A comparative study into the effects of pre and post catalyst exhaust gas recirculation on the onset of knock

Abstract

Exhaust gas recirculation (EGR) is proven as a valuable technology for controlling knock whilst maintaining lambda one operation, and is also capable of providing efficiency gains at low load. Despite this few studies in the literature address the question of EGR composition effects, namely whether the EGR gas is sourced from before or after the catalyst, and this remains an area which is often overlooked whilst investigating EGR performance. This paper demonstrates a novel method combining experiment air-path emulation and in-depth data processes to compare the effect of EGR catalysis on the angle of knock onset in a 1L GDI engine. Since initial temperature and pressure have a significant impact on knocking behaviour, an artificial boosting rig replaced the turbomachinery. This enabled fine control over the engine boundary conditions to ensure parity between the catalysed and un-catalysed cases. To overcome the difficulty of comparing stochastic phenomena in an inherently variable dataset, a pairing method was combined with Shahlari and Ghandhi’s angle of knock onset determination method to assess the effects of EGR composition on knock onset for EGR rates ranging from 9% to 18%. The air path emulation system stabilised the engine combustion to provide a suitably rich dataset for analysing knock using the pairing method. Catalysed EGR improved the mean knock onset angle by 0.55 CAD, but due to the inherent variability in cylinder pressure data this only equated to a 58.3% chance of a later knock onset angle for catalysed EGR in any given pair of comparative cycles.

Evaluation of ASTM D6424 standard for knock analysis using unleaded fuel candidates on a six cylinder aircraft engine

“…Retarding the combustion timing is an effective method for knock intensity reduction as this reduces the peak pressure and end-gas temperature; however, it could have an adverse effect on engine output power and fuel efficiency. Techniques like EGR 78 and water injection 62,79 can be used for combustion timing and knock control in aviation SI engines. Both methods lower flame temperature and speed, so giving useful reductions in knock intensity with delayed combustion timing.…”

Experimental assessment of ignition characteristics of lubricating oil sprays related to low-speed pre-ignition (LSPI)

“…Hence, the use of Exhaust Gas Recirculation (EGR) has demonstrated to reduce its frequency. [17][18][19] The injection strategy [20][21][22][23][24] may also influence LSPI, the presence of spray-wall interaction 9,25,26 increasing its likelihood.…”

A methodology to study the interaction between variable valve actuation and exhaust gas recirculation systems for spark-ignition engines from combustion perspective

“…Apart from the fuel consumption improvement at low loads thanks to lower pumping losses, such systems have shown to reduce the need to enrich at high loads [8] to protect turbine components, reducing fuel consumption and emissions of hydrocarbons and carbon monoxides [9,10]. Besides, EGR can help to reduce the knock tendency [11,12], which can be translated into an efficiency increase [13] thanks to a better combustion phasing [14] or a compression ratio increase [15]. However, it has to be considered that a negative effect in laminar flame speed and combustion duration is induced by EGR, which can actually overcome the specific heat ratio and heat transfer benefits for some engine conditions, resulting in an indicated efficiency deterioration [16].…”