This paper presents a systematic approach to diagnose and resolve cold-start knocking noises (1800-2300 rpm) in a V6 engine cylinder block. The transient metallic “clicking” sound – perceptible both internally and externally – diminished with rising coolant temperature and disappeared above 90°C.
1. Vibration Signal Analysis
Triaxial accelerometers (50 kHz sampling) mounted on cylinder block surfaces revealed impact signatures at Cylinder 5 through time-domain analysis. The knocking periodicity followed engine firing intervals:
$$T_{knock} = \frac{120}{n \cdot N} = \frac{120}{2000 \times 6} = 0.01\,s$$
where n = engine speed (rpm), N = cylinder count. Wavelet analysis identified dominant frequency components:
$$W(s,\tau) = \frac{1}{\sqrt{s}} \int_{-\infty}^{\infty} x(t) \psi^*\left(\frac{t-\tau}{s}\right) dt$$
showing energy concentration at 3500-5500 Hz (Figure 1). The continuous wavelet transform scalogram confirmed transient impacts synchronized with combustion events.
2. Angular Domain Correlation
Synchronizing vibration data with cam/crank signals (0.1° resolution) revealed knocking occurred at 486°CA after Cylinder 1 TDC. The piston positions were calculated using:
$$\theta_{cyl} = \theta_{ref} + 240°(k-1) \mod 720°$$
where k = cylinder firing order position. Critical positions at knocking instant:
| Cylinder | Position | Phase (°CA) |
|---|---|---|
| 5 | Compression TDC-114° | 486 |
| 2 | Expansion TDC+6° | 246 |
| 6 | Intake TDC+126° | 366 |
3. Engine Cylinder Block Structural Analysis
Modal testing revealed abnormal 2nd order Fourier coefficients in the problematic engine cylinder block:
$$F_2 = \frac{1}{N}\sum_{k=0}^{N-1} x_k e^{-i2\pi \cdot \frac{2k}{N}}$$
showing 32% deviation from specification. Metallurgical analysis identified insufficient bonding between cylinder liner and block (Figure 2), with void fractions up to 15% in critical areas. The thermal deformation mismatch was quantified as:
$$\Delta \varepsilon = (\alpha_{Al} – \alpha_{Fe}) \Delta T = (23.1-11.7) \times 10^{-6} \times 300 = 0.34\%$$
where α = thermal expansion coefficients, ΔT = casting cooling range.
4. Optimization Solutions
Three process improvements were implemented for engine cylinder block production:
| Parameter | Original | Optimized | Effect |
|---|---|---|---|
| Liner Preheating | Ambient | 250°C induction | ↑ Bonding strength 62% |
| Sand Core Drying | Air dried | 120°C×4h | ↓ Gas porosity 78% |
| Pouring Method | Single gate | Dual risers | ΔT < 15°C |
The improvements reduced cylinder bore distortion (2nd order) from 18μm to 5μm, calculated as:
$$D_2 = \sqrt{A_2^2 + B_2^2}$$
where A₂ and B₂ are Fourier coefficients. Post-optimization engines showed complete elimination of knocking noises across 20 validation prototypes.
5. Thermal-Structural Verification
Finite element analysis confirmed reduced thermal stresses in the optimized engine cylinder block:
$$\sigma_{thermal} = E\alpha \Delta T = 71\,GPa \times 23.1\times10^{-6} \times 150^{\circ}C = 24.6\,MPa$$
versus 58.3 MPa in original design. The safety factor improved from 1.8 to 3.2 against aluminum yield strength (250 MPa).
This systematic approach combining advanced signal processing, angular domain analysis, and structural optimization effectively resolved complex knocking noises in the V6 engine cylinder block, demonstrating a 100% success rate in production validation.