Does your automotive engine “knock”?

By JAY VETZEL,
Panther Airboats

Ever wondered why we use the phrase “knock” to describe the sound some engines make? Actually, it’s a very good description of the sound heard from an engine using fuel whose octane level is too low for that engine. This “knocking” sound comes from an engine that, if not given the right octane fuel, will rapidly be destroyed.

In 1912 Thomas Midgley, Jr. was determined to find the exact cause of engine knock. Using a Dobbie-McInnes Manograph, he demonstrated that the knock did not arise from pre-ignition, as previously assumed, but from a violent pressure rise after ignition. The Manograph was not suitable for further research so in 1927, Graham Edgar used two hydrocarbons that could be produced in sufficient purity and quantity and were known as “normal heptane,” available from the distillation of Jeffery Pine Oil. He combined this with an octane named 2,4,4-trimethyl pentane which was also known as iso-octane. This octane had a high anti-knock value and Edgar used the ratio as a reference fuel number.

Having decided on standard reference fuels, a wide range of engines and test conditions were available. Today, the most common number we see is the Research Octane Number(RON) and the Motor Octane Number (MON). We see these at the pump as (RON+MON)/2 and know this indicates the “antiknock index.” It remains the most important quality criteria for motorists today.

There are several fuel properties that affect knock; the most significant determinant of octane is the chemical structure of the hydrocarbons and their response to the addition of octane enhancing additives.

Other factors include: front end volatility and paraffins which are the main components in gasoline.

The octane number decreases and there is a significant reduction in octane front end volatility with improper long term storage of fuels. Fuel economy on short trips can be improved by using a more volatile fuel if the driver is willing to risk carburetor icing and increased evaporative emissions.

Many times the question is asked “Can higher octane fuels give me more power?” The answer is that with today’s modern engine and their sophisticated fuel management systems, engines can operate efficiently on fuels with a wide range of octane rating. In these engines, the engine “learns” which fuel is being used and adjusts accordingly, in most cases, if you are already using the proper octane fuel for your engine, you will already be operating at optimum settings and using a higher octane fuel will not give your motor a power increase.

So, it’s important to identify the fuel that keeps your engine on its best settings. There is not advantage in moving to a higher octane but there is a potential penalty if you get the wrong fuel and octane settings this can result in expensive engine damage.

Aviation fuels are commonly used for airboat applications and tend to be highly leaded and graded using two numbers. The typical grades of fuel are: 80/87; 100/130 and 115/145. The first number is the aviation rating (also known as lean mixture rating), the second number is the supercharged rating (also known as the rich mixture rating.)

The aviation rating is determined using the automotive motor octane test procedure, and then converting that to an aviation number. Aviation numbers below 100 are octane numbers while numbers above 100 are performance numbers.

The second avgas number, the rich mixture method performance number, is determined using a supercharged version of the CFR engine which has a fixed compression ratio. The performance number indicates the maximum “knock-free” power obtainable from a fuel when compared to iso-octane = 100.

The list of factors that an engine encounters with gasoline and gasoline additives can vary with every aspect of the environment. Engine builders are keenly aware of this and are always trying to find the “perfect” air-to-fuel ratio.

In summary, we should keep these factors in mind when we hear an engine “knocking.”

Knock occurs when the cylinder pressures are high. It is normal for an engine to have very little ”knock” at full throttle conditions because cylinder pressures are very high.

High octane gasoline burns slower than low octane gasoline. This slow burn prevents engine knock when cylinder pressures are high.

There is not advantage to be gained by using higher octane fuel if your engine runs well and does not knock on low octane gasoline. 2.2.4- Trimethylpentane (iso-octane).

Jay writes a regular column for Marsh Rider. You are invited to subscribe to get ALL his knowledgeable ramblings!

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