Variable Valve Timing discussion thread

[paras211]

how is the lift varied?(in bmw)

[vasudeva]

I remember reading sometime back (perhaps 3-4 months back) that Toyota has developed a next-generation engine valve mechanism that can improve the fuel efficiency of petrol-powered vehicles by about 5-10%. The new system is also claimed to reduce carbon dioxide emissions, boost engine output by at least 10% and enhances acceleration responsiveness. The new system is called `Valvematic', and will be launched in the near future, starting with a new vehicle model with a 2.0 litre engine. The system adjusts the volume of air taken in by continuously controlling the intake valve lift volume and the timing of the valve's opening and closing. Toyota has also targeted to completely revamp its engine and transmission lineup by 2010 to reduce emissions.

[srishiva]

Ultimately the intention is to vary the amount of fuel/air mixture let into the cylinder.
Wouldn't the future be to go for valveless engines ? Or at least for the valves to be operated not from crank/cam but electrically through other means ?

[anandpadhye]

OK.

Understood that "any variable valve technology has to achieve variation on the timing, duration and lift of the ait intake and exhaust valves."

Is this corect?

Now, may I know how this impacts the performance and efficiency in the following 3 conditions:
1. Low speed (steady)
2. Accelration
3. High speed (crusing)

I am looking for the theory.

Reading various sources (including Honda website), I came across some explainantion on condtion No. 3 (High Speed - steady). Let's say cruising at 80kmph @2500rpm in 5th gear on flat surface (no incline). One needs to press accelrator only little to maintain the speed and hence throttle opens only little resulting in very little air getting into the cylinder. I understood this much. What I did not understand is:
- Why is this a problem?
- Why is this called pumpng loss?
- What is being lost here? Fuel, i guess. Am I correct?
- How does VTEC solve this?

And I did not find any info on how VTEC improves condition No. 1 and 2.

Thanks in advance.

[doomsday]

Originally Posted by paras211

how is the lift varied?(in bmw)

Check out the following links(It is too detailed to post here)

BMW World - Technology

AutoSpeed - BMW's Valvetronic!

Originally Posted by srishiva

Ultimately the intention is to vary the amount of fuel/air mixture let into the cylinder.
Wouldn't the future be to go for valveless engines ? Or at least for the valves to be operated not from crank/cam but electrically through other means ?

Yup. Ducati is already using it. Its called the Desmodromic Valve control system, wherein the valves are controlled by a set of rocker arms. Also another approach involves using camless engines wherein the valves will be completely controlled by the ECU, which will achieve what all VVTs seek to achieve: infinite variable valve timing.

[doomsday]

Originally Posted by anandpadhye

OK.

Understood that "any variable valve technology has to achieve variation on the timing, duration and lift of the ait intake and exhaust valves."

Is this corect?

Partly correct. A VVT system seeks to achieve variation in either in the timing or the duration and lift. If a VVT system is able to vary all the three parameters, it will be more efficient. Also, it will be more complex. So a manufacturer tries to seek a balance between the efficiency and the complexity/cost involved while designing a VVT system.

Originally Posted by anandpadhye

Now, may I know how this impacts the performance and efficiency in the following 3 conditions:
1. Low speed (steady)
2. Accelration
3. High speed (crusing)

I am looking for the theory.

Reading various sources (including Honda website), I came across some explainantion on condtion No. 3 (High Speed - steady). Let's say cruising at 80kmph @2500rpm in 5th gear on flat surface (no incline). One needs to press accelrator only little to maintain the speed and hence throttle opens only little resulting in very little air getting into the cylinder. I understood this much. What I did not understand is:
- Why is this a problem?
- Why is this called pumpng loss?
- What is being lost here? Fuel, i guess. Am I correct?
- How does VTEC solve this?

And I did not find any info on how VTEC improves condition No. 1 and 2.

Thanks in advance.

1. Low speed: At low speeds, less air and fuel is needed to run the engine. This translates to lower valve lift and duration. An engine just deisgned for economy will work fine, until you demand performance from it. The valve lift and duration (due to a lower cam profile) will not be enough to fuel the engine. So you get economy but bad performance.

VTEC: Since VTEC uses two cam profiles per cylinder, the lower cam profile is the economy cam. So less air and fuel needed by the engine below a certain RPM/throttle load. Translates to good efficiency.

2. Accelaration: While accelarating, i.e. pushing the engine RPM beyond the point where the VTEC engages. In OHC VTEC, it is 4200 rpm. As soon as you cross this threshold, the solenoid powered system engages the higher cam profile. Instantly, your valves lift and duration increases. This allows more air and fuel into the combustion chamber, since the valve open quicker and stay open for a longer time.

3. High speed cruising: This depends on what your definition of high speed is. For instance, in the OHC VTEC, if you cruise around 140 kph in 5th, you will still be below the RPM threshold where the VTEC engages. This will result in very good economy since the air and fuel going into the combustion chamber will be minimum, just enough to keep the car at that constant speed. However, you are looking at top speed blasts, the VTEC will help you reach the top speed sooner if you shift right and always stay above the RPM threshold on the rev-counter.


PUMPING LOSS:

When the throttle is partially pressed, the throttle butterfly closes partially or even almost fully. The pistons are still running, taking air from the partially closed intake manifold. The intake manifold between the throttle plate and the combustion chamber has a partial vacuum, which resists the sucking and pumping action of the pistons, which in turn wastes energy. The slower the engine runs, the more the throttle butterfly closes, and more energy is lost.

Here is where i-VTEC comes in. What i-VTEC does is that it keeps the throttle butterfly fully open; while the lower cam profile is being used. This does not create a vaccum between the throttle plate and the combustion chamber, which in turn does not resist the pumping actions of the piston.

Note that this is implemented only in i-VTEC and not in the prevoius performance-oriented only VTEC implementations. (Eg. DOHC VTEC)

[Sparkie]

Since you guys are talking broadly about controlling valves, this might be of some interest to you. I remember reading a couple of years ago, Lotus is working on camless technology, basically the valves are eletronically actuated.

The benefits are many. To start with you can lift the valve and close it at exactly the right time and for an appropriate amount of time, basically you are in control of all parameters and you have an engine which works like it has an infinite number of cam profiles to work with. So lesser inertial losses because of lesser components to rotate. Lesser clatter because of no rocker arms reciprocating back and forth. A lighter and more compact engine. Hopefully lesser valve float. And then you can also start it with less energy. You simply lift all the valves and rotate the engine(at which point it is like a straight through duct) till it reaches the desired rpm and then leave the valves in the desired position. Think of the possibilities. Displacement on demand too will become an instant possibility, which will work more efficiently than the current system too because as far as I know in the current system, it's simply the fuel supply that is cut off, the valves do operate so you still have pumping losses.
So you finally won't have to bother with hot cams, soft cams all that. And if they do tune it softly, simply retune, and extract the best amount of torque your engine can give you at each and every rpm. Of course, it robs off a bit of the charm of an engine huh ?
Originally Posted by anandpadhye
Now, may I know how this impacts the performance and efficiency in the following 3 conditions:
1. Low speed (steady)
2...
If you design a cam for high speeds, at low speeds the engine will not be too torquey. This is because a cam designed for a higher speed will have more lift or open for more time. Hence at lower speeds, the air will come in slowly and with less turbulence. You want to give the air sufficient velocity. So for low speeds you open the cam for lesser duration or height and to fill the same cylinder the air travels in faster, so thats more turbulence and better combustion which translates to better torque.

[sanchits]

Guys...check out this link...its a detailed comparison between toyota vvt and honda vtec.
link - Bill Sherwood's VVT Vs VTEC Page

[doomsday]

Originally Posted by Sparkie

Since you guys are talking broadly about controlling valves, this might be of some interest to you. I remember reading a couple of years ago, Lotus is working on camless technology, basically the valves are eletronically actuated........ Of course, it robs off a bit of the charm of an engine huh ?

Yup easy to say, its gonna be difficult to implement. Think of how complex the valve train will have to be. More complexity=More reliability issues. So its gonna take some time before an infinitely variable valve timing engine comes onto a production car.

Originally Posted by sanchits

Guys...check out this link...its a detailed comparison between toyota vvt and honda vtec.

link - Bill Sherwood's VVT Vs VTEC Page

Yup, read this article long back. Very well explained for a novice to understand.

[Sparkie]

Originally Posted by doomsday

Yup easy to say, its gonna be difficult to implement. Think of how complex the valve train will have to be. More complexity=More reliability issues. So its gonna take some time before an infinitely variable valve timing engine comes onto a production car...

You will not really have much of the valve train in this system. The valves will not be mechanically actuated. There will be solenoids on top of the valves, so mechanically it will make the system much simpler - no more timing belts, camshafts and cams, rocker arms etc. The complex bit will be calculating the appropriate amount of lift, duration and timing at each speed of the engine, in other words the software which will be an all new thing for the designers. Lotus already has prototypes up and running.

Just did a google and got the link for you guys:

Lotus Exige 265E gets Japanese debut - AutoblogGreen

[manolis]

Here is a quick explanation of the improvement the various VVAs bring to the low revs – partial load operation.
Take a look at the following plot (low revs and partial load in various valve train systems).
The plot is not accurate, nevertheless it provides the answer.
In all cases we start from the condition at point A (Top Dead Center, 1 Atm pressure inside the cylinder) and go to the condition at point B (1 At pressure).



The Red curve is the case of the conventional.
With almost closed throttle valve, the intake valve opens at TDC.
Soon the pressure drops to the pressure of the intake manifold.
As the piston is pulled downwards, the crankshaft has to overcome the vacuum into the intake manifold and into the cylinder.
The mixture enters slowly: there is neither significant speed nor turbulence nor swirl (the charge is like dead).
At BDC the intake valve closes and the compression of the charge starts.
The area between the suction part and the compression part of the red curve is the energy spent going from the condition A to the condition B.

The blue curve is the case of an engine with Lost Motion VVA.
There is no throttle valve, so the pressure before the intake valve is 1 Atm.
The opening of the intake valve at TDC does not change significantly the pressure.
As the piston moves downwards, the pressure drops and air/mixture enters into the cylinder through the small passage between the valve and the valve seat.
The high speed at the passage makes the mixture homogenous. The turbulence and swirl into the cylinder are high.
Very soon the intake valve closes. From that volume to the BDC and back to that volume, the total energy from/to the crankshaft is (nearly) zero.
A side effect of the process is that there is much time, from the closing of the intake valve to the BDC and then to TDC, to slow down the charge motion. This was a serious problem for the first Lost Motion VVAs (check the web for the SAE technical paper 2000-01-1221 for Delphi Automotive systems VVA).

The green curve is the case of an engine with Constant Duration VVA.
There is no throttle, so the pressure before the intake valve is 1 Atm.
The opening of the intake valve at TDC does not change significantly the pressure.
As the piston moves downwards, the pressure drops and air/mixture enters into the cylinder through the small (significantly smaller than in the Lost Motion VVA) passage between the valve and the valve seat.
The extreme speed at the passage makes mixture homogenous. The turbulence and swirl into the cylinder is strong. The suction process continues until the intake valve closes at BDC, leaving short time to the charge to settle down before the spark at the end of the compression.

The cyan curve is the case of a VVA engine equipped with the Pattakon Electromagnetic Idle Valve.
The VVA keeps closed the normal intake valves.
There is no throttle, so the pressure before the Electromagnetic Idle Valve is 1 Atm.
As the piston moves downwards, the pressure drops but there is no induction of air/mixture because the Electromagnetic Idle Valve is kept closed. The piston pass the BDC. The vacuum into the cylinder is strong and “pulls” the piston upwards to the TDC (returning the energy absorbed for the downwards motion). The piston moves upwards and after the middle stroke the Electromagnetic Idle Valve opens for a few degrees, allowing air/mixture to enter the cylinder at high speed / turbulence /swirl, and then the electromagnetic Idle Valve closes. The motion of the charge have no time to “die” until the spark ignition occurs at the end of the compression.

The area between the suction part and the compression part of each curve (red, blue, green and cyan) is proportional to the energy spent to go from the condition A to the condition B.

As you see, in the case of the conventional engine the percentage of the generated on the piston (during expansion cycle) energy that is consumed to overcome the pumping loss is significant and, furthermore, it increases rapidly as the load gets lighter.

For a specific amount of power output, the VVA engine needs not to pass from the condition B, but from a condition C (representing by an intermediate point in the line interval A-B). This is because the additional pumping loss of the conventional, in case of the VVA engines is added to the energy delivered by the crankshaft. This way the difference between conventional and VVAs increase further.

I hope you understand now the difference (in energy consumption) of “the throttling exactly on the intake valves” from “the conventional throttling”.

You can find more at Pattakon Greece

Thanks
Manolis Pattakos

[manolis]

Here are a few graphs taken with Hondata’s s200 data logger.

The B16A2 engine has the roller version of pattakon VVA on intake and exhaust valves (full version). All logs were taken with “Open Loop”.

The video at http://www.pattakon.com/vvar/OnBoard/A1.MOV(4MB) corresponds to the following log files:
http://www.pattakon.com/tempman/A1MOV.PNG and the http://www.pattakon.com/tempman/A1MOV.pdf
This data logging was one of the first, with the engine still having the original valve springs on it, and the Injection / Ignition maps finishing at 1 Bar.


The http://www.pattakon.com/tempman/no70.PNG , the
http://www.pattakon.com/tempman/no70.pdf and the http://www.pattakon.com/tempman/no70Analysis.pdf is another indicative data logging.
The egg shape Toda valve springs were installed. The columns from B11 to B16 of Hondata’s system (typically used for turbo applications) are used in Injection and Ignition tables (the ECU does not care if there is turbo or not).
The no70Analysis.pdf file, is an Analysis of the data.
The blue curve is the lambda value times the injection duration value, i.e. it is the quantity, per cycle, of air that is handled by the engine. At a first approach, the blue curve gives the form of the torque curve.
The red curve is the value of the blue curve times the revs (horizontal axis). At a first approach the red curve gives the form of the power curve.
The cyan curve is the MAP sensor. It gives an idea of how much the gas pedal was pressed at the specific revs.


The http://www.pattakon.com/tempman/ja21.PNG , the
http://www.pattakon.com/tempman/ja21.pdf and the http://www.pattakon.com/tempman/ja21Analysis.pdf is another indicative data logging.
The car moves slowly with 3rd gear at a steep uphill road.
In the ja21Analysis.pdf file the blue curve is the lambda value times the injection duration. This curve simply says that the engine at 750 rpm can handle per cycle some 130 units of air quantity, when the same engine at medium and high revs can handle some 150 units of air quantity.
The red and cyan curves are as in the no70 graphs.


The http://www.pattakon.com/tempman/de96.PNG , and the
http://www.pattakon.com/tempman/de96.pdf is another indicative data logging.
The car accelerates at an uphill road (the same as in the no70 logs).


Thanks
Manolis Pattakos