Stirring the Pot on Fuel (again!)

[jluetjen]

The October Edition of "Race Engine Technology" had an excellent (and quite readable) article on race fuels. This is the sister publication of Race Tech Magazine -- admittedly a bit pricey, but the amount of information that they provide is great. The article was writen by John Coxon of Independent Powertrain Projects along with William Betts of PetroScience Ltd (suppliers of fuel to F1), and I'll just hit the highlights here. For those people who take this sort of stuff seriously, I believe that the magazines are worth the price of the subscription.

Background: Crude oil is composed of a large range of hydrocarbons with a wide range of boiling points. About 50 different hydrocarbons (aka: cuts) defined by very narrow boiling points are used in the average race fuel.

In Europe street fuels are controlled by EU directives. In many cases the race authorities dictate that race fuels have to pass the same standards as street fuels. But that doesn't mean that they're the same.

Energy Value:
The components of fuel with the highest energy values (per kg) are found in shorter chain alkanes (paraffins) and alkenes (olefins) -- which make up a major component of race fuels.

Ratio of Products to Reactants:
Fuel designers also consider the stoichiometry of the air fuel mixture and the ratio of the products of combustion to the reactants. Put simply, the amount of air necessary to burn exactly our given mass of fuel and the ratio of the space that would be ocupied by the final burned gases to that of our air-fuel mixture. Assuming that an engine burns all of the mixture that it ingests, the heat released will be greater the more fuel that is put in. A fuel with a lower stoichiometric air-fuel ratio will consequently release more heat per unit mass of air. (Of course performance can be increased further by enriching beyond stoichiometry to a degree). Are you still with me??? So if the ratio of products to reactants for fuel B is greater then fuel A, then these are going to apply a greater pressure in the combustion chamber and hence produce more power. As a result, any fuel designed for racing will almost certainly have a stoichiometric ratio different to pump fuels. It is therefore imperative when using a new fuel to recalibrate both fuelling and ignition characteristics to compensate.

Octane:
This is been covered at length in other threads, but some key points discussed includes the "sensitivity" of a fuel, which is the difference between the RON and MON (RON-MON). Most road fuels have a sensitivity of around 10. Purposed designed race fuels have sensitivities much lower then this. The other key point is octane distribution. Remember that gas is a mixture of different components with different boiling points. The lighter parts of the fuel (with the lowest boiling points) may also have the lowest octane then the heavier components. Under certain transient circumstances (ie. a sudden throttle opening), the lighter ends will be swept into the furthest reaches of the combustion chamber (think of an early 2.0S combustion chamber) first by the air stream. If the octane is insufficient, then it is possible for the engine to run into transient knock until the heavier components catch up. While not necessarily damaging to the engine, it will be felt as poor pick-up of the throttle. This is especially true of engines with carbs.
Also engines running at high speed can have fatal detonation in a very short amount of time. Retarding the ignition helps, but once detonation starts, you have to drastically retard the ignition to stop it which costs HP.

Speed of Burn:
As the speed of an engine increases, there is less time for the mixture to burn. For an engine to produce peak performance, the peak cylinder pressures should occur around 10 degrees ATDC. Anything after this will result in reduced cylinder pressures. Fuels with a high burn rate require less ignition advance which reduces the "negative work" on the piston and greater effect on the power stroke. It's not just a high flame speed that counts, but a consistent flame speed which is where race fuels outperform street fuels. Pump gas varies more drastically from supplier to supplier and batch to batch then race fuels -- especially when you consider the difference between summer and winter formulations of pump fuel. A fast and more consistent burn rate also results in less heat being rejected to the coolent system and exhaust. Generally fast burning fuels contain more olefins and lighter naphthenes.

The cooling effect an volatility:
The individual boiling points of the up to 200 components which define the fuel and it's octane matter. For a typical pump fuel, the boiling points will range from ambient up to 215 degrees C. For race fuels the boiling points will range from 20-35 degrees C up to (and often less then) 180 degrees C. Since it is not the gasoline liquid that burns, but the vapor, as it evaporates it also cools the incoming charge which becomes denser and offsets the air displaced by the evaporating fuel. This increases the density and the VE of the engine. Altering the mixture of the components (each of which have their own boiling point remember) can often have a significant impact on the VE of the engine. If you were to plot the percentage of the total fuel evaporated against the temperature, you'll have a graph called the distillation curve. In a single component fuel, the curve will be essentially a straight horizontal line. In a fuel with a large number of components, the curve will be "S" shaped. If the curve is displaced downward on the left (low temperature side), the fuel will have good cold start properties, but poor drivability under hot conditions due to vapor lock in the lines. On the right side of the graph, components with a high boiling point will increase fuel economy, but increase deposits within the engine. Generally race fuels are blended for specific applications and thus have tighter specification limits, unlike street fuels. So a street fuel with a RON specified as 97 may in fact have batches that range from 98.6 to 98.2. Both are legal, but the second would kill a race motor tuned to the 98.6 RON. Race fuels also tend to have tighter control of the specific gravity, year after year.

My Conclusions:
Race fuels are a very specific tool in the engineer's "tool chest". Small changes can have a drastic affect on a tightly tuned race engine. In general race fuels are overkill for a street engine, and may reduce mileage and cold temperature running since these are outside the range of conditions that race fuels are designed for. Much like a close ratio gear box, using race fuel can make a big difference if the rest of the system is optimised around the specifics of the fuel. The flip side is that it's much easier to find yourself outside the ideal conditions if you are not careful.


Enjoy your next fill-up!

[dhoward]

In the circle of people I used to ride dirt bikes with, there was a certain 'chemist' from the Chicago area who would blend specific fuels for use based on a particular day's meteorlogical parameters.
Very knowlegeable.
Plus, we burned the daily spoils in the campfire that night.

Mmmmm
Beer and dangerous campfires....

[jpnovak]

Great read. Some great chemistry in there. If you could ID the fractions of petroleum that come off and know how much branching each molecule has you could easily come up with the stoichiometric ratios. Would be some fun experimentation to find out what works best. Of course, would be even better to take advantage of the drivability from someone else's work.