Fuel

[riff_raff]

Engine control systems are very sophisticated now. Current 32-bit ECU's can modify engine operating parameters on a cylinder-by-cylinder and cycle-by-cycle basis.

How an engine is operated is also very important. Take for example, the Toyota Prius engine. It has a compression ratio of 13.0:1, which is normally only seen in race engines running on 105 octane race fuel. Yet the Prius happily accepts 87 octane pump gas. Why? Because the Prius combustion engine, with its electric motor assist, is never required to operate under conditions where it might encounter detonation. So 87 octane fuel is OK.

[West]

Rev Head:

The "octane" rating of gasoline is a relative value of it's resistance to detonation when run in an SI engine. There are two common methods of establishing octane rating with gasoline: the "research" method (RON) and the "motoring" method (MON). The research method involves igniting the fuel in a combustion bomb, under lab conditions. The motor method involves running the fuel sample thru a variable compression engine, and noting where it begins to detonate.

The rating scale is based on an octane fuel sample as being 100, and a heptane fuel sample as being 0.

The amount of power an engine produces from a particular fuel is a result of the theoretical LHV (Lower Heating Value) of the fuel and the BTE (Brake Thermal Efficiency) of the engine. An SI (spark ignited) engine only needs a fuel with an octane rating high enough to prevent detonation, under all operating conditions. Using a fuel with a higher octane rating does not produce additional power.

Regards,
Terry

My professor discovered the (R + M)/2 method, Professor Samuelsen. They said the "Samuelsen number" was too long a name.

How does higher octane prevent knock? I haven't studied combustions in a while.

[manchild]

Fuel with higher octane number has better resistance to self ignition under high pressures and temperatures while "knock" is a detonation - self ignition of fuel caused buy previously mentioned conditions before it was supposed to be ignited by ignition system.

[riff_raff]

"How does higher octane prevent knock? I haven't studied combustions in a while."

The combustion kinetics occuring in an SI engine's combustion chamber are incredibly complex and I doubt that anyone even remotely understands the process.

When you use the term fuel, I assume you mean gasoline. Unfortunately, there is no established (chemical) definition for gasoline. Gasoline can be composed of various organic molecules, as opposed to fuels such as methanol (CH3OH) or ethanol (CH3CH2OH) which have a fixed chemical definition. Thus the need for a performance rating, like octane number, for gasolines.

The octane rating of gasoline can be improved thru additives. The most common additive used to be tetraethyl lead (TEL), but it caused problems due to toxicity. Benzene and tolulene also work very well, but they too are very toxic.

As a side note, Honda ran an 85% tolulene race fuel in their 1.5L turbo F1 engines back in the 80's.

[manchild]

...As a side note, Honda ran an 85% tolulene race fuel in their 1.5L turbo F1 engines back in the 80's.

As much as it can be found on the web in 1983 BMW used fuel that was originaly invented for rockets launched on London from Peenemunde during WWII.