Ignition

Making ignition  go with a BANG

Firstly some very basic electrical facts:

 If you pass a magnetic field across a wire, a voltage is formed. The faster the rate of field change, the higher the voltage generated. Also the more wires (turns) you have the higher the voltage.

 Most people know the ignition system provides the spark to ignite the fuel and air mixture. There are various factors that influence how and how well an ignition system will work for a given application.  Starting with the most basic points driven system. The low voltage side of the system comprises of the points, a condenser and the primary windings of the HT coil. Starting from the time the points close, a current starts to flow in the primary of the coil. This causes a magnetic field to build up. As this magnetic field increases, it passes across the same primary winding, generating a voltage that opposes the applied voltage, and slowing the rate of increase of magnetic field.  This is a basic feature of all electromagnetic coils and is known as reluctance. In other words it takes time to allow the coils magnetic field to reach its maximum level. The rising magnetic field from the primary also cuts the secondary windings but relatively slowly so no high voltages are generated. The final level of magnetic field is controlled by the resistance of the coil of wire in the primary and the current that can flow with a 12-volt supply.  At this point the magnetic field no longer increases and the coil is now ready to discharge.

 The charge time for the coil may only be a matter of a few milliseconds but it has a direct influence on how the ignition performs. On say a V8 or V12 engine, so many sparks are required per second at high rpm, that the coil does not have time to become full “charged”. This is where setting the dwell angle is important, as the number of degrees the points are closed for alters the amount of  “charge” time for the coil at a given RPM. An example would be a V8 running at 6000 rpm requires 400 sparks a second. That only gives the coil 2.5 Milliseconds to go through a full charge and discharge cycle. As there is only about half that time available from the dwell angle for the coil to recover, the setting in degrees becomes critical if the spark intensity is to be maintained. Some multi cylinder engines use 2 coils and 2 sets of points to overcome this problem. If the dwell is too long, this means the points are not opening fully, and may not switch the coil off correctly as the points gap arcs for longer, again leading to a poor or erratic spark.  On a twin cylinder Fiat 126 engine however this is hardly an issue only needing 1 spark per revolution, and at 6000 rpm allowing a full 10 milliseconds per charge and discharge, so the points can run quite a wide cap without the coil performance suffering.

 Now as the points open, the magnetic field collapses very rapidly as the current stops flowing in the primary. Two things happen as the magnetic field collapses across the both primary and secondary winding. The only path for the voltage generated in the primary to dissipate through is the condenser across the points. (Known as back EMF, about 300 volts). Luckily this is just what condensers are good at is passing through rapidly rising or falling voltages. The value of this condenser has a controlling factor on how quickly the coil can dissipate this energy in the primary, as the faster the collapses of the magnetic field, the greater the voltage generated. Some of the excess energy appears as the arc across the points as it looks for somewhere to go. If the value of the condenser is wrong or it is defective, it can lead to a very high voltage but brief spark, or insufficient voltage to create a spark.  This can lead to some very odd misfires or bad arcing across the points. The coil voltage does not simply then drop to zero volts but overshoots to produce a negative voltage, before swinging back up again. This is know as ringing, but has little effect once the main spark has taken place.  Meanwhile the secondary is subjected to the same collapsing magnetic field, but the high voltage generated has no where to go except the gap across the plug, so the voltage keeps rising until it can jump the spark gap. Once the plug starts to arc, its resistance drops to almost nothing, so the energy from the coil is dissipated very rapidly. Net results, a very hot, but very brief spark. Fitting high resistance HT leads slows the rate of discharge of the coil once the spark starts, so the result is a weaker but longer lasting spark.  More importantly it reduces the level of radio interference the coil produces, as the voltage spikes produced as less intense. Taken to is extreme case, with high resistance HT leads, resistive rotor arms, plug caps and plugs, most of the sparks energy is wasted in these items, leading to a feeble white spark at the plug tip, that may not be able to fire the fuel air mix. Some of the more expensive HT leads feature an extra shield of wire around the outside of the cable, that prevents the interference from escaping, so a low resistance core can still be fitted. Fitting copper HT leads dramatically improves the spark as much as any electronic ignition, but a cost to the local radio users. It may also cause rev’ counters or other electrical devices on the ignition system to behave strangely and is also probably breaking some law about radiated EMF signals.

Wasted spark systems.

This system is frequently used on motorcycle engines. It has the big advantage of removing the need for a distributor and rotor arm, with its associated problems. This system uses one coil per 2 cylinders. This coil has one primary winding plus a secondary winding with an EHT lead at each end. The spark path starts at one end of the winding, travels down to the HT lead  to one plug, through the cylinder head to the second plug, then jumps the gap in the opposite direction to the plug tip and back up the HT lead to the other end of the coil. This means if you get a break anywhere in this loop, you loose both plugs. You cannot simply split an HT lead into two, as once a plug starts to spark it goes to a very low resistance, and will take all the available energy, so the second plug will never spark. Although this twin system can  fires both plugs at once,  only one will ignite the fuel / air mixture in one cylinder as the other cylinder is on the exhaust stroke, so there is nothing to ignite. Hence the “wasted spark”. 

 Ballasted Coils.

 This is a system that makes for better starting. Normally a coil is run at battery voltage or about 12 volts. The problem occurs when you use the starter motor to turn the engine, and this then drops to about 9 volts under the massive current being drawn from the battery. This makes the spark weak just when it needs to be strong. The answer is to use a resistor and a coil that needs only 9 volts to work. In normal use the resistor drops 3 volts from the 12 available to feed the coil with its 9 volts. During starting, this resistor is bypassed so the coil gets the full battery voltage available (now 9 volts) so the spark remains strong. IF YOU RUN A BALLASTED COIL WITHOUT A RESISTOR IT WILL BURN OUT.

 Spark types

 The type of spark required will change depending on the type of engine you are running. A full race engine with a high compression ratio will need a very hot, brief spark. This is because its much harder for the spark to ignite in the high-density mixture of fuel and air, so a high voltage is needed. Then once the explosion has started, it will rapidly pass through the high compression combustion chamber, and will burn fiercely so the spark has done its job. With the modern “lean burn” type engine, the amount of fuel available to ignite is lower, so it may take longer to get the explosion started and keep it burning, so a longer spark time is required, with a lower intensity.

 Basic Electronic ignition.

 Firstly the advantages. No points to arc, get dirty or wear. Also semiconductor switches turn off the primary current very quickly and cleanly, and they do not arc like points. These leads to more power being available for the spark as the magnetic fields collapse very quickly. The units replace the points with either a photo sensor with a light chopper disk of some kind (like the Luminition units) or magnetic “hall” sensor, with a rotating magnetic disk, that then in turn switch on or off a special transistor that drives the coil. This type of system is called an inductive or inductive discharge system, as it switches only the 12 volts across the coil, and the coil induces the high voltage. A few years ago, a new system came out called CDI or capacitor discharge ignition. This used a transistor oscillator and transformer to take the 12 volts from the battery and transform it up to about 300 volts. This was then stored in a large capacitor as energy store. When it was time to spark, the 300 volts was then switched into to primary of a coil that normally took 12 volts. Bingo a huge secondary voltage and very intense spark. Problem is the capacitor discharges very quickly, so the spark is very brief, and only of use to very highly tuned engines that need a very hot but short spark. In real terms no advantage over the inductive system, but a lot more complex, and consequently are not seen much theses days. There are also various other systems like “plasma burn”. These provide a very high voltage and long spark duration, so in theory give the best of both worlds (apart from shortening the spark plug life as the tip arcs away). This can be a mixed blessing. In an ideal world the rapid spark starts the explosion, that spreads out from the spark plug tip to fill the whole combustion chamber in a smooth and controlled manner, (called the flame front) burning all the fuel as it goes. If the spark continues, this can start up secondary explosions and flame fronts. These have the effect of interfering with the first explosion, not leading to the best possible burn and power. This type of system would however help if the mixture was not optimum or a bad combustion chamber shape was preventing a good flame front developing to start with. Many claims have been made over the years to the merits of various electronic systems, but the manufactures have stuck to the basic coil and switch to ignite the mixture. (Albeit electronic switch).

 Spark timing.

 This is critical to get the maximum from the engine. There are multiple factors that influence the best setting. Firstly the spark has to ignite the mixture so the maximum part of the explosion occurs as the piston starts to descend. It requires the spark to occur before this point so the explosion has time to build to its maximum at this point.  As the piston speed increases, it still takes the same amount of time for the explosion to build, so you have to start the explosion earlier and earlier in the pistons cycle. This is the automatic advance system. For years this was nothing more than bob weights in the distributor that advanced the spark timing as the rpm increased. Crude at best, as the timing once set would not change from a standard advance curve. The timing needs to alter if you modify the engine (like high compression heads) or run different octane fuels. Judging the maximum power point takes some juggling. If the spark is too early when the piston is rising, the main explosion starts, but as the pressure in the chambers rises rapidly, this sets of spontaneous secondary explosion, as the pressure wave moves through the mixture. These are very undesirable and cause the engine to pre ignite or “pink”. This is a heard as a light rattling sound at high throttle openings at low rpm and high engine temperatures. You only get maximum compression at full throttle opening, and the piston is moving relatively slowly, so the pressure remains higher for longer. The high temperatures also  lead to the mixture being more unstable.  This is bad news for an engine as these mini explosions damage the piston crown. The timing needs to be set just before this occurs, although this point changes with throttle opening, type of fuel (High octane fuels are more stable against this pre ignition), and temperature. It’s all a bit too much for our simple bob weights, so with this system nothing more than a compromise can be reached.

Raising the heat.

 Once an engine is tuned and the compression raised new factors come into play. As the mixture is compressed further it will allow the final explosion to be hotter, more rapid and more intense, hence produce more power. This improves the engine efficiency.  BUT, if you do not improve the fuel octane rating , pre ignition becomes a problem with the standard spark timing. Many high compression engines require the spark timing to be retarded to less than optimum power to prevent pre ignition. (Ask any early Golf GTI driver when the cars had to be reset to run the low octane unleaded fuel)   Luckily Shell came to our rescue with Optimax, that has a good octane rating and can still be used in modern engines. If you add some of the octane boosters to this and levels approaching the old 5 star can be reached (100-octane approx), so high compression’s can still be used without the use advanced engine management. On the Fiat air-cooled engine the limiting factor is the rate at which the heat can be removed from the head with the fan, as you cannot fit a bigger radiator. This limits the absolute maximum to 9.5: 1.  Even then it may be necessary to back the timing off a little from standard even when running high-octane fuel. The old rule of thumb is to advance the timing with the engine hot until it “pinks” and then back it off 1 or 2 degrees. On the other side of the coin if the spark timing is late, the piston is now traveled two far down, so the mixture is now de compressing. This gives a less efficient burn, less power and the engine may run hot. Modern engine management’s overcome these variable conditions by using special “knock” sensors and electronically mapped spark timing. The basic timing is set with sensors giving information on revs, temperature and throttle opening, and then taking the timing values from a pre determined fuel map. Fine adjustment then takes place with the knock sensors. These sit in the cylinder head or block and listen for these mini explosions. When one is picked up, it automatically backs the spark timing off a fraction, but will always run at just the edge of this point. This will maximize the engines efficiency for any given octane fuel.