Capacitors in Gasoline Engine Ignition Systems

Capacitors play a critical role in the reliable operation of gasoline engine ignition systems, with paper-based capacitors being a traditional and widely used type. This article will detail the structure, wiring configuration, core functions, performance requirements, and capacity-related considerations of these capacitors, while also noting the emerging trend of metal film capacitor applications.

Structure of Paper Capacitors

The paper capacitor typically consists of two long, narrow metal foils serving as electrodes. These foils are interleaved with two equally long and narrow strips of thin insulating paper. The assembly is then rolled into a cylindrical shape. Air between the layers is removed in a vacuum, after which the entire roll is impregnated with capacitor oil or paraffin as the insulating medium. Finally, the impregnated roll is enclosed in a cylindrical metal casing.

Wiring Configuration

One of the aluminum foils is connected to the metal casing. A rear ground mounted on the distributor housing or circuit breaker fixing plate establishes a connection between the casing and the fixed contact on the iron circuit breaker. The other aluminum foil is connected via a No. 5 wire to a central terminal block insulated from the metal casing. This terminal is further connected to the circuit breaker wiring and then to the primary winding circuit, forming a parallel loop with the circuit breaker contacts.

Core Functions

Protect Circuit Breaker Contacts:When the circuit breaker contacts separate, the capacitor prevents damage from sparks generated by the self-induced electromotive force (ranging from 200V to 300V) and current in the primary winding.

Enhance Ignition Performance:It accelerates the decay of primary current and magnetic flux, increasing their rate of change and preventing a drop in secondary voltage, which is essential for reliable spark generation.

Performance Requirements

When the contacts open, the capacitor operates in conjunction with the self-induced electromotive force. To ensure reliable operation, the capacitor must not break down when subjected to a 600V AC voltage for 1 minute.

Impact of Capacitance on Ignition

Capacitance has a significant impact on ignition system performance:

Excessive Capacitance:If the capacitance is too large, the capacitor discharges slowly when the contacts open, causing the primary current and magnetic flux to dissipate gradually, which leads to low secondary voltage. When the contacts close, slow charging of the capacitor results in a slow rise in primary current, also reducing secondary voltage. This issue is particularly pronounced at high engine speeds, increasing the likelihood of misfires. Additionally, excessive capacitance is ineffective at eliminating or reducing sparking between contacts, making the contacts prone to burning.

The typical capacitance range for such capacitors is 0.17-0.35 μF. Currently, metal film capacitors are gaining widespread popularity due to their smaller size and lower cost.

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