Friday, July 19, 2013

Ignition Advance and Octane rating and then some

Until the day nuclear fission can be safely done in an engine compartment, most engines still have to run on internal combustion. (aka Otto Cycle).

The petrol, being one of the 2 primary components needed for combustion, has an Octane Rating that affects the way that the engine runs. Because of the fact that it is not a constant speed engine, ignition advance is dynamically adjusted to reach the expected rpm.

The ignition advance or how much the ignition can be advanced, is largely dependent on the octane rating of the fuel used. ( and also dependent on existing condition/health of your engine )

When using a higher octane fuel, it means that the atomized air-fuel mixture in the combustion chamber can withstand a higher compression (during the compression stroke of the engine cycle) before it detonates and the combustion will cause the sudden volume expansion of gases and exerts the kinetic energy on the piston.

Between the ignition of the spark plugs and the flame front reaching and exerting the force on the piston within the combustion chamber, there is a slight lag. That slight lag is the reason for requiring to ignite the spark plug at a certain angle (degrees) before the piston reaches the top dead center of its travel. That angle required to cater for the lag, is the "ignition advance".

Longer compression stroke distance required between
ignition and detonation for higher octane petrol

Because of the higher compression(longer stroke distance between ignition and detonation) required to achieve better combustion, some might say higher octane fuels has a slow burn. On the other hand, lower octane fuel require less compression to achieve combustion and hence the explosive forces [ sudden expansion of gases from the combustion ] is slightly lesser, hence considered fast burn as the flame front reach the piston faster, however with slightly lesser power.

Video showing propagation of flame front from ignition point

With modern cars, ignition advance is solely controlled by the ECU, is tested rigorously with all kinds of conditions [ temperate, altitude, air quality, etc ] and has engine knock detection sensors that will retard the ignition timing table or even alternate different ignition tables according to the knocking(pinging) detected.

FYI, knocking occurs when the flame front hits the pistons, exerting downwards forces on it when the pistons are still in the upwards travel motion.

Older cars running spark distributors (aka dizzy) are tuned with an initial idling advance that determines the lower limit of the ignition advance.

The additional advance required (that determines the upper limit) is dependent on 2 other components.
One of which is the vacuum advance unit that varies the additional advance, based on the engine vacuum, added onto the lower limit. The vacuum advance has a maximum degree of advance upon full vacuum [ie wide open throttle].

The other ignition advance added onto the vacuum and idle(base advance timing) is by means of dead weights on the distributor spinning plate which is spring loaded. At higher rpms, the dead weights provide more advance due to the spinning inertia of those weights.

And for the "classic" dizzy(spark distributor based) cars, the only adjustable advance is the base advance as the other 2 (vacuum advance and mechanical advance) determines the higher limits and is not dynamically adjustable within its range.

With the above elaboration of octane level and ignition control, will higher octane fuel suit your car?

It all depends if the car engine controls is able to advance the timing effectively to maximize the effect of using higher octane fuel.

For modern cars, this is a definite yes. But for maximizing the longevity of the engine, the timing may not be optimized fully. This is because with too much advance, the risk of knocking/pinging runs higher as the tolerable limits of other factors affecting the combustion becomes narrower. As such, most ECU's conservatively retards 2-3 degrees from maximum allowable advance for the octane level used.

For older dizzy driven cars, the ignition advance is not dynamically correctable ( lack of knock sensor and ECU controls and mechanically driven by nature), once the base advance is set in accordance to a certain octane level, it is necessary to continue using that octane fuel until the owner decides to set the base advance manually again for higher octane.

For the classic cars, if their ignition advance timings are tuned for example to petrol of octane 95. Will there be significant difference if they are to use octane 98 fuel without changing their base timing?

By knowing now that the higher octane fuel requires a longer compression stroke for achieving the optimum combustion and that if the car ignition timing is tuned for shorter compression stroke for 95 octane, it can be understood that by using 98 octane fuel in a classic car tuned for 95 octane, the combustion will not be optimal, resulting in the full flame front reaching the pistons past the most effective position and as such, some of the energy transferred is wasted in the power stroke.

And now if the car engine encounters pinging/knocking, most will recommend a higher octane fuel as remedy. This is often the most valuable advice. By using a higher octane fuel, you prevent the flame front hitting the piston too early in its upward travel during the compression stroke. Pinging usually occurs when the flame front reaches the pistons too early. It may be due to the ignition timing itself or due to the heat buildup within the chambers that causes fuel pre-detonation.

What about the myths about octane boosters in small 50ml bottles?

Some products may be truly efficient in their claims in improving the octane level of the fuel in your fuel tank.

But to condense high octane compound into 50ml bottles to be diluted into more than 50 liters lower octane fuel and claiming to increase the octane level by at least 2-3 octane ratings, seems a bit far fetched, considering the costs of such magic potion.

Using them may reduce pinging or even prevent the pinging from happening altogether if the engine was previously encountering pinging constantly. But the true effect of these "potions" may be merely slower burning fuel grade contaminants that effectively slows down the burn to achieve the similar effect of requiring a longer compression stroke before the flame front hits the pistons. But with that contaminant, the combustion may produce lesser power [ lesser explosive expansion of gases ].

Using them "octane boosters", it may be possible to hide the symptoms of pinging/knocking, but if we were to compare current prices of 95 and 98 octane rating fuel in Singapore.....

Fuel prices in Singapore on 19 July 2013

A full tank of 60 liters of octane 98 vs octane 95, the costs is less than SGD$6 for a full tank.

To purchase a 50ml magic potion that "claims" to increase octane 95 to become octane 98, I doubt it will cost you less than SGD$6.

And if your engine pings even with octane 98 fuel, it is strongly recommended to get your ignition timing and individual cylinder compression checked, rather than spending additional $$$ on magic potions at every refill of your fuel.

For more about ignition timings, a past article can be found here.

A video in YouTube showing and explaining octane levels and compression [ elaboration ]