50cc Gas to Diesel Engine Conversion
 

Hello Everyone,

I was referred to this forum by someone over at It Is the Ride forum. I am in my senior year as a mechanical engineer and for my senior design project I am on a team working on building a 50cc diesel engine. We have a supermileage program here at school that competes in the Shell Eco-Marathon every year and the engine is going to go in a car and run at the marathon. I'm not sure if any one is familiar with the Eco-Marathon but the cars are all small one seaters. The diesel category is new and the recent years and not many teams have cars that are running diesel. The ones that do are pretty much all running stock 219cc or bigger engines. We are trying to be the first school to put a 50cc diesel into a car.

As of now we are thinking we are going to start with a Yamaha Zuma 50cc engine. We like the head on that engine quite a bit and we run that engine using gas in another one of our cars. The biggest challenges we are seeing right now is the fuel injection/pump system. We want to swap pistons to increase our compression ratio to around 20:1. We are also hoping to put our fuel injector where the spark plug originally was.

I am wondering if anyone here has any advice to offer? Has anyone tried to convert a gas engine to diesel? I realize most of you probably are working on much bigger cars and engines but any feedback on the challenges and feasibility of this project would be appreciated.

Not sure the bottom end could handle the pounding a diesel conversion would create.

regards
mech

Might depend on how advanced of a fuel system you are going to use, a single bulk injection... I would be concerned about the bottom end like old_mech, if you're going to run multiple injections I would feel better about the bottom end.
Assuming this is the newer 4stroke zuma?

We are hoping to get multiple injections but we may end up with just one. Yes I' am referring to the newer 4 stroke.

What do you guys think would give in the bottom end?

We have considered just taking the head and possibly the cylinder from the Zuma and putting it in a beefier case. Do you think that would be a problem?

Yes, General Motors sorta did years ago & it was a disaster. A diesel engine needs to be designed as a diesel engine from the start. Gasoline engines can't withstand the extreme compression inherent with diesels.

This project sounds interesting to say the least. Not really willing to discourage you, but I'd also be kinda skeptical regarding the Zuma engine as a base for a Diesel conversion. Anyway, if you're really set into it, you should not forget that you'd probably need some sort of heater (either a glowplug or an intake "grid heater") to enhance cold-start ability.

Other aspect to consider is regarding the compression ratio: Mazda has the SkyActiv-D engine with a 14:1 compression ratio, reportedly in order to get the ignition more centralized inside the combustion chamber.

BTW I guess you have already seen something about those Evinrude spark-ignited MFE outboards used by the U.S. Department of Defense, that are also able to run on Diesel fuel for short periods.
workaround ideas to discuss among friends: Can the advantages of both spark-ignition and compression-ignition (Diesel cycle) get conciliated at the same engine platform?

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.

I spoke to one of the teams who had a diesel in the competition last year and one of the things they wanted to add was something to warm the fuel. Do you think there is much advantage between heating the diesel vs the air? Last year the competition was in Houston so it was not a problem but next year it will be in Detroit in May so it may be decently cold.

How does Mazda's lower compression centralize the ignition?

Heating the fuel has to do with getting it flowing easily, avoiding it to jellying. Heating the air intake reduces the time required to reach operational speeds right after the start-up.


It has something to do with the injector spraying pattern, and lower combustion temperatures resulting from the lower compression

Bigscoot, welcome to Ecomodder! There are many posts on here about the Shell Eco marathon and we have active members that are extremely knowledgeable in a wide variety of areas, so you definitely came to the right place!

Would it be feasible to 3D print a new engine? That sounds like far too much work, but you are reinventing the wheel here!

How hard are engines used in this competition? If they are not used hard enough, maybe that will be enough to make them last longer. As I recall, they only hit 30 MPH and then coast down.

Once you have five posts, you can share links and attachments, so you could reply two times to this thread, just writing:

Four

Five

I look forward to hearing more about your project! Good luck! :)

The bowl design in the piston and the ignition timing via multiple injections events and high injection pressure help to bring compression down and still be able to cold start. You may be able to use less compression if you're willing to crank on it until it builds heat from compressing and maybe using a intake air heating method. Using ether would probably wreck the ring lands or split a rod.
There was a German diesel bicycle motor that used a mix of diesel and gasoline. That may help you get it started better. I've used gasoline to thin motor oil so I could filter it and run it in an old Benz I had some time ago. Gasoline was allowed (in that motor) to prevent fuel gelling in winter in the owners manual IIRC.
The combustion events carried a very strong wave through the rods and crank--enough to ring the flywheel in my 06 Jetta. That was the rattle you would hear at idle in neutral if you didn't have a dual mass flywheel to make it quieter.