Mini diesel engine.

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Steamchick

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Sorry, Min. Somewhere in a text book, but I have been too busy to research it. There are 2 basic formulae, one for a pin fitted in the con-rod and rotating at the piston bearings (diagram 4. SIMPLE BEAM- UNIFORM LOAD PARTIALLY DISTRIBUTED ) , one for the pin fitted in the piston and rotating at the con-rod small-end bearing (diagram16. BEAM FIXED AT BOTH ENDS- CONCENTRATED LOAD AT CENTER).
See: http://www-classes.usc.edu/engr/ce/457/moment_table.pdf
For Fatigue I would use max calculated stress must be below 0.1% of the proof stress for the pin material.
Of course the bearing loads are relatively easy to calculate.
or:

Getting the ratio of piston bearing width to con-rod small-end bearing width is very important for optimum design.
K2
 

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GreenTwin

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I remember looking at Find's website a long time ago, and as I recall, there were photos of his true injector build.

If my memory is correct, Find said that the injector has to be designed such that the pressure within the engine cylinder forces the injector closed, and the pump pressure forces the injector open.

This situation is similar to a valve on any standard 4-stroke engine, where the valve opens into the cylinder when lifted by the cam, and then closes with the cylinder pressure pushing against it.

I assume that Find uses a light spring to keep the injector valve seated, and the tiny (I guess mushroom-head) valve lifts off it seat when the pump reaches sufficient pressure.

I think this is one of the secrets of making a miniature injector work.

.
 

minh-thanh

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Hi All !
A update :
I assembled the engine , I don't have a small copper tube, so I used a 6mm copper tube - which is quite large - to test the pump and injectors (I will replace the small copper tube when I buy it).
with a high pressure oil leak in the coupling, I see oil vapor coming out of the exhaust, which means the injector has injected oil into the cylinder - whether it's good or bad I don't know
I have a question: the injector will inject when the piston is 20 degrees in front of TDC, so: where will the pump lobe be located ? will it start pumping or in the middle of pumping or ?
Thanks !
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Mechanicboy

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Connect the lever for amount of fuel to the centrifugal regulator and the ekstra lever with spring connected to the lever for amount of fuel. Then the engine are running with right regulated amount of fuel in the right rpm range.
 

Peter Twissell

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Regarding your injection timing question:
If the system is sufficiently rigid, i.e. no bubbles in the fuel, no movement in the pipework etc., then the injector should open almost immediately when the pump piston begins to move.
If you are controlling pump displacement by raising the piston away from the cam, injection timing will be retarded as position displacement is reduced.

Pete.
 

minh-thanh

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Connect the lever for amount of fuel to the centrifugal regulator and the ekstra lever with spring connected to the lever for amount of fuel. Then the engine are running with right regulated amount of fuel in the right rpm range.
Mechanicboy !
My first diesel engine, so I just need it running. when it runs i will think of other problems

Pete !

Useful information !
Thank you !
 

reichpaul630

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Hi All !
A update :
I assembled the engine , I don't have a small copper tube, so I used a 6mm copper tube - which is quite large - to test the pump and injectors (I will replace the small copper tube when I buy it).
with a high pressure oil leak in the coupling, I see oil vapor coming out of the exhaust, which means the injector has injected oil into the cylinder - whether it's good or bad I don't know
I have a question: the injector will inject when the piston is 20 degrees in front of TDC, so: where will the pump lobe be located ? will it start pumping or in the middle of pumping or ?
Thanks !
i'm a bit curious when you say that you "see oil vapor coming out of the exhaust". you should be shooting a very fine mist.
the quantity of fuel per stroke should also be very small, only a few milliliters. you may have too large or too coarse of spray
from your injector. i suggest you "fire" your injector over a glass jar. it must have a very fine mist with no droplets. a proper
injector will make a popping sound when the injector nozzle opens. otherwise, the fuel won't atomize properly. high pressure
and proper nozzle manufacture makes building injectors difficult. (but Find Hansen did find a way.)

your 6mm copper tubing is too large. copper is also "soft" compared to steel tubing, and will affect fuel atomization. smaller
copper (or better, steel)) tubing should improve the performance of your fuel system.

by convention, "20 degrees BTDC" would be the *start* of injection (noszsle opens). the fuel spray will continue until the nozzle
closes. the *effective* timing is determined by the start of injection, the duration of injection and the injection quantity. i.e. if the
fuel is "dumped" through the injector quickly, the effective timing will be more advanced, etc. depending on your actual setup, 20
degrees may be a bit much, 5-10 degrees might start easier.

earlier there was some question about how much fuel pressure was needed. my diesel VW has an injection pressure of
3,000 PSI. the newest models use a different system that can achieve 30,000 PSI. at these pressures, pieces of copper
soldered together will not be able to hold that pressure for long. very high pressure plus very good fuel atomization is what
makes a diesel go!

paulr
 

minh-thanh

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reichpaul630 !
i'm a bit curious when you say that you "see oil vapor coming out of the exhaust". you should be shooting a very fine mist.
the quantity of fuel per stroke should also be very small, only a few milliliters. you may have too large or too coarse of spray
When I tested : I could smell oil from the exhaust pipe , but i am not sure it was oil from the exhaust pipe , so i increased the oil pressure a little ( piston stroke at the beginning of the test was the lowest - only increased about 0.5 - 0.7 mm extra piston stroke) to be sure: the oil pump generates enough pressure to open the injector
When I assembled the engine, I noticed that the pressure in the cylinder was quite high so I am not sure if the oil pump created enough pressure to open the injectors.
I only have a 6mm copper tube and I only use it for testing
Yes, I will replace it with a small copper tube when I buy it

by convention, "20 degrees BTDC" would be the *start* of injection (noszsle opens). the fuel spray will continue until the nozzle
closes. the *effective* timing is determined by the start of injection, the duration of injection and the injection quantity. i.e. if the
fuel is "dumped" through the injector quickly, the effective timing will be more advanced, etc. depending on your actual setup, 20
degrees may be a bit much, 5-10 degrees might start easier.
That's what I'm really thinking about
Thank you !
 
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Steamchick

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The 6mm copper tube is only rated for 6bar - if regular "domestic" pipe - but will have a burst pressure well above that. However, it will be yielding if you attain anything like 50bar... But you want 200 bar pressure in the pipe... - all be it for small pulsations - otherwise you won't attain "atomisation" pressures at the injector. Solder won't hold that. The copper will flex and as it expands you won't get the pressure at the injector. Go to a scrap yard and get some old steel injector pipes from a diesel car or truck and use that material... cheap - easy to source - the right stuff!
Try and remember: You need 25 bar air pressure in the engine from the adiabatic compression stroke. Just to open the injector you need to exceed that. But to get atomisation you'll need to achieve 100bar plus in the pulsation from the injection pump. The volume is so small you need a very "stiff" pipework and connector system. I.E. THICK walled steel tubing with very small bore.
Have you checked the actual compression you are achieving in the cylinder? Have you checked the pressure you are achieving from the pump?
I think you need to know more so you can direct your efforts accordingly.
But enjoy it! - We are...
K2
 

Badhippie

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I am wondering how you are getting air flow into the engine. Since a Diesel engine produces no vacuum at all due to the fact there is no throttle plate. A true Diesel engine has to have forced air.
Thanks
Tom
 

Steamchick

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Hi Badhippie, I disagree. There are millions of naturally aspirated diesels running around the roads... without superchargers, turbochargers, etc. The "vacuum" (actually partial vacuum) is developed by the piston moving down the cylinder drawing air through the inlet valve. This may be only a fraction of the "vacuum" (partial) seen on petrol engines with a closed throttle - but comparable to the (partial) vacuum when the throttle on the petrol engine is wide open.
Hope this explanation helps... (someone will correct me when I am wrong - they always do. It's just the best way to learn!)
K2
 

TonyM

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Surely doesn't need forced air for a diesel to work. Air is drawn in through the inlet valve on the induction stroke simply by the downward movement of the piston causing the vacuum.
 

minh-thanh

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Steamchick !

Go to a scrap yard and get some old steel injector pipes from a diesel car or truck and use that material... cheap - easy to source - the right stuff!
Why didn't i think of that !?? I will get some old tubes from my friend

Try and remember: You need 25 bar air pressure in the engine from the adiabatic compression stroke. Just to open the injector you need to exceed that. But to get atomisation you'll need to achieve 100bar plus in the pulsation from the injection pump. The volume is so small you need a very "stiff" pipework and connector system. I.E. THICK walled steel tubing with very small bore.
Have you checked the actual compression you are achieving in the cylinder? Have you checked the pressure you are achieving from the pump?
I think you need to know more so you can direct your efforts accordingly.
Yes, you are right !
I can't buy what I need right now because of the pandemic and travel restrictions
I will find a way...
 

Badhippie

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You are correct air is drawn in by the downward stroke of the piston on a 4 stroke Diesel engine. Or on any engine as far as that goes. But there is nothing there to allow it to build vacuum. Pretty sure in my 30 yrs of building Diesel engines omg it’s getting closer to almost 40 yrs now. I just realized I am getting old. they will not produce enough air flow without having some sort of a throttle plate to allow it to build. So in other words it would be like a gas engine running wide open throttle and producing almost no vacuum. And with the vast amount of air a Diesel engine needs to run. So what I guess I am saying is there has to be some sort of a restriction in the air intake. And yes before you go there the size of the cylinder and the stroke will help determine the amount of air flow into the engine. This is also why Diesel engine have to have a separate vacuum pump which is either belt driven or gear driven most are gear driven. To run vacuum operated accessories. Is because Diesel engines produce very little vacuum. Sorry if this don’t make any sense it’s 3:30 am here and I am a little drunk and might not be able to write the words correctly that are trying to flow out of my head
Thanks
Tom
 
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