نشریه تحقیقات موتور
پیاپی 32 (پاییز 1392)

One of the suggested methods, to overcome drawbacks of HCCI engines, is using of the variable valve timing technology (the VVT system). The VVT system can affect HCCI engines characteristics due to the EGR's main role in controlling the HCCI combustion. The auto-ignition in HCCI engines can be facilitated by adjusting the timing system of exhaust-valve-closing and, to some extent, the timing system of intake-valve-opening in order to capture a proportion of hot exhaust gases in the engine cylinder during the gas exchange process. In the present paper, a comparative analysis is performed to investigate the effect of the VVT system on the combustion, the performance and the emission characteristic of HCCI engines. For this purpose, a multi-zone model was developed and coupled with a gas exchange model. Results were compared to experimental results, obtained from a single-cylinder engine. Finally, it can be concluded that if the duration of opening of the intake and exhaust valves were kept constant, positive-valve-overlapping was an reliable strategy for the HCCI combustion as it could improve combustion characteristics such as the IMEP, the work and engines efficiencies and reduce exhaust emissions.

In recent years, HCCI engines have been in the center of attentions due to their high thermal efficiency, low emission rate (e.g. NOx) and low fuel consumption. The main drawback in these engines is the combustion control, which happens in a limited operation range between the knock and misfiring. The natural gas is a commonly used fuel in internal combustion engines (ICE). However, using the natural gas in HCCI engines is challenging due to its low tendency to the self-ignition. Hence, approaches like the reformer gas enrichment should be employed. In this paper, a zero-dimensional model was introduced in order to predict combustion timing and investigate effects of the variable composition reformer gas with different contents of H2 and CO. Experimental results from a single-cylinder CFR engine was used to validate the proposed model. Results showed that the addition of synthetic gas (RG) and changes in the H2 content of RG could affect ignition timing and could be used in the combustion control in HCCI engines, which expanded the limit of the operation, increased the engine efficiency and reduced the fuel consumption especially in lower air/fuel ratio and the inlet temperature.

In this study, the process of producing Ruston RK215 diesel engine rings is investigated by using the hot rolling process. In order to insure results, simulation results, concluded by the ANSYS workbench software, have been compared to experimental results. Details of the analysis and tensile tests were obtained and used in the simulation process. The hot rolling process was applied in order to create the necessary residual stress to provide the spring back force, which was required for the installation in the engine. Results of physical tests on produced parts, confirmed the accuracy of the residual stress as well as the created necessary pressure required for the spring force, when the cylinder was aligned.

Crankshaft is an important engine component that is working under high forces and stresses. Crankshaft loading consists of the moment inertia of the rotary mass, the moment inertia of the reciprocating mass and the combustion force that cause bending and torsional stresses in crankshaft. Bending and torsional stresses in crankshaft lead to fatigue loading. In this research, for a diesel engine, stresses in crankshaft were calculated by finite element method. And then, in order to validate these results, strain gauges were installed to measure strains and stresses. According to finite element results, the maximum value of the Von-Misses stress equaled to 580 MPa. In addition, the stress was about 8 MPa, where the strain gauge was installed. Data of the strain gauge measurement resulted in stresses less than 8 MPa. This result showed a good agreement with finite element model results. The maximum error between observed values and the finite element analysis was not more than 3.5%.

The homogenous charge compression ignition (HCCI) is a promising concept for internal combustion engines to reduce the fuel consumption and exhaust emissions. The main target of the current study is developing a multi-zone thermo-kinetic model for HCCI engines in order to investigate the engine operation in different conditions and effects of using the DME-methane blended fuel on combustion characteristics. The developed 13-zone model included detailed chemical kinetics of DME and methane combustions and clearly showed the two-stage heat release of DME. Comparisons to experimental data showed the precision of the model in predicting SOC and the pressure trend. Five different compositions, varied from the pure DME to the pure methane, were selected to study effects of the blended fuel composition. Results showed that in an initial condition that the pure methane would not ignite, adding DME would lead to a combustion phase. Changing the amount of added DME do not change the SOC distinctly, but strongly affected the engine output power.

In recent years, natural gas and gasoline blended fuels have been considered as alternative fuels in spark ignition (SI) engines. In order to utilize their potential benefits, a precise identification of those alternative fuels is attractive and inevitable studies to predict their performance in SI engines. In this study, the laminar flame speed of natural gas and gasoline blended fuels in the constant volume chamber at different equivalence ratios of 0.8 to 1.2, via the Schlieren optical method was measured and the un-stretched laminar flame speed by following the theory of the weekly stretched flame was obtained. Direct injectors, were installed at the top of the chamber and were used to inject liquid and gaseous fuels. To ensure that the liquid fuel would evaporate completely, the chamber wall was heated with four band heaters and liquid injection was done in the vacuumed chamber. The result showed that adding the natural gas to the gasoline would increase its flame speed at low equivalence ratio, from 0.8 to 0.95. Adding the natural gas to the gasoline in high equivalence ratio caused the flame instability that originated from cracking and the cellularity of the flame surface occurred at higher equivalence ratio.