E: Engine Comet Mongrel

[Cyborg]

That does surprise me a bit. I would have thought that with lighting the fire later, there would be less pressure buildup before TDC and being later, there would be less mechanical advantage for what is pushing back.

 

[eglijim]

That does surprise me a bit. I would have thought that with lighting the fire later, there would be less pressure buildup before TDC and being later, there would be less mechanical advantage for what is pushing back.

Suppose it depends what the rpm is, you need the advance to get the fires lit just at the right time for the power stroke. Always applied extra care when hand propping an aeroplane with a mag and never tried without first checking the impulse had not stuck so still have a full set of hands and fingers.

 

[passenger0_0]

That does surprise me a bit. I would have thought that with lighting the fire later, there would be less pressure buildup before TDC and being later, there would be less mechanical advantage for what is pushing back.

Assuming it's an adiabatic (very quick) process, any reduction in flywheel kinetic energy required to compress the burning mixture BTDC is fully recovered by the additional gas pressure acting on the piston after TDC. (gas spring). I think the real issue here is that the burning mixture temperature gets very high when ignited too early so it causes thermal issues for mechanical and combustion processes. There are many factors here as also suggested by eglijim.

 

[eglijim]

Assuming it's an adiabatic (very quick) process, any reduction in flywheel kinetic energy required to compress the burning mixture BTDC is fully recovered by the additional gas pressure acting on the piston after TDC. (gas spring). I think the real issue here is that the burning mixture temperature gets very high when ignited too early so it causes thermal issues for mechanical and combustion processes. There are many factors here as also suggested by eglijim.

Absolutely agree, it is not really possible to isolate each individual factor due to the variables involved even on a day to day basis, temperature fuel grade air pressure et al. Hardly surprising that "modern" engines rely on a range of sensors to constantly adjust fuelling and ignition many times a second.

 

[Cyborg]

Well... what if any difference it makes to my engine will have to remain a mystery. Even if I had free access to a dyno, I don't think I could stomach watching that many full pulls on a poor old Comet. That second plug lead will haunt me now, although my knee might appreciate it. I would like to know more about what is going on in there.... Anyway, I need to focus on getting wheels attached to this lump. I scrounged a Ducati swing arm that I thought might save me some time, but evidently not.

 

[eglijim]

Well... what if any difference it makes to my engine will have to remain a mystery. Even if I had free access to a dyno, I don't think I could stomach watching that many full pulls on a poor old Comet. That second plug lead will haunt me now, although my knee might appreciate it. I would like to know more about what is going on in there.... Anyway, I need to focus on getting wheels attached to this lump. I scrounged a Ducati swing arm that I thought might save me some time, but evidently not.

Just realised I did not say that we had best results with the maximum advance at 35 degrees on single plugs and 30 degrees on the twin plugged engine. Both were 90x90mm and 9:1 compression, JE pistons with the squish bands machined into the head casting. Unless I have missed something what will you use as a chassis. You also have the advantage with your ignition to change where it starts to spark if you fit an electric starter

 

[Cyborg]

I haven't made a final decision on a chassis. I have a few different options but leaning towards one that currently has all of the necessary paperwork. Unfortunately it would likely cause the club to issue a fatwa. If it doesn't work out or I come to my senses, there is a nice Monster trellis frame that came attached to that swing arm. At first glance, it does seem wide and would probably look odd.

 

[Oldhaven]

we had best results with the maximum advance at 35 degrees on single plugs and 30 degrees on the twin plugged engine.

Jim,

At what RPM was maximum advance reached for single and twin plugs?

 

[eglijim]

Jim,

At what RPM was maximum advance reached for single and twin plugs?

Whatever the standard advance curve was for my single plugged bike ( Lucas RITA), think the customer bike was on twin points and atd so both likely fully advanced before 2500. Not really an issue in the limited rpm range to which we are restricted assuming even a highly tuned Vin runs past the power peak by 6400 rpm at most. As I have said our race bikes ran OK without an atd on fixed ignition but could be a pain to start from cold but seldom came under 4000 rpm when warmed up and in action. Does depend on the intended purpose of the machine.

 

[vibrac]

Well... what if any difference it makes to my engine will have to remain a mystery. Even if I had free access to a dyno, I don't think I could stomach watching that many full pulls on a poor old Comet. That second plug lead will haunt me now, although my knee might appreciate it. I would like to know more about what is going on in there.... Anyway, I need to focus on getting wheels attached to this lump. I scrounged a Ducati swing arm that I thought might save me some time, but evidently not.

Yes it is stomach churning here is an early vid of the first time we used one....

http://www.oldracer.co.uk/video-gallery.html#prettyPhoto[group1]/6/