How to ensure the correct nitromethane concentration for top fuel bikes

To ride a top fuel bike requires skill, experience, daring and the right fuel composition. Measuring the fuel’s density helps avoid disqualification, a burnt motor or even a serious accident due to too much or too little nitromethane in the fuel. The top fuel mixture has to be prepared anew before each drag race. With accurate measurements, drag racers like Christian Jäger determine the density of their fuel and ensure that their fuel leads them to victory and still meets the official limits and regulations.

A passion for top fuel bikes means a passion for speed

The name Christian Jäger stands for the highest class drag racing. His way to the top was accompanied by a well-chosen education and passion for motorcycles from a very young age. As a boy he already showed a keen interest in motorcycles and when he happened to watch a drag race at Hockenheim, a German town with a well-known motor racing course, the interest turned into passion.

He started off with a street bike. His next bike was a Funny Bike, a four-cylinder jet bike with a turbocharger, and then – finally – his Super Twin bike with a two-cylinder piston engine. Today he not only rides top fuel bikes, but also organizes the NitroHunter Racing Team, a German team of around 5 high-end technicians, mechanics, and engineers – and a DMA™ 35 Ex Petrol density meter to measure the density of own fuel mixtures.

Picture 1: Drag racing with top fuel bikes requires passion and time. Christian Jäger invests both with his NitroHunter Racing Team.

Constant teammate at drag races: the weather

The Fédération Internationale de Motocyclisme (FIM, Federation of International Motorcycling) is the responsible authority for global regulations on motorcycle racing. “According to them,” states Christian Jäger, “a top fuel bike has to be operated with a mixture of at least 90 % nitromethane and 10 % methanol. A nitromethane content below 90 percent corresponds to a lower racing class.” Ambient conditions like temperature affect the combustion, which is why drag racers mix their own fuel before each race.

When the bike’s engine is running, methanol (CH3OH) is burnt according to

2 CH3OH + 3 O2 à 2 CO2 + 4 H2O,

and nitromethane (NH3NO2) according to

4 CH3NO2 + 3 O2 à 4 CO2 + 6 H2O + 2 N2,

2 CH3NO2 → 2 CO + 2 H2O + H2 + N2,

the hydrogen being responsible for the “header flames” because it burns when exposed to ambient air.

During combustion, methanol consumes more oxygen from the ambient air than nitromethane. Depending on the temperature, the density of the ambient air changes, and so does the amount of oxygen molecules per air volume.

“If the ambient temperature is very high, the motor heats up. To cool it down a little I increase the percentage of nitromethane. Is the weather cool, I decrease the amount of nitromethane as the aspirated air is already cooler and contains more oxygen molecules,” states Christian Jäger. “The fuel for my bike contains, depending on the weather, between 97 % and 99 % nitromethane.”

Picture 2: Checking the fuel density of his custom mixture only takes one minute for Christian Jäger.

To check if his custom fuel has the desired concentration and meets FIM requirements, Jäger uses a DMA™ 35 Ex Petrol density meter from Anton Paar. It measures the density of fuel and, provided that an additional customer function is installed, displays the result in %w/w nitromethane or methanol. The instrument is certified for use in environments with high explosion hazards and all its components withstand the aggressive fuels. Knowing the exact composition of his fuel is also important to Christian Jäger because if the amount of nitromethane is too high or too low this can cause severe danger to bike and racer. Fluctuations in the fuel composition could lead to severe motor damage that may result, as an extreme case, in explosion of the motor.

The fuel composition is adjusted prior to each race to yield more power. If tuned more aggressively, the risk increases. Therefore, it is essential to keep the percentage of nitromethane constant.

A life insurance called DMA™ 35 Ex Petrol

Know-how, experience, caution, and a DMA™ 35 Ex Petrol: that’s what the pilot needs to create optimized and safe conditions. There are narrow margins between which tuning is possible to achieve the highest possible speed under the respective ambient conditions. The density of the fuel mixture must be controlled precisely and the measurement must be fast.

Before every race the fuel mixture has to be prepared anew. “You cannot compare the motor with the ones used in a standard car,” explains Christian Jäger. “With the bike, settings are pre-programmed before the race. Once the race has started, the settings cannot be changed any more. The bike does not have an intermediate throttle, it is either all or nothing.”

“A guy I once bought a bike from recommended the DMA™ 35 density meter to me,” remembers Christian. „And I have heard that a race colleague also uses the instrument. Plus, from time to time, the FIM takes samples before a race to make sure that the fuel composition is according to requirements. They, too, use a DMA™ 35.”

According to Christian Jäger, you do not need to be a chemistry guru to get the composition of the dragster fuel right. He considers the instrument very user-friendly and powerful, especially when compared to the formerly used hydrometer. Most of all, he likes the speed of analysis that the DMA™ 35 Ex Petrol provides. “The instrument is ready for use within seconds, the measurements are fast and the results allow me to directly deduce whether the fuel density is right,” says Christian. A measurement takes less than one minute.

Picture 3: One thing that leads Christian Jäger to victory: the perfect fuel composition, measured with a DMA™ 35 Ex Petrol density meter.

How to measure the density of fuel with DMA™ 35

In addition to the standard DMA™ 35 version, two intrinsically safe product versions – DMA™ 35 Ex and DMA™ 35 Ex Petrol – are available which are suitable for use in hazardous areas, even where flammable or explosive gases or vapors are present.

The instrument weighs less than 400 g and its backlit display makes it a useful tool for measurements directly on-site, even with insufficient light. Filling is performed easily with the built-in pump.

The fuel’s density is measured based on the oscillating U-tube technology at ambient temperature. The instrument automatically calculates and displays % concentration by using the pre-programmed tables. Up to 1024 results can be stored and downloaded at a later stage. Operating DMA™ 35 Ex Petrol is easy even when wearing protective gloves while measuring fuels directly on-site or in the lab. For quality control on fuel tanks, tags can be attached to the tank for easy recognition. Changing methods and sample IDs for tanks containing different products can be done within seconds.

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