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Old 09-07-2007, 06:18 PM
masterp2 masterp2 is offline
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Join Date: Nov 2004
Location: Desert SW, Arizona USA
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Default WI on a duramax

I have spent much of the last year working through a problem with coolant overheating on GM diesel work vehicles. Since some of my findings are applicable to this effort, I will elaborate.

What I found was that the smallish compressor was working in the northeast part of the map, and in some thermal conditions, "off the page". On further investigation, the temperature of the compressure discharge emerged at near 600 F. I calculated efficiency around the 50% mark. Needless to say, this is in a word, ridiculous! Further investigation led me to question what the effect of this heatup (and expansion) is on the downstream plumbing. Something told me that I was seeing extaordinary head losses with this huge heatup. In other words, the velocity in the IC/CAC plumbing would be increasing dramatically, leading to higher frictional losses. It turned out, the difference between the compressor discharge pressure (work) and the intake plenum pressure (downstream) was nearly 6 psi! This at a plenum requirement of 32 psi. On the diesel this is around 60-65 lb/min of airflow.

Now hopefully I have not lost you. The plumbing restrictions when considering the 2.5" IC plumbing and the IC itself, totalled 6 psi. (2.5" is way too small)

After working through the compressible flow equations in a 2.5" conduit, it turns out the increased discharge temp was creating much of this loss, via increased air flow velocity. With fluid flow, there is apoint where the force required to push the fluid (air) through the straw (or IC pipe) becomes exponential, the curve quickly rising vertically at some flow rate. When this happens it is time for a diameter increase from the engineers. (what in fact happened, is that GM reduced the diameter from 3.0" in previous models, and to this day there is no known explanation why)

Adding insult to injury, that high temp product was leading to dramatic ambient temp increases behind the grill. The IC sits in front of the radiator, and measured ambient in front of the radiator, on the hot side of the IC, was 240 degrees! So the CAC was acting like a torch to heat coolant. I calculated something like 290,000 BTU/hr of heat exchange with the IC. Nuts!

But back to hot discharge product. The predictions for this non-adiabatic behavior, show that velocity increases dramatically, and adds a lot of added restriction in the plumbing. Clearly cooling the charge increases density, reduces velocity, and hence pressure losses. This means that, for a given desired intake plenum boost pressure, less work (discharge pressure) is required. This compressor now works less, which means higher efficiency. The improvement is cyclical in nature.

it appears that pre-compressor WI, PCWI, can be a performer or a deterent to performance. If you already operate in the efficient islands, on a humid day, then cooling charge can move you to lower efficiency on the left. It also leads to huge condensation effect in the IC. So PCWI would have limited usefulness on a properly designed forced induction platform with large stock amounts of charge cooling. But the undersized compressor, operating on a dry day, with excessive air box temps, should benefit big.

From my point of view, I do not share the idealized concept that WI provides quasi-isothermal compression. I believe that all the inefficiencies of non-adiabatic compression are in place even with WI. But naturally the beneficial impact of cooler charge can be seen in my explanation. But I don't believe that WI improves the efficiency of the compressor, from a purist sense. This assumes that there is no appreciable evaporation prior to compressor, as is the case in an axial mount nozzle in front of the nut.
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