CFM query [SOLVED]

[Carolyn]

Putting head briefly above the parapet.

I think CGB is saying it will be true, for the range it is calibrated for.
If 0 = no air and 100 = air.

Not much point calibrating to measure a vaccum. If there is no air. Engine starting will be the least of your worries.

So anything below a set target does not need to be measured. But between whatever the range is. Will be accurate.

Ducking back down quickly.

On that note, how does the car measure manifold pressure? Is there a sensor I don't know of? That shows vacuum on the "gauge"

The standard car doesn't measure manifold pressure.
 

[shnazzle]

Putting head briefly above the parapet.

I think CGB is saying it will be true, for the range it is calibrated for.
If 0 = no air and 100 = air.

Not much point calibrating to measure a vaccum. If there is no air. Engine starting will be the least of your worries.

So anything below a set target does not need to be measured. But between whatever the range is. Will be accurate.

Ducking back down quickly.

On that note, how does the car measure manifold pressure? Is there a sensor I don't know of? That shows vacuum on the "gauge"

The standard car doesn't measure manifold pressure.
 

Indeed. So it has to be a calculated value.

[ChrisGB]

So taking what you said, really 100% VE, on our engines with VVTi and 4 valves per cylinder, is quite bad.
But, that's based on the MAF signal, which as you say, may not be calibrated for the full VE range. So, 100% VE might actually be higher. The only way to measure VE then is to have a calibrated device that accurately measures the actual airflow.

Why is the MAF no calibrated to be true?

I think you are still confusing VE with airflow. VE is very specifically the amount of mixture in the cylinder at the time the valves are both closed at the end of an induction cycle. For a 500cc cylinder, this would be 100% if there is 500cc of mixture in there. This has very little to do with airflow. The VE changes depending on the various physical attributes of the engine and its load condition. It is generally a guide to how effectively an engine is setup and it's peak value will only occur at a very limited speed and load range.

It is difficult to explain, but this may make sense... Imagine your 500cc single cylinder engine as an air pump. At full throttle lets assume that the engine as a system draws in 500cc of air for every intake stroke. You'd think that this would mean 100% VE. However, what happens if some of this air gets down the exhaust port before the exhaust valves close? If 10% goes down the exhaust port, the engine has pulled 500cc per intake cycle, but VE is only 90%. Another loss could be that some of the air gets pushed back into the inlet system as a pulse before the inlet valves close, eg 5%. It has still pulled 500cc for that intake stroke, but another 5% is sitting in a higher pressure area in the inlet system, you are now down to 85% VE.

On the subject of air metering, the MAF consists of two elements. A hot wire assembly, which is cooled by inlet air, the more it is cooled, the more airflow it reports. A thermistor, which tells the ECU how warm the air is. So if you know the cooling effect and the air temperature, you can easily work out how much fuel to add. MAF specifically refers to Mass Air Flow, so it is done by weight. For any number of reasons, the airflow metering system could be non linear in output, so the readings would need to be scaled electronically. It is not unusual to have sensors with a full range value of say 0-5v being used between 1v and 4v to represent minimum and maximum output within their application.

VE is a slippery concept to pin down and not easy to measure directly.

[SV-3]

How come that most cars sr said to run about 80-85% volumetric efficiency?

Because there are flow losses. It is after all ´driven´ by súcking in air through a filter and then pushing it out through another restriction. There is a pressure difference needed to drive the flow, hence the volume pumped is less than the swept cilinder volume.

How sure are you that the number indicated is the actual air mass? In other words is the value calibrated?

Have a look at this folks:
https://www.motor.com/magazine-summary/driveability-corner-june-2003/

[Petrus]

How come that most cars sr said to run about 80-85% volumetric efficiency?

Because there are flow losses. It is after all ´driven´ by súcking in air through a filter and then pushing it out through another restriction. There is a pressure difference needed to drive the flow, hence the volume pumped is less than the swept cilinder volume.

How sure are you that the number indicated is the actual air mass? In other words is the value calibrated?

Have a look at this folks:
https://www.motor.com/magazine-summary/driveability-corner-june-2003/

Peak VE under optimum circumstances is not a usefull perspective for modern road cars. Air filtration, sound regulations and catalists are but three real world restrictions lowering VE for an n.a. engine.
The about 10 extra hp the 1ZZ in the SW30 pumps out with the cats and restrictions in the muffling removed is a good example.
Take the air filter into account ánd the proven limited flow though the head.
And remember the MAF mod and the OEM inlet elbow.


Monitoring and tuning for VE from the MAF ís usefull though. Just not as an accurate real value measuring instrument.
It is as with apps on the mobile phone for tuning purposes. Gréat comparative tools, just not for real numbers.

[shnazzle]

So taking what you said, really 100% VE, on our engines with VVTi and 4 valves per cylinder, is quite bad.
But, that's based on the MAF signal, which as you say, may not be calibrated for the full VE range. So, 100% VE might actually be higher. The only way to measure VE then is to have a calibrated device that accurately measures the actual airflow.

Why is the MAF no calibrated to be true?

I think you are still confusing VE with airflow. VE is very specifically the amount of mixture in the cylinder at the time the valves are both closed at the end of an induction cycle. For a 500cc cylinder, this would be 100% if there is 500cc of mixture in there. This has very little to do with airflow. The VE changes depending on the various physical attributes of the engine and its load condition. It is generally a guide to how effectively an engine is setup and it's peak value will only occur at a very limited speed and load range.

It is difficult to explain, but this may make sense... Imagine your 500cc single cylinder engine as an air pump. At full throttle lets assume that the engine as a system draws in 500cc of air for every intake stroke. You'd think that this would mean 100% VE. However, what happens if some of this air gets down the exhaust port before the exhaust valves close? If 10% goes down the exhaust port, the engine has pulled 500cc per intake cycle, but VE is only 90%. Another loss could be that some of the air gets pushed back into the inlet system as a pulse before the inlet valves close, eg 5%. It has still pulled 500cc for that intake stroke, but another 5% is sitting in a higher pressure area in the inlet system, you are now down to 85% VE.

On the subject of air metering, the MAF consists of two elements. A hot wire assembly, which is cooled by inlet air, the more it is cooled, the more airflow it reports. A thermistor, which tells the ECU how warm the air is. So if you know the cooling effect and the air temperature, you can easily work out how much fuel to add. MAF specifically refers to Mass Air Flow, so it is done by weight. For any number of reasons, the airflow metering system could be non linear in output, so the readings would need to be scaled electronically. It is not unusual to have sensors with a full range value of say 0-5v being used between 1v and 4v to represent minimum and maximum output within their application.

VE is a slippery concept to pin down and not easy to measure directly.

Gotcha. Thanks Chris. That explains it perfectly. Two completely different concepts and explains a lot of questions I had about Helen's standalone map.


So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

[Petrus]

So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

Tricky Patrick. Temperature change in the inlet tract also messes up the ´real´ volume.
In itself metering the fow through a tube is a good base for calculation but again, it is flów metering and then calculation the quantity of air, compensating for variables such as temp.

Yes, the actual value is not important nor the goal is it?! For a given egine set up you mess with the variables to get max. flow over a wide as possible rev range with max torque and hp the resiltants.

[Joesson]

So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

Tricky Patrick. Temperature change in the inlet tract also messes up the ´real´ volume.

As explained by Mr Boyle.

[Call the midlife!]

So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

Tricky Patrick. Temperature change in the inlet tract also messes up the ´real´ volume.

As explained by Mr Boyle.

Frankie?? I've not heard his opinions on VE...🤓

[Joesson]

So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

Tricky Patrick. Temperature change in the inlet tract also messes up the ´real´ volume.

As explained by Mr Boyle.

Frankie?? I've not heard his opinions on VE...🤓

Frankie Boyle, four letter, rather than four syllable words are apparently more his forte. I had no knowledge of Frankie Boyle until I looked at the www. However, I do believe that Roberts words will endure longer than Frankie's

[ChrisGB]

Gotcha. Thanks Chris. That explains it perfectly. Two completely different concepts and explains a lot of questions I had about Helen's standalone map.

So, is there any way to measure VE actually? Or, do you just aim to get peak torque and it is what it is? Whether it's 100% VE, or 125%,or 85% is irrelevant I guess.

I think the only way you could home in on a value would be to measure a known quantity of fuel and air flow, adjust whatever variable inlet and exhaust parameters you can, optimise the ignition timing and gather information like unburnt HC / CO EGT / torque etc. To be fair, there are bags of calculations you can make when designing your engine and these generally get you into the right ball park.

The fun starts once you start adding boost. It is possible to have a huge airflow and relatively poor VE if inlet charge goes down the exhaust port. This is an issue for people boosting the 2zz by large amounts IIRC. Cams need changing to keep the charge in!

[Carolyn]

The problem with gases is that they are elastic.  Volume, in a dynamic environment, is all over the place and pretty meaningless.  In a liquid, mass and volume are much easier to tie down.

In rocketry, we only ever measure mass.  That isn't always easy either.

[ChrisGB]

The problem with gases is that they are elastic.  Volume, in a dynamic environment, is all over the place and pretty meaningless.  In a liquid, mass and volume are much easier to tie down.

In rocketry, we only ever measure mass.  That isn't always easy either.

So very true! Mind you, it is that elasticity that makes it possible to cram in more than 100% cylinder volume with good design. When that tune all comes together it just sounds "right". Four stroke setup is tricky, two stroke even more potential, but trickier still.

I'd really love to know more about rocket engines. Do they, like a well set up petrol engine, have that grin inducing sound that tells you when everything is just "right"?

[Carolyn]

The problem with gases is that they are elastic.  Volume, in a dynamic environment, is all over the place and pretty meaningless.  In a liquid, mass and volume are much easier to tie down.

In rocketry, we only ever measure mass.  That isn't always easy either.

So very true! Mind you, it is that elasticity that makes it possible to cram in more than 100% cylinder volume with good design. When that tune all comes together it just sounds "right". Four stroke setup is tricky, two stroke even more potential, but trickier still.

I'd really love to know more about rocket engines. Do they, like a well set up petrol engine, have that grin inducing sound that tells you when everything is just "right"?

Not really.. Mind you they do leave you rather deaf - at least the ones I worked with did.  The exhaust plume, (colour, steadiness and shock-waves) is where that satisfaction  is to be had.

[ChrisGB]

The problem with gases is that they are elastic.  Volume, in a dynamic environment, is all over the place and pretty meaningless.  In a liquid, mass and volume are much easier to tie down.

In rocketry, we only ever measure mass.  That isn't always easy either.

So very true! Mind you, it is that elasticity that makes it possible to cram in more than 100% cylinder volume with good design. When that tune all comes together it just sounds "right". Four stroke setup is tricky, two stroke even more potential, but trickier still.

I'd really love to know more about rocket engines. Do they, like a well set up petrol engine, have that grin inducing sound that tells you when everything is just "right"?

Not really.. Mind you they do leave you rather deaf - at least the ones I worked with did.  The exhaust plume, (colour, steadiness and shock-waves) is where that satisfaction  is to be had.

I'd assumed that loud is good and louder is better. Something on my bucket list it to get to see a launch from Cape Kennedy, Baikonur or Kourou.

I can see the satisfaction in seeing / feeling shock waves is always visceral!

[shnazzle]

The problem with gases is that they are elastic.  Volume, in a dynamic environment, is all over the place and pretty meaningless.  In a liquid, mass and volume are much easier to tie down.

In rocketry, we only ever measure mass.  That isn't always easy either.

So very true! Mind you, it is that elasticity that makes it possible to cram in more than 100% cylinder volume with good design. When that tune all comes together it just sounds "right". Four stroke setup is tricky, two stroke even more potential, but trickier still.

I'd really love to know more about rocket engines. Do they, like a well set up petrol engine, have that grin inducing sound that tells you when everything is just "right"?

Not really.. Mind you they do leave you rather deaf - at least the ones I worked with did.  The exhaust plume, (colour, steadiness and shock-waves) is where that satisfaction  is to be had.

I'd assumed that loud is good and louder is better. Something on my bucket list it to get to see a launch from Cape Kennedy, Baikonur or Kourou.

I can see the satisfaction in seeing / feeling shock waves is always visceral!

I got that just from watching the top fuel drag bikes at Santa Pod. Never experienced such a visceral rumble before that.


Watching a launch would be epic

[ChrisGB]

I got that just from watching the top fuel drag bikes at Santa Pod. Never experienced such a visceral rumble before that.


Watching a launch would be epic

Go and see top fuel dragsters if you like a bit of noise!

[Call the midlife!]

https://youtu.be/EhzTvN4UEKY
The last comment reminded me about this, feel free to remove it or reposition it as necessary Patrick, just thought it might appeal..

[shnazzle]

https://youtu.be/EhzTvN4UEKY
The last comment reminded me about this, feel free to remove it or reposition it as necessary Patrick, just thought it might appeal..

We saw those at the pod as well! 

I remember having a good look at one and saying a CBR600 engine with a turbo and NOS was absolutely mental.
.... Which it is.

[shnazzle]

Thread revival;
Was reading another forum and someone else was struggling with the concept of VE.

His struggle was around an example of a 5L engine being 100% VE if it displaced 5L of mixture through one cycle. So the cylinders filled perfectly. Nothing up the intake, nothing out the exhaust,perfect valve timing, nothing blowing. Each cylinder filled with all the mix it could, to be compressed at whatever rate the engine allowed. 

But, then went on to say, if it only sucked in 2.5L of mixture, because of an intake restriction (throttle body) would it have a VE of 50%?
This is where things got funny 
The cylinders were still filled with 5L of mix. But that mix was 50% as dense. 
So in that case... What's the VE? Is it still 100%?

[Carolyn]

Thread revival;
Was reading another forum and someone else was struggling with the concept of VE.

His struggle was around an example of a 5L engine being 100% VE if it displaced 5L of mixture through one cycle. So the cylinders filled perfectly. Nothing up the intake, nothing out the exhaust,perfect valve timing, nothing blowing. Each cylinder filled with all the mix it could, to be compressed at whatever rate the engine allowed.

But, then went on to say, if it only sucked in 2.5L of mixture, because of an intake restriction (throttle body) would it have a VE of 50%?
This is where things got funny
The cylinders were still filled with 5L of mix. But that mix was 50% as dense.
So in that case... What's the VE? Is it still 100%?

That's why, in rocketry, everything is done by mass not volume. Guess what - the car uses a MASS airflow meter.