Pre-turbocharger methanol injection 1

[dbecker]

Hello,


I am trying to understand what the limits are of pre-compressor methanol injection on turbocharged cars.

From my understanding, erosion occurs when you have the wrong combination of compressor speed and droplet size.

I have a turbocharger and I calculated the tip speed of the compressor to be at about 1158 ft/s at maximum boost.

My question is if the inlet tubing to the compressor is 3 feet long and I am flowing 100 ml/min of pure methanol (tiny amounts) will I see erosion if my ambient inlet temps are 80 degrees F?

I did a quick hand calc and the speed the air is moving through the inlet tube is about 160 ft/s so it will take about 20 ms for the air to reach the compressor.

I dont know how to compute the content of liquid left in this mixture, the methanol is injected via a super atomizing nozzle. So inlet droplet size can be assumed some value. I just dont know droplet size at the end of the tube feeding the compressor.

Thank you,

- D

 

[1gibson]

Centering the nozzle in the pipe via a spider so that it sprays directly on the center of the compressor, is one method I've seen to reduce erosion.

http://www.aquamist.co.uk/vbulletin/

is a very good resource

 

[tbuelna]

dbecker,

Methanol has a high latent heat value, so it would provide a nice reduction in charge air temps.

The droplet size would not likely be an issue, since the small amounts of methanol you propose to use would readily vaporize before contacting the compressor wheel. But methanol is also very corrosive to some metals/materials. So you would need to check for compatibility.

You would also need to make sure that your turbo's compressor shaft seals are capable of keeping the methanol out of your lube oil.

Good luck.
Terry

 

[WhizzMan]

Why would you want to inject at pre-compressor stage? The higher the temperature difference, the more efficient cooling will be. I can see a case where you'd argue to inject before any intercooler, to use that gain, but not before the compressor.

In general, methanol is injected either directly into the cylinders or into the inlet ports of the engine, when used as "regular" fuel. This does prevent most effects of long term corrosion. When risking the chance of condensation of your methanol onto the compressor wheel, intercooler and inlet tract, you will probably expose your aluminium much more than when injecting further towards the inlet valves.

I don't think that experimenting with the size and concentration of the droplets will make a fundamental difference. Maybe a practical, but you're still exposing aluminium to (heated) methanol. It will corrode in the end. I don't know if cavitation plays a part in this process, but purely chemical, concentration and temperature are your main factors. If cavitation plays a part, you'd want the stuff evaporated before it hits your compressor wheel. If not, you'd want large droplets so only a small portion of the alcohol is directly exposed to the aluminium and most will just fly past and evaporate further down. Maybe the high impact speed at the tips of your compressor wheel would cause cavitation in larger droplets?

Remember that the evaporation of the methanol is what cools the mixture, once it's gaseous, it's just another gas that you are heating. It makes sense to evaporate it at the point where temperatures are highest. That would be either just before the intercooler, or during the compression stroke inside the cylinder.

 

[patprimmer]

Latent heat of vaporisation remains constant irrespective of delta T. Rate of evaporation does depend on temperature but not delta T.

If you inject after the inter cooler you get the coolest charge as the inter cooler is dependant on delta T.

Erosion of the compressor wheel is always a risk but depends on a number of variable including droplet size, wheel speed, number of droplets, material used for wheel temperature of the methanol and wheel and construction of wheel.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

http://eng-tips.com/market.cfm

for site rules

 

[WhizzMan]

You are right about the temperature dependency of the vaporization. I got confused with delta T because you start with the same temperature in all cases, that of external air. What I also forgot is that the total amount of energy (heat) absorbed by the vaporization remains the same, regardless of the starting temperature. As long as all the methanol vaporizes, that is.

Your argument to maximize the temperature drop in the intercooler and thus to put the methanol injection after that, is what I was looking for in my original reply. Thanks for making it clear.

 

[izzmus]

"Why would you want to inject at pre-compressor stage?"

Simplicity - no high pressure pumps required. If you use a boost-pressurized tank, a pump isn't even required.

Uneven water distribution becomes more of a problem, admittedly.

 

[WhizzMan]

I think it would be quite a challenge to control the amount of methanol or the size of the droplets if you don't use a high pressure pump. At the very least, you'd need extra calculations in the ECU to correct for the pressure fluctuations.

 

[1gibson]

Pre-turbo injection gives you wet compression, which has many benefits, a major one being increased volumetric efficiency. Changing the inlet density will also alter the location on the compressor map, which can be good or bad, but needs to be considered.

Liquid methanol is corrosive to aluminum (don't store it in an aluminum tank) but you are not very likely to see actual corrosion with vapor.

There is a lot of information in the forum I linked to earlier.

You will want a pump and a check valve, low pressure is not the way to go. Will siphon or drip without a check valve.

 

[dbecker]

Thanks for the responses all.

I am injecting pre turbo because the turbos sized for this vehicle are small and loose a lot of top end power. The turbos come from the factory running at about 8 psi boost, I am running about 17 psi boost with methanol injection.

about 900 ml/min at the throttle body and the other 100 ml/min are split to each turbo (there are two) 50 ml/min each, a very small amount.

I am trying to create a cooler inlet air charge to the compressor to shift the efficiency up towards topend engine speed where these turbos are poor. Cooling the air before the turbos lets them create target boost at a reduced compressor RPM.

I am just worried about erosion, right now I am using a super atomizing nozzle to do this. The nozzle is over 3 feet from the compressor with ample time to vaporize. Methanol vaporizes very fast, if anyone has any other suggestions please let me know.

Thanks for your time

 

[WhizzMan]

Is there a reason you keep running these smaller turbos? The obvious solution here would be to just put bigger turbos on.

I think that if you reduce the pressure behind the turbo, the total amount of air flowed will still be the same. The turbo has to create a pressure difference between pre and post turbine. If you lower the pressure of the compressed air with methanol injection, that should give the same effect as injecting pre turbine? Is there a flaw in my reasoning here?

 

[dbecker]

I cannot get bigger turbos or change the turbos. They have to stay the same.

I am not injecting methanol before the turbine I am injecting it before the compressor.

Reducing the inlet temperature to the compressor will allow me to hit target boost at a lower compressor speed and greater efficiency, that is my goal.

I am just worried about erosion over long term.

 

[tbuelna]

dbecker,

Here's a picture of the compressor inlet from a Cosworth XB turbocharged, methanol fueled Champ Car engine (This engine uses a very small PR and no intercooling):

http://www.lolachampcar.com/images/XB Engine Pictures/PCI.jpg

Note the location of the (4) injectors......

Regards,
Terry

 

[dbecker]

tbuelna,

Thanks for the pic, I have seen that pic many times and admittedly I studied that setup. It turns out they dont care about long term erosion (im talking 50k miles not a 24 hour race).

So they can essentially dump fuel into the compressor like that and it will live, but long term I have seen compressors get torn up because of droplet impact.

I am just looking for a way to quantify erosion vs. droplet size and speed or even rules of thumb for this application. I have checked out aquamist website many times and there isnt any theoretical data on the subject. Just that "it's good to use".

The whole purpose of this injection method is to reduce the inlet temperature of the compressor and that will greatly aid in compressor efficiency, because as I mentioned earlier the turbos are undersized units from the factory and I don't intend to replace them with anything bigger.

 

[yoshimitsuspeed]

I don't know what rules you are trying to comply to but you could also consider coating the turbo. I use a lot of products from

http://nicindustries.com/.

They have a coating called turbo coat. It's primarilly for the turbine side but can also be used on the intake side. I coated my intake mani runners with it. They claim it flows better than an uncoated surface. Since the greater majority of the car world agrees a slightly rough intake flows best I emailed them and asked them if they did flow comparisons of their coating vs different surface finishes and that totally threw them. Whatever the case though it's good stuff and should be very resistant to alcohol. It also goes on very thin. If you apply it right it shouldn't throw off the balance of your turbines or cause interference between the turbine and housing. Recommended application is 1 mil.

 

[gruntguru]

For each 1% by mass of methanol evaporated in air, the density of air will increase by about 3% and about 1% of volume is lost to methanol vapour so your compressor flow capacity improves by about 2%. The key for you is to vaporise as much methanol as possible before entering the compressor. It may be worth considering counterflow injection - locate the injector in a bend at the comressor inlet, spraying up the centre of a straight section of pipe. This maximises the velocity difference between the droplets and the air. Also self regulates to some extent as the larger droplets with lower surface to volume ratio travel further before reversing and moving with the airflow.

Engineering is the art of creating things you need, from things you can get.

 

[dbecker]

Thanks for the info grunt, I will take that into account when designing my system.