imzjustplayin

I was thinking about how important it is for NA builds to have properly sized runners for the power band you plan to run in, with appropriately chosen valve timing to go with it in order to maximize scavenging. Now I had an idea for a turbo that would attempt to keep the intake pressure exactly the same at all engine speeds instead of just boosting once it is revved up and bleeding off excessive pressure. This would also mean that you wouldn't have to worry about having appropriately sized bore and length of your intake runners because if the intake is at a constant pressure, scavenging isn't necessary.

So with this in mind, would utilizing a system like this allow for the use of valve timing and lift intended for a high power band engine (6K-7K rpm+) but have near peak performance and torque in the low power band (500-2K rpm) like as if the engine had valve timing and lift intended for that power band?

If this doesn't make sense, let me try to explain the details. Normally if you're tuning an engine, you want valve timing and lift that goes with the intended power band, so a lot of lift and advancement of timing (I think) for peak power, but for an engine that operates at 500-2K rpm, then you'd want shallow lift and less timing advancement (I think). Yes Exhaust scavenging matters but for the sake of simplicity, I want to not include those variables. The point of having the right valve lift and timing is for when just as the cylinder achieves maximum pressure, the valves close, bringing in the maximum charge so more fuel can be burnt. If you use shallow lift and not enough valve timing advancement, the cylinder will be starved of that extra fuel and air it could be getting, limiting power. If you use too much lift and timing, then at low speeds, you'll have no torque and might even idle badly if at all because too much air comes in too early, pressure equalizes in the cylinder and intake, then the air and fuel is pushed out, leading to (I think) a situation where there isn't enough fuel and air in the cylinder by the time the valves close.

So this turbo would just constantly keep the intake pressurized at a pre-defined bar and attempt to maintain this at all times, which while wouldn't lead to peak performance in any given situation like you would have in other scenarios, what I'm hoping it would do is flatten out the power and torque curve.

So, what do you guys think of this idea? If you think it's dumb and or my understanding how how things work is wrong, please, feel free to clue me in.

Scott_Tucker

If the intake pressure was constant then you would need to throttle the engine using the valves. Kind of like BMW's valvetronic which has variable lift (although it still has a throttle body on the intake). BMW's system does not utilize variable valve duration but it does have the ability to steplessly vary both intake and exhaust cam phasing.

imzjustplayin

Um, yeah... Well wouldn't my idea allow for super low idling speeds though? I mean isn't one major issue with the valve timing is that most of the time, it is limiting how low you can idle, that unless you want to idle at an extraordinarily slow RPM, you'd need to have appropriately tuned valve lift and timing for that?

Scott_Tucker

So is your idea that the engine utilizes variable valve lift and timing to throttle the engine? This would certainly allow you to idle the engine lower. However you would need a relatively heavy flywheel to keep it running at low speeds. Also, although the mechanism to provide variable valve timing and lift has been invented, no one has produced a system that is reliable enough or is low priced enough for mass production.

imzjustplayin

Uh.. Well why then is it not possible for a car to idle at a really slow speed regardless of the flywheel unless it was because the valve lift and timing wasn't optimal for it? Wouldn't any engine that normally idles at 650rpm be able to idle at 100rpm if the valve lift and timing (cam's profiles being molded to) had been appropriate for that?

Scott_Tucker

I think to determine how heavy the flywheel would have to be you would have to determine what the valve timing at idle would need to be. The inertia of the flywheel is necessary to complete the 3 out of 4 strokes that aren't contributing power to the engine (intake, compression, and exhaust). These are called pumping losses. You would want the intake valve to close late to reduce the compression and thus reduce the pumping loss of trying to compress the mixture. However, if you close it too late there will not be enough compression to raise the air/fuel mixture temperature to the point of flammability and the engine would have a tendency to misfire.

imzjustplayin

So for the ability to run at ALL, valve timing is very important while valve lift is important for efficiency and power, but you could get away with a LOT of lift but perfect duration at those low RPMs? If the valve lift and timing was perfect at those lower speeds, the flywheel weight still wouldn't matter, right?

Scott_Tucker

Yes, valve timing would be very important. Lift is also important and needs to be controlled during idle. Under idle conditions the valve lift would be very minimal to just let in enough air to produce the amount of power required to keep the engine running. This is refered to as 'throttling'. If the engine is not throttled in some way it RPM will just keep increasing.

I think realistically the lower the idle speed is going to be, the more important is the need for a decent amount of mass in the flywheel. I'm not saying it would be more than a stock flywheel from a normal engine but I don't think eliminating the flywheel would be possible. There are too many variables that have not been discussed that would make it difficult to idle an engine at 100 rpm. At engine speeds that low there is so little velocity in the intake air stream that it would be difficult to have a homogeneous air/fuel mixture because there is so little turbulence. This would make misfires likely. Also, and engine that idles that low would not be performance oriented since there is so little torque being produced at 100 rpm that it would take a little while for the engine to get going from a stop once the accelerator is pressed.

imzjustplayin

Well are you saying that at low RPM, if you had appropriate valve timing but too much lift, you're saying that the car will "rev" too high because too much air and fuel is getting in? I thought the reason why you want small lift at low RPMs is for scavenging and you don't end up with stagnant air inside the cylinder, causing an equalization between the intake manifold and in the cylinder walls opposed to negative pressure which keeps more of the air fuel mixture inside. Why else do cars with lot of valve lift idle poorly and therefore have to be idling at 1000 rpm? You make it sound like cars with lots of lift idle high because so much air and fuel is getting in that is isn't physically capable of reducing the RPM unless you were to lean it out or do something similar.

Scott_Tucker

Engines that have high valve lift idle poorly mainly because in order to achieve that lift they need to have lots of valve overlap. This causes poor scaveging at low rpm and can cause exhaust gases to revert back into the chamber which interferes with combustion. This is similar to having an EGR valve stuck open at idle. What I was trying to say is that the engine you are proposing has to be throttled somehow. If you plan on having an intake manifold which is at a constant pressure then you would not be able to have a throttle body. So the engine would need to be throttled my some other mean, such as variable valve lift and timing.

imzjustplayin

Uh, crap.. Ok in most turbo builds, are they boosting a constant amount of air at all engine speeds and just bleed off excess speed when it exceeds a certain amount (full speed and no speed) or does the boost rise over a given range and then finally top off and bleed off excess speed? I'm starting to think my idea is seriously flawed as I seem to have a misconception of how things work for what ever reason that may be..I guess the real issue in all of this is that an engine with lots of lift ends up having valve overlap which leads to unburnt fuel leaving the chamber by going in the intake and out the exhaust before things close up.

Is there a site that says in general terms what is the optimal valve timing (you advance it this for this performance range) and optimial valve lift (you have this much lift for this performance range)?

xander1100

turbos are driven by exhaust, more exhaust = more boost until the WG cuts it off. turbos build boost up to a certain point then drop off. in general the larger the turbo, the higher RPMs it needs to start to boost. (thats my understanding of it, someone correct me if im wrong)