Electronic Ignition - an explanation on combustion

By Martin Williamson & Paul Wiley

Why electronic ignition? It's far more accurate than the old points system, and never needs resetting is the short answer.  Other benefits include a fatter spark, thus better burn and performance, as well as cleaning up the power meaning the spark plugs last longer.  Apart from that, many distributors will be worn and hence the timing will be difficult to maintain accurately.  Replacing the distributor with a new one will often see benefits in normal running, but if you are wanting to optimise engine modifications then it is essential to consider upgrading the distributor and coil.

So as a performance and reliability benefit, electronic ignition systems make sense.

See also:

Using existing distributor:

• Lumenition Magnetronic - fixed advance
• Lumenition - fixed advance
• Newtronic - fixed advance
• PerTronix - fixed advance

Using new components and replacing the existing distributor (and possibly the coil):

• The Cheaper Alternative - fixed advance (an article by Thomas Wykes using Maestro/Montegos distributors)
• 123 - 16 optional Advance curves
• EDIS - the fully mappable option

And, for problems with connecting a rev counter to your electronic ignition if using EDIS or similar

• Modifying a Tachometer

Also: Fuels

On this page:

• Why Advance/Retard?
• Bobweights and Springs - Centrifugal Advance
• Vacuum Advance
• Fuel & Air
• Detonation & Pre-ignition
• Run-on

The following discussion relates to the theory of the internal combustion engine rather than the B directly but whilst the principle applies to all engines, the difference is in the execution.

But firstly what is timing and advance/retard?

The fuel entering the combustion chamber needs time to allow the flame front to propagate and burn completely. The idea is that the full push from the combustion is timed to happen just after the piston passes Top-Dead Centre (TDC - the point at which the piston stops before starting its downward stroke), so that it is already moving down. Hence knock is when the combustion happens just before TDC, the noise being the effect of the force exerted on the piston as it is still moving upwards.

Advance and retard apply to at what point Before Top Dead Centre (BTDC) that the spark fires and starts the flame propagation resulting in the complete combustion.

So if you think of the crank rotating on a single cylinder unit for ease of explanation, then the flame propagation will happen in milliseconds. So at low revs, the milliseconds relate to small amount of piston travel, in other words looking at the crank, only a few degrees of turn. But at higher revs, the same few milliseconds of flame travel now relate to a much greater movement in terms of the piston travel and therefore the arc through which the crank turns.

So to ensure the combustion completes at the appropriate point After TDC, the advance must be increased with increasing revs. If the spark firing point was held constant then at higher rpm the complete combustion would happen much later after TDC.

Bobweights and Springs

Thus, the bob-weights and springs in the distributor adjust the spark firing point accordingly, in other words on a B Series, it typically increases the spark firing point up to 34° BTDC as the revs rise. Obviously you have to set a basic or static timing at 10°, for example, which caters for the low revs and you would do this by rotating the dizzy accordingly to get the static timing set, and when you rotate the dizzy you can either retard or advance the timing as you are simply adjusting the position when the points open and close to trigger a high tension spark to the plug.

As the revs build up the static timing is advanced by the fact the centrifugal weights throw out and cause the points to move on the base plate to further advance the spark. The amount by which the advance happens as the revs rise is controlled by the weight itself, and the tension of the springs holding the bob weights.

Now, all this assumes a constant level of mixture. But you need to allow for the time when the load changes and the fuel mixture richens accordingly, as the flame travel will be affected and hence the point at which the spark fires needs adjusting. 

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs. 

Vacuum Advance

Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero. At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum) is activated by the high manifold vacuum, and adds spark advance, on top of the initial static timing setting.  The same thing occurs at cruise, the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed.

When you accelerate, the mixture is instantly enriched by the accelerator pump, burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can return to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

Vacuum advance, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions.

This all holds true for modern cars, too, but in their case, the timing is normally monitored by a position sensor on the crank pulley or flywheel that let's the ECU know where the piston is at in terms of ° BTDC. It also uses Manifold Absolute Pressure (MAP) and a Throttle Position Sensor (TPS) to look at load/fuelling needs, and with this can then set up the advance electronically with much more resolution, resulting in improved tick-over and cleaner, more efficient burn of the fuel.

Fuel & Air

Now the whole thing takes on a new meaning when you examine the way the flame travel works. 

Fuel and air mixture should burn in a steady, even fashion across the chamber, originating at the spark plug and progressing across the chamber in a three dimensional fashion, and should progress in an orderly fashion. The burn moves all the way across the chamber and, quenches against the walls and the piston crown. The burn should be complete with no remaining fuel-air mixture.

To improve the combustion process the fuel suppliers put an anti-knock additive in the fuel so that the flame is induced by the spark and travels smoothly across the chamber. Lead was the usual method but that has been replaced by better/safer substitutes. The more anti-knock properties, or in other words the higher the Research Octane Number (RON), the less the risk of pre-ignition. This then allows greater advance and higher compression ratios. Higher compression ratios create more power, and allowing the complete combustion to occur as close as possible after TDC allows more use of the burn power, rather than it happening later when the piston is already a fair portion of the way down the cylinder.  In a B series designed for 4 or 5 star leaded petrol, then the current 95 unleaded is really totally unsuitable.  The cylinder head would benefit from redesign to match the needs of today's fuels but at best most heads will be converted to unleaded specification through the fitting of new seats and valve guides.  For performance, the best option is to use Shell's V-Power at 99 Octane, or at least to use an Octane booster with standard unleaded fuel at 95 Octane.  However, for cleaner fuels, Shell V-Power and BP Ultimate (97) (note: it is possible to confuse the Ultimate pumps and put diesel in) is hard to beat meaning cleaner combustion chambers and less risk of deposits.

See also Fuels

Detonation and Pre-ignition

Detonation is the spontaneous combustion of the end-gas (remaining fuel/air mixture) in the chamber. It always occurs after normal combustion is initiated by the spark plug. The initial combustion at the spark plug is followed by a normal combustion burn. The end gas in the chamber spontaneously combusts. The key point here is that detonation occurs after you have initiated the normal combustion with the spark plug.

Pre-ignition is defined as the ignition of the mixture prior to the spark plug firing. Anytime something causes the mixture in the chamber to ignite prior to the spark plug event it is classified as pre-ignition. The two are completely different and abnormal phenomenon.

Detonation causes a very high, very sharp pressure spike in the combustion chamber but it is of a very short duration. If you look at a pressure trace of the combustion chamber process, you would see the normal burn as a normal pressure rise, then all of a sudden you would see a very sharp spike when the detonation occurred. That spike always occurs after the spark plug fires. The sharp spike in pressure creates a force in the combustion chamber. It causes the structure of the engine to ring, or resonate, much as if it were hit by a hammer. Resonance, which is characteristic of combustion detonation, occurs at about 6400 Hertz. So the pinging you hear is actually the structure of the engine reacting to the pressure spikes. This noise changes only slightly between iron and aluminium. This noise or vibration is what a knock sensor picks up. The knock sensors are tuned to 6400 hertz and they will pick up that spark knock.

The reason why pistons do not melt is because of thermal inertia and because there is a boundary layer of a few molecules thick next to the piston top. This thin layer isolates the flame and causes it to be quenched as the flame approaches this relatively cold material. That combination of actions normally protects the piston and chamber from absorbing that much heat. However, under extreme conditions the shock wave from the detonation spike can cause that boundary layer to breakdown which then lets a lot of heat transfer into those surfaces.

Engines that are detonating will tend to overheat, because the boundary layer of gas gets interrupted against the cylinder head and heat gets transferred from the combustion chamber into the cylinder head and into the coolant. So it starts to overheat. The more it overheats, the hotter the engine, the hotter the end gas, the more it wants to detonate, the more it wants to overheat.

With pre-ignition a glowing spot somewhere in the chamber is the most likely point for pre-ignition to occur. It is very conceivable that if you have something glowing, like a spark plug tip it could ignite the charge while the piston is very early in the compression stoke. For the entire compression stroke, or a great portion of it, the engine is trying to compress a hot mass of expanded gas. That obviously puts tremendous load on the engine and adds tremendous heat into its parts. Substantial damage occurs very quickly. You can't hear it because there is no rapid pressure rise. This all occurs well before the spark plug fires.

There is a situation called detonation induced pre-ignition. Detonation continues for a long period of time. The plug heats up because the pressure spikes break down the protective boundary layer of gas surrounding the electrodes. The plug temperature suddenly starts to elevate unnaturally, to the point when it becomes a glow plug and induces pre-ignition.

Run-on

Run-on is usually caused by deposits accumulating in the combustion chamber area, or very hot surfaces, such as the valves and plugs. Usually, too, after a good thrash. This is sufficient to ignite the fuel air mixture, but because it happens randomly rather than in a controlled fashion it is a very rough idling.

Unlike fuel injected cars that just simply shut off the fuel supply, the carb has a bowl of fuel that allows the fuel to still be drawn in as the engine runs on. An anti-run on valve can reduce the effect as it opens and breaks the vacuum to prevent further fuelling.

With modern fuels, too, the anti-knock properties have changed, and you are looking at RON 95 for unleaded or 97 for super or even 99 for Shell's V-Power or 99 from Tesco. The cars were designed to run on 100 (5*), so run-on is a problem with older cars to some extent unless you have a modified head that helps eliminate the problem.

Unless the timing is too retarded or advanced, causing it to run hotter than normal, then adjusting the timing will not help. However book spec on timing is usually unreliable given the discussion above on today's fuels.

What can help is increasing the spark plug gap and valve clearance by about 3 thou. This worked on my A series in a hot South African climate.

Generally though, allowing the engine to idle for about 30secs to a minute before switching off will help. In worst case, put in the clutch to load the engine and if this doesn't work, ensure the handbrake is on, and put it in 4th and let the clutch out slowly to stall it.

If you modify the engine, of course, this affects all of the above, and thus the timing needs, so with a distributor you need to play with the spring tension and bob weights to modify this. With a modern car you hook up the laptop!

The mechanical approach worked well, but with increasing demand for more fuel efficient engines with a cleaner burn the distributor and carburettor were dropped in favour of the modern fuel injectors and electronic ignition systems.

Note to Oversees readers - references to the RON apply only to the UK and do not confuse the higher value RON with the normally quoted Motor Octane Number (MON) in the US. 

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