New energy-efficient turbosystems run at high temperatures but can still benefit from use of engineering plastics.
DSM Engineering Plastics
Edited by Jean M. Hoffman
Turbocharging makes it possible to use smaller engines to get a given amount of power output. This makes fuel combustion more efficient and puts less unburned fuel in the exhaust system. Turbocharged diesels in passenger vehicles have gained ground in European markets and are expanding into other regions of the world.
However, the high temperatures inherent in turbocharging and the ever-smaller engine compartments bring new challenges for materials used in these systems. At the same time, turbocharger technology is evolving. Ongoing efforts aim to squeeze more performance and bigger environmental benefits out of turbocharged engines.
The introduction of variable-turbine geometry (VTG) technology is one example. Here, guide vanes in front of the turbine wheel regulate turbine output by changing the inflow angle and speed at the turbine wheel inlet. This helps maintain a high level of efficiency over a broad engine operating range, from acceleration to high-speed cruising. The vanes are activated through a simple on/off actuator or alternatively by a completely variable system with actuation through a combination of gears, a dc motor, engine control unit (ECU), and sensors.
Turbosystems with VTG technology are candidates for high-performance engineering plastics such as polyamide 46 (PA46). Polyamides have excellent high-temperature resistance. They retain mechanical properties (stiffness, fatigue resistance, and wear and friction properties) while operating at excess of 200°C (390°F). This makes them useful for turbosystem air-inlet elbows, VTG gears, hot charge air ducts, and charge air cooler end caps. Further, they offer processing advantages because of superior flow and moldability.
PA46 TAKES ON ALUMINUM
Stanyl PA46, for example, has been widely used for charged air cooler end tanks. Cost savings as high as 30% are possible over conventional end tanks made from aluminum. Additionally, the PA46 drops end-tank weight by 20 to 30% and has a higher strength-to-weight performance than aluminum and other competing thermoplastics.
The polyamide end caps can be crimped on to the end tank. This saves production time and costs associated with welding on aluminum end caps. Additionally, the inherently easy flowing Stanyl PA46 also lets molders use less injection pressure and a single gate when molding large lengths. This helps make the end tanks flatter and less prone to warp. And it reduces the risk the tanks will leak during service. Injection molding also lets designers easily add brackets for attaching the end caps to the vehicle body.
AIR DUCTS AND ELBOWS
The air duct from the turbo compressor side to the inlet side of the charged air coolers delivers compressed air to the inlet side of the intercoolers at a temperature as high as 200°C with a pressure of up to 3 bars. Conventional turbo systems use aluminum air ducts that are either cast or hydro-formed. Silicone rubber with textile inlays is another option. But air ducts made from these materials are expensive and don't easily integrate brackets or sensors.
In contrast, two-shell welded charge air ducts made from PA46 cost half as much as those made from aluminum or silicone. PA46 air ducts are also lighter than aluminum and have several advantages over those made from silicone. The cross section of the PA46 air ducts can take the shape of the space available, whereas, cross sections of silicone rubber ducts can only be round. PA46 is also recyclable while silicone parts are not.
In comparison to other thermoplastics, Stanyl PA46 has better fatigue resistance and thus gives a higher safety factor. Elbows are used for connecting the turbo compressor side to the air ducts. When made of PA46 they offer similar advantages, as mentioned above, compared to aluminum elbows. The part can be fully molded in a single-shell design and directly bolted onto the turbo compressor thanks again to the outstanding heat resistance of PA46.
PA46 TAKES THE HEAT
The actuation of the VTG system is another area that could benefit from PA46. The resin's high heat resistance, stiffness retention at high temperature, and excellent wear and friction properties are key. The end cap of a VTG motor consists of an end bracket, brush holder, and also acts as a base for the gear reduction mechanism. With PA46 it is possible to combine all three features in a single injection-molded part. The PA46 part will withstand high ambient temperature as it sits in close proximity to the turbine side of the turbo charger.
During operation, temperatures in the brush holder area of the end cap can rise to 290°C (554°F) briefly. It's important that the holder doesn't deform or melt at this temperature. If so, it could jam the brush and stop the motor. Stanyl PA46 has a heat-distortion temperature (HDT) of up to 290°C and withstands this short-term peak temperature more readily than other thermoplastics.
There is a rotary electric actuator (REA) that adjusts the position of the turbine vanes. It does this by means of a gear train that converts its output to linear motion. The gears operate at high frequencies in temperatures above 150°C (302°F). PA46 retains its stiffness at these elevated temperatures and demonstrates superior wear resistance, contributing to optimum durability and extended life cycles
The REA executes a closed loop using a feedback set of sensors that monitor the speed of the compressor and the position of the vanes. The ECU manages the motor and the movement of the vanes. Sensors in the system must retain dimensional stability at a peak temperature of 230°C (446°F). The manufacturer must warranty them for life against oil leakage through the threads and resistance to automotive fluids. The sensors, of course, should not break during installation.
Sensors are also candidates for PA46. Here, the resin's ability to retain stiffness at peak temperatures and its resistance to creep and stress relaxation are key. Additionally, PA46 demonstrates excellent fatigue and chemical resistance at elevated temperatures. Other important advantages in this application include an opportunity to reduce weight by eliminating metal parts and parts consolidation via injection molding.
The ECU uses surface-mounted electronics soldered to a PCB through reflow processes where peak temperatures can exceed 265°C (509°F). Thus, the ECU housing must withstand this temperature. PA46 with its high HDT and outstanding toughness is an ideal candidate for this application. It is economical and can also save space via the use of snap-fit fasteners.
DSM Engineering Plastics, (812) 435-7539, www.dsmep.com