By ANN Columnist Eric Tucker

Two main questions come up, continually, regarding oil injection. In an attempt to answer them we will look into the function and practical use of the system. Rotax has made this system available for the type 582 and 503 for many years; however, misunderstanding and distrust of the system still is evident among users and manufacturers.

The first question is always, "Why have oil injection?" This is followed by the statement, "I what to mix my gas because I know it will have oil in the engine." The "why" needs to be explained in the light of today's modern fuels, and the needs of the engines.

On the most basic level, we can all understand the convenience of not having a fuel that needs mixing before we fuel our plane. The fuel stays separate from the oil in the fuel tank and the oil is added in a separate reservoir that can be mounted on the airframe or attached by some bracket to the engine. The pilot needs only to add fuel to the tanks and oil to the oil reservoir.

Who could possibly object?

The complaints installers have when installing the oil reservoir are twofold. On one hand, the manufacturer must make accommodation for brackets, access, and even a different cable system for the installation, in order to connect the oil injection pump and throttle and the same time. The mounting of an oil reservoir is difficult if the aircraft has no structure higher than the engine, as we would see, for example, in some high wing pusher applications. The mass of the oil tank requires that the manufacturer supply a quality mount system that can resist vibration, yet be accessible to the pilot for refilling. The oil tank must at all times be higher than the oil pump itself, since, by design, the pump cannot draw the oil in; it can only push the oil out from the pump.

Advantages:

From an engine design standpoint, the oil injection is a blessing when we consider the failures that can be avoided by the use of an oil injection pump. These would include the carbon buildup within and on all the engine components. The best feature of the pump is therefore the ability to reduce oil when the engine is at low power settings. When the engine is not producing much energy, and therefore not much heat, very little lubrication is required. The pump is variable in delivery and can reduce the oil to about a 70 to 1 ratio for idle and increase the delivery to a normal 50 to 1 for high power settings. This reduces the carbon deposits in the engine and allows a much cleaner combustion in the engine. Rings, piston crown, and even the inside of the exhaust system remain much cleaner with this variable delivery.

Ecology vs Chemistry

On should not forget, especially in the North American market, that fuels today have a great deal of measurable materials, such as alcohol, that do not readily want to mix with the oil. In the case of Ethanol, present in many fuels in North America, some oils conflict with the blended fuels, and when we pre-mix, not using the injection, the fuel system will not be mixed 100% as we would ideally like. Aside from the problems of water with alcohol enhanced fuels this issue of an incomplete mixture should be considered. Another point is the fuel deterioration, which is accelerated, by the addition of oils into our main fuel supply. When the fuel evaporates, as it does so in any vented tank system, the oil ratio continually changes to an oil rich one. This makes more carbon in the engine, and less octane in the fuel. It also means more gums, varnish, and other undesirable 'crud' can form in the fuel system from the tank to the carburetor.

Cleaner, More Positive, and Better

Injection offers us the balance of clean fuel in the system, from tank to delivery into the intake port, and an oil mixture that is optimized for the work load of the engine. Oil conflicts are not an issue since we add the oil at the last instant into the intake port, after the carburetion.

Reliability, reliability, reliability. (Remember: we're talking 2-strokes, here.)

On a related note, reliability always is a question that is part of the mistrust of the injection. This can easily be addressed by some simple facts on the number of oil injection systems in use. Let's not talk about the 3 decades of use in motorcycle applications, or the use of injection systems in other brands of engines using the same basic pumps. 

If we get really simple, and just restrict it to Rotax engines, the numbers are staggering: well into the hundreds of thousands of Rotax two-strokes have been produced with oil injection, in applications for both watercraft and snowmobile engines. Snowmobile engines, very nearly identical in design of the oil system, have used pumps on their Rotaxes for some 20 years, and watercraft models made since 1988 have all had oil pumps. 

You may argue that 'some pumps do fail,' and yes, that is possible; however, the most common cause of failure is the contamination of the oil delivery to the pump itself. This causes the pump to run dry and then fail, when an internal part seizes. As long as the oil is not contaminated and the pump has oil in it, the system will pass oil and deliver a metered amount of oil into the engine. (One important service note that should be made is that the vinyl lines that deliver the oil should be replaced whenever they become hard, or about every 2 years, whichever comes first.) Oil pumps run with no real load, they do not take any measurable power, and therefore have no heat and are immersed internally in a cool bath of oil. The result is that there's no measurable wear or service life limit to worry about.

How does it work?

The other, more technical question that is asked is how can it work? How does the pump vary the oil delivery to the engine? Let's look at the pump adjustments first.

Oil injection adjustment for idle position, 2000 rpm setting.

Oil injection for full delivery. (Any point beyond the dot gives full oil volume.)

Oil pump cam engagement to oil pump offset adjustment.

Summary

The injection pump has some requirements for installation that need be considered. The cable and the oil tank must be suitable for the aircraft design. Builders must correctly mount the tank to gravity feed the pump with a bracket system that will accommodate these requirements.

The Oil injection system provides tangible befits to the operator in that they can avoid mixed fuel contamination of the tanks and fuel system. The degradation of the fuel due to oil in the mix will be none issue, and the use of alcohol in some fuels, unavoidable in some areas, will not become a blending problem. Carbon deposits will be reduced, with the obvious benefits for the engine and maintenance intervals. Clearly the oil injection is superior, once you address the installation issues.

FMI: http://www.kodiakbs.com