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The aim of this work was to carry out a sensitivity analysis of a valve recession model. For the sensitivity study, the effects of the parameters on the valve recession model were investigated, for both, light duty and heavy duty engines. For light duty engines, it was observed that the impact wear of component parameters had the greatest effect on valve recession and for heavy duty engines, the sliding wear of component parameters had an increasing contribution to the overall valve recession.

Diesel engines are subjected to high thermal and mechanical stresses, and are exposed to loads resulting from impact on valve closure, combustion pressures and high temperatures [

Not many models exist to predict valve or seat insert life. One study [

The aim of this work was to investigate the effect of the parameters on a valve recession model [

A valve recession model [

where k is a sliding wear coefficient, P is the load at the valve seat interface, N is the number of cycles, h is the hardness, ^{2}, where m is the valve and follower mass and v is the valve closing velocity), A_{i} is the initial valve/seat contact area, A is the contact area after N cycles and j is a constant determine dempirically using bench and engine test data [

where R_{i} is the initial seat insert radius, _{i} is the initial seat insert seating face width (as measured).

On the other hand, the objective of the sensitivity analysis is to identify sensitive recession model parameters whose values can be changed without changing the final recession. The methodology used in this work was as follows [

Identify all the parameters that can change the response of the model.

Set a range of values for each parameter based on two engine types (“light duty” and “heavy duty” in addition to a typical set of baseline values).

Use a version of the model described above set-up in MatLab to simulate the recession response, varying each parameter within their established ranges, leaving all other parameters at their baseline value.

Determine for each parameter the recession values, and calculate the difference from the baseline recession value.

Finally determine which parameters had the greater and least effect on recession value.

As explained, two components contribute to the overall recession, which is produced by the sliding effect of the valve on the seat insert in the combustion stage and the recession caused by the impact of the valve on the seat.

In light duty engines, where the valve closure velocity is relatively high, a greater effect of the impact component than the sliding component is apparent. The opposite happens for heavy duty engines, where the valve velocity is relatively low and the sliding between valve and seat is very large due to the combustion and the higher sliding due to the increased valve head size.

It is observed that for light duty engines (

Calc. | Parameter Name | Baseline Value | Parameter Ranges | ||
---|---|---|---|---|---|

Light Duty Engines | Heavy Duty Engines | Light Duty Engines | Heavy Duty Engines | ||

Sliding Wear Volume | Friction Coefficient, μ | 0.1 | 0.1 | 0 - 0.5 | 0 - 0.5 |

Max. Combustion Pressure, P_{p} (MPa) | 13 | 13 | 1 - 30 | 1 - 30 | |

Valve Head Radius, R_{v} (m) | 0.018 | 0.1 | 0.008 - 0.03 | 0.05 - 0.5 | |

Valve Seating Face Angle, θ_{v} (˚) | 45 | 45 | 20 - 50^{ } | 20 - 50^{ } | |

Seat Insert Hardness, h (MPa) | 4900 | 4900 | 1000 - 9000 | 1000 - 9000 | |

Sliding Wear Coefficient, k | 5.0E−05 | 5.0E−05 | 0 - 1.0E−03 | 0 - 1.0E−03 | |

Slip at Interface, δ (m) | 8.0E−06 | 0.2E−03 | 0 - 1.0E−05 | 0 - 5E−03 | |

Impact Wear Volume | Impact Wear Constant, K | 5.0E−14 | 5.0E−14 | 1.0E−18 - 1.0E−11 | 1.0E−18 - 1.0E−11 |

Impact Wear Constant, n | 1 | 1 | 0.1 - 4 | 0.1 - 4 | |

Valve Closing Velocity, v (m/s) | 0.3 | 0.01 | 0 - 2.5 | 0 - 1 | |

Mass, m (kg) | 0.182 | 10 | 0 - 0.75 | 0 - 350 | |

Total Number of Cycles, N | 50,000,000 | 50,000,000 | N/A | N/A | |

Recession | Seating Face Angle, θ_{s} (˚) | 45 | 45 | 20 - 50^{ } | 20 - 50^{ } |

Initial VSI* Facing Width, w_{i} (m) | 2.0E−03 | 0.012 | 0.8E−03 - 6.0E−03 | 5E−03 - 80E−03 | |

Initial VSI^{*} Radius, R_{i} (m) | 0.0168 | 0.095 | 0.008 - 0.03 | 0.05 - 0.5 | |

Wear Constant, j | 10 | 10 | 3 - 20 | 3 - 20 |

*VSI = Valve Seat Insert.

corresponding to the impact component (n, v, K) have the greatest effect on the overall recession, because in these engines the valve moves at high velocity, increasing the impact energy. On the contrary, it can be seen that, the sliding wear components have a lower effect on the recession.

On the other hand, _{p} and h.

Moreover, the parameters corresponding to the impact component, v, n, K and m, gave maximum and minimum recession values very close to the baseline value, indicating that their impact on the overall recession is very small, this is because this type of engines work at low velocities, due to its large mass resulting in a low effect on the energy of impact. These results coincide with those pre- sented in other work reported [

In the work reported by Blau [_{p}) in a profile of the valve/seat combination after n cycles [

where A_{disp} is the projected area of displaced volume in the plane that contains the valve seat incline, μ_{T} is the kinetic friction coefficient at the valve/seat interface at temperature T, p is the maximum combustion pressure in the cylinder, c_{a4} is the valve geometrical constant, k* is the projected area (profile) abrasive wear rate of the surface at a seat angle of 45˚ and S(n) is the cumulative sliding distance.

Valve specimens made of martensitic low alloy steel were put in frictional sliding tests against seat insert specimens made of cast tool steel. The work reports the sensitivity analysis carried out on a valve recession model, concluded the following.

For light duty engines, the impact wear of component parameters had the greatest effect on valve recession.

For heavy duty engines, the sliding wear of component parameters has an increasing contribution to the overall valve recession.

Modelling applied to tribological systems provides a reliable alternative in situations where experimental process is not easy to carry out, especially when there are many variables that affect the system.

The work described in this paper was supported by CONACyT México and by the Tribology Group of The University of Sheffield, UK.

Vera-Cardenas, E.E., Lewis, R. and Slatter, T. (2017) Sensitivity Study of a Valve Recession Model. Open Journal of Applied Sciences, 7, 50-56. https://doi.org/10.4236/ojapps.2017.72005