VE Table for turbo engine

[alexx_88]

Hi,

I've seen quite a few VE tables for turbo engines that had values in them well above 100% and very different to their NA equivalent.

My thinking tells me that the volumetric efficiency shouldn't vary this much from a naturally aspirated engine to a turbo one, as the air pressure is measured and used separately when calculating the air quantity.

Am I wrong?

Thanks!

[spacer]

Higher VE is exactly what you're after when (turbo)charging an engine.

If I were to really, really simplify things; a NA engine running 100% VE, would get 200% VE if running 1Bar of boost.

[J.A.W.]

Higher VE is exactly what you're after when (turbo)charging an engine.

If I were to really, really simplify things; a NA engine running 100% VE, would get 200% VE if running 1Bar of boost.

Indeed, & a commensurate BMEP/torque increase accordingly..

[alexx_88]

Let's stick to VE for now, please

What's the explanation, then? Why would increasing the pressure of the air coming into the cylinders modify the relative volume occupied by that air in the cylinders?

Starting from the ideal gas law: n = p * V / R * T . Compressing the air that comes into the engine modifies the 'p' term, thus increasing the air quantity, but it can't modify the maximum volume that the air can occupy compared to the NA route.

Starting from your example, having a 1bar boost pressure and let's assume a 1000cc engine.

NA:
p = 1 bar
Veng = 1000cc
Vair = VE * Veng = 1000cc

NA: air quant = p * Vair / R * T = 1000 / R * T

Turbo:
p = 2 bar
Veng = 1000cc
Vair = VE * Veng = 2 * 1000cc = 2000 cc

Turbo: air quant = p * Vair / R * T = 4000 / R * T


As you can see, increasing the VE and still accounting for air pressure translates a doubling of the air pressure into aquadrupling of the air quantity, which is simply not correct.

[guffe]

You are multiplying your equation with the pressure twise. Also the cylinder volume does not change due the pressure, only the air density changes.

[alexx_88]

I was just showing the flaw that I consider to be in the 'VE doubles with pressure' camp.

I'm saying that, as long as you take MAP (manifold absolute pressure) into account when calculating the air quantity, modifying the VE table is incorrect.

[guffe]

Ok, I see what you are meaning.

Yes, the engine VE doesn't chance, the density of the air in the intake manifold changes and with MAP + IAT you can calculate the density so your fuel injection will be correct. But if you just look at the cylinder without thinking of the surroundings, you could say that the VE has changed.

[gruntguru]

It would help if you posted the VE tables you are talking about.

If the VE is calculated from intake airflow, there will be an effect caused by a change (reduction) in trapping ratio. If the turbo engine has boost greater than exhaust back pressure, there will be a larger quantity of scavenged air during valve overlap. This will make the VE look higher.

[xxChrisxx]

I feel I may be missing nuance of the question. However a doubling of pressure doesn't mean the same as double the volume of air.

VE = Volume of atmospheric air actually moved / swept displacement


Compressing air is an (semi) adiabatic process, and a doubling of the pressure by compression doesn't mean doubling the pressure*volume after compression.
http://en.wikipedia.org/wiki/Adiabatic_ ... ompression

So the compression follows the following: (P2/P1)^(1/gamma) = V2/V1
Gamma for air is 1.4.
P2 = 2 (1 bar boost - so double atmospheric)
P1 = 1

Normally you would use that equation to find V2 based on a known V1. However we know we fill V2 (set V2 = 1), and want to find the required volume of pre-compressed air to do that.

So:
V1*((P2/P1)^(-1/gamma)) = V2
V1*((2/1)^(-1/1.4)) = 1
V1*0.609 = 1
V1 = 1/0.609
V1 = 1.64

So for ideal air, 1 bar boost will give a 64% increase in Volumetric efficiency

In reality the process is not adiabatic so effects like temperature, flow, dynamic compression, valve timing, etc, etc, etc. will give you considerably less than this figure.

[gruntguru]

I feel I may be missing nuance of the question. However a doubling of pressure doesn't mean the same as double the volume of air.

VE = Volume of atmospheric air actually moved / swept displacement

Compressing air is an (semi) adiabatic process, and a doubling of the pressure by compression doesn't mean doubling the pressure*volume after compression.
http://en.wikipedia.org/wiki/Adiabatic_ ... ompression

So the compression follows the following: (P2/P1)^(1/gamma) = V2/V1
Gamma for air is 1.4.
P2 = 2 (1 bar boost - so double atmospheric)
P1 = 1

Normally you would use that equation to find V2 based on a known V1. However we know we fill V2 (set V2 = 1), and want to find the required volume of pre-compressed air to do that.

So:
V1*((P2/P1)^(-1/gamma)) = V2
V1*((2/1)^(-1/1.4)) = 1
V1*0.609 = 1
V1 = 1/0.609
V1 = 1.64

So for ideal air, 1 bar boost will give a 64% increase in Volumetric efficiency
In reality the process is not adiabatic so effects like temperature, flow, dynamic compression, valve timing, etc, etc, etc. will give you considerably less than this figure.

1. If the compressed air is intercooled back to ambient temperature the density ratio will equal the pressure ratio (2.0 in your example not 1.64).

2. Although the most common definition of VE is the one you gave:
VE = Volume of atmospheric air actually moved / swept displacement

it is useful to consider an alternative definition
VE = Volume of air (at intake plenum conditions) actually retained in the cylinders per cycle / swept displacement.

Useful because the engine as a positive displacement pump doesn't really care about air density or pressure at the intake. Consequently its VE by this definition does not change when throttled or supercharged.

Also note the change from "air actually moved" to "air actually retained in the cylinders". Air actually moved can change significantly when intake pressure is different to exhaust pressure, due to scavenge air bypassing the combuustion process during valve overlap.

[alexx_88]

So my hunch was correct, as long as you use a MAP sensor to correct for changes in pressure due to turbocharging, there's no logical reason to increase the VE table by the same ratio (i.e. changing a 100% VE on a NA to 200% VE on an engine with 1 bar of boost).

@gruntguru: There's no specific example, just maps that I've seen which seem to follow this trend.

Alex

[gruntguru]

The NA VE table will hold true when turbocharged - with two conditions:

1. Volumetric scavenging does not change. (True if Exhaust pressure = Manifold pressure)

2. "VE" is assumed to be based on Volume flow of air at manifold conditions.

So if "1." is true, you could use a 1D (VE vs rpm) NA VE table and for boosted or throttled conditions, apply a correction based on manifold air density (measure MAT and MAP).

[J.A.W.]

Or in other words.. & notwithstanding the sophistications of fluid dynamics absolutes..

An efficient forced induction set-up running +1bar over equivalent atmospheric - will effectively ~double the output..

[toraabe]

Or in other words.. & notwithstanding the sophistications of fluid dynamics absolutes..

An efficient forced induction set-up running +1bar over equivalent atmospheric - will effectively ~double the output..

Just look at this car https://www.youtube.com/watch?v=qsBOUJ5MQrI

just insane ..

[guffe]

Or in other words.. & notwithstanding the sophistications of fluid dynamics absolutes..

An efficient forced induction set-up running +1bar over equivalent atmospheric - will effectively ~double the output..

Just look at this car https://www.youtube.com/watch?v=qsBOUJ5MQrI

just insane ..

Going a bit offtopic, but oh the internet how you make things bigger than they are. Nothing is fast or give you a buzz in youtube if it doesn't have 1000hp... And in this case the car has a lot of power, but not the internet troll's wet dream of 1000hp+.