Kazuie Nishiwaki

Internal-combustion engine heat transfer

Abstract The introduction to this review outlines the reassons why engine heat transfer is important, but also very complex. As can be understood from the review, the current status of both experimental and theoretical findings leaves much to be done. In the following we will try to briefly summarize the status quo and indicate what we feel needs to be done for future research.

Numerical analysis of heat transfer in heat insulated diesel engines

Abstract The diesel combustion process has been simulated by an axisymmetric CFD computation for analyzing the wall heat transfer at elevated wall temperatures in heat insulated diesel engines and at conventional wall temperatures in normally cooled ones. A new model has been introduced in the computation to describe the reactive thermal boundary layers. The results show that in most cases tested the wall heat fluxes are lower at an elevated wall temperature than at a conventional wall temperature. It has been found, however, that the inclusion of deposits thermal resistance at a conventional wall temperture results in higher heat fluxes at an elevated wall temperature where the deposits are not formed. Besides, higher wall heat fluxes are estimated in heat insulated engines, if there is a large change in the combustion process due to a higher gas temperature.

Combustion and Emissions Characteristics of a Polypropylene Blended Diesel Fuel in a Direct Injection Compression Engine

The authors investigated the formulation, combustion and emissions of polypropylene (PP)–diesel fuel mixtures in a direct injection diesel engine. The fuel has been obtained by an original technology they developed, in which the low or high density polypropylene (LDPP, HDPP), have been mixed in a nitrogen atmosphere at 200 °C, 10–40% by wt. in diesel fuel. The kinematic viscosity of the polypropylene-diesel fuels was investigated between 25–250 °C and the results showed that viscosity of the plastic mixtures is much higher than that of diesel alone, ranging from 10 cSt to 500 cSt, and depending on the plastic structure, content, and temperature. The TGA and DTA analysis has been conducted to investigate the oxidation and combustion properties of pure PP and polymerdiesel fuels. The results showed that at about 125 °C, the LDPP melts, but does not decompose up 240 °C, when the oxidation starts, and has a peak of heat release at 340–350 °C, and the process is completed at 400 °C. The engine’s injection system used, was a piston-barrel type pump, capable of an injection pressure of 200 bars. The injector had 4 × 0.200 mm nozzles with a conical tip needle. The 25% PP-diesel mixture had a successful ignition in a direct injection 110 mm bore, omega combustion chamber engine. The ignition delay for polypropylene-diesel mixtures was longer by about 0.5 ms (at 1200 rpm), compared with diesel. The heat release showed a different development compared with the reference diesel fuel, the premixed phase being inhibited while a slow diffusion combustion phase fully developed. The maximum combustion pressure has been 83 bars for diesel and decreased by 2 bars for the blended fuel, while the bulk gas maximum temperature (calculated) reached about 2500 K for diesel vs 2600 K for polypropylene mixture. The heat flux calculated by the Annand model has shown lower values for diesel fuel with a maximum of about 2.7 MW/m2 compared with 3.0 MW/m2 for PP blended fuel with similar values for convection flux for both fuels at about 1.57 MW/m2 and a higher radiation flux of about 1.44 MW/m2 for PP fuel versus 1.27 MW/m2 for diesel. The heat lost during the cycle shows low values for the premixed combustion stage and increased values for the diffusion stage for both fuels. The exhaust temperatures have been practically identical for both fuels for all loads, with emissions of NOx, and CO reduced by 40% for the alternative fuel, while the CO2 exhibited almost the same values for both fuels. The smoke emissions decreased by 60–90% for the polypropylene blended fuel depending on the load, The engines’ overall efficiency was slightly lower for PP fuel at low loads compared with diesel combustion but at 100% load both reached 36%. The study showed that the new formulation process proposed by the authors is able to produce a new class of fuels from diesel blended with low density polypropylene, and resulted in hybrid fuels with very promising combustion prospects. The engine investigation proved that 25% PP fuels can be injected and burnt in a diesel engine at a residence time of about 5 ms from the start of injection, and the engine’s nominal power could be reached, with lower emissions than reference diesel fuel.Copyright

A diagnostic bi-zonal combustion model for the study of knock in spark-ignition engines

Abstract A bi-zonal model for the combustion process in spark-ignition engines was developed to provide thermodynamic information, using experimental pressure records. The model was applied to the knock analysis, introducing a reduced kinetic mechanism. The unburned gas temperature in an adiabatically compressed core region is shown to be a good approximation for the reaction temperature. The activation energy of the forward rate of the isomerization reaction was determined for each of seven fuels by comparison with the experiments. It is shown that the activation energy has an almost linear dependence on the research octane number.

Reduction of Nitric Oxide in Diesel Exhaust With the Addition of Methylamine

A chemical gas-phase process capable to reduce the nitric oxide in diesel engine exhaust was studied. In this process, monomethylamine (CH 3 NH 2 ) was added to the exhaust gas in a molar ratio to NO varying between 1:1 and 4:1. Experiments were conducted in electrically heated quartz reactors in a temperature range of 200°C to 600°C. Diesel exhaust gas and simulated exhaust gas were used in this experiment. The results showed thorough mixing of methylamine into the exhaust effectively breaks NO down into nitrogen and water, enabling more than 80 percent NO reduction in a reactor temperature range of 400°C to 540°C and at a molar ratio of 1. On the other hand, imperfect mixing between methylamine and exhaust gases results in excess ammonia and reduced NO decomposition. Consequently, it is suggested that the mixing is a very important factor in this technique. The results also show that the coexisting gases such as carbon monoxide, carbon dioxide, hydrocarbons, and water vapor in the diesel exhaust have no effect on NO reduction by methylamine. However, the presence of oxygen in excess of 10 percent in the exhaust is needed for an 80 percent Nox reduction. Furthermore, the mechanisms of the methylamine process were discussed.

A Numerical Prediction Method for the Auto-Ignition Process in a Homogeneous Charge Compression Ignition Engine

An auto-ignition process in a homogeneous charge compression ignition engine has been numerically solved by the Very Large Eddy Simulation (VLES) which integrates a reduced kinetic model for the low temperature oxidationof hydrocarbon fuels. We employ a new method to set the initial turbulent velocity field, reforming the velocity field so that the turbulence dissipation process may fit the results of the K-e model simulation. The phase-averaged quantities of the VLES agree well with those of the K-e model simulation. The VLES exhibits the spatially random appearance of auto-ignition sites, which is similar to experimental observations shown in the reference literature.

Modeling of flame-wall interaction for combustion and heat transfer in S.I. engines

Abstract A numerical combustion model was developed for an engine CFD code in spark ignition engine combustion chambers. The combustion model features the following new concepts: (1) The introduction of preheated reactants which are produced in the turbulent mixing process and consumed in the reaction process; (2) the modification of the probability of burned and unburned gas blobs meeting each other in consideration of a change in turbulence length scale, which becomes smaller in the near wall region. With the first concept, the model deals with turbulence-reaction controlled combustion in the near wall region as well as turbulence-controlled combustion in the core region. The second concept avoids the unrealistic flame shape due to the near wall acceleration of the turbulent flame propagation, which is inherently seen in combustion models based simply on the turbulent mixing time such as the Magnussen model. It is also shown that the present model gives more realistic local heat fluxes.

Simulation of Artificial Turbulence by the Vortex Method

First, a simple and accurate numerical method is presented to produce velocity fluctuations that are determined by the prescribed physical quantities and qualities of turbulence such as longitudinal and lateral spectra, and integral scales. The fluctuations are obtained by solving a system of nonlinear equations that are derived from the equations of energy spectra and of root mean square of the fluctuations. This method requires as many computer memories and computations as one-dimensional case even for the three dimensional calculations. It is shown that there is a strong resemblance of the simulated velocity fluctuations and experimental data. The energy spectra of these velocity fluctuations are quite accurate with less than 0.01% relative errors to the prescribed spectra. Secondly, these solutions are used to examine the capability of the vortex methods to produce turbulent flows with the prescribed parameters. It is found that although the energy spectra by the vortex method scatter to some extent, they are distributed along the prescribed spectra. It can be said that the vortex methods are able to simulate the target turbulence fairly well. Also it is found that the solutions with the LES model increase and deviate from the target spectrum at the higher frequency regions. This may suggest the nonessentiality of the LES model for the vortex method.Copyright

The analysis of amplification and damping of turbulence subjected to a rapid compression : expansion in an engine-like geometry

Abstract The dilatation effect on turbulence kinetic energy was analyzed by experiments and calculations for a rapid compression-expansion machine. Theoretical analysis shows that whether turbulence kinetic energy increases or decreases during compression depends on the turbulence characteristic time at the beginning of compression. The turbulent flows were measured under several different initial strengths of turbulence with a hot-wire anemometer and a LDV. Combined theoretical/experimental analyses validate the theoretical finding.

Determination of Thermal Conductivity and Thermal Diffusivity of Diesel Engine Combustion Chamber Deposits

Thermal conductivity and thermal diffusivity of combustion chamber deposits in a diesel engine have been measured as fundamental data for heat transfer analyses. The deposits were built-up on the surface thermocouples under different engine loads, speeds and locations. After removing the sooted thermocouples from the engine, the thermal properties were determined by the combination of several techniques which were established in a previous study. The results show that thermal diffusivity is 2.0∼3.4×10-6m2/s without clear dependencies on engine loads and locations. It is also shown that thermal conductivity is influenced by the engine loads resulting in 1.50∼1.84W/(m·K) at 80% load and 1.14∼1.41W/(m·K) at 60% load. This fact contrasts with the previous results for s.i. engine deposits whose thermal conductivity does not depend on engine loads. It is also found that that the thermal conductivity of the diesel engine deposits is larger in most cases than that of the s.i. engine deposits which is 1.06∼1.21W/(m·K).