EndTuning>Resources>Ignition, Advance and Detonation

The Ignition systems are one of the most important factors in engine tuning, so we'll spend quite a bit of time on this. From what you will already know from Engine Basics, the spark plug initiates the burn that ultimately drives the engine.

Advance - In the rotation of an engine, positions are recorded in Degrees. A full cycle for a four stroke engine is 720°, which is as follows-
0° - Known as TDC (Top Dead Centre), Piston #1 will be at the highest point, and starts moving down. The Inlet valve opens and allows the air / fuel mix to be sucked in.
180° - Having reaching the far end of it's stroke, the piston starts to move upwards to begin compression. For the start of this, the inlet valve is still open, utilising a fluid momentum technique described in Intake Systems.
360° - Once again, the piston is at TDC, but this time, it is the end of the compression stroke. This would be the "historic" ignition point, and the piston would start to move down on the power stroke.
540° - The end of the Power stoke. and at BDC (Bottom Dead Centre). This 180° is where all the power comes from (from this particular cylinder). It now returns to TDC on the final Exhaust stroke
720° - The piston arrives back at TDC ready to start again.

As mentioned at 360°, when the piston is at TDC, this would represent the point to start ignition, but in practice, more torque is made by starting the burn earlier.

Combustion - One Major point to consider is that the Air / Fuel mix does not explode, but burns in a controlled wave from the ignition point out. The advancing burn is called the Flame front and it's speed is usually a steady factor in the combustion cycle. The expanding gasses push the piston down, and this turns the crankshaft in a similar way to pedaling a bicycle.

To get the most work out of the expanding gasses, you need to get the timing right. Going back to the bicycle idea, if the pedal was a 12 o'clock, you'd be able to balance yourself on top of it, and all your force won't create any movement. If the pedal was at 3 o'clock, you'd get the most possible leverage on it, and ride off into the sunset.
By now, you might be thinking that the ideals are starting to contradict each other. The point of maximum compression (0°) is 90° away from the angle you want to get the most efficiency (the 3 o'clock position) Not only that, but in actual fact the most efficient point to have your maximum cylinder pressure (most weight on the pedal) is at about 14°. This is due to only having 180° to get your gasses to do any work, maximising at 90° would be wasting energy, and maths and science show that for a crankshaft with a gasses expanding on top, the piston moving down and increasing the volume all the time, you get the most work done by having maximum pressure at 14°. Although you won't get the peak pressure at the 90° sweet spot, you get the best average pressures, and therefore the best use of the bang.

It's all about......Timing - So where are we now? We now know that we want to get the peak pressure at the 14° mark, and the fuel mix takes time to burn. This is why we have ignition timing, and ignition advance. At a low engine speed, you can quite possibly start the burn at TDC, the pressures increase, the piston moves, and by the time we get to 14° we have the peak pressure. As the engine speed increases, that 14° sweet spot comes round quicker and quicker, the fuel mix still takes the same time to burn. You'll need to start the burn earlier, so the timing becomes advanced. Most engine spend most of their time advanced at between 15-30° which, if you've been keeping up, you'll realise you are burning and expanding gasses when the piston is still trying to compress the charge. Who ever said the internal combustion engine was simple? Factors are starting to add up now, so we'll look at Detonation and Pre-ignition next

Petrol is quite dangerous. It's volatile, likes to form flammable vapour clouds and burns releasing a lot of energy. Occasionally inside an engine, it will ignite by itself and explode. This is Detonation. Detonation can cause a number of problems, the shock of the explosion can chip metal off pistons and make the familiar ringing noise that people call pinging. The temperatures can be far greater, expending the piston into the bore and causing excessive wear, and you can end up with holes melted through pistons. Pre-Ignition is a different creature all together.

Pre Ignition - As the name suggests, Pre Ignition is when the fuel air mix ignites before the sparkplug fires. It's name is well known, but it is infact very rare to occur. Pre Ignition needs another heat source to initiate the burn, often from carbon buildup that becomes glowing hot. A common myth is that it can occur from a high compression engine crushing the mix and it ignites a few degrees before the Ignition Angle (the point measured in engine degrees where the spark plug is fired).

What actually happens is the mix ignites way before this point. The fresh air/fuel mix being sucked in comes into contact with a burning carbon ember at it's hottest (just after the last charge has left the cylinder) and the burn starts. The gasses have already expanded, and the engine now tries to compress a cylinder that is already under a lot of pressure. This causes some real damage, the piston's connecting rods (con rods) can be bent, the cylinder head gasket can get pushed out and at the very least, you'll be requiring a lot of engine work.

There isn't a corresponding large explosion like in a detonation event, you won't hear anything unusual, and the first you know about it is when a con rod decides it's had enough and exits the side of the engine block.

Detonation - This is an uncontrolled burn inside the cylinder. The fuel mix inside the engine does not explode, but burns in a wave starting from the spark plug, and expanding out into the now compressed volume of the cylinder. If the Ignition event is too early, the pressure wave from the expanding flame front of the controlled burn will compress the rest of the mix, increasing it's temperature (from the effect of the increased pressure) to above it's auto-ignition temperature. A chain reaction of burning air/fuel increases the pressure further, and the remaining gasses explode.
It is the supersonic shockwave compressing, heating and auto igniting the gasses that changes the event from a controlled burn, or deflagration into an explosion.

The exploding gasses scrub the inside of the cylinder, breaking away a cooler layer of gasses called the boundary layer which would normally insulate the cylinder walls from a large proportion of the heat. The temperatures can be over 2000°C which will melt the steel components, increase the load on the cooling system, which will leave temperatures higher for the next cycle, and further increase the chances of detonation again. The shockwave can shatter the spark plug's ceramics, and vapourise surfaces leading to pitting, and finally a complete melt through of the piston head.
The increase of temperature can then also provide a pre-ignition point at a sharp edge, or at the tip of the spark plug.