Autoignition Characteristics of Primary Reference Fuels and their Mixtures

Abstract

This study investigates the autoignition of Primary Reference Fuels (PRFs) using a detailed kinetic model. The chemical kinetics software CHEMKIN is used to facilitate solutions in a constant volume reactor and a variable volume reactor, with the latter representing an IC engine. Experimental shock tube and HCCI engine data from literature is compared with the present predictions in these two reactors. The model is then used to conduct a parametric study in the constant volume reactor of the effect of inlet pressure, inlet temperature, octane number, fuel/air equivalence ratio, and exhaust gas recirculation (EGR) on the autoignition of PRF/air mixtures. A number of interesting characteristics are demonstrated in the parametric study. In particular, it is observed that PRFs can exhibit single or two stage ignition depending on the inlet temperature. The total ignition delay, whether single or two stage, is correlated with n-C 7 H 16 /O 2 ratio. Thermodynamic and chemical effects are identified for the EGR constituents CO 2 , H 2 O, and N 2. The former effect is due to the difference in specific heats and is well known. Introduction of each diluent reduces the mole fraction of O 2 which has the chemical effect of delaying ignition. In addition, H 2 O as a diluent is shown to directly affect key species and reactions involved in high temperature kinetics, resulting in the enhancement of the ignition process relative to N 2. The Engineering Meetings Board has approved this paper for publication. It has successfully completed SAE's peer review process under the supervision of the session organizer. This process requires a minimum of three (3) reviews by industry experts. All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or transmitted, in any form or by any means, electronic, mechanical, photocopying, recording, or otherwise, without the prior written permission of SAE.

Ignition Delay Correlation for Predicting Autoignition of a Toluene Reference Fuel Blend in Spark Ignition Engines

“…The higher energy release from richer mixtures during the first stage ignition speeds up the reactions leading up to the second stage ignition, resulting in shorter overall ignition delay times. Sjöberg and Dec [33] observed experimentally an increase in cool flame activity with increasing ϕ for PRF 60 and PRF 80, and Little et al [34] demonstrated computationally an increase in first stage heat release with richer fuel-air mixtures for PRF 20. The ability of the ignition delay predictions for varying ϕ to replicate the trends observed experimentally in the literature lends credence to the methodology adopted in this study in terms of incorporating the effects of equivalence ratio into the "TRF 91" ignition delay correlation.…”