Two-Stroke Engine Warming
Two-stroke engines must be warmed because different metals expand at different rates as they are heated. When heating steel and aluminum, the aluminum parts expand faster than the steel parts. This becomes a problem in two different areas of many two-stroke engines. The first place is in the cylinders of the engine.
The cylinders have steel walls that expand slowly, compared to aluminum pistons that expand quickly. If an engine is revved too quickly during takeoff before warming up, a lot of heat is generated on top of the piston. This quickly expands the piston, which can then seize in the cylinder. A piston seizure will stop the engine abruptly.
The second area of concern is lower in the engine around the crankshaft. This is an area where parts may get too loose with heat, rather than seizing up. Additionally, the crankcase has steel bearings set into the aluminum which need to expand together or the bearings could slip. Many two-stroke engines have steel bearings that normally hug the walls of the aluminum engine case. The crank spins within the donuts of those steel bearings.
If the engine heats too quickly, the aluminum case out-expands those steel bearings and the crank causes the bearings to start spinning along with it. If those steel bearings start spinning, it can ruin the soft aluminum walls of the case, which is very expensive. If heat is slowly added to an engine, all parts will expand more evenly. This is done through a proper warm-up procedure. Many two-stroke engines are best warmed up by running the engine at a set rpm for a set amount of time. Follow the instructions in the POH; however, a good rule of thumb is to start the engine initially at idle rpm, get it operating smoothly at 2,500 rpm for 2 minutes for initial warm-up, and then warm the engine at 3,000 rpm for 5 minutes. The cylinder head temperature or coolant temperature must be up to the manufacturer’s recommended temperatures before takeoff. This may require running the engine at higher rpm to reach required temperatures on some engines.
Once the engine is warmed up and the aircraft is flying, it is still possible to cool down the engine too much. This happens when the engine is idled back for an extended period of time. Even though the engine is running, it is not generating as much heat as the cooling system is efficiently dumping engine heat into the atmosphere. An immediate power application with a cooled engine can seize the engine just as if the engine had not been warmed in the first place.
In water-cooled engines, on a long descent at idle, the coolant cools until the thermostat closes and the engine is not circulating the radiator fluid through the engine. The engine temperature remains at this thermostat closed temperature while the radiator coolant continues to cool further. If full throttle is applied, the thermostat can open, allowing a blast of coolant into the warm engine. The piston is expanding due to the added heat, and the cylinder is cooling with the cold radiator water, resulting in piston seizure. To prevent this, slowly add power well before getting close to the ground where power is needed. This gives the system a chance to open the thermostat gradually and warm up the radiator water.
Just as it takes time for the engine crankcase and bearings to warm up, it also takes those steel parts a long time to cool down. If a pilot lands, refuels, and wants to take off again quickly, there is no need to warm up again for 5 minutes. The lower end of the engine stays warmed up after being shut down for short periods.
Any engine restart is an example in which it would be appropriate to warm the engine up until the gauges reach operating temperatures. The lower end of the engine is warm and now a pilot needs to be concerned only with preventing the pistons from seizing.
Four-Stroke Engine Warming
A four-stroke engine must also be warmed up. The four-stroke engine has a pressurized oil system that provides more uniform engine temperatures to all of its components. Takeoff power can be applied as soon as the water, cylinder head temperature (CHT), oil temperatures, and oil pressure are within the manufacturer’s recommended tolerances for takeoff power applications.