Randy E. Herold

High speed engine gas thermometry by Fourier-domain mode-locked laser absorption spectroscopy.

We present a novel method for low noise, high-speed, real-time spectroscopy to monitor molecular absorption spectra. The system is based on a rapidly swept, narrowband CW Fourier-domain mode-locked (FDML) laser source for spectral encoding in time and an optically time-multiplexed split-pulse data acquisition system for improved noise performance and sensitivity. An acquisition speed of ~100 kHz, a spectral resolution better than 0.1 nm over a wavelength range of ~1335-1373 nm and a relative noise level of ~5 mOD (~1% minimum detectable base-e absorbance) are achieved. The system is applied for crank-angle-resolved gas thermometry by H(2)O absorption spectroscopy in an engine motoring at 600 and 900 rpm with a precision of ~1%. Influences of various noise sources such as laser phase and intensity noise, trigger and synchronization jitter in the electronic detection system, and the accuracy of available H(2)O absorption databases are discussed.

Thermodynamic Benefits of Opposed-Piston Two- Stroke Engines

A detailed thermodynamic analysis was performed to demonstrate the fundamental efficiency advantage of an opposed-piston two-stroke engine over a standard four-stroke engine. Three engine configurations were considered: a baseline six-cylinder four-stroke engine, a hypothetical threecylinder opposed-piston four-stroke engine, and a threecylinder opposed-piston two-stroke engine. The bore and stroke per piston were held constant for all engine configurations to minimize any potential differences in friction. The closed-cycle performance of the engine configurations were compared using a custom analysis tool that allowed the sources of thermal efficiency differences to be identified and quantified. The simulation results showed that combining the opposed-piston architecture with the twostroke cycle increased the indicated thermal efficiency through a combination of three effects: reduced heat transfer because the opposed-piston architecture creates a more favorable combustion chamber area/volume ratio, increased ratio of specific heats because of leaner operating conditions made possible by the two-stroke cycle, and decreased combustion duration achievable at the fixed maximum pressure rise rate because of the lower energy release density of the two-stroke engine. When averaged over a representative engine cycle, the opposed-piston two-stroke engine had 10.4% lower indicated-specific fuel consumption than the four-stroke engine.

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.

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.

Time Resolved Particle Image Velocimetry Measurements in an Internal Combustion Engine

High frame rate particle image velocimetry (PIV) measurements were performed in a motored engine at speeds of 600 and 1200 rpm under both throttled and unthrottled conditions. Data were acquired at 1 kHz throughout the entire engine cycle, allowing the temporal and spatial evolution of the flow to be observed. The data were both temporally and spatially filtered to study the turbulent flowfield. The mean (over the spatial domain) kinetic energy of the high-pass filtered data, and its evolution with cutoff frequency or length, was used to quantitatively compare differences between operating conditions and different cycles at the same condition. The difference in fluctuation kinetic energy, when normalized, between different operating conditions was found to be comparable to the difference between cycles. A comparison between spatially and temporally filtered data at the same level of fluctuation kinetic energy was performed. The turbulent structures were found to be quite comparable for both filtering methods.

Residual gas homogeneity measurements

Abstract Planar laser-induced fluorescence (PLIF) of a dopant added to the fuel, which was well mixed with the intake air upstream of the engine, was used to investigate inhomogeneities that arise from the incomplete mixing of the intake charge with the residual gas left in the cylinder from the previous cycle. The residual gas fraction was independently measured using a fast-acting solenoid valve and a cylinder dumping technique. The experiments were performed on a four-stroke homogeneous-charge spark-ignition engine operating at light load. The PLIF data were shot-noise-limited, permitting filtering to reduce some noise artefacts; and to further reduce system-induced biases associated with pulse-to-pulse laser variations the data were ray-mean-normalized, i.e. normalized by the mean of the data along the laser propagation direction. The filtered results had excellent noise characteristics, with a signal-to-noise ratio in excess of 50:1, and allowed the low levels of mixture unmixedness to be identified. The fresh charge was found to exhibit inhomogeneities up to 30° before top dead centre, the latest time investigated. The second moment of the ray-mean-normalized intensity distribution was found to be relatively insensitive to the total residual fraction up to a residual fraction of approximately 30 per cent, having a value of ∼0.04. At higher levels of residual there was a monotonic increase in the intensity distributions second moment.