Made in China Fuel Injection Pump Plunger S1100 8.5mm Pump Elements Engine Accessories

To meet the wear, corrosion, and fatigue requirements of high-pressure oil pump plungers under extreme operating conditions, this paper investigates plunger fabrication technologies using novel ceramic-based composites, such as silicon carbide (SiC)/aluminum oxide (Al₂O₃) and carbon fiber-reinforced silicon carbide (C/SiC). By comparing the mechanical properties, tribological characteristics, and service performance of traditional metal plungers (38CrMoAlA), this paper reveals the advantages and adaptability of ceramic-based composites in plunger applications. Experimental results show that the C/SiC composite plunger exhibits over three times the wear resistance of metal plungers, a lower coefficient of friction (0.12), and improved high-temperature stability (up to 600°C), providing a new solution for material upgrades in high-pressure oil pump plungers.

Introduction
High-pressure oil pump plungers operate in a highly demanding environment characterized by high contact stress (peak value up to 500 MPa), high reciprocating speed (up to 8 m/s), and high operating temperature (300-400°C), while also enduring corrosion from diesel fuel and particle erosion. Despite surface hardening, traditional metal plungers still lack the wear resistance and high-temperature stability required by next-generation engines for long life (targeted at 15,000 hours). Wear-induced degradation of injection precision can increase engine fuel consumption by over 10%.
Ceramic matrix composites (CMCs), with a ceramic matrix and fiber or particle reinforcement, combine the high hardness (HV 1500-3000) and high-temperature resistance of ceramics with the toughness (fracture toughness 4-10 MPa・m¹/²) of composites. They have proven themselves in extreme operating conditions such as aircraft engine turbine blades. Their application in plunger manufacturing could overcome the performance bottlenecks of metal materials, but key challenges remain, such as controlling molding precision, interfacial bonding strength, and cost control. This article focuses on two typical types of ceramic matrix composites for plunger applications, systematically evaluating their performance compatibility.

Ceramic Matrix Composite Material Selection and Preparation Process
2.1 Material System Selection
Based on the service requirements of the plunger, two candidate materials were selected:

SiC/Al₂O₃ Particle-Reinforced Composite: This composite material uses Al₂O₃ as the matrix (70%) and SiC particles (5-10μm in size, 30%) as the reinforcement phase. This utilizes particle dispersion strengthening to improve wear resistance and is relatively low-cost (approximately three times that of a metal plunger).

C/SiC Fiber-Reinforced Composite: This composite material uses carbon fiber cloth (T700) as the reinforcement (30% by volume) and a SiC matrix fabricated via chemical vapor infiltration (CVI). This composite material exhibits high specific strength (strength/density > 200 MPa・cm³/g) and excellent thermal shock resistance. 2.2 Plunger Forming Process

SiC/Al₂O₃ plungers: Gel casting is used. Ceramic powder and organic monomers are mixed to form a slurry (65% solids content). This slurry is then poured into a plunger-shaped mold. After in-situ polymerization and curing, the mold is removed and sintered at 1600°C in air for 2 hours. Finally, the mold is precision-ground to IT5 dimensional accuracy (tolerance ±0.01mm).

C/SiC plungers: Fiber winding and chemical vapor infiltration are used. Carbon fibers are wound into a plunger-shaped preform. A mixture of CH₃SiCl₃ and H₂ is introduced at 900°C, gradually depositing the SiC matrix on the fiber surface. After multiple infiltration and densification cycles, the surface roughness is achieved to a Ra of 0.05μm.​
2.3 Surface Modification
To reduce the coefficient of friction, both composite plungers underwent surface treatment:

The SiC/Al₂O₃ plunger was treated with a 1μm-thick diamond-like carbon (DLC) coating deposited by plasma spraying, reducing the friction coefficient from 0.6 to 0.25.

The C/SiC plunger was treated with a silane coupling agent (KH550) to form a nanoscale SiO₂ transition layer, enhancing compatibility with the lubricating film.

The results showed that ceramic-based composites have significantly higher hardness than metals, but lower fracture toughness, necessitating structural design to avoid stress concentration. The low density of C/SiC (67% lighter than metal) reduces the reciprocating inertia of the plunger and reduces vibration and noise. 3.2 Friction and Wear Performance Evaluation
Tests were conducted using a reciprocating friction tester (simulating plunger-liner friction):
Experimental conditions: cast iron cylinder liner, load 50N (contact stress 300MPa), diesel lubrication, temperature 100°C, and test time 50 hours.
Wear Results:
SiC/Al₂O₃ plunger: wear loss 0.8μm, wear scar width 0.3mm, primarily due to abrasive wear.
C/SiC plunger: wear loss 0.3μm, smooth wear scar, indicating slight adhesive wear.
Metal plunger: wear loss 3.5μm, wear scar showing obvious plowing and material transfer.
The excellent wear resistance of C/SiC is attributed to the "pullout effect" of the carbon fibers—the fibers dissipate friction energy as they pull out of the matrix, inhibiting crack propagation.​
3.3 High-Temperature and Corrosion Performance

High-Temperature Stability: After 100 hours at 400°C, the strength retention of C/SiC was 90%, while the strength of SiC/Al₂O₃ decreased by 15% due to grain growth, and the hardness of the metal plunger decreased by 20% due to oxidation.

Corrosion Resistance: After immersion in sulfur-containing diesel (sulfur content 500 ppm) for 300 hours, the mass loss of C/SiC was less than 0.01%, and that of SiC/Al₂O₃ was 0.03%. The metal plunger showed pitting (depth 5μm) due to corrosion.

Bench Tests and Application Compatibility Analysis
4.1 High-Pressure Fuel Pump Bench Test
Two composite plungers were assembled into a high-pressure common rail pump and subjected to a 1000-hour endurance test (rail pressure 200 MPa, speed 2000 r/min):

SiC/Al₂O₃ plunger: After 500 hours, slight chipping occurred (due to insufficient impact toughness), and the injection volume deviation increased to ±2%, still meeting the requirements.

C/SiC plunger: After 1000 hours, performance was stable, with wear of 0.5 μm and injection volume deviation less than ±0.5%, showing no obvious signs of failure.

Metal plunger: After 300 hours, wear resulted in excessive leakage (>0.3 mL/min), requiring premature withdrawal from the test.​
4.2 Failure Mode Analysis
The SiC/Al₂O₃ plunger failed due to brittle fracture: microcracks developed at the rounded corner of the plunger head (a stress concentration area), which propagated and led to localized collapse.
The C/SiC plunger had a lower failure risk: the bridging effect of the carbon fibers effectively prevented crack propagation, and only minor fiber wear was observed after the experiment.

4.3 Cost and Life Cycle Assessment

Initial cost: The C/SiC plunger (approximately 800 yuan/unit) is four times that of a metal plunger (approximately 200 yuan/unit), but its service life is more than three times longer.
Life Cycle Cost: Based on 15,000 hours of service, the C/SiC plunger's unit cost (0.053 yuan/hour) is lower than that of a metal plunger (0.1 yuan/hour), offering an economic advantage.

Conclusions and Application Recommendations
Performance Advantages: C/SiC composite plungers offer optimal wear resistance, high-temperature stability, and lightweight performance, making them suitable for high-end engines (such as commercial vehicles meeting the China VII standard). SiC/Al₂O₃ plungers offer a more cost-effective solution and can be used in mid-range applications (such as agricultural machinery).

Improvements: The SiC/Al₂O₃ molding process needs to be optimized to improve toughness. The fracture toughness can be increased to 5.5 MPa m¹/² by adding a ZrO₂ phase transformation toughening phase (5%).

Application Limitations: The brittle nature of ceramic-based composite plungers makes them unsuitable for applications subject to severe impact (such as cold-start conditions in construction machinery). Flexible transmission mechanisms are required to cushion the load.

New ceramic-based composites offer a viable path to high-performance plungers. With the reduction in manufacturing costs (expected to drop by 40% within five years), they are expected to achieve large-scale application in the high-pressure oil pump industry.