Fuel Tank Pressure Part Two By James Goss      I am still looking for improvements in the fuel delivery system found on our two stroke glow engines.  I still think pressure control in the fuel tank is the key to better performance and reliability.  As I discovered in part one of this article, we are working with extremely low pressure in our fuel tanks, only .02 psi at idle and about .8 psi at max rpm's.  With a check valve in the pressure line the pressure doubles to about 1.5 psi.  To improve even further without adding any major components, such as fuel pumps, what else can be done?      First I would like to try using a larger pressure tap on the muffler. Normally mufflers come with a 6-32 fitting.   This entails that the screw is a number 6 in diameter and the 32 states the number of threads per inch.  I will first try an 8-32 and then a 10-32 fitting while measuring the tank pressure with a manometer.  In order to use a larger fitting I will drill and tap the muffler each time.  All engine parameters are the same as was in part one of this article. Here are the results: ( Check Back  Later )      Next I thought about increasing the diameter of the pressure line from the muffler to the fuel tank. It works just like current flowing in a conductor. The smaller the awg gauge wire, the more voltage drop you will have for any given current flowing in that wire. We all likely know that a number 12 awg copper wire will carry much more current than a number 22 awg. On this same train of thought, a smaller diameter pipe or tube will create more pressure drop than will a larger pipe when a liquid or gas is flowing. The question is, will this short piece of tubing make any noticeable difference in tank pressure? Also important is the fact that pressure drop varies in direct portion to the length of the tubing. If you are using a lot of extra tubing for your pressure return line then you are not getting all you could be getting. Keep it as short as possible for maximum pressure return to the tank.  The tubing being used for this experiment is the standard Great Planes fuel tubing. It measures 3/32” inside diameter. I will change this tubing to 1/8”, and then to some 5/16”. Here are the results: ( Check Back Later )      Next I thought about using a larger diameter supply line from the fuel tank to the needle valve on the carburetor.  The idea here was to reduce the flow resistance from the tank. This alone will not increase the tank pressure but will work toward the same goal, getting more fuel to the engine. The larger tubing would also act like a small reservoir for the fuel, so when the engine needs more fuel, such as when you accelerate, the fuel would be closer to the engine. Sometimes when you accelerate your engine the fuel is just not there and the engine hesitates. This increased line size would tend to improve fuel regulation. Or better yet, why not have a very small in-line tank just before the fuel input to the needle valve? We could call this a pre tank, or surge tank, maybe a small tank that would only hold 1/10 of an ounce or less. If you are using an expanded fuel filter at the location just before the needle valve, you already have this small surge tank. How about on an engine with a remote needle valve, placing the fuel filter between the needle valve and the carb input nipple. This would give us a reservoir of fuel at the front door of the carburetor to handle peak demands. With a surge tank installed the fuel could be delivered more on demand and would reduce the overall line resistance between the main fuel tank and the engine. Here are the results: ( Check Back Later )      I remember 25 years back I was working on a project to increase a car's mileage so it would get double the miles per gallon. It involved preheating the fuel before it entered the carburetor. This is the same theory we use on a smoke system to create more smoke, so why not preheat our glow fuel before it enters the carb. I know this for a fact; hot fuel is more ignitable than cold fuel when it enters the cylinder. Preheating the fuel would be about the same as using a higher nitro percentage. The nitro we have in our glow fuel is added to make the fuel more ignitable by providing more oxygen, which increases the burn rate of the fuel.  It would be like having a timing advance on the system. Timing would be retarded while starting your engine and as the fuel heats up timing will advance for better high-end operation. At low-end operation the engine needs to fire late in the upstroke because the piston doesn't have very much energy and if fired too early the piston would be blown back down before it reaches TDC. At high rpm's the cylinder can fire early into the cycle and the inertia will carry it over TDC. So you see in a way it would be a simple timing advance that would allow the engine to run better in cold weather.  By running the fuel line around the head of the engine, two or three loops, the fuel will pick-up heat from the engine and ignite more readily when it enters the cylinder. At the same time the oil in our fuel will also be heated which means it will be thinner and the engine will turn over with less resistance.           Here is an idea I really got excited about.  In some cases where you don't have enough room to install a large enough fuel tank as needed, you can take two smaller tanks and put them in series, end to end, which will fit into a narrow fuselage. You could even use three if needed. Here is how it works when using two tanks: The first tank is sometimes called a header tank and is connected to the carburetor through its normal output line. Its vent line, which would normally go to the muffler, connects to the output from the tank behind it and the vent of tank number two will connect to the muffler. This puts them in series with each other.  You will fill the tanks the same as always. While in use tank number two will empty first and then tank one will empty.  As fuel is removed from tank number one its inside pressure will be lowered which allows it to draw fuel from tank number two. Of course the pressure exerted on tank number two will also be felt by tank number one because pressure in a liquid is exerted throughout the liquid and transmitted to all points of the container.      Knowing about the multiple tank system I thought about using two tanks, but use the second tank for pressure only.  Using the same setup as above, fill the first tank with fuel. Do not connect the pressure line from the second tank to the muffler. Instead connect it to a small hand pump and pressurize it to about three psi and plug the line. I may have to crank the engine first and then pressurize the system to prevent over flow at the carburetor.   A check valve in the pressure line will enable you to remove your pump without loosing any pressure. You could now connect to the muffler pressure tap if you wanted or leave it off for a cleaner run. I think this system will work better than an oscillating type pump because at low rpm's the oscillating pump is not creating very much pressure. It depends on vibrations of a diaphragm to move fuel so pressure is directly related to the rpm's. Another advantage would be that you are not returning all the muffler's by-products such as carbons and oils back to the fuel tank to go back through the engine. This is what I meant when I said a cleaner run. This along would make it worthwhile; remember that at the end of the day most of us will pump the fuel from the tank back into our fuel jug. No wonder that we sometimes have bad fuel when our fuel container gets close to being empty.  Knowing this I still find it hard to waist that small amount of fuel .  Here are the results:      Another idea is to use two pressure taps on the muffler instead of one, and return both lines to the fuel tank. Now I know that one larger line would do the same job as two smaller ones, but larger lines and fittings are harder for us to come by. Most fuel tanks will have three nipples for input and output lines to be connected. Using two 1/8 inch lines will be equivalent in area to .024 square inches. You might think that using two 1/8 inch lines would be equal to a ¼ inch line, but this is not the case. A ¼ inch line has .049 square inches of area.  A ¼ inch line can support four times more flow than the 1/8 inch line, but using two 1/8 inch lines will give you twice the area for fuel flow as did one line. Here are the results:      Here is another experiment I would like to perform on the muffler of any size two-stroke engine.  This experiment concerns the size of the exhaust port found on the muffler.  The exit from the muffler is required to be as large as it is so it can accommodate the engine at max rpm's. If the opening were too small, it would place more load on the engine and result in a lower rpm, so it needs to be the size it is only for max rpm. On the other hand, at low rpm settings we do not need all that exhaust area. To give you an idea of how much air is needed at an idle just look in the carb at it's barrel opening. It is almost closed during idle so the exhaust port could also be this size. On a .46 size engine the airflow through it may surprise you. While running at 12,000 rpm's the engine requires over three cubic feet of air per minute. At 2500 rpm's it requires only .6 cubic feet of air per minute. Having the exhaust port open to its max during idle is just a waist of energy that could be used to improve the engine's performance.         First, the fuel tank will receive more pressure and improve the fuel delivery system if the exhaust port is closed during idle. Second, during idle the glow plug will stay hotter because the hot gasses can't escape as readily and produce a more reliable engine run. The question is how can we achieve this? I guess the simplest way would be to place a spring loaded damper on the output of the muffler that will create a partial closure at idle and open at maximum rpm's. I am sure there will be many ways to approach this. Another idea is to use a linkage from the throttle push rod. I remember 30 years ago a Veco engine used a butterfly on the exhaust port of the cylinder head for a better idle and was activated by the throttle push rod. Mufflers were not used at that time. The engine performed well on both high and low ends.                      The End

Back