Design of Car Hood _(Frontal BiW Closure)

Objective:

To design the Car Hood (bonnet) as per the given parameters and styling surface input, with the help of UG-NX CAD software.

Hood:

Hood is a part of the subsystem of a car that provides closure to the engine compartment and acts as a styling body-member. It plays an important role in pedestrian safety as well as passenger safety. The hood’s outer surface is designed to reduce the aerodynamic drag and it plays an important role in improving the NVH characteristics. Hood is hinged at the rear end and a latch–striker locking mechanism is provided for ensuring the hood in the closed position. A supporting mechanism is provided for keeping the hood in an open position and doing the maintenance works of the car.

Parts of Hood:

Conventional Hood consists of the following parts.

1. Inner & Outer panels
2. Reinforcements
3. Hinge & Mountings
4. Latch-Striker lock
5. Mastic Sealant
6. Stiffener Pads/Acoustic pads
7. Support rod/Gas stay

In addition to this, modern cars also have an “Active Hood pedestrian safety system”.

1) Inner panel

The inner panel act as the backbone of the hood. It provides strength, support, and helps in dissipating the energy in case of collision. The inner panel consists of Embosses, holes, and areas for mastic structure. Different grades of Stainless steel & aluminum alloys are commonly used materials for the inner panel. Stainless steel hoods are produced by the Deep drawing process while the Aluminium panels are made by Casting.

Design considerations of the inner panel

a) Geometric requirements:

Thickness: 0.75 mm

b) Manufacturing requirements:

• Minimum fillet radius should be 3mm+thickness
• No curvatures of less than 5 mm
• Small emboss depth should be less than 6 mm
• Mounting and welding areas should be flat
• Appropriate corner relieves on curvatures
• Excellent weldability
• No sharp corners

c) Safety requirements:

• Passenger Safety: The geometry should be designed for dissipating the energy outwards (i.e., to the body instead of to the passenger compartment) during a collision.

• Pedestrian safety: avoid rigid members at impact areas. The child impact areas should be made to have less stiffness so that the force will be absorbed by the hood thus reducing the impact on the child.

HIC- Head impact criteria of Adult & Child

WAD- Wrap around distance

• A minimum space must be provided between the inner panel and the engine components in order to create a safe Hood.

In modern automobiles, an Active Hood pedestrian safety system is also being used.

This system is going to be mandatory in India under the new pedestrian safety norms from October 2020. With the use of this system, it's expected to reduce the Pedestrian head injuries to a great extend.

d) Functional Requirements

• The hinge and striker should be positioned for the perfect opening and closing of the hood.
• The support member (Hood stay) should be positioned to reduce the interference.

Design Methodology for the inner panel

The hood is designed to meet the above-listed requirements.

Step 1: Creation of the Master Section

Master section sketch

The Master section is created as per the given dimensions and ensured that there are no curvatures of less than 5 mm and no sharp corners are present.

Step 2: Creation of Geometry & Emboss

The geometry is created to meet the design, manufacturing and safety requirements such as small emboss depth should be less than 6 mm, Mounting and welding areas should be flat, Minimum fillet radius should be of 3mm+thickness, a design that diverts impact force away from the passengers and maximizing the pedestrian safety. After performing the initial stage of design, the local embosses are provided to strengthen the component. Embosses is then smoothened to improve the manufacturability of the part. Landing areas are defined and mastic points are allocated on the required areas.

Step 3: Thicken the surface

The surface is then thickened to 0.75 mm using the thicken command.

2) Reinforcements

The reinforcements are provided on Hinge, Latch- Striker areas to improve the strength. They are simple metal sheets in the required size & shape.

Design Considerations:

Thickness- 1.2mm

Design Methodology:

step 1: Creation of surface geometry

The primary surface models of the reinforcements are created.

step 2: Thicken

The surface model is then thickened by 1.2 mm.

The reinforcements are attached to the inner panel by the process of spot welding.

Outer Panel

The outer panel is the styling element of the hood. It is designed to achieve less aerodynamic drag and to have good aesthetics. The CAD model of the outer panel is designed based on the inputs from the designing team. The outer panel is mounted to the inner panel by the process of hemming. The commonly used materials for the outer panel are Steel and Aluminum alloys.

Design considerations

Geometry:

• Thickness: 0.75 mm

Hemming:

• Hemming tolerance: 0.2 mm
• Hemming clearance: 2 mm
• Flange Length: 6 mm

Corner relief:

Design methodology

step1: Creating the Outer panel surface from the given input skin.

step2: Hemming the Outer panel

Hemming is the process of wrapping the outer panel around the outer edge of the inner panel to hold them together. The hem is created as per the given dimensions.

step3: Giving corner reliefs

Corner reliefs are provided to prevent tearing & to make the hemming part more manufacturable.

step4: Thicken

The Outer panel surface is then thickened including the hemming region by 0.75 mm.

Other Components of Hood:

Stiffener Pads/Acoustic pads

They are provided to improve the NVH characteristics of the hood. The stiffener pads are installed in between the outer and inner panel.

Support rod/Gas stay

This will support the hood in an open position. Support rods are commonly used for lightweight hoods whereas Gas stays are commonly used for high performance and luxury vehicles. Support rods can be installed in two ways, either they can be installed on the inner panel or they can be installed within the engine compartment. They are positioned in such a way to reduce interference while doing maintenance works. They are designed to achieve the required opening angle of the hood.

Mastic Sealant

They are elastic glue-like materials provided between the outer & inner panels to hold them together. They are defined at certain locations called mastic points. Each point will have an effective circular area of 80 mm in diameter. The mastic sealant helps to improve the elasticity of the outer panel. They also improve the NVH characteristics of the hood.

Mastic points:

Latch & Striker locking mechanism

This will ensure the hood in the closed position and easy opening of the hood when required. The striker is generally mounted on the inner panel. The positioning of the latch is done with respect to the hinge axis position and trajectory of the striker.

Draft analysis

We use Draft analysis to see that the panel can be drawn or not. It helps to find the negative draft present in the component which will cause difficulty in manufacturing. The result of the draft analysis of inner panel:

The limiting angle is set to 5 degrees.

The model has no negative drafts at all, so the part has good manufacturability.

Final Model :

Cad_Model: https://drive.google.com/file/d/19r09n68OoWdt6myApJEfYyMM2Wr7IyED/view

Result:

The Hood is designed successfully considering all the geometric, manufacturing, safety, and functional requirements.