Search Results

An experimental investigation was conducted to explore the impact in-cylinder pressure oscillations have on piston heat transfer. Two fast-response surface thermocouples embedded in the piston top measured transient temperature and a commercial wireless telemetry system was used to transmit thermocouple signals from the moving piston. Measurements were made in a light-duty single-cylinder research engine operated under low temperature combustion regimes including Homogeneous Charge Compression Ignition (HCCI) and Reactivity Controlled Compression Ignition (RCCI) and Conventional Diesel (CDC). The HCCI data showed a correlated trend of higher heat transfer with increased pressure oscillation strength, while the RCCI and CDC data did not. An extensive HCCI data set was acquired. The heat transfer rate - when corrected for differences in cylinder pressure and gas temperature - was found to positively correlate with increased pressure oscillations.

Many combustion researchers use peak pressure rise rate or ringing intensity to indicate combustion noise in lieu of microphone data or using a combustion noise meter that simulates the attenuation characteristics of the engine structure. In this paper, peak pressure rise rate and ringing intensity are compared to combustion noise using a fully documented algorithm similar to the ones used by combustion noise meters. Data from multiple engines operating under several low-temperature combustion strategies were analyzed. The results suggest that neither peak pressure rise rate nor ringing intensity provides a direct correlation to engine noise over a wide range of operating conditions. Moreover, the estimation of both metrics is often accompanied by the filtering of the pressure data, which changes the absolute value of the results.

A simple model to simulate cycle-by-cycle variation that is suitable for use in Monte-Carlo approaches has been developed and validated with a wide range of experimental data. The model is intended to be diagnostic rather than predictive in nature, with a goal of providing realistic in-cylinder pressures. The individual-cycle cumulative rate of heat release was curve fit with a four-parameter Wiebe function. It was found that the distribution of the Wiebe b-parameter was quite small, so its value was obtained from the ensemble-averaged condition. The remaining three Wiebe function parameters, θig, θcomb and m were found to be distributed over a moderate range, and were linearly correlated to each other. Using the cumulative density function of θig, and the linear fit of θcomb and m to θig, with a random component added, a Monte-Carlo scheme was developed.

The current work investigates the in-cylinder mixing of a fluorescent tracer species inducted into the engine through a small-diameter tube mounted along the inner port wall and the remaining inlet stream in a small-bore utility engine. Planar laser-induced fluorescence (PLIF) measurements were acquired on a single plane, parallel to and approximately 4 mm below the cylinder head deck, throughout the intake and compression strokes. The data were analyzed to qualitatively and quantitatively describe the evolution of the mixture stratification. The highest degree of stratification in the mean field was observed at a timing of 90 crank angle (CA) degrees after top dead center (aTDC) of the intake stroke, which corresponds closely to the point of maximum intake valve lift (105 CA degrees aTDC).

In this paper, three models for the prediction of knock onset timing are compared: an ignition-integral model using a simple ignition delay correlation, an ignition-integral model using a pre-computed lookup table of ignition delays, and the direct integration of a detailed chemical kinetic mechanism. All three models were found to compare well with experimentally measured results; the correlation-based knock-integral model was found to be as accurate as the other methods and was computationally far more efficient. The direct integration approach correlated very well with the experimental data but was delayed by 1-2 crank angles. The simplified models have been used in conjunction with a Monte-Carlo approach to assess the cycle-by-cycle variations in knock onset timing. A statistical comparison between the Monte-Carlo predictions and experimental results showed a good prediction of the distribution widths, and some modest phasing issues over a wide range of ignition timing.

The objective of this work is to compare and contrast standard metrics that are used to quantify engine knock phenomena. Several methods found in the literature were investigated, and the fundamental bases for the methods were elucidated. A new knock onset metric was developed and compared to the commonly used threshold value exceeded metric. The standard knock intensity metrics were categorized based on the underlying signal (pressure, heat release), domain of analysis (time or frequency), and the calculation method (single-valued or integral). Each category was evaluated based on the fundamental advantages and disadvantages of that category. Single-value methods were found to be inadequate because they are prone to biases due to wave interference and beating. Heat release-based metrics were found to be redundant and be based on flawed assumptions.