Hood design-Week 2

Objective

1.To design and model a Passenger Car Hood with the Outer Panel, Inner Panel, Hinge and Striker assembled

2.Provide the required emboss definitions and the Mastic data.

Software Used

CATIA V5

Introduction

The Hood is an opening body panel which is the frontal BIW enclosure component in a front engine vehicle. It consists of an outer and inner panel. The inner panel provides strength while the outer panel functions as metal cover. It is a very critical component because of the frontal crash requirements and pedestrian and passenger safety requirements. The design of the hood is vital since it affects the aerodynamic flow around the car.

Major parts of a Hood

Support Rod/Gas Stay

It is a device used for supporting the engine hood in its open position. This component plays the primary role in determining the opening angle of the hood. Its length varies for different regions depending upon the average human height. European regions require a larger opening angle compared to Asian regions. Gas stay requires no man power and is engaged into support position automatically as the hood opens, unlike the Support Rod. It can be kept unmounted on the Grommet in the hood or in the engine compartment. The Support Rod can be given curvatures for increased strength, not exceeding 35mm which is the industry standard limit.

Inner and Outer Panel

The inner panel provides strength while the outer panel functions as metal cover. They are of 0.65-0.75 millimetres (mm) thick. To improve NVH (Noise, Vibration and Harshness) i.e to reduce these levels, Mastic Sealants are used.

Mastic Sealants improves the strength locally at around 80mm diameter. They are positioned between the inner and outer panel of the hood in such a way that it covers most of the hood region. They are made of hard Rubber and are introduced as a paste thereby joining the inner and outer panel with a thickness of 5mm.

Fig 1- Outer and Inner Panels

Mastic Sealants are preferred over Spot Welding to join the inner and outer panel due to the fact that the latter reduces the aesthetics of the car, especially the outer panel which is the styling surface.

Fig 2-Mastic Data ( shown in white circles)

Hinges and Striker

Hinges enable the opening and closing of the Hood. The Latch & Striker forms the locking mechanism of the hood with the engine compartment. One part of the Hinge is mounted to the engine compartment and the other to the inner panel of the Hood, on both sides.

Fig 3- Hinge and Striker

The Hinge Axis point, which is the midpoint of axis of the Hinges, should be aligned with the striker through the trajectory of the Hood opening and closing. If the Striker is exactly perpendicular to the trajectory, then the Hinges can be fixed accordingly on the inner panel. Embossing is done at the Hinge and Striker placements to compensate for the additional load by adding reinforcements.

Fig 4- Hinge Axis point aligned (through the dotted line) with the Striker (perpendicular to the trajectory)

Designing

There is a Master Sketch for every region which would be adoptable on different situations which gives us the dimensions of the part to be designed. The inner panel including all its embosses are designed by offsetting the outer panel. The maximum gap between the inner and outer panel in the master sketch is 25mm, under which the Engine compartment is situated. This specification is given taking into consideration pedestrian safety. Process of Deep Drawing is used in manufacturing the inner panel and the design considerations are kept in mind for the same.

Fig 5- Master Sketch

Embossing

Embosses are added to the inner panel to improve the stiffness of the part. The embossing are done in such a way as to perform the Deep Drawing direction smoothly.

Fig 6 (a)- Emboss definitions on both sides of the inner panel

The design and direction of the embosses should be the guide for the force to travel in case of a collision, dissipating the force away from the passengers to the outside of the vehicle (towards the Hinge) and thereby reducing the impact force drastically.

Fig 6 (b)- Embosses acting as a guide for the dissipative forces to travel

Hemming

Hemming is a metalworking process in which a sheet metal edge is rolled over onto itself.

In this case Hemming is done to assemble the inner panel with the outer panel. Here there are predefined nomenclatures to follow w.r.t gap (0.2mm) between the outer and inner panels on both sides where sealants are applied to achieve water tightness.

Fig 7- Hemming done on the outer panel

Hemming is preferred over Spot Welding given its superior aesthetic finishing.

Relief is provided on the hood corners to avoid intolerance issues arising from the rolling process in Hemming operation.

Fig 8- Hemming relief at corners

Pedestrian Safety – EuroNCAP Standards

The hood is designed in accordance with the Euro NCAP (European New Car Assessment Programme) standards. The Adult and Child impact zone is taken into consideration and calculated by plotting the regions from the model data. The model, in this case the outer panel, is offset to generate the deformation space in the Engine compartment below the styling surface and thereby determine the interference.

Package components that penetrate the generated deformation are considered critical and need to be relocated or tuned to fill. This relocation or tuning leads to the decrease in the impact force exerted on the pedestrian from the compartment during a collision( since there are no critical components under the hood in this case). This is done to reduce the impact on the pedestrians head in particular.

Brackets that gets easily deformed during a collision are also installed on the Fender and Engine to reduce the impact on the pedestrian.

The share of Pedestrian Safety criteria for a 5-Star rating has increased significantly from 25% in 2009 to 60% in 2012. Implementing these considerations while designing the Hood will result in a good rating under EuroNCAP Standards under Head Injury Criteria (HIC) (HIC ₁₅ less than 1000- better rating).

Fig 9- Front view render

Fig 10- Side view render

Fig 11- Exploded view

Conclusion

Thus the Passenger Car Hood comprising the inner and outer panels, hinges and striker was modelled using CATIA V5 according to the industry as well as EuroNCAP standards.