Hi everyone ,
I put my Super Tigre .45 ABC engine on my trainer ! I have just started landings ! 
The first start of the day , or a cold start , the engine starts in just one flick , But when we start it after one flight where the engine is hot , it just doesn't start for about 10-15 flicks ! We have to flick it quite hard , then it fires ! 
Any idea as to what is wrong or what can be improved ? 
Compression is not the issue since this engine is quite new and has run only about 25 flights .
Thanks 
Sahevaan

dont worry ......

IT HAPPENS TO THE BEST OF US...........!!!!!

if u getting exhausted by flicking you can always use a starter.....

let the engine cool for 10 mins.... then flick.. try choking some excess fuel or u can always prime it (i will not recommend priming, if it is flooded it will be a big pain)

then you can even use a hotter glow plug (like OS no 8 )

We generally take a break of about 10 minutes between 2 flights but the engine is still quite hot. Other engines start one or two flicks wheather hot or cold !

change the plug then..... try OS no 8

Is is a new engine ??

Don't think it will make a difference. It is most likely the characteristic of the engine. Many engines are reluctant to "flip start" when hot.

i had a same prob the engine would never start after 1st flight.... even if you take a break of 1hr... change of plug to the above solved it.... i even tried A3 did not bear any result... only no8 did..!!
 

What would be a good technical/logical reasoning for this behavior ?  Wouldn't one expect sure shot ignition when the engine is already hot ? Or would it be some thing like the idle being too rich, that too much fuel was left behind in the chamber from the previous run ?

PS:  Let us learn more about glow engines

This is because of lower compression. Hot engines have lower compression due to different contraction rate of piston and sleeve.

the reason why this happens is when the engine is hot and expands on the inside(mostly the newer OS or TT or any other newly manufactured ) and has uneven expansion compared to the piston and the cylinder this creates a low compression on the next start..... even adds up keeping low pressure in the crankcase for the fuel supply in comperssion zone.....

thus to over come this you will have to get the engine pretty rich by choking or directly prime it by pourin 3-4 drops of fuel in the carb..... which might end up flooding the engine and making the pain worst

or use a hotter plug which can fire pretty quickly if the fuel is low.....

PS: if i am wrong please correct....!!

EDIT: the above post came as i was typing....

But wouldn't the small difference in compression be adequately be compensated by the hot chamber itself, allowing the fuel to ignite easily ?  After all, it is well known that not much help of a glow plug is necessary once an engine is running.  It is known for engines to fire by themselves (rarely though) even without a glow igniter/booster connected. So it would seem that a hotter engine should start faster ?

Hmmm, use a hotter plug ? While the engine is already hot, and aiding with the glow plug action already ?

Check

1. Have you put the right fuel in your gig. This is the most common problem for beginners.

2. Next is your glow plug - is it getting hot enough? A faulty glow plug, or a flat Glow starter could be it. Take the Glow plug plug it into your Starter and check that the plug shines nice and bright. If it doesn't glow then you could have a faulty glowplug or a flat Glow Starter.

3. See if the engine is getting any fuel. Hot engine tends o form vapour in the fuel line causing vapour lock. prime it with carb closed ensure fuel is coming in easily

4. Check tuning, too lean or rich can cause it.

5. if it doesn't start all then the one-way bearing could be broken. This is the part that makes the engine turn in the right direction, so take out the glowplug, look into the cylinder head and slowly rotate the prop. If it doesn't move the piston, its broke.

Hot or Cold

Hello Everyone,
There can be many reasons for the painful hot start but their is one thing that I noticed in few engines:

After the first run if the fits of engine are still minorly tight (i.e. it needs more run in)   or there is a minor (negligible) misalignment in the ball bearings of engine the crank case gets heated up & when we choke for the fuel or fuel comes to the engine & as methanol is highly volatile & of course the engine is hot, a part of methanol vapourises leaving an improper mixture to start the engine & as in above post this vapoured methanol can cause vopour lock in the tube.

This problem occurs more with the engines in couling. Now there is a DESI REMIDY to this i.e pour some fuel  on the engine to bring down its temprature & then try it. But this thing will need frequent (outside) cleaning of the engine.

Second this (may be not applicable for this case) keep your engine clean  from outside. The black carbon deposits on body, head & silincer of engine really creates problem in the cooling of engine.   

HAVE A NICE DAY

Many thanks & Regards,
Vivek

i will never recommend this.. when you force cool by this method the cylinder is cooled and the piston remains hot..... making a tight fit .. tighter than it is designed for.... thus damageing the engine which is equivalent of 4-5 hrs of running ..!!!
 

RC Vish, you are right to the some extent but in case of ABC engines the Brass Cylinder & Aluminium Piston's contact area is so much that while loosing heat they can be onsidered as a same member. The cofficeint of ecpension of this brass & Aluminium is almost the same.

Now when you have first launch done it will take at least 2-3 miniutes for the next start & for this time the tempratures are lowered to the extent where it wont happen.


Secondly in the ABC engines the piston is tight fit at only Top Dead Centre & below that it is approximately 30-50 microns loose. So there is no question of getting engine damaged as for of 4-5 hours running.

Warm Regards,
Vivek

If the coefficient expansion of the brass sleeve and piston would have been same then we would not get into this situation (of loosing compression) in the first place. I don't know much about this, but my guess is; all the reasons that will cause an engine to loose compression when heated should also cause damage to the engine if the cylinder is forced to cool before the piston does.

SLS thanks for the support.... i thought i had a misconception....!!!

PS: found 2mm brass rods will they work...!!!

The glow plug is fine , the booster is the new one , which is connected to a 12v battery and then the voltage is reduced to 1.5v. The fuel I am using is 20% caster and 80% Methanol . The fuel was prepared by captain so its all good . Engine is a bit on the richer side though its performace is very good. While priming , the fuel does reach the carb .
Thanks ,
Sahevaan

does the crank shaft heat up after the running....... you can check that after the engine has stopped by touching the cankcase underside(below the mounting lugs. under the piston-cylender) and let us know...!!!

Going flying tomorrow , will check and let you know

No , its not that hot also. Its a little less hotter than the cylinder head

Gusty,
Which is the one way bearing in a model engine? All my 2T and 4T engines run either way (sometimes) and nothing is broken yet.   . In fact a common trick when this happens with my 2T is to flip the engine clockwise and pronto it starts in the right direction

In my opinion the reason is as explained by Vivek, vaporising of the meth in a hot engine. Now even though you are allowing time between restarts do remember that you are at peak of Indian summers and even ambient temp is high so your engine would take considerable time to cool down.
The remedy is fly another model, let this one coold down and then restart