Charge cooling and mass flow

[Mark911]

Here’s another one for you guys. You hear all the time how an intercooler/aftercooler produces more mass flow because it increases density. I need this explained. The way I see it the Forced Induction device produces a given mass flow X at a given temperature Y. Now, pass that mass flow through a charge cooler and what do you get, the same mass at a lower temperature! Granted, the pressure will be reduced for a given mass flow and that’s always good, but the density is no different with these assumptions. Could it be that the reduction in temperature and subsequent pressure decrease produces a pressure gradient across the IC (not to be confused with pressure drop due to flow restriction) that generates additional flow through the IC which in turn reduces the pre IC pressure which in turn forces the turbos wastegate closed which THEN increases the overall mass flow up to the original wastegate setting? So it’s not the IC producing the extra flow/density, it’s the subsequent increase in turbo speed and extra mass flow. Given these assumptions one would expect a supercharger to only show the reduced air temps and pressures and no additional flow. In this case, the only benefits to HP coming from possibly less pumping work (dynamic CR) and the ability to tune a bit more aggressively than with higher air temps. That is unless one increases the boost back using a different pulley ratio. What am I missing?

 

[Warpspeed]

You are right, mass flow through an intercooler does not change. Every air molecule that goes in, comes right out the other end.

The effect is different with a positive displacement supercharger than with a turbo. With a supercharger the mass flow through the blower stays pretty much the same, but the intercooler shrinks the air volume and measured boost pressure will reduce at both blower outlet and engine. Blower Rpm must then be increased by altering the drive ratio to recover this perceived boost loss. That will increase mass flow.

With a turbo, the wastegate will automatically do a similar thing and increase turbo Rpm to maintain the original boost level setting. Once again mass flow increases.

 

[CESSNA1]

MARK911: Sometimes we run across terms that sound good, but do not make sense. The thing with intercooling/aftercooling/charge cooling/etc. is that the air temperature is lowered. By lowering the air temperature we can produce more power out of the engine. For example: Let us say that an engine has a maximum exhaust temperature or 1500 Deg F. If the air entering the engine is at say 1000 Deg F. we can get 1500 Deg F. minus 1000 Deg F. worth of power out of the engine. If, however we cool that air to say 500 Deg F. then we can get 1500 Deg F. minus 500 Deg F. worth of power out of the enigne. The other reason is that although the mass flow does not change the density does change, so we can "pack" more air into the cylinderat a lower temperature. It is the air, not the fuel that determines how much power and engine puts out. The fuel only heats the air to the necesssary temperature. Needless to say this requires more fuel and the fuel consumption goes up. Conversely if you want economy then do not cool the air. Fuel consumption and maximum power will go down.

Hope this helps
Regards
Dave

 

[21121956]

Hello everybody:

Then, what about turbos without wastegate in large Diesel engines? If there is not a possibility to increase the mass of air, so the only benefit to cool the air that passes through the cooler to the cylinders is to avoid high compression temperature (which induces early ignition) and in turn results in high combustion temperature.

Is it possible to accept that the mass of air addressed by these large turbos does not suffers any change on its way down from the inlet air filters to the cylinders?

Ricardo

 

[patprimmer]

Yes

The intercooler cools the charge. This drops the boost pressure for the same mass of air. This allows the turbo to pump in more air as there is less pressure to pump against.

Regards

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[CESSNA1]

Large diesel engines are two cycle engines. The turbo and superchargers not only increase the amount of air in the cyclinder, they also scavange the cylinders to remove the previous products of combustion, and introduce a fresh air charge. Diesel engines are compresion ignition engines, the timing is done at the injector. There is no fuel in the cylinder until the injector sprays it in. Typically turbocharged engines have a slightly lower compression ratio to compensate for the extra pressure of the turbocharger.

Regards
Dave

 

[21121956]

Hi Dave:

When I said large engines I really want to say, more precisely, medium speed Diesel engines of four strokes.

Certainly, I do know how does, two and four strokes, reciprocating engines works.

In the combustion process, the time between the start of the injection and the start of the ignition is known as "ignition delay". In medium speed Diesel engines, during the compression stroke, the temperature of the AIR reaches values of + 700ºC (depending obviously of the compression ratio and the inlet air temperature to the cylinders).

When for one reason or another, the inlet air temperature is higher (inlet air hot or not well cooled by the intermediate cooler), the "ignition delay" practically disappears provoking an instantaneous ignition (explosion or detonation), on the contrary of the normal gradual inflamation of the mix. With this detonations one engine easily can be destroyed.


Ricardo

 

[SBBlue]

A couple of observations here. . . . .

Gas engine turbochargers and diesel engine turbochargers don't quite work the same.

First of all, a gasoline engine turbocharger really only functions when you put the "metal to the pedal". If the pedal isn't down to the metal, then the engine is throttled and adding boost doesn't really make much sense.

A diesel engine is not throttled, however. A diesel turbocharger works all the time. The amount of boost varies depending on the engine speed and the exhaust gas temperature, which depends on fuel flow and load.

A diesel engine of the size that is used in semi-trucks (400-500 hp) may normally run at boost pressures of 20-30 psi.

The point Ricardo made is quite correct. That is why diesel engine intercoolers are used.

Consider a turbocharger that is producing 30 psi of boost. If the ambient temperature is 80 deg F, then the post-compressor charge air temperature will be 290 deg F.

Let's say that our diesel engine has a 18:1 compression ratio. If the intake air has a temperature of 80 deg F, the amount of work required to do the 18:1 compression is 119 BTU/pound of air. The temperature after compression is 750 deg F.

How about if we have a turbocompressor and not an intercooler? After an 18:1 compression, the gas temperature is 1150 deg F -- and we start seeing the problems that Ricardo mentions. In addition, it takes more work to do the compression at the higher temperature; instead of 119 BTU of work, it requires 163 BTUs.

Hope this helps!!!