Jabiru Power

The automatic mixture control

To date, those aircraft engines not employing fuel injection have used carburettors, primarily due to their simplicity and reliability. Carburettors are tasked with supplying the correct ratio of atomised fuel-to-air to the engine for subsequent ignition within the engine’s cylinders. However, carburettors are not ideally suited to compensating for changes in air density, meaning that as the atmospheric conditions change the engine will run leaner or richer than is ideal. A carburettor also cannot control how equally the fuel is divided between the individual cylinders; this is affected by the idiosyncrasies of each induction system. These defi ciencies can lead to a number of issues depending on whether the mixture is too rich, or too lean, ranging from engine roughness, fouled spark plugs and high fuel consumption for an over-rich mixture, to overheating and lack of power if over-lean.

In an effort to address these issues, Jabiru engineer Dan Mulder has developed an automatic mixture control system in association with its Gen4 engine. The aim of this system is to improve the mixture provided by the carburettor to each individual cylinder or overall. The fundamental logic of the system is that it is more straightforward to accurately regulate the large volume of air than the minute amount of fuel that comprises the fuelto- air ratio. Towards this goal, the system admits additional air to the inlet pipe on each cylinder. The air can be fi ltered or unfi ltered and come via carburettor heat or bypass carburettor heat. The amount of air is controlled automatically with a valve for each cylinder.

The valves are controlled with a microprocessor. The microprocessor estimates the fl ight phase such as taxi, takeoff, climb, cruise and descent using data received from engine management systems.

‘Jabiru has continued to move ahead with engine development in the form of the Gen4’

The system can control or infl uence any engine variable affected by fuel-toair ratio, such as EGT, CHT, fuel fl ow, fuel economy etc. It is also failsafe, using one or more fail-closed valves to shut off the additional air supply on electrical failure, reverting back to the basic carburettor.

Many have asked why Jabiru Aircraft didn’t pursue a fuel injected engine as opposed to inventing the new automatic mixture control system. The company explains that the answer stems from a number of considerations. Simply put, the new system can achieve 90 per cent of a fuel injection system for about 20 per cent of the cost. But there’s more to it than that, and it relates to design standards.

Pivotal is the fact that the automatic mixture control is not the engine’s primary fuel system, so in the event of system failure it reverts, as previously mentioned, to a basic carburettor. By comparison, design standards dictate a fuel injection system would need to be fully redundant or possess a carburettor as a back-up – the latter option effectively calling for a second fuel system. Certifi cation of both of these options extends beyond the fi nancial reach of a company that is the size of Jabiru Aircraft. Hence, Jabiru has again opted for the simplest path.

It is early days in the development of the automatic mixture control but its potential is attracting a lot of attention.

The Gen4 engine

Jabiru Aircraft has continued to move ahead with engine development in the form of its Gen4 engine, with a large number of the engine components manufactured by a network of companies throughout Australia. To date, they have produced 150 of their new Gen4 engines, with 50 delivered in Australia and the remaining hundred distributed across sixteen countries around the world.

The Gen4 was certifi ed in March 2017 to the ASTM International Light Sport Aircraft (LSA) standard F2339 – an international technical standard for the Design and Manufacture of Reciprocating Engines for LSA.

The qualifying test for the engine was the ‘accelerated overhaul test’.

This involved an engine being run for 200 hours, with 83 hours at full power (3,300rpm) and the remainder at cruise power (2,850rpm), while maintaining at least one cylinder-head operating at red line temperatures for the entire test.

Furthermore, the process called for the engine to be cycled through the start sequence, cruise, and maximum power settings at regular intervals. Following this, the engine was stripped and inspected with a requirement that the engine wear did not exceed the limits expected after 1,000 hours as specifi ed in the overhaul manual This Gen4 engine was conceived under Jabiru’s trademark philosophy of safety and simplicity and, with that purpose, it has incorporated a number of new features. Firstly, with the average recreational aircraft owner only flying 30 to 40 hours a year, dormant periods on the ground leave the engine susceptible to rust in the steel barrels located within the cylinders. Consequently, the cylinder heads are cast aluminium and screwed to aluminium cylinders to form a one-piece assembly. Also, there are no cylinder head bolts that are needed to be re-torqued which reduces maintenance. The cylinder barrels are electrolytically coated with silicone carbide (ceramic) in a nickel matrix.

This material is very hard wearing and has low friction qualities and the aluminium cylinders dissipate heat at three times the rate of the previous steel barrels.

The new engine fits into most Jabiru models with new air-cooling ducts, which are included with the engine.

Reducing weight and maintenance requirements, the propeller remains directly crankshaft driven and does not use a reduction gearbox (the same as previous Jabiru engine generations). Furthermore, the engines are supplied with a pair of cooling ducts which have been designed to draw air directly from the propwash to critical sections of the engine, notably the cylinder heads and barrels.

The ‘JabTwin’

First considered in 2012, the concept of a twin-engined Jabiru was not immediately fathomable.

To mount an engine on each wing would not only call for signifi cant design and construction changes but, ergonomically, access and egress via the cabin doors would be virtually impossible. Undeterred, the concept began to become a form and rather than mounting the engines on the wings, the design called for the two powerplants to be mounted on the nose upon a canard.

The simplest means of achieving the confi guration was to retain the airframe of a Jabiru J430, aft of the fi rewall. An aluminium and epoxyglass composite engine mounting platform then spans the width of the airframe, through where the cowling would be situated in a singleengined version. At each end, an 80 horsepower Jabiru 2200 engine would then be mounted. In this form, the prototype fi rst fl ew in South Africa in August 2013.

With the engines close to the longitudinal axis, or centreline, one can conceive that the asymmetric yaw in the event of an engine failure would be relatively minor. However, in early test fl ights, it was discovered that at high angles of attack, the canard generated lift, pitching the nose further up. Consequently, a section was cut out of each side of the canard to counter this lift and address the issue.

Since then, the JabTwin has continued to be a ‘work in progress’ with a redesign of the engine mounting arrangement and the cowlings just a sample of the changes.

Still, there is a lot to do on the road ahead including load testing of the engine mount, fabricating and fi tting both cowlings and fi tting the engines.

New fairings will need to be built and engine cooling re-examined before the aircraft is weighed and balanced and ready to test fl y in its new form.

It may be some time until that fl ight takes place but there is no doubt that it will generate tremendous interest in General Aviation circles when it does.

An ongoing process

Jabiru Aircraft has made a substantial contribution to aviation, not merely in Australia, but around the globe.

Across three decades, there have been awards and accolades for Jabiru in an industry where many have fallen by the wayside. Even so, the company continues to refi ne its existing aircraft and engines with one eye to the future.

Whether it is inventing an automatic mixture control, enhancing its existing engine stable or introducing a new Twin, it is done through a philosophy of simplicity and many hours of research, development and construction.

The past 30 years have been full of milestones and, with engineers like Dan Mulder contributing, the years ahead look just as bright.