Mini diesel engine - New discussion

[minh-thanh]

Mini diesel engine - New discussion
I will start this topic with what I said in my other thread about mini diesel engine
I put my topic in the " Discussion " section , that means : Everyone is welcome to comment !!


Sharing some of my diesel discoveries:
1/ Injection time: More precisely the time the lobe begins to contact the pump piston: 22 to 45 degrees before TDC
2/ Engine runaway : Should be worried?
If your design calculates lobe timing, intake and exhaust ports....exactly like a full size diesel engine then you should worry. But with a small engine you can easily stop the engine.
With many tests, my engine can not runaway
3/ Injection : Fuel vaporization-Atomized fuel . How to inject fuel
3A/ Fuel vaporization-Atomized fuel : It is necessary but not all.
Fuel vaporization-Atomized fuel will give you an objective view of the efficiency of the pump and injectors: the pressure generated by the pump, the tightness of the injectors....and when the engine is running it will have very little smoke.
" Not all " because when i make some test with my engine the injectors just "inject " fuel - as in the picture (Some cases are even worse)- -and the engine is still run

This leads to 2 cases: a/ the engine runs but a lot of smoke and some unburnt fuel (it looks like mud) comes out the exhaust pipe and b/ the engine a lot of smoke . Solve this problem: simply increase the compression force of the needle spring
3B/ How to inject fuel :
fuel injection : radiates from the center - good
Fuel injection is left or right: it doesn't matter
With a full size diesel engine, the fuel vaporization-Atomized fuel, the fuel injection, ... needs to be really good because it concerns the performance ... of the engine, and that's what is really needed.
But with a homemade diesel engine, you can stick to that rule if you want, and that's how I do it with my engine.
If you only need one engine running then it shouldn't be too important. And the engine has running, you can easily adjust the injectors...to make the engine run better

One important thing is : the injector must be completely sealed .
If the injector is not completely sealed it will affect the pressure in the cylinder and the oil pressure in the oil pipe

Next : Fuel and Fuel Pump !

Fuel and fuel pump
You must determine what fuel you will use for your engine !
If using fuel or a volatile fuel mixture - low temperature like Ether it is very easy to vaporize in the pump system and that results in the pump system being completely useless.
To avoid fuel vaporization in the pumping system you need the "inlet port" to be sealed - but do not create too much negative pressure on the fuel during the suction stroke.
"inlet port" here means the inlet port on the pump cylinder and the ball valve., but apparently the inlet port on the pump cylinder seems to be useless with the plunger stroke being too short in my pump systems. That means all the task is on the input ball valve

 

[minh-thanh]

Next : Fuel and Fuel Pump !
Why determine what fuel your engine will run on??
A low temperature, volatile fuel or mixture pump system you can use with kerosene or diesel fuel. But a system that uses kerosene or diesel fuel may not be able to use volatile fuels or fuel mixtures
The inlet ball valve needs to be perfectly sealed and must be tested with compressed air (about 1 to 2 bar) before and after assembly, including the outlet ball valve.
Why test it after assembly?? Because sometimes there is slight deformation of the valve seat caused by the force of tightening or fixing it...
The compressed air test must also be applied to the injectors: nice fuel injection does not mean that the injectors are completely sealed.
I could do some nice spray patterns videos, but that doesn't mean my injectors are completely sealed.
Build a diesel engine, make one yourself: it's not easy, yes it's not easy but it's not impossible
What I said above is what I have learned with my engine and pump system, and it does not mean that it will be true for your engine or anyone else's and should be taken as a suggestion.
Hope the above will give someone more confidence when building a diesel engine
PS: Will update more as I learn more

 

[Lloyd-ss]

Minh-Thanh,
Thanks very much for compiling this list of "lessons-learned" from your diesel builds. They will prove to be very helpful to myself and others.

One thing that you mention several times is that the fuel supply system needs to be sealed. And by that, I take it to mean that in particular, sealed so that any back-pressure from the cylinder does not blow the fuel backwards. Is that correct? You also mention strongly about making sure that the inlet check ball or check valve feeding fuel to the pump must seal 100%, and that it should be tested with air pressure to verify.

Honestly, getting things sealed properly has been difficult for me. Take the inlet check for example. I can get it to seal almost 100%, but then if I increase the spring pressure on the check ball to get it to fully seal, then it also has a difficult time opening to let fuel enter into the pump. Most of this boils down to proper attention to detail on my part. And making sure things are correct, and not just marginal.

The sealing against cylinder back-pressure is something that might be fairly obvious during operation of the engine, but not prior to that. That makes that particular hint pretty important.

Thanks again, Lloyd

 

[minh-thanh]

Hi !

One thing that you mention several times is that the fuel supply system needs to be sealed. And by that, I take it to mean that in particular, sealed so that any back-pressure from the cylinder does not blow the fuel backwards. Is that correct? You also mention strongly about making sure that the inlet check ball or check valve feeding fuel to the pump must seal 100%, and that it should be tested with air pressure to verify.

Yes , and including injectors must be sealed . They have to be really sealed
If your ball valve or injector is sealed with compressed air, you will be sure of one thing, it will be more sealed with oil !

Honestly, getting things sealed properly has been difficult for me. Take the inlet check for example. I can get it to seal almost 100%, but then if I increase the spring pressure on the check ball to get it to fully seal, then it also has a difficult time opening to let fuel enter into the pump. Most of this boils down to proper attention to detail on my part. And making sure things are correct, and not just marginal.

That's really the ball valve problem. Valve ball must really be sealed with seat
lapping the ball valve seat carefully to remove all major scratches, from grit 100 -> 600 -> 1500, finally with toothpaste - I always use toothpaste at the last step when lapping the ball valve seat and it always gives me a perfect sealed result . The grit size ( 100, 600, 1500 ) for the first steps depends on the material you choose as the valve seat - for me it is really important to choose the abrasive , - lastly : 1 ball and a small hammer, the angle of the valve seat I usually choose about 45 - 50 degrees

 

[Roger B]

Thank you for starting this thread A few comments/thoughts.

Ball inlet valves are a problem with fuel volumes that are less than the volume of the ball. The ball has to move and as it returns to it's seating it effectively increases the volume of the pump chamber. This is why I have chosen an inlet port closed by the pump plunger.

Having adequate compression and minimising heat loss is important for starting and running. When I was experimenting with the 2 stroke diesel it was very easy to make a new head with a different size combustion chamber. When the compression pressure was too low it would need a very long cranking time or ether to get it too fire. when the pressure was finally high enough it would fire immediatley there was fuel available.

The four stroke engine will also start immediately. I always start cranking with the fuel shut off and then open up the fuel rack until it starts.

 

[minh-thanh]

Thank you for starting this thread A few comments/thoughts.

Ball inlet valves are a problem with fuel volumes that are less than the volume of the ball. The ball has to move and as it returns to it's seating it effectively increases the volume of the pump chamber. This is why I have chosen an inlet port closed by the pump plunger.

Hi !
Yes, it's important but don't worry too much about it.
With my 10cc 1-cylinder engine, I tested with 3mm diameter balls and the engine runs fine - high and slow speed.
As for closing the inlet port with the plunger, as I said before it's almost completely useless with a short stroke = about 1mm - unless I make a cylinder for the pump with a diameter of 1.5mm. Your engine is larger so it allows longer plunger stroke so it is more effective to cover the inlet port

 

[minh-thanh]

Hi All !

A bit of information
With a 1 cylinder engine and I use diesel, I do not adjust the injection timing as I said above. when the piston is at 25 before TDC, I set the pump lobe to the "max pumped" position - the plunger is in contact with at the top of the lobe
With a 4-cylinder engine, when the piston is at 40 before TDC, I set the pump lobe as above
The difference could be :
In a 4-cylinder engine, the piston stroke is 6 mm shorter than in a 1-cylinder engine. But I don't know for sure if that could be the cause - shorter piston stroke means less heat but how much and what effect it has I don't know.
It could be something like a spring...in the pump system that is the problem. But definitely not the cylinder and plunger because I make it better than a 1 cylinder engine
It's definitely not an injector either, because each injector I adjust to it in a different way
Up to this point, all my knowledge about small diesel engines came back to 1 cylinder engines.
If someone intends and is building a diesel engine: It is recommended to make a lobe for the fuel pump so that you can adjust it - at least from 20 to 50 before TDC - This gives you a huge advantage !!

 

[ruben]

donde puedo encontrar los planos del motor