Gasoline engines ignite the fuel-air mixture through the spark from the spark plug. The theoretical cycle indicates that the moment to produce this spark is when the piston is at the top. That is, with the mixture fully compressed. However, this is not the case in practice, because the fuel does not fully ignite instantly. That is why there is what is known as ignition advance.
The problem of theoretical cycle that explains the operation of an engine, is that it does not take into account some considerations that physics gives us. The time that it takes time for the mixture to burn is around the 2 milliseconds. A time that may seem negligible, but that is a lot if we take into account the great speed with which things happen in the combustion chambers.
How it works
To give you a clear idea of how fast an engine moves, when it rotates at 2.000 rpm, the piston goes up and down about 33 times per second. That is to say, that your crankshaft takes around 30 thousandths of a second to complete a turn. Therefore, the piston takes about 15 thousandths to go up or down.
If that sounds fast to you, imagine how much the times are reduced if the engine revs up to 4.000 rpm, 6.000 rpm or even more. In the following video you can see the Camshafts of an engine turning from idle to 14.000 rpm (a regime typical of Formula 1):
Knowing this, the 2 thousandths it takes for fuel to burn doesn't seem like such a short time. Especially if you want to take advantage of the impulse that each explosion gives. For this reason, the ignition advance is a very important technique, which is nothing more than starting the burning of the mix when the piston it's still going up.
The ignition advance in degrees
The way an engine's ignition advance is measured is not in milliseconds. Actually, to calibrate it well It is measured in degrees of rotation made by the crankshaft.. We remind you that when the piston goes down due to the explosion, the crankshaft rotates 180º.
El ideal moment for the fuel-air mixture to finish burning is between 17º and 23º of rotation. So for this to be achieved on time, you have to advance the ignition a few degrees before the piston is at Top Dead Center (TDC). In other words, before the crankshaft is at 0º of rotation.
But…we can't overdo it either, because if we do it too early, the explosion will push the piston before reaching TDC and therefore it will try to make the engine turn in the opposite direction to which it was going. This situation where the ignition advance is too great (in degrees) is known as minced from Rods, which is shown by those characteristic metallic sounds. A very damaging effect that causes imbalances and vibrations that are very damaging to internal parts. In extreme cases, the piston may bounce downward. Something unlikely in multi-cylinder engines, due to the inertia they carry, but in one or two cylinders it can cause very serious damage.
ignition advance systems
Since the motor does not always rotate at the same speed, it requires some system that modifies the ignition advance as necessary. In the past, more or less complex elements were used to achieve this mechanically, although today everything is controlled by the car's computer. There are mainly three types of mechanisms responsible for doing this:
Electronic ignition advance
We start with him, because the current engines they work thanks to electronics and have left behind many mechanical devices that are no longer necessary. The one in charge of choosing the moment of ignition is the car's computer, which receives a multitude of data to accurately guess: the temperature, the amount of air, the speed of the engine thanks to the crankshaft sensor, etc.
Vacuum ignition advance
In a old system used to adjust the ignition advance based on the amount of air received by the engine. The less air you get, the longer it takes for the mixture to ignite, so you need to spark a little earlier to get the most out of the explosion.
It works by using the vacuum, or to be more exact depression, that is generated in the intake manifold. This depression affects sagging membrane with the pressure difference on both sides. This moves a spring which alters the spark advance calibration. In addition, it usually has two cameras, one in charge of the ignition advance and another for the delay if necessary.
Centrifugal ignition advance
This system only adjusts the ignition advance according to the revolutions that have engine at all times. The more rpm, the more you must anticipate the ignition of the spark plugs, so that the fuel finishes burning at the right time.
It works thanks to two centrifugal weights or platinums. The faster the element in which they are located is rotated, the more they move out. Its position determines how far the ignition advances in the distributor.
Consequences of delayed ignition
If the fuel-air mixture ignites late, less kinetic energy will be used of gas inflammation. To understand this, a comparison can be made with a bicycle: if we only apply force when the pedal is already very far down, we will hardly get any momentum. However, if we push the pedal when it is higher, we will get much more speed.
However, this is not the only consequence of delayed ignition. As the explosion remains the same even though we do not take advantage of it well, the generated heat is transmitted to a greater extent to the cylinder walls. In other words, getting less kinetic energy from the explosion means transferring more heat energy. What in engineering is called a less thermal performance.
Also, if the ignition delay is very pronounced, there may still be flames when they open Válvulas exhaust, which sends extra heat where it's not supposed to be. Something that can seriously damage the engine.
faults and problems
- In current engines where electronics prevails, failures in ignition advance are usually caused by a sensor malfunction that report to the computer. Whether it is the one that counts the revolutions of the crankshaft, or others that measure values related to air, temperature, etc. It can also be due to a bad programming of the switchboard supposing values have been changed factory.
- When we talk about older engines, problems may be due to failure or poor adjustment of the mechanical elements of the Ignition system. For example, him incorrect setting of the fixed advance that the dealer has. That is, before any vacuum or centrifugal ignition advance system acts. The fixed advance is the one that keeps the engine running at idle, to later be modified as required. In the electronic ignition advances, it would be a bad programming of the fixed advance.
- Breaker cam problems. That is, the cam that makes contact to send the current to the plugs is damaged, poorly adjusted or has the wrong shape. Square for four-cylinder engines, hexagonal for six, octagonal for eight, dodecagonal for 12, etc. As you can imagine, in the latter case the engine imbalance will go beyond a problem with the ignition advance. Also, even if the cam is correct and well calibrated, there may be a alteration in the distance that separates the breaker contacts.
- La chain o belt that keeps the ignition system coordinated with the rest of the engine has been stretched. Therefore, the advance is not harmonized and the motor works abnormally.
- In systems of vacuum ignition advance that maladjustment the pneumatic advance element of the membrane. This element is made of rubber, so it can end up being damaged over time.
- In systems of centrifugal ignition advance, that the two masses that we have described above do not move freely and therefore do not respond well to speed of rotation. So you have to proceed to a platinum adjustment.
ignition advance and fuel
Another peculiarity is that the fuel used affects how far the ignition can be advanced (and get more power). 98 fuels octane they have less tendency to burn, therefore, while the mixture is compressed when the piston rises, they tend less to self-detonate. Therefore, a higher ignition advance can be applied, without the fuel exploding before Top Dead Center (TDC).
Incidentally and anecdotally, E85 has an octane rating of 105, which will allow the compression ratio to be increased in future engines, but I'll explain this another day. Although that would have to go through the wide acceptance of this fuel in the market. In addition, fuels with 97 y 100 octane, instead of the traditional 95 and 98 octane. With what could be achieved greater ignition advance or other advantages, such as a higher compression ratio.
Images – Kevin Rheese, Robert Couse-Baker, Emilio Küffer, Eduardo Otubo