Jaal Ghandhi

On the fluorescent behavior of ketones at high temperatures

Results are presented of the effect of elevated temparature and pressure on the laser-induced fluorescence of ketone molecules. The fluorescent yield of ketones was found to decrease with temperature but was unaffected by pressure for a 266 nm excitation and broadband collection.

Passive scalar mixing in a planar shear layer with laminar and turbulent inlet conditions

The effect of inlet conditions on the mixing of a passive scalar was investigated in a planar shear layer with inlet boundary layers that were laminar, tripped and naturally turbulent–transitional. Planar laser-induced fluorescence measurements of acetone were used to directly evaluate the shear layer structure, and were processed to determine probability density functions (PDFs) of the mixture fraction. The results agree well with previous studies in aqueous and gaseous systems for laminar inlet conditions. Large-scale structures of a nearly homogeneous composition were found, and the structures spanned the mixing layer width giving rise to a nonmarching style PDF. A high-speed boundary layer that differed from the laminar state (produced by tripping a laminar boundary layer, from the naturally turbulent–transitional state at high flow rates, or by tripping the turbulent–transitional condition) gave rise to a hybrid-style PDF that was markedly different from either the laminar nonmarching case, or the ma...

Fuel Film Temperature and Thickness Measurements on the Piston Crown of a Direct-Injection Spark-Ignition Engine

Fuel film temperature and thickness were measured on the piston crown of a DISI engine under both motored and fired conditions using the fiber-based laser-induced fluorescence method wherein a single fiber delivers the excitation light and collects the fluorescence. The fibers were installed in the piston crown of a Bowditch-type optical engine and exited via the mirror passage. The fuel used for the fuel film temperature measurement was a 2x10 -6 M solution of BTBP in isooctane. The ratio of the fluorescence intensity at 515 to that at 532 nm was found to be directly, but not linearly, related to temperature when excited at 488 nm. Effects related to the solvent, solution aging and bleaching were investigated. The measured fuel film temperature was found to closely follow the piston crown metal temperature, which was measured with a thermocouple. A detailed analysis of the fiber-based laser-induced fluorescence technique was used to ascertain film thickness based on a single-point calibration. The calibration methodology also accounted for the effects of fuel film temperature. A 4% by volume solution of 2,3-hexanedione in isooctane was found to be a suitable choice for fuel film thickness measurement because it was verified to be co-evaporative. The fuel film thickness was found to be quite small, less than 10 μm, for both motored and fired conditions performed at the same piston temperature. The 2,3-hexanedione was found to leave a viscous residue on the piston crown, which carried over from cycle to cycle and limited the results.

An experimental study of spray mixing in a direct injection engine

Abstract The effects of injection timing, injector type, injection pressure and engine flowfield on fuel spray mixing in a direct injection engine were investigated using planar laser-induced fluorescence. The fluorescence images had sufficient resolution and quality to permit, for the first time in an engine, the calculation of the scalar dissipation rate. The probability density function of the scalar dissipation scaled by the mean showed excellent agreement with turbulent jet and shear layer data for late injection conditions, indicating that the same fundamental mixing process existed in the different flows. The effect of shot noise limited such comparisons for the more homogeneous early injection conditions. A dual-metric method was developed to characterize the degree of mixedness. The two metrics employed were the spatial variation, which describes the homogeneity of the scalar population, and the mean scalar dissipation, which describes the average magnitude of local scalar gradients and represents the rate of fine-scale mixing. Using this method, it was found that the presence of a strong bulk flowfield dominated the mixing rate in the test engine, while injector characteristics showed lesser effects. The data set averaged results of the two metrics for a wide range of conditions were found to define a single, unique curve that was accurately described by a quadratic relationship. This curve defines the path that turbulent mixing follows from an initial segregated state to the fully mixed limit.

Fuel unmixedness effects in a gasoline homogeneous charge compression ignition engine

Abstract Fuel stratification, independent of thermal and residual gas stratification, was studied in a gasoline homogenous charge compression ignition (HCCI) engine. The unmixed charge was created by injecting fuel (iso-octane) into the intake port after being prevaporized and heated to the same temperature as the intake stream. Planar laser-induced fluorescence measurements showed that local equivalence ratios in the charge differed from the mean equivalence ratio by up to 50 per cent for the latest possible injection timing. Experimental results showed little to no change in combustion phasing and performance between prevaporized port (unmixed) or premixed (homogeneous) fuelling. Increases in NO x and CO emissions were observed with the prevaporized port fuelling and are believed to result from the regions richer or leaner than the mean equivalence ratio. These results indicate that fuel stratification in the absence of thermal and residual stratification does not appear to be a viable method for HCCI combustion control for gasoline-type fuels.

PIV Measurements of In-Cylinder Flow in a Four-Stroke Utility Engine and Correlation with Steady Flow Results

Large-scale flows in internal combustion engines directly affect combustion duration and emissions production. These benefits are significant given increasingly stringent emissions and fuel economy requirements. Recent efforts by engine manufacturers to improve in-cylinder flows have focused on the design of specially shaped intake ports. Utility engine manufacturers are limited to simple intake port geometries to reduce the complexity of casting and cost of manufacturing. These constraints create unique flow physics in the engine cylinder in comparison to automotive engines. An experimental study of intake-generated flows was conducted in a four-stroke spark-ignition utility engine. Steady flow and in-cylinder flow measurements were made using three simple intake port geometries at three port orientations. Steady flow measurements were performed to characterize the swirl and tumble-generating capability of the intake ports. In-cylinder flows were investigated using Particle Image Velocimetry (PIV). Two-dimensional PIV measurements were made in a vertical plane and a horizontal plane of the cylinder with the engine motored at 1200 RPM. The steady flow swirl and tumble characteristics were similar for the three port geometries, but differed significantly with port orientation. The swirl direction and magnitude measured on the steady flow bench correlated well qualitatively with the ensemble-averaged velocity distributions in the horizontal PIV plane. The PIV results showed that the in-cylinder flows generated by the three ports were complex, three-dimensional flows with no dominant large-scale fluid motion. Significant cycle-to-cycle variation was observed in the flow field. The orientation of the intake port was also shown to have a significant effect on the flow field.

Effect of combustion regime on in-cylinder heat transfer in internal combustion engines

A multi-dimensional model was applied to investigate the influence of combustion regimes on heat transfer losses in internal combustion engines. By utilizing improved turbulence and heat transfer sub-models, the combustion and heat transfer characteristics of the engine were satisfactorily reproduced for operation under conventional diesel combustion, homogeneous charge compression ignition, and reactivity controlled compression ignition regimes. The results indicated that the total heat transfer losses of conventional diesel combustion are the largest among the three combustion regimes due to the direct interaction of the high-temperature flame with the piston wall, while the heat transfer losses of reactivity controlled compression ignition are the lowest and nearly are independent of combustion phasing because of the avoidance of high-temperature regions adjacent to the cylinder walls. Compared to conventional diesel combustion, homogeneous charge compression ignition shows more potential for the reduction of exhaust energy and improvement of fuel efficiency. In reactivity controlled compression ignition combustion, the reduction of heat transfer and exhaust losses outweigh its increase in combustion losses and offer the opportunity for further improvement of fuel efficiency. Furthermore, by evaluating the widely used Woschni and Chang et al.’s empirical heat transfer correlations, it was found that both correlations considerably overestimate the heat transfer rate for the reactivity controlled compression ignition regime. It is necessary to improve empirical heat transfer models to take account of the flow and combustion characteristics under various combustion modes.

Experimental investigation of piston heat transfer under conventional diesel and reactivity-controlled compression ignition combustion regimes

The piston of a heavy-duty single-cylinder research engine was instrumented with 11 fast-response surface thermocouples, and a commercial wireless telemetry system was used to transmit the signals from the moving piston. The raw thermocouple data were processed using an inverse heat conduction method that included Tikhonov regularization to recover transient heat flux. By applying symmetry, the data were compiled to provide time-resolved spatial maps of the piston heat flux and surface temperature. A detailed comparison was made between conventional diesel combustion and reactivity-controlled compression ignition combustion operations at matched conditions of load, speed, boost pressure, and combustion phasing. The integrated piston heat transfer was found to be 24% lower, and the mean surface temperature was 25 °C lower for reactivity-controlled compression ignition operation as compared to conventional diesel combustion, in spite of the higher peak heat release rate. Lower integrated piston heat transfer for reactivity-controlled compression ignition was found over all the operating conditions tested. The results showed that increasing speed decreased the integrated heat transfer for conventional diesel combustion and reactivity-controlled compression ignition. The effect of the start of injection timing was found to strongly influence conventional diesel combustion heat flux, but had a negligible effect on reactivity-controlled compression ignition heat flux, even in the limit of near top dead center high-reactivity fuel injection timings. These results suggest that the role of the high-reactivity fuel injection does not significantly affect the thermal environment even though it is important for controlling the ignition timing and heat release rate shape. The integrated heat transfer and the dynamic surface heat flux were found to be insensitive to changes in boost pressure for both conventional diesel combustion and reactivity-controlled compression ignition. However, for reactivity-controlled compression ignition, the mean surface temperature increased with changes in boost suggesting that equivalence ratio affects steady-state heat transfer.

The effects of intake charge preheating in a gasoline-fueled hcci engine

Experiments were performed on a homogeneously fueled compression ignition gasoline-type engine with a high degree of intake charge preheating. It was observed that fuels that contained lower end and/or non-branched hydrocarbons (gasoline and an 87 octane primary reference fuel (PRF) blend) exhibited sensitivity to thermal conditions in the surge tanks upstream of the intake valves. The window of intake charge temperatures, measured near the intake valve, that provided acceptable combustion was shifted to lower values when the upstream surge tank gas temperatures were elevated. The same behavior, however, was not observed while using isooctane as a fuel. Gas chromatograph mass spectrometer analysis of the intake charge revealed that oxygenated species were present with PRF 87, and the abundance of the oxygenated species appeared to increase with increasing surge tank gas temperatures. No significant oxygenated species were detected when running with isooctane. The presence of the oxygenated species for PRF 87 fueling indicated that reactions were occurring in the intake surge tanks which resulted in needing lower intake charge temperatures to achieve autoignition.

Scavenging Measurements in a Direct-Injection Two-Stroke Engine

The scavenging process in a direct-injection two-stroke research engine was examined by using an electromagnetically controlled poppet valve to sample the trapped charge. A physical model was developed to characterize thescavenging based solely on the measured trapped gas composition. This method obviates the need to measure the post-combustion composition of the trapped charge, which significantly eases the sampling valve requirements. The valve that was developed proved to be very robust and was able to sample over 30% of the trapped mass at 3000 rpm. The measured scavenging efficiency was found to agree well with the non-isothermal two-zone perfect mixing limit of scavenging. The scavenging efficiency was found to increase with delivery ratio, and was nearly independent of speed. The scavenging efficiency was found to decrease with increasing air-fuel ratio at a constant delivery ratio, but the effect was found to be the result of the exhaust gas temperature reduction more than the increase in the blowdown pressure.