SuperCharger and intercoolers. 2

[chastain]

Hello, all;

I am new to the forum, which I found while searching for an answer to a minor question. I am hoping that someone here could help with the question. Is there any advantage to the use of an Intercooler with a Centrifugal type Super Charger? I would think that any gains you may get from the cooler air (assuming it would be cooler), would be counter-acted by losses from the added pip-work. Any thoughts on this?

 

[SBBlue]

A well designed aftercooler will have a boost of 15-20 psi and a pressure loss of only one or two psi. The only real disadvantage to using an aftercooler is that it will cost money.

 

[chastain]

Do you think there will be a gain in power/torque due to the cooling it provides when used with a Cetrifugal Supercharger.

 

[rixindley]

The compressed air from the supercharger will be warmer than the intake air due to adiabatic heating ("heat of compression&quot. An intercooler (aftercooler) will cool the compressed air and make it denser. The denser air allows more fuel to be added thus more horsepower can be generated. If the intake plumbing and intercooler are sized correctly there is much to be gained.

A cooler charge temperature will help prevent detonation, too. Also, an intercooler will permit the engine to operate more consistently under high boost conditions regardless of intake air temperature.

Typically, the intake air temperature is measured upstream from the supercharger. The heat contributed by the supercharger is "unknown" by the engine's fuel management system, which results in less-than-ideal fuel management. An intercooler will help to raise the boosted air temperature closer to the measured air temperature, thus improving fuel management. Obviously, if the engine's intake air temperature sensor is located in the intake plenum, then an intercooler likely will not improve fuel management.

Rick

 

[chastain]

All well know points. I guess I am not associating heat generation with Supercharging. 5-15 PSI doesn't seem to be enough compression to create a significant amount of heat. I had always thought that the Intercooler was developed to reduce heat from turbos because of the Exhaust temp.

Cooler is always better, for sure.

 

[GregLocock]

You might be surprised. An adiabatic compression leading to a doubling in pressure, will also increase the temperature by 22% or so, ie reducing your 'volumetric efficiency' from a nominal 200% to 156%... quite a persuasive argument in favour of intercoolers.





Cheers

Greg Locock

 

[patprimmer]

Greg
Do you know of any charts or formulas for working this out.

I guess by doubling you mean 15 psi gauge and 22% in degrees Kelvin. I guess i can figure it out from there if I look up the combined gas laws.

I think I just answered my own question.

Honest, I don't touch the stuff

Regards
pat

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[chastain]

THX, all. I'm not an engineer, just a manager with a technical background who loves to tinker with TOYS, but I spent some time in the military as an automotive/electronic technician and I work half-way through an Electronic Engineering Degree before opting for Business/Management, so I can grasp what you are explaining.

Next question...
Do Turbos and Superchargers induce the same amount of heat into the incoming air charge. I had always thought that the Intercooler was developed becuase the Turbos use of exhaust gas heated the air to several hundered degree(f). A guy in another forum is telling me that this is not so. He is telling me, because the air moves so quickly through the Turbo it does not pick-up heat, so the only increase in temp is from the compression. Can this be true? Makes sense to me if I ignore the fact that the exhaust gas is 1000+ degrees.

 

[GregLocock]

Pat, yes you have to use absolute temp.

for adiabatic expansion of an ideal gas

T2/T1=(P2/P1)^((gamma-1)/gamma)

gamma=1.4 for air





Cheers

Greg Locock

 

[maniacle]

Superchargers and turbos both will raise the temp of your charge, thats already been covered, but a turbo doesnt raise the temps solely based on exhaust gas temps. The majority of the elevation in temps is from compression. There can be some other heat exchages through the expansion and compression stages in the turbo, but not enough on a practical basis to consider.

You mentioned youre using a centrifugal blower, which in a lot of ways is goverened by the same rules as a turbine compressor. A posotive displacement supercharger is a little easier to work with, because the flowrate is linear with rotating speed, whereas a turbine compressor wont always share this attribute.

and yup...intercooling is always a goood practice if its in the budget. Basic thermo...the greater the difference between your incoming charge temps and peak cylinder temps, the greater the efficiency.

just my 2 cents worth.

 

[maniacle]

if you use a k value (gamma)of 1.3 or 1.25, im pretty sure thats a but closer to the value of air and gasoline in a stoichiometric mix.

 

[chastain]

Not actually using yet, just concidering.

Thanks all. I am glad I found this forum.

So why wouldn't the incoming air charge pick-up the heat from the exhaust side of the Turbo. That seems like a lot of heat in close proximity to the incoming air.

 

[turbododge]

Rixindley mentioned that the temp of the air is usually taken upstream of the supercharger, which is probably true on a MAF system conversion to supercharging, but most of the systems that I have seen, both turbo and supercharger, have used MAP systems and have the temp sensor after the compression, as it is the only way to get accurate fuel calculations. In my twin turbo setup, I read temp in the plenum, after the compressors and throttle body.

 

[patprimmer]

It does, but not much compared to the adiabatic heat from compression.

Roots blowers and belt driven centrifugal blowers (read same compressor as torbo, but driven by belt instead of exhaust tibine) all create significant heat in the compressed charge.

Heat is comeing into or out of all the manifolds and air ducts, and often the inside and outside temps are both constantly varying. It all gets to complicatedto quick to consider every minor detail. Adiabatic heat is by far the largest effect when at full power (ie when it matters), so you can safely ignore the rest.

An efficient intercooler significantly reduces the charge temp and pressure, allowing the compressor to move a greater mass of air for the work it does, as it is not fighting such a high head of pressure. Lower charge temps also reduce the tendency to detonate, so higher boost or compression can be run.

Greg

Thanks for the formula. I was aware of the PVT thing and Charles and Boyles laws, but I was unaware of the gamma factor. Is gamma effected by methanol as the fuel. I studied this stuff in 1960 something, so it's sometimes a bit vauge now. I also did calculus in 1960 something, and it's completely and utterly lost now.

Regards
pat

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[chastain]

I like the name TurboDodge. This thing started over a discussion about Supercharging a 1st Gen 3.5 Liter from an LH body. I know someone who is building the engine, but not for an LH. He is designing it for a customer who wants to patent the idea so he is being quiet about specifics, but he refered to it as a Street-Rod, so I am betting it is for a '97 Prowler which uses the same iron-block 3.5 as the 1st gen LH. He has said the mounting will not work on the LH, but is hoping the customer will allow him to modify the design when he is done.

hehehe, I'm already drooling over it... lol

 

[GregLocock]

Gamma is the ratio of the specific heats for the gas, at constant volume and constant pressure. A mixture of gases behaves as the weighted average of the constituents by volume, I expect.

A quick perusal of the Bosch book gets me no further than that, they use 1.4 as well in their handy charts.



Cheers

Greg Locock

 

[SBBlue]

Here's some calculations using the air-standard tables.

For five pounds of boost: If you have a "ACME-100" perfectly adiabatic compressor it will require 11.24 BTU of energy to increase the pressure of one pound of air by five psi from ambient temperature (80 deg F) and pressure (14.7 psi). The temperature of the air as it leaves the compressor will be 127 degs F.

Unfortunately, all the "ACME-100" compressors have been recalled, and the best compressor you will be likely to find -- at least for automotive purposes -- will only be 80% efficient. The amount of work to pressurize the same amount of air will be 14.05 BTU, and the resultant temperature will be 138 deg F. The increased temperature is the result of the inefficiency of the compressor. It takes part of the mechanical energy and converts it into heat energy.

If you use a Roots type supercharger, the efficiency will only be about 50%. The resulting compressor work is 22.48 BTU per pound of air, and the discharge temperature of the air is 174 degress F.

To put this into perspective, consider a six liter (350 ci) engine with a volumetric efficiency of 90% that is turning at 3000 rpm with the throttle wide open. That engine will use about 23 pounds of air per minute. If a supercharger is used with 5 psi of boost, the air mass will increase to about 31 pounds/minute. The amount of power required for an 80% efficient compressor will be about 10.3 hp; for the Roots supercharger with an efficiency of 50%, 16.4 hp will be required for supercharging purposes.

Now let's look at the numbers for 15 pounds of boost. The mythical 100% efficient adiabatic compressor will require 28 BTU of energy for each pound of air compressed by 15 psi. The post compressor temperature will be 200 deg F. An 80% efficient compressor will require 36 BTU per pound of air, and the discharge temperature jumps to 230 deg F. The roots compressor takes 57.6 BTU, and the exit air is at a temperature of 322 deg F.

Looking at our six liter engine, the air flow through the engine is now increased to 46.7 lbs/min (assuming a 100% efficient aftercooler, but that's another story. . .). A 80% efficient compressor takes 39.7 hp, while a Roots supercharger will require about 63.5 hp.

Well, I'm getting tired of typing. If anybody wants, I will quantify the amount of heat removed by an aftercooler and how much aftercooling decreases engine work and thereby increases engine output.

 

[aviat]

The following site provides another formula that accounts for compressor efficiences, which can add to the temperature increase.
Values for gamma I have are: Only air 1.4 (ie fuel injection), air/fuel mixture 1.314, and methanol 1.2

http://www.gnttype.org/techarea/turbo/turboflow.html

 

[aviat]

Actually the power above should be ((gamma-1)/gamma). That is how they get the 0.263 in the formula in the site above. They are using gamma of 1.357.

 

[patprimmer]

((gamma-1)/gamma) is how I read Gregs post

I expect that latent heat of vaporisation also needs consideration, especially for methanol as it has a very high latent heat and is also needs a an AF ratio of about 5.5:1

Regards
pat

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