Investigation of the Knock Limited Brake Mean Effective Pressure for Propane, 03-9439

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Principal Investigators
Timothy J. Callahan
Jess W. Gingrich

Inclusive Dates:  10/08/03 - 02/08/04

Background - The Port Terminal Rail Authority (PTRA) currently operates 24 MP 1500D switcher locomotives within the Houston nonattainment area. The annual oxides of nitrogen (NOx) output from these 24 locomotives are 418 tons per year. A study conducted by SwRI indicated that the NOx produced by the PTRA switchers could be reduced by 120 to 200 tons per year from current levels, if the locomotives utilized propane for fuel. Like natural gas, propane-fueled engines offer significant emissions benefits over traditional diesel engines. Propane, a liquid at moderate pressures, does not have the fuel storage problems associated with natural gas. The octane number of propane, however, is lower than that of natural gas, making it more prone to knock in heavy-duty applications. Lean or dilute combustion is required to mitigate knock, but ignition of these highly dilute mixtures is difficult.

Approach - This program examined the use of pilot ignition of dilute propane-air mixtures to obtain high power densities required in heavy-duty applications. A single-cylinder research engine was converted to propane operation with pilot ignition. Engine performance was first recorded with natural gas. The lean misfire limit and the knock-limited brake mean effective pressure (BMEP) were determined. Engine performance on propane was then investigated. Initial results showed that operation on propane was severely limited by knock in comparison to the natural gas performance. Several operational and design variables were considered for improving the knock tolerance of the engine on propane. Compression ratio changes were eliminated based on the requirement for ignition of the pilot and prevention of knock for the propane. A 12:1 compression ratio piston was optimum for these conflicting requirements. The combination of the effects of manifold air temperature, injection pressure, injection timing, and pilot quantity was investigated.

Accomplishments - High BMEP engine operation on propane was demonstrated using pilot ignition. By increasing the injection pressure, minimizing the pilot, and reducing the manifold air temperature, load conditions comparable to locomotive service were achieved. The results from this project supported a $2.5 million promotional effort for a propane locomotive with the Texas Railroad Commission.

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