Improved model for the analysis of the Heat Release Rate (HRR) in Compression Ignition (CI) engines

The accuracy of the Heat Release Rate (HRR) model of Internal Combustion Engines (ICEs) is highly depended on the ratio of specific heats, gamma (γ). Previous γ models were largely expressed as functions of temperature only. The effects of the excess air ratio (λ) and the Exhaust Gas Recirculation (EGR) rate on γ were neglected in most of the existing γ functions. Furthermore, previous HRR models were developed for stoichiometric or near – stoichiometric air - fuel mixtures in an engine condition. However, Compression Ignition (CI) engines operate over a wide range of λ. No work has been done to model the HRR of CI engines under non – stoichiometric conditions. Also, no work has been done to investigate the accuracy of existing γ functions specifically with respect to the modelling of the HRR of CI engines for non – stoichiometric conditions. The aim of this work was to develop an improved HRR model for the analysis of the HRR of CI engines for non – stoichiometric conditions (λ>1). In this work, a modified γ(T,λ), was used to model the HRR of a 96 kW, multiple fuel injection, Euro V, Direct Injection (DI) engine. The modified HRR model (Leeds HRR model) predicted the fuel consumption of the engine with an average error of 1.41% confirming that the accuracy of the HRR model of CI engines is improved by using γ(T,λ). The typical average error in the prediction of the other models was 16%. The much improved HRR model leads to more accurate prediction of fuel consumption, which enables the development of and enhances better fuel consumption management strategies for engines and fuels. It was also ascertained in this work that EGR has insignificant effect on the HRR of CI engines at low and medium loads

Assessment of elliptic flame front propagation characteristics of iso-octane, gasoline, M85 and E85 in an optical engine

Premixed fuel–air flame propagation is investigated in a single-cylinder, spark-ignited, four-stroke optical test engine using high-speed imaging. Circles and ellipses are fitted onto image projections of visible light emitted by the flames. The images are subsequently analysed to statistically evaluate: flame area; flame speed; centroid; perimeter; and various flame-shape descriptors. Results are presented for gasoline, isooctane, E85 and M85. The experiments were conducted at stoichiometric conditions for each fuel, at two engine speeds of 1200 rpm (rpm) and 1500 rpm, which are at 40% and 50% of rated engine speed. Furthermore, different fuel and speed sets were investigated under two compression ratios (CR: 5.00 and 8.14). Statistical tools were used to analyse the large number of data obtained, and it was found that flame speed distribution showed agreement with the normal distribution. Comparison of results assuming spherical and non-isotropic propagation of flames indicate non-isotropic flame propagation should be considered for the description of in-cylinder processes with higher accuracy. The high temporal resolution of the sequence of images allowed observation of the spark-ignition delay process. The results indicate that gasoline and isooctane have somewhat similar flame propagation behaviour. Additional differences between these fuels and E85 and M85 were also recorded and identified

Assessment of effective thermal product of surface junction thermocouples on millisecond and microsecond time scales

Surface junction thermocouples are used extensively for transient heat flux measurements, but their accuracy is dependent on the effective thermal product (TP) of the gauge and this can be a function of the time scale of interest. In the present work the response of surface junction k-type thermocouples was investigated experimentally using a water droplet calibration technique (for millisecond times scales) and a small shock tube (for microsecond time scales). Different junctions formed by scalpel blade scratches and abrasive paper were investigated. When scratches from scalpel blades were used to form the junction, the TP identified from the water droplet calibrations consistently differs by approximately 20% depending on whether the junction was made on the chromel or alumel substrate, in accord with existing thermal properties data. However, the shock tube calibrations indicate that for scalpel-scratched junctions there is considerable variability in thermocouple response time due to effective junction depth variations produced during construction. In contrast, junctions formed with abrasive paper produced rise times consistently less than 1s, but the water droplet and shock tube experiments both indicated significant variability in the effective TP for these gauges. The consistency in TP for scalpel-scratched junctions for millisecond time scales and the variability for junctions created with abrasive grit for both the millisecond and microsecond time scales is attributed to the differences in the effective proximity of the junction to the insulation between chromel and alumel substrates. For junctions created with abrasive grit, the effective TP is approximately 30% smaller for microsecond time scales than it is for millisecond time scales