Masters Theses

Date of Award

3-1987

Degree Type

Thesis

Degree Name

Master of Science

Major

Mechanical Engineering

Major Professor

Jeffrey W. Hodgson

Abstract

The prospect of future fuels' (from coal and oil shale) containing more nitrogen than existing fuels, coupled with the possiblity that automobile manufacturers may resort to increased use of supercharging, has motivated this study. The objective is to determine whether high levels of fuel-bound nitrogen lead to increased emissions of nitric oxide in a supercharged engine. This study of the fuel-nitrogen conversion in a supercharged, sparkignition engine consisted of three phases. Phase I involved determining the effect of fuel-nitrogen on the nitric oxide (NO) emissions for every combination of compression ratio (6, 8, 10), intake manifold pressure (20, 30, 40, 50 in. Hg.), air-fuel ratio (12, stoichiometric, 16, lean misfire), and fuel-nitrogen level (0, 0.3, 0.6, 0.9$ by mass) using iso-octane as the base fuel. Phase II involved determining the effect of the base fuel aromatic content on the NO emissions for every combination of base fuel (low, medium, high aromatic content), air-fuel ratio (12, stoichiometric, 16, lean misfire), and fuel-nitrogen level (0, 0.6, 0.9% by mass). Phase III involved testing Solvent Refined Coal (SRC), a synthetic gasoline containing no nitrogen, for air-fuel ratios of 12, stoichiometric, 16, and lean misfire. In both Phase II and Phase III, the compression ratio was 8 and the intake manifold pressure was 30 in. Hg. For each phase, the engine was run at 1800 rpm and MET (Minimum for Best Torque) spark timing. The fuel-nitrogen levels were created by doping the base fuel with pyridine. Plots of MNOMF (mass flux of nitrogen in the NO per mass flux of fuel) versus PHI (equivalence ratio) for every combination of each phase were made. The fraction of the fuel nitrogen converted to nitric oxide was also calculated for Phase I data.

In evaluating the plots and fuel-nitrogen conversion values, it was found that uncontrolled parameters (spark advance and inlet air humidity) played a more significant role in NO emissions than the fuel-nitrogen in Phase I. Although the spark advance was maintained at MBT, it was found that a difference of just a few degrees from this setting (particularly in the fuel-lean region) could result in MNOMF value changes which significantly altered the fuel-nitrogen conversion results. At the combination of high compression ratios and high intake manifold pressures, it was necessary to reduce the spark advance to avoid knock, thus, resulting in lower MNOMF values. Any effects of the individual parameters (compression ratio, intake manifold pressure, air-fuel ratio) on NO emissions were masked by the effects of these uncontrolled parameters. In Phase II, it was found that the aromatic content of the base fuel had no effect on NO emissions. In Phase III, the results were similar to those in Phase I and Phase II. However, in the fuel-rich region. Phase III data had consistently higher MNOMF values than those in Phase I and Phase II. No explanation is given for these results.

Other conclusions drawn were that the fuel-lean operating regime held the most interest, that in the fuel-rich regime one curve represents all the data of Phases I and II, and that the addition of pyridine to a base fuel changes the stoichiometry and MBT spark timing. A comparison with the results of other researchers suggests that the results of this study may be associated with stratification of the fuel-air mixture.

It is recommended that additional tests be conducted to determine whether better fuel-air mixing changes the results.

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