the following reference may be useful

Understanding octane number evolution for enabling alternative low RON refinery streams and octane boosters as transportation fuels

Abdülvahap Çakmak

This sentence in the book is correct?

Engine knocking is favored by low fuel octane number, low engine RPM, high engine load, leaner mixtures (lower F/A ratio), high compression ratios, too much turbocharging, high ambient temperatures, in-cylinder carbon deposits, insufficient cylinder wall cooling, etc)

In this context, you can avoid knocking by using a more rich mixture (which tends to reduce in cylinder temperatures).

You can do it also by using a higher octane number fuel or an anti-knocking additive mixed in the tank.

Or you can cool a bit more the compressed air coming out of the turbocharger

Or you can address any issue of the above that can cause knocking.

So, the sentence you quote is correct but you can not take it generally out of context. A leaner running engine will also run hotter, that is why a higher octane number fuel would be required than when the engine is running richer.

Hope this helps

For evaluating possibility of knocking, flame speed and knocking reaction duration should be interpreted together. When operating rich condition, flame speed increases but knocking reaction duration increases, too. In this situation, the flame consumes quickly air-fuel mixtures without starting knocking reaction (since reaction duration is high). Mainly, when operating rich condition, the possibility of knocking is low. Therefore, lower octane can be tolerated when the engine is running rich.

The book is true :)

According to E. F. Obert, "Internal Combustion Engines" : " Maximum knock would appear to be in the vicinity of the chemically correct mixture if boost pressure were held constant" (constant boost pressure implies a constant throttle/manifold pressure). So according to this source, the statement is basically correct.

why? S.C. Sorenson

How about my explanation?

The auto-ignition theory, widely accepted for knocking, suggests that not only the flame propagation speed but also the in-cylinder temperatures influence it.

In general, at phi ~ 1.2 the mixture experiences least flame development phase, maximum flame speed and minimum flame propagation time.

As we make the mixture richer (say, from phi=0.8 to phi=1.2), the ignition lag decreases, flame speed increases and flame propagation time decreases. Beyond phi=1.2, the richer the mixture, more is the ignition period and less is the flame speed.

So, at phi~1 (strictly speaking, at phi=1.2) the flame speed would be maximum.

By this, we misinterpret that the knocking decreases because the flame speed increases when we enrich the mixture. But, the combustion of enriched mixture also yields products of higher temperatures and apparently the products are at their highest temperatures at phi=1 (strictly speaking, at phi=1.2). Therefore, though the flame speed is maximum at phi=1, the product's temperatures (maximum at phi=1) favor auto-ignition in the end gas region and hence knocking is more observed at phi=1.

Hence, highest octane number requirement is more at phi=1 (to resist knocking) than at leaner/richer mixtures.

YES Kiran Raj Bukkarapu

Fuel Octane number for gasoline fuel or SI engine's application. The Octane number refer to Ignition quality of the fuel. Normally Octane number varies from 90 to 120 sometimes minimum 95. For Stoichiometric application Octane number 95 would be enough. Two high Octane number could affect on NOx emissions.

look into value of K for antiknock index... You will understand the whole funda....

You can run lower octane fuel if you retard the ignition spark but there is not much power. Octane improvement comes by 'neutralizing' free radical products during combustion. By improving octane # you can advance the timing such that radical combustion peak occurs just after TDC. Contrariwise, you can increase the octane # if you don't want to change (advance) the timing.

Dear Steve Koelzer

Thank you for your interesting.

I wanted to ask this question without changing ignition spark time

The answer in youre qustion