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Old Apr 28, 2012, 08:23 PM
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Need info on a small diesel engine to run on biodiesel

Hello,

I'm working on a project for school that involves building a small-scale vehicle. My goal is to utilize a small rc airplane diesel engine and run it with homemade biodiesel. Unfortunately, I don't know too much about these types of small engines, and I was hoping someone would be able to help me out.

My questions right now are as follows:
- "Biodiesel" is a broad term. What are the ideal properties of the fuel required for this? If possible, I would like to synthesize the fuel in a sustainable manner, even if it's not the easiest way of going about it. I have access to a fully furnished lab, including distillation columns, burners, stirrers, hot plates, glassware, chemicals, etc.

- What qualities should I look for in the engine? I have no clue which brands are considered "quality" and what brands are just selling cheap unreliable junk. My budget for the engine is tentatively set at around $70-$100, but is flexible if the extra cost is justified.

Any help and/or information would be greatly appreciated. I look forward to hearing what people have to say.

Thank you.
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Old Apr 28, 2012, 11:46 PM
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You can mix model diesel fuel using biodiesel and pretend it's burning. The reality is that biodiesel needs to be atomized by high pressure injection into a cylinder with air compressed to 1000F to burn well. The properties of biodiesel are such that it's actually much more viscous and much less volatile than DF2. While this is still within spec of most full size injected diesels, model diesels do not meet the needs.

Perhaps you could just "synthesize" ethanol and put it through a glow engine without any modifications at all.

Greg
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Old Apr 29, 2012, 12:04 AM
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Unfortunately there aren't any inexpensive diesel engines that can run on biodiesel, especially model diesel engines.

The name diesel as applied to model diesel engines is a misnomer. They are really compression ignition engines that do not need a glow plug to run. Unfortunately they use a special model diesel engine fuel. the basic formula for model diesel fuel is 33% diethyl ether, 33% kerosene, and 33% oil preferably castor oil. There are many variations of the formula too, it depends on how large the model diesel engine is. The larger the model diesel engine the less ether you need to make it work.

As mentioned biodiesel isn't as combustible as kerosene, so that would be a problem if you simply substituted biodiesel for kerosene. But one might be able to use something like 40% ether, 39% biodiesel, 20% oil and 1 to 2% Amsoil cetane booster. I don't know for sure but it ought to work in a 40 to 60 size model diesel engine.

You can get a diesel conversion head for many model glow engines from Davis Diesel Development. http://davisdieseldevelopment.com/
You can then test run the engine as a glow engine for a while to familiarize yourself with how it works. Then test run it using Davis Diesel model engine fuel. If that goes well then try your biodiesel fuel mix and see if it runs like that. You should get the unique biodiesel engine exhaust smell from it like that then.
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Old Apr 29, 2012, 12:57 AM
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Greg, the final fuel just needs to contain a significant percentage of biodiesel, not necessarily have the engine running on it straight. I was figuring I would need a high alcohol content or include some other volatile to get the necessary combustibility.

Quote:
As mentioned biodiesel isn't as combustible as kerosene, so that would be a problem if you simply substituted biodiesel for kerosene. But one might be able to use something like 40% ether, 39% biodiesel, 20% oil and 1 to 2% Amsoil cetane booster. I don't know for sure but it ought to work in a 40 to 60 size model diesel engine.
This sounds like a very viable route. With enough testing, there should be some usable fuel ratio that contains enough biodiesel to get that "french fry" exhaust smell.

So now I'm working under the assumption that the bigger the engine, the higher the compression, the hotter it gets, the easier it is to burn biodiesel. With this in mind, how big would be big enough. You said 40 to 60, but is that an optimistic estimate? It would be better for me to shell out a bit more for a larger engine if it meant a much greater chance of success.

I'm not sure I can figure out exactly what Davis Diesel is selling. Is it a replacement head for the compression engine? What exactly is being replaced or added to the compression engine that allows it to burn diesel fuel?
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Old Apr 29, 2012, 08:14 AM
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It is a replacement head with a counter piston to set compression.
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Old Apr 29, 2012, 08:24 AM
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ChemEng. Ok what Davis Diesel offers is a replacement head for the glow engines. What you do after breaking in a new engine, or making sure a used engine is running good first off of glow fuel, is replace the glow head with a diesel head. Inside of the diesel head there is what they call a "contra-Piston". A screw is used to adjusted the contra-piston so it changes the compression ratio to run with different fuels and propellers on the engine. On most model diesel engines, they have a control-piston setup on the head to allow them to adjust the compression ratio which makes the engines more versatile. Anyway, once you replace the glow head with the diesel head you then use model diesel fuel in the engine to have it run without needing a glow plug or a spark ignition system.

For example, in these two pics is a Fox .45 glow engine, being run with a glow head and then with the Davis Diesel head as a model diesel engine.




Here is a pic of the diesel head with the adjusting screw for the contra piston.
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Old Apr 29, 2012, 08:47 AM
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If you look on Youtube you can probably find something on it. I remember seeing a guy running it. It will be hard to start without ether, but I think the guy did it that way, and used a starter. When you run the diesel motor be careful to not overcompress it as you can easily bend the rod or break the crank. As another side note, I was at a festival where a guy had a diesel Rabbit with a fuel valve from a Ford pickup for switching tanks. He started it on the diesel and when it warmed up switched it over to a one gallon container of vegetable oil from the grocery store. It ran quite well on that and smelled like french fries. Before he shut it off, he let the diesel go through for a bit to fill the lines and injectors.
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Old Apr 29, 2012, 09:14 AM
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Here is a cutaway drawing pic of a typical small model diesel engine. You can see the two stroke engine design and the head with the contra-piston and adjustment screw on top.
This is from a article at Model Engine news. http://modelenginenews.org
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Old Apr 29, 2012, 10:09 AM
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If I'm going to use some form of alternative fuel it better be at least as efficient as what it's replacing. We've gone down this route quite some time ago. It turn out that most of the biodiesel wasn't being consumed even when mixed with ether.

There is no doubt the engine will run if there is ether in the fuel. The question is how much fuel will it consume. You probably find it's burning much more fuel that it would on a similar fuel blend containing kerosene in place of the biodiesel. What would that prove? Using a renewable fuel just to waste it?

Norvel .15 on Biodiesel Fuel (0 min 9 sec)
Go to the youtube page and read the description.

Greg
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Old Apr 29, 2012, 11:04 AM
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Thanks gkamysz, I had vaguely remembered some of the details about your tests a long time ago, but I had forgotten too much about it to mention here.
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Old Apr 30, 2012, 04:47 AM
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Originally Posted by earlwb View Post
Thanks gkamysz, I had vaguely remembered some of the details about your tests a long time ago, but I had forgotten too much about it to mention here.

There is quite a lot of material available in the RCU archives model diesel forum about experiments with biodiesel.

A preheater for the biodiesel fuel would probably help considerably with its lack of a willingness to burn problem.


Ed Cregger
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Old Apr 30, 2012, 11:14 AM
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Ed, to what temperature did you preheat the biodiesel to get it to burn successfully? Which engine did you use? I've not seen any reports of biodiesel burning well in anything but an diesel injection engine.

Greg
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Old Apr 30, 2012, 10:55 PM
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Interested as well. Also curious if anyone would know how large I would need to get engine-wise in order to properly burn biodiesel?

And thank you all for the information. I have a much better idea of what these engines comprise of and what they're capable of. Insufficient temperature (or pressure... essentially the same thing), is our enemy.

Quote:
There is no doubt the engine will run if there is ether in the fuel. The question is how much fuel will it consume. You probably find it's burning much more fuel that it would on a similar fuel blend containing kerosene in place of the biodiesel. What would that prove? Using a renewable fuel just to waste it?
It wouldn't be a complete waste if it does, in fact, burn. I'm discouraged by the notion that the biodiesel isn't burning at all in these blends though. If the biodiesel is providing power, and can come from a source that benefits the environment (think fuel made from algae that removes CO2 from the air) and burns cleaner than the fuel it replaces... You can see how it is still something worth pursuing.
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Old May 01, 2012, 10:48 AM
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The first thing I'm going to want to see in a project like this is fuel conversion efficiency. If it's poor, what is the real "well to wheel" efficiency of the fuel? If you're not familiar with the debates surrounding ethanol and biodiesel it may do some good to research it.

Engine already exist which can burn biodiesel efficiently. There is no point in building an engine which does it poorly. If the study is about the process of making oil, published engine data will suffice. If the study is about designing an engine to burn such fuel, there is much more research to be done, because this project will be short on details.

Usually though these school projects about the learning process itself and being able to apply it to the outcome, good or bad. Carry on. Claiming that biofuel burned inefficiently is still cleaner than fossil fuel burned efficiently is a bit shortsighted, however.

Greg
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Old May 01, 2012, 07:16 PM
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Quote:
Originally Posted by gkamysz View Post
The first thing I'm going to want to see in a project like this is fuel conversion efficiency. If it's poor, what is the real "well to wheel" efficiency of the fuel? If you're not familiar with the debates surrounding ethanol and biodiesel it may do some good to research it.

Engine already exist which can burn biodiesel efficiently. There is no point in building an engine which does it poorly. If the study is about the process of making oil, published engine data will suffice. If the study is about designing an engine to burn such fuel, there is much more research to be done, because this project will be short on details.

Usually though these school projects about the learning process itself and being able to apply it to the outcome, good or bad. Carry on. Claiming that biofuel burned inefficiently is still cleaner than fossil fuel burned efficiently is a bit shortsighted, however.

Greg
The project involves making a small, self-propelled vehicle that is powered by novel chemical reactions. Our aim is not to create a new engine, and efficiency is not explicitly factored into the judging. I am aware of the debates surrounding biodiesel as a transportation fuel.

The project just requires a working car, powered by chemical reaction. The biofuel aspect is something we want to include to be above and beyond, and hopefully attract the attention of corporate sponsors.

"Efficiency" is not the end all, however. I never said that burning biofuel inefficiently is cleaner than fossil fuel burned efficiently. My point is that even if the efficiency of burning the biofuel is not equal to the efficiency of diesel (ex - 2 gal biodiesel burns to get the power of 1 gal diesel), that doesn't mean that the biofuel is necessarily worse for the environment than the diesel. If the biofuel can come from a crop that consumes CO2, releases significantly less pollutants when it burns, and can be produced locally on small scale farm/gardens, then the total environmental impact can potentially be favorable when compared to standard diesel.

The engine efficiency is a somewhat misleading measure of environmental impact.

This is a digression, however. Would you or anyone know the temperature/compression necessary to burn biodiesel (either pure or in solution) as an engine fuel and whether or not this is achievable in a small-scale airplane or other small engine?
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