Triple triumph01 March 2012
Andrew English reports on turbocharging developments, including BMW's innovative M50 tri-turbodiesel
So why would you want to power a supercharger with fuel you have burned in the engine at an efficiency of maybe 36%? Okay, a turbo has parasitic losses, but this [turbocharging] is a highly efficient process..." Dr Nikolai Ardey, BMW's diesel engine development director, manages not to spit on the floor as he says this, but he's clearly not impressed with superchargers.
Hardly surprising, as he has just unveiled the Munich engine maker's most powerful diesel to date, the M50 tri-turbodiesel, which, with outstanding numerical illiteracy, is referred to in the press pack as "an extension of the company's TwinPower Turbo technology".
But three turbos – really? Is it absolutely necessary to bolt a trio of highly expensive compressors onto a six-cylinder oil burner? There's logic here, but it's complicated. This three-litre straight six has turbos with different characteristics, which boost at different engine speeds, with the aim of flattening the torque curve from low engine speeds to increase drivability and reduce turbo lag, increasing the power at the top end of the engine's operating range.
Near instantaneous response
As far as automotive turbocharged diesels are concerned, this is the state of the art. The first, small variable geometry, high pressure turbocharger starts to boost at just above idling speed through its own duct, which means an almost instantaneous response to the accelerator. That duct closes as soon as the intake air isn't being dragged round the blades of the second, larger low-pressure turbo, which is starting to provide useful amounts of positive boost at about 1,500rpm.
That's the way things stay, with a 740Nm torque peak maintained between 2,000rpm and 3,000rpm.
At high engine speeds, however, the smaller turbo starts to act as a flow restriction, which limits ultimate power. To prevent this happening, at 2,700rpm an exhaust valve opens to spin up a third, small, high-pressure variable geometry turbo to increase the charge pressure even further and maintain maximum power of 280kW from 4,000rpm to 4,400rpm. An electronic wastegate valve prevents the larger turbo from over-pressuring the system at very high speeds.
It works, too, though, as Ardey admits, you need four-wheel drive to contain the torque of this super diesel and BMW's eight-speed automatic transmission to allow the engine to work at its most efficient, and to avoid spikes of noise, vibration and harshness. When you examine the complex ducting at the front of the engine, you can't help thinking this is an expensive solution to get
a flat torque curve.
"Boosting an engine is the second most expensive thing you can do to an engine, after fuel injection," says Steven Johnson, a turbo technical specialist with Ford.
"So you need to get over that cost disadvantage, and you also need to optimise the turbo for low and medium-speed operation, rather than high speed, because the launch performance and transient response are key."
Big news once more
Yet, despite these cost disadvantages, turbos, as Eighties as big hair and shoulder pads, are automotive big news again. Downsizing, reducing emissions and the search for part-load efficiency are the reasons for turbos making
a big comeback. The global automotive turbocharger market is currently estimated at about 20 million units, with 15 million diesel applications. German turbo maker Continental reckons that, by 2016, that market will have expanded to about 35 million units, with 15 million petrol applications.
"Turbos, with new designs and materials, are the future," says Dr Herbert Kohler, Mercedes-Benz
vice president of research and development. "We have done, and are doing, a lot of research in this area."
There are several distinct areas of research, however, as Craig Balis, global vice president of engineering for Honeywell Garrett, the world's biggest turbocharger manufacturer, explains. "Yes, they are a big story and they are going to remain so,"
he says. "We tend to divide our business into three: the gasoline market, the light vehicle [or diesel] market and the commercial vehicle market."
Second pillar of fuel economy
Balis explains that the large commercial market has been through "a decade of emissions reduction", but now is concentrating on differentiating product through fuel efficiency. The small diesel market faces similar pressures, whether corporate average fuel economy requirements in the US and China or CO2 emissions targets in Europe. "The market has about 60% penetration in Europe and we don't think that's going to get much higher," he says.
"With Euro V and VI requirements, every time you have to eat meat… and it's a challenge to maintain the fuel efficiency gains with each subsequent emissions drop."
And so to the petrol engine market and it's here that Balis sees big gains in the next few years. "We need to do something about gas, with smaller, more efficient turbocharged engines; it's like the second pillar of fuel economy."
Optimum fuel/air mixtures
Ford's latest one-litre, three-cylinder, turbocharged Ecoboost is the acme of such thinking. It uses a tiny, 38mm Continental turbocharger atop the engine, bolted to water-cooled aluminium exhaust stubs in the manner of an old fighter aircraft – the engine has no conventional exhaust manifold. The advantages, as outlined by Ford's Johnson, "are improved warm-up and response, and a reduction in the over-rich running at high rpm. With exhaust gases of up to 1,050°C running through the turbine, we can operate optimum fuel/air mixtures across the engine rev range".
Part of the secret is the active electro-pneumatically operated waste gate, which, as well as limiting overall boost, is also opened at part throttle openings to reduce back pressure. "We spent a lot of time on the three-cylinder waste gate," says Johnson. "We scraped back a lot of parasitic losses in part-load conditions and that amounts to about 2% better fuel economy."
Udo Schwerdel, Continental's head of turbo production, says the Ford installation is the result of a "focus on the close interaction of design, application, manufacturing, simulation and validation engineering". He adds that "response behaviour time-to-torque was also optimised" and that the other advantage of the installation was "the robust and modular design", which allows the turbo to be easily adapted for other engine sizes.
If the Ford engine epitomises some of the latest developments in gasoline boosting, there is also the twin-scroll turbo, which simply exploits the exhaust pulses from the engine by ducting pulses into separate ports. Used almost exclusively on gasoline engines, where the robust design is able to withstand and exploit the larger expansivity of the hotter exhaust gases, the additional inertia of the twin scroll turbine wheel is more than outweighed by the improved aerodynamic efficiency. Twin-scroll turbos go some way to the responsiveness and efficiency of a twin turbo set-up, but with a much lower proportionate costs.
Altering the aerodynamics
Another way of getting efficiency is to alter the aerodynamics within the compressor housing. Traditionally, this has been done with axial and radial feeds into the turbine wheels. Axial is where the intake/exhaust air flow is in line with the turbine wheel shaft and is used for larger commercial installations. Radial is where intake air comes in at right angles, on a similar tangent to the path of the turbine blades. Automotive installations tend to use radial intake, axial exhausting, which makes for a robust and efficient blower.
Honeywell's DualBoost turbo, used on the Ford 6.7-litre Powerstroke diesel engine on the F150 series pickup, uses mixed radial and axial technology, plus
a handful of new technologies, including a dual-sided turbine wheel that reduces the overall size enough to be able to fit the turbo into the centre of the cylinder banks.
Like many diesel turbos, Honeywell's DualBoost exploits its radial intake flow with moveable vanes, directing exhaust gas into the turbine wheel at high engine speeds. Known as Variable Geometry Turbos (VGT), the result is a much greater efficiency and reduced turbo lag at low engine speeds, as well as assisting exhaust-gas recirculation systems.
With exhaust gases up to 100°C hotter, VGT technology has rarely been tried on petrol engines. Honda and Garrett made them for small production runs and the 997 Porsche turbo uses a Borg Warner VGT turbo, but there is a drawback. "It is very expensive," sighs Wolfgang Hatz, development chief at Porsche and head of engines at the VW Group.
Bearing the load
Simple solutions, such as ceramic ball-bearing turbine bearings, which reduce the friction of the centre bearing and give greater bearing stability, improved transient response and higher ultimate turbine speeds. These are already in production for a range of Garrett after-market turbos and in Honeywell OEM production on a 3.0 V6 Mercedes-Benz diesel. This technology is also used in its DualBoost engine, with a single pedestal mounting to reduce noise transmission.
Another way of reducing turbine wheel weight and inertia is to make them out of titanium/aluminium alloy – Ti/Al. "It has a similar effect to the ceramic ball bearings," says BMW's Ardey. "The turbine wheel weighs a third less, but it is expensive."
Honeywell's Balis agrees and so does Ford's Johnson, but adds a note of caution over their reliability. "They have a failure mode of creep growth," he says, "and you end up with a very big casing and big clearances."
Continental's Schwerdel thinks the weight advantages of Ti/Al wheels can be as high as 50%, but "the first challenge with TiAl is hot-gas corrosion, especially when we look for a gasoline application with [high] exhaust gas temperatures. The second is the industrialisation process."
Ford's Johnson also likes the idea of ceramic turbine wheels, if only the durability could be solved. "They are a great idea," he says, "with reduced density and inertia. But they are brittle and I don't think anyone has yet solved the durability issues."
Other developments? What about compound turbocharging, where you use the engine as a supply pump for the turbo, which has a power take-off? Balis is adamant that Honeywell is working on this technology for outside clients, then goes silent.
"I'm not even 5% sure that we can recover that sort of energy from the exhaust gas," says VW's Hatz.
Ford's Johnson is sceptical, too. "It's still out there on large CVs," he says, "but trying to get that kind of power back into the crankshaft on a small engine is not really sensible."
Ceramic ball bearings and lots of calibration seem to be the mid-term future for turbo applications, though, as Balis says, "the march of technology continues for passenger-car diesels. The spotlight is on petrol at the moment, but diesel turbo developments are coming".
Wolfgang Hatz should, perhaps, have the last word. "Our TDi engines have been a big success. We need to do the same for gasoline. As I said, turbos give driving fun, but also refuelling fun."
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