Experimental Analysis of Breakdown With Nanosecond Pulses for Spark-Ignition Engines

Michelangelo Balmelli,Davide Bleiner,Patrik Soltic,Laura Merotto,Jurgen Biela,Raphael Farber,Christian M Franck

doi:10.1109/access.2021.3095664

Abstract

The influence of pulse rise rate and pulse duration for ignition purposes in engines is investigated. A constant volume cell is used to characterize the breakdown voltage under nanosecond pulsed voltages with automotive sparkplugs having electrode gaps ranging from 0.2 mm to 1 mm. Two pulse generators are used to compare pulses with durations of 10 ns and 50 ns. Different pulse amplitudes are used, and air gaps with breakdown voltages ranging from 4 kV to 15 kV are investigated. The cell is filled with synthetic air with densities gap distances products that are relevant for internal combustion engines. This study shows that the pulse shape and rise rate influence the breakdown voltage. Under pulsed discharge, the breakdown voltage is always above the static breakdown voltage. The probability of pulsed discharge breakdown increases as both the pulse amplitude and duration increases. Furthermore, the breakdown voltage value increases with increasing pulse rise rate. The delay time between reaching the static breakdown voltage and the actual breakdown voltage decreases with increasing overvoltage. The delay time is constituted by statistical and formative times. Both the statistical and formative times decrease with increasing overvoltage. For ignition purposes, the pulse rise rate should be as high as possible to deliver a larger energy input in the breakdown phase. Furthermore, for reduced electrode erosion, the pulse duration should be short (10-20 ns) to reduce the probability for a transition to an arc.

Highlights

In spark-ignition engines, increased efficiency is often limited by the ignition process
When fast-rising voltages are applied to the gap, as with the nanosecond pulsed discharge under investigation, the time needed to reach breakdown influences the value of the measured breakdown voltage U [15]
Theoretical estimates of the formative times for the Townsend and streamer mechanisms suggest that only the streamer mechanism is compatible with the observed delay times under pulsed voltage conditions

Summary

Introduction

In spark-ignition engines, increased efficiency is often limited by the ignition process. Three distinct phases constitute the discharge: breakdown, arc, and glow. The breakdown is defined as the first very short (nanoseconds) phase In this phase, the current rises to reach its maximal value, which is given by the ratio of the ignition voltage and near-gap impedance. The gap voltage drops to low values [2]. The energy stored in the parasitic cap capacitance (sparkplug) is transferred, with low heat losses, from the electric field gap to the electrons and subsequently to the heavy particles (molecules and ions). The breakdown phase ends when the electron supply mechanism changes from photo or field emission (glow) to thermionic emission, i.e., when cathode hot spots are formed, which turns the discharge into an arc [4]. The electrode erosion that diminishes the sparkplug lifetime is most significant once a cathode spot, and an arc is formed [3]

Methods

Results

Conclusion