Laser Engines - Technical questions

[thebluemax]

44 minutes ago, Jon - Laser Engines said:

I hope this gives a bit of an insight into how you can trouble shoot just by listening to the engine, understanding what all the little noises mean, and the clues they give regarding the problem and how to solve it. In my case, i did all the above diagnosis and knew the cause of the problem almost before the video ended and instructed the customer on how to fix it. But given the handy video i thought it might make a useful learning tool and thought i would give this group game a try as i have seen over the years that folk really struggle with engines that will not start at all. All the clues are there as to why and i hope this gives a few pointers people find helpful.

 

Let me know if it was of any use. 

 

Definitely useful.  Saved for future reference. Never thought about listening to why it doesn't start.  It was always looking at what it was going on with it.

 

[Jon H]

1 hour ago, Don Fry said:

Jon, why is the silencer close to the firewall bad, and what is the minimum distance. My winter build, 180 powered, is tight, as designed. Can be altered at this stage.

 

Exhaust close to firewall is only a problem if there is enough mount flex for it to it it and/or burn it. Carb close to the firewall is worse though as you get turbulence at the intake and this can make the engine run rather badly. I had this on my La7 when it was laser 180 powered. Worked fine on the ground but not in the air once above a certain speed. Cut out the area round the carby intake and problem solved. 

[Jon H]

1 hour ago, thebluemax said:

 

Definitely useful.  Saved for future reference. Never thought about listening to why it doesn't start.  It was always looking at what it was going on with it.

 

 

Its all about the listening ? 

[John Stainforth]

Jon, it's easy for you to say that, as the world's expert in the language of Laserspeak! Most of us are still struggling with the basic vocabulary.

[thebluemax]

4 hours ago, Jon - Laser Engines said:

 

Its all about the listening ? 

Only listened to it when it was running and sounding so sweet.....

[Jon H]

1 hour ago, John Stainforth said:

Jon, it's easy for you to say that, as the world's expert in the language of Laserspeak! Most of us are still struggling with the basic vocabulary.

 

yea i get that but the idea is to point out the vocab in a video like this so you guys can hear what im hearing and understand what it means ?

[Jon H]

Right then

 

Round 2

 

 

 

Whats up with it now? 

 

Get those ears flapping.  

[Alan Hilton]

3 minutes ago, Jon - Laser Engines said:

Right then

 

Round 2

 

 

 

Whats up with it now? 

 

Get those ears flapping.  

Sounds and looks rich 

[J D 8]

              Plus one on above and may well have stopped if glow off.

[Denis Watkins]

Yes JD 

surely the connected glow plug messes with the timing too

[Jon H]

Rich you say. 

 

Ok, why? what makes you say rich. 

 

And in fact we should probably have all of the diagnoses come with an explanation of why. 

 

Its like being at school, gotta show your working ? 

[Don Fry]

I reckon it rich on the main needle, Smokey Joe exhaust smoke. That abrupt choking when he closes the throttle is I think a very rich idle needle, putting the plug out.  

[David Davis]

I agree with Don.

 

[Jon H]

 

This is proving to be quite interesting and does highlight the issues people have with identifying faults. Its not a criticism of anyone with the wrong answer (everyone so far!), it just seems to be challenging to many and perhaps i take it more for granted than i should that the language engines speak is understood.  

 

So, exhaust smoke. This is not and never will be a reliable means of tuning an engine. There are many reasons for an engine to smoke more/less and most are down to the oil used and how much. An engine with 10% castor will probably smoke more than 15% synthetic no matter the tuning. Also how much of the visible vapour is condensation? Car exhausts 'smoke' in the winter and glow engines do the same. An engine with worn rings may also smoke as oil from the crankcase can more easily get back up the top. Combustion efficiency will also play a role. 

 

Leaving the glow on. This can be a problem but i would expect to hear the engine knocking if it were. 

 

But we are back to looking for the problem and not listening for the problem. 

 

The key points in the video for diagnosis are the exhaust note, and what happens when the chap advances the throttle.

Overall, the exhaust note is a smooth, soft and has a constant thrub to it. This means a lean mix. The note only changes when he advances the throttle. When he does this, the engine dies.

 

When it dies, ignition has stopped and the engine is running down under its momentum so again its a lean cut symptom just like pinching off the fuel line when an engine is idling. As we talked about yesterday a rich cut usually stops dead and will often bounce backwards. This lean cut is the same as we had when he was trying to start it in the other video. Same symptoms, same audio cues, same problem. 

 

With the engine at a fast smooth idle, throttle is opened allowing more air in. the slow run should open as well to give enough fuel for mid range running. But the engine falters and we have a flame out. Clearly we have lean mixture in the midrange and this is controlled by the slow run needle. Reducing the throttle brings us back to good enough mixture and we end up firing again as the glow is still connected, just very lean. 

 

There is also an element of logic to this if you assume this was your own engine and you were working on it yourself. Its the same engine you were working on yesterday, it was lean then, been richened a bit, now its better but not right... You can sort of assume its still lean based upon that alone. Also the standard test for a lean slow run mix is to shove the throttle open. If it cuts, its lean, if it coughs and splutters then its rich. As we got a straight cut, we can again assume our slow run mix is lean. 

 

So we are still chasing a lean slow run needle on this example. 

 

Everyone with me?

 

 

 

 

 

 

[Nick Somerville]

This is very helpful. More homework please.

[David Davis]

Having seen the two videos and got the diagnosis wrong in both cases, I'm surprised that I've ever got an i/c engine to run at all! ?

[Jon H]

27 minutes ago, David Davis said:

Having seen the two videos and got the diagnosis wrong in both cases, I'm surprised that I've ever got an i/c engine to run at all! ?

 

What was it that lead you astray on the diagnosis?

[Jon H]

I was testing a 150 after repair today and was able to shoot this. Idle tuning is correct to begin with and i just lean it off. It then ends up sounding like our 180 in the other videos and behaves in the same way. Richen up again and its off to the races

 

 

 

Compared like this the sound of the lean condition is very clear. 

[Alan Hilton]

4 hours ago, Jon - Laser Engines said:

I was testing a 150 after repair today and was able to shoot this. Idle tuning is correct to begin with and i just lean it off. It then ends up sounding like our 180 in the other videos and behaves in the same way. Richen up again and its off to the races

 

 

 

Compared like this the sound of the lean condition is very clear. 

Never could tell the difference between a lean or rich cut !! 

[Don Fry]

I think I can, but on an I pad, I’m struggling to hear the differences. Perhaps full volume helps.
Now I’m writing in the kitchen, having been ejected from the lounge.
And the lean cut, earlier this afternoon, still lacks the dry sucking sound you get trying to open a lean engine.

Caviate, my top note hearing is damaged. To much time shooting rifles.

[Frank Skilbeck]

So looking at that Jon, it would appear that at idle the engine needs to be slightly rich otherwise it will starve when the throttle is opened and fuel demand suddenly increased, back in the old days carbs on cars had the accelerator pump to overcome this.

[Jon H]

8 minutes ago, Frank Skilbeck said:

So looking at that Jon, it would appear that at idle the engine needs to be slightly rich otherwise it will starve when the throttle is opened and fuel demand suddenly increased, back in the old days carbs on cars had the accelerator pump to overcome this.

 

 

Exactly right Frank. 

 

Glow engines will run with a much leaner mix at idle but they then fall over on acceleration. There are a number of ways around this, but they all add significant complexity to the carb. 

 

OS did a range of engines with 3 needle carbs (low, mid, high) at one point for helicopter use but this again adds to the complexity and its another thing to get right when tuning so maybe not a great idea. 

 

One thing that can be done to allow the leanest possible slow run mixture is to be gentle with your throttle. This gives the engine time to spool up and the lower the acceleration load, the less fuel you need to accelerate the load. In the past i have suggested people who dont trust themselves to move it slower in panic situations add a delay to their tx of 1-2 seconds so if they slam the throttle up the engine has a slow ramp over that 2 second period. It has virtually no effect with small changes, but large ones are damped out a bit. 

 

Although much maligned the super tigre carbs we used to use were better at idle/transition performance than our current carb due to the way they metered fuel. The only snag was, they did vary a bit and they either worked just fine or not at all so have got a bit of a bad reputation. Also, folk love to fiddle and they would mess about with the position of the jet and screw the whole thing up

[Nigel R]

37 minutes ago, Jon - Laser Engines said:

OS did a range of engines with 3 needle carbs (low, mid, high) at one point for helicopter

 

The mid range needle did not have a great range of adjustment, if I remember. Choppers tend to run constantly at part throttle, too, so I guess the mid range was to optimise mix at hovering? Rather than throttling per se.

 

[Jon H]

I never used one Nigel so i have no idea what the adjustment range was. Clearly it must have been significant enough otherwise it would not have been worth the rnd cost. 

 

I think it was a 3d thing where you didnt want any dead bands in the throttle and, as you say, heli's spend most of their time around that mid throttle point...which is just where most carbs struggle

[Nick Cripps]

That last video is very instructive, Jon.

A lot of people adjust the low needle to give the highest idle speed but this clearly shows the tuning is then too lean for clean acceleration.