Analysis of performance and knock phenomena in a converted spark-ignition medium-duty engine fueled with different LPG formulations

Since traditional fuels are being phased out, engine conversions to more sustainable fuels are being performed to address local regulations. Due to its potential to reduce CO2 and local pollutants, liquified petroleum gas (LPG) stands out as an alternative fuel for internal combustion engines. LPG chemical structure allows for higher compression ratios (CR) and improved thermal efficiency. However, studies indicated that increasing the CR may lead to knocking combustion. In this investigation, an engine conversion to LPG was analyzed using two LPG formulations, commercial propane (93.91% propane) and autogas (38.42% propane − 60.37% butane). First, using a numerical methodology previously proposed for estimating the event of knock based on the thermochemical characteristics of the fuel. Low-order methods are utilized in combustion simulations to estimate autoignition delay (AID) and laminar flame speed (sL). Second, an experimental validation was performed with a 4-cylinder turbocharged SI engine. It was tested with three different CRs: 12.45:1, 11.05:1, and 9.86:1. Simulation indicates that autogas exhibits a significantly higher tendency to knock compared to commercial propane. Experimental results confirm the occurrence of knocking when using autogas with a CR of 12.45:1, resulting in a 29% reduction in the engine maximum torque compared to commercial propane. Reducing compression to 9.86:1 minimizes the occurrence of undesired knocking. It also leads to a decrease in thermal efficiency by 3.8% for autogas and 4.5% for commercial propane.