Advanced Ignition for the 21st Century
Overview of the LASAR® electronic ignition system and its operation – Part I
By Harry Fenton
What is it?
LASAR, which stands for Limited Authority Spark Advance Regulator, is the first and only stand alone microprocessor-based engine control system approved by the FAA for GA piston aircraft. It consists of a controller, two magnetos, a low voltage harness, a special status cockpit light, and a CHT probe. The controller is a sealed box, which mounts to the firewall and contains the electronics and software for controlling ignition. The magnetos are similar to conventional magnetos but also include electronic circuitry. The low voltage harness electrically connects the controller to aircraft power, the magnetos, the cockpit light, the ignition switch, and the CHT probe. The cockpit light provides an indication of system status (ON or OFF). The CHT probe is used as a control input in the LASAR system.
This system optimizes the timing of the engine by electronically advancing the timing in response to the absolute manifold pressure and the engine RPM, resulting in better fuel efficiency and improved horsepower. It can provide ignition timing advance angles from zero to 50 degrees before top dead center (BTDC). The net effect is that the engine can develop more power output for a given amount of fuel input. An additional benefit is a smoother running engine.
During engine start, LASAR provides a high energy spark discharge. The higher available energy can contribute to better engine starting if the spark plugs are wet or carbon-fouled, and in cold temperature conditions when the fuel is less likely to properly atomize.
In order to provide optimum engine performance while at the same time providing ignition system redundancy as required by FAR 33.37, Unison elected to incorporate the best features of magnetos with the advantages of electronics into the architecture of the overall system. The overriding concept is simple: electronics are used to advance and enhance ignition, but if the electronics fail or lose electrical power, then the fail-safe backup is the tried and true magneto.
LASAR magnetos are similar in concept to conventional magnetos because in the back-up mode, they must function just like conventional magnetos. However, they are also a very integral part of the system’s operation when in the LASAR electronic mode. The engine rpm and crankshaft position inputs, the generation and distribution of the spark, and the fail-safe switching from LASAR magneto to conventional magneto all take place in the LASAR magnetos.
In operation, a relay in the magneto circuitry serves as the switching mechanism between LASAR mode and backup magneto mode. When the LASAR controller is providing power to the LASAR magneto relay, the relay contacts, which connect the controller to the magneto coil, are closed. If power fails or the controller commands backup operation, then the relay contacts open, connecting the magneto coil to the backup magneto circuit.
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.
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.
The CHT probe, which is mandatory on all applications that require the CHT controller, is available as a single- or dual-point pickup and must be installed on the hottest running cylinder of the engine. The LASAR CHT probe is a special purpose designed Resistance Thermal Device (RTD), as opposed to a traditional bi-metallic thermocouple.
LASAR Fault ModesLASAR Fault Modes
Latching Fault: A fault that will force LASAR to stay in backup mode until power to the controller is cycled. The cockpit light is illuminated in this fault mode. Latching faults are the most serious of the fault modes and can be indicative of a major component or software malfunction. If the fault is determined to be a latching type, the system needs to be diagnosed before the next flight. In some cases, though, the latching fault can be a simple problem, such as a low battery or insufficient voltage at start. This can cause the LASAR system to not initialize.
Healing Fault: LASAR diagnostics commands backup mode and illuminates the cockpit light, but returns to LASAR mode and extinguishes the light, providing the fault co0ndition does not re-occur. Healing faults are typically a short time fault indication, such as a LASAR input signal subjected to a rapid change in a parameter, such as manifold pressure, that is outside of the programmed range of values.
LASER Mode Fault: LASAR commands illumination of the cockpit light, but continues to operate in LASAR mode at default timing and does not command backup mode. Typical faults in this category are a broken LASAR CHT connection or a discontinuity in the manifold pressure line from the engine to the sensor in the LASAR controller.
In the Next Issue of AMT
The LASAR discussion continues! A step by step installation on a Textron Lycoming O-320 powered aircraft will be documented along with in-depth troubleshooting and diagnostics procedures. An update on the Unison and Textron Lycoming EPiC electronic engine control will also be covered.
About the author
Harry Fenton is Manager of Customer Support for Unison’s Piston Engine Products. He is a pilot and an aircraft technician with an A&P and IA.. You can reach Harry at Unison’s Rockford, IL facility at (815) 965-4700 or at [email protected].