Rio Shimizu

Effects of High Turbulence Flow on Knock Characteristics

In enhancing the performance of automotive internal combustion engines, increasing the compression ratio offers an effective means of improving engine thermal efficiency. If the compression ratio is increased, however, the problem of knock occurs in exchange for improvement in engine thermal efficiency. In other words, an increase in compression ratio causes in-cylinder compressive end gas temperature to rise, resulting in the occurrence of knock. This in turn requires ignition timing retard to combat the knock. This trade-off makes it difficult to achieve the theoretical maximum combustion efficiency. In this paper, we clarify the feasibility of suppressing the occurrence of knock by increasing the burn rate. Specifically, we increase the burn rate by injecting high-pressure air directly into the combustion chamber, causing highly turbulent in-cylinder flow. To optimize the generation of this turbulent flow, we examined the following parameters of direct high pressure air injection (DHPAI): injection timing and the arrangement and quantity of injector nozzles. After modifying a commercially available, 2.0-liter 4-cylinder gasoline engine, we made performance comparisons at full load between running with and without DHPAI. The actual ignition timing advance comes close to MBT (minimum advance for best torque) by selectively improving the burn rate in the later stages of the combustion period. As a result, the output torque is increased 10% in the low and medium engine speed conditions.

An investigation for reducing combustion instability under cold-start condition of a direct injection gasoline engine

In order to meet recent stringent emission regulations, the exhaust catalyst should be heated as early as possible to activate the purifying reactions. In a direct injection spark-ignition engine, a combination of late fuel injection during the compression stroke and late ignition in the expansion stroke is a common strategy to quickly raise exhaust gas temperature for subsequent rapid activation of exhaust catalysts. However, this approach under cold start-up of an engine often results in incomplete and unstable combustion. In this study, to explore the conditions of stable ignition and combustion, the effect of injection timing on indicated mean effective pressure and early combustion duration (MBD0.5) are first investigated by an analysis of the pressure indicator diagram. As this analysis shows a strong correlation between indicated mean effective pressure and MBD0.5, the mechanism of initial flame propagation is investigated intensively using optical diagnostics. Namely, mean equivalence ratio of mixtures in the propagating flame front is measured by focusing on the ratio of C2* to CH* emission intensities. The flow velocity and turbulence intensity around the spark electrode are measured by the back-scattering laser Doppler anemometry. Two major conclusions are derived from this study: First, when the injection timing is retarded, the mean equivalence ratio increases as the time for the injected fuel to travel and diffuse is shortened. The most preferable mean equivalence ratio for fast initial combustion is found to lie in a range from 1.2 to 1.4. Second, when the second injection timing is retarded further, the mean equivalence ratio increases exceeding 1.4, and this results in slower and more fluctuated initial flame propagation. But, if the turbulent intensity is increased by means of the spray induced air motion, the slowed initial combustion can be recovered.

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.

Development of CFD Method for Spray Shape Estimation

Although many CFD analyses have been applied for the prediction of spray behavior in the direct-injection gasoline engines, a priori estimation of the spray characteristics including the penetration length and the Sauter mean diameter is difficult, and the empirical determination of the model parameters are usually necessary. The purpose of this study is to develop a CFD modeling scheme to estimate the engine performance from the nozzle characteristics for the front-end loading design of injector nozzles. We have developed a method to predict the spray shapes by applying the LES turbulence model for the internal flow of the nozzles. The method has been applied to estimate the engine performance in conjunction with spray models.