Effects of intake-port throttling on combustion behaviour in diesel low-temperature combustion

Abstract

Citation: SOGBESAN AbstractThis paper describes the effects of intake-port throttling on diesel low temperature combustion (LTC) at a low and medium load condition. These conditions were known for their characteristically high hydrocarbon (HC) emissions predominantly from over-mixed and under-mixed mixture zones respectively. The investigation was carried out to supplement current findings in literature with valuable information on the formation of HC emissions with increasing swirl levels generated by intake-port throttling. This was achieved through the use of cycle-resolved HC measurements in addition to cycle averaged emissions and in-cylinder pressure-derived metrics. While there was negligible overall effect at the moderately-dilute lowload conditions, increasing swirl has been shown to be beneficial to premixing efficacy under highly dilute conditions with extended ignition delay. This potential advantage was found to be nullified by the swirlinduced confinement of fuel and combustion products to the central region of the cylinder leading to poor late cycle burn rates and increased smoke emissions. HC emissions from the squish and head quench regions were reduced by an increase in swirl ratio.

The effects of increasing FAME biodiesel content on combustion characteristics and HC emissions in high-EGR low temperature combustion

“…In order to obtain intra-cycle hydrocarbon emissions, a fast flame ionisation detector (Cambustion HFR 400 fast-FID), with a response time of ∼1-2 ms, was positioned in the exhaust port 20 mm downstream of one of the two exhaust valves as shown in Figure 2 [26]. The raw fast-FID data was time-shifted in post-processing to account for the sample transit time and the instrument's response time to yield crank-angle aligned HC data [27].…”

“…In all cases the AHRR data shows a clear low temperature heat release (LTHR) region followed by a rapid high temperature heat release (HTHR) phase corresponding to the combustion of the pre-mixed fraction of the charge. Note that, in contrast to convention, the pre-mixed phase is defined at this particular operating condition as the crank angle interval between 5 % and 70 % mass fraction burned, i.e., as shown in previous works [26], this is a substantially pre-mixed combustion condition. The pre-mixed phase is then followed by a significantly slower mixing-limited heat release phase as the in-cylinder charge begins to cool with expansion.…”

“…The pre-mixed phase is then followed by a significantly slower mixing-limited heat release phase as the in-cylinder charge begins to cool with expansion. The oscillatory heat release behaviour shown in the mixing-limited phase of combustion, which is characteristic of low to intermediate EGR rate diesel combustion [26], is attributed to the very high rate of pressure rise associated with pre-mixed combustion. These oscillations are not seen at high-EGR LTC conditions where the pressure rise rate is limited by oxygen availability [44].…”

“…Figure2: Cross-section though exhaust manifold and cylinder head detailing the positioning of the fast FID probe within the exhaust port with respect to the exhaust valve[26] …”