Back to posting… maybe

I know I still need to finish the JD 3hp series, and the Turbo jet math needs some work, but right now I have a short-term computer project that I am going to post.

This is the preliminary layout for my new Portable HTPC. It is being built in an aluminum tool box that i picked up from Home Depot for 20 bones. Not bad considering the cheapest computer cases are barely that cheap and not even as close to being this rugged. 

The purpose for this project is to have a computer that will do the following:

• Live music effects processing- plug in a guitar and pedals
• Live music multi-track recording
• Blu-ray Home Theater PC
• DX10 gaming PC

This is quite a lot to ask of a computer, but I think I managed to put together the hardware that will do it:

• mATX board
• Core 2 Duo 7300
• 4GB OCZ ram
• Radeon HD4850
• 160 GB hard drive
• HD/Blu-ray combo drive
• 500 W power supply
• 19″ touch screen
• Custom case

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Turbo Charged - The Thrust part 1

So… my attempt at a simple blog on how to turn a turbo into a useful engine is spilling into an uber-multi-page super rant. Oh well… time to suck up and crunch the numbers I promised in the last article. For this I will be using this NASA page as a general reference.

First we state the givens/assumptions:

• Thrust = 100 lbs(force)
• operating fluid - air
• ambient conditions - STP (standard temperature and pressure)
• exhaust pressure = ambient
• engine is fixed to ground, no forward velocity
• maximum operating temperature =

Then the variables:

• T = temperature
• p = pressure
• h = specific enthalpy (look this up- it’s an amazing property)
• V = velocity
• cp = specific heat - you used this in high school
• y = specific heat ratio
• n = adiabatic nozzle efficiency (adiabatic = no heat is absorbed or escapes the system)
• NPR = nozzle pressure ratio
• A = nozzle exit area
• R = gas constant
• anything with “i” after it = condition at the front of the nozzle
• anything with “e” after it = condition at the exit of the nozzle
• anything with a “t” after it = “total” which means that there are multiple parts to the variable (pt = total pressure = velocity pressure + static pressure)

Next we have the formulas:

1. F = mdot * V
2. pte/pti =(Tte/Tti)^(y/(y-1)) = 1
3. h = cp * T
4. hte =he + Ve^2/(2n)
5. Ve = sqrt(2*cp*Tte*n*[1-{1/NPR}^(y-1)/y])
6. mdot = (A*pt/sqrt(Tt))*sqrt(y/R)*((y+1)/2)^(-(y+1)/2(y-1)

Ok, so now that we have a bunch of formulas we need to decide what we want them to tell us and then which ones to use to get it. So, looking at the nozzle, we have a state point going in to the nozzle, and a state point coming out of the nozzle. We know that we want our maximum thrust to be 100 lbs-force, and since that is a maximum, we know that the flow is choked at the nozzle exit (mach = 1).

The easiest formula is #1, which can be solved for mdot, with velocity = mach 1:

mdot = F/V = 100 lbf / speed of sound at sea level = 2.88 lb/s

So the total mass flow through the nozzle (and through the engine) is 2.88 lbs of air every second. That’s quite a bit of air! Now that we have the mass flow, we need to figure out the area of the nozzle exit. That requires 4 more variables: total pressure, total temperature, the Mach number, and gamma for air. We already know the Mach number is 1 for choked flow, gamma for air = 1.4 for stp, and total pressure and temperature are limited by material and design properties. Total pressure is limited by the compressor- most turbocharger compressors have pretty good efficiency at 2 bar pressure ratio. Total temperature is limited by the highest temperature that the housing material can withstand. Lets call it 400F.

Using the calculator on this page, the area of the nozzle is going to be 1.21 sq ft. This is huge! There are a number of ways to reduce the size of the nozzle, such as revising the total temperature, or total pressure, which I might look at in the next post. Or, since I have been more interested in making an engine with shaft power output, I think I will probably just try to design that… But who knows.

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Turbo Charged

For a while now I’ve been interested in using turbochargers as stand-alone engines. While there are many sites on the internet documenting how to turn a turbocharger into a jet engine, I have yet to see one that did anything really useful- at least in my opinion. Nearly all of them are only designed to make heat, noise and quickly burn through fuel. Sometimes people try to get some thrust out of them… but rarely does one ever turn into a viable source of power or propulsion.

This is really a shame considering that turbocharger research has been going on for over a century. The technology in modern turbochargers must rival that found in the jet propulsion industry… right? So what if we consider that turbochargers can be hand selected for the purpose of creating an engine. Instead of just getting a turbocharger and throwing some sort of combustor on it, lets think about what we want it to do, and build something useful.

So what can a turbocharger based engine do? We already know they can create heat, but unless that heat is put to use, that isn’t a reason to make a turbo-jet. So not heat. Well, if it isn’t already obvious, thrust would be the next easiest thing to create. Finally, the most useful output would be shaft power. Since producing shaft power is really using thrust to move an impeller (fan), lets look at thrust first.

So far, every turbocharger jet that I have seen is set up the same way: compressor -> combustor -> turbine -> nozzle. Sometimes there’s an afterburner between the turbine and the nozzle. While this works, it is really an inefficient setup. The problem is that people who build these don’t know what all these things do and how they work at the root, and therefore don’t use them to their advantage.

Lets break it down math/engineering-wise: Thrust is created as a reaction force. Specifically, thrust force is equal to the mass-flow of exhaust gases multiplied by the velocity at which they exit the engine. This is apparent if you have ever used a spray nozzle on the end of a garden hose; when there is no nozzle, the water comes out at a low velocity, and the hose sits there– put the nozzle on and you might get the wild comical snake effect as the hose thrashes around. The same amount of water (or possibly even less) comes out with the nozzle on, yet there is obviously more thrust throwing the hose around.

So now that we know that we want lots of mass-flow and the highest velocity we can achieve- how do we get that? Well, people always stick a nozzle on the end of a turbo jet- why do they do that? Well a nozzle trades pressure in exchange for velocity. At the entrance to the nozzle the pressure is high, and at the exit, it is usually very close to atmospheric. I am not an expert in this, but I just found a cool link to a NASA explanation. Careful with that link though, if you aren’t a math person the equations will make blood shoot out your eyes. Using those equations, it is actually possible to get a turbocharger based jet to provide supersonic thrust. YES SUPERSONIC THRUST. That does not mean that your go-cart will break the sound barrier, but it sure will make more noise than you could ever imagine! Of course, the average Joe doesn’t have the technology to make one work, so I doubt we’ll ever see this. Since it is easier to calculate subsonic thrust, I’m just going to use a standard converging nozzle in my calculations.

If you read yet another NASA web page on nozzles, you will discover that the nozzle design is the most important feature of a turbocharger based jet, yet it is usually the part that receives little or no thought at all compared to the remainder of the engine. In case you haven’t figured out from the NASA equations, the nozzle sets the remaining variables for the whole engine. I am going to go ahead and post this article, but in my next article, I am going to crunch some numbers for the nozzle design required for a turbo charger engine to produce 100 lbs of thrust.

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Long time no see

Quick update. Since my last post I have been settling in to my new apartment, and spending all my free time getting www.blendergood.com up and running. The site is only just coming to maturity, so hopefully I will have some time to get back on and blog.

I still have a few episodes left in the John Deere series, and my next project was started last Saturday.  It is a 1984 BMW 325e that is going to get stripped and loaded out with a twin turbo Ford small block. I’ll start posting the pictures and progress reports when I get into it full swing, but I have to get the blender site going strong so that I can actually afford the project car!

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Tanked

This past weekend was quite productive. I didn’t get BlenderGood.com fully up and running Friday as I wanted to, but it’s close. I’m waiting on GoDaddy to restore my shopping cart right now; it’s really a pain being held up by someone else, especially since I have quite a bit of money tied up in this thing now.

Saturday was pretty lax. I did some research on rifles and was going to head to the gun show, but ended up going over to Jake’s place. He was building a bar in the loft of his house where there is currently a pool table. I helped him put the top on and lay down black granite tile. We also figured out some slick rope light mounting that should illuminate the lower bar without seeing the rope light itself. I snapped some pictures, but haven’t uploaded them yet.

A couple of my cousins got into town this weekend for spring break so I got to hang out with them for a little bit. I’m starting to fill the week nights again so I don’t know how much I will see them the rest of the week. The place they are staying in is huge, which is good since they have 10 people staying there for the week!

Sunday I was going to get up early and go help set up at Shoreline, but my phone didn’t switch time and woke me up an hour late. I got there just about the time they had finished setting up, so I just sat and watched. They were struggling to get set up and sound checked since they had 10 musicians this week, and ended up only having time to run through 2 songs before the service. As good as they had done in practice, they really struggled the whole first service. Then they tanked the last song horribly. It was “Basic Instructions” and had a pause in the intro that they all messed up so bad that they had to start over- I’ve never seen a band start a song over. Even at Crosspoint when we screwed up really bad someone always kept it going and pulled it back together. It was kind of funny seeing the reality check after being intimidated by how good they were at practice.

After the first service I met Jake at the gun show. I ended up shopping around and buying a Marlin 822. It’s a .22 Magnum, which is a caliber that is relatively new, but picking up popularity. Casey has the same gun but he has a trigger kit on his that brings the pull from a herculean 8 lbs to a  super-light .5 lbs. I’d like to get mine down to 1-1.5 lbs if I can. (Pull is how hard you have to squeeze the trigger to fire the gun. The harder you have to squeeze the more you shake and mess up your aim.) We’ll probably hit the range this weekend and see how different they shoot.

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