I don't know if this was ever posted in FI before, but here goes.... great article nevertheless....

Turbo Revolution - The Coming Technologies



Variable nozzle turbines (VNT) were supposed to be the killer turbo technology - the wastegateless design able to be matched to engines to give excellent response throughout the rev range. And, while they have been adopted in huge numbers in diesel engine applications, problems associated with the high exhaust gas temperatures of petrol engines have continued to prevent their more widespread use in passenger cars.

Ball bearing turbos? Yes, it's a technology that's now being applied widely - although it hasn't seemed to have made quite the impact that was tipped for it.

But now, waiting in the wings, are two turbo technologies that have the potential to absolutely revolutionise turbocharging in passenger car petrol engine applications.

The technologies?

Air bearings and electrically assisted turbos.

Major turbo manufacturers are feverishly working on turbos that have the potential to produce boost at literally idle revs, and to spool up to speed in a shorter time than would be possible with any conventional turbo design. Not only that, but the electrically-assisted turbos are designed to also function as generators for the car's electrical system, potentially removing the need for what is becoming an increasingly expensive and heavy alternator.

Now if a turbo manufacturer were to make a presentation to an OE car company showing a device that needs no lubrication, has a life longer than that of the car, gives the car no weight penalty (the alternator possibly gets left off, remember), improves bottom-end and transient torque very substantially, has the ability to improve fuel economy and emissions - well, put that list of attributes together and you can see why the technology is so exciting!

For this two-part special series we've scoured resources across the world - studying patent applications, trawling through scientific and engineering papers, and talking to turbo company personnel. The result is a story like you'll find nowhere else - a detailed look at the technologies that you can expect to see in turbos five years down the track.

Turbos and Oil

Current turbos use one of two centre bearing designs. (The centre bearing supports the shaft which has the compressor at one end and the turbine at the other.) Sleeve bearings like the pictured design are still the most common, while more modern turbos use ball bearings. One of the major advantages of ball bearings is that they can survive with a far smaller oil supply - and less oil means less drag. In fact, it is suggested by turbo engineers that the reduction in oil flow through the bearing is at least as significant as the lower friction of the ball bearings in giving quicker spool-up onto boost.

But having any oil in the centre bearing gives rise to a number of significant disadvantages:

The possibility that oil can enter the intake air has potential to have a negative impact on exhaust emissions.
Even the small amount of oil supplied to a roller bearing creates hydrodynamic drag.
There is the potential for the oil to coke if the engine is turned off when the turbo is still very hot.
The need for oil supply and drain hoses increases the cost and the complexity of fitting a turbo.
Failure of the oil supply results in the destruction of the turbo bearing.
With both sleeve and ball-bearing designs, there is a relatively large blade tip clearance.
Another point is that in that in a fuel cell application, the supply of air to the cell has to be scrupulously clean - less contaminated than can be achieved with a conventional oil-bearing turbo.

Quoted in Automotive Design and Production, Robert Gillette, president of Garrett Engine Boosting Systems, says, "Preventing oil from entering the intake charge is important in any gasoline or diesel engine, but vital in low emission vehicles. Any amount of oil in the system potentially could foul the system."

So a turbo bearing system that doesn't require any oil supply has some potentially major benefits - and air bearings look like being the best way of achieving that.

Air Bearings



Air bearings are being developed for high-speed turbomachinery not just because that will give us a better spool-up time on the road in our turbo cars, but because the new bearings are very suited to aircraft turbine engines. In aircraft applications, studies have shown that there could be a 15 per cent weight saving if the requirement for a bearing oil supply was removed.

A number of companies - including Capstone Turbines, Pratt & Whitney, Mohawk Innovative Technology Inc and Honeywell - are currently working on the development of high-speed, high-temperature air bearings.

And Honeywell owns Garret, one of the best-known names in turbos.

There is also major government research interest, with the Oil-Free Turbomachinery Program at the NASA Glenn Research Center performing major work on the bearings.

So, what actually is an air bearing?

An air bearing consists of a shaft surrounded by very thin shaped foils. The foils both provide support for the shaft when it is stopped and also direct and control the airflow when the shaft is spinning. At high speeds, foil air bearings maintain the air film between moving parts by actually pumping air between the rotating and stationary surfaces. Air is drawn in and adheres to the bearing surface, no matter how fast it is moving. The pumping of air between the moving surfaces creates an air pressure that generates a load-carrying capacity. At high speeds, a very thin layer of air, just under one-thousandth of an inch thick, can support hundreds of kilograms.



This animation shows that as the journal shaft (grey) starts to rotate, it drags a film of air between it and the top foil (purple). As the hydrodynamic pressure increases, a force is exerted on top foil. This pressure pushes this foil away from the journal, moving it back a tiny distance against the backing bump foil. After the lift-off speed, the journal 'floats' on this hydrodynamic film of air without touching the top foil.

In one example of the forces involved, an electric motor was used to turn the shaft. Prior to lift-off, the shaft required a torque of about 4Nm to turn it. After lift-off, this dropped to just 1.5Nm, then on touchdown the torque peaked at about 6Nm.

While the technology has been known for many years - air bearings are used in the hard-drive read/write heads in computers - it is only recently that there has been sufficient development to produce bearings that can cope with the speeds, loads and temperatures associated with turbos.

Bearing Generations

The First Generation of foil bearings used either flat leaves...



...or bump foils. The bumps on this First Generation foil bearing are uniformly spaced across and around the bearing. Note that the foils are only 100µm thick!



The Second Generation of air bearing foils have variable compliance characteristics, and are split circumferentially (ie around the shaft).



In this Second Generation design, a foil backing spring uses a variable pitch.



The Third Generation design (the one that is currently being used) has an even more complex foil with variable-pitch bumps and a circumferential split.



The maximum weight that the bearing can support (ie its load capacity) is dependent on the design of the top and bump foils; and the diameter, length and speed of the rotating shaft. A faster shaft speed increases the load capacity by pumping more air between the top foil and the shaft, while a larger bearing increases load capacity by providing a bigger area over which the load can be spread. The design of the foils has a different affect - it determines how quickly the load capacity increases with speed for a particular bearing.



The First Generation of foils used a uniformly-corrugated bump foil, which resulted in air leakage at the edges. As a result, this type of bearing had a relatively low load capacity. The current Third Generation design uses features which are better at trapping and maintaining the air film between the shaft and top foil. These refinements have resulted in great improvements in the weight that can be supported by foil air bearings. This graph shows the improved performance of the latest design of bearings when compared with the First Generation.

One of many interesting aspects of these bearings is that not only does the load carrying ability alter with shaft speed, but so do other characteristics. Mohawk Innovative Technology, Inc suggests that under light loads and speeds their foil air bearing, "maintains its softer character, while under high load and speed, it stiffens up to provide stability and high load capacity. Additionally, the higher the amplitude of vibration, the greater the damping value becomes to dissipate vibrations and make for a smoother operating system; as vibrations are reduced and operation becomes smoother, the bearing system damping automatically reduces as well."



Coatings Technology

Until recently, a major limitation of foil air bearings was that their use was limited to applications where they would be subjected to temperatures below 300 degrees C. This was the case because during startup and shutdown, the bearing and shaft make rubbing contact until sufficient rotational speeds pump air into the bearings to develop the running clearance. To reduce wear when contact is being made, a dry lubricant coating is applied to the foil and shaft. Most current foil-bearing applications use a polymer coating such as Teflon® that is limited to about 300 degrees C.

However, the researchers at the NASA Glenn Research Center have pioneered new surface coatings that have lifted the upper temperature limits to over 650 degrees C. One of the coatings (US Patent No 5,866,518 'Self-Lubricating Composite Containing Chromium Oxide'), consists of chemicals acting as a binder, hardener, high-temp lubrication and low-temperature lubrication.

The lubricant has been successfully used in a foil bearing application with more than 100,000 start-up/shut-down cycles successfully completed! Making the conditions even harder is that the test was carried out with a turbine inlet temperature of 650 degrees C and 2.5 times the normal static start-up loading. The test mule was a 150hp turbocharger equipped with the air bearings.

One hundred-thousand hot start/stop cycles..... with no oil!

A plasma spray, as shown above, is used to apply the solid lubricant coating. The actual process of coating the shaft is shown in the steps below.

Firstly, the shaft is undercut.



Then it is sandblasted, providing a surface with good grip characteristics.



The plasma spray is then applied...



...before it is ground back to a 0.005-0.0010 inch thickness. In normal use, wear particles fill any surface pits or voids, resulting in a mirror smooth surface.



Further development of the coating is expected to provide a lubricant which:

Provides start/stop wear protection for foil bearings
Operates from cold start to 815°C
Has no vaporization or emissions
Can be machined

How Advanced?



Test air bearing-equipped turbos have already been taken to speeds of 121,500 rpm - a limit that was set not by the turbo, but by the testing equipment! In this test, the journal bearings operated at 4.4 million DN (diameter in mm times speed in rpm), which is well beyond the capability of even advanced oil-lubricated ball bearings. (At high speeds, ***** and rollers are forced away from the shaft and place large stresses on the bearing.)

The test turbo (shown here) was based on a 150hp Caterpillar diesel engine turbo that normally spins to 95,000 rpm. It has a rotor 2kg in mass and 254mm long. The special turbo used a 95.5mm thrust runner and two double-acting foil thrust bearings. Journal bearings consisted of a pair of 36.5mm air foil assemblies.

In addition to turbochargers and aircraft gas turbines, the air bearing technology also has potential benefits for flywheel energy storage mechanisms, aircraft auxiliary power units and gas expanders used for cooling. Given the wide applications of the technology and the progress that has already been made, it's very likely that air bearing turbochargers for car applications aren't far away.

Garret personnel unofficially hint that air bearing turbos are in fact much closer than you might imagine....

TURBO SHORTCOMINGS



No matter how well it is matched to an engine, a turbocharger still has poor low-load performance. This is because the performance of the compressor (the part of the turbo that pumps air into the engine) is dependent on its speed of rotation. Since it is connected straight to the turbine, before the compressor can turn quickly enough to move significant amounts of air into the engine, there needs to be sufficient exhaust gas flow to whiz the turbine around.

In a steady-state situation, there's no problem. The engine develops lots of exhaust flow and the turbo is spinning quickly, pushing fresh air into the combustion chambers. The resulting burn produces vast quantities of exhaust gas which keeps the turbo spinning, allowing it to continue to pressurise the intake.

But in cars, a steady-state high-load condition is quite rare. Instead, it's much more likely that full power will be needed following a time of quite light load. For example, you're driving along at 60 km/h and then put your foot down hard.

This situation is quite different - it's the transient performance of the turbo that becomes critical in determining how quickly the engine torque output will increase after your accelerator movement.

When a driver opens the throttle wide at low load, there will be a time delay while the exhaust gases of the engine build in volume, the turbo responds by increasing in speed, more air (mixed with fuel) is forced into the engine, a greater amount of exhaust gasflow is developed, the turbo spins harder, and so still more air is forced into the engine.

This delay between the movement of the driver's right foot and the actual increase in engine torque output is known as 'turbo lag'.

In addition to the chicken-and-the-egg process of waiting for the volume of exhaust gases to rise before more intake charge can be forced into the engine, the speed of turbo reaction is also affected by the rotor's inertia and the frictional bearing drag on the turbo shaft. The inertia is decreased by the use of exotic light materials (although ceramic in this application has developed an unfortunate reputation for shattering when the turbo is taken beyond its strict design limitations) and bearing drag is reduced by the use of ball bearings, rather than the older sleeve bearing design. Air bearing technology looks set to reduce bearing drag even further.

Currently, if strong bottom-end turbo response is required, a small turbine is fitted - which reacts more quickly to lower exhaust gas flows. However, such a turbo also causes quite great exhaust backpressure, choking flow at high power outputs. On the other end of the turbo, a large compressor that is spun up to speed too early can result in compressor surge - a problem where the aerodynamic behaviour of the blades changes as a result of the air not being accepted by the engine.

So with the current state of turbo engineering, the bottom line is to use a small turbo if you want the engine to be responsive at low loads and are prepared to trade-off some top-end performance, or fit a much larger turbo if peak power is important but bottom-end response doesn't matter.

An alternative with current turbo technology is to use two turbos, in either parallel or sequential configurations. In a parallel approach (eg the Nissan Skyline GT-R or Toyota twin turbo Soarer) both turbos blow together all of the time. The benefits of this approach come from the reduction in rotor inertia of using two smaller diameter turbo assemblies, rather than one big one. A sequential system is different - one turbo initially gets all of the exhaust flow (and so comes on boost quickly) and when the exhaust gasflow of the engine is sufficient, the second turbo is brought on-stream. The Mazda RX7 twin turbo and Toyota Supra twin turbo are examples of cars using this approach.

Parallel twin turbos give reduced lag, however packaging two turbos under the bonnet is expensive and difficult. Sequential twin turbos can work very effectively indeed but the costs of engineering a complex changeover system in addition to the expense and packaging disadvantages of using two turbos means that this approach is very seldom carried out.

To overcome the problems of turbo lag during transients and a lack of turbo response at low loads, the concept of an electrically-assisted turbo is now being developed. It has the potential to give boost much more quickly after the driver requests more torque, and to develop that boost at much lower engine loads. In fact, at least one prototype electrically-assisted turbo can make boost at idle...

Electric-Assist Turbochargers



A point that needs to be made from the outset is that electric-assist turbochargers can make use of all the technologies also available to improve traditional turbochargers. So, a variable nozzle turbine, air bearings, twin scroll turbines, lightweight compressor wheels and so on can all be integrated into electric-assist designs. In fact, the two most recent technologies - air bearings and electric assist - will almost certainly be used in the one production unit.

So what does an electrically-assisted turbo consist of?

Surprisingly, the prototype designs look very much like normal turbos, with the exception of a slightly larger centre housing. Within the centre housing, and located just behind the compressor wheel (ie towards the cooler end of the turbo) is a permanent magnet, brushless motor. The permanent magnets are carried on the rotor, while around the shaft are stator windings formed on conventional laminated steel cores.

Heat is a major enemy of such a design, so to cool the motor, air passages are present that allow the compressor wheel to push air past the windings and permanent magnets. In addition, the magnets mounted on the rotating shaft are insulated from it. (If the temperature of the permanent magnets exceeds what is termed their 'Curie point', the magnets lose their magnetism!)

In some patented designs, the permanent magnets are mounted on the back of the compressor wheel, housed within a machined shoulder and held in place by a steel sleeve. One advantage of placing the magnets on the compressor wheel is that it is easier to keep them cool - the compressor being cooled by the ambient airflow passing through it. (The temperature rise due to the air being compressed takes place well away from the magnet location.) However, the rotating inertia of the turbo is increased with the magnets in this position, and the extra mass placed so far outboard of the turbo bearings can make the turbo shaft unstable. Recent designs have not followed this path, instead placing the magnets directly on the main shaft.

The use of permanent magnets and individually wound stator assemblies allows the motor to be run without the need for brushes or a commutator - both would be impossible with the speed and power required of the motor. To operate the motor, the windings are energized in sequence, with the speed of the sequencing determining the speed of shaft rotation. The motor can also easily become a generator, ie be made to work electrically backwards.


So that the minimum of exhaust heat is transferred to the motor assembly, much attention is paid to the way in which the oil flow through the centre bearing of the turbo is controlled. In addition, some patents for electrically-assisted turbos show an ancillary air cooling pump which is brought into action after a hot shut-down. In this design, a hollow rotor shaft is used, with air blown from the electrically-driven external pump into the compressor-end of the shaft by a nozzle mounted just distant from the end of the shaft. The air that is blown into the turbo's hollow shaft travels through it and out the other end, so cooling the motor assembly. If required, this forced air-cooling can also be used during normal turbo operation, with a bleed of pressurised air from the turbo compressor being used to provide the airflow. Yet another patented design places passages carrying engine coolant adjacent to the motor stator windings in order that their temperature rise is limited.

The latest prototype Garrettt electrically-assisted turbo does not use external air cooling or water-cooled windings.

The placement of an electric motor within the turbo also allows the easy location of speed and position sensors. In fact, to switch the windings in a brushless electric motor, an internal sensor is always required. A Hall Effect sensor, that responds to the presence of a magnet, can be easily integrated - the magnets are, after all, already there! With such a sensor, the speed of the turbo can be closely monitored, with this information being fed back to the control electronics. In addition, the turbocharger control electronics can have normal engine management inputs, eg manifold pressure, intake air temperature, air/fuel ratio and so on.

No Wastegate

One reason that having lots of sensor inputs is advantageous is that the electric-assist turbo can not only be brought up to speed more quickly than a conventional turbo, but its top-end speed can also be finely controlled - without the necessity for a wastegate. This diagram shows how the system can work. (The example is for a diesel engine fitted with an electric-assist turbo, but the same idea can be applied to a high performance petrol passenger car engine.)



The solid line shows the turbo response of a conventional turbo fitted with a wastegate. As can be seen, boost pressure rises relatively slowly at full throttle, increasing until it reaches the boost pressure limit, where the wastegate opens, bypassing exhaust gas around the turbine and so limiting boost pressure to this value. This is of course what happens in every turbo petrol engine - but what's often not considered is that any wastegated exhaust gas is not being made use of. Instead of doing work, it's just being shot out of the exhaust pipe like in a naturally aspirated car. But the reason that it needs to be bypassed around the turbine is shown by the long dash/short dash line: if all of the exhaust gas continued to pass through the turbine, boost pressure would continue to rise excessively.

But by using the electric motor contained within the electric-assist turbo as a generator, an additional load can be placed on the turbine. Even with all of the exhaust gas passing through the turbine all of the time (ie no wastegate being used), the turbo shaft speed can be controlled by the motor, which is now acting as a generator. So, as can be seen by the dashed line, the electric-assist turbo can generate boost noticeably earlier than the conventional turbo, and then hold the turbo back at the speed at which the full boost has been achieved, generating electricity for the car's battery as it does so.

Note that not all prototype electric-assist turbos use the motor/generator to limit boost - some still retain wastegates or variable nozzle designs.

The way in which the electric-assist is used depends on the application of the turbo, but one approach would be to use full electric assist when the driver requests (via the accelerator pedal) lots of torque at low engine loads, and then for just short bursts during transients (ie when quick in-gear acceleration is needed).

On the electric generator side of things, it may even prove more efficient to electrically load the turbo during cruise to generate power for the car's battery rather than take power directly from the crankshaft via an alternator.

How Much Power?

The problem with electric supercharging is the massive electrical power needed. For example, 10kW of electrical power in a car using a nominal 12-volt electrical system (ie 13.8V running) would need a current flow of at least 700 amps. This amount of current has major implications for how quickly the battery would be flattened (you'd certainly expect the headlights to dim when the assistance was on!), how heavy the wiring needs to be, and for the design of the electronic switching control unit. In addition, the electric motors need to be very special if they are to attain the required very high rotational speeds (eg 100,000 rpm) and survive the huge current flows.

The Detroit Diesel Corporation and the US Government's Oak Ridge National Laboratory carried out some testing to ascertain the power that the motor in an electric-assist turbo would need to have if it was to be effective. Their goal was to improve turbo transient performance by 50 per cent, ie halve the time for the turbo to come on boost.



Some of the results of the research can be seen in this graph, which shows turbo speed against time. The red line indicates the response of the turbo with no assistance, the brown line the response with 10hp (7.5kW) of assistance, dotted blue - 15hp (11kW) of electric motor power, and the green line - 20hp (15kW) of assistance.


Obviously, the greater the power of the electric assist motor, the quicker the turbo accelerated - no surprises there! However, the team decided to concentrate their efforts on the development of a 7.5kW design, with the prototype electric motor shown here. In testing, the motor achieved a speed of 60,000 rpm. Note that the research was carried out on a fairly large and heavy truck turbo.


The Honeywell/Garrettt electric-assist turbo design that is currently being developed is not aimed at truck applications; instead it is expected to be suited to petrol and diesel engines of around 2 litres. As a result, it doesn't need as powerful an electric assist motor - in fact, it's tipped to be about 1.8kW - or 1800 watts. This amount of power still represents a current flow (with a 100 per cent efficient electric motor) of 130 amps, but that's a lot more manageable than 700 amps!

Still, dissipating this much power in a small electric motor - one which is already mounted within a hot turbo - is likely to mean that the time that the electric assist can be switched on continuously will be limited. Suggestions are that a 3 minutes on, 1 minute off cycling may be needed at this power level. The Garrettt electric-assist turbo will be able to be used as a generator, delivering about 1.2kW continuously (ie not needing any rest period to cool down). Transient boost response is tipped to be improved by at least 50 per cent over conventional turbos.

It's thought that Garrett is currently working with Saab on introducing a production electric-assist model featuring the technology.

 

Interesting reading! Has anybody seen anything about electromagnetic bearings? Imagine an electromagnetic bearing in a vacuum! Zero drag! No friction! I have heard that this technology has been researched. I wonder if anyone has developed and tested them? Incidentally, all of this technology could apply to bearings inside of internal combustion engines...or maybe replace internal combustion engines making the turbo obsolete. That is blasphemous! With zero friction bearing and an electromagnetic field that could act like an electric motor, you could eliminate the need for an engine. It could happen! We may see this in our life time.