50cc Gas to Diesel Engine Conversion

[bigscoot]

Thank you everyone for the help so far. I don't think printing the engine would be feasible for us and its not really the direction our adviser is wanting us to go. At competition we typically run the engine wide open til we get to about 20ish mph and then coast down to 10 mph. We may be running this one a little faster.

I have been doing some research on valve timing and if there is any difference between CI and SI engines. It seems to me that CI engines typically don't have overlap where as SI usually do. What do you guys think about this? Can you use a gas cam shaft with overlapped valve timing in a diesel?

[Xist]

Originally Posted by bigscoot

Our problem is there really isn't any good base for this engine. As I mentioned before, the smallest diesel we have found is 219cc.

Are you familiar with the The Avero Ultra-Lite by Advanced Engine Technology Ltd?

(UL) 300W to 2.5 kW AC/DC Diesel Powered Generator Set
Features a light weight, compact, rugged, electric power source (35 lb.) for applications where size and weight are critical;

AVERO Ultra-Lite - Advanced Engine Technology Ltd.

However, they do not have a page for it anymore, so I do not know if they still make it.

Is the 219cc engine single-cylinder? If there were two, you could deactivate one. Could you take the 219cc engine and install shims and custom parts to reduce its displacement? Hopefully the diesel's efficiency would balance out all of that weight!

This is .2L, which is only 10% smaller: Tractor Engine (R165)>>Jiangsu Changfa Refrigeration Group Co.,Ltd.>>

[AndrzejM]

bigscoot,

Diesel engines are different from gasoline engines in many ways. First of all it's not a question of compression ratio, that can kill your gasser bottom. It's the way different burning characteristics of injected diesel fuel. It's like a tetonation, that's why all diesels engines have to deal with that. Old engines with indirect compression had small compustion chamber combined into the head of the cylinder to prevent destroying the piston. That chamber had only a small part of it opened to the engine's combustion chamber and piston itself. Modern engines has combustion chamber build in the piston itself and injection of the fuel is divided in few stages. First stage is to heat up the combustion chamber and move piston slightly down, then the second one (and every other following the first one), gives the power and torque. That's why diesel engines have such high torque, because combustion last much longer than in gasoline engines.

You can see the difference on these pictures:

Indirect injection diesel


Notice the glow plug in the precombustion chamber.

Direct injection diesel


Like the guys said before you'll need some solution to heat up the combustion chamber to start the engine. It's possible to start diesel engine with just cranking it but it'll be much easier with the air heated up before.

Another thing you've mentioned is the camshaft. First of all you need to make sure that your valves won't meet the piston, which will be much closer to the cylinder head than in gasser.

Another idea that you could consider is to build a two stroke diesel engine. That was done many times before, maybe not in such small scale, but maybe that will be easier to achieve? Two stroke diesel with big and heavy flywheel. Should work IMHO. It won't be so fuel efficient though. But you'll have work cycle per every rev of the engine.

You can find some info here: DIESEL LOCOMOTIVES

Anyway interesting project!

Last thought of mine. You can find some more info of two stroke diesel cycle engines looking for small engines for RC models. Maybe you even could use one for your project? These engines are very small, light the only disadvantage is they rev like crazy

Good luck!

[BobS]

Bigscoot
Since you are thinking outside the box, take a look at the Youtube video "Vegetable Oil Vapor Carb - Running a gas engine on grease". Diesel should be easier to vaporize than Straight Vegetable Oil (SVO). Depends on your definition of a diesel engine as to whether it would qualify. Good luck, whichever path.

[Xist]

http://www.youtube.com/watch?v=dW6K-Nw63VI

Vegetable Oil Vapor Carburetor - Running A Gas Engine On Grease! - YouTube

He does not have any newer videos and does not link to plans or anything.

[elhigh]

RC engines were my first thought when I saw this, but I think they're more like a hot bulb two-stroke than a true diesel. Nevertheless it's a way to think about the problem.

A conventional gas engine won't hold up to the continued pounding of a diesel, but this thing only has to hold up long enough to survive the competition. Scooter engines aren't terribly expensive, get three: one for testing and prototyping, two for the competition (one primary and one backup).

This guy talks specifically about modifying existing four-stroke RC engines into diesels, he could add some fuel to your fire.

Unless I misrecall the most competitive teams are now insulating their engines to keep the heat in, and do low speed pulse-and-glide to maximize matching their engine's loading to its best BSFC point.

[bigscoot]

Originally Posted by Xist

Are you familiar with the The Avero Ultra-Lite by Advanced Engine Technology Ltd?


However, they do not have a page for it anymore, so I do not know if they still make it.

Is the 219cc engine single-cylinder? If there were two, you could deactivate one. Could you take the 219cc engine and install shims and custom parts to reduce its displacement? Hopefully the diesel's efficiency would balance out all of that weight!

This is .2L, which is only 10% smaller: Tractor Engine (R165)>>

I was not familiar with the Avero Ultra-Lite. It looks pretty interesting. Have not been able to find much about it yet.

The 219cc is single cylinder. We are looking pretty hard at taking either a gas 125cc or a diesel 219cc and sleeving the engine to decrease the displacement. Our thought is that this would give us a beefier bottom end that could handle the diesel.

We are still trying to understand why a diesel engine needs to be beefier than a gas. Can someone help me understand this? If a 125cc gas engine can get a more power from a 125cc diesel engine why does the diesel need to be beefier? I guess we are not understanding why the diesel pounding is more damaging than a gas engine's pounding.

For the competition our engine must be 4 stroke and run and start on plain jane diesel.

elhigh you are correct in your statement.

Unless I misrecall the most competitive teams are now insulating their engines to keep the heat in, and do low speed pulse-and-glide to maximize matching their engine's loading to its best BSFC point.

We can warm the engine up before we run it in competition. During competition we only run the engine for short times so we are turning it on and off often.

[Xist]

Well, if you sleeve sixty percent of the displacement of a small gas engine, you will have 2.5% as much metal as you need for that displacement.

250% of what a gas engine needs should still be enough for a diesel, right?

[2000mc]

Originally Posted by bigscoot

We are still trying to understand why a diesel engine needs to be beefier than a gas. Can someone help me understand this? If a 125cc gas engine can get a more power from a 125cc diesel engine why does the diesel need to be beefier? I guess we are not understanding why the diesel pounding is more damaging than a gas engine's pounding.

what rpm would you be running each of these theoretical engines?
a 2015 escalades 6.2L can make 460lb-ft@4100rpm ...and 420hp
a 2015 duramax 6.6L can make 765lb-ft@1600rpm ...and 397hp
if you could get that escalades engine to make 765lb-ft@1600rpm by what ever means necessary ... i wouldnt be surprised if it could take it for a while, but i think the knock/ rattle of a diesel on top of that, from an increase of pressure so sudden it bangs, is what i think would really shorten its life expectancy. which was why i was thinking pilot/ multiple injections would be helpful

[BobS]

Bigscoot
My understanding of the differences between gas and diesel engines is that the gas engine's fuel mix is set on fire by a spark, effectively a point source, and a flame front moves away from that point source in every direction, burning the mix.
A diesel's mix is set on fire by compression, with the same pressure occurring everywhere in the chamber. I believe complete simultaneous ignition goes by the label "explosion" and, combined with the higher energy of the diesel fuel, stresses the engine more.
That is why I suggested the grease powered video - it uses a point source to start the burn.