V8 nitro engine

[Parksy]

Using pre mixed nitro fuel. It does huff and puff and get hot. I'm wondering if regular spark ignition will solve my problems.

 

[canadianhorsepower]

Using pre mixed nitro fuel. It does huff and puff and get hot. I'm wondering if regular spark ignition will solve my problems.

and your final CP /r is

 

[Parksy]

I believe it's about 8:1, but this is according to the maths. Real life figures could vary. I should test with a gauge. What sort of PSI should I achieve?

 

[dsage]

I guess with glow plugs you have no control of timing. Does it "fight back" when it fires or does it just go through the motions and get hot. I guess if it's not fighting back but it is getting hot then it is burning fuel and it seems reasonable it would fire at maximum compression. So my assumption is it is not producing enough power to overcome it's own friction. It might need more fuel to increase the power or a higher nitro mix.
Try a spray of starting fluid. It should at least react differently on that. If it attempts to run then it might just be a carb adjustment.
I'm not sure what compression you SHOULD have but if you think it is 8:1 then it should be somewhere around 8 x 15 psi = 120 psi. But that's going to be highly theoretical as well. Cam timing / rings / cranking speed etc. etc. are going to drastically affect that. Are you sure your cam timing is proper? I would expect for that engine you'd want the intake valve closed very near (if not before) bottom dead center to get maximum compression.
I've never built a nitro engine. Are they supposed to run like a diesel or model airplane engine? With no control of timing (or compression) how do you control when it fires?

Sage

 

[petertha]

I guess with glow plugs you have no control of timing... Are they supposed to run like a diesel or model airplane engine? With no control of timing (or compression) how do you control when it fires?
Sage

There is no compression 'control' in a glow engine. (Or maybe you are referring to what are loosely labelled diesels with the tommy bar?)
Aside form these, regular glow engines are the same as any spark conventional gasoline engine in this regard. Once the parts are machined & assembled, the resultant physical geometry dictates CR. You can tweak it up/down with head shims or skimming surfaces no different than most engines, but by & large the (mathematical) compression is set once assembled. One small exception, glow plugs come in long & short & mid range lengths & copper washers. So if this incremental volume (which occurs in the threaded section) is a large percentage of head volume, it can affect CR. But a displacement of this size I wouldn't think is hugely significant presuming the true CR is in the ballpark.

The glow plug element type + fuel composition + compression + I/E factors collectively affect ignition timing. Once you get the magic recipe, it just works, 2S or 4S. If any of the above is too far out of range, it frustratingly wont work. The trick is to isolate the culprit(s). If you can at least try & mimic the methanol glow engines in existence on this basis (fuel, timing, CR, plugs, carburation) that's probably the diagnostic route. Because there are millions of them out there. Well.. not that many multi-cylinders, but I'm just talking about trying to make a cylinder fire here.

One thing that I cant explain out of inexperience, but can at least point out. The running methanol/glow Vee's I see predominantly seem to have multiple carbs. Yet the same displacement radial glow engines can get away with one carb in the rear, but the same length or longer induction headers going both uphill & downhill without any artificial boost & same I/E timing. There must be a reason for this. I don't think the designers would bother ganging multiple carbs just for the cool factor & even more tuning equalization challenges. Yet OTOH there are examples of model gasoline/spark Vee's with one carb & intake manifolds that are scale reductions of their FS counterparts. ????

http://www.cad-jung-shop.de/epages/62479729.sf/de_DE/?ObjectPath=/Shops/62479729/Categories/Baupl%C3%A4ne/Bauplane_Modellmotore

http://www.engineman.de/

 

[petertha]

I believe it's about 8:1, but this is according to the maths. Real life figures could vary. I should test with a gauge. What sort of PSI should I achieve?

Might be worth validating this with a syringe of methanol if you haven't already? Its amazing how small variations can affect CR at this scale.

Fill cylinder volume on top of piston at TDC. Then fill head volume with your existing glow plug assembly in place. Then compare to BDC volume for actual CR. If the physical numbers agree with 8:1 then that's one diagnostic check off your list. But if it turns out to be 6:1 for whatever reason, this will make starting & sustained running much more challenging.

Trying to get gage pressure can be done with the right do-dads & would be good to know, but its a little more involved,

 

[Parksy]

Thank you guys, I appreciate your effort. I will make a compression gauge out of an old glow plug some how and see what I get.

Sage, something you mentioned intrigued me right at the start of your post. When rotating the engine by hand, i can feel the cylinders firing but far too early. So it's pushing the piston back down before its reached TDC. Could this be a factor? It's not far off TDC. I suppose this is where I need to start playing with head shims to set the timing correctly.

 

[petertha]

Were you able to validate valve intake open/close positions relative to TDC like using a protractor wheel? All it would take is a smidge of cam lobe misalignment or tappet gap or pushrods coming off at an angle or rocker ratio... resulting in something different in real life vs. on paper.

Head shims will affect compression but not really advance ignition point a lot on glow. If you are 8:1 now & de-shimmed to 10:1, you'd still be the range of many ordinary RC glow that run at that CR. Increasing nitro % will give you some more pop, but save that for later. If its not sustained running my gut feel is it something more fundamental.

Also, what do the plugs look like when they come out? Light tan? Totally clean & fuel soaked? Dark brown & sooty? Silver grey & distorted wire? These are all signals indicating different issues.

 

[Parksy]

The plugs look brownish on inspection. I didn't use a protractor wheel, instead I looked underneath at the crank to check for tdc for cylinder number one and went from there. A protractor wheel may be a better option I think.

 

[Parksy]

Ok, had another play and one thing that is evident is that I need to work on the cam timing. I played with just one bank, ran with the drill, then connect the battery to drive the glow plugs and it sounded like an engine.
But the problem is that I'm guessing here. Loosen the cam sprocket set screw, rotate a couple of degrees and re tighten. How does one use a degree wheel?
I initially timed the engine by setting number 1 piston to TDC, then set the intake cam to start open. Then rotate the engine so number 2 was at TDC, and again, intake to just start open. But this setup doesn't work and the engine doesn't run.
For fun I set the exhaust valves to open on compression stroke. Makes for a very loud engine when rotating with the drill. This sound I was able to replicate when I altered the timing and connected the glow plug drivers. With glow plug drivers connected, the exhaust Sound changed dramatically indicating that I'm actually getting firing, where as before I wasn't(despite thinking I was). But the issue with this is I have no idea where my timing is. This doesn't help my cause at all, so if anyone can point me in the right direction, I'd be very happy.

 

[Parksy]

My firing order is 1-2-5-7-8-3-4-6. Bank angle is 67.1 degrees. So because I'm running two cams, and 2 fires directly after 1, I need to set number 2 cam 33.55 degrees from number 1 cam?

 

[grapegro]

Hello Parksy, A method I used ver many years is to get piston at top dead centre with the two valves rocking, exhaust nearly closed and inlet starting to open. You can check each cylinder in this manner and save a lot of anguish.
Norm

 

[Parksy]

Cheers grapegro. I have set timing to what I think is correct, removed all glow plugs and checked valve openings against piston position. All good in this regard.

Got close to starting tonight. Played with carb adjustments and got all cylinders firing and hot(very hot). Just didn't want to continue running. Cylinders continued firing even when I disconnected the glow drivers, so long as I continued cranking the engine.
Just need to keep tweaking.

Cheers

 

[Brian Rupnow]

Try setting it up with your exhaust valve just beginning to open at about 25 degrees before bottom dead center, and the intake valve beginning to open at about 20 degrees before top dead center. The amount of time that the valves actually stay open is dictated by the profile of the cam itself. There is an entire world of science around cams and cam timing, but the numbers I have given you are in the ballpark.---Good Luck.---Brian

 

[stevehuckss396]

My firing order is 1-2-5-7-8-3-4-6. Bank angle is 67.1 degrees. So because I'm running two cams, and 2 fires directly after 1, I need to set number 2 cam 33.55 degrees from number 1 cam?

The best way is to find top dead center of each cylinder and make sure that the exhaust valve is fully open 110 crankshaft degrees before TDC and the intake should be fully open 110 crank degrees after TDC. This TDC is the end of the exhaust stroke and the beginning of the intake stroke. Use a degree wheel. This is not the time for the eyeball method.

Because of the oddball cylinder bank angle I would just check all cylinders individually just to be sure. Shouldnt take long after the heads are off.

This assumes your cam is 110 degrees lobe angle.

 

[petertha]

You're miles ahead of me parksy, but I'm collecting links for when my special hair pulling day inevitably comes.

Here is a setup pictorial that maybe you can re-jig for your own purposes.
http://philsradial.blogspot.ca/2013/02/valve-timing.html

From other setup pics I think the procedures are similar, but obviously vary by the engine configuration & accessibility. To my eye, for any given cylinder, looks like:

- with an indicator against a reference piston top, measure highest position in bore

- 'mark' that position as TDC. Some use a protractor degree wheel mounted on CS against a fixed pointer reference, some make up their own wheels from cardstock because they have pre-figured out valve I/O clock positions relative to their engine. I think this fellow has a block attached to CS end & then measures rotation with a mag digital level device.

- assemble head & cam/valve assembly

- ?somehow? put indicator on intake or exhaust valve rocker or tappet or some part of the valve train to reliably indicate when valve begins to lift & has closed as the CS is rotated through TDC-BDC motion. Now you have independently validated I/O timing for that cylinder as a function of CS rotation degrees & can compare to original design or whatever.

- now whether that's the end of it for remaining cylinders because the camshaft (or cam ring) and crank throws are fixed as a function of how they were machined & assembled, or how that relates to firing order I/E on other cylinders I cant say. But one should be able to repeat this procedure for remaining cylinders & relate it to the #1 cylinder TDC reference if that original position can be preserved.

Hope I've got this right, otherwise maybe the experienced guys will correct me.

 

[Parksy]

Thank you guys. Almost had it started today. It ran for a couple of revolutions. Had condensation form on the intake it got so cold. Unfortunately the loctite holding the crank together hasn't held up and the flywheel now wobbles. This I will need to address before I go any further. I will correct it, but at this stage I'm giving this project a break.

Thank you all for your help. I will have it running eventually.

Andy

 

[t.l.a.r. eng]

You may have posted a clue. If it is trying to run, open the throttle all the way and use a heavy rich mixture when trying the first start.

If white smoke is coming out the exhaust it is still a little lean. New engines tend to have raw liquid exit the exhaust when running on glow fuel until things settle in. Another clue is when you said it will try to keep on firing without the glow driver.

Open the throttle wide, richen the mixture, and leave the glow driver on and it should run if the cam timing is correct.