Yoshihiro Nomura

Analysis of cycle-by-cycle variation in a direct injection gasoline engine using a laser-induced fluorescence technique

Abstract To analyse the cycle-by-cycle variation of combustion in a direct injection gasoline engine equipped with a fan-shape spray nozzle and operated with exhaust gas recirculation (EGR), the fuel mixture distribution was measured at a time of spark and during the combustion period by the laser-induced fluorescence (LIF) technique. It was found that in the case of advanced or retarded injection timing, the initial combustion period tends to extend and the indicated mean effective pressure (i.m.e.p.) becomes low when lean mixtures appear at the spark position and at the spark timing. This suggests that the cycle-by-cycle variation of combustion under these conditions is dominated by the fuel concentration at the spark position and spark timing. In contrast to this, for the best injection timing, which allows the lowest cycle-by-cycle variation, the i.m.e.p. fluctuation is affected not by the initial combustion period but by the main combustion period. The observation of LIF images revealed that the i.m.e.p. fluctuation at this condition is strongly correlated to the unburned mixture quantity at the side area of the piston cavity during the latter half of the combustion period. It was shown by a computational fluid dynamics (CFD) calculation that the combination of a uniform spray pattern and a compact cavity shape is effective to reduce the over-lean mixture region in the edge of the piston cavity, which is responsible for the cycle-by-cycle variation of combustion at the condition of best-tuned injection timing.

Evaporation characteristics of fuel spray and low emissions in a lean premixed-prevaporization combustor for a 100 kW automotive ceramic gas turbine

A lean premixed-prevaporization combustor (PPL-1) for a 100 kW automotive ceramic gas turbine has been developed to meet the Japanese emission standards for passenger cars without using an aftertreatment system. The design of a fuel injector and a prevaporization-premixing tube (PP-tube) in the PPL-I combustor is a key subject for promotion of evaporation of fuel spray. The Sauter mean diameter and the non-evaporated mass fraction of fuel spray in the PP-tube were measured employing a phase Doppler particle analyzer varying inlet air temperature, air pressure, air velocity and swirl number. The evaporation of the fuel spray in the PP-tube was promoted by higher swirl number, air velocity, air temperature and increased atomization of the fuel injector, but was suppressed by higher air pressure and fuel properties such as distillation in high temperature. The characteristics of NO x , CO and HC emissions were measured with a combustor test rig and discussed influences of the evaporation characteristics of fuel spray. Results show that a mass fraction of non-evaporated fuel of less than 10% and lean fuel-air mixtures, i.e. having equivalence ratios from 0.15 to 0.5, reduce the PPL-I combustor emissions of NO x and CO.

Modeling of Wall Impinging Behavior with a Fan Shaped Spray

The experiment-based droplet impinging breakup model was applied to a fan shaped spray and the impinging behavior was analyzed quantitatively. Evaluation of the quantitative results with validation tests verified the following. The model enables prediction of fan shaped spray thickness after Impingement caused by the breakup of fuel droplets, which could not be represented with the Wall-Jet model, widely used at present. Fuel film movement on a wall is negligible when the injection pressure of the fan shaped spray is high and the spray travelling length is not too short. The proposed heat transfer coefficient between fuel film and the wall is too small to represent the vaporizing rate of the fuel film.

A fractal-based flame propagation model for large eddy simulation

A novel combustion model for large-eddy simulation (LES) for gasoline engines has been developed. Unlike conventional models based on Reynolds-averaged Navier–Stokes (RANS) models, the new model takes a unique approach; it is described by the fractal characteristics of flame front and a universal expression for the subgrid scale (SGS) flame speed. The present fractal combustion model was applied to calculations of a spark ignition engine. Both the 0–10 per cent and 10–90 per cent combustion periods agree well with the experimental data. Because the modelling of the SGS turbulent speed is based on fractal analysis with experimental observations, the SGS combustion model is able to apply a wide range of engine operating conditions. The present model was applied to a multi-cycle simulation of a single-cylinder engine. The fluctuations at the instant when the heat release rate peaked were compared with data that was obtained experimentally. The calculated magnitude of the fluctuations was found to be close to the experimental values. It is thought that the flow variation generated during the intake stroke significantly influences the cyclic variations.

A quasi-theoretical predictive 0D combustion model for 1D gasoline engine simulation

Fig. 1 shows a typical development process with CFD. For the left bank in this figure, an accurate predictive model, which does not rely on specific measured data, is required. 3D-CFD can be used for the prediction (1), however, most conventional (0D) combustion models (2) for 1D-CFD usually require calibration process for each engine. Although the calibrated model is applicable to the similar engine type, there is a limit to different new engines.

Numerical Simulation of Combustion Processes in Homogeneous and Stratified Charge Spark Ignition Engines

A three-dimensional simulation technique for stratified combustion process in direct injection gasoline engines is developed. The laminar flame speed for wide range of mixture equivalence ratio and EGR condition is modeled taking into account the reference temperature intermediate between unburned and flame temperature for chemical reaction. This new laminar flame speed model and the coherent flame model are incorporated into a CFD code. The calculated flame propagation process, heat release rate and exhaust emissions are validated by measurements including LIF technique. The good agreement obtained for various conditions shows the availability of this method.

Ignition and Exhaust Emission Characteristics of Spray Combustion in a Pre-Chamber Type Vortex Combustor

The lean ignition limit, the lean blowout limit and the exhaust emission characteristics of spray combustion have been investigated experimentally using a pre-chamber type vortex combustor developed for a 300KW large-bus gas turbine engine. It has been verified that these depend on the spray characteristics of the fuel injector and the air flow pattern or the distribution of air in the chamber.Ignition succeeds through three processes. The first step is the formation of a flame kernel near the sparking ignitor, the second step is the propagation of the flame kernel into a flame holding region, and the last step is the formation of a rotating flame in that region. The lean blowout limit of the rotating flame depends on the air flow pattern in the pre-chamber when the air temperature in the combustor inlet is under 470K, while a constant fuel-air ratio of less than 0.001 is maintained at 470K and above. With no or a little secondary air, the NOx emission index does not increase in proportion to the fuel-air ratio, because both the gas temperature and residence time decrease due to the radiative heat loss caused by soot formation and reduction of a recirculation region in the main-chamber.These phenomena were evaluated with 3 dimensional numerical simulations taking account of spray combustion, soot formation, the extended-Zeldovich thermal NO formation and radiative heat loss.Copyright

Catalytic combustion with practical liquid fuel.

The characteristics of catalytic combustion below 1000°C with gas oil and kerosene were measured using practical catalysts, Pd-Al2O3 or Pt-Al2O3. A one-dimensional steady-state combustion model was developed to predict the temperature distribution in the catalyst. The following results were obtained. Pre-heating of the mixture up to about 300°C was required to start the catalytic combustion. The fuel oxidation was controlled by the fuel diffusion from the mixture to the catalyst surface below about 700°C; the combustion efficiency was determined to be a function of the non dimensional number, Re·Sc·d/l. The gas-phase reactions were activated and combustion efficiency was increased at the burnt gas temperature higher than 700°C. The developed model was capable of predicting the combustion efficiency, the temperatures of the burnt gas and the catalyst.