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Understanding Ignition timing

Copyright Liam Venter, FastBikeGear.co.nz, 2013

(You need a beer now!)
Many people think that simply running the maximum advance you can run before you get pre-ignition or detonation will give you the maximum power. This is not correct.

Probably the best way to explain ignition timing is tell you a bit of a story. Recently I got asked by an experienced motorcycle dyno operator why I had twin sparked the FastBikeGear project Ducati 900SS? His belief is that twin sparking wouldn't provide a performance advantage. But here's the theory for doing it. A bit of a brain dump really. Hopefully it will stimulate your thinking.

Most twin spark Ducatis fitted with twin spark heads fire both spark plugs in the cylinder head at the same time with the primary aim of increasing performance. Other manufacturers sequentially fire each of the plugs in their twin spark heads one after the other. Their primary aim is to improve emissions – not improve performance!

A petrol engine can be thought of as just a self powered air pump. The pump can be optimised (tuned) for economy or performance. There are many ways to improve the power output of a pump, especially if we are willing to compromise on fuel efficiency, service intervals and reliability.

I will go out on a limb here and state there are only four ways to increase engine performance:

  1. Reduce engine losses (including pumping, 'inertial' and thermal losses)

  2. Increase the number of air and fuel molecules you burn in the cylinder.

  3. Increase the speed of the controlled burn in the cylinder.

  4. Improve the timing of peak combustion pressure.


Every performance advantage I can think of fits into one of the above four categories.

A mixture that burns (not explodes!) at the fastest rate produces the maximum power!
Mixing: Burn speed is dependent on several factors including how well the air and fuel is mixed on it's way into the cylinder and within the cylinder.

Air/Fuel ratio: An air-fuel-ratio mixture of between 12.5 to 13.5 burns fastest. A mixture either leaner or richer than this burns slower.

Charge Density: Burn speed is also dependent on how many air and fuel molecules are packed together in the combustion chamber. The closer they are packed together in the same volume, (charge density) the easier it is for the fire to jump from one set of molecules to the other and burn faster.

There are many ways to improve air fuel
mixing, optimise the air/fuel ratio and improve the charge density, including the following: increase valve sizes, increase valve overlap, increase valve lift, increase the size of the inlet and exhaust ports, increase the number of valves, optimise the location of the spark plugs, use multiple spark plugs, optimise the air/fuel ratio, install a tuned exhaust system, increase compression. ALL of these techniques increase the the burn speed and consequently provide an increase in performance.

On a bike with carburettors, high load conditions like opening up the throttle in a high gear while going up a hill can can also increase the charge density.

Timing ignition for power and minimising pumping losses.
Due to the geometric relationship between items such as pistons, conrods and cranks, to develop maximum power, starting the fire (ignition) needs to be timed so that the peak combustion pressure occurs somewhere between 15 and 20 degrees ATDC. To achieve this the fire must be started by sometime BTDC (Before Top Dead Centre).
As the piston is coming up on the firing stroke the mixture is ignited. This causes some extra increase in pressure within the cylinder over and above that caused by compression due to piston movement alone. This extra compression causes pumping losses as the inertia of the engine must fight the force of this increased pressure trying to reverse its direction. Which results in decreased horsepower. This loss in power is a referred to as a pumping loss.

A starter motor must also fight this increased compression when trying to turn over an engine. To make the starter motor's job easier, modern ignition systems usually retard the timing at revs below idle speed.

The heat build up problem.
Heat build up within the combustion chamber increases the likely hood of pre-ignition and detonation issues. Remember a mixture that burns (not explodes!) at the fastest rate produces the maximum power. Pre ignition is when the fuel air mixture explodes at some point before the spark plug has fired. Post ignition detonation normally just referred to as 'detonation' is when the fuel/air mix explodes at some point after the spark plug has fired. Unfortunately the onset of pre-ignition and detonation cause dramatic and rapid temperature increase inside the combustion chamber which can become self sustaining. Pre ignition and detonation explosions can quickly damage or destroy an engine

Unfortunately ALL
of the things we do to increase performance by increasing charge density, also increases heat build up within the combustion chamber. (Higher charge density means that we have a greater number of molecules burning, which means that we have more energy being released.) Uneven mixing of fuel and air also increase the likelihood of pre-ignition and detonation.

There are several strategies to counter this problem including:

  • Optimizing squish bands to improve the fuel air mixing and helps counter unevern fuel/air mixing. There seems to be near universal agreement that 40" or 1 mm is the ideal squish gap to achieve this for a number of complex reasons.

  • Conduct the heat away from the cylinder head using richer than optimal fuel air mixtures. Because petrol has about twice the specific heat (heat absorption ) capacity of air, richer fuel mixtures allow more energy to absorbed and not transmitted to the cylinder walls pistons and exhaust valves etc. Unfortunately using sub optimal fuel air mixtures also decreases our burn speed which means that we also loose performance!

  • Conduct heat away using water cooling and improved oil cooling.

  • Reduce friction using slipper pistons and special low friction coatings.

  • Retard the ignition timing to reduce temperature.

As mentioned the need to starting the fire BTDC so that the maximum combustion pressure occurs somewhere between 15 and 20 degrees ATDC. Means that there is a fight between forces as the engines momentum drives the piston up to towards Top Dead Centre and the already ignited mixture tries to expand. This fight between opposing energies is a cause of surprisingly large temperature increases. One method of reducing heat build up to avoid avoid pre-ignition and detonation is retarding the ignition timing. Unfortunately we often need to retard the timing to such an extent that the peak combustion pressure occurs after the ideal geometric power transfer time (15 and 20 degrees ATDC.)

Kicking the horse after it has bolted.
If we retard the timing too much to decrease pumping losses and prevent heat build up, our maximum combustion pressure occurs somewhere later than our ideal of 15-20 degrees ATDC. It is like trying to kick the horse after it has bolted – we loose horsepower.

Because richer fuel ratios, retarded timing, and improved cooling have inherent performance trade offs, achieving maximum performanceby balancing these variables is like trying to solve simultaneous equations.

However using a twin spark set up allows less advance to be used and still have maximum combustion pressure arrive at between 15 and 20 degrees ATDC for the simple reason it reduces the distance the flame front has to travel from each spark plug in order to set fire to all of the mixture. There is no performance trade off to using twin spark heads. In engines with a greater distance between the plug and furthest point of mixture (e.g large bore engines, engines with high domed pistons, or two valve engines with non centrally positioned spark plugs ) converting to a twin spark set up is even more of an advantage.

Complicating matters
As noted earlier a bikes maximum performance ignition requirement at any given revs is dependent on the burn speed, which is faster at higher charge densities. Typically charge density will be higher at greater throttle openings. This is why you are far more likely to have detonation at full throttle at low revs than at partial throttle at the same revs. Unfortunately this means on bikes fitted with crude ignition systems often have to run far less ignition advance at any given revs than would be ideal for maximum power (and torque) at partial open throttle. For this reason all modern bikes have some way of varying the ignition advance not only for different revs but also for different charge densities. The easiest way to this is to use a Throttle Position Sensor (TPS).

Using a Throttle Position Sensor to modify the ignition advance means you can optimise when you ignite the mixture to maximise performance at different throttle openings. Some bikes will run as much as 10 degrees more advance on half throttle than they do at full throttle at 3000 rpm for example. Installing a Throttle Position Sensor and an ignition system that can vary the advance according to throttle position, typically provides very noticeable gains in partial open throttle performance. The bikes performance will also be much smoother. From first hand experience I can advise that installing a TPS and an Ignitech ignition system to dynamically modify the ignition curve on my own Ducati 900SS has made an incredible difference to the bikes performance when rolling on the throttle coming out of corners!

See I warned you at the start of this appendix that you needed a beer!

If you want to know more on the subject of maximising ignition timing for performance I recommend having a look at these two excellent resources:

http://www.vehicular.isy.liu.se/en/Publications/PhD/99_PhD_580_LE.pdf (Pages 27-38)

http://www.gonefcon.com/whatifdyno/combustion.htm

 

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