Air to LPG Intercooler/Vaporiser - NOT!!

[cribbj]

My project for the 1UZ includes a dedicated LPG based SFI system and mild boost from a supercharger. I thought I had a cunning plan for intercooling the charge air by putting a fintube cooler in the intake manifold, and flashing the LPG on the tube side from 10-15 bar down to 3-4 bar. I felt that the JT effect of the pressure drop, coupled with the state change in the LPG from liquid to vapor would have a dramatic cooling effect. Enough, I hoped, to chill the charge air down, as well as vaporise the propane, Win-Win for both sides as it were. I consulted a Spanish colleague of mine who has probably forgotten more about heat transfer and enthalpy than I'll ever know, and he was happy to run a process simulation for me (I think he'd secretly like to be building high performance motors, too). He used a process simulator software called HYSIS, which is highly regarded in the gas processing industry. He also did a reality check of the HYSIS results by running the same equations of state manually.

For those of you who aren't into thermodynamics and heat transfer, the bottom line is that my cunning plan won't work. There's simply not enough mass flow of LPG in comparison to the air flow, to accomplish significant intercooling. So, I present this only as free data for anyone who might consider trying this, or something similar, in the future.

Here are the parameters I gave him to work with (sorry for all the -----'s I can't figure out how to space the characters to keep the table formatted)
:
------------------------Pressure-------------Temperature
-------Flow-----------In-------Out----------In--------Out
LPG: 1.2kg/min-------15bar----4bar---------25C-------????
Air: 25 kg/min---------2bar----2bar--------140C-------????

Here is his analysis:

OBJECTIVE OF THIS CALCULATION:

FIND MINIMUM AIR OUTLET TEMPERATURE THAT CAN BE ACHIEVED

Imagine you manage to expand the LPG liquid through a valve just to the bubble point, this is to say, that starting to boil liquid can enter the exchanger. This is the best case you can have, as vaporization heat can be completely given to the other fluid (air). Normally this is desired as vaporization heat is in the order of 1000 times the equivalent non-state change heat flux

Of course, a real isenthalpic expansion through a valve will cause LPG partially to vaporize (or all LPG, because of the high amount of a single component, starting and ending boiling points are very close, near 4ºC difference).

Let’s assume we have the first case and no vaporization is found at the entrance of the exchanger. In order to be in a more optimistic case I assumed bubble point at pressure 2.75 bara (temperature -8ºC).

The pinch analysis is an easy tool to determine thermodynamic constraints within an installation. It is a comparison between the available energy of both sides of the exchanger and which is the maximum temperature (for cold fluid) and minimum (for the hot one) that can be achieved in an exchanger of infinite area an infinite residence time (this is to say, the thermodynamic limit)

As you may find in the attached "Pinch" graph, the limit is 109 ºC. This means that, the outlet air temperature will be always over 109 ºC (this is in accordance with the hysis result of last days, outlet air temperature was around 130 ºC).

As heat capacities are in the same order for both air and LPG, significant difference in the slopes of the curves you can see in the graph, are due only to the ratio of mass rate (25:1). We will conclude that the mass rate of LPG is not enough to cool down the air.

Fuel composition, is 90% propane, 5% propylene, 5% butane. All physical properties are close enough to propane so as to use them for the study. Please find below, Pressure- Temperature curve for the automotive fuel (maybe you find it useful for further calculations)

 

[cribbj]

The original post seems to have lost the graphs that I had attached, so I'll try again here.

 

[turboandrew]

"We will conclude that the mass rate of LPG is not enough to cool down the air"

since you don't need intercooling all the time, what about heat exchanging a small reserve of water, and only flowing that water through the heat exchanger in the intake when at wot? That way you have much more time (idle, cruise) to remove the heat frm the water. I'm an not sure what the ideal way to plumb this would be, but, hmm, maybe cooling the returning water with ambient air first, then cooling it further with the heat of LPG vaporization, then returning it to the heat exchanger in the intake? Myabe the "normal" procedure involves just the ambient air cooling, but at WOT, the reserve of "cool" water gets routed to the intake manifold heat exchanger for extra intercooling only whenmost needed.

 

[cribbj]

Thanks for trying to salvage the idea, but I think it's still a sinker. Pity too, because we all know what happens when we let gas free flow from an orifice. Although it's a dramatic deltaT, there's just not enough of it (BTU's) to cool down the charge air from our hair dryers ;-).

Next try will be with methanol injection. Many people think that FI with methanol shouldn't require ANY intercooling. Let's see what a super duper process simulator, driven by an expert process engineer comes up with. I'll ask him to inject increasing quantities of methanol into the charge air stream and plot the change in temperature.
John

 

[turboandrew]

If you have any ideas on port methanol injection, I would like to hear them. In particular, how to get all the components that are methanol safe: regulator, pump, lines, injecotrs, etc, at a reasonable cost.

IMO, my rational is if you have a high enough octane rating, don't intercool at all, just get the timing correct for the application. I suppose coating the pistons just in case wouldn't hurt, either.

 

[urtwhistle]

cribbj, i was thinking on the same lines that u are a while ago mine were quite as educated to yours tho. http://www.lextreme.com/forums/showthread.php?t=1929&page=2&pp=20&highlight=gas

i still beleive there has to be a way of utilizing the liquid gas to ur benifit. if u could some how inject liquid gas into the cylinder it would be awsome but would probly freeze the injecter lol.

 

[cribbj]

If you have any ideas on port methanol injection, I would like to hear them. In particular, how to get all the components that are methanol safe: regulator, pump, lines, injecotrs, etc, at a reasonable cost.

IMO, my rational is if you have a high enough octane rating, don't intercool at all, just get the timing correct for the application. I suppose coating the pistons just in case wouldn't hurt, either.

Does Klotz help the corrosion problem at all? I would think that by choosing the right materials, there shouldn't be problems with the tank or lines, and the injectors should be some flavor of $$ or unobtanium, so they're probably OK. Even the intake manifold and intake ports of the heads could probably be teflon coated to resist the stuff, so the pump & regulator are the weak links, right?

Here's an idea - with an LPG fuel system, there will need to be a small surge tank of vaporised propane at an intermediate letdown pressure of 3-4 bar in order to meet the demands of the injectors at the occasional WOT burst. Why not use that "surge" quantity of propane to blanket or pressurise the methanol tank, and eliminate the pump and regulator?

The only downside I can see, is that refilling the methanol tank could be a pain because you'd have to depressure the blanket gas system, but with an isolation valve on the blanket gas line, right at the tank, only the quantity of gas in the tank would be lost.

Even if the system ran out of methanol, and the blanket gas got into the lines and injectors, the worst that would probably happen is the engine would run way rich and probably die. A level gauge in the methanol tank, with a low level cutoff switch might be a good idea.

If the blanket gas idea is a non-starter, then we need to find a pump & regulator with $$ internals.

I think the benefits of cooling the charge air are too great to give up; I hope we can find a solution.

Comments/Suggestions?

John

 

[urtwhistle]

hmmm i wonder if u could some how have ur normal injecters and a realy tiny pipe screwed into the head which would let liquid gas into the cylinder at wot. no injecters just some sort of secondary fuel rail with no gas coonverters that would open a valve and let liquid gas flow. would be rather simple realy. if u could stop it from freezing up.

 

[cribbj]

My colleague, the process engineer ran a simulation with methanol injection on the conditions I gave him previously, ie 25kg air flow per minute, at 2.0 bar, and 140 degrees C. Following is a graph that shows the reduction in air temperature vs quantity of methanol injected. This approach works much better than the LPG intercooler.

Sorry urtwhistle, we haven't gotten to the liquid LPG injection yet, simply because I don't know how we would keep the stuff from boiling in the piping once it's in the engine bay.

Now I'm trying to figure out how to do a sensitivity analysis on this to come up with the optimum amount of methanol t/b injected in order to:
1. Maximise air cooling and BHP
2. Minimise the air displaced by methanol (which decreases the BHP)

I have a hunch this sensitivity analysis will have to be done empirically, ie with the butt dyno....

 

[turboandrew]

Perhaps a "mechanical" port injection of methanol combined with a finely controlled gasoline fuel injection could be the best solution. Kinsler has tons of this stuff: http://www.kinsler.com/ The "injectors" or nozzles really, are available in tons of sizes and configurations and can be fit in some pretty smalll/hidden places. I imagine they would have to have methanol safe pumps, or at least we could use the SS pumps a lot of people use on the supplememtal meth injection kits. We could even maybe skip the regulator and just control the pump's output by PWM'ing the power to it. So, we do a medium size stainless tank, hard stainless lines to pump and to some sort of distribution block, then hard stainless lines to the nozzles.

The great thing about the port nozzles is that hopefully most of the vaporization happens inside the cylinder, where the greatest heat absrobtion can happen. We also eliminate some of the space taken by the methanol vapor if some of it does not vaporize before the intake valve closes.

I don't think this would work well without a "base" fuel injection, running only methanol, as getting the methanol that finely controlled might be difficult (but I guess not impossible). However, using a base fuel injection, and then at a certain boost level, rpm, and/or throttle position, combining the meth injection with a gasoline delta map in the fuel injection (like AEM has), you could get a great solution.

This could give you pretty much all the detonation resistance you need, and let you run around on cheap, readily available gosoline 90% of the time.

 

[herbivor]

lpg cooled intercooler

Well, its been some time in the planning but i have eventually got around to building an LPG cooled intercooler. I have to agree with the other posts, the math does not support a great result. I need to cool the air intake something like 10c below ambient temperature to gain a 2.5% power increase. Fluid dynamics is not my strong point, but my gut feeling the power gain figure quoted should be a square root sum, as we are talking about a volume here.
As for the build, i have purchased 120x 10mm elbow compression joints, 15m of 10mm copper pipe and an intercooler. I wanted to keep the build as tight as possible, hence the compression joints. I am going to construct a water-housing jacket to surround the intercooler for the best heat transfer. I have also just purchased a multi-zone recordable thermometer, so i will immerse a probe in the water jacket, and the air inlet and outlet zones.
Finally, i have an infrared camera to record the results. I just want to record some more baseline data to verify any additional changes have made an impact.