Octane Numbers And Ignition Timing

Before we start talking about ignition timing and all that, it makes sense to briefly review the mechanics of a 4-stroke internal combustion engine. There's a nice animation here. Anyway, for our discussion, we will only consider the compression and combustion strokes, i.e., the piston comes up compressing the air-fuel mixture, the spark plug produces a spark, and the piston is forced down due to the pressure of the combusting air-fuel mixture.

I sometimes wondered: how does the piston come up in the cylinder and compress the air-fuel mixture anyway? Well, in a multi-cylinder engine it is one of the other cylinders firing and doing the work, but what about 1-cylinder engines? Well, once it is running, the axle will keep rotating enough to force the piston to compress the air-fuel mixture until the spark plug fires again. I think.

The engine heats up due to this combustion. This is rather obvious, but kind of important for the discussion.

Actually, the gases in the cylinder heat up because of two reasons. One is the aforementioned combustion, but there is also the thermodynamic effect of gases heating up when they are compressed. You may have noticed this when inflating a bicycle tire or a ball with a hand pump: the pump gets hot at the bottom. A refrigerator uses the same phenomenon, but then in reverse: expanding gases cool down.

We could just accept this as fact and go on, or we could dive into thermodynamics to explain this. But there is a wonderful explanation, which I stole from the late Richard Feynman, genius and brilliant educator. The explanation goes something like this. The velocity of the molecules in a gas determines the temperature (please take this on faith...). So, the faster the molecules move, the hotter the gas. Now, consider a ping pong ball. Throw it at a ping pong bat. The ball will bounce back with a certain velocity. Now, do the same thing, but throw the ball at a bat that is moving toward you (someone is swinging it). The ball will bounce back at a higher velocity. Or, consider a baseball player: there is a significance difference between bunting and "regular" hitting.

Now, consider the gas molecules bouncing off the piston in our cylinder. If the piston is moving up (compressing the gas), the molecules will bounce back with a greater velocity than they had before the bounced off the piston, since the piston is moving toward them. So, the average velocity of the gas molecules will increase, due to the movement of the piston, therefore the gas gets hotter.

Note that the temperature rise due to compression is indirectly caused by the combustion, of course, both because of one of the other cylinders, and because of the work put into the system during a previous combustion stroke. So in the end, the heating of the gas is caused by combustion.

Ignition Timing

Back to the combustion cycle. As the piston moves up and compresses the air-fuel mixture, it will reach a certain highest point. The air-fuel mixture is compressed to the maximum at this point. This point is called Top Dead Center or TDC for short. This is the ideal time for the spark plug to fire and to ignite the air-fuel mixture, since the effective work will be maximal.

Well, almost. This would be the case if the air-fuel mixture would ignite instantly. However, it takes a certain amount of time before all of the air-fuel mixture is burning and the pressure will be maximal. So, the ideal time to ignite the air-fuel mixture is a little before the piston reached TDC. The amount of this time is called spark advance, and it typically specified in degrees, one axle rotation being 360 degrees.

This is where a minor problem comes in. The time for the air-fuel mixture to combust is constant for all practical means and purposes. It is typically independent of the RPM of the engine. Unfortunately, the piston will be moving up faster if RPM increases.

So, the bottom line is that spark advance, or ignition advance, is dependent on the RPM. This is basically taken care of by the ignition timing mechanism: the higher the RPM, the more ignition timing will be advanced. In older cars it is accomplished by mechanical means, in newer cars the electronic ignition timing system takes care of it (and can do so more accurately). Fine.

It is very important to stress that all this is part of normal engine management. There is nothing special about this ignition advance, it happens in all cars, all the time.

There is something very important that is a consequence of all this.

For a given RPM, there is an optimal ignition timing. Any sooner or later, and the engine will not be performing optimally. This is the crux of the rest of my arguments.

Knock Sensors And Ignition Retardation

Modern internal combustion engines have gizmos called knock sensors. Basically these things "listen" to the engine and detect if there is any knocking going on. If so, the ignition timing is adjusted to stop the knocking.

Typically this is achieved by retarding the ignition timing. This means that the spark plug fires slightly later than the optimal moment. This causes the air-fuel mixture to reach maximum pressure when the piston is already on its way down. This causes the piston to get slightly less of a "push" than under optimal circumstances, causing the temperature in the cylinder head to drop slightly, and thus reduce the likelihood of knocking on the next cycle. This retardation continues until knocking stops.

Why not advance the timing rather than retard it, if there is an optimum ignition moment? Especially given the fact that the air-fuel mixture ignites prematurely, you would think that you should try to beat the pre-ignition by sparking before the optimum moment rather than after. Well, as it turns out, that is not a good idea. If the spark plug sparks before the optimum moment, the piston will still be moving upwards when the air-fuel mixture burns. This would be very bad for the engine. For example, it is very possible that connection rods may become bent.

This ignition retardation causes engine power to drop. This is where the following widespread (in my humble opinion) misconception comes from: If ignition retardation causes power to drop, then ignition advance must cause power to rise. So, an engine with a sophisticated Engine Management System can produce more power on high octane fuel.

I beg to disagree. Like I said, there is an optimal ignition moment. Assuming that the car manufacturer has designed the engine properly (i.e. to run without knocking, even under high load, with the recommended fuel), then there is no benefit to be had from higher octane fuel, except maybe for the minimal difference in heating value that was mentioned earlier.

In other words, an Engine Management System can only improve performance with higher octane fuel if the engine is knocking when running on the recommended grade! So, that would mean that the fuel recommendation is wrong! This seems rather unlikely, also given the fact that engine manufacturers are typically quite conservative when recommending octane numbers, i.e., its better to recommend a fuel grade that is slightly higher than necessary than to have customers ruin their engines.

So, why do the guys at the race track put 100+ octane fuel in their cars? Two possible explanations. First, they also believe the widespread myth that higher octane fuel is better. Second, they use modified engines. If you modify an engine by putting another cylinder head on, then yes, you may have increased the engine's octane requirement, and therefore a higher octane fuel is needed to prevent knocking. If you added or modified a supercharger so that the pressure and temperature in the cylinder head are higher than what the engine was designed for, the yes, again, you have increased the octane requirement.
Note that the higher performance at the race track is not caused by the higher octane fuel. The higher performance is caused by engine modifications, which then require a higher octane fuel to prevent knocking.

Bottom line is: if your engine is not knocking (which should be the case with the recommended fuel), then putting higher octane fuel in will not give you better performance. Again, with the caveat that the possible higher heating value might give a very small possible increase.

Quotes And Links

"13. What will happen if I use the wrong octane number gasoline in my vehicle?
Using a gasoline with an AKI or RON lower than that required by your vehicle will cause the engine to knock or, if the engine is equipped with a knock sensor, decrease the vehicle's power and acceleration. Using a gasoline with an AKI or RON higher than that required by your vehicle is a waste of money."
Source

"On modern engines with sophisticated engine management systems, the engine can operate efficiently on fuels of a wider range of octane ratings, but there remains an optimum octane for the engine under specific driving conditions."

"If you are already using a fuel with an octane rating slightly below the optimum, then using a higher octane fuel will cause the engine management system to move to the optimum settings, possibly resulting in both increased power and improved fuel economy."
Source

Some interesting stuff in this USA Today article

Many thanks to Ole Larsen for supplying various links.

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