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.

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?

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

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.>>

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
http://ecomodder.com/forum/attachmen...1&d=1410420811

Notice the glow plug in the precombustion chamber.

Direct injection diesel
http://ecomodder.com/forum/attachmen...1&d=1410420811

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!

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.

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.

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.

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.

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.

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?

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

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.

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.

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?

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.

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
.

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

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.

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?

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.

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.

Trying to alter an engine for faux Atkinson cycle with a late intake valve only works with fuel injection. Carbs don't like the back and forth air flow. And they are trying to build a diesel out of a gas engine. Which means doubling the stock compression.

Seems to me that the most difficult part of your project would be to find or manufacture a fuel pump for the required displacement.

A diesel engine usually has a bore to stroke ratio very much towards long stroke because the diesel burns slower so is basically useless above 5K rpm so the torque advantage of the longer stroke has to be used.

If building a small capacity diesel, I would seek for this bore to stroke ratio in bigger capacity gas engine and then building a top end with home made head and cylinder built from small capacity engines.
(doing the CR is gonna ask for a very shallow chamber volume hence a flat head sounds a good idea)

Sendler, in post 27, bigscoot said that they would use a mulit-inject system so no carb and, since the Mazda Skyactiv diesel uses the gas engine ratio of 14:1, perhaps no increase in compression increase is needed. However, Mazda uses a glowplug for ignition and Bigscoot also has the sparkplug as an optionignition source
Bigscoot, looking for a bigger engine, I would like to recommend the relatively inexpensive Lifan 110cc engine. It has a stroke of 4.95 cm compared to the Zuma 4.36 cm stroke. I calculate a bore size of 3.535 cm to keep the 50cc displacement and the Lifan starts with a bore of 5.24 cm so a sleeve would have to be .85 cm thick or about 1/3" thick. As sendler notes, this could wellt interfere with the valves.
On the other hand, using the zuma cylinder, piston and head with the Lifan's longer stroke means that you would exceed the 50cc displacement without a (very thin) sleeve.
Well, at least your possibles have been clarified for you. Good luck.

One word of caution around Mazda's Skyactiv low comp diesel :
it is a turbo diesel !
edit : so volumetric efficency lost in low comp is restored because it is under boost


If building a 50cc engine, turbo is out because there is no appropriate unit available.

It doesn't mean you can't boost it but not with a turbo. (there you need to check the rules because the next best setup would be an electric blower on a battery)

If it has to stay N/A, then you're back to square one and a 18:1 CR.
In fact your benchmark setup is a VW SDi engine with corrections towards capacity applied.
http://en.wikipedia.org/wiki/SDI_(engine)

edit :
wich means you are in the 1.8 hp region.

also you need a fancy piston with a bowl in it to center combustion in the cylinder.

I remember Peugeot made some scooters with a Roots-type supercharger.

Never mind, results were fake :(

Sorry to bring this back. How did it go?

My first surprise is how a senior in any mechanical engineering program knows so little about the principles of combustion engines and fuels used.

What are they teaching these days?

You mean the Lohman 18cc diesel engine. That one was capable of getting 125km/l.