Recently, a diesel mechanic advised that the return line was improperly connected. So, he fixed it while advising that most of the fuel pumped to the engine is not burned but returned via such line. This was news to me but was not until much later that I realized I should have asked why? Apparently, much of the fuel is filtered over and over. Can anyone explain why this is so?
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Lots of variations too
You can put lots of filters on the lines and have a fuel polisher, or you can install an additional pump to fill a 5 gal "day tank" and then extra polish this fuel as the return line goes to there. This fuel also gets warm and leaves the main tank fuel cold which tends to make the main tank fuel stay microbe free longer.
You can put lots of filters on the lines and have a fuel polisher, or you can install an additional pump to fill a 5 gal "day tank" and then extra polish this fuel as the return line goes to there. This fuel also gets warm and leaves the main tank fuel cold which tends to make the main tank fuel stay microbe free longer.
The reason is...
OK, The real reason for the fuel return line is that the extra fuel pulls heat off of the injector and makes it last a long time. Remember that the injector is in direct contact with the very hot cylinder head and combustion chamber. If this heat wasn't carried away the diesel fuel might vaporize inside the injector and allow the parts to seize instead of sliding against each other. Hopes this helps.
OK, The real reason for the fuel return line is that the extra fuel pulls heat off of the injector and makes it last a long time. Remember that the injector is in direct contact with the very hot cylinder head and combustion chamber. If this heat wasn't carried away the diesel fuel might vaporize inside the injector and allow the parts to seize instead of sliding against each other. Hopes this helps.
Lube.
It is not because the fuel pump capacity, and has nothing to do with fuel polishing. The pulling away of the heat is the closest. Diesel is an oil. It is used as a lube in a diesel engine, and does not get burned in the chamber. This leftover oil is sent back to the tank. If you think the fuel pump is providing too much fuel, and decide to reduce the fuel pump capacity with a smaller pump, you diesel engine will freeze and die from lack of lube.
It is not because the fuel pump capacity, and has nothing to do with fuel polishing. The pulling away of the heat is the closest. Diesel is an oil. It is used as a lube in a diesel engine, and does not get burned in the chamber. This leftover oil is sent back to the tank. If you think the fuel pump is providing too much fuel, and decide to reduce the fuel pump capacity with a smaller pump, you diesel engine will freeze and die from lack of lube.
Lube??
"It is used as a lube in a diesel engine" well.... I'd have to say this it's used as a fuel in a diesel engine. Because of it's oily qualities, it will certainly lubricate the fuel system, pump, etc, but it is a fuel. "and does not get burned in the chamber" ... huh? what does then? The fuel is injected into the cylinder, compressed, and will combust as the temperature rises. Unburnt fuel (which you hope to avoid), is primarily sent out the exhaust. There are probably lubricating effect of the injected fuel in the cylinder, but inadequate fuel supply will certainly cause starting and running problems before anything else bad will happen. Diesel engines have crank case oil as the primary engine lubricant. Your statement "you diesel engine will freeze and die from lack of lube" is also hard to understand. Your engine will fail to start, or run properly if your fuel pump not is providing adequate fuel. In fact, people suffer from this all the time as their fuel filters get plugged up. The typical symptom is that the engine will run poorly, or not start, but NOT freeze and die. JS, maybe you can explain your points in more detail to help us understand. Chris
"It is used as a lube in a diesel engine" well.... I'd have to say this it's used as a fuel in a diesel engine. Because of it's oily qualities, it will certainly lubricate the fuel system, pump, etc, but it is a fuel. "and does not get burned in the chamber" ... huh? what does then? The fuel is injected into the cylinder, compressed, and will combust as the temperature rises. Unburnt fuel (which you hope to avoid), is primarily sent out the exhaust. There are probably lubricating effect of the injected fuel in the cylinder, but inadequate fuel supply will certainly cause starting and running problems before anything else bad will happen. Diesel engines have crank case oil as the primary engine lubricant. Your statement "you diesel engine will freeze and die from lack of lube" is also hard to understand. Your engine will fail to start, or run properly if your fuel pump not is providing adequate fuel. In fact, people suffer from this all the time as their fuel filters get plugged up. The typical symptom is that the engine will run poorly, or not start, but NOT freeze and die. JS, maybe you can explain your points in more detail to help us understand. Chris
I worded it badly, I will rephrase my comment
The amount of total fuel coming from the tank to the engine, only a small percentage gets burned as fuel in the chamber. Fuel coming back out can be a very big percentage of what went into the engine because not so much fuel is burned; instead, it’s used for lubrication near the injectors, which are not lubricated by the lube oil. If you eliminated this extra diesel fuel, you engine will die. Yes, diesel is a fuel, but it is also an oil. Compared to gasoline, which is not a oil, and is only a fuel. Think about the history of diesel engines, and when it was designed, and what it was designed to burn as fuel. It burned "oil", as in poorly refined oil of those days, plus corn oil, nut oil, motor oil, lub oil, and french fry oil from McD. At that time, they hadn't even refined gasoline yet. What do you think biodiesel is? Now, biodiesel can attack a modern diesel engine's other parts, such as seals, but you can probably burn B10 with little problems. The very old diesel engines had internal parts that were not easily damaged by bio-oil.
The amount of total fuel coming from the tank to the engine, only a small percentage gets burned as fuel in the chamber. Fuel coming back out can be a very big percentage of what went into the engine because not so much fuel is burned; instead, it’s used for lubrication near the injectors, which are not lubricated by the lube oil. If you eliminated this extra diesel fuel, you engine will die. Yes, diesel is a fuel, but it is also an oil. Compared to gasoline, which is not a oil, and is only a fuel. Think about the history of diesel engines, and when it was designed, and what it was designed to burn as fuel. It burned "oil", as in poorly refined oil of those days, plus corn oil, nut oil, motor oil, lub oil, and french fry oil from McD. At that time, they hadn't even refined gasoline yet. What do you think biodiesel is? Now, biodiesel can attack a modern diesel engine's other parts, such as seals, but you can probably burn B10 with little problems. The very old diesel engines had internal parts that were not easily damaged by bio-oil.
Like Fred
I will try to refrain. But here are a few things to consider. First, one has to understand the difference between how a diesel engine develops its power and how a gas engine develops its power. Gas is based on expanding volume and varying pressure. Diesel on the other hand is expanding volume and constant pressure. This is why a diesel has more power than a gas engine. To obtain this a diesel needs a different type of fuel delivery system than a gas engine. I can explain this if need be. Second, there are gas mechanical fuel injection engines (Porsche 911E and S 66-75) whose injectors work very similar to a diesel engine and they have no lubrication what so ever. I'm not saying there aren’t some lubrication properties to diesel fuel but it isn't its primary function. Third, there are diesel engines that have no return lines as well. The Yanmar 3GMF in my last boat did not have a return line to the tank just a fuel ring. Hope this helps. Your turn Fred. :>) Vinny
I will try to refrain. But here are a few things to consider. First, one has to understand the difference between how a diesel engine develops its power and how a gas engine develops its power. Gas is based on expanding volume and varying pressure. Diesel on the other hand is expanding volume and constant pressure. This is why a diesel has more power than a gas engine. To obtain this a diesel needs a different type of fuel delivery system than a gas engine. I can explain this if need be. Second, there are gas mechanical fuel injection engines (Porsche 911E and S 66-75) whose injectors work very similar to a diesel engine and they have no lubrication what so ever. I'm not saying there aren’t some lubrication properties to diesel fuel but it isn't its primary function. Third, there are diesel engines that have no return lines as well. The Yanmar 3GMF in my last boat did not have a return line to the tank just a fuel ring. Hope this helps. Your turn Fred. :>) Vinny
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