2011 IR&D Annual Report

The Investigation of the Formation, Composition, and Durability of Zinc Dialkyldithiophosphate Films in a Spark-Ignited Gasoline Engine, 08-R8215

Principal Investigators
Eric Liu
Vasiliki Poenitzsch
Robert Mason
Fred Gerhart

Inclusive Dates:  03/01/10 – Current

Background — Zinc dialkyldithiophosphates (ZDTPs) are common additives used in engine lubricants to primarily improve their antiwear (AW) performance and secondarily serve as an antioxidant in the oil. Although this technology has been in existence since its invention more than 60 years ago, little is understood about the mechanism that allows ZDTPs to have such great AW performance. Past research has shown that ZDTP forms a sacrificial film that separates two metal components in relative motion to mitigate or prevent metal-on-metal contact. The film composition, formation and durability are shown to be affected by bulk temperature of the lubricant, loading on the two metal surfaces and exposure to nitric oxides (NOx) in an aqueous solution. However, many of these experiments are restricted to bench-top test setups whose test conditions are set at coarse increments that may or may not reflect actual engine operating conditions. Also, no quantitative model exists that gives a general overview of the behavior of ZDTP films. This project seeks to study the changes in composition of ZDTP films while exposed to the harsh conditions of a fired engine test. Specifically, temperature of the oil in the valvetrain area, valve spring loads and crankcase NOx concentration will be the test variables in these experiments. The relationship between the composition of ZDTP films and the specified test variables will be quantitatively characterized in a response surface model developed from the data obtained during the experiment.

Approach — The data required to formulate the response surface model will be obtained from the results of an experimental matrix involving three levels of variation in each of the following three test variables: oil temperature, valve spring loads, and crankcase NOx concentration. Control over oil temperature and valve spring loads are relatively simple to achieve, but control over crankcase NOx concentration will require a separate response surface model that requires a separate test matrix to develop. The test matrix for crankcase NOx concentration will consist of three levels of variation in each of the following five test variables: engine load, fresh air flow to the rocker arm cover, spark advance, air-fuel ratio and piston ring gap. During the experiments, these five test variables will be independently controlled. The data obtained from this test matrix will be used to formulate a response surface model that characterizes crankcase NOx concentration at various levels of each of the aforementioned five test variables.

Accomplishments — The investigation team has successfully established control over the test engine's operating temperatures, AFR and spark advance. A response surface model of the crankcase NOx concentration with respect to engine load, fresh air flow to the rocker arm cover, spark advance, AFR and piston ring gap has been established. The investigation team will proceed to use this response surface model to establish operating conditions that can generate high, medium and low levels of crankcase NOx concentrations for the main experimental matrix that tests for the effects of oil temperature, valve spring loads and crankcase NOx concentration on the composition of ZDTP films.

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Southwest Research Institute® (SwRI®), headquartered in San Antonio, Texas, is a multidisciplinary, independent, nonprofit, applied engineering and physical sciences research and development organization with 10 technical divisions.