The brain of this system is the controller. The controller is a small, but powerful computer that is programmed to optimize engine performance based upon the parameters of the Engine Personality Map (EPM), which is electronically stored in the controller. The map, as it is commonly called, is a highly detailed set of programmed parameters that is unique to the engine series to which it is installed. Inputs to the controller are absolute manifold pressure, rpm, crankshaft position, and, in most applications, cylinder head temperature (CHT).
To obtain the EPM, the base engine model is set up in a calibrated dynamometer and a LASAR mapping unit is installed on the engine. Along with the mapping unit, the engine is fitted with a number of sensors to measure torque, rpm, and numerous engine heat and detonation parameters. The mapping exercise is extremely meticulous and all of the sensor values are recorded as the engine is run at various rpm and manifold pressures. Ultimately, the goal of the mapping exercise is to determine the ignition timing that provides the maximum amount of torque while maintaining the anti-detonation margin and normal operating temperature range of the engine.
The system software is initialized when airframe power is applied to the controller. A diagnostics checksum software subroutine executes immediately, and verifies that the software is correct and the LASAR system is ready to start the engine. The magneto needs to turn one full revolution above 8 rpm before the controller enables the ignition function for starting. Once the start is commanded, the system delivers a uniform high-energy pulse to each cylinder at the top dead center (TDC) of each compression stroke.
Once the engine is running, the controller processes the data from the magneto, manifold pressure and CHT inputs and optimizes engine performance according to the EPM, making two million ignition management decisions per second. For example, the LASAR system will not advance timing unless the engine is developing less than 27 inches hg of manifold pressure. The system also won’t advance beyond base engine timing unless the CHT input is within nominal parameters.
During LASAR mode, all ignition events are commanded and executed by the controller. The diagnostics program continually monitors for faults in the software and electronic circuitry of the system. If the LASAR diagnostics detects a fault condition, then the cockpit indicator light is illuminated, and the system automatically defaults back to magneto operation or LASAR mode at default base timing.
The first step in the LASAR installation process is to determine whether a particular engine and airframe combination is approved for installation, and to select the proper combination of components to complete the installation.
The primary document to reference is the latest revision of LASAR Service Letter SL1-96. Consult Unison to confirm that you are working from the latest revision before performing any work. SL1-96 contains all required part numbers, installation instructions and installation eligibility for the components. Be aware that LASAR is approved for most, but not all Lycoming 320, 360 and 540 applications, so be sure to have the correct system before getting too deep into the project.
The two basic types of LASAR magnetos are sensor and non-sensor. The sensor magneto is configured with a Hall effect transducer that provides an rpm and crankshaft position signal to the controller. The non-sensor magneto provides no engine performance feedback and its functions are driven by the controller.
Although the LASAR magnetos are not impulse coupled, there are models listed for impulse coupled applications. The drive gears between impulse and non-impulse magnetos are different and the LASAR magnetos are simply configured to accept the correct drive gear. Also, some magnetos are configured to work on either 20 or 25 degree base timing engines, so be certain of the base timing of the engine before selecting a magneto model.
The controller also has a number of models from which to select. Controller maps are based upon the basic engine model and airframe voltage – 12 or 24 volts. Once again, one should be certain of the engine base timing as certain engines in the Lycoming IO-360 series are approved for either a 20- or 25-degree base timing. Lycoming Service Instruction SI-1325A is a good source document to determine the engine’s base timing, as is the engine’s data plate. Finally, with LASAR, there is the option of using a CHT or non-CHT controller. Unison recommends using a CHT controller with a CHT probe for all installations, but there are a handful of applications in which the CHT controller is optional.
Low voltage control harness
Another component to be selected is the low voltage control harness, more commonly referred to as the LH. The LH is the communication cable between the magnetos and the controller and features a left and right lead to run to the sensor and non-sensor magnetos. Given that the controller may be mounted on the left, center or right side of the firewall, the left and right leads will need to be biased to lengths that provide for a minimum of excess length between the magnetos and controller.