Lot of folks have asked questions about smoke systems, and after fooling around with them for a few years, I thought I'd let you in on a few of the basics. Maybe it will help you to avoid some of the failures that I have gone through.
First let me say that if you are the type of person who doesn't like constantly adjusting or changing things, don't bother reading any further. In my experience, a smoke system is not something you can "Set and Forget" (My apologies to Ron Popeil). But, if you don't mind the occasional adjustment, a Smoker will add a "WOW Factor" to your plane that is hard to beat.
To make smoke you only need to do one thing: Get oil to reach its Smoke Point. The bad news is: That's a lot harder to do than it sounds. You only have one source of heat - your engine, and you can only access that source at one place - the muffler. The problem is that the exhaust is constantly moving through the muffler, so the oil doesn't get much time to get hot before it is blown out into the atmosphere, so you need to keep the oil inside the muffler for as long as possible. This is one reason why small 2-Stroke engine don't usually work well with smoke. Each stroke of the piston blows the contents of the muffler out, so the oil doesn't get very much time to get hot. Plus, the type of plane that would be powered by a .40 size 2-Stroke would probably suffer under the extra weight of a Smoke System. With larger (Gas) 2-Strokes, the amount of "Time-In-Muffler" is not a problem, due to their excessive heat. I have also had much success with Glow 4-Stroke engines.
Let's take a look at what is involved. The major components that we need are:
- Fuel or Gas Tubing
- Flow Regulator
- Check Valve
Now let's look at each component one at a time:
Nothing too tricky here. I use a standard 10 oz Fuel Tank, but the oil I use is non-petroleum based, so I can use the regular stopper that comes with the tank. If you use a petroleum-based oil, you'll have to use a stopper that is approved for petroleum use (More on Oil later). One thing to note is you should probably consider a 3-line system (Or at least give yourself access to the feed line before it reaches the pump). You may find that due to Murphy's Law, you will often land with a full oil tank and need to empty it by hand.
There are lots of Smoke Oils on the market. Super-Dri is one of the popular brands, but I must confess that I have never tried it. Be that as it may, I have heard many good things about it. Many people also make their own by mixing 3 parts Diesel Fuel with one part Automatic Transmission Fluid. This is a classic example of Petroleum Based Smoke Oil. Personally, after having tried a few Commercial Oils, as well as a few "Home-Brews", I have settled into a product called "Ultra Smoke". It gives great Smoke, and it is not Petroleum based, so I can use any type of fuel lines, and the airplane doesn't stink up my car and house afterward. The bad news is, last time I checked, the distributior of Ultrasmoke was out of stock and having a hard time finding the ingredients, so I may be looking for another soon (Fortunately Last time I called I bought a case!)
The only thing that needs mentioning here is that if you use Petroleum based Oil, you need to use a fuel line that is approved for Gasoline, as the Petroleum based oil will eventually deteriorate standard silicone tubing.
Now we get to the heart (no pun intended) of the system. There are 3 methods of pumping the oil that I know of.
- 1) Electric Pump
- 2) Crankcase-Pressure Activated Pump
- 3) Crankcase Pressure alone
Now, let's take a look at these three methods, along with their pros and cons.
The Electric Pump is by far the easiest setup. It simply plugs into an auxiliary channel on your receiver, and into a separate (usually 4.8v) power supply. There are many commercially available pumps on the market, but I use a windshield washer replacement pump from the J.C Whitney website and turn it on and off with a servo-activated microswitch.
- Pros: Easy to set up and use.
- Cons: Requires separate battery (which will also require charging before each trip to the field), and it can cause radio interference if placed too closely to radio components.
The Crankcase-Pressure Activated Pump is in essence, a fuel pump from a motorcycle. They work very well, and do not require a battery, and they work best on Gas Engines where you have a lot of crankcase pressure.
- Pros: Delivers good oil flow. No risk of radio interference. No need to charge extra Battery pack.
- Cons: Requires an extra servo to operate on/off valve, and they only work on Gassers.
And finally, there are a few systems that work by tapping the engines crankcase and applying the pressure directly to the oil tank. I have never tried this method, but I have doubts as to its reliability to deliver steady oil pressure (which is a MUST).
- Pros: Probably the lightest of the systems. Least complex.
- Cons: Steady oil supply questionable. Requires servo-operated on/off valve.
The amount of oil that is pumped into the Muffler is critical. Too little oil means not enough smoke, and too much will cool the exhaust below the Smoke Point of the Fluid. Ideally, the pump will put out a steady, constant flow of oil in a larger quantity than is needed, which can then be regulated by a restrictor - similar to the way a Needle Valve on your engine works.
The right combination of heat and oil will give a beautiful stream of Smoke.
Not enough oil will produce too little Smoke.
Too much oil will cool the exhaust so the oil can't reach the smoke point.
A Flow Restrictor can be as complicated as a needle valve, or as simple as the one I use, which is simply a hole drilled in a 3/8" dowel, with a pinch screw that clamps down on the line. Some commercial pumps have adjustable flows built right into them, which can be regulated at the transmitter.
A Check Valve is needed because a muffler contains a lot of pressure, and we only want that pressure to go one way - INTO the muffler, and not OUT from it.
Last but not least, we have the Muffler. This need not be near as fancy as you might think. All that is really needed is a way to get the oil into the exhaust stream. I have tried adding copper tubing to the inside to "Pre-Heat" the oil, but in most cases, I have found this practice to be detrimental to making good smoke.
Why? Let's think about it. When you heat the oil with muffler heat, you are at the same time COOLING THE MUFFLER! Heat is the key to good smoke; so don't rob your heat source.
Now, I'm not saying not to preheat, anything that makes your system work better is good, I'm just saying that IF you need to preheat, try using the engine head instead of the muffler. I have found that in both of the engines I use a Smoke System on (YS 91 AC and Zenoah G-62) I don't need to preheat.
You can buy a commercially available smoke muffler, but in many cases, it's easy enough to customize the one you are using now. The trick is in knowing where to introduce the oil. There are two factors to consider:
- The amount of time the oil will spend in the heat
- "Bernoulli's Principle"
The first one is easy, introduce the oil in a place where it will spend the longest amount of time in the muffler (I.E. Near the engine's Exhaust Port)
The second is not nearly as complicated as it sounds.
Whether you remember it or not, back then you were taught about a thing called "Bernoulli's Principle". In a nutshell, this states that if a fluid is moving at a high velocity, it will have low pressure, and if it travels at low velocity, it will have high pressure.
So what does that mean? Take a look at the diagram. I remember seeing many diagrams like this as a kid in my Science books in school. It didn't mean much then, but as I got older, I saw many practical examples of Bernoulli's Principle in action.
Simply stated, if you have a pipe that has wide and narrow sections, the narrow sections will have lower pressure that the wide sections (Ever notice how the tank pressure fitting on your muffler is always located on the fattest part of the Muffler?).
Now, let's look at two mufflers; a 2-stroke, and a 4-stroke.
The taps are placed where they are for two reasons:
- This position gives the oil the longest amount of time inside the muffler
- This area has lower pressure that the fat area, which means your pump won't have to work so hard.
For a gasser, you need to get a little more creative, but it's still not tough. All you need to do is drill a hole and insert a piece of copper tubing So that it ends right at the exhaust port. If you can open the muffler, it's a lot easier, if not, you can do it with the muffler removed from the engine so you can see the tubing through the exhaust port (Or just take a guess if you're feeling lucky). Once in place, hold the tubing secure with JB Weld.