High Pressure vs. Low Pressure zones for air induction

[R_Lefebvre]

What do you mean by total system VE? I'd just like to explore this line of thinking, because maybe I'll be able to explain myself better, or you'll open up a flaw in my logic.

Also I should note, diesel engines would be heavily affected by flow restrictions, since they are running wide open on the air side all the time, and VE comes into effect. Also, turbocharged fuel injected engines will react differently, depending on the turbo boost control circuit. My comments were only about naturally aspirated fuel injected engines with modern electronic controls utilizing mass airflow, or manifold absolute pressure sensors.

 

[ntsqd]

Most VE numbers, as I understand them, are concerned with from the throttle plate to the header collector. Left out of the measurement are the rest of the intake and exhaust systems.
So those restrictions are not factored into the VE number. That is probably appropriate given how VE's are used, at least in racing oriented circles. For the purposes of this discussion they don't tell the whole story.

I brought up VE because I got the impression that there is this notion that an engine will always fill it's cylinders to full displacement capacity all of the time. That isn't really true with an NA engine. Due to all of the necessarily restrictive elements in the system the intake valve will close before 100% cylinder filling has occurred. I'm not even sure that it is always true all of the time for an AA engine (turbocharged, supercharged, turbo-supercharged), but obviously they come closer to doing this.

 

[R_Lefebvre]

VE actually has nothing to do with the exhaust system at all, at least, not directly. VE, the scientific definition, is purely cylinder fill relative to atmospheric density. 100% VE simply means that you have exactly the same density (at BDC) in the cylinder, as you do outside. There's really no consideration for intakes and exhausts in the calculation. However, both those items have an effect on VE. Ram air to tail pipe, soup to nuts, it all plays a role. But VE is simply: BDC Density/Atmospheric Density. And it's always calculated at full throttle.

Typical Naturally Aspirated engines in the 60's might have had a VE of 60%. Some as low as maybe 40, or up to 80. Modern hi-po engines, with variable cam timing, duration, etc can actually achieve a VE of 100%, or even over 100% in some of Honda's really hot applications. Yes, you can achieve a VE over 100% without forced induction. That gets into some crazy concepts that I'll leave out.

As I said, it's as simple as "BDC Density/Atmospheric Density". But intake and exhaust play a role. The intake has a very direct and obvious effect on VE. The exhaust also plays a role, because if the exhaust can't clear the combustion chamber to make room for oxygen, VE suffers.

Now, when cruising, most vehicles require something on the order of 20 hp. Not 200, or whatever the full power is. If you had a 100% VE engine that produced 200hp, you would require 10% VE to produce 20hp, assuming no frictional or thermal losses. Let's also assume it requires 10% throttle to let 10% air density into the intake manifold which results in the 10% VE. This is grossly over simplified, obviously.

Ok, so now we install a restrictive intake system, or a backwards snorkel or whatnot. This immediately reduces the full load VE from 100%, to 95%. At a 10% thottle setting, you might see more like 9.5% density in the manifold, producting 9.5% VE, and actually only making 19hp. So, the truck starts slowing down. What does the driver do? He opens up the throttle a little more. So, you open it to 10.5%. The extra throttle angle now allows 10% air density into the manifold again, which again produces 20hp.

So, to cruise down the road using 20hp, the engine has 10% air density in the manifold whether it is operating at 100% VE (measured at full load, VE doesn't get measured when throttled), or whether you have restricted it to 95%. That 10% air density requires the same amount of fuel to burn at stoichiometric ratios, and the same fuel economy results.

Obviously, the big things I've left out are: Engines have friction, and thermal losses. This is seen as an "energy overhead" which needs to be overcome. It is constant at a given speed and load. It might mean you actually need 30% air density in the manifold to achieve 10% power at the wheels. All of this is transparent to the previous discussion.

The other thing left out, is what I mentioned in my previous post. The computer looks at the throttle position to determine load demand. There is a cross-over point when the computer will switch from 14.7:1 Air/Fuel Ratio for emissions, to a "high power" AFR of about 13:1. It richens up the mixture to make more power from a given air density in the manifold. So fuel economy suffers. That is how a restrictive intake can affect a fuel injected vehicle.

One thing you may be interested in is the concept of "Load". Which is the instantaneous cylinder fill of an engine. At full throttle, VE is necessarily equal to the VE of the engine. At lower throttle settings, Load is closer to the air density in the intake manifold.

Again, to travel down the road at a given speed, you require a certain HP, which means a certain load on the engine. When you restrict the intake, you must open the throttle a little more to achieve the same load. This has no effect on fuel consumption, unless you cross-over into the high-power setting of the fuel injection system. The worse you restrict the intake, the higher throttle setting required, and thus the more likely you are to cross into the open loop setting of the ECU.

This can be a little hard to grasp, and I've seen many "professional" aftermarket engine tuners who don't really get this stuff. Once you start working with an elegant fuel injection system (I've tuned Pectel, and Ford's EEC IV) it's much more obvious what's going on.

 

[ntsqd]

I've not been using Volumetric Efficiency in the strict Engine guy's definition. I'm using it to describe the overall pumping efficiency of the complete engine and all of it's related sub-systems. Perhaps I should have said "Pumping Efficiency", but some how that seems less clear to me. All of the restrictions in the various engine support systems (induction, filtration, exhaust, muffling, etc.) reduce the in-use cylinder filling from 100% of it's calculated volume. To me that is best expressed as "Volumetric Efficiency."

I think that we can agree that anything done to any part of the engine and it's support systems will have an impact on how well the engine breathes. Some things will have a negative impact and others will have a positive impact. Putting the intake point of the induction system in a region of partial vacuum will be a negative, but since the magnitude of the vacuum depends on vehicle speed the amount of impact will vary with vehicle speed. Additionally, airflow over the exterior shape of the vehicle combined with the location of the intake point can influence the amount of vacuum or positive pressure at the point of intake and what that absolute pressure might be at any given speed is not always an easy to guess or obvious answer.

 

[R_Lefebvre]

I've not been using Volumetric Efficiency in the strict Engine guy's definition. I'm using it to describe the overall pumping efficiency of the complete engine and all of it's related sub-systems. Perhaps I should have said "Pumping Efficiency", but some how that seems less clear to me. All of the restrictions in the various engine support systems (induction, filtration, exhaust, muffling, etc.) reduce the in-use cylinder filling from 100% of it's calculated volume. To me that is best expressed as "Volumetric Efficiency."

You know, I think we're exactly on the same page here, it's just a matter of semantics. The only part I really took exception to is this:

Left out of the measurement are the rest of the intake and exhaust systems.
So those restrictions are not factored into the VE number.

They aren't in the calculation directly, because the calculation is very simple. But they do have a direct measurable effect on the VE, and shouldn't be left out of any discussion.

It should also be pointed out that when discussing an engine's VE, that is usually on it's peak VE being referenced. In fact, an engine has a variable VE that changes with RPM. The peak VE occurs close to the torque peak rpm.

The reason I mention this, is just because you said that something like an exhaust system is left out of the measurement. On a 4 cylinder, the entire system plays an important roles. Just changing between 2.25 and 2.5" tubing can move the VE peak around, noticably affecting the peak torque and hp figures.

I think that we can agree that anything done to any part of the engine and it's support systems will have an impact on how well the engine breathes. Some things will have a negative impact and others will have a positive impact. Putting the intake point of the induction system in a region of partial vacuum will be a negative, but since the magnitude of the vacuum depends on vehicle speed the amount of impact will vary with vehicle speed. Additionally, airflow over the exterior shape of the vehicle combined with the location of the intake point can influence the amount of vacuum or positive pressure at the point of intake and what that absolute pressure might be at any given speed is not always an easy to guess or obvious answer.

Agree 100% with this. That's why I said I point mine forward in the blind trust that it's the best direction in my application. I have no proof.

 

[ntsqd]

I was correct in assuming that we were in, as my old boss used to put it, "violent agreement."

I agree and it was what I was trying to get across, that the external systems should not be left out of the discussion even if they are left out of the true VE calculation.

 

[R_Lefebvre]

Amazing what happens when people can discuss technical details with a level head.

 

[xtfritz]

Just to stir things up abit, I actually run my snorkel forward most of the time with the exception in snow, high dust or heavy rain for the pure sake of ensuring NOTHING goes down the snorkel. That said I also have a computer measuring my inputs/outputs on my FJC and I actually ran my snorkel turned around for a week driving in Houston traffic just to see if I could get a change in performance. I noticed the fuel efficiency drop slightly from my standard 17 mpg to around 15.5-16 mpg range and obviously with that I had to tank up one extra time that particular week, all of this running between near parked at stoplights and up to 65 mph on the interstate. From a feel standpoint I 'felt' that my engine could race better with the head pointed forward taking advantage of a more ram effect, but this is a tough point to back up since I don't have dyno to actually prove it.

Though if someone really is concerned with backwards/forwards of a snorkel my thought is to get the prefilter bowl that you can find on sierra expeditions website or like the OE toyota snorkel filters that are found on 70 series LCs. It covers the intake from branches, debris, etc and the intake itself is unidirectional, taking from any which way air can be drawn into the snorkel head. No need to flip the darn thing around everytime you see a heavy storm on the horizon.

Cheers!

 

[dust devil]

Pre-filters, themselves, create a degree of restriction in the intake system. Most marketed snorkels are marginal for air passage to start with, being only barely sufficient to feed, at high speeds, the engine they are intended to mount to. Adding extra filters makes for cleaner air, but a bit less of it because of restricted air flow, and they provide no ram effect. I find that a good pre-filter bowl results in air supply similar to turning the head around backwards on the highway. I also find them, personally, to look goofy so I don't use one anymore. They do, however, take a lot of dust out of the incoming air. Unless you are racing, driving in really dusty conditions is usually done at slow speeds where the low pressure from a reversed head or from a pre-filter has a very negligible effect on air delivery.

I don't find having the snorkel pointing forward adds anything in the way of extra dirt to the filter system in dusty driving. If the dust is there, your intake will find it. The intake under the hood seems quite able to find as much dirt in the air as otherwise. If, however, the snorkel puts the intake at a height which, due to conditions, is above the bulk of the air borne dust, there is an advantage, and it is this advantage that the snorkel is intended to provide (that and high water crossing). Pointing it backwards in heavy dust helps nothing. Just as dust accumulates on your back window due to the low pressure zone created there, turning the head around in dust only puts the intake in the zone of highest accumulated dust as well as in a low pressure site because as the air decelerates in the low pressure turbulence, the dust settles out and is taken into the intake. Not the best combination.

If it really bothers you to have the snorkel pointed forward in dust, turn it sideways. You won't avoid the dust because it is dispersed in the air the engine needs, but it might make you feel better about the situation and the air pressure drop is not as severe as turning the head around backwards.

If you drive in a lot of dust, clean your air filter regularly, snorkel or no snorkel.

 

[ntsqd]

Dust particles are normally heavier than air. As such they have more inertia than does air, so pointing the scoop backwards may not eliminate all dust from being pulled into the filter, but at least some it will not be able to make the 180* turn into the snorkel. That would reduce the dust inducted via a rear facing scoop vs. a forwards pointed scoop in the same conditions. It won't eliminate the dust from getting in, but it will reduce the volume vs. time of operation.

 

[bigreen505]

Interesting theories, but it would be easy enough to tape some strands of yarn on the windshield and the snorkel and go for a drive to see where air is actually going. If you do it, take pictures and post them up.

 

[ntsqd]

I thought that I suggested doing that early in the thread. For the most part a little aero education will go quite a ways in predicting what the airflow will look like, but there are ALWAYS surprises.

Once I'm to the point where it is coming up on important I plan to do this test on the hood of my VW Ribbit diesel. The idea is to confirm what I think the flow is in order to suitably place the intake for the aftercooler's exchange air. Don't hold your breathe though, I'm a long ways from that point.