BMW Valvetronic System
BMW Valvetronic System
(OP)
Is the BMW valvetronic system, or any other variable valve train system, truly THROTTLE-LESS? They promote it as such.
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RE: BMW Valvetronic System
It's my understanding that the BMW system is truly throttleless. I haven't taken one apart personally, but I don't see any reason to disbelieve the published material.
RE: BMW Valvetronic System
RE: BMW Valvetronic System
I thought that one of the main advantages of going to direct injection was the elimination of the throttle?
RE: BMW Valvetronic System
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http://www3.bc.sympatico.ca/Volvo_Books/specart10.html
"Mitsubishi 1.8 litre Gasoline Direct Injection (GDI) engine ... A direct injection, stratified charge engine need not have a power-robbing thottle plate. "
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http://www.carenthusiast.com/20020901_roadtest_mitsubishishogunpinin.html
" In essence, the idea is to control the work output by the amount of fuel injected (not by adjusting the throttle, which reduces the engine's efficiency)."
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http://www.fev-et.com/03eng/01co/e_co_ot_df.html
"In comparison with a well optimized, conventional PFI engine, there is a potential benefit of up to 25% in fuel consumption due to the throttle free and stratified operation."
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The middle article seems to be a review of a MY2002 throttle-less GDI-powered SUV... is that not correct?
RE: BMW Valvetronic System
Several "tech" sites state emphatically that the Mitsu GDI doesn't have a throttle, yet this Mitsu site
http://www.mitsubishi-motors.co.jp/inter/NEWS/9704-09/9709.html
says that at least one of their GDI engines has an electronically controlled throttle valve.
RE: BMW Valvetronic System
Cheers
Greg Locock
RE: BMW Valvetronic System
Rod
RE: BMW Valvetronic System
On the 156 JTS (Jet Thrust Stoichiometric) Alfa-Romeo uses the stratified operation up to 1500 rpm only and most of the others up to about 3000 rpm under low load. In the stratified mode, the throttle is wide open, but still a throttle is needed in the homogenous mode at higher loads and revs.
The stratified charge operation brings a load of problems such as NOx emissions control and top ring durability, due to higher combustion temperatures and increased heat transfer to the piston because of the larger surface of its specifically shaped head – at least in the wall control system. To full benefit of a stratified charge engine, sulfur free gas is needed.
On the opposite, control of the engine output by infinitely variable valve lift and duration seems to me a more logical and better approach since it allows optimum cylinders filling under every condition, without the drawbacks of the stratified charge mode.
Cheers
Aorangi
RE: BMW Valvetronic System
power wasted to needlessly lift the valves? you mean valvetrain friction?
RE: BMW Valvetronic System
RE: BMW Valvetronic System
Cheers
Greg Locock
RE: BMW Valvetronic System
So a significant amount of energy is lost in the system via something other than friction? Where does it go?
can't recall the exact number, but I think it was around 2 hp
2hp sounds a bit high to me, but I'll have to look at some numbers tomorrow before I say anything definite. Even at it's max, valvetrain friction is pretty small compared to some of the other sources of friction within an engine (slider assy, journal bearings, oil pump, etc)...
RE: BMW Valvetronic System
RE: BMW Valvetronic System
Cheers
Greg Locock
RE: BMW Valvetronic System
RE: BMW Valvetronic System
Pistons, rings against cylinders: 44% (50%)
Main and crank journals: 22% (24%)
Gas exchange: 20% (14%)
Valvetrain: 8% (6%)
Oil, water and fuel pumps: 6%
Also some old data about a small 4 cylinder of 1100cc which I think was a Renault. The difference of losses with and without lifters are detailed at different revs. Total mechanical losses at 6000 rpm amounted to 32 hp:
1000 rpm : 0.5 hp
2000 rpm : 0.7 hp
3000 rpm : 1.3 hp
4000 rpm : 2.04 hp
5000 rpm : 3.5 hp
6000 rpm : 8.9 hp
I suppose there's a great difference if the lifters or rockers are flat or roller type. Anyone having up to date data ?
Cheers
Aorangi
RE: BMW Valvetronic System
The numbers that you mention seem close enough to be correct for a particular engine, but there are significant variations between engines. If I get a chance before turkey day (the US holiday "thanksgiving"), I'll look up some more info from testing of modern engines. I won't, of course, be able to share specifics about the engines, but I can at least try to generalize based on what I find (same as above).
RE: BMW Valvetronic System
Another DOHC, I6, roller followers, very high torque at cranking speeds, say 5 Nm (to be safe) at 20 deg C, falls to 1 Nm/shaft, at 3000 crank rpm, 110 deg C, double that if cold.
Note the gotcha when converting camshaft torque to power.
The second set of results was obtained by motoring the shaft only, so there are no combustion forces involved.
I think the difference between my hazy recollections and Aorangis data, and ivymike's low figures and these test results, is probably the roller followers.
After all we wouldn't have switched to these expensive items unless they saved a measurable amount of friction.
Cheers
Greg Locock
RE: BMW Valvetronic System
RE: BMW Valvetronic System
It's a long time since I did any text book physics, but I guess any effect that gasflow past the valves has on valvetrain drag is also inertia.
I expect that if the range of adjustment is sufficient, then controlling valve lift and duration will give suitable speed control, with less overall valve train losses, particularly at low speed, however I am not aware of the details for the valvetronic system
Regards
pat
RE: BMW Valvetronic System
RE: BMW Valvetronic System
I assume that the assumption you're making here is that when the follower comes back into contact with the cam, the cam has turned so far that none of the impact force is converted into torque on the cam, and all of the energy is lost as heat? That would be a pretty extreme case of "valve float," and I doubt that an engine could really power itself up to such a speed. Usually separation happens "over the nose," with an impact that occurs on the closing flank. Depending on the cam shape, a great deal of the energy can be recovered. With some cams, even when "floating," there are no significant impacts to speak of (think of a skier landing on a steep downhill after a jump), and in these cases I would expect nearly all of the energy to be recovered.
Of course, if your valves are not closing when you want them to, you're probably going to get less-than-optimal power output...
RE: BMW Valvetronic System
Cheers
Aorangi
RE: BMW Valvetronic System
RE: BMW Valvetronic System
Sure, it takes more energy to accelerate. That's why you get more energy back from it when it decelerates. The energy is stored, not consumed.
When you decelerate in a vehicle, the energy that is dissipated by the (friction) brakes is lost to the atmosphere. You can recover the energy if you slow the vehicle down in another manner. Driving an electric generator is one way. If you come to a stop by coasting to the top of a big hill, then you haven't lost the energy (except the standard losses of a moving vehicle). If you come to a stop by lifting big weights to the top of a tower, or by accelerating a flywheel, then you haven't lost the energy.
In a valvetrain, when a cam lobe forces the valve line to move, torque applied to the cam lobe is converted into kinetic energy of the valvetrain components, and stored energy in the spring(s). Some of the energy is lost to friction, of course. As the valvetrain components are slowed by the spring, their kinetic energy is transferred into it. On the closing flank, when the valve gear begins to accelerate back towards the closed position, the stored energy in the spring is transferred back to both the moving components and the camshaft (negative drive torque). As the components slow to a stop (with their velocity controlled by the cam lobe), their kinetic energy is converted back into cam torque. In a frictionless valvetrain with no separation, you would have zero net drive torque regardless of the component inertia. If you have separation, you'll have some amount of energy lost in the impact when the components come back together, but not all of it.
Try to imagine the drive torque required if the energy stored in the valve spring and the moving components was not recovered. I can do a detailed calc when I get back to work on monday, but here's a quicky for the time being:
Peak lift: 8mm (intake and exhaust)
valves per cylinder: 4
4 cylinders
spring fitted force: 150N
max working force: 500N
crank rpm: 3000
valve spring stiffness (constant rate spring): (500N-150N)/8mm = 43.75 N/mm
work to lift a single valve = f_0*x + 0.5*k*x^2
w = 150N*8mm + 0.5*(43.75*N/mm)*(8*mm)^2 = 2.6J
work to lift all 16 valves = 16*2.6J = 41.6J
frequency at which all 16 are lifted = 0.5*3000*cyc/min
freq = 25/sec
power consumption (frictionless) = 1.04kW
so a frictionless valvetrain as described above (all spring energy is lost) would consume 1.04kW to run.
RE: BMW Valvetronic System
If we want to calculate the drive power given the assumption that the kinetic energy of the valvetrain components is lost instead of recovered, let's try the following approach:
same engine
average valve velocity = freq*2*8mm = 0.4m/s
let's guess that the peak valve velocity is 2.4m/s
valve mass = 60gm
peak valve KE = 0.173J
valve "power loss" = 69.1W
So the energy lost (if this really happened) would not be of great interest anyway...
RE: BMW Valvetronic System
RE: BMW Valvetronic System
Of course, in case of valve float, the landing of the follower occurs on the closing side of the cam and not by an impact on the came base circle.
But it's difficult to understand how at high revs the torque recovered on the closing side can be as great as on the torque spend to open the valve, even if there's no separation. So the difference would only be due to friction?
There's a direction reversal, I mean the valve is projected down and some of the spring force is used to counterbalance the kinetic energy and to send back the valve. OK, at low rev and if we neglect frictional losses, the torque recovered is the same. But before separation occurs, before the valve floats wildly, there must be a gradual reduction of the force applied on the closing side at high lift. So, as revs increase, more and more of the energy stored in the spring is used to accelerate back the valve and follower and is recovered later by the camshaft, but nearer to the shaft axis and so with less leverage.
Do you mean that the produce of this force by this leverage would produce anyway the same recovered torque as long as the force isn't applied to the cam base circle, so that only an extreme valve floating wouldn't allow the recovery of the full torque – still neglecting the friction losses, of course?
Thanks for your clarifications.
Aorangi
RE: BMW Valvetronic System
A good way to picture this is to hold a ball in your hand, and make a motion as though you are going to toss it. If you move your hand slowly up and down, gravity holds the ball in your hand. If you gradually increase the speed of this motion, you will find that there is less and less force between your hand and the ball, not when your hand is moving down, but when it's acceleration is in the downward direction. Eventually you will find that the ball leaves your hand completely for a while, and you must catch it. If it weren't for wind resistance, the energy transferred from your hand to the ball when you accelerate it will be the same as the energy transferred from the ball to your hand when you decelerate (catch) it. There may be a period of time when the ball leaves your hand completely, but the end result will be the same.
RE: BMW Valvetronic System
On a more practical note, maybe you can explain why valve springs get hotter than their environment if they do not absorb kinetic energy and convert it into heat by shear at the molecular or crystal interface. Shear as I understand it is friction, so I guess the losses due to inertia really come back to frictional losses in the end, but still, the greater the valve lift and acceleration and deceleration rates, the greater the loss (though still quite low) to friction
Re the original question, I guess this system can control speed effectivly without a throttle butterfly or slide in the traditional sense, but if the valves are only opened slightly to control airflow so as to control speed, doesn't the valve then become an effective throttle. As I understand it, a throttle is a device to restrict or control air flow.
Re flywheels.
As it is in the real world that valve float is never so extreme, that the follower lands on the base circle, also virtually no one drives a car with a system that converts energy stored in the flywheel "normally lost in brakeing" into an energy form that can be reused for acceleration. Everyone I know, simply applies a friction brake and converts energy stored in the flywheel into heat which is then, deliberately lost to atmosphere, as quickly as possible
Regards
pat
RE: BMW Valvetronic System
So the point was that you can possibly make the valvetrain do something such that the kinetic energy of the components would be lost?
On a more practical note, maybe you can explain why valve springs get hotter than their environment if they do not absorb kinetic energy and convert it into heat by shear at the molecular or crystal interface.
Sure, there is some hysteresis that happens in a valve spring. I wouldn't say that it has a whole heck of a lot to do with the pushrod or valve inertia, though. Do you disagree?
As it is in the real world that valve float is never so extreme, that the follower lands on the base circle, also virtually no one drives a car with a system that converts energy stored in the flywheel "normally lost in brakeing" into an energy form that can be reused for acceleration. Everyone I know, simply applies a friction brake and converts energy stored in the flywheel into heat which is then, deliberately lost to atmosphere, as quickly as possible
I brought up the regenerative braking example to illustrate the difference between energy stored and energy lost, because it was starting to seem like the distinction needed to be made.
An important difference between the valvetrain example and the braking example is that people don't want to do the former, and do want to do the latter, if possible. I expect that more and more people will be driving vehicles with regenerative braking systems in the future. At present, I know (personally) four people with regenerative braking systems on their "daily drivers." There's also a person who owns a Honda Insight living on my block, but I don't know him.
RE: BMW Valvetronic System
Now about the pumping losses reduction with infinitely variable valve lift and duration, BMW say that:
" Variable valve lift adjusts the “effective cam” and, accordingly, the cross-section and opening times of the valves. This, in turn, eliminates two factors partly responsible for the pumping losses: First, the process of filling and emptying the collector between the throttle butterfly and the valve. Second, the need to build up a vacuum. While the shorter opening times under part load also generate a certain vacuum in the cylinders during the intake stroke of an engine with VALVETRONIC, this vacuum acts like a spring in the subsequent compression stroke, setting off the balance of energy."
I suppose they mean the required amount of charge is allowed early during the intake stroke, so that the cylinder is under a reduced vacuum in the first part of that stroke, most of the vacuum being established late in the stroke. As the intake valve is closed before BTC instead of several degrees after, the cylinder remains under vacuum during the first part of the compression stroke.
BMW phrasing again:
"Under normal driving conditions, this concept keeps valve lift relatively small between 0.5 and 2.0 mm.
With this kind of small valve lift, fuel is only able to reach the combustion chambers through a narrow slot. This ensures ideal atomisation of the fuel/air mixture, while a conventional engine draws in relatively large drops of fuel. Such a fine mist is however the best prerequisite for rapid, constant and effective ignition and combustion.
A further advantage is the unusual spontaneity of the engine transient response. This is because load management is near the point of action, that is right next to the combustion chambers. So there is no more delay between the process of giving gas and the actual acceleration attributable so far to the time required for filling up the intake manifold between the throttle butterfly and the combustion chamber"
Cheers
Aorangi
RE: BMW Valvetronic System
RE: BMW Valvetronic System
RE: BMW Valvetronic System
http://www.histomobile.com/histomob/tech.asp?
id2=17&id1=5&lan=1
And other pics about the Valvetronic system two pages away:
http://www.histomobile.com/histomob/tech.asp?id2=17&id1=3&lan=1
Cheers
Aorangi
RE: BMW Valvetronic System
at the end of the link change to lan=2 for english
or any number up to 6 for various other languages
RE: BMW Valvetronic System