Zhijun Peng

Understanding of controlled autoignition combustion in a four-stroke gasoline engine

Abstract Controlled autoignition (CAI) combustion has recently emerged as a viable alternative combustion process to the conventional spark ignition or compression ignition process for internal combustion engines, owing to its potential for high efficiency and extremely low NOx and particulate emissions. Since CAI combustion is a process dominated by chemical kinetics of the fuel-air mixture, an engine simulation model with detailed chemical kinetics has been developed and applied to a four-stroke gasoline engine fuelled with isooctane. After calibration and validation, the engine simulation model was used to study the effects of the intake temperature, exhaust gas recirculation (EGR), the air-fuel ratio, the compression ratio and the engine speed on CAI combustion in a four-stroke gasoline engine. The characteristics of CAI combustion investigated include the autoignition timing, the partial burning and knocking combustion and NO emission. Results show that CAI combustion could be achieved within a limited speed and load range. The lower end of the CAI combustion range was affected by partial burning, and the higher end of its operation was limited by knocking combustion. Among the engine parameters investigated, the intake charge temperature and EGR had the greatest effect on the CAI combustion process. The effect of EGR was further analysed in terms of its thermal (increase in heat capacity), dilution, chemical and charge heating effects by means of a series of simulation studies. It was found that the charge heating effect caused advanced ignition timing, faster heat release rate and moderate reduction in the CAI combustion duration. The thermal effect (increased heat capacity) retarded ignition, extended combustion duration and slowed down heat release rate. The dilution effect also resulted in longer combustion duration and slower burning but it did not affect the ignition timing. The chemical effect was found to accelerate the combustion process when the percentage of EGR was large.

Particle image velocimetry measurement of in-cylinder flow in internal combustion engines : Experiment and flow structure analysis

Abstract A cross-correlation digital particle image velocimetry (PIV) system has been developed and applied to study the in-cylinder flow in a single-cylinder engine with a production-type cylinder head. The PIV system set-up and its optimization are described in the first part of the paper. Two-dimensional velocity distributions measured over 100 cycles are analysed to obtain ensemble-averaged mean and fluctuating velocities, the turbulent length scale, vorticity and strain rate distribution in the measurement plane. In particular, a spatial filtering scheme is developed in order to obtain the cycle-resolved velocity measurements. The cycle-resolved analysis shows that the low-frequency velocity fluctuation component (i.e. cyclic variation) is mainly responsible for the spatial variation in velocity and turbulent kinetic energy distributions. The integral length scale calculated from the PIV data is between 6 and 10 mm and the strain rate is estimated to be within 1000 s-1 in most areas of the measurement plane.

Numerical investigation of soot reduction potentials with diesel homogeneous charge compression ignition combustion by an improved phenomenological soot model

Abstract An improved phenomenological soot model coupled with a reduced n-heptane chemical mechanism was implemented into KIVA-3V code to describe soot formation and oxidation processes in diesel homogeneous charge compression ignition (HCCI) combustion. This model was first validated by the shock tube experiments with a rich n-heptane mixture over wide temperature and pressure ranges. The computational results demonstrate that the phenomenological soot model is capable of predicting the soot yield, particle diameter, and number density with satisfactory accuracy. Then the model was applied to investigate the influence of the orifice diameter and injection pressure on soot emissions in a constant-volume combustion vessel under typical diesel combustion conditions. The predictions showed qualitative agreement with the measurements on the soot volume fraction distribution. The results also indicate that the soot formation can almost be suppressed as the local equivalence ratio is kept lower than 2.0. Finally, the model was used to explore the potentials of soot reduction with HCCI combustion for diesel engines. The overall trend of soot with the variations in the start of injection timing was well reproduced by the model. With the help of an equivalence ratio—temperature map, it was found that nitrogen oxide emissions could be markedly reduced by applying a high exhaust gas recirculation rate and relative low compression ratio for diesel HCCI engines. However, the mixture preparation by using a multi-hole injector with early injection strategy remains a limitation for further reduction in soot emissions.

CHARACTERISTICS OF HOMOGENEOUS CHARGE COMPRESSION IGNITION (HCCI) COMBUSTION AND EMISSIONS OF n-HEPTANE

ABSTRACT This paper reports the outcome from a systematic investigation carried out on HCCI (Homogeneous Charge Compression Ignition) combustion of a diesel type fuel. The n-heptane was chosen in this study to study the HCCI combustion characteristics of diesel engines with premixed charge by port fuel injection. Measurements were carried out in a single-cylinder, 4-stroke and variable compression ratio engine. Premixed n-heptane/air/EGR mixture was introduced into the cylinder by a port fuel injector and an external EGR system. The operating regions with regard to Air/Fuel ratio (A/F) and EGR rate were established for different compression ratios and intake temperatures. The effects of compression ratios, intake temperatures, A/F and EGR rates on knock limit, auto-ignition timing, combustion rate, IMEP and engine-out emissions, such as NOx, CO, and unburned HC, were analysed. The results have shown HCCI combustion of n-heptane could be implemented without intake charge heating with a typical diesel engine compression ratio. The attainable HCCI operating region was mainly limited by the knock limit, misfire, and low IMEP respectively. Higher intake temperature or compression ratio could extend the misfire limit of the HCCI operation at low load area but they would reduce the maximum IMEP limit at higher load conditions. Compared with conventional diesel combustion, HCCI combustion with diesel type fuels would lead to extremely low NOx emissions (less than 5 ppm) and smoke free exhaust, but would produce higher HC and CO emissions. An increase in intake temperature or compression ratio helped to reduce HC and CO emissions.

Effects of Air/Fuel Ratios and EGR Rates on HCCI Combustion of n-heptane, a Diesel Type Fuel

The effects of Air/Fuel (A/F) ratios and Exhaust Gas Re-Circulation (EGR) rates on Homogeneous Charge Compression Ignition (HCCI) combustion of n-heptane have been experimentally investigated. The experiments were carried out in a single-cylinder, 4-stroke and variable compression-ratio engine equipped with a port fuel injector. Investigations concentrate on the HCCI combustion of n-heptane at different A/F ratios, EGR rates and their effects on knock limit, engine load, combustion variability, and engine-out emissions such as NOx, CO, and unburned HC. Variations of auto-ignition timings and combustion durations in the two-stage combustion process are analyzed in detail. Results show that HCCI combustion with a diesel type fuel can be implemented at room temperature with a conventional diesel engine compression-ratio. However, its knock limit occurs at very high A/F ratios, although high EGR rates can be tolerated. It was also found that auto-ignition timings or start time of both the low- and high-temperature combustion stages of diesel HCCI combustion are very sensitive to EGR rates. But combustion durations dominantly depend on the A/F ratios. Since the combustion temperature is very low, NOx emissions is at near zero ppm level among all attainable operating regions. However, HC and CO emissions are high and increase with incomplete combustion caused by misfire. Copyright

Analysis of Tumble and Swirl Motions in a Four-Valve SI Engine

Tumble and swirl motions in the cylinder of a four-valve SI engine with production type cylinder head were investigated using a cross-correlation digital Particle Image Velocimetry (PIV). Tumble motion was measured on the vertical symmetric plane of the combustion chamber. Swirl motion was measured on a plane parallel to the piston crown with one of intake ports blocked. Large-scale flow behaviours and their cyclic variations were analysed from the measured two-dimensional velocity data. Results show that swirl motion is generated at the end of the intake stroke and persists to the end of the compression stroke. Tumble vortex is produced in the early stage of the compression stroke and distorted in the late stage of the stroke. The cyclic variation of swirl motion is noticeable. The cyclic variation in tumble dominated flow field is much greater. Distribution of two-dimensional fluctuation kinetic energy field for swirl motion gradually becomes homogeneous in the late stage of the compression process, while for tumble motion the distribution of velocity fluctuation field is inhomogeneous during the whole compression stroke. Copyright

Evaluation of Spray/Wall Interaction Models under the Conditions Related to Diesel HCCI Engines

Diesel homogeneous charge compression ignition (HCCI) engines with early injection can result in significant spray/wall impingement which seriously affects the fuel efficiency and emissions. In this paper, the spray/wall interaction models which are available in the literatures are reviewed, and the characteristics of modeling including spray impingement regime, splash threshold, mass fraction, size and velocity of the second droplets are summarized. Then three well developed spray/wall interaction models, ORourke and Amsden (OA) model, Bai and Gosman (BG) model and Han, Xu and Trigui (HXT) model, are implemented into KIVA-3V code, and validated by the experimental data from recent literatures under the conditions related to diesel HCCI engines. By comparing the spray pattern, droplet mass, size and velocity after the impingement, the thickness of the wall film and vapor distribution with the experimental data, the performance of these three models are evaluated. The results indicated that the predicted mean droplet diameters by HXT model are in better agreements with measurements due to the consideration of the gas density. However, the film thickness and fuel vapor distribution near the wall region are not significantly affected by the spray/wall interaction models, and all the models present inaccurate predictions relative to the experimental results.

Effect of recycled burned gases on homogeneous charge compression ignition combustion

ABSTRACT Homogeneous Charge Compression Ignition (HCCI) also known as Controlled Auto-Ignition (CAI) combustion, has recently emerged as a viable alternative combustion process to the conventional spark ignition (SI) gasoline and compression ignition (CI) Diesel engines, due to its potential for extremely low emissions and good fuel economy. One of the most important means of achieving and controlling HCCI combustion is to use recycle burned gases (EGR). In order to understand better the effects of recycled burned gases on such combustion process, detailed analytical studies were performed. Since HCCI combustion is a process dominated by chemical kinetics of the fuel-air mixture, an engine simulation model with detailed chemical kinetics has been developed and applied to a four-stroke gasoline engine fuelled with isooctane. After calibration and validation, the engine simulation mode was used to investigate the effects of EGR on HCCI combustion in a four-stroke gasoline engine. The characteristics of HCCI combustion investigated include the autoignition timing, the partial burning and knocking combustion, and NO emission. The heat capacity, dilution, chemical, and charge heating effects on the HCCI combustion process were studied individually by means of a series of analytical studies. When isothermal EGR is used, such as in HCCI combustion with diesel type fuels, the heat capacity has the largest effect on extending the combustion duration and slowing down the rate of the combustion. The larger heat capacity of EGR gases tends to retard the start of autoignition. The dilution of oxygen by EGR has little effect on autoignition but it has a similar effect on extending the combustion duration to that of the dilution effect. The dilution effect of EGR on lowing heat release rate is only noticeable at high concentrations. Finally, the chemical effect of CO2 and H2O was found to be negligible. When hot EGR is used in HCCI combustion, the charge heating effect of EGR has much greater effect on ignition timing and some less significant effect on the combustion duration and heat release rate than the other effects.

Tumbling flow analysis in a four-valve spark ignition engine using particle image velocimetry

Abstract Tumble motion in the cylinder of a four-valve spark ignition (SI) engine with a production-type cylinder head was studied using cross-correlation digital particle image velocimetry (PIV). The in-cylinder flow field was measured on three planes: the vertical symmetric plane of the combustion chamber, the vertical plane through centres of the intake and exhaust valves, and a horizontal plane 12 mm below the cylinder head. Ensemble-averaged mean velocity, velocity fluctuation distribution and cyclic variation of the instantaneous velocity field were analysed. Analysis results show that the tumble vortex is formed in the early stage of the compression stroke and distorted in the late stage of the compression stroke. The tumble centre is nearly in the centre of the cylinder when the tumble forms. Then it moves gradually to the underneath of the exhaust valves as the piston moves up. It is found that the cyclic variation of the tumble motion at a tumble ratio of 0.9 is so great that the ensemble-averaged flow characteristics hardly represent any individual cycle flow behaviours. Distribution of the velocity fluctuation field is inhomogeneous during the whole compression process. As the engine speed changes the large-scale flow structure seems to remain unaffected.

In-cylinder air motion characteristics with variable valve lift in a spark ignition engine. Part 1: swirl flow

While variable valve actuation or variable valve lift (VVL) is used increasingly in spark ignition (SI) engines to improve the volumetric efficiency or to reduce the pumping losses, it is necessary to understand the impact of variable valve lift and timing on the in-cylinder gas motions and mixing processes. In this paper, the in-cylinder flow characteristics for reduced maximum valve lifts (MVLs) were examined in a modified four-valve optical SI test engine in which the combustion system is suitable for gasoline direct-injection (GDI) operations. Three different MVLs of 6.8 mm, 4.0 mm, and 1.7 mm were tested and particle image velocimetry was employed for the measurements. It is expected that the investigation will be helpful in understanding and improving GDI combustion when a VVL system is used. The results showed that a reduced MVL could significantly enhance the in-cylinder swirl motion in all three measured horizontal planes. By comparing the swirl ratios for different MVLs, it can be found that a reduced MLV can considerably increase the swirl ratio during the intake and compression processes. In general, the swirl centre induced by a lower MVL is more stable and can stay closer to the cylinder centre. In addition, the reduction in the MVL can also increase both the high-frequency fluctuating kinetic energy and the low-frequency fluctuating kinetic energy. These may contribute to an improvement in the air-fuel mixing but also to an increase in the cycle-to-cycle variation.