel and diesel became insignificant at low injection pressures roughly below 100 MPa.