Stochastic preignition (SPI) or low-speed preignition (LSPI) is an abnormal
combustion phenomenon observed in downsized turbocharged direct-injection
spark-ignition engines at highly boosted conditions. SPI results from the
ignition of the air-fuel mixture from a fuel or oil droplet or a detached
deposit before the spark discharge, and its occurrence can lead to extremely
high peak pressures and severe knock, which can cause physical damage to the
engine. This phenomenon limits the downsizing and boosting potential of
direct-injection spark-ignition engines, thereby constraining the efficiency
benefits that can be achieved.
The propensity for SPI to occur is impacted by engine operating conditions as
well as the properties of the fuel, fuel additives, lubricant, and lubricant
additives. To mitigate its occurrence, it is important to understand the factors
that impact the frequency of SPI events. As this abnormal combustion phenomenon
is relatively recent, there was a lack of a standard procedure to detect the
impact of a parameter on SPI frequency. This study details the development and
validation of an engine dynamometer test procedure—the TOP TIER™
Standardized Dynamometer Test Method to Evaluate Additized Detergent
Gasoline for SPI—approved by the Center for Quality Assurance (CQA),
to evaluate gasoline additives for their impact on SPI.
In this project, the newly validated SPI test protocol was used to compare the
relative SPI tendencies of four TOP TIER™ fuel additives at maximum retail
concentration against unadditized SPI test fuel, which served as the baseline.
All four fuel additives were tested three times in randomized order. The results
revealed that none of the TOP TIER™ additives tested had a statistically
significant impact on the SPI rate.
