is the speed at which your starter rotates the engine's crankshaft during a start. You may think that a higher cranking speed would be better but this is not always the case. Your starter is part of the engine's starting system which also includes the fuel and ignition systems. These systems have been designed to provide good starting characteristics for a particular starting speed range. For most engines this speed is between 120 and 180 rpm. Cranking your engine faster than 200 rpm may disengage the magneto impulse couplings thereby advancing the ignition timing which will result in poor engine starting and can even induce kickbacks! A typical 6 cylinder Lycoming engine requires 50-60 ft*lbs of torque to start. For this load condition, consider the following comparison (see chart, point B for Edrive and point D for Sky-Tec).
Sky-Tec - Spins the engine too fast
295/305 rpm at 50 ft*lbs of torque.
E-Drive - Ideal cranking, well below limit
145/155 rpm at 50 ft*lbs of torque.
directly affects the amount of heat that builds up in your starter when cranking your engine. The more current your starter pulls, the more heat that is generated in the starter. Increased heat reduces cranking performance, shortens the overall life of your starter and drains your battery. The rate of temperature increase also decreases the duty cycle of the starter which means fewer starts before the starter reaches its temperature limit. Current draw also affects other components in the aircraft starter electrical system. Contactors, terminals, and cables are all adversely affected by high current draw. Current draw is particularly important to pilots who have the battery located in the rear of the aircraft. The cable between the battery and the starter acts as a resistor which causes a voltage drop over its length. As current increases so does this voltage drop. The resulting electrical power lost to heat over the length of the cable is not available for use by the starter. Higher current draw results in greater voltage drop and less power to the starter. For cranking a typical Lycoming 6-cylinder engine with a 24-volt system and assuming a 50 ft*lb torque load the following comparison should be considered (see chart).
Sky-Tec - Pulls about 2-1/2 times as much current, shortening starter life
E-Drive - More than Double the number of starts and duty cycle of Sky-Tec!
Current Draw also determines the number of starts you can get from your battery. For a given charge condition, your battery is capable of delivering a given number of starts and that number is directly related to the amount of current your starter draws while cranking. A starter drawing twice the current will only give you half the number of starts before battery depletion.
is important as it reflects how well the starter can tolerate repeated starting attempts and over-cranking abuse. Warranty data from starter manufacturers indicates that the principle cause of in-service starter motor failures is over-cranking abuse. A starter with a duty cycle twice that of another will be able to deliver twice the number of starts within a given window of time without overheating. This is an important performance indicator when considering a starter as it will have a direct impact on starter longevity.
The operational limitations for the E-Drive are 10-seconds of engagement time followed by 20-seconds of rest for 20 consecutive starts! No other starter can match that kind of performance!
The Hartzell starters have undergone rigorous durability & laboratory tests including operating at temperatures ranging from -45 to 180°F shock tests, sinusoidal vibration sweeps, load cycling, normal starting, abusive starting, starting with induced kick-backs, and other environmental/operational extremes. The new starter lines from Hartzell Engine Technologies are capable of providing maintenance free service to your engine's TBO... and beyond.