Experimental study of recompression reaction for low-load operation in direct-injection homogeneous charge compression ignition engines with n-heptane and i-octane fuels

Abstract

Experiments have been reported in the literature in which the low-load limit of a retention-mode HCCI engine operating on gasoline has been significantly extended by preprocessing of the fuel during negative valve overlap. This paper presents experimental studies in which this low-load-limit extension is demonstrated and characterized using simple, singlecomponent hydrocarbon fuels with relatively well-known chemical kinetics. The model fuels were n-heptane and i-octane and this choice was made both because of the extensive work that has been undertaken to develop their chemical kinetic mechanisms and because these fuels span the range of ignitability that is likely to be of interest for HCCI engines.The experimental results reported here show that both fuels exhibit load extension to as low as 1 bar net indicated mean effective pressure when operated at high residual mass fractions, low equivalence ratios, and an appropriate choice of compression ratio (13 for n-heptane, 18 for i-octane). Near the low-load limit, combustion is stable, exhibits slightly advanced timing, has relatively low unburned hydrocarbon emissions, negligible (, 5 ppm) NO emissions, and slightly increased CO emissions (compared to higher load conditions). The indicated efficiency of low-load operation with recompression reaction is somewhat reduced, mainly due to increased heat transfer and decreased combustion efficiency.

Understanding the Dynamic Evolution of Cyclic Variability at the Operating Limits of HCCI Engines with Negative Valve Overlap

“…Our work show that, for higher CV, the effect of unburned fuel creates an additional coupling that must be taken into account. The use of injecting fuel during the nvo period for control was mentioned in [9] and experimentally investigated in [21,22] and used in [23,24] to extend the low load limit of HCCI. In the present work, the injection parameters are constant for each operating point and the effect of the unburned fuel, recycled from incomplete combustion in previous cycles, is investigated.…”

Cyclic Variability and Dynamical Instabilities in Autoignition Engines With High Residuals

“…Reducing CV by controlling the fuel injection during the nvo period is investigated experimentally in [19] and [20] for extending the low load limit of HCCI. Linear controllers for reducing CV are developed in [21] and [22] using variable valve timing and in [23] using fuel injection timing.…”

A study on combustion control and operating range expansion of gasoline HCCI

“…Song et al extended the low-load limit by injecting fuel during NVO (negative valve overlap). Direct injection and NVO injection strategy were also simultaneously used and it was possible to expand low-load limit drastically [10]. And also intensive numerical study was also performed in order to understand the chemical effect during recompression process [11].…”