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Analysis: The N55B30 engine

Featured Posts, Interesting | February 24th, 2010 by 20
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The BMW N55B30 represents the current workable technical edge of gasoline engine performance and economy. It is a state-of-the-art powerplant available in the 535i GT …

n55-turbo-twin-scroller1

The BMW N55B30 represents the current workable technical edge of gasoline engine performance and economy. It is a state-of-the-art powerplant available in the 535i GT and soon the 535i from BMW. It is possible, that before long the N55B30 will supplant the N54B30 in BMW’s entire lineup.

To gain a better understanding of the technical advances contained in this engine, it is necessary to develop a simple example of a gasoline Otto cycle engine. This will be used to help explain the benefits of the technology contained in the N55B30.

Consider, for example, a simple stationary engine. It is designed to run at peak torque. The engine must start and then quickly hit its optimum RPM. The air/fuel mixture is restricted for starting (for which the amount of fuel to air is increased). This will ‘light’ the engine off and, once started, the restriction or ‘choke’ can be removed. From here, the throttle is moved to provide operation at the optimum RPM. There isn’t much more to it than that. In addition, the valve timing is optimized for the single RPM requirements of the engine.

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Now, depending on the method with which the air/fuel mixture is introduced to the engine, the performance and emissions characteristics of the engine will vary significantly or slightly. It is most apt to vary significantly if a carburetor is used to supply the air/fuel mixture. That is due to the inherent variability of the mixture arriving in the combustion chamber. The carburetor does not optimize each and every air/fuel pulse, it simply hopes for a happy medium.

Carburetors work on the Bernoulli principle. They draw air past a venturi and the vacuum created draws fuel from a hollow needle (jet) creating an air/fuel mix. The amount of air/fuel mixture allowed to flow to the engine is controlled by a throttle plate. The carburetor, inherently, does not provide optimum fuel economy and emissions control.

A mechanical fuel injector is better than a carburetor in that the fuel will be metered relatively precisely.

Fuel injection utilizes a nozzle that can be opened and closed to inject pressurized fuel into the air stream. However, some early mechanical fuel injection, as used in the old sprinters and Indy cars, was often described as a controlled leak. Electronic fuel injection, however, delivers more precision over the air/fuel mixture.

In the history of gasoline engines, electronically controlled port fuel injection was a key enabler of emissions compliant performance engines.

Now, an engine optimized for a single RPM has it uses, as a lawn mower for example, but it is useless for an automobile. (Even hybrids that use an IC motor for an electrical generator allow for some variation of engine RPM).

The engine has to perform perfectly over a range of RPMs sufficient to meet any potential driving condition. One approach to this is to vary the intake and exhaust valve openings in relation to the combustion process so that they are optimized over a range of engine RPMs. BMW accomplishes this with its double-VANOS technology.

n55_eng_3

VANOS changes the relative position of the cam lobe in relation to the combustion cycle based on engine load and RPM. It can smooth out the power band of an engine, making an engine feel less ‘peaky’, for example.

Variable lift of the intake valves can alter the use of a throttle in the engine. When a throttle is used to vary the volume of the intake charge allowed into the cylinder, the engine has to work against the throttle during partial throttle opening. This results in lost work, known as pumping losses. Imagine the work required to pull a sheet of paper away from the nozzle of a vacuum cleaner. That, in a very simplistic way, is what pumping losses are.

Varying the lift of the intake valve can change the volume of charge allowed into the combustion chamber, this can require less work than the tugging against a closed throttle. Of course that depends on the amount of work needed to provide the variable lift mechanism. The variable lift mechanism has to return power at a rate better than the potential pumping losses to be effective. What variable valve lift buys is increased fuel economy.

BMW uses Valvetronic to provide variable lift to the intake valves of an engine. It utilizes an intermediary between the rocker arm and cam lobe whose shape is designed to change the total lift of the valve depending on its position in relation to the rocker arm and cam lobe. This allows for a variable lift profile, within a specific range, unlike the original VTEC which is a simpler (yet effectively clever) solution.

So with double-VANOS, Valvetronic and port fuel injection we have the foundation technology for the N52B30 engine familiar to NA BMW owners as the x28i models.

But let’s take that basic suite of engine technology, enhance the performance and miniaturization of the Valvetronic components and add direct injection and forced induction like the N54B30 and you have a recipe for real success.

Direct injection works well in an engine that allows significant variations in valve timing and forced induction. Since it must deliver fuel directly into the combustion chamber, it subsequently utilizes higher fuel pressures than port injection to overcome the pressure of the compressed air charge in the cylinder. It is a key enabler in the new round of emissions compliance and fuel economy requirements that would otherwise inhibit engine performance.

One more piece of technology merits a brief discussion and that is the Twin Scroll turbocharger used in place of the twin turbochargers used on the N54B30. By using a single turbocharger, that maintains exhaust pulse separation ahead of the turbine, an exhaust pulse reaches the turbine every 120 degrees of crank rotation rather than every 240 degrees of rotation for the dual turbocharger setup on the N54B30.

Coupled with Valvetronic and VANOS, direct injection and the twin scroll turbocharger, BMW provides the same performance across a wider RPM range for less fuel consumption in the N55B30 than the preceding N54B30. Quite an accomplishment.

n55_eng_2

  • Brandon

    Why oh why cant we get a naturally aspirated version of this motor?

    250hp, 250lbft with the most precise response this side of a Porsche boxer 6.

    • Babak

      The sad part is that even engines like the N52 will be gone by the time the F30 comes around. It will very likely be replaced with a 2.0 I4 making 260hp/260lb-ft or something like that, but it’ll be a turbo 4. It wouldn’t make business sense for BMW to keep a N52 (or a successor) when a turbo 4 can make the same power and consume less. Sucks, but it’s like what Audi was forced to do with the A4; drop the 3.2L NA V6 because no one bought it.

    • lennardt

      i think you can’t do engines without turbo anymore, because eu laws and restrictions constrain manufacturers to lower the consumption on the paper and the most economic way is using turbos. seems to me that no one cares that a naturally aspirated engine can take less fuel than a turbo charged when driven the right way. i’m happy bmw uses not so high pressures right now so their engine’s temper still is very good, although i really love engines, that get their power through high rpm levels. sorry for my english, i was trying hard ;-)

      • atr_hugo

        Your English is very good. Better than my German by far. :-)

        • lennardt

          nice to hear, thanks!

  • Bolfer

    No one makes engines like Bmw. That’s why they were awarded with so many engine of the year awards.

  • eracer

    Great write-up!

    That picture at the top is of the N54B30, not N55

    • http://www.bmwblog.com Horatiu B.

      you’re right, thanks. Made the change

  • Artmic

    I think the idiotic Global Warming fanatics are to blame for any idiotic decision BMW makes in regards to engine sizes and types….

    Our idiotic minuscule contribution of CO2 in the atmosphere is nothing compared to other sources …. Yet this is never covered on tv/magazines in the media in general, why?

    Global Warming, now climate change is the biggest rip-off on the planet, I hope most people will wake up soon, because this is getting so idiotic I want to puke every time I see these rich shittard movie stars, or singers talking about climate change like they knew what they were talking about, and the worst thing is that people gobble that crap up as truth since they saw it on tv, and some popular star said it, so it MUST BE TRUE.

    • adc

      Well, since you argue so eloquently, you have convinced me.

      • Habs

        ^^ Agreed.

        I’d rather believe people who make their points with research and data than people who make their points by shouting and swearing. You talk about people who “don’t know what they’re talking about” but I haven’t seen you provide any empirical support for your arguments that outweighs empirical support on the other side of the debate.

        BMW is taking a step in the right direction. While I love my NA 6, the days of waste and inefficiency are coming to an end. Creating the same power and smooth response in a turbocharged engine as a naturally aspirated engine is BMW’s goal. They’ve done a great job with the N54/N55, especially to provide low-end torque and minimal turbo lag, and I for one can’t wait to see what’s coming down the road.

    • Doug

      Artmic, that’s a fair point about people not evaluating what they read or hear. But— your counterpoint is what, exactly?

  • ARON

    This probably a byproduct of the fact that I drive a 330 w/ an N52 engine… but I’m not a huge fan of the new turbos. Sure, the extra power—and thus faster launch times—are sexy. If I could have afforded the 335, I would’ve gotten one. But the N52 just feels so much more natural. It’s efficient, bullet proof and superior in dicey conditions when paired with a manual transmission.

    I guess everyone is just crazy about the turbos. But every time I drive the 335 (usually a loaner), I find myself missing my car and they way it handles. With all this new technology, BMW’s are starting to feel “overly-assisted.” The connection between man, machine and the road is quickly loosing favor to elaborate gadgetry.

    Am I the minority here?

    • another aaron

      You miss your 330i and it’s handling b/c you are fully familiar with how your car feels…and that the loaner may be of different spec. compared to your car. When I get to drive a loaner Bimmer it feels odd as hell mostly b/c of those damn run-flats. I have pilot sport a/s on mine.

      • Habs

        I drove a Geo Metro for many years in my late teens/early 20’s. After buying my 128i, driving it for a few months, and then going back to the Metro (which my brother is now stuck with, haha), it felt totally different to me. It’s a crappy car to begin with, but you develop a “feel” for the car you’re driving that makes all others feel different when you jump into them for the first time. Same thing goes with my girlfriend’s Accord, which I used to drive quite a bit before my Bimmer. Just feels different, not like I remember. I think after a while of driving a 335i, you’d get just as used to it, would learn its quirks, the way it handles, the throttle response, etc.

      • ARON

        That’s very true. And since I drive an Xi, I also ditched the stock run-flat tires for a set of Continental Extreme Weather performance tires. The difference is unbelievable. Feels like a totally different care.

  • Doug

    Very interesting article, Hugo. Lots of good info in there. Too many questions to ask but… I’ll try one —

    the graph at the bottom seems to indicate that there’s up to 1.75s of “lag” to when the engine can achieve its maximum torque at any rpm. They also indicate that despite this, there’s 10% more immediate torque than the equivalent non-turbocharged engine. Is that to say that the engine performs basically the same (10%?) as its non-turbocharged sibling in terms of performance in dynamic scenarios (within 0s..1s aka “responsiveness”), but it achieves it’s potential only under sustained throttle? It’s not bad at all, many turbo’d engines underperform their non-turbo counterparts in this regard. But is this actually justification for a supercharged engine that can achieve full performance at any RPM instantaneously?

    Porsche used to (i believe) maintain a compressed reservoir of air in the intercooler for this purpose — for instantaneous output that actually exceeded the engine’s rated performance at that rpm.

  • atr_hugo

    I wonder if the ‘10% above NA’ comment is a typo on the graph. It appears they are comparing the N54 to the N55.

    The graph also implies that Valvetronic has some play in the reduced lag. But that doesn’t make a lot of sense to me. I do know that the twin scroll single turbo has three exhaust impulses per 360 degrees of crank rotation, versus the individual turbos three exhaust impulses per 720 degrees of crank rotation. That may keep the pressure up better at lower RPM.

    A turbocharged engine’s ‘throttle’ response will generally be not as ‘linear’ as an NA engine, but which would you rather have going up the Berthoud pass? ; -)

    Neat tidbit about the Porsche trick. I hadn’t heard of that one. I’ve got Ludvigsen’s three volume Porsche tome on the bookshelf, maybe I should read it. :-)

  • farid

    want to know, why not BMW uses the valvetronics and turbo charger together in the previous engines. there was any dis advantages in using these two systems together.

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