I posted this on a thread I probably shouldn't have. I've been messing with them and others have to. I have heard of mpg gains and better running but if the proper function of them is true. We should see gains when they are properly set and working correctly. Just trying to figure out wether to acuate or not to acuate. lol.
Here is what I found...first is the def of the Miller Cycle...this has been around a long time and used in many import cars. The second is what I found from CAT about the function of them.
So what I'm trying to decide...is it for emissions...or increased efficiency or maybe both. They can be a headache when the malfunction...they do put added pressure on the rockers from what I have read. Why if the miller cycle adds efficiency do we seem to see gains in mpg from backing them off?
Here is the def.
A traditional Otto cycle engine uses four "strokes", of which two can be considered "high power" ' the compression stroke (high power consumption) and power stroke (high power production). Much of the internal power loss of an engine is due to the energy needed to compress the charge during the compression stroke, so systems that reduce this power consumption can lead to greater efficiency.
In the Miller cycle, the intake valve is left open longer than it would be in an Otto cycle engine. In effect, the compression stroke is two discrete cycles: the initial portion when the intake valve is open and final portion when the intake valve is closed. This two-stage intake stroke creates the so called "fifth" stroke that the Miller cycle introduces. As the piston initially moves upwards in what is traditionally the compression stroke, the charge is partially expelled back out the still-open intake valve. Typically this loss of charge air would result in a loss of power. However, in the Miller cycle, this is compensated for by the use of a supercharger. The supercharger typically will need to be of the positive displacement (Roots or Screw) type due to its ability to produce boost at relatively low engine speeds. Otherwise, low-rpm torque will suffer.
A key aspect of the Miller cycle is that the compression stroke actually starts only after the piston has pushed out this "extra" charge and the intake valve closes. This happens at around 20% to 30% into the compression stroke. In other words, the actual compression occurs in the latter 70% to 80% of the compression stroke.
In a typical spark ignition engine, the Miller cycle yields an additional benefit. The intake air is first compressed by the supercharger and then cooled by an intercooler. This lower intake charge temperature, combined with the lower compression of the intake stroke, yields a lower final charge temperature than would be obtained by simply increasing the compression of the piston. This allows ignition timing to be advanced beyond what is normally allowed before the onset of detonation, thus increasing the overall efficiency still further.
An additional advantage of the lower final charge temperature is that the emission of NOx in diesel engines is decreased, which is an important design parameter in large diesel engines on board ships and power plants.
Efficiency is increased by raising the compression ratio. In a typical gasoline engine, the compression ratio is limited due to self-ignition (detonation) of the compressed, and therefore hot, air/fuel mixture. Due to the reduced compression stroke of a Miller cycle engine, a higher overall cylinder pressure (supercharger pressure plus mechanical compression) is possible, and therefore a Miller cycle engine has better efficiency.
The benefits of utilizing positive displacement superchargers come with a cost. 15% to 20% of the power generated by a supercharged engine is usually required to do the work of driving the supercharger, which compresses the intake charge (also known as boost).
I also found this from CAT
An Intake Valve Actuator or IVA (also known as a Varible Valve Actuator or VVA) is a device that contacts the intake rocker arm and holds the intake valve of an ACERT Cat engine open in an effort to lower the cylinder temperatures of the engine. This is done to help lower NOX emissions.
The Actuator is electronically controlled and hydraulically activated very similar to a Jake Brake. Your engine has 6 of them. One for each cylinder.
The variable valve actuators are used in order to control the closing of the inlet valves. The variable valve actuators do not operate until the engine oil has reached a desired temperature. The oil for the variable valve actuator flows from the oil filter base to an oil rail that is outside of the head. If the oil temperature is below the desired temperature, the diverter valve in the oil rail is open. The open diverter valve allows the oil to drain back into the head. When the oil temperature is increased, the diverter valve is closed. This closing pressurizes the oil rail and the housings for the variable valve actuators. The pressure in the oil rail will be 250 ± 50 kPa (36 ± 7 psi) higher than the rest of the lubrication system. Bleed holes are located in the housing in order to exhaust the pressurized oil.
The variable valve actuators hold the inlet valves open. The valves would normally close with the profile of the camshaft lobe. The solenoid is energized while the inlet valves are open. The solenoid allows pressurized oil to fill the cylinder. The pressurized oil pushes down the piston. As the inlet valve starts to close, the valve rocker arm for the inlet valve contacts the piston. The piston holds the inlet valves open.
In order to close the inlet valve, the solenoid is de-energized and the oil is allowed to leave the cylinder. The valve spring force pushes up on the rocker arm. The rocker arm pushes the piston back into the normal position of the piston. The inlet valves are then closed.
Apparantly what I was told about them being open and letting exhaust back into the intake to be reburnt was wrong. But seems like if you are lowering combustion chamber temps this is good. Your turbo is giving you compression as is the cylinder. From what I read the actuator works only during the first 15% of the compression stroke...Accorrding to Miller Compression actually doesn't happen except during the last 70-80% of the compression stroke. So in theory you should not be loosing any compression from the IVA's....you should not be loosing any power from the IVA's...you should gain effieciency. You are not pushing out any burnt gasses into the intake...it is clean air that was just sucked in by the engine. So I do not see how it would cause the soot in your intake?
I backed off a set on an 05 T600 with an accert when I did the overhead. Doing this as an experiment. I think I'll let the truck run as it is for now...and just see if we have any changes. But it looks as though we should be gaining from the system.
CAT IVA (Intake Valve Actuator) Defined...Kind of
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If intake stroke is complete the piston is then coming back up and beginning to build pressure in the cylinder for compression stroke. Why you you want that intake valve left open and relieve some pressure that was in that cylinder?
Lets say it was a good running Acert so it runs around 50psi of boost under heavy load, so you have 50psi of boost in your intake and your having a cylinder beginning to produce pressure upwards of over 500psi. What good is that 50psi gonna do against the 500psi on the other side of the valve. Your gonna be releasing some of that cylinder pressure into the intake and therefore losing efficiency by not having as high of compression in the cylinder before the fuel is injected.
UNLESS that first 15% of the compression cycle the pressure is below the boost pressure at which it would allow more air in through the intake before the piston creates more pressure then whats in the intake. Now that would make sense. Is it what happens? I have no idea but It has to be one or the other either its relieving pressure or its letting a little extra in.
If it is letting a little bit of boost in through the intake at the beginning of the compression cycle then it should increase efficiency by allowing more air in. In which, why would anyone wanna mess with their IVA's when they should be making the engine run better.
"Caterpillar currently uses a form of VVA termed Inlet Valve Actuation (IVA) on C-13 and C-15 engines. A hydraulically actuated piston assembly is used at certain engine conditions to increase the opening duration of the inlet valve past the period that the normal cam profile would provide, as shown in Figure One. This operation reduces the effective compression ratio of the engine meaning higher boost is required from the turbocharger to achieve equal power. The benefit arises from a reduction in temperature within the cylinder, which subsequently lowers the formation of Nitrogen Oxides, NOx (dependent on temperatures in excess of 2000 Kelvin), whilst also reducing block heat rejection. Providing the energy required to compress the air by way of the turbocharger is less than the energy consumed by the piston to do the same work, then engine efficiency is not compromised and a significant reduction in NOx emissions and heat rejection is achieved."
The Acert is running a Miller Cycle set up. The original post is one of the better explanations of how it works that I've seen.
You have to remember that all the engine manufacturers had to lower NOX emissions on Oct 1 2002. The Acert was the answer to meeting the emission standard, it wasn't about creating a higher efficiency engine. By blowing some of the air back into the Intake Manifold you lower cylinder pressures. When you lower cylinder pressures you lower the temperature of the combustion of the fuel being burnt. This lowers NOX emissions. Cummins and Detroit used EGR systems, the inert exhaust gases mixed in with the air/oxygen lowered the combustion temperatures, which lowers NOX emissions. CAT felt that to bring exhaust gases back into the engine for the combustion cycle with an EGR system would ultimately cause early engine failures with much higher soot levels in the oil. This is why CAT went to the Miller Cycle and the VVAs and twin stage compound turbos. An Acert will build boost at a high speed idle with no load on the engine, the system reacts very similar to a supercharger. If CAT had built a VVA system that could handle the constant hours a truck is run without failures, the engines would survive a million miles without any problems.
The dirty looking intake is a byproduct of the volume of air these engines move through them compared to a standard engine. I haven't bothered to research the CFM of air they use at full load compared to a 6NZ, but I'd bet it's at least 20% more for the same Hp rating. Install a new turbo on a 6NZ and see how long it takes for the compressor wheel to turn black.......not long. The air filters don't catch everything
Wow...good information. I'm not on the "back off" or "set right" side. Trying to educate myself on it and its function so I make the right decisions concerning them when running an overhead.
Still trying to figure out where the so called increase in mpg people have reported from backing them off when the miller cycle increases effciency. I've read where CAT messed up by adding this instead of going with an EGR like others...Cummins was going to I read...but dropped it to go with the ISX. From where I stand its a great concept and CAT should be applauded for it...keeping exhaust out of the intake stream seems like a great idea to me!!
Thats what I'm gathering anyways.underpsi Thanks this.
The Acert does run a high compression piston. You are also dealing with much higher boost pressures.
One of the things that may also cause the accumulation of what looks like soot in the intake manifold is valve overlap. Every engine has a certain amount of this. The reason is during the exhaust stroke the exhaust valve is open, but at soon as the piston reaches the top of the bore and before it can go down on the intake stroke, the intake valve can start to open. It's not uncommon for engines to have both valves open at the same time depending on how the camshaft is ground and its duration. With the high boost pressure of the twins, it also generates high exhaust gas back pressure in the exhaust manifold. It is very possible with the valve overlap of the intake and exhaust valves, that as soon as the intake valve starts to open that you get exhaust gases going past the intake valve and into the intake manifold. Unfortunately everytime I've heard of anybody doing a Exhaust Gas Back Pressure test on an Acert, the pressure in the exhaust manifold is higher than the intake. So if there is a substantial amount of valve overlap in the camshaft, then you can very well be pushing exhaust gases into the intake. You can't control the valves with high ramp rate speeds to eliminate valve overlap easily. If you would try this then you'd have to increase valve seat pressures to keep the cam followers on the lobes of the cam. Once you increase valve seat pressures than you have to combat the extra heat the spring will generate, the heat will cause springs to break eventually. Its a complex puzzle, change one thing and it effects another.
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