BMW Valvetronic and Audi FSI combined

[djvernon]

Does anyone know - would it be possible to combine BMW's Valvetronic system with direct injection such as Audi's FSI, to give a double improvement in fuel economy and performance? Or are the two systems just different methods of achieving the same?

Dan

 

[GraviMan]

Well, Valvetronic effectively replaces throttle by clever use of valvegear, as well as optimised valve timing. Direct inject improves AFR control, but does not affect airflow. In principle there is no reason why the two cannot be combined.

Mart

 

[aorangi]

The new BMW V12 has the Valvetronic system and direct injection, but no charge stratification.

See (it's in French, but Google can translate it not too badly):

http://www.histomobile.com/histomob/tech.asp?id2=17&id1=3&lan=1

(Valvetronic, 2 pages)

http://www.histomobile.com/histomob/tech.asp?id2=17&id1=6&lan=1

(BMW V12)

http://www.histomobile.com/histomob/tech.asp?id2=2&id1=16&lan=1

(direct injection)

I don't think Valvetronic and charge stratification are compatible because Valvetronic is a throttling system – at the intake valve – while charge stratification works with no throttling, like in a Diesel engine. The Audi FSI and likes have a throttle butterfly, but it's used in the homogeneous mode only.

Cheers
Aorangi

 

[ivymike]

I don't think Valvetronic and charge stratification are compatible because Valvetronic is a throttling system – at the intake valve – while charge stratification works with no throttling, like in a Diesel engine.

Let's not forget that diesel engines also throttle flow at the intake valve - that's what the intake valves are there for, after all.

 

[aorangi]

Ivymike, I would expect intake valves being there to keep the charge and combustion gases in the cylinder during compression, expansion and exhaust parts of the cycle, preventing the gases to flow back into the intake… The less the valves strangle the flow, the better ! I mean when they're open, of course…

Did I misuse the word throttling? Do you mean throttling the reverse flow? Or are you joking since we are the 1st of April?

Cheers
Aorangi

 

[aorangi]

And before you catch me wrong I'll ad "the less they strangle the flow, the better *at full load*"!

 

[ivymike]

The intake valves are used to control gas flow between the cylinder and the intake manifold. When the intake valve closes, it prevents gases from traveling in either direction. It's not necessarily true that inward flow would already have stopped, or that more flow would be desirable. The valve timing (intake and exhaust) is selected to strike a balance between efficient operation, power output, desired operating speed, etc.

Additionally, I don't think that throttling flow with a butterfly valve upstream of the intake has the same effect on pumping work that closing an intake valve earlier would have. Your previous statement seems to imply that you expect pumping losses to be the same (or similar) regardless of whether a butterfly valve is used, or early IVC. It does seem clear to me that using reduced lift (rather than reduced duration) to decrease air intake would have an effect similar to a standard throttle. To summarize, I think shorter intake duration is preferable to intake throttling, whether by intake valve or throttle valve, from the perspective of reducing pumping work.

 

[enginead]

The two things are different ways of trying to get the same effect, ie. reduce the pumping losses during the inlet stroke. At part load Valvetronic works by only opening the inlet valves enough to draw the desired charge without having to pull down the whole inlet manifold (in a normal throttle gasoline engine flow is limited by the throttle, thus a lot of the valve area is not used). In a stratified direct injection engine the pumping losses are reduced by not throttling the airflow but by injecting just enough fuel to do the work required and ensuring that it's at an appropriate lambda to burn well (this is the real trick). This means that under normal conditions you would use one or the other not both together. However, all stratified gasoline engines have to run homogeneous lambda 1 conditions for most of the operating envelope which requires the mass flow to be reduced by some means of throttling (due to torque, combustion and catalyst limitations). In these conditions then you could use valvetronic and stratified injection together. The reason that this is unlikely in practice is cost, both systems on their own are very expensive so putting them together would be inhibitive.

 

[aorangi]

You can use Valvetronic and direct injection together in homogeneous mode and that's what the BMW V12 does.

Stratified charge engines could benefit from a Valvetronic system in most of their operating envelope… when they don't run in the stratified mode.

But Valvetronic equipped engines would hardly gain from a stratified charge because they use shorter inlet valve duration (and lower lift) at part and low load to limit the amount of charge admitted in the cylinder while the stratified charge principle works with a cylinder's full charge of air, without throttling.

Cheers
Aorangi

 

[enginead]

I think the BMW V12 is a homogenous only GDi rather than stratified so it doesn't attempt to use stratification as a means of reducing pumping loop work. I'll have to check if I'm correct about it being homogeneous only though.

 

[aorangi]

Yes, you are correct.

Cheers
Aorangi

 

[aorangi]

The articles I gave the links in my post of Mar 31 are available in English.

http://www.histomobile.com/histomob/tech/2/123.htm

Cheers
Aorangi