High-Boost and Two-Stage Turbo Power Systems
The latest regulations for reducing emissions and raising gas mileage have again put car designers into a race to reduce, reduce, and reduce. Engines and power potential are out front in design centers, with research dollars going to miniaturization of turbochargers and overall engine displacement. One major focus is downsizing the internal combustion engine, mainly from six to four cylinders. In order to compensate for the drop in power and torque, car companies are adding turbochargers on engines that did not use them as standard equipment in the past.
The new Euro 6 rules will affect large and small vehicles and, while downsizing equipment for cars is a priority, companies are testing new designs for all vehicle classes. Diesel high-boost turbo technology will help make lower emission vehicles the standard on European roads. Two-stage turbo technology continues to look at the ways these mechanical components can add power while not adding prohibitive weight and size to the overall structure.
Turbo manufacturers are working on ways to increase inlet pressure and boost the power of an already reduced-size engine. Euro 6 emissions standards go online in September, 2014, and vehicle weight reduction along with more efficient power supplies will both be necessary in all vehicle types. Cummins Turbo Technologies, based in Huddersfield, UK, is addressing this challenge by providing a new range of turbos for 2.3 L to 5.0L diesels. The new Variable Geometry (VG) configuration of the diesel turbo for smaller vehicles in this range is designed to mimic its larger counterpart for durability and performance, the company said. (1) Cummins has net sold a turbo for engines this small in the past. They have determined that newly engineered turbos will be as robust as their larger products.
The Cummins product will replace a swing-vane with sliding wall technology, a part the company said will be:
• More durable
• More reliable
• Facilitate thermal management of exhaust
• Used for engine exhaust braking
The company also stated that automobile products currently on the market will not meet the heavier requirements that light commercial vehicles using these diesel engines need. Testing competitive turbos on these engine sizes produced failure at an average of 300 hours, which was "attributed to wear on the swing vanes used in the VG turbine. The wear occurred on the vane pivot axles, introducing play that led to the vanes jamming."
Next, Cummins interviewed engine manufacturers in this range to determine their highest priorities with diesel turbos. Rating "high" in priority were more power from smaller engines, faster transient response, improved operation, meet NOx requirements, deliver reliability, be more robust, and reduce complexity of the build. While these categories are often vague, the totality of the survey showed a desire for a new product line with largely different engineering, Cummins said.
Better two-stage turbocharging is also being developed for long-stroke, large bore diesels for seagoing vessels and stationary machinery. These engines need lower pressure equipment than other diesels. Two years ago, MAN Diesel and Turbo announced (2) a new two-stage process using an alternative diagonal turbine design that performs well in low-pressure systems needed for very large bore diesels. Adding another turbocharger to the engine to complete a two-stage system requires more space and equipment, including pipes and mounts. MAN compacted the entire unit by installing the turbos at right angles to each other.
Honeywell International Inc. stressed the benefits of two-stage design when it began building alternate geometry double turbo configurations. Since diesel systems need a high degree of exhaust gas recirculation (EGR), the two-stage unit helps better regulate the intake and temperature of return gasses by increasing pressure ratios. Two-stage diesel turbos can operate on lower RPM and split the compression between two units. (3) This increases part life particularly in bearings, compressor wheels and turbine wheels. Honeywell is pursuing smaller turbo technology in diesels from 4.4L small trucks to 64L mining equipment.
Weight and size problems
As power demand rises while engine and component size and weight falls, engineering in lighter, stronger materials and geometry must find more miniaturizing possibilities. Most designers of turbos agree that the main negative concerning two-stage units is the size and weight added to the overall engine configuration. Manufacturers are developing new alloys that can withstand the heat and movement of engine environments. Ford Motor Co. is using a truly space-age alloy for its turbo on the EcoBoost engine. (4) A super alloy used in rocket engines is being used to lengthen the life of turbos.
The nickel-cobalt-tungsten alloy is used on the now retired space shuttle’s main booster rocket. It has an upper temperature range of 1050°C, which raises the normal limit on turbos for the 2.0L being fitted to Edge and Explorer models. The material is used in the turbine world. Ford notes that this upgrade will help the turbo last years longer without degrading performance, but that is as yet unproven.
Turbo boosting and technology
Let’s take an in-depth look at turbo boosting in view of the new regulations and the compression factors necessary to safely run diesel engines more efficiently. In two-stage systems, as discussed earlier, the first turbo in the line boosts pressure as high as is safely possible, the second must compress to optimal pressure while reducing lag in a vehicle with continuous change in acceleration. While this technology has been used extensively in diesel engines, it is being vigorously pursued for gasoline engines as these become smaller and lighter.
First, a project completed by Honeywell (5) was conducted to raise the level of performance and durability with "breakthroughs in transient engine performance without the use of exotic materials such as Titanium Aluminide or the additional complexity of variable geometry turbines." The new designs address the problem in gasoline engines: air mass flow varies widely and far more than that in diesel engines. Pressures at the intake are up to 80:1 from idle to rated power, according to the report. So, looking at the overall problem, the Honeywell team developed DualBoost® technology. They replaced the radial turbine with an axial one, redesigned blade angles and added a "double-sided parallel flow compressor."
Tests on this design showed lower stress at low-speed torque, and better use of turbine exhaust energy throughout the acceleration process, the company said. The results are dramatic in smaller vehicles. Using a Ford 1.6 L I4 gas engine, the DualBoost created 90 percent torque at a rate 40 percent faster than a standard turbo.
Further boosting the powertrain
Geometry, angled vanes and precise air/exhaust control are several ways smaller turbos and combinations are dealing with the need to reduce emissions while boosting acceleration and sustained power. The action of inflowing and outflowing chemically charged hot air is a detriment to all components under the hood, including electronic parts.
Along with bringing ideal boost to the new turbos, companies want to reduce boost spiking and creep. These phenomena occur in acceleration when the waste gate cannot handle pressure changes or volume of exhaust used to boost the intake. The smaller the power plant, the more critical are the deviations in pressure management during acceleration. Developers and mechanical designers are trying to solve boost spike, or overboosting, and boost creep. Although it is a relatively small problem in most turbochargers, spike and creep use fuel and power significantly over time. Solutions for this include very accurate hose diameters, since spike occurs when the feed hose cannot accommodate the volume and pressure going through it.
In an effort to further control boosting, manufacturers are using the engine control modules on newer cars to map boost and electronically control the valve opening for the waste gate. Haltech Engine Management Systems, based in New South Wales, Aus., manages the gate with a short-throw solenoid, in turn managed by the engine electronics system. It traces the spike and creep activity much as dwell and advance of the ignition are monitored. "A high force spring return is used to produce swift valve closing and to prevent valves "blowing" open under high boost applications. Valve movement control (Duty Cycle) map is accessed via the ECU programming software. Raising the duty cycle increases boost pressure, while lowering the duty cycle will reduce boost pressure. The boost control feature allows boost response adjustment for varying turbo sizes."
This is one example of the need for instant response from components that can’t be achieved through strictly mechanical means. As time goes by and miniaturization of engine components continues, electronics will certainly play a bigger role in management of combustion engineering. The manufacturers of high-precision engines and turbochargers, and many others, will need to develop not only lighter and stronger materials but the software to operate it all under smaller hoods and inside smaller engines.
Al Tuttle is news and features editor whose experience includes Media News Group, Reed Elsevier and New York Times New England. He specializes in industrial and commercial writing. He was in sales for industrial companies and manufacturers for 15 years.