Running on

[Peter Cobbold]

What I don't understand, all TR 4 pot engines are basically the same but some run on (and it's a seemingly incurable problem) and some don't (mine for example). Why is that?

 

Rgds Ian

 

Autoigintion needs both time to light up the mixture, the mixture itself, and a high enough temperature somewhere in the combustion chamber to speed up the chemical reactions of autoignition. The time is only available during the very slowest turn of the crank, at around "1 rev per second ". Any faster than that and the compression is dropping after TDC and the flame-precursor chemicals dissipate and combustion ceases. In order to run on there must be mixture during that last slow turn of the crank, so factors that supply fuel and air come into the equation. The temperature has to be high enough to speed up the chemistry so that it happens at the right point in the cycle, whcih is to deliver peak combustion pressure just after TDC ( or in Rogers reversing engine, just before).

So a lot of factors have to coincide to get running on. They will act in combination. This gives many possibilities that could differ between engines. For instance, if an engine decelerates fast upon ignition switch-off it will reach the 1 rev per sec speed ealier in time, with more mixture in the cylinders, with more temperature in the hot spot ( usually the plug). This means a lightened flywheel coule lead to RO, or more drag from the clutch. or more frition from the pistons. Alone a rapid deceleration may not trigger RO, but combined with say a plug a grade too hot, and/or retarded spark that heats up the plug or exhaust valve just a tad more than normal autoigntion might happen sufficient to give RO. There are many many other combiantions.....

Autoignition is subtle but the most importnat contributor is temperature. Autoignition is a normal part of combustion: it happens in every engine all the time, on every stroke. But in normal running it has no time to alter pressures, because the extremely high temperature of spark then speed up the chemistry enormously. The chemistry is altered by the octane of the fuel. High octane has the effect of raising the temperature at which autoignition happens by about 50deg C. ( 99 vs 93 RON IIRC) This is also how high octane fuel kills detonation. The difference in end-gas temperature between a knocking engine and detonation is a mere few degrees C.

 

So temperature is very important in RO and is the first thing to track down. Letting the engine idle for say 10-20 seconds should allow heat to dissipate. The spark plug is a prime candidate for changing to colder grades. If those don't work check the spark timing as engines running retarded run hot - plugs, exhaust valves etc. (The TR4 ignition curve is very odd, static is 4BTDC compared with other engien of similar design and compression ratio ( static 10-12BTDC). If the centrifugal advance doesnt start in at strangely low rpm (450-700rpm) then the tickover at 800rpm will be retarded.) Beyond that almost anything that might heat up the mixture going into the engine, or impair heat removal from the combustion chamber walls will act in the direction of promoting RO. Steve's CHT measurement and simple swap from waterless coolant ( post 7) is a beautiful example. Which then points the finger at cruddy rusted-up water jackets in some cylinder heads, not others...

 

Changing carbs is a big change, and I am not inclined to put the blame on SUs. After all, most SU'd TRs dont run on. But a carb change also means a new design of manifold...

Peter

Edited September 26, 2016 by Peter Cobbold

[monty]

Autoigintion needs both time to light up the mixture, the mixture itself, and a high enough temperature somewhere in the combustion chamber to speed up the chemical reactions of autoignition. The time is only available during the very slowest turn of the crank, at around "1 rev per second ". Any faster than that and the compression is dropping after TDC and the flame-precursor chemicals dissipate and combustion ceases. In order to run on there must be mixture during that last slow turn of the crank, so factors that supply fuel and air come into the equation. The temperature has to be high enough to speed up the chemistry so that it happens at the right point in the cycle, whcih is to deliver peak combustion pressure just after TDC ( or in Rogers reversing engine, just before).

So a lot of factors have to coincide to get running on. They will act in combination. This gives many possibilities that could differ between engines. For instance, if an engine decelerates fast upon ignition switch-off it will reach the 1 rev per sec speed ealier in time, with more mixture in the cylinders, with more temperature in the hot spot ( usually the plug). This means a lightened flywheel coule lead to RO, or more drag from the clutch. or more frition from the pistons. Alone a rapid deceleration may not trigger RO, but combined with say a plug a grade too hot, and/or retarded spark that heats up the plug or exhaust valve just a tad more than normal autoigntion might happen sufficient to give RO. There are many many other combiantions.....

Autoignition is subtle but the most importnat contributor is temperature. Autoignition is a normal part of combustion: it happens in every engine all the time, on every stroke. But in normal running it has no time to alter pressures, because the extremely high temperature of spark then speed up the chemistry enormously. The chemistry is altered by the octane of the fuel. High octane has the effect of raising the temperature at which autoignition happens by about 50deg C. ( 99 vs 93 RON IIRC) This is also how high octane fuel kills detonation. The difference in end-gas temperature between a knocking engine and detonation is a mere few degrees C.

 

So temperature is very important in RO and is the first thing to track down. Letting the engine idle for say 10-20 seconds should allow heat to dissipate. The spark plug is a prime candidate for changing to colder grades. If those don't work check the spark timing as engines running retarded run hot - plugs, exhaust valves etc. (The TR4 ignition curve is very odd, static is 4BTDC compared with other engien of similar design and compression ratio ( static 10-12BTDC). If the centrifugal advance doesnt start in at strangely low rpm (450-700rpm) then the tickover at 800rpm will be retarded.) Beyond that almost anything that might heat up the mixture going into the engine, or impair heat removal from the combustion chamber walls will act in the direction of promoting RO. Steve's CHT measurement and simple swap from waterless coolant ( post 7) is a beautiful example.

 

Changing carbs is a big change, and I am not inclined to put the blame on SUs. After all, most SU'd TRs dont run on. But a carb change also means a new design of manifold...

Peter

Thanks Peter for an excellent explanation, which all makes sense to me.

Does he occasional run on do any harm?

Cheers.

[Peter Cobbold]

Thanks Peter for an excellent explanation, which all makes sense to me.

Does he occasional run on do any harm?

Cheers.

Monty, Thank you.

Running on forwards is unlikely to do harm. There wont be much mixture to burn, so there's little pressure on the pistons. Despite the oil pressure being low the bearings should not suffer. Every time a cold engine is started there's low oil pressure, plus slow-speed scraping of piston against bore, but without the benefit of the oil films that will be present in a RO engine. On the other hand the sharp, metallic clunking sounds during RO suggest it is best cured.

RO backwards might be nastiest for the timing chain: a sudden snapping taut of the slack side. I really dont know.

 

 

A lot of web sites blame RO on a glowing hot spot triggering combustion. That's not right. Any engine that has glowing hot spot - a carbon ember is often quoted - is a very few seconds away from piston melt-down. This is 'preigntion' and it is silent, but feels like the engine is not rsponding to the throttle, and it make a strange exhaust note - the 'bark' is lost and it sounds 'hollow'. Preignition is rare, fortunately. Most common cause is using a too-hot grade of plug. Different plug manufacturers use different - even opposite - scales. This is worth bookmarking:

https://www.boschsparkplugs.net/learning-center/article/225/heat-range-conversion-chart

 

I holed three pistons in the time it took to go around the second half of the hairpin at Aintree - preignition is fast ! In my case the cause was running very lean, under boost. Combustion was dramatically retarded, the plugs overheated and triggered preignition. I drove home on three cylinders....slowly.

 

Peter

Edited September 26, 2016 by Peter Cobbold

[Peter Cobbold]

RO needs fuel, where's it coming from? Do all TR4 stock manifolds have a hot spot where the exhaust touches the inlet, as in this photo? It looks like a bolted-on pad:

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/SMwAAOSwZQxW3yhR/$_1.JPG

That hot spot will speed up evaporation of liquid fuel wetting the intake walls. When the ignition is switched off the fuel will evaporate quickly, leaving no fuel to cause RO a second or so later. If an after-market exhaust manifold is fitted the lack of a hot spot may allow liquid fuel to persist long enough to cause RO. For the same reason fitting a heat shield might make RO worse.

 

If Weber manifolds slope down more towards the inlet valves the liquid fuel wont puddle as much as in a near-horizontal SU manifold, so curing RO without a hot spot.

 

Peter

Edited September 26, 2016 by Peter Cobbold

[stuart]

RO needs fuel, where's it coming from? Do all TR4 stock manifolds have a hot spot where the exhaust touches the inlet, as in this photo? It looks like a bolted-on pad:

http://i.ebayimg.com/00/s/MTIwMFgxNjAw/z/SMwAAOSwZQxW3yhR/$_1.JPG

That hot spot will speed up evaporation of liquid fuel wetting the intake walls. When the ignition is switched off the fuel will evaporate quickly, leaving no fuel to cause RO a second or so later. If an after-market exhaust manifold is fitted the lack of a hot spot may allow liquid fuel to persist long enough to cause RO. For the same reason fitting a heat shield might make RO worse.

 

If Weber manifolds slope down more towards the inlet valves the liquid fuel wont puddle as much as in a near-horizontal SU manifold, so curing RO without a hot spot.

 

Peter

Thats an earlier manifold and sidescreen cars dont seem to suffer so much as 4/4a. Performance manifold sometimes dont provide good clearance to the inlet manifold and often arent matched well to the ports either. Weber manifolds are pretty much straight on no slope either way.

Stuart.

[Nobbysr]

I used an anti run on valve on a stage 2 MK11 GT6 the valve was an MGB pt number, but was essentially an overdrive solenoid with a filter. When the ignition was switched off power activated a relay, timer which opened the run-on valve for 10 second then closed .

Its not as complicated as it sounds but completely stopped run on , The valve was mounted on a bracket and a hole drilled and taped into the manifold was connected via rubber hose. Will try and dig some

photos out . I did fit one to a TR6 and again it sorted the problem . I believe the MGB valve was part of kit which was developed for the US market

 

cheers

Steve

[greasemonkey]

We had a 4a in a while ago that all sorts of people had tried to cure its run on including fitting heat shields and a dump valve. Careful setting of the carbs (Float levels/needles/springs/damper oil level) correct setting of the distributor and the A/R capsule connection and bob weights freeing up and the right springs for them, proper routing of fuel lines and correct plugs stopped it completely. Often its a combination of all of that that will make them run on like a damn. Totally agree that fitment of Webers/Dellortos never have that problem at all.

Stuart.

Stuart - when you say "proper routing of fuel lines" , do you mean as the factory did i.e. along the engine block, or another route away from the heat??

Thanks,

Phil.

[stuart]

Stuart - when you say "proper routing of fuel lines" , do you mean as the factory did i.e. along the engine block, or another route away from the heat??

Thanks,

Phil.

I have never had a problem with the original factory run, round the front under the thermostat housing and then round to the front carb and then looped from there to the rear carb. Check the run in the workshop manual illustrations.

Stuart.