Multiple statistical methods were evaluated to determine the most suitable
approach to gauge whether a gasoline deposit control additive (DCA) causes a
statistically significant increase in rate of stochastic preignition (SPI) in a
direct injection, turbocharged, spark ignition (SI) engine. SPI is an abnormal
combustion phenomenon observed at highly boosted conditions in turbocharged SI
engines. The high in-cylinder pressures observed during an SPI event have the
potential to cause physical damage to the engine.
The SPI rate is a function of engine operating conditions as well as fuel and oil
properties. DCAs are commonly used in commercial gasoline at very low
concentrations (20–200 ppm m/m) to prevent the buildup of carbon deposits in the
combustion chambers and fuel systems. Since the effect of DCAs on SPI rate can
be subtle and the data inherently stochastic, a controlled engine test
methodology with robust statistical analysis is required to conclusively
determine whether a gasoline detergent increases or mitigates the rate of SPI
occurrence. Therefore, the TOP TIER™ Standardized Dynamometer Test Method to
Evaluate Additized Detergent Gasoline for Stochastic Pre-Ignition (hereafter
referred to as the TOP TIER™ Additive SPI Test Protocol) described herein is
designed to gauge whether a gasoline additive causes a statistically significant
change in the SPI rate at a controlled operating condition as compared to
unadditized gasoline.
This study focuses on the statistical analysis portion of the test protocol.
Multiple statistical approaches were evaluated using data from the TOP TIER™
Additive SPI Test Protocol to determine the most appropriate and robust approach
for detecting a statistically significant difference in SPI rates. The analysis
included the two-sample (unpaired) t-test, linear mixed-effects models (LMMs),
the Mann–Whitney U test, and the permutation test. The results indicated a
combination of permutation paired with Wilkinson’s method as the most suitable
approach.
