Pre-Heating Your Aircraft Engine
More Than Just Warming the Oil – The True Purpose of Pre-Heating.
While many pilot-owners have heard of engine pre-heating, only a minority practice it. Fewer still understand its true purpose, often attributing it solely to warming up the engine oil. While it’s correct that in cold conditions, single-grade oil requires heating to reach the appropriate viscosity, this isn’t the primary concern with multi-grade oil. Multi-grade oil is formulated to maintain suitable viscosity across a range of temperatures.
However, the need for pre-heating goes beyond just the oil. It’s crucial for accommodating the thermal expansion of the different metals that make up an aircraft engine. Components like the crankcase, pistons, and cylinder heads are made of aluminium alloys, whereas the crankshaft, connecting rods, piston pins, and cylinder barrels are crafted from steel.
The Challenge: Dissimilar Coefficients of Linear Thermal Expansion
Thermal expansion is the tendency of matter to change its shape, area, volume, and density in response to temperature variations. This phenomenon typically excludes phase transitions.
Temperature correlates directly with the average molecular kinetic energy of a substance. As substances heat up, their molecules vibrate more intensely and tend to drift further apart. While some materials contract as temperatures rise, such behaviours are rare and usually limited to specific temperature ranges. The coefficient of linear thermal expansion represents the relative expansion divided by the temperature change. This coefficient can vary with temperature fluctuations. Essentially, as particle energy intensifies, their movements become more rapid, thereby reducing the bonds between them and leading to the expansion of the substance.
Significantly, aluminium’s thermal expansion coefficient is nearly double that of steel. In the following section, we’ll discuss two scenarios demonstrating how differing thermal expansions could jeopardise your aircraft engine if not properly managed.
Example #1: Crankshaft/Crankcase
An aircraft engine’s steel crankshaft is mounted between thin bearing shells, which are supported by a cast aluminium crankcase. These bearing shells fit snugly into the crankcase bearing seat and are meticulously positioned to align with the oil galleries for lubrication. A minimal gap exists between the crankshaft and the bearing shells, facilitating lubrication. These gaps are designed for normal operating temperatures.
When the engine is cold-soaked, the parts contract at different rates due to their respective coefficients of linear thermal expansion. Notably, the aluminium alloy crankcase contracts nearly twice as much as the steel crankshaft. This differential contraction might result in a “negative” gap, especially in newer engines with tighter tolerances. Thus, the crankshaft becomes trapped between the bearing shells, preventing any lubricating oil from reaching it, regardless of oil pressure.
This phenomenon might be familiar if you’ve ever tried to manually rotate the propeller of a cold-soaked aircraft engine, possibly even finding it immovable. The issue isn’t thick oil – particularly not with multi-grade oil – but the minimised gap between the crankshaft and bearing shells, potentially even causing the crankshaft to seize.
Starting the engine in such a state can lead to expedited bearing wear and potential scuffing of the crankshaft journals until the engine warms up. In the worst-case scenario, the bearing shells could be displaced, cutting off oil lubrication, resulting in total bearing destruction and significant damage to the crankshaft journals.
Example #2: Piston/Cylinder Barrel
Contrastingly, an aircraft engine’s pistons, crafted from aluminium alloy, move within steel cylinder barrels. When cold-soaked, the gap between the piston and the cylinder enlarges, but it narrows as the engine reaches operating temperatures. This is why compression tests should be conducted when the engine is warm.
Upon starting a cold engine, pistons heat up much more rapidly than cylinder barrels. This is because the piston, being relatively smaller with a lower thermal mass compared to the larger, fin-cooled cylinder, reaches operating temperature faster. As a result, the piston expands to its full operating size before the cylinder has a chance to fully warm up and expand. Consequently, the piston’s fit within the cylinder barrel can become abnormally tight shortly after start-up. If the engine is cold enough, the piston-to-cylinder clearance might be reduced to zero, leading to metal-on-metal scuffing between the piston and the cylinder barrel. Due to the tapered design of most cylinders, the most severe cold start-induced scuffing usually occurs at the top dead centre (TDC).
When To Pre-Heat
In general, if the engine core is cold-soaked below freezing (0°C), it’s considered a cold start. If the temperature dips below -10°C, it should be seen as critically detrimental. Cold starting your aircraft engine is especially harmful for new or rebuilt engines with tight factory tolerances. While it’s less damaging for high-time engines (where the bearings might be worn and thus have more clearance even at cold temperatures), it’s still inadvisable. We recommend pre-heating your aircraft engine whenever the engine core is cold-soaked below 10°C, and it’s absolutely essential below 0°C.
By now, it should be evident that merely warming up the oil is insufficient to prevent crankshaft-to-bearing and piston-to-cylinder scuffing, especially when clearances can reach zero. It’s crucial to pre-heat not only the crankcase but also the cylinder barrels, particularly the top sections where they connect to the heads. Several methods are available for this purpose, with varying degrees of effectiveness.
Pre-Heating Methods
Insulated, Temperature-Controlled Hangar
The premier option remains an insulated, temperature-controlled hangar. This not only evenly warms up various parts of your aircraft engine, including the crankcase, crankshaft, cylinders, pistons, and oil, but also the windscreen (preventing it from fogging up upon breathing), gyro instruments, and even the pilot’s seat. It’s highly recommended to house your aircraft in such a hangar. When away from your home base, while it’s not necessary to consistently store your aircraft in an expensive heated hangar, consider doing so for at least eight to twelve hours prior to your departure – ideally overnight.
Multipoint Electric Heater
A close second, especially beneficial when combined with an insulated aircraft cover during particularly cold and windy conditions, is the multipoint electric heater. Equipped with multiple heating elements, these devices attach to the oil pan, crankcase, and each cylinder, ensuring that these components are warmed up adequately when connected to AC power for several hours – optimally, overnight.
Two prominent brands offering multipoint electric heaters for various engine brands, including Continental, Lycoming, Franklin, and Rotax, are Tanis Aircraft Products and Reiff Preheat System.
Tanis Aircraft Products
Renowned primarily for their multipoint electric heater, Tanis Aircraft Products also offer avionics and cabin heaters, alongside replacement parts and accessories. Their preserving kit, for aircraft engines that remain inactive for more than 30 days, comes highly recommended.
The preheat kit, designed for Continental, Lycoming, and Franklin engines, features threaded heat elements placed in each cylinder assembly (acting as replacements for intake bolts or rocker cover screws) and two pad heat elements for the engine crankcase, oil sump, or oil tank. This configuration ensures comprehensive preheating of the engine and its accessories. Power is routed to the elements via a dedicated wiring assembly equipped with LED system power indicators and circuit overload protection. Self-regulating through design, the heated components achieve an average state of thermal equilibrium in approximately six hours.
Compatible with all engine monitoring systems, the threaded heat elements can replace either rocker cover screws or intake bolt fasteners. Tanis Aircraft Products boast STC approvals from the FAA, TCCA, and EASA.
Reiff Preheat System
The Reiff Preheat System adopts a different strategy. They employ band heaters, which clamp around the cylinders, and an aluminium HotStrip element, which adheres to the oil sump. Their system warms the engine roughly 80°F above ambient temperature overnight. Notably, the Reiff system doesn’t include a crankcase heater.
Though less pricey and simpler to install, it takes slightly longer for the Reiff system to heat-soak the cylinder heads fully. The Reiff Preheat System only possesses STC approval.
Insulated Aircraft Cover
In milder conditions or when the aircraft is sheltered, a multipoint electric heater might suffice to heat soak your aircraft engine. However, in very cold and/or windy conditions, especially when pre-heating on an exposed ramp, you should consider using an insulated aircraft cover to retain the heat within the engine cowlings. At the very least, an insulated engine cover is essential. For extremely cold and windy situations, propeller and spinner covers should also be considered.
Insulated aircraft covers are not only advantageous for assisting the pre-heating process of your aircraft engine in conjunction with a multipoint electric heater, but they also offer protection for your paint job. These covers are particularly beneficial during trips to shield the aircraft from weather elements. Furthermore, installing the covers soon after shutdown – thereby maintaining engine warmth for three to four hours – might eliminate the need for pre-heating altogether.
Heat Gun and Other Devices
While heat guns and similar devices are available online, we do not recommend any of them. Whether electrically operated or fuelled by some combustible material, they don’t seem designed for prolonged use, which is crucial to properly heat soak an aircraft engine. We don’t have extensive experience in this area, but it appears that a well-designed system isn’t readily available yet.
Mother Nature
When left with no alternatives, it might be worth tapping into Mother Nature’s resources. While this might mean postponing an early departure, with proper planning, you can utilise the sun’s radiant energy to warm up your engine during a spring or fall day.
Conclusion
Ensuring the optimal operation and extending the lifespan of your aircraft engine demands more than routine maintenance. Pre-heating a cold-soaked engine is a fundamental step towards its reliability and longevity. The true purpose of pre-heating is not merely to warm the oil, but to ensure that all engine components achieve a uniform temperature. This is crucial because of the challenge posed by dissimilar coefficients of linear thermal expansion among engine components. Without uniform heating, these differences can lead to internal stresses, resulting in potential damage or reduced engine life. Whenever temperatures dip below freezing, it becomes imperative to pre-heat for several hours, ensuring that the entire engine – including the crankcase, crankshaft, cylinders, pistons, and engine oil – is thoroughly heat-soaked.
Recommendation
Our top recommendation is to store your aircraft in an insulated, temperature-controlled hangar, especially at your home base. This not only ensures a warm start but also shields your aircraft from the elements. In the absence of such facilities, the next best option is to utilise multipoint electric heaters, with those from Tanis Aircraft Products being particularly noteworthy. Enhancing this approach with an insulated aircraft cover can serve a dual purpose: retaining heat within the engine cowling and safeguarding your aircraft against harsh environmental conditions.
About Quest Aeronautics
Quest Aeronautics is a state-certified engineering office for aviation, dedicated to shaping the future of general aviation by providing innovative and cost-effective solutions to enhance aircraft performance and operations. With a focus on CS/FAR-23 and experimental/amateur-built (E/A-B) aircraft, Quest Aeronautics provides a range of services including flight testing, aircraft operations and maintenance consulting, high-quality aviation products, and tailored support for E/A-B projects. Collaborating with industry-leading partners, Quest Aeronautics is committed to delivering unparalleled support and expertise to individuals and organisations in the general aviation market.
About Author
Sebastian, the founder of Quest Aeronautics, is a driven and enthusiastic individual with a passion for aviation. Before delving into aviation, he gained valuable experience as a chemical process engineer and laboratory technician. Sebastian holds a Master of Science in Engineering and a commercial pilot licence, with several fixed-wing aircraft ratings under his belt. He has also completed an introduction course for fixed-wing performance and flying qualities flight testing at the National Test Pilot School in Mojave, CA and is compliance verification engineer for flight.