• Jabiru's new automatic mixture control. (Jabiru Aircraft)
    Jabiru's new automatic mixture control. (Jabiru Aircraft)

Jabiru Aircraft announced last week that it has developed an automatic mixture control for its engines.

Fuel mixture controls for carburettor engines in aircraft are adjusted manually to take into account air density that varies with altitude.This process not only means using an optimal fuel-air mixture, but also reduces fuel consumption in the cruise.

In Jabiru's new system it is the air volume that is controlled rather than the fuel volume. The inlet pipe to each cylinder is tapped with a separate air supply hose. The extra volume of air admitted to each cylinder is controlled by simple valves operated by a tiny microprocessor.

The microprocessor uses data from the engine management system to calculate the phase of flight and sets the appropriate air flow to each cylinder.

The amount of air admitted is determined by a calculation that can include variables such as those on an engine management system or EFIS or attitude and heading reference system (AHRS) or other measurements such as exhaust gas temperature, manifold pressure or fuel flow.

"The mission of Jabiru is to develop and enhance affordable flying throughout the world," Jabiru's Sue Woods told Australian Flying.

"The automatic mixture control system is in keeping with this philosophy, being simple, inexpensive, but extremely effective. The automatic Mixture control does 90% of what fuel injection can do for 20% of the cost and complexity, so it ticks all the boxes for Jabiru."

Woods said the system was developed as an alternative to fuel injection, which the company sees as adding complexity.

"Over the years, we have had customer requests for fuel injection, as is expected when everything we drive on the ground is fuel injected," she said.

"However, meeting the requirements of the airworthiness standards for aircraft engines, which call for full redundancy for fuel injector systems, is extremely arduous and expensive. Even when a fuel injection system is installed, expensive calibration and tuning of the injectors is required to produce an even mixture across all cylinders.

"This path was not one that fitted the Jabiru philosophy of keeping the design simple.

"The simplicity of the automatic mixture control system comes from recognising that it is a lot easier to accurately control a large volume of air than a small volume of fuel."

According to Woods, the system can control or influence any engine variable affected by fuel-air ratio, and is failsafe, using one or more closed valves to shut off the additional air supply on electrical failure.

The system then reverts to the basic carburettor.

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