Scilight Press

1. Introduction

Engine braking, as an effective auxiliary braking method, has gained widespread acceptance and application among domestic truck users since its introduction to the Chinese market in the early 21st century. It plays a crucial role in improving transportation efficiency, reducing costs, and enhancing safety. Currently, however, the core technologies, design expertise, and application experience in engine braking remain largely held by foreign companies or enterprises with foreign backgrounds. China still faces notable gaps in related areas such as engine design, testing, patents, and technical understanding in this field [1,2,3].

Dongfeng Commercial Vehicle Co., Ltd. (Wuhan, China) stands as one of the domestic main engine manufacturers that early adopted engine braking technology and undertook independent research and development in this area. The company has successfully commercialized a significant number of self-developed engine braking technologies. As a state-owned enterprise, it bears the responsibility of providing informational support to other enterprises engaged in the independent development of engine braking. This paper shares a development case of integrated brake rocker arm failure that occurred several years ago at the company and offers relevant reflections based on the experience.

2. Analysis

Compression-release engine braking is the prevailing mainstream design scheme. For a four-stroke engine, its operating principle involves opening the exhaust valve with a small lift before the compression top dead center when the engine is motored without fuel injection. The phase duration for this event typically spans approximately 60° of crankshaft angle. The mainstream implementations of compression-release engine braking can be categorized into two types: dedicated rocker arms and integrated rocker arms. Dedicated rocker arms benefit from more generous spatial allowance and exhibit minimal lost motion during positive power operation, thereby ensuring the reliability of their adjusting bolts [4,5,6].

The integrated brake rocker arm must incorporate the main lift into its brake cam profile design. In terms of technical approach, there are two schemes available: one with a maximum braking lift of approximately 2.2 mm without exhaust braking integration, and the other with a maximum braking lift of about 1.6 mm combined with exhaust braking (the two differ in cost and application scenarios, which will not be elaborated here). If the larger-lift braking scheme is selected, it may result in a lost motion of 3–4 mm in the exhaust rocker arm under positive power conditions. When high-stiffness, high-preload springs cannot be feasibly installed in the available space to restrain the exhaust rocker arm, valve float is likely to occur during overspeed operation [7,8].

2.1. Failure in Overspeed Test

The adjusting bolt of the independently integrated brake rocker arm in one of the company’s medium-sized engines fractured during an overspeed test, as shown in Figures 1 and 2.

Figure 1. Fracture of the adjusting bolt.

Figure 2. Comparison between the original and finalized cam profile schemes.

2.2. Cause Analysis

The initial step was to conduct a material analysis of the failed components, including hardness testing (high-strength bolt material) and composition testing. The results were found to comply with all specified requirements, as detailed in Table 1.

Table 1.

Surface hardness characteristics of parts.

No.	Vickers Hardness (HV1)	GBT3098. Requirements
1	298	290–360 HV1
2	304
3	310

The second step involved conducting a metallographic analysis of the fracture surface. The examination of the fracture morphology and crack initiation site confirmed a high-cycle bending fatigue failure. No material or processing defects were observed at the crack origin, as illustrated in Figure 3.

Figure 3. Adjusted bolt crack initiation region metallograph.

The third step involved reviewing overspeed test reports for the valve train (without the early-stage engine braking function) as well as various reliability and durability test reports under conditions with the brake rocker arm installed. No anomalies or related failures were observed in these reports.

Based on the results from the third step, the fourth step concluded that this failure is strongly correlated with both the braking system operation and the overspeed condition.

2.3. Detailed Design in Development

The dynamic analysis revealed that the original brake cam profile exhibited significant negative acceleration at the BGR and CR positions. This caused severe rocker arm float, resulting in forceful collision between the adjusting bolt and the valve bridge and generating substantial impact loads. While a spring-assisted solution to suppress float was initially proposed, it ultimately did not pass design review. Consequently, it was decided that the issue would be resolved through cam profile optimization, as shown in Figures 4 and 5.

After optimization, the impact load was significantly reduced. However, given the uncertainty in the accuracy of the calculated impact forces and the lack of established evaluation criteria, a further strength assessment of the adjusting bolt was carried out. Its reliability and durability were subsequently verified through testing.

Figure 4. Correlation between cam.

Figure 5. Comparison of elephant foot profile and impact force impact forces.

3. Optimization Design

The optimization design proceeded primarily in two directions: first, by refining the existing cam profile to minimize or eliminate the impact force; second, by reducing the brake cam lift, as a lower-lift design more readily achieves a non-impact operational state, as shown in Figure 6.

Figure 6. Comparison of cam profiles of the optimization schemes.

By further reducing negative acceleration in the CR segment to lower impact forces, the following design approaches were proposed:

Scheme 1-1: Maintain the brake lift unchanged and slightly reduce negative acceleration in the CR segment, as shown in Figure 7.

Figure 7. Comparison between determined scheme and Scheme 1-1.

Scheme 1-2: Maintain the brake lift unchanged and reduce negative acceleration in the CR segment by half, as shown in Figure 8.

Figure 8. Comparison between the determined scheme and Scheme 1-2.

Scheme 2: Reduce the brake lift from 2 mm to 1.6 mm while simultaneously halving the negative acceleration in the CR segment, as shown in Figure 9.

Figure 9. Comparison between the determined scheme and Scheme 2.

A comprehensive analysis of the calculation results, factoring in design modifications, cost considerations, previous development assessment tests, and project timelines, led to the selection of Scheme 1-2 as the optimal choice. Nevertheless, to ensure systematic development rigor, it remains necessary to conduct mechanical component-level validation tests to verify the reliability of the computational findings.

4. Test and Measurement

Given that directly measuring the force on the elephant foot would require attaching strain gauges—a relatively complex procedure—the acceleration of the rocker arm shaft near the exhaust rocker arm was instead measured to indirectly reflect the impact load induced by the cam profile. To capture the phase relationship, the valve lift of the corresponding brake exhaust valve was also measured (sensor range limitations allowed only partial lift data to be recorded). Emphasis was placed on the acceleration in the Z direction, with the vertical upward direction defined as positive Z.

Scheme 1-2 subsequently passed overspeed test verification, and no further fractures of the brake rocker arm adjusting bolt have occurred in field applications, as shown in Figures 10 and 11. The results of schemes 1-2 correspond to a significant reduction in the vibration amplitude in the Z direction at the phase position

Figure 10. Vibration acceleration of the rocker arm shaft of Cylinder 1 in determined scheme.

Figure 11. Vibration acceleration of the rocker arm shaft of Cylinder 1 in Scheme 1-2.

5. Conclusions

(1) When space constraints limit the design layout, the reliability issues of the integrated brake rocker arm system can be effectively addressed by optimizing the cam profile to appropriately reduce braking power.

(2) Dynamic simulation and analysis must be conducted early in the design phase, and any rocker arm float under overspeed conditions should be strictly avoided.

(3) In determining the design configuration for the integrated brake rocker arm, it is recommended to adopt a small-lift CR/BGR profile combined with exhaust brake functionality to achieve an optimal balance between strong braking performance and high reliability.