High-performance resin survives under the bonnet

Edited by the Engineeringtalk editorial team Jun 23, 2006

The German firm of Mann+Hummel uses DuPont Zytel HTN polyphthalamide for the air-intake manifold in the 4.2 litre diesel motor that powers the Audi A8.

The German firm of Mann+Hummel, a major global partner for the automotive and engineering industries, picked DuPont Zytel HTN polyphthalamide for the air-intake manifold in the 4.2 litre diesel motor that powers the Audi A8.

This high-performance polyamide's good mechanical properties withstand high temperatures as well as aggressive gases and liquids such as fuels, lubricants, combustion products and brine, over long periods.

Parts moulded from this resin show outstanding dimensional stability.

The motor's improved environmental cleanness created a challenge for the manifold's designers, in the form of higher thermal resistance requirements for this part.

The optimised exhaust gas recirculation system alone raises the temperature in the manifold to over 200C, and it reaches 250C for several minutes when soot is burned off to regenerate the particle filter.

Such operating conditions require the use of a thermoplastic material whose performance is substantially better than that of conventional nylons.

Says Achim Rehmann, team leader for Air-Intake Manifold Systems at Mann+Hummel: "Together with DuPont, we chose Zytel HTN 52G35HSL for this application".

"This semi-aromatic heat-stabilised polyphthalamide (PPA) reinforced with 35% glass fibre stands out through its especially high melt point (310C) and a correspondingly high heat distortion temperature".

"Its resistance to aggressive exhaust-gas components, mainly sulphurous acid, is in line with that of nylon 6.6, the standard material for the industry, and would be good enough for use with bio-diesel fuels".

Each intake manifold of Zytel HTN supplies one bank of four cylinders in the V-8 TDI motor.

Each cylinder has two inlet valves and the manifold has eight outlets to match.

Four of the canals (those with rectangular openings) are unobstructed; these feed the air into the cylinder tangentially, creating an optimal air-fuel swirl in the combustion chamber.

At low power requirements, electronically operated valves keep the circular openings closed; as the load on the motor increases, the valves gradually open.

To simplify the complex manifold's production, Mann+Hummel first injection-moulds the (upper) housing and the (lower) connecting flange as separate parts which are easy to demould.

The two parts are then joined by friction welding.

Thanks to the good weldability of Zytel HTN, the weld produces strong seals which allow no pressure loss from the system.

The moulding's smooth surface quality helps to minimise air-flow losses in both air-flow canals.

In a separate operation, the bearings, also made of polyamide, are positioned on the two-part valve shaft; the shafts with the bearings are then placed into a moulding tool where the valve-flaps are overmoulded.

This creates a positive connection, so that the flaps cannot slip or twist out of place on the shaft.

Comprehensive simulation calculations were needed for the manifold's design.

"Together with DuPont we calculated the manifold's deformation behaviour under operating conditions, for example", Rehmann notes.

"Based on this, Mann+Hummel was able to optimise the design to ensure tightness of the joint at the interface to the cylinder head even under a 'worst case scenario' of pressure, temperature and motor speed combinations".

Rehmann sees considerable development potential for Zytel HTN for intake manifolds.

"Weight and cost considerations mean that more and more plastic intake manifolds will replace aluminium manifolds, in spite of ever-higher thermal resistance requirements".

"A manifold of Zytel HTN has the necessary safety margin and weighs only half as much as a corresponding metal part".

"As a multifunctional component it is substantially less costly to produce, and it has proven long-term reliability".