Observing simultaneous low temperature heat release and deflagration in a spark ignition engine using formaldehyde planar laser induced fluorescence

Low temperature heat release (LTHR) and its underlying chemistry is of particular interest for its potential to mitigate knock in spark ignition (SI) engines and enable advanced combustion strategies that rely on end gas autoignition. It has been proposed that, in SI engines, LTHR can occur volumetrically in the end gas, after ignition, whilst deflagration occurs elsewhere in the cylinder, however, current pressure-based heat release metering techniques are unable to distinguish such LTHR from high temperature heat release (HTHR) due to the overlapping pressure rise characteristics. Planar laser-induced fluorescence (PLIF) of formaldehyde, a known product of LTHR which is consumed during HTHR, offers an opportunity to detect end gas LTHR simultaneously with deflagration but is challenging to implement, as end gas is often located closer to cylinder walls and away from typical optically accessible locations. An optically accessible SI engine was used to show formaldehyde PLIF signal intensity under motored conditions is well correlated to cumulative LTHR intensity, using a recent method to isolate LTHR in SI engine conditions. An alternative ignition method using four side-mounted spark plugs was implemented to generate end gas close to the cylinder axis. This enabled measurement of LTHR within the end gas during the deflagration process of a SI engine, demonstrating the utility of formaldehyde PLIF to optically measure LTHR under conditions where pressure-based diagnostics cannot isolate the contribution of LTHR. • Low Temperature Heat Release (LTHR) has been studied widely by pressure methods. • LTHR occurring during deflagration in end gas cannot be measured by pressure methods. • LTHR is investigated using formaldehyde Planar Laser Induced Fluorescence (PLIF). • This is validated comparing PLIF to isolated LTHR using a pressure-based method. • LTHR in end gas is observed ahead of the flame front using PLIF for the first time.