Xiaobei Cheng

Investigation to Charge Cooling Effect of Evaporation of Ethanol Fuel Directly Injected in a Gasoline Port Injection Engine

ABSTRACT Ethanol direct injection plus gasoline port injection (EDI+GPI) is a new technology to make the use of ethanol fuel more effective and efficient in spark ignition engines. It takes the advantages of ethanol fuel, such as its greater latent heat of vaporization than that of gasoline fuel, to enhance the charge cooling effect and consequently to increase the compression ratio and improve the engine thermal efficiency. Experimental investigation has shown improvement in the performance of a single cylinder spark ignition engine equipped with EDI+GPI. It was inferred that the charge cooling enhanced by EDI played an important role. To investigate it, a CFD model has been developed for the experimentally tested engine. The Eulerian-Lagrangian approach and Discrete Droplet Model were used to model the evolution of the fuel sprays. The model was verified by comparing the numerical and experimental results of cylinder pressure during the intake and compression strokes. Mesh density and time step sensitivities have been tested. The verified model was used to investigate the charge cooling effect of EDI in terms of spatial and temporal distributions of cylinder temperature and fuel vapor fraction. Compared with GPI only, EDI+GPI demonstrated stronger effect on charge cooling by decreased in-cylinder temperature. The cooling effect was limited by the low evaporation rate of the ethanol fuel due to its lower saturation vapor pressure than gasolines in low temperature conditions.

Investigation of the combustion and emission characteristics of partially premixed compression ignition in a heavy-duty diesel engine

Partially premixed compression ignition in a diesel engine is a combustion mode between diffusion combustion and homogeneous charge compression ignition combustion, the combustion controllability and the emission performances of which are close to those of a homogeneous charge compression ignition combustion engine; the mixing period plays a key role in realizing partially premixed compression ignition, and a long premixed process before combustion is necessary to make the fuel and air mix well. Partially premixed compression ignition combustion in a boosted six-cylinder heavy-duty diesel engine is realized by adjusting the injection timing and the rate of exhaust gas recirculation based on a single injection. The effects of the injection timing, the exhaust gas recirculation rate and the load rate on the combustion and the heat release pattern are studied. The factors which influence the mixing period and the ignition delay and its regularity are researched. Endoscope technology and the two-colour method are also used to gain an insight into partially premixed compression ignition combustion. The results show that both early injection and late injection have a long mixing period, which helps to form a more homogeneous mixture, and no diffusion combustion is found in the heat release rate curves. Premixed combustion and low-temperature combustion are the key factors in reducing the particulate matter emissions and nitrogen oxide emissions simultaneously. However, the low-temperature combustion and the dilute mixture may lead to incomplete combustion; consequently, for relatively late-injection conditions, the hydrocarbon and carbon monoxide emissions increase dramatically and the fuel consumption becomes worse. In these partially premixed combustion patterns, the effect of the injection pressure on the particulate matter and nitrogen oxide emissions is not clearly observable. At lower load rates, partially premixed compression ignition combustion displays a low-temperature and a high-temperature two-stage heat release. When the engine load rate is increased to 50%, diffusion combustion appears in the early-injection modes, which leads to higher nitrogen oxide and particulate matter emissions.

Tests and Numerical Simulations on the Thermal Load of the Cylinder Head in Heavy-Duty Vehicle Diesel Engines

The paper has explored the solutions to the thermal overload in the cylinder head of a heavy-duty vehicle 6-cylinder diesel engine and the thermal cracks in the valve-bridge of the engine. The experiments include measuring the temperature of the cylinder head bottom and testing the flow distribution of coolant through the upper nozzles of cylinder head bottom. The follow-up analysis was conducted on the causes of the excessive thermal load of the cylinder head bottom, the thermal cracks in the valve-bridge region, and the rationality of the structure of the water jacket for the cylinder head. The mechanism of the water jacket of cylinder head was further inquired. Then 3-D CFD numerical simulation of water jacket in the sixth cylinder, which is in the worst cooling condition, is performed. To enhance the flow form in water jacket and lower the cost of enhancement, we proposed 4 schemes of water jacket and conducted the numerical simulations to these schemes. It was identified that all these schemes have efficiently improved the flow field in water jacket. In the typical proposed scheme 1 in which 6 nozzles of all the 10 upper nozzles were blocked, the coolant flow rate on the bottom of the water jacket and in the cylinder head valve-bridge region increased by about 68.73%. The measuring results of the cylinder head bottom temperature show that the maximum temperature in the valve-bridge region of cylinder head is reduced by 9.2 °C and the temperature gradient reduction is 19.55 percent, suggesting that the thermal load and thermal stress of the studied diesel engine cylinder head has been significantly lowered.Copyright

Study of Particle Size Distribution Emitted From a Diesel Engine by Diesel and Biodiesel-Diesel Blends Fuels

The effect of diesel and biodiesel blends on particle size and number concentration distributions were studied in diesel engine under different operating conditions, including speeds, loads, and injection timing. The results showed that the engine load was more influential on particle size distribution than the engine speed. At the high load, diesel fuels produced mainly accumulation-mode particles, and at medium or lower load, diesel fuel produced more nuclei-mode particles. The injection timing had obviously influence on particle size distribution and number concentration. Advanced injection timing induced higher number concentrations of nuclei-mode particles for the low load and more accumulation mode particles for high load. Compared to the neat diesel fuel, the combustion process was improved when fueled with diesel-biodiesel blends. The oxygen contained in the biodiesel fuel may improve combustion. The number and mass concentration of PM was greatly decreased with the increase in biodiesel blend ratio. Biodiesel blends had an early start of injection, and particle size distributions tended to be ultra-fine particles with the increase in the ratio of biodiesel blend. The average mid-diameter range of particles was significantly affected by the change in fuel injection timing.Copyright