SMA’s Compression-Ignition Engine

Jet A fuels the new piston engine Cessna JT-A

Saint Loup says the SMA engine is a “fly-by-wire” engine since the engine control unit (ECU), which controls the rack position on the Bosch fuel injection pump(s), is electrically connected to the pilot’s primary power lever. Two potentiometers — for redundancy — on the power quadrant feed the ECU with the pilot’s desired power. The ECU combines that signal with several signals from two variables: 1. altitude and air temperature data at the inlet of the turbocharger compressor from the P1/T1 sensor, and 2. fuel temperature ([Tfuel] for fuel density) to arrive at an adjusted fuel flow. The ECU then powers a servo motor that controls the fuel pump rack position which results in injecting the right amount of fuel directly into the combustion chamber to deliver the desired power. In addition, rpm sensor data is used by the ECU during starting and low speed (800- to 1,200-rpm) operations.

The fuel flow/power output curve in the SR305-230E-C1 engine is linear, just like a turbine engine. The ECU, unlike carburetors or fuel injection systems on avgas engines, does not richen the mixture at transition from idle to power up or at high power settings for detonation protection. Compression ignition engines run lean by design with a permanent excess of air (the battle about rich or lean of peak is over!).

The ECU does not monitor EGT, CHT, TIT, or oil pressure or oil temperature since they are not necessary for the fuel flow management. All those parameters are displayed on the G1000, except for EGT, for the pilot to monitor and ensure operating limitations are not exceeded. After an SMA maintenance software download to a laptop computer, data from the engine control unit (ECU) can be displayed live or downloaded through a cable through an aircraft RS-232 port during inspections.

Downloaded data includes faults (if any), sensor’s data, and engine operating time. This data can then be entered into a spreadsheet type program to chart engine change parameters. It can also be sent to Lycoming or Cessna tech experts to aid in troubleshooting if needed. Lycoming has contracted with Cessna to support the SMA engine. This includes spare parts distribution, warranty administration, and field support.

In addition to the RS-232 data download cabling, a second specialized cable permits ECU map updates. The ECU and the continuously governed propeller results in a pilot workload that is greatly reduced — there’s no mixture to adjust and no prop control knob to push and pull. Unlike avgas turbocharged engines, there is no waste gate so erosion and adjustment maintenance is nonexistent. All the exhaust gas goes through the turbine at all power settings.

However, on the far left side of the pilot’s switch sub panel, there’s a glow plug switch. It’s used to activate an automated preheat sequence before starting the engine and provides the most significant benefit when starting in cold conditions.

ECU/engine fault modes and manual mode

All the engine sensor wiring is shielded. Engine control redundancy is provided with primary and secondary sources or sensors. If the ECU diagnostic system detects a fault in either the primary or secondary inputs, the pilot is alerted by a fault annunciation on the instrument panel.

If one sensor or component in any data leg fails for any reason, a “minor fault” light is illuminated. This lets the pilot know that there’s a minor problem but that the ECU is still controlling the engine. The airplane can continue to be flown in the normal mode. After completion of the flight an A&P needs to find the fault and fix it. If the “major fault” annunciator is “on” it signals the pilot that the failure has compromised the ability of the ECU to control the engine. The pilot must switch to the manual mode of controlling the engine when the major fault annunciator comes on.

The engine control console has two levers. The left one is the power lever and has a T-configuration. The right one is smaller, is red, and is the mode change lever. The switchover to manual control requires the pilot to pull the power lever back to a clearly marked spot in lever travel. The mode change lever is then moved aft to the manual backup position which disengages the electronic control from the pump, providing the pilot with direct control of the fuel flow through a cable connection between the power lever and the fuel pump. A power chart must be referred to to adjust the manual lever to the correct manifold pressure. There’s no requirement to land immediately if a fault light is illuminated.

Economy and ease of operation

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