The use of Auto Gas in Radial Aircraft Engines

//The use of Auto Gas in Radial Aircraft Engines

The use of Auto Gas in Radial Aircraft Engines

Q.A. Manager Covington Engines

We at Covington Aircraft Engines, Inc. have been in the radial aircraft engine overhaul business since 1972. We have been supporting the Aviation Industry with technical support in the form of engine overhaul, repair and maintenance, parts sales, and engine operation and service recommendations for twenty-five years. During this time span we have witnessed the changes to the oils and fuels available to the industry with varying degrees of approval or dismay. Approval if the changes brought on improved circumstances, dismay if circumstances worsened. One change that has resulted in considerably worsened circumstances, ever since it was first introduced, was the elimination of tetraethyl lead from gasoline’s available to the aviation community. The engines we work on were designed to operate on leaded aviation gasoline (approx. 3.5 grams/gal). The lead provided a cushion for the exhaust valves and seats, which operate at extremely high temperatures, in air-cooled, radial aircraft engines. The removal of this lead cushion in engines operated on auto gas has resulted in wear problems in many engines / cylinders.

This unusual wear (and related problems) has been consistently documented at our facility through teardown findings, malfunction and defect reports to the F.A.A., etc. These findings are made known to each customer through teardown reports following overhaul / repairs. Aviation gasoline presently contains around 2.006 milliliters of tetraethyl lead per gallon of fuel. This provides sufficient cushion to the valves and seats of these engines in all but the leanest operating conditions. Automotive fuel, on the other hand, provides no cushion at all. This alone is not the only reason for concern on the differences between auto and aviation fuel, another major difference is the Quality control aspect. 

Phillips Petroleum’s, Mr. Fred Cornforth, and Team Leader of Fuels and Lubricants at R & D provided the following information.  The manufacturers are required to meet ASTM D-910-99 Specification for Aviation Gasoline. This requirement standardizes aviation fuels through out the industry.  The Automotive fuel manufacture is required to intentionally vary several characteristics of the fuel, such as Vapor Pressure, Octane Rating, Ethanol content and Oxidant levels.  These are all varied due to seasonal climate conditions; geographical area requirements such as metropolitan cities versus rural communities; mountainous regions as opposed to sea level areas. Another factor is the handling and shipping guidelines. Aviation fuel is required to be filtered at each handling i.e. manufacture to bulk plant, bulk plant to transport truck, transport to FBO (no pipelines!).  On the other hand auto fuel is not required to filtered at each handling due to cost restraints.

The STC’s for using auto fuel were approved while listing fuel obtained from certain vendors rather than going to the nearest convenience store and purchasing fuel.

This factor has caused us to take the position of opposing the use of auto gas in these engines under any circumstance. We have been faithfully warning our customers of the possible / probable consequences of using auto gas for years. Such problems as detonation, pre-ignition and backfiring (causing undo stress throughout the engine), exhaust valve and valve seat damage resulting in valve recession into the seat, piston ring and ring land failure, and cylinder head cracking and failure. This backfiring and detonation places unnecessary stress on the valve train (valve lifters, pushrods, cam and rollers, and cam drive gears), possibly resulting in premature failure of its components and housing. Detonation and pre-ignition connected with auto gas usage in aircraft engines does not have the same “signature” that everyone has come to recognize in their automobile. The pinging noise so commonly associated with use of inferior fuel in a car is drowned out by the propeller noise and wind noise typical of all aircraft. The only signs a pilot may have are a roughness in the engine and possibly some black smoke at periods of high power demand. He may experience a noticeable increase in performance with reduction of throttle / manifold pressure because the reduced combustion chamber pressure reduces the tendency of the fuel to detonate or pre-ignite.

Over the last few years we have begun to see the same operational problems associated with auto gas usage with some of our Canadian customers. The dwindling supplies of leaded fuels in Canada are resulting in increased economic temptations to use auto gas. When we realized operators were going to utilize auto gas (even when warned), we admonished those operators to at least mix auto gas with av-gas at a 50/50 ratio. This technique proved undependable and the recommendation to mix was dropped. The increased cost of overhaul / repair due to engine damage resulting from use of improper fuels such as unleaded auto gas has resulted in our placing a red tag in every engine logbook, preceding our overhaul entry, stating “Warranty is void if engine is run on automotive fuel.” The weight of evidence is against continued use of auto fuel in ag aviation. Our efforts, as well as the efforts of other agricultural aviation allied industry members, to curtail its use, is the reason we feel the customer / pilot / operator is responsible for the problems that arise when using automobile fuel in aircraft

By |2002-08-04T00:00:26+00:00August 4th, 2002|Flying-Wire|Comments Off on The use of Auto Gas in Radial Aircraft Engines

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