Search Results

Computer models of engine processes are valuable tools for predicting and analyzing engine performance and allow exploration of many engine design alternatives in an inexpensive fashion. Knowledge of the flame initiation period and subsequent propagation periods are the most important parameters of the combustion modeling. At the present work, a simple experimentally based model for flame propagation period was developed for a natural gas engine. This model does not require any knowledge of the flame shape and propagation speed to calculate the burning rate. The model was used along with the mass and energy conversation equations to calculate cylinder pressure traces for various engine operating conditions. In all cases the model's results and the experimental data agree well in trends and magnitudes.

This paper reports on an experimental study conducted to determine the effect on the knock limited operating map of a natural gas engine when propane is added to the fuel. The map involves engine parameters such as BMEP, spark timing, equivalence ratio, and propane fraction. The map shows that to maintain its design BMEP, the maximum and minimum equivalence ratios that the engine can operate with natural gas are 0.78 at a timing of 25 degrees BTDC and 0.73 at 20 degrees BTDC, respectively. However, when the propane percentage of the fuel is increased to 15% of the fuel by mass, the maximum and minimum equivalence ratios that the engine can operate are 0.75 and 0.70, respectively, which corresponds to spark timings of 22 and 20 degrees BTDC. The map demonstrates that knock is not a major constraint for typical natural gas. Spark timing retard is limited by the exhaust gas temperature and minimum equivalence ratio is limited by the BMEP requirement of the engine.

Knock-induced pressure waves in the combustion chambers of spark-ignited engines cause the engine block to vibrate at the same frequencies. These vibrations have different amplitudes at different locations on the engine block. This paper describes a project to find a location on the engine block where the amplitudes of the knock-induced vibrations are high enough to use in a knock control system. To find this location, six piezoelectric knock sensors were located on suitable regions of the engine block. Data were collected from the sensors at both knocking and non-knocking conditions using a high speed data acquisition system. After the data were transformed into the frequency domain, comparison of the knocking and non-knocking condition data indicated the frequencies and amplitudes of the knock-induced engine block vibrations. The location where knock-induced vibrations were transferred with the greatest amplitude was determined.