Intercooling a Pos.Displacement blower, effect on boost? 1

[Turbinator]

Running some calculations, it looks like putting an intercooler after a positive displacement supercharger will result in lower pressure after the blower, all else being equal (vs. non-intercooled).

My reasoning is that the blower alone is what determines the mass air-flow into the engine, and increased air density after the intercooler would result in a greater volume flow before the intercooler, which would reduce the volume ratio before and after the blower, meaning a lower pressure ratio (and lower heat from compression and less parasitic loss, etc).

Am I correct here? I know the intercooler would have no effect on mass air flow if nothing else was changed, but doing some searches I haven't found anyone saying the manifold pressure would be lowered.

 

[patprimmer]

As a roots type blower is positive displacement, ignoring leak past loss changes due to different pressure profiles, you are correct. The mass air flow will be determined by the induction into the blower and will not be effected by anything downstream of the rotor.

The temperature drop will decrease the manifold pressure at the same mass, so there will be less work required to drive the blower meaning lower parasitic losses.

The differences you have not considered are that a lower charge temperature will be less inclined to detonate, thereby allowing more boost to be used if available, or a lower octane fuel, or a higher compression ratio. Also you need to consider the effect on airflow, and therefore back pressure in the inlet manifold caused by the intercooler


Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SBBlue]

An intercooler will decrease the charge pressure, but only because there is a pressure drop from the resistance of the air flowing through the intercooler.

Pat is right in that the lower manifold/charge temperature will be much less likely to detonate. I will also note that decreasing the charge air temperature results in less work during the compression stroke, resulting in a slight, essentially trivial increase in engine efficiency.

I suspect that your goal in putting a supercharger on your engine is to increase power output. If that's the case, intercooling also helps because it results in more air per piston stroke, which means more fuel can be burned and consequently a higher engine output will result.

 

[patprimmer]

SBB

What you say about more air per piston stroke can be true, and most definitely is true for a turbo or centrifugal blower.

For this to be true for a Roots type blower, you need to change drive ratio of the blower, which can be done to optimise the manifold pressure once an intercooler is used.

A Roots blower is a positive displacement pump. It takes in a fixed volume of ambient air per revolution of the blower, and manifold pressure depends on the resistance to flow of that air on leaving the blower.

An intercooler reduces temperature and consequently pressure for the exact same mass of air. If you do not alter the drive ratio of the blower, you still get the same mass of air, but it is easier to pump, as you are pumping the same mass to a lower pressure.

Your observation on energy lost to the compression stroke due to higher compression is quite astute.

As I see it, the energy lost to compress the gas, is mostly, but not completely, regained by the pressure of the compressed gas pushing down on the power stroke. This pressure will be on top of combustion produced pressures.
There will also be some extra losses as heat to the cooling system and the higher pressure on the compression stroke will increase ring friction slightly

Regards
pat pprimmer@acay.com.au
eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[SBBlue]

Pat wrote:

As I see it, the energy lost to compress the gas, is mostly, but not completely, regained by the pressure of the compressed gas pushing down on the power stroke. This pressure will be on top of combustion produced pressures.
There will also be some extra losses as heat to the cooling system and the higher pressure on the compression stroke will increase ring friction slightly

====================================

I thought I would do a few calculations on this.

The compressed gas pushing down on the power stroke is a pure force * distance traveled type of equation. Calculating the work for compression requires the air standard calculations.

It comes out like this; amount of work required to increase the pressure by 100% (ie, double the pressure) starting at an ambient temperature of 80 deg F: 28.4 BTU/lb air. This is for 100% efficiency. Since we don't have very many compressors that are 100% efficient, we will use a very good turbocharger compressor which is 80% efficient. That gives us about 35.4 BTU/lb air.

The amount of power produced by gas pushing down on a piston with a pressure of two atmospheres of pressure:
33.7 BTU/lb air. Taking into consideration the efficiency --say 85%, you wind up with about 28.6 BTU/lb air.

The morale of the story -- For spark ignition engines, a turbocharger or supercharger does not increase the engine efficiency. However, it does increase the amount of air and therefore the amount of fuel that can be used.