A Comparative Low Speed Pre-Ignition (LSPI) Study in Downsized SI Gasoline and CI Diesel-Methane Dual Fuel Engines 2014-01-2688
Low speed pre-ignition (LSPI) in downsized spark-ignition engines has been studied for more than a decade but no definitive explanation has been found regarding the exact sources of auto-ignition. No single mechanism can explain all the occurrences of LSPI and that each engine should be considered as a particular case supporting different conditions for auto-ignition.
In a different context, dual fuel Diesel-Methane engines have been more recently studied in large to medium bore compression ignition engines. However, if Dual Fuel combustion is less knock sensitive, LSPI remains one of the main limitations of low-end torque also for dual fuel engines. Indeed, in some cases, premature ignition of CNG can be observed before the Diesel pilot injection as LSPI can classically be observed before the spark in gasoline engines.
This article aims at highlighting the similarities and discrepancies between LSPI phenomena in SI gasoline and dual fuel engines. The comparison of both cases bring a novel perspective on some proposed LSPI mechanisms in literature and help in understanding possible conflicting theories.
The first essential characteristic of dual fuel engines lies in the use of a gaseous fuel (port-injected in the studied case). The mixture homogeneity within the combustion chamber is thus expected to be much better than with a liquid fuel. Compared to classical direct injection gasoline engines, fuel impacts on the walls don't exist and fuel / lubricant interactions must be reassessed as if to explain LSPI.
The high octane number and auto-ignition delay times of methane must also be considered but counterbalanced by the high compression ratios used. Critical conditions for auto-ignitive LSPI can thus be compared for SI and dual fuel engines.