Efficiency without compromise
By Jason Kavanagh Email
Date posted: 05-08-2006
BMW's reluctance to place the whirling propeller on a full hybrid to date is centered on three things: 1) Existing full hybrid systems famously haven't lived up to their promise of stellar real-world fuel economy; 2) and 3) are that their drivability and weight compromises are, well, simply un-BMW-like.
According to the company, an equally effective route to fuel savings is one that couples key features of hybrids with efficiency-oriented powertrain and chassis advances. By shotgunning this technique, right now, to existing high-volume vehicle platforms, the company-wide net fuel savings realized are similar to that provided by a single full hybrid model. Oh, and in the process, performance will improve.
Too good to be true? We spoke with BMW engineers during Innovations Day, held at the BMW Research and Development Center in Munich, Germany. There, we snuck a peek at what is immediately around the corner in their powertrain department.
The single biggest factor in BMW's strategy is direct injection (DI). The 760Li was introduced a few years ago with a DI V12; however, its system operated at lower pressures and was not capable of lean-burn operation. The company's second release of DI technology, equipped with piezoelectric fuel injectors, is debuting in the twin-turbocharged 3.0-liter, inline six-cylinder engine of the 2007 335i.
DI's roots are in the Mercedes-Benz 300SL of the 1950s, but it took 40 years for another manufacturer to embrace the technology. It was Mitsubishi's GDI system that placed stratified charge DI engines in the hands of consumers. Since then, other manufacturers like Audi, Mazda, Nissan and Peugeot have mass-produced DI engines. What BMW brings to the table in its latest iteration of DI (called "High Precision Injection" in BMW-speak) is a larger envelope of lean-burn operation than first-generation stratified charge DI systems, like Mitsubishi's GDI were capable of.
Here's what DI does: It moves the fuel injectors a few inches. By relocating the injectors from the intake ports to the combustion chambers, the fuel is no longer vaporized by the hot walls of the intake ports. Instead, it is vaporized by the intake air rushing into the combustion chamber. Like a swamp cooler, the vaporization of the fuel cools the air, allowing for a higher compression ratio, directly improving performance and efficiency.
And since the intake ports are no longer constipated with fuel vapor, there's more space for air. This increase in volumetric efficiency afforded by DI further increases performance.
Overall, BMW reckons that the addition of DI increases an engine's output by roughly 10 percent. Do you smell a 550-hp DI-equipped V10 for the next version of the M5? BMW's lips are sealed. However, the company confirmed this new generation of DI will be proliferated into several engine platforms after the inline six. We're betting that it will pop up next on the upcoming M3, which is expected to have a high-revving normally aspirated V8 based on the current M5's V10.
Wait, there's more
Unlike conventional port fuel injection, DI allows the fuel to be injected into the chamber after the valves are closed. This capability opens the door for "lean-burn" stratified charge operation. In fact, such a lean-burn mode will be introduced in normally aspirated European-market inline six-cylinder engines, where it was found to reduce fuel consumption by 3 percent on the EU drive cycle (the European fuel-economy standard, which differs from the EPA). Unfortunately, it's difficult to translate what this means in American mpg terms since there's no worldwide standard.
For now, DI-equipped U.S.-market 3 Series will forgo the lean-burn mode, as our fuel's high-sulfur content is incompatible with the catalysts required to support stratified charge operation. Today, sulfur-free gasoline is widely available in Europe. The U.S. is getting there, with all gasoline meeting a standard of 30 ppm or less as of January 2005, but that's still a far cry from 0 ppm.
Valvetronic is BMW's infinitely variable valve lift technology, which is unique in its ability to throttle the engine. The driver's right foot actually controls the amount of valve lift instead of opening and closing a traditional butterfly valve in the intake tract. Unfortunately, Valvetronic's additional valvetrain bits add reciprocating valvetrain mass, limiting max engine speed. Good news for M-car addicts — DI does not have the engine speed limitations of Valvetronic, which has been a major reason it hasn't appeared on M-badged vehicles to date.
Speaking of which, Valvetronic will initially be absent from DI-equipped I6 engines, although the marriage of the two technologies is viewed as a "very good thing" by Dr. Hans Rathgeber, senior vice president of Vehicle Concepts and Integration.
Turbos on gasoline engines are gaining traction among automakers, and improved fuel consumption is a big driver. Among other advantages, they promise the performance of larger-displacement normally aspirated engines, minus the associated weight and fuel consumption penalties.
To illustrate this, BMW reports that its new twin-turbo inline six-cylinder engine weighs 154 pounds less than a normally aspirated 4.0-liter V8 of equivalent power. Thanks to DI, it offers 10-percent better fuel economy than an otherwise similar port-injected turbo I6 engine.
BMW hinted that turbocharged gasoline V8s and V12s are under development, too. Philosophically, the company's strategy for turbocharged variants of a given engine family is not to outperform the base version of the next engine family. For example, future versions of the turbo I6 would not exceed the horsepower of the base V8 available. These corporate marching orders place an artificial cap on the ultimate horsepower potential of the turbo engines.
Accordingly, the company confirmed — in a move that will disappoint some enthusiasts — that M-badged vehicles will not be equipped with turbocharged engines of any kind.
Active electrical management
Just as turbos recoup energy that would otherwise be wasted as heat, the energy dissipated while slowing down can be put to good use. Gee, brewing up a nice hefeweizen would be — no? OK, how about producing electricity, then? It takes horsepower to make electrical power. Thanks to the alphabet soup of electrically powered doodads in modern cars, the alternator sucks down an ever increasing amount of horsepower.
Called "Brake Energy Regeneration," BMW's solution is to plop in an oversized battery and engage the alternator only during deceleration. By actively controlling the alternator and battery charge electronically, the alternator just freewheels happily during cruise and acceleration while the battery supplies the necessary juice for the car.
Referring to this system as regenerative braking is something of a misnomer. True regenerative braking implies that the batteries are the prime motive power source, when in fact BMW is still using the battery in a conventional manner to power auxiliaries. The system is rather clever nonetheless. And in the EU drive cycle, Brake Energy Regeneration alone reduced fuel consumption by 3 percent and freed up to the drive wheels the power that otherwise would have been used to turn the alternator. It's a kind of free lunch. Just without the beer.
Pump it up
Another source of inefficiency is an engine's water pump. Water pumps need to supply enough flow to cool an engine at full throttle. But the vast majority of driving is at part throttle, where an engine is producing less heat. Without a means to vary pump speed independently of the engine, the water pump is effectively too big for the task at hand. A pump that is flowing more than it really needs to is consuming more horsepower than it needs to…and more fuel.
BMW's solution? Mount the water pump remotely and spin it with electrons, not the engine, only as fast as it takes to regulate engine temperature. Introduced on the latest 3 Series, the electric water pump netted 1.5-percent fuel consumption improvement on the EU drive cycle.
Conventional water pumps also circulate coolant when the engine's stone cold. Conversely, the electric water pump is switched off while the engine is warming up. This strategy allows the engine to reach its operating temperature quicker, reducing emissions and warming your buns faster.
On the Road
We drove a preproduction 3 Series equipped with a stratified charge normally aspirated inline six, Brake Energy Regeneration, an electric water pump and electric-assist steering. Special screens in our car were rigged up to show whether the lean-burn or regen modes were active. Although traffic on public roads prevented spirited driving, we got a flavor for its drivability.
The transition from lean-burn to homogeneous charge operation was seamless; the engine was smooth and tractable when pulling from any engine speed. Most notable was the large envelope over which lean-burn operation was active. If it weren't for the screen, we'd have never known. Regenerative braking, however, was detectable, but barely — it felt like the brakes would continue hauling the car down a bit faster than we really intended. We expect this will be fully sorted by the time it reaches production.
What was most significant was that there was no indication that we were driving anything other than a normal 3 Series. This seems to validate BMW's short-term powertrain strategy, though we'll have to take BMW's word on the total real-world fuel savings it produces. The march of full hybrids is inevitable, and the technology has already been integrated into vehicles of several types. Just as BMW is spreading the incremental efficiency advantages of select technologies over various vehicles, so can full hybrid technology. Accordingly, BMW's joint venture with GM and DaimlerChrysler in developing full hybrid systems is in full swing, and will bear fruit in 2008. Whirl, baby, whirl.
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I'm not disapointed that M will not be using turbo's, would be very off brand if they did and I know they'll continue to make strides in N/A engine development. It's also pretty cool that BMW is making it's own hybrid system w/ out having the inefficencies of a hybrid engine, though I'm looking forward to seeing just what the GM & DM venture bears as fruition of the partnership.