Lab 3 - Ignition of Premixed Gases

(Recommended material: Chapter 5 – Ignition)

 Video demonstrations of this lab can be found on YouTube at http://www.youtube.com/user/FndmtlsofCombustion

Most automobiles are powered by engines that use gasoline or diesel as fuel. The fuel is sprayed either into the intake manifold (gasoline engines) or the combustion cylinder (diesel engines) and evaporates, forming a gaseous fuel-air mixture. The mixture is compressed in the combustion cylinder then ignited by a spark. On a cold morning, the mixture sometimes fails to ignite and we are forced to feed excess gasoline to start the engine (choke). This indicates that the ignitability of the premixed mixture depends on temperature, ratio of air to fuel, spark energy, and other parameters. In this experiment, the ignitability of premixed methane-air mixtures will be in- vestigated by measuring the energy required to ignite these mixtures inside a sealed combustion chamber. The ignition energy will be measured over a range of equivalence ratios which are calculated from the partial pressures of air and methane. The pressure history in the combustion chamber is acquired using a pressure transducer. The mixture is ignited using an electric spark from a capacitor dis- 1 2 charge ignition system where the energy of the electric spark is given by 2 CV . The capacitance (C) of the ignition system is 2.2 μF and the voltage is given by a digital readout. By testing over a range of equivalence ratios, we will be able to find the flammability limits of methane.

Procedures: 1. Set igniter voltage to 35 V. 2. Set capacitor gap to d = 1.8 mm. 3. Evacuate the combustion chamber to 0.1 atm. 4. Fill combustion chamber with air to 1 atm. 5. Repeat above 2 times to flush the system. 6. Fill with air to 0.5 atm. 7. Fill with a given pressure of methane (0.07, 0.08, 0.09, 0.10, and 0.11 atm) 8. Fill with air to 1.0 atm. 9. Close all valves. 10. Increase the voltage while triggering the spark until ignition occurs. 2

11. Record the voltage and pressure history. 12. Repeat for different equivalence ratios.

Exercises

Based on your experimental observations and measurements: a) Calculate the equivalence ratios for the different partial pressures tested and the energy for the different igniting voltages. b) Make an energy vs. equivalence ratio plot and discuss your findings.