On natural gas combustion split into diesel like architecture of a compression ignition engine retrofitted for spark ignition operation

In commercial areas that no longer favor diesel engine, such as Europe, it might be interesting to convert an existing compression ignition (CI) engine to the spark ignition (SI) operation and to use natural gas (NG) because of its advantages: availability of still abundant NG supplies worldwide and environmental benefits of NG compared to conventional liquid fossil fuels. This paper presents experimental results on NG combustion inside such a converted engine with diesel-like architecture dedicated to light-duty vehicles (LDV) and passenger cars (PC). Particularly, our study carried out at the engine test bed revealed that in certain operating points (low speed and load, stoichiometric mixture and rather high spark advance), the combustion is split into two distinct events, which is not specific to the standard SI engine. This is clearly illustrated by a rate of heat release (RoHR) profile with two peaks. This kind of RoHR profile appeared following a spark advance sweep carried out in stoichiometric and lean mixtures. The explanations for such combustion split are supported by a 3D CFD study showing the air-NG mixture formation and the in-cylinder NG distribution before and after the spark production. Thus, the CFD results support the idea of combustion split into, first, a fast combustion inside the piston bowl and then, a slower combustion occurring outside the bowl-in combustion chamber, in other words, in the squish region. Consequently, when setting up the calibration of such an engine, reducing the fuel mass burning in the squish region into this second stage of combustion should be aimed in order to reduce the formation of carbon monoxide (CO), unburnt hydrocarbon (UHC) and particulates (PM, PN).