Lean Limit Expansion up to Lambda 2 by Multi-Point Microwave Discharge Igniter

The main challenge to run the engine at ultra-lean or high EGR conditions depends on robust ignition of the mixture i.e. generation of a repeatable and robust ignition kernel to subsequently ignite the fuel-air mixture. We had developed microwave enhanced ignition system in which regular spark is augmented by microwaves which generates a bigger size plasma where. large pool of active radicals effectively increased the initial flame speed, engine efficiency, extended the lean limit and resultant decrease in COV of IMEP. Recently, we developed another plasma-based ignition device named Microwave Discharge Igniter (MDI) which works on the principle of microwave resonation within a cavity. MDI is a compact 𝜙 4.5 mm plug with a quarter coaxial cavity resonator built into its structures. It receives the microwave (MW) pulse signal at 2.45 GHz from a semiconductor-based MW generator which can be controlled to produce very precise pulse characteristics such as pulse width, pulse number and pulse frequency, with time resolution down to 0.1 μs. The MDI has been shown to have very good combustion performance, including dilution and lean limit extensions. ... mehrAn igniter for gasoline engine also needs to be robust. Hence, the MDI was put under stress and endurance tests. The tests were carried out inside a constant volume chamber at non-reactive condition up to 0.5 MPa. The MDI was controlled to discharge continuously for more than 20 million times, 124-hour straight, mimicking the standard lifetime of 20,000 km highway driving. The compact size of the igniter means that multi-point ignition inside the combustion chamber is possible. In this study, a 3-point MDI plug with M12 size was developed and tested inside a practical commercially available multi-cylinder engine to evaluate the performance of multi-point ignition. The lean limit was compared with a standard spark ignition system at 1460 rpm engine speed and 20 Nm engine torque condition. As expected, 2-point ignition performed better than single-point, reaching the air-fuel ratio of 31 (approximately Lambda, λ = 2.1) in cylinders #1 and #3. However, the variance in IMEP of cylinder #2 was higher than that of cylinder #1 and #3 at the same airfuel ratio for both spark ignition and multi-point MDI. This is caused by cylinder difference of combustion due to the mounting platform constraint imposed on the intake manifolds. Even though the engine used for this study was not optimally designed for higher lean limits at the chosen conditions of this study, the multi-point MDI demonstrated a better load and emission performance tests maintaining exhaust gas temperatures below 300°C and achieving single digit ppm of NO compound emission.