Search Results

To improve performance and exhaust emissions of a converted dual-fuel natural-gas engine, the effects of basic parameters were experimentally investigated. The results show that diesel fuel operation is favorable at very low loads and that a small amount of pilot fuel with a moderate injection rate is effective for suppressing knock at high loads. As for the charge air throttling, there is an optimal combination of charge amount and equivalence ratio to obtain high thermal efficiency and reduced emissions. An optimal strategy for fueling is demonstrated based on the results. Adequate control of pilot fuel amount, injection timing and throttle opening area gives diesel-equivalent thermal efficiency with very low smoke emission over a wide range of loads.

In diesel engines, pilot injections and injections at a reduced initial injection rate with high-pressure fuel injection have a potential to reduce particulate, NOx and noise emissions simultaneously. For this reason, various shaping methods of injection rate waveform have been proposed. The present authors also propose such a high-pressure injection system with variable injection rate that relies on spool acceleration and oil-hammering in the injection pipeline. This paper first describes the injection rate shaping, including injections with pilot and reduced initial injection rate, and elucidates their effects on reducing exhaust and noise emissions. A pilot injection can be achieved by either installing a fuel spill path in a plunger body or elongating prelift of the spool. Computer simulations and bench tests of such injection systems show that pilot injection quantity is small enough and the pilot injection pressure is much lower than that of the main part of the injection.

In diesel engines, high-pressure fuel injection is very effective to reduce emissions of particulates and oxides of nitrogen. For this reason, all efforts have been directed to develop suitable high-pressure injection systems. However, high-pressure injection often increases internal leakage of the working fluid, increases power for pumping, and is sometimes still short of injection pressures at lower engine speeds. To remove these faults, the present authors developed a fuel injection system named KD-3 that relies on a novel principle using oil hammering in a convergent pipeline. The dynamic behavior of the proposed system was analyzed by the method of characteristics and computer simulations. A prototype injection system was designed and tested with success. Using a source pressure of 20 MPa, this system boosts pressure well to higher than 120 MPa at the inlet of injection nozzle.

This paper deals with the effect of combustion chamber shape and the role of pressurized injection in high-speed direct-injection diesel engines. First, the previously reported good performance and emission characteristics of the reentrant chamber were confirmed in a single-cylinder engine test. To obtain a better insight into this excellence, a high-speed gas-sampling method was applied to determine the local fuel-air equivalence ratios and mass fractions of substances having higher boiling points during combustion. The results showed that even at a retarded injection the reentrant chamber suppressed the outflow of gas into the clearance space from containing a lot of higher-boiling-point substances, like raw fuel and carbonaceous matter, thereby assuring a less heterogeneous state than the ordinary deep-bowl chamber. This is attributed partly to the suppressed outflow of unburnt gas from the cavity and partly to the enhanced mixing near the entrance.

The soluble organic fraction (SOF) of particulates emitted from a high-speed four-cycle DI diesel engine at various loads for commercial diesel fuels and some blended and distillate fuels is studied under steady operating conditions. The fuels examined have various final boiling points and aromatic content. Emissions of lubricant, fuel fractions, and combustion products in the SOF are evaluated by chemical analysis. Exhaust emission data indicates that the addition of aromatic hydrocarbons to fuel does not cause increases in SOF or in solid fraction, and that heavier components of fuel are responsible for high level of SOF emission at medium load mainly due to the deposition of fuel on the wall of the piston cavity. Furthermore, flow reactor experiments under atmospheric pressure have been conducted to look at the temperature effect of forming SOF. The results successfully explain the dependence of SOF emission on load and fuel properties.

The combustion process in high-speed direct-injection diesel engines is characterized by random turbulent mixing between turbulent eddies having different fuel concentrations. Nitric oxide and soot are formed in hot eddies and fuel-rich eddies. In the present study, the authors elucidate the diesel combustion process, from the viewpoint of such heterogeneity and turbulent mixing, by analysis of high-speed flame photographs. Based on this study the following points are suggested: jet-like flames are formed just after ignition but soon disintegrate into random turbulent flamelets as each flame quickly expands. In the middle and later stages of combustion, uniform and isotropic turbulent motions prevail over the entire space, gradually decaying with time. Such turbulent motions favor the destruction of fuel concentration heterogeneity. Gas expansion due to combustion enhances such random motions, and the swirl prevents their early decay in every burning stage.

A laser homodyne technique is applied to measure turbulence intensities and spatial scales during compression and expansion strokes in a non-fired engine. By using this techinique, relative fluid motion in a turbulent flow is detected directly without cyclic variation biases caused by fluctuation in the main flow. Experiments are performed at different engine speeds, compression ratios, and induction swirl ratios. In no-swirl cases the turbulence field near the compression end is almost uniform, whereas in swirled cases both the turbulence intensity and the scale near the cylinder axis are higher than those in the periphery. In addition, based on the measured results, the k-ε two-equation turbulence model under the influence of compression is discussed.

A systematic study on particulate mass emission from a high-speed direct-injection diesel engine was conducted using a mini-dilution sampling method. Effects of fuel-air equivalence ratio, engine speed, injection timing, and swirl intensity are presented and discussed with special regard to soluble organic fraction (SOF) and hydrocarbons. Results show that these concentrations are greatly affected by ignition delay or by temperature level in the engine cylinder. As the sources of SOF and hydrocarbons, local and bulk quenching of the charge, interaction of the fuel spray with the combustion chamber walls, and slow thermal decomposition of fuel are considered and discussed. Among them, the significance of the fuel decomposition is pointed out, by separate experiments on a simulated engine by using an in-cylinder gas-sampling technique.

A high-speed gas-sampling technique has been applied to reveal the time development of spatial distributions of several chemical species produced during combustion in the swirl and the main chambers. To enable traversed gas-sampling in each chamber, experiments have been made by using a double-scavenged two-stroke cycle engine with a simulated two-dimensional chamber configuration. By these experiments the following matters have been examined in detail; the extent of fuel-rich zones, their decay with time, the action of swirling air motion, the formation of nitric oxide, the formation of hydrocarbons and of carboneous substance, the state of the outflow of gas from the swirl chamber into the main chamber, and the flame spread within the main chamber. Besides, the influences of some operating conditions and of design parameters, such as overall fuel-air ratio, injection timing, dimension of connecting passage, and direction of the fuel spray, upon these items have been investigated.