Many people feel they live where it is dry enough that they do not need to be too concerned about rust and corrosion inside their engines. This is not a very informed conclusion if one considers some data.
First, rust does not need condensation to form before oxidation will begin. Rust will form on bare metal anytime the relative humidity (RH) of the air exceeds 50%, and is very aggressive above 80% RH. This article provides a brief description of the chemistry of rust.
Every location in the United States exceeds 50% relative humidity during some period of almost every day each year. And, many exceed the 80% threshold most days of the year during the overnight hours. The following link lists the average morning and afternoon relative humidity by month for all major US cities: Average Relative Humidity for US Cities
So how dry does the EICU Engine Dehydration System keep an engine?
During testing of each system, electronic data loggers are utilized to verify the performance of each system. These data loggers capture the temperature, relative humidity and dew point of the air from both inside the engine and outside the engine. They also log each time system pumps dry air into the engine. This information is loaded into charts for analysis, and for system calibration.
The following two charts cover the same three day period, and demonstrate the typical performance each EICU system is calibrated to achieve. This system was connected right after returning from a 1.5 hour flight, so the left side of each chart shows how quickly the system pulled the moisture out of the engine during the cool down period.
The chart titled “Duty Cycles,” shows the percent of time the system ran the blower, compared to the daily outside temperature, relative humidity, and dew point. Each small black box is an instance when the system ran the blower to pump dry air into the engine. These boxes scale to the right side of the chart. A 1% reading indicates the blower ran for about 10 seconds each 16 to 17 minutes. The first 12 hours on the left side of this chart show the system running the blower longer and more often during the engine cool down period to quickly pull all the moisture out of the engine that was left after engine shutdown. After that 12 hour period, this chart shows how the system duty cycle measurements track the outside air relative humidity very closely. This shows how the system dynamically responded to the outside temperature and humidity changes during each 24 hour period during this three day test. The net effect of this dynamic control is the system runs the blower more when the outside humidity is higher, and less during the warmer, drier periods of the day. This effectively keeps the engine the driest when the outside humidity is the highest, the time when rust is most likely to form.
This chart compares the same outside air temperature, humidity and dew point readings, to those measurements from inside the engine. (The temperature & humidity probe is fed down the dipstick tube to just above the highest oil level in the sump.) The black line is the internal engine temperature measurement. On the left side of the chart this black line shows the engine cool down period from connection, at about 7:00 PM on March 15th, until around 8:30 AM the next morning, when the black engine temperature line finally crossed the red outside air temperature line. After that point the chart shows how the internal engine relative humidity and dew point track against the outside air conditions. The negative spikes on the dark blue engine dew point line are the result of the dry air entering the crankcase each time the system ran the blower. These spikes correspond to the black boxes on the first chart.
The average relative humidity inside the engine for this entire three day period was 4.8%. And, this is in Houston, TX, one of the most humid locations in the US. (This average is typical of all the data that has been gathered from inside the engine while connected to an EICU Engine Dehydration System.)
So, how does this 4.8% relative humidity average compare to other locations? This article describes the driest place on earth. Its average relative humidity is 17.3%.