Tuesday, 5 February 2013

Valvetrain

The valvetrain on the F10 M5 has all the gadgets in BMW's arsenal.

The valvetrain is what operates the valves in the cylinder heads, two intake valves from the intake manifold to let fresh air into each cylinder, and two exhaust valves into the exhaust manifold to let the exhaust gasses escape out of each cylinder.

The illustration below could represent one of the two cylinder banks in a V8 (if only it wasn't an inline 4 crankshaft!)

The design shown is DOHC, meaning "Dual OverHead Camshafts", which is the system most V8's use nowadays and how the F10 M5 valvetrain is organized.

What pushes down on the valves to open them are called "cams". The cams sit on camshafts that are turned at half the speed of the crankshaft by gears and a chain. It goes at half the speed because the valves only open once every two turns of the crankshaft. There are two camshafts per cylinder bank in a DOHC arrangement. One camshaft controls all the intake valves, and one all the exhaust valves on that cylinder bank. The engine shown above has 2 each of intake and exhaust valves per cylinder, so the cams are essentially doubled up and in-phase with one another for any given cylinder, and out of phase on the other cylinders as each cylinder fires at a different time.

The principle of the cam is very simple and is shown below.

As the camshaft spins, an eccentric (egg shaped) cam rotates around and pushes down on the valve at a certain point to open it. A spring then pushes it closed again. It's actually a bit more complex than that in practice, as suggested by the diagram below.
All the mechanism must remain well-lubricated, and modern systems are all built with mechanisms that will automatically re-adjust things over time to keep everything working smoothly.

There are two additional features on most BMW engines: Double-VANOS and Valvetronic, both BMW terms.

VANOS is a German acronym that means "variable valve timing".  Double-VANOS is when variable valve timing exists on both the intake and exhaust camshafts. What this means is that the rotation of each camshaft can be advanced or delayed under computer control. VANOS was first introduced in the early ninties, with Double-VANOS following in the late 1990's. We discuss the VANOS mechanism in more detail later.

The other technology is Valvetronic, which is "variable valve lift". This applies only to the intake valves. What this means is that the computer controls how widely the valves open. Because the valve lift is variable, it can be used in place of a conventional throttle. In other words, when you push on the "gas" pedal in the M5, it causes the intake valves to open more widely allowing more air into the cylinders.

Valvetronic was first introduced in the early 2000's. My old 2005 E60 545i had both Double-VANOS and Valvetronic. When the turbocharged gasoline direct injection engines came out, Valvetronic had to be dropped as it needed more development. An important innovation of the S63top over the S63 and base N63 engines was adding back in a more compact form of Valvetronic variable intake valve lift. The system is now being retrofitted into those other engines as well (the N63 technical update from 2012, as an example). We will discuss Valvetronic in more detail below as well.

First, however, let's get an overview of the entire system.

The illustration above shows the valve trains on one of the cylinder banks. To the inside of the V are the exhaust valves, and to the outside the intake valves. This is opposite the conventional placement on a V engine, and is done to shorten the path from exhaust to turbochargers which are nestled inside the V in order to improve throttle responsiveness.

The VANOS mechanism is used on both the intake (1) and exhaust (2). The intake timing can be shifted up to 70 degrees of camshaft rotation, and the exhaust valves up to 55 degrees. In addition, the intake valves have "Valvetronic III" variable valve lift between 0.18mm and 9.9mm (its servo-stepper-motor is labelled 8). More details on both to follow.

Camshaft timing is done as follows.
The camshafts (1 & 2) are driven by two separate roller-type chains (5) from the crankshaft (6) with an automatic chain tensioner (3) operating on the tensioning rail (4). This chain drive was re-developed specifically for the S63 to minimize chain chatter at high engine speeds. Note the variable valve timing units (1 & 2). The chains spin these, and these spin the camshafts.  These components are housed behind the chain case cover (11 below).


In front of the cover go the following pieces.

The alternator (1) is driven by the belt (2) with tensioner (5) from the crankshaft (6). The same belt drives the coolant pump (3), the power steering pump (4), and the A/C compressor (7).  The alternator is of a special design which spins freely, without resistance, when the battery does not need charging and demands are being placed on the engine. When the car is braking, or the engine is slowing it down, the alternator engages. BMW calls this "Brake Energy Regeneration", which is a very misleading term for this! The other side of the crankshaft is attached to a heavy flywheel to smooth out the rotation, and a chain drive for the oil pump.


Getting back to the valvetrain, more detail is illustrated below.
This diagram shows both intake and exhaust valvetrains. The VANOS wheels are (1) and (16) (explained below). It is on both intake and exhaust camshafts. We also see on both sides the valves (6,7), cams (2,15), and springs (5,9).

In order to keep the valves at exactly the right starting height at all times, there is a mechanism for automatically adjusting this called HVCC (Hydraulic Valve Clearance Compensation) (4,8), and the roller cam followers (3,10) that go with them. In older engines, valves needed to be constantly adjusted by hand during a tune up. Otherwise they would get noisy and inefficient over time. Nowadays this is no longer necessary, and a car can run its entire lifetime without ever needing its valve clearances adjusted.

Those were all the common components. Everything else in the illustration above is related to the Valvetronic variable valve lift system, exclusively on the intake side.

The Valvetronic servo motor (11) is what's known as a "stepper motor". Each time a digital signal is sent to it, it moves either a step clockwise or a step counter clockwise. The size of the steps is determined by tooth spacing inside the motor.


Electromagnets around the outside are fired in a particular sequence (all under digital control) to align the teeth and move the wheel by one tooth on each cycle.

As the servomotor moves a worm gear turns an eccentric shaft (12) which moves an intermediate lever (14). A spring (13) is used to assist in moving forward and back so as to avoid solid connections that will get out of adjustment over time. As the intermediate lever moves, the amount of valve lift is varied. Note that the valve lift is adjusted for all intake valves simultaneously, as there is only one servo motor for each intake camshaft.

The M5 has Valvetronic III, which incorporates "phasing" and "masking" to promote a better air-fuel mix. Phasing is opening one of the intake valves slightly in advance of the other to increase turbulence. Masking is designing the valve seats to align the incoming air so as to get the required movement in the charged air inside the cylinder head.

The Double-VANOS (variable valve timing) is hydraulically operated using engine oil. The basic principle is that an inner wheel rotates the camshaft, and an outer wheel is driven by the crank chain, and the angle between them can be varied hydraulically.


The VANOS system moves by applying differential oil pressure to either the right or left side of an oscillating rotor (6 in the photo). Referring to the hydraulic diagram below, oil from the sump (1) is taken up by the oil pump (2), directed through the oil filter (3) and via non-return valves (4,5) to fine filters (6,7) and into both intake and exhaust solenoids (8,9) that control the flow to either the pressure chamber for advancing (5 in the photo), or the pressure chamber for retarding (7 in the photo). This system (times 4) requires considerable oil flow during operation. We shall discuss the oil delivery system in a later post.
The combination of continuously variable valve timing and valve lift allow the system to operate more efficiently, doing away with the conventional throttle mechanism that hampers intake airflow at low engine speeds, thus improving fuel economy.

The DME computer controlling Valvetronic, Double-VANOS, spark timing, and turbo boost via the wastegate allows BMW to "engineer" the torque curve to be so broad and flat, to start at so low an RPM, and to hold horsepower right to the redline.

The remarkable improvements in fuel efficiency, emissions control, and power all stem from the Digital Motor Electronics (DME) computer having access to readouts from dozens of sensors around the engine and then using those to alter key engine parameters on the fly (valve timing, valve lift, fuel injection timing, spark timing, throttle position, and turbo wastegate). Before computers and technologies like those described here, key engine parameters such as valve timing and lift were completely fixed. How terrible to be made to compromise so much by having to choose just one setting for these things across so many different temperature, speed and load situations!

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