Takayuki Fuyuto

Low Emissions and High-Efficiency Diesel Combustion Using Highly Dispersed Spray with Restricted In-Cylinder Swirl and Squish Flows

A new clean diesel combustion concept has been proposed and its excellent performance with respect to gas emissions and fuel economy were demonstrated using a single cylinder diesel engine. It features the following three items: (1) low-penetrating and highly dispersed spray using a specially designed injector with very small and numerous orifices, (2) a lower compression ratio, and (3) drastically restricted in-cylinder flow by means of very low swirl ports and a lip-less shallow dish type piston cavity. Item (1) creates a more homogeneous air-fuel mixture with early fuel injection timings, while preventing wall wetting, i.e., impingement of the spray onto the wall. In other words, this spray is suitable for premixed charge compression ignition (PCCI) operation, and can decrease both nitrogen oxides (NOx) and soot considerably when the utilization range of PCCI is maximized. However, in diffusive combustion, especially at full load, a low-penetrating spray potentially causes higher soot emissions and results in lower maximum torque. In this case, item (2) is applied to recover full-load performance. The lower compression ratio enables diffusive combustion phasing to be advanced more with an earlier injection timing because of a larger margin between the compression-end pressure and the allowable maximum in-cylinder pressure. This results in lower soot emissions because enough time is created to oxidize soot before the end of the combustion period. A lower compression ratio often leads to worse cold-condition engine performance aspects, such as cold startability, unburned hydrocarbons, and white smoke. Item (3) is applied to compensate for such practical problems. Drastically weakened in-cylinder flow keeps the compression-end temperature to the same level as a conventional engine with an ordinary compression ratio by decreasing heat-flux escaping through the chamber wall (i.e., heat-loss). Although a weak in-cylinder gas motion might lead to higher soot emissions due to slower fuel-air mixing, it should be noted that the highly dispersed spray of item (1) enables PCCI-dominant combustion in which the fuel-air mixing process is less dependent on in-cylinder flow. In this way, these three items act mutually to compensate for each other’s drawbacks, while maximizing their advantages. Consequently, NOx emissions in the New European Driving Cycle (NEDC) can be reduced drastically to less than 1/4 of the level of a conventional engine, or less than half of the Euro 6 standard without deteriorating fuel consumption, full-load torque, or cold-condition performance.

In-cylinder stratification of external exhaust gas recirculation for controlling diesel combustion

Abstract A technique for achieving the in-cylinder stratification of external exhaust gas recirculation (EGR) gas in direct-injection (DI) diesel engines has been developed to reduce toxic exhaust emissions. The external EGR gas is supplied from one of the two intake ports which can create a swirl flow in either the upper or lower portion of the cylinder during the intake stroke. In the final stage of the compression stroke, a squish flow conveys the vertically stratified EGR gas into the piston cavity, generating a radially stratified EGR gas in the piston cavity at the end of the compression stroke. This strategy for achieving EGR gas stratification in the piston cavity was developed by using an unsteady computational fluid dynamics (CFD) code. Prior to the exhaust emission tests, the accuracy of the simulation was evaluated by planer laser-induced fluorescence (LIF) imaging. The exhaust emission tests showed that there was less smoke emission under medium load conditions when the EGR gas was delivered to the inner part of the piston cavity. The mechanism of this smoke reduction was investigated using CFD simulation, which is based on a series of calculations related to the internal flow of the injector nozzle, the in-cylinder fuel spray, and mixture formation and combustion. It has been shown that, at the beginning of the combustion, the higher concentration of EGR gas in the inner part of the cavity lowers the combustion temperature and reduces the soot formation rate. Air, which exists in the outer part of the cavity at the start of fuel injection, enhances the oxidation of the soot cloud in the piston cavity periphery in the latter half of the combustion period.

Laser-based temperature imaging close to surfaces with toluene and NO-LIF

Two novel techniques based on Laser-Induced Fluorescence (LIF) were applied to measure gas-phase temperature distributions in boundary layers close to wall surfaces. Single- line toluene-LIF thermometry was used to image temperature in a nitrogen gas flow above a heated wall. The nitrogen gas flow was doped with evaporated toluene. When excited at 266 nm, the toluene LIF-signal shows an exponential dependence on temperature. This behavior was used to calculate absolute temperatures from LIF images after calibration at known conditions. The second technique, multi-line NO-LIF thermometry was applied to image temperature in the quenching boundary layer close to a metal wall located on a flat flame burner. A small amount of nitric oxide was mixed into the air/methane mixture. NO molecules were excited in the A-X (0,0)-band at 225 nm. NO-LIF excitation spectra were acquired by tuning the excimer laser wavelength and recording the NO LIF-signal with an ICCD camera. Absolute temperatures were calculated for every pixel by fitting simulated excitation spectra to the experimental data. Temperature distributions close to the wall surface were measured at two different flow-rate conditions. A high nominal spatial resolution of 0.016 mm/pixel in direction perpendicular to the wall was reached. Wall surface temperatures were recorded simultaneously by embedded thermocouples and compared with gas-phase temperature near the wall surface.

Spark ignition and early flame development of lean mixtures under high-velocity flow conditions: An experimental study:

This study set out to experimentally investigate spark ignition and the subsequent early flame development of lean air–fuel mixtures of A/F = 20–30 under high-velocity flow conditions using a uniquely designed swirl chamber. The swirl chamber realizes a high-velocity flow of 65 m/s at the spark plug gap, as well as internal temperature and pressure histories that are equivalent to those of spark-ignition engines, being equipped with an optically accessible engine. The designed swirl chamber clearly captures the characteristic behavior of the spark channels and flames in the vicinity of the spark plug. The results show that the spark channels stretch downstream following the flow and are subject to short circuits or restrikes. In the case of a high ignition energy of 200 mJ, short circuits of the spark channels occur in the early part of the discharge, while restrikes occur in the later parts. With a decrease in the ignition energy, restrikes occur in the earlier parts of the discharge. With a low ignition energy of 65 mJ, restrikes can occur immediately after the electrical breakdown without any significant spark stretch. At a sufficiently low dilution degree of A/F = 20, flames can hold behind the ground electrode of the spark plug, which significantly suppresses the cycle dispersion while also enhancing the combustion in the early stage. With further air dilution, that is, A/F > 20, flames develop, flowing downstream without flame holding. However, temporal flame attachment to the ground electrode is observed during the discharge duration even at A/F = 30, while the attached flames eventually blow off downstream at the end of the spark discharge.

Measurement of vibrational and rotational temperature in spark-discharge plasma by optical emission spectroscopy: Change in thermal equilibrium characteristics of plasma under air flow

The vibrational and rotational temperatures in a spark-discharge plasma were measured using optical emission spectroscopy, and the influence of the air flow velocity and ambient pressure on these temperatures was investigated. The optical emissions from the plasma were led to an imaging spectroscope through an optical fiber. The temperature was estimated by fitting a theoretically calculated spectrum to that which had been acquired experimentally, formed by nitrogen molecule emission from 372 to 382 nm. The spark-discharge plasma was examined with a flow of ambient air at a discharge energy of 80 mJ. The air flow caused the spark-discharge channel to elongate downstream. At the center of the spark plug gap, the vibrational temperature in the plasma was 4000 K, whereas the rotational temperature was 2000 K. This plasma can be regarded as being in non-thermal equilibrium because the vibrational temperature was higher than the rotational temperature. At a position approximately 3 mm downstream from the spark plug gap, the vibrational and rotational temperatures increased to 4500 and 4000 K, respectively, while approaching each other. Both temperatures reached a maximum value. These results show that the plasma transitions from non-thermal equilibrium to thermal equilibrium as it is elongated by the air flow. Ignition efficiency improvements can be expected if the time required to transition from non-thermal to thermal equilibrium can be shortened.

Set-off length reduction by backward flow of hot burned gas surrounding high-pressure diesel spray flame from multi-hole nozzle

The backward flow of the hot burned gas surrounding a diesel flame was found to be one of the factors reducing the set-off length (also called the lift-off length), that is, the distance from a nozzle exit into which a diffusion flame cannot intrude. In the combustion chamber of an actual diesel engine, the entrainment of the surrounding gas into a spray jet injected from a multi-hole nozzle is restricted by the combustion chamber walls and the adjacent spray jets, thus inducing the backward flow of the surrounding gas toward the nozzle exit. The emergence of this backward flow was measured by particle tracking velocimetry in the non-combusting condition. A new momentum theory for calculating the backward flow velocity was established by extending Wakuri’s momentum theory. Shadowgraph imaging in an optical engine successfully visualized the backward flow of the hot burned gas. The hot burned gas is re-entrained into the spray jet in the region of the set-off position and shortens the set-off length in comparison to that of a single free-spray flame which does not induce the backward flow.