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In modern diesel and gasoline direct injection systems, multi-injection strategies such as pilot, main, and post injections are widely applied to improve combustion efficiency and reduce emissions. While significant attention has been paid to electronic control accuracy and hydraulic response, the influence of assembly tolerance stack-up on multi-injection consistency remains insufficiently investigated. This article analyzes how assembly-related deviations affect injector performance and proposes a tolerance-tolerant structural design approach.

Fuel injectors consist of multiple precision components, including the nozzle needle, control valve, armature, springs, and shims. During assembly, small dimensional deviations in these components accumulate, resulting in variations in preload force, magnetic air gap, and effective needle stroke. These variations have a limited impact on single-injection operation but become critical under multi-injection conditions, where extremely short dwell times and precise needle positioning are required.

A coupled mechanical–hydraulic model is established to evaluate the effect of tolerance stack-up on injection delay, minimum controllable injection quantity, and injection separation stability. Simulation results show that axial preload variation of the control valve spring is the dominant factor influencing pilot injection repeatability. A preload deviation of only ±5% can cause more than 15% fluctuation in pilot injection quantity, leading to unstable combustion initiation.

To address this issue, a tolerance-tolerant injector structure is proposed. The design introduces a floating compensation element between the armature and control valve, allowing partial decoupling of magnetic force transmission from assembly-induced preload variations. Additionally, a dual-rate spring system is adopted to reduce sensitivity to shim thickness errors while maintaining sufficient closing force at high pressure.

Experimental validation is carried out using injectors assembled with intentionally varied component tolerances. Test results demonstrate that the proposed structure significantly improves multi-injection consistency, reducing pilot injection dispersion by approximately 30% compared with conventional designs. The separation stability between consecutive injections is also enhanced, especially under high rail pressure and short dwell conditions.

Furthermore, the tolerance-tolerant design simplifies assembly quality control by relaxing critical dimensional requirements without sacrificing performance. This provides practical benefits for mass production, reducing rejection rates and improving injector-to-injector uniformity.

In summary, assembly tolerance stack-up plays a crucial role in determining the multi-injection performance of fuel injectors. By integrating structural compensation mechanisms into the injector design, it is possible to enhance injection consistency, robustness, and manufacturability. The proposed approach offers a valuable reference for next-generation injectors designed for complex injection strategies and high-volume production.