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MG MGB Technical - Advanced thinking ?....

Hi all.

I have been trying to understand spark advance for some time, and I think I have discovered something that is really quite obvious, but my addled brain has only just grasped ; I offer it here in the hope that it might help others.

It is usual to express spark advance in terms of degrees of crakshaft rotation, probably because this is relatively easy to measure using a strobe or similar.
I believe that this is unhelpful when we try to understand what is really going on, let me try to explain :

It is well known that it takes time for the flame to propogate across the cylinder, and that the spark must be applied before TDC so that maximum pressure will occur at the ideal time of approximately TDC.

Take a hypothetical single cylinder engine running at 600 rpm (2 stroke to make the maths easier). The sparks will be generated at a rate of 10 per second. Now suppose that the flame takes 0.001 seconds to build to maximum cylinder pressure, this will equate to a required advance of 3.6 degrees.

Now increase the revs to 6000 rpm and assume that the charge mass is the same. The fuel burns at the same rate, so the advance required is still 0.001s, which equates to 36 degrees at this speed.

The key factor for me is that the advance TIME is unaltered, it is only the advance expressed as an ANGLE that has changed.

HTH... Don

Don

Actually a bit more complex than that. By starting the combustion prior to TDC, the expanding gasses actually cushion the reciprocating piston before it comes to the abrupt stop at the top of it's stroke. That's one reason the timing advances as speed increases, the fire takes the same amount of time to start, but has less time to do what it needs to do.
R. L Carleen

The mixture doesn't explode, it burns over time, and that time is constant irrespective of engine revs (alhough may vary due to other factors). If the timing is too advanced you get pinking or detonation which is when the mixture *does* explode due to the effects of continuing compression after ignition, which is damaging to the engine. So that sets the maximum timing or angle at idle at typically 8 to 10 degrees before TDC. But as the revs increase the burn time extends more and more into the expansion stroke of the piston and more and more energy is lost to forward motion and instead becomes wasted heat. And because of the reducing *time* between ignition and TDC as revs increase the risk of pinking is similarly reduced so the point of ignition can be brought forward, so converting more of the energy of the burn into forward motion instead of wasted heat. So yes advancing the timing changes the angle, but whether this reduces, increases or merely restores the duration in time between ignition and TDC depends on the specific engine design. Even low and high compression versions of the same engine can have a huge difference in advance at a given rpm i.e. the advance TIME *does* vary from specification to specification. Take the 18V 581/582 for example. Both have the same strobe timing - 13 degrees at 600 rpm - but whereas the HC reaches 20 degrees at 2200 rpm the low compression does not reach that until about 3500 rpm.

Charge mass also has a great effect on pinking, part-throttle cruising with a relatively small mass can take much more advance than full throtle at the same rpm, which is where vacuum advance comes in, adding up to 24 degrees of advance on its own, greatly inmproving crusing economy and reducing waste heat.

The two together try to maintain the maximum possible advance without pinking over the whole of the three-dimensional map that is throttle opening, load and revs. But the very crude electro-mechanical arrangements of the MGB only make a poor approximation of the ideal. What *would* be ideal is a closed-loop knock-sensing ignition-retarding system.
Paul Hunt

Hi.

Thanks for the responses, especially your detailled one, Paul.

So, the 3000rpm pinking problem (mentioned in previous threads) is probably one of compression by the piston PLUS pressure due to fuel burn resulting in an explosion of the remaining fuel / air charge.. yes, I think I understand that now, it certainly explains why it occurs with a relatively low charge mass.

The next questions are why, and what can we do about it ?. Is it true to say that once a single 'ping' has occurred then added heat makes further 'pings' likely?.
I found that a decoke fixed the problem, but it returned very quickly, suggesting that the thermal insulation of even light coking may cause probs, there certainly weren't any significant deposits (ie enough to glow or reduce the cylinder capacity) when the pinking came back.

I am currently considering water injection, maybe using the slight vacuum after the air filters to draw water in rather than pumping it in. I guess it is a good idea to keep paper air filters dry as they might collapse or may be drawn into the carb.

Interesting stuff... Don
Don

Decoking can help if the carbon is resulting in red-hot pieces. But certain eras of MGB heads are particularly prone to thin lands of metal glowing red-hot, and some head specialists remove these. Other than that you just have to set the timing to max without pinking, which is what I have to do. These hot-spots also contribute to running-on, which my 73 suffered badly from especially running standard unleaded. The MGOC anti-runon valve as supplied did nothing to cure this, but plumbed in like the North American valve it is now 100% effective. By contrast the Rover V8 engine is extremely resistant to both pinking and running-on. Water injection will cool the charge which lowers its density so you get a greater mass which makes for a bigger bang! Cooling the charge it also tends to damp down hot-spots which should reduce pinking. Steam injection, OTOH, does the opposite and results in loss of performance, its main reason is to improve economy ... which could be achieved at no expense by using a lighter right foot.

On water injection I well remember an airliner crash landed on an autobahn many years ago because it couldn't gain enough height. The cause was found to be oil contamination of the distilled water used for injecting into the engines on take-off. the contamination reduced the effectiveness of the water injection hence not enough power for take-off. I never realised how marginal flight was.
Paul Hunt

This thread was discussed between 09/02/2004 and 11/02/2004

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