Multipulse Ballistic Injection: A Novel Method for Improving Low Temperature Combustion with Early Injection Timings

Nowadays, increasingly stricter regulations on emission reduction are inducing rapid developments in combustion science. Low-temperature combustion (LTC) is an advanced combustion technology that increases an engine’s thermal efficiency and even provides low emissions of nitrogen oxides (NOx) and particulate matter (PM). The technology often uses early direct injections to achieve sufficient mixture homogeneity. This leads to increasing wall wetting and lower combustion efficiency. This paper introduces the Multipulse ballistic injection (MBI) method to improve combustion with early injection timings. The research was carried out in a four-cylinder medium-duty diesel engine with high-pressure exhaust gas recirculation (HP-EGR). The investigation was divided into two experiments. In the first experiment, MBI was examined without EGR, and in the second, EGR was applied to study its effects. It was found that the MBI strategy decreased wall wetting and increased homogeneity and the indicated mean effective pressure (IMEP) at early injection angles.

Experimental research on the wall-wetting effects on the frictional property of the cylinder liner–piston ring pair

The application of novel injection strategies (high-pressure injection, early injection, retarded injection, etc.) in combustion engines has made the wall-wetting problem severer. As the splashed fuel dilutes the lubricating oil, the tribological performance of the cylinder liner–piston ring pair will be affected. In this research, the viscosity and wettability tests were conducted firstly by mixing diesel into lubrication oil. It was found that the dynamic viscosity of the mixture drops with more fuel diluting the oil, and a small quantity of diesel mixed will cause a remarkable decline in lubricant viscosity; also, the contact angle shows a downward trend with the increasing diluting ratio. Then based on several typical diluting ratios, the reciprocating friction tests were carried out to measure the instantaneous friction force of the production ring/liner pair. The experimental results showed that under a mixed lubrication state, the peak friction force of the ring/liner pair occurs around the dead centers, while the minimum force occurs at the middle position of the reciprocating stroke; with more fuel diluting the oil, the bearing capacity of oil film degrades, resulting in the increase of friction force. In addition, the average friction coefficient of the ring/liner pair shows an upward trend with the increasing diluting ratio, and the Stribeck curve moves toward the upper-left, which means the lubrication condition of this pair tends to transit from mixed lubrication to boundary lubrication, causing negative effects on the frictional property of the cylinder liner–piston ring pair. Therefore, the diluting ratio should be controlled under 20%.

Numerical simulation of fuel dribbling and nozzle wall wetting

The present work describes a numerical methodology and its experimental validation of the flow development inside and outside of the orifices during a pilot injection, dwelt time and the subsequent start of injection cycle. The compressible Navier-Stokes equations are numerically solved in a six-hole injector imposing realistic conditions of the needle valve movement and considering in addition a time-dependent eccentric motion. The valve motion is simulated using the immersed boundary method; this allows for simulations to be performed at zero lift during the dwelt time between successive injections, where the needle remains closed. Moreover, the numerical model utilises a fully compressible two-phase (liquid, vapour) two-component (fuel, air) barotropic model. The air’s motion is simulated with an additional transport equation coupled with the VOF interface capturing method able to resolve the near-nozzle atomisation and the resulting impact of the injected liquid on the oleophilic nozzle wall surfaces. The eccentric needle motion is found to be responsible for the formation of strong swirling flows inside the orifices, which not only contributes to the breakup of the injected liquid jet into ligaments but also to their backwards motion towards the external wall surface of the injector. Model predictions suggest that such nozzle wall wetting phenomena are more pronounced during the closing period of the valve and the re-opening of the nozzle, due to the residual gases trapped inside the nozzle, and which contribute to the poor atomisation of the injected fluid upon re-opening of the needle valve in subsequent injection events.

Influence of wall-wetting conditions on in-flame and exhaust soot structures in a spark ignition direct injection petrol engine

Great attention to the efficiency benefits of spark ignition direct injection engine has been averted due to its problematic particulate emissions. In the present study, the fundamental knowledge of wall-wetting-induced spark ignition direct injection soot particles is enhanced through direct particle sampling from pool fire on the piston top surface and cylinder liner as well as from the exhaust stream. The sampled soot particles are imaged using transmission electron microscope, and the image post-processing for statistical morphology and internal structure analysis is performed to better understand the soot formation and oxidation processes. The experiments were performed in a single-cylinder optical spark ignition direct injection engine where diffusion flame luminosity was recorded using a high-speed camera through the cylinder liner window, with which the overall sooting level was understood, and the pool fire location was identified. Given the in-flame soot sampling experiments in the spark ignition direct injection engine were new, error analysis was conducted in terms of the number of fuel injections and engine run-to-run variations. This sampling technique then was applied for various injection timings in the intake stroke. The data analysis and physical interpretation was focused on a piston-wetting condition at the most advanced injection timing of 320 °CA bTDC and a liner-wetting condition at the most retarded injection timing of 180 °CA bTDC in the present study. Between these two different wall-wetting conditions, it was found that the piston-wetting condition has larger soot primary particles and soot aggregates. The internal carbon-layer fringe shows longer length, less tortuosity and smaller gap, indicating more mature and carbonised soot. This was consistent with more significant and wider distributed pool fire and thus longer soot residence time within the flames. When the exhaust soot particles were analysed, however, it was found that the reduction in soot aggregate size was much higher and the carbonisation was more progressed for the piston-wetting condition than those of the liner-wetting condition. This suggested higher soot oxidation later in the expansion/exhaust stroke for the piston-wetting condition, which potentially can be better utilised for engine applications.