Interesting. I suppose this wouldn't work so well if you were driving a car through a gearbox, your speed could be high with the throttle closed (e.g. going downhill in gear) and the mixture would then become excessively rich. But of course many model carbs would do no better.
Injected Diesel 56cc 2 Stroke, Will it ever work?"
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MORE COMPRESSION AND TEMPERATURE TESTING
I have been trying to come up with a better set-up to drive the engine to do some testing. This testing is just something that I "have a need to know" the results of, even though it might be slowing the forward progress.
Right now it is set up to be riven at up to 500 rpm. It is a 2 stroke and the exh valves are locked closed. The intake port is open from about 30 deg BBDC until about 30 deg ABDC. And the intake is not pressurized at this time.
Both the pressure and temperature seem are low and I suspect that it is because the cylinder is starved for air and will not build full pressure..
After about 10 seconds of being driven by the 500rpm motor, the pressure rises to about 220 psi and the temperature rises to 90C. Both numbers are way too low. I think the next step will be to pressurize the intake ports and try again.
Lloyd
I have been trying to come up with a better set-up to drive the engine to do some testing. This testing is just something that I "have a need to know" the results of, even though it might be slowing the forward progress.
Right now it is set up to be riven at up to 500 rpm. It is a 2 stroke and the exh valves are locked closed. The intake port is open from about 30 deg BBDC until about 30 deg ABDC. And the intake is not pressurized at this time.
Both the pressure and temperature seem are low and I suspect that it is because the cylinder is starved for air and will not build full pressure..
After about 10 seconds of being driven by the 500rpm motor, the pressure rises to about 220 psi and the temperature rises to 90C. Both numbers are way too low. I think the next step will be to pressurize the intake ports and try again.
Lloyd
Roger B
Well-Known Member
It is certainly being starved of air. Can you add the blower/compressor or is that too much work? Have you measured/calculated the actual combustion chamber volume? Tolerances can build up in the wrong direction.
TIME TO CHARGE AHEAD!
I have gone thru a couple of iterations today with varied success. The window is now machined for the inlet to the air chest for the scavenge ports. The picture shows 2 of the 14 scavenge ports. After doing that I taped a 1.5" x1.5" x .75" 12vdc fan over the hole and tried the tests again. No change of temp or pressure of significance. Then I tapes the exhaust hose from the shop vac to the port and tried again. Same results. I then started checking the CR calcs as they relate to the port timing. The scavenge ports are open from 44 deg BBDC to 44 deg ABDC, just about where they should be. But the CR didn't calculate out like I expected. I made a filler disk for the top of the piston which might have bumped the CR up 3 or 4 points depending on how you calculate it. For a 2 cycle, can't really calc from BDC because the scavenge ports and exhaust valves are still open. But anyway, after that change to the CR, the pressure easily went up to 350 psi but the temp stayed the same. However, the 1/8 steel tube going to the pressure gauge was too hot to hold. There may be a limitation or lag in thermocouple.
350psi is getting quite close to being right. I think it is time to take @minh-thanh 's advice to heart and just go ahead and try to get it to fire. If I leave a few things undone, that can happen within several days. But the weather has to cooperate because I can't run this in my shop, LOL.
Here is the window that the output from the roots blower will be connected to. you can see 2 of the 14 ports. The piston is at BDC in the picture.
Thanks for taking a look and offering advice and encouragement!
Lloyd
I have gone thru a couple of iterations today with varied success. The window is now machined for the inlet to the air chest for the scavenge ports. The picture shows 2 of the 14 scavenge ports. After doing that I taped a 1.5" x1.5" x .75" 12vdc fan over the hole and tried the tests again. No change of temp or pressure of significance. Then I tapes the exhaust hose from the shop vac to the port and tried again. Same results. I then started checking the CR calcs as they relate to the port timing. The scavenge ports are open from 44 deg BBDC to 44 deg ABDC, just about where they should be. But the CR didn't calculate out like I expected. I made a filler disk for the top of the piston which might have bumped the CR up 3 or 4 points depending on how you calculate it. For a 2 cycle, can't really calc from BDC because the scavenge ports and exhaust valves are still open. But anyway, after that change to the CR, the pressure easily went up to 350 psi but the temp stayed the same. However, the 1/8 steel tube going to the pressure gauge was too hot to hold. There may be a limitation or lag in thermocouple.
350psi is getting quite close to being right. I think it is time to take @minh-thanh 's advice to heart and just go ahead and try to get it to fire. If I leave a few things undone, that can happen within several days. But the weather has to cooperate because I can't run this in my shop, LOL.
Here is the window that the output from the roots blower will be connected to. you can see 2 of the 14 ports. The piston is at BDC in the picture.
Thanks for taking a look and offering advice and encouragement!
Lloyd
ninefinger
Well-Known Member
getting exciting. I'm following closely.
I usually check out your thread when there's something new
Personal opinion, it is difficult to measure the temperature of compressed air in the cylinder because the delay of the mechanical thermometer is quite large. To measure temperature from 30 , 50 degrees Celsius to 150, 200 degrees Celsius in an extremely short period of time - a few milliseconds !!?? - impossible
Roger B
Well-Known Member
Good luck with the next steps
When I built my two stroke I had to reduce the combustion chamber to about half the original calculated value to achieve ignition. I had compensated for the port position but the clearance between the piston and head was more the planned and I think heat loss also reduced the pressure.
Yes, I already reduced it by filling the depression at the top of the piston. I will eventually have to make a new piston.Good luck with the next steps
When I built my two stroke I had to reduce the combustion chamber to about half the original calculated value to achieve ignition. I had compensated for the port position but the clearance between the piston and head was more the planned and I think heat loss also reduced the pressure.
I was fortunate enough to talk to a diesel mechanic who has been working on 2 stroke diesel longer than some of us have been alive. His first suggestion for "hard to start" was to use a heatgun to blow hot air into the intake while cranking it. He says it makes a big difference.
minh-thanh. Thank you (and everyone else) for following along on this journey (death march??I usually check out your thread when there's something new
Personal opinion, it is difficult to measure the temperature of compressed air in the cylinder because the delay of the mechanical thermometer is quite large. To measure temperature from 30 , 50 degrees Celsius to 150, 200 degrees Celsius in an extremely short period of time - a few milliseconds !!?? - impossible
)I agree with you about the thermocouple, but I was hoping that the temp might increase like the pressure does.
Roger B
Well-Known Member
Holding a burning diesel soaked rag in front of the air intake is a traditional way of starting cold recalcitrant diesels. You can't plug in heat gun in the middle of a field.
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Not related to your build, but in my daily YouTube grazing, came upon this converted motor / brake adapted to a (commercial) model engine.
Here is the plan for the gear train.
There are 2 idler gears in the gear train for 2 reasons. First, the idlers are offset and adjustable so that the backlash between the gears can be controlled. Second, the The crank, cam, and balance shaft gears are only 50 teeth with the idlers installed. If no idlers were used, those gears would all have to 120 teeth and would look way out of proportion to the rest of the engine.
The crank, accessory, cam, balancer shaft, and both idler gears are all in the front-most plane at the front of the engine. The belt drive tot he blower is directly behind the balance shaft gear. The 2 blower drive gears are behind the belt drive. Although the belt drive to the blower is not conventional, it made everything else work out, space-wise, and also makes it possible to change the blower drive ratio if needed. It also helped the the directions of rotation all work out.
The gears are all Mod 1 metrics which is similar to a 24DP 20 deg PA gear. I wanted to use a small multi V-groove belt to drive the blower, but couldn't find any that were short enough. But there are tons on little toothed timing belts that will work.
Now I have to get these new parts on order.
Thanks for taking a look. Please let me know if I am off on anything.
Lloyd
There are 2 idler gears in the gear train for 2 reasons. First, the idlers are offset and adjustable so that the backlash between the gears can be controlled. Second, the The crank, cam, and balance shaft gears are only 50 teeth with the idlers installed. If no idlers were used, those gears would all have to 120 teeth and would look way out of proportion to the rest of the engine.
The crank, accessory, cam, balancer shaft, and both idler gears are all in the front-most plane at the front of the engine. The belt drive tot he blower is directly behind the balance shaft gear. The 2 blower drive gears are behind the belt drive. Although the belt drive to the blower is not conventional, it made everything else work out, space-wise, and also makes it possible to change the blower drive ratio if needed. It also helped the the directions of rotation all work out.
The gears are all Mod 1 metrics which is similar to a 24DP 20 deg PA gear. I wanted to use a small multi V-groove belt to drive the blower, but couldn't find any that were short enough. But there are tons on little toothed timing belts that will work.
Now I have to get these new parts on order.
Thanks for taking a look. Please let me know if I am off on anything.
Lloyd
IT WAS JUST TURNING OVER TO CHECK IT OUT BUT IT REALLY WANTED TO FIRE !
Its not finished and I was just turning it over with a motor that was way too big at about 600 rpm and it kept trying to fire.
There is no fuel connected to the injector. My guess is some residual oil in the cylinder or a little fuel in the injector.
The video is in slow motion.
No matter what, I was not expecting it to fire !
Its not finished and I was just turning it over with a motor that was way too big at about 600 rpm and it kept trying to fire.
There is no fuel connected to the injector. My guess is some residual oil in the cylinder or a little fuel in the injector.
The video is in slow motion.
No matter what, I was not expecting it to fire !
Love the sound.
Fire : a good sign
For me, that's enough to give me confidence for the next goals
Please update......
Roger B
Well-Known Member
Excellent, you obviously now have enough compression. If there was oil in the blower it could have produced a mechanically atomised mixture that would fire.
The gears for the front gear train won't be arriving for a few weeks and I just used some timing belts and pulleys that I had on hand. They are on the other side of the view in the video in post #313. They work for a few minutes and then get slack on the idler and go out of adjustment, so I am kind of stuck until the gears arrive.
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