1989-02-01

The Influence of Pneumatic Atomization on the Lean Limit and IMEP 890431

Lean limit characteristics of a pneumatic port fuel injection system is compared to a conventional port fuel injection system. The lean limit was based on the measured peak pressure. Those cycles with peak pressures greater than 105 % of the peak pressure for a nonfiring cycle were counted.
Experimental data suggests that there are differences in lean limit characteristics between the two systems studied, indicating that fuel preparation processes in these systems influence the lean limit behaviors. Lean limits are generally richer for pneumatic fuel injection than those for conventional fuel injection. At richer fuel-to-air ratios the pneumatic injector usually resulted in higher torques. A simple model to estimate the evaporation occurring in the inlet manifold provided an explanation for the observed data. Apparently, larger droplets formed by the conventional injector collect on the hot surfaces in the inlet manifold, and undergo evaporation at a much faster rate than the fine droplets suspended in the air which were formed by the pneumatic injector.
Pneumatic port injection is expected to improve combustion characteristics of engines, particularly at cold start and light load (idle). In the pneumatic injector the fuel is finely atomized by the air used to transport it. The presence of finer droplets may generally alter the character of the charge and possibly enhance the energy release rate. These changes in the charge could influence the lean limit.
Investigation of lean limit operation is of interest because of the potential for increasing engine efficiency while decreasing emissions. Other advantages of pneumatic atomization are expected to appear in the experimental results. An analytical approach to relate in-cylinder droplet size history to the operation of spark-ignition engine is deemed unrealistic because of its complexity. A study to obtain experimental data to determine the effect of fuel droplet preparation on the operation of a single cylinder spark-ignition engine is therefore warranted.
The purpose of this work is first to experimentally determine the influence of pneumatic fuel injection, on the lean limit and performance characteristics of the CFR engine, by comparing the results against a conventional fuel injection system. Second to understand the reasons for the observed phenomenon. This paper is based upon the work presented in the Master's thesis by R. Nussio [1].
In order to establish an appropriate basis for comparison, a pneumatic port fuel injector was compared to a conventional port injector. The conventional injector is representative of current port injection technology. Generally, the results are presented as a comparison between the two systems. Experimental data was obtained from an air-to-fuel ratio of approximately 14.5 to 1, to the lean limit for both injection systems for a range of engine speeds and air flow rates.

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