50cc Gas to Diesel Engine Conversion

[sendler]

Seems like too much work and non optimized parts to convert a motorcycle gas engine to diesel for this event where Engine off Coasting, which alleviates much of the loss of efficiency of a throttled gas engine, is entirely feasible. I would probably start with a Honda 49cc injected engine out of a Ruckus or Metro as Honda is the champ of FE right now. Retune the mixture with a piggyback fuel controller, Retune the intake pulse for your desired rpm range, use hot air intake to save heat energy and use a high tech coolant to boost engine temps to 200C? with minimal vaining on the water pump.

[bigscoot]

Originally Posted by 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?

Sorry Xist, but I'm not sure I understand quite what you are saying with this.

So a diesel with multiple injections would not be as hard on the block as an engine with one injection? Our goal is to get up to 7 injections but it is hard to say if we will be able to accomplish that in the time we have.

2000mc and BobS, let me know if I am understanding this correctly. One cycle in a diesel engine will have higher pressure and torque than a gas engine. Because the pressures and torque are higher it is harder on the engine. A gas engine of the same displacement can get higher power than a diesel (Escalade vs Duramax) because gas engines rev much higher than a diesel?

[Xist]

Bigscoot, I was asking if a .125L gas engine would be sturdy enough to be a 0.5L diesel, but Sendler did not think that was a good idea. He seems to know what he is saying.

However, I think that Elhigh had a point, a gas engine converted to diesel might not last very long, but very well might last longer than your competition.

[sendler]

One of the Honda "GX" engines might be worth testing if you wanted something really small and air cooled.
.
Honda Engines | GX35 Mini 4-Stroke Engine | Features, Specs, and Model Info
.

[2000mc]

Yes, typically gas engines make more power than diesels by revving significantly higher, diesels typically have a greater output per revolution.
I was thinking you could limit the total amount of diesel fuel per cycle to keep the output down to a similar level of when it was burning gas.
I think gasoline combustion is relatively slow compared to diesel, so if you run a single injection, what is normally already a greater output also comes in one sudden punch. So if you can break that up with more injections I think you'll be better off

[BobS]

bigscoot
I agree with 2000mc on the particulars. I would add to his first statement that diesels, to take the pressure of the instantaneous combustion, are made massive - thus more mass leading to lower rpms. I also would like some limit on the air/fuel mix - optimum ratio just at lower pressure at the intake. And we offer two solutions to your quest.
He suggests multiple injections which I believe is very much a workable solution. I, however, believe that you can use a vaporizer (perhaps ultrasonic with a heater) to modify the characteristics of the diesel fuel to allow it to burn in an otherwise unconverted gas engine. I suggest this since you seem to be time bound and this is the most rapid answer to your project need.

[bigscoot]

BobS, It is now looking like we will be able to get an injector with multiple injections so that has been a big step for us. We have not made the final decision but we are currently leaning towards starting with something in the 80-130cc range and sleeving it down to 50-70cc. We like the beefier bottom ends from the bigger engines and parts are much easier to find.

Do you all see any major issues with sleeving it and swapping out the piston? Is it workable to swap just the piston and leave the crank and connecting rod alone?

[sendler]

Are you going to make a new piston? Or remake the cylinder head? how will you double the compression ratio of the donor gas engine to make it diesel? If you take a big engine and sleeve away half the displacement, won't the valves hit the sleeve?

[bigscoot]

Originally Posted by sendler

Are you going to make a new piston? Or remake the cylinder head? how will you double the compression ratio of the donor gas engine to make it diesel? If you take a big engine and sleeve away half the displacement, won't the valves hit the sleeve?

Thanks for the input sendler.

We don't have all the answers yet but we will most likely have a piston made for us. We will probably use a head from a 50cc range engine and make that work. Our piston will give us the higher compression ratio. In the past for our gas engines we have welded on top of them and then took off material til we get the ratio we wanted.

The head is one of our biggest question marks right now so I don't really have a good answer for all your questions yet.

[BobS]

bigscoot,
To my mind, the easiest way to get 50cc from something like the popular Lifan 140cc engine is to delay the intake valve closure, which reduces the displacement. However, this also decreases the compression since compression only starts when the valve closes. This can be compensated for by shaving the head to get what would be a disastrous compression ratio if the intake valve closure were not delayed. (There is a limit here as the valve has to get out of the piston's way or the piston has to be designed to be the closure mechanism for the valve.) If you have good machine shop access, this might be a viable answer for the project.