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The Rechlin cold start method was an improvised method for cold starting piston aircraft engines at low ambient temperatures, which did not require complex heating devices for preheating and only a short lead time for the engine to start the flight. The process, which was used by the German Air Force during the Second World War, was developed before the war in the Rechlin testing center.
The basis was the addition of fuel to the engine oil, which reduced the viscosity of the lubricant and made it remain thin even when cold. After starting the engine, the fuel evaporated as it warmed up to operating temperature, causing the engine oil to regain the viscosity required for load operation. The time needed to warm up until full engine power (starting power) could be accessed was around three to six minutes, depending on the mixture ratio and the outside temperature. After around 30 minutes of flight operation, the fuel content in the lubricant was still around four to five percent, and after around one to two hours the fuel had completely evaporated. Motor gasoline with 80, 87 or 100 octane was used for the admixture; diesel fuel was generally not used.
The fuel was added after landing and the engine oil had cooled to a temperature of around 20 to 40 degrees Celsius. In older aircraft, this was done by pouring fuel directly into the aircraft's oil tank and then briefly run the engine; newer aircraft models were equipped with a permanently installed mixing system. The mixing ratio depended on the engine type, the previous flight duration (in relation to any remaining residue from the previous cold start) and the expected outside temperature at the time of the next start. Depending on the oil level in the oil tank of the dry sump lubrication, a mixing valve had to be kept open when the engine was idling for a defined time, which can be found in the mixing table, so that the admixture took place. The Rechlin testing center developed corresponding tables for this purpose in series of tests. The tests carried out by the testing center did not reveal any damage to the engines as a result of using the process; in many cases, even less engine wear was found.