H   ANDS ON FIREWALL FORWARD

Exceeding engine operational limits can have disastrous results.

How Hot Is Too Hot? Engine cooling, part II BY MIKE BERRY

A DISCUSSION ON AIRCRAFT engine cooling would not be complete without mention of just how hot is too hot and what to do about excessive temperatures in an air-cooled engine. Basic aircraft engine cooling system design has changed little in the past 50 years. The basis for this is that aircraft engines must be light in weight, reliable, exhibit long life, and rotate a propeller at an efficient speed. It is for these reasons that the selection of materials used in the design and manufacture of an aircraft piston engine is limited. The specific design, materials used, and method of construction of an aircraft engine have much to do with limiting temperature.

provided us with synthetic oil that can withstand higher operating temperatures; however, the problem is that unless the engine is designed to operate at these temperatures, the problem of increased wear may not be mitigated with this oil. It is again the material used in the engine that dictates the limit—not just the oil. The bottom line is that high oil temperatures (as well as high cylinder head temperatures) indicate there is a problem that needs correcting.

OIL TEMPERATURE

Aircraft operations have always had limitations, and temperature is unfortunately no exception. As modern aircraft equipped with electronic instrumentation become the standard, it is even more important to know the limits for your engine—what the effects will be of exceeding limits and how to reduce temperatures. A prominent aircraft piston engine manufacturer places recommended limits on oil temperature of a minimum of 180 degrees F and an absolute maximum of 245 degrees F. Let me remind you that these limits are not regulations, but physical limits associated with the design and materials used within the engine. Operations outside of this range can have significant effect on the reliability and longevity of an engine. Lower than recommended temperatures will not cook or boil off the byproducts of combustion (moisture), causing internal engine corrosion. Oil temperatures approaching or exceeding limits will cause increased wear, which will cause decreased performance and impact reliability. Aviation oil formulated from petroleum will oxidize and break down when subjected to higher temperatures. Modern chemistry has

88 Sport Aviation June 2011

Exhaust leaks can cause excessive cylinder temperatures and cracks in the aluminum cylinder head.

PHOTOGRAPHY BY MIKE BERRY

H   ANDS ON FIREWALL FORWARD

Taking corrective action may be challenging, but ignoring the problem will only exacerbate the situation. Aircraft of all designs and performance categories have operated successfully for years without exceeding oil temperature limits, indicating that it is possible to achieve success in controlling oil temperature. CYLINDER HEAD TEMPERATURE

The design, materials, and construction of cylinders used on air-cooled aircraft engines have Efficient cowling installations require accurate engine instrumentation to not changed significantly for detect and correct overheating. many years either, and this overall design has proven itself in reliability, light weight, and efficiency. Cylinder heads are constructed of an aluminum alloy that is strong yet light in weight and capable of transferring heat away from the combustion chamber in an effective manner. The foundation material of the cylinder barrel is typically made of steel and is usually shrunk fit or threaded onto the cylinder head. The absolute cylinder head temperature limit of 500 degrees F (and desirable limit of 435 degree F), as published by one engine manufacturer, is typical and is imposed as a result of the physical properties of the material. Exceeding these temperature limits on an aluminum cylinder head will weaken the metal over time and cause cracking and eventually complete failure of the metal. INSTRUMENTATION

Engine temperature instruments installed as original equipment by an aircraft manufacturer have met the requirements but are of no real value in tracking actual temperature readings as many are marked with green, yellow, and red lines having only a reference to an exact number for the limit. While this gives some indication of temperature, these are really trend instruments and are not even linear on the scale. Over time these gauges often become inaccurate, and typically no one bothers to check their calibration. This can pose a problem as conditions such as low oil pressure may be associated with high oil temperature, yet if a gauge is inaccurate, one could be troubleshooting incorrectly. Any time a temperature or pressure is near or at a limit always verify the accuracy of the gauge. However, electronic instrumentation has eliminated much of the inaccuracy inherent to these earlier instruments. The position of a temperature sensor is also important. Mounting a sensor close to an Turbocharging places additonal demands on the engine cooling system. exhaust manifold (or even an

90 Sport Aviation June 2011

exhaust leak onto a temperature probe) can cause an inaccurate reading. Cylinder head temperature readings and the published limits specify how the reading should be taken—for example, from a bayonet sensor installed in the well within the cylinder head. Spark plug gasket type sensors will indicate a different temperature than a sensor placed in the bayonet position on the same cylinder head. Some low-powered aircraft were designed without the use of oil cooler and had one or more blast tubes directed at specific locations on the engine. In one installation the oil temperature sensor was mounted on the engine in such a way that the oil temperature reading during operation was largely dependent on the angle of the blast tube and ram air across the engine, and was not a true representation of the oil temperature. The oil temperature that one observes on the instrument is most likely not the hottest the oil gets, but rather somewhat of an average of the oil temperature within the engine. COOLING SYSTEM DESIGN

To properly maintain and troubleshoot a piston aircraft engine cooling system, it is necessary to understand its design and how it is employed on a specific aircraft. The downdraft cooling system is technically part of the airframe and is the principal design in use today in air-cooled, horizontally opposed, piston aircraft engine installations. The cooling airflow in the downdraft system enters through openings in the forward cowling and flows over and down through the cylinders, eventually exiting in the lower aft cowling area. The cooling airflow is created in-flight by ram air and is complemented by a low-pressure exit incorporated in the design of the lower aft cowling. Adjustable cowl flaps may also be a feature of the design capable of significantly enhancing cooling; these flaps may be closed to reduce overall aircraft drag when a large volume of cooling air is not required. When designing and maintaining an aircraft cooling system consideration must be given to the effects of fire, smoke, and hot gas. Materials and methods used for construction or repair must be capable of withstanding these elements during flight to allow for a controlled descent and safe landing. Firewalls must be completely sealed,

PHOTOGRAPHY BY MIKE BERRY

Insulated hoses can help control heat transfer near exhaust system components.

and items such as fuel and oil tanks, electrical components, and wiring must not be directly attached to the aft firewall unless protective measures are employed. TROUBLESHOOTING

Troubleshooting and correcting high temperatures can be challenging, but a solution to the problem should always be available. Many times a solution is a simple change to what may seem like a complex task that you are tempted to ignore. Before attempting corrective action, take time to think about what the exact problem is: Make written notes as to when it occurs, and list possible steps that may be taken to correct the trouble. Is there really a problem? Is the instrumentation correct? What are other owners/builders experiencing and what have they done to correct a similar problem? When troubleshooting, always start with the easy and inexpensive items. Since engines do create heat, is the heat produced within the parameters for the particular engine installation? Are the ignition timing and fuel air mixture correct? Is the engine in good health, with acceptable compression and oil consumption? Is the propeller and spinner the correct one for the aircraft and engine combination? What about the exhaust system; is it a design of your own or one that has been successfully used by others with the same aircraft and engine installation? How about the oil cooler and associated hoses and routing of the hoses to an external cooler? Are the lines or tubes insulated or routed away from the exhaust and turbocharger? Can heat shields or fire sleeves be installed to prevent heat transfer? FINAL THOUGHTS

Throughout my years of aviation experience I have seen the unfortunate effects of excessive heat on aircraft engines. The causes of excessive heat can typically be traced back to improper construction, maintenance, or operating techniques. Operating an aircraft within acceptable limitations and discontinuing operation should something happen is not only the safest plan of action, but may also be the most economical. Mike Berry, EAA 264353, is a retired airline captain with 16,600 hours of flight time and

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