5.5HP Turbo Charged Fuel Injected Engine

For a while now, I've been thinking about some of the shortcomings of my original fuel injection project.  I wanted to go back and redesign this project adding some of the missing pieces.  So, that said, the idea was to take a small carbureted engine and convert it to fuel injection, then add a turbo charger.  All of this would be controlled with a stand-alone ECU, and PC software to tune the parameters.

The Engine

There were a couple of options when it came to selecting the engine.  The main criteria was to select an engine that was small enough to bench test.  Although a larger engine would have helped when the turbo was installed (more displacement, would be able to reach higher boost) this one was good enough to prove out the concept.  It's a 4 cycle, 5.5hp engine with a horizontal output shaft.  It is similar to ones that you would find on a go-kart (GX160).

Controlling the Spark

These engines are equipped with a transistorized magneto, and the ignition is fixed at around 25 degrees BTDC (before top dead center).  Being able to set the spark advance gives you more control over the engine.  This is especially true if you can vary it according to engine load and RPM.  So the first step is to get some sensors mounted to engine to report on the cam shaft position, and when the engine has reached TDC.

Normally, we could use Hall-Effect sensors mounted near the teeth of the cam gear.  In this case, there wasn't an easy place to do this (at least I couldn't find one).  So instead, I chose to use a shaft encoder (CAM Position) and a break-beam IR Tx/Rx pair (TDC).  Future engines (yes, I plan on doing this again) will instead have the traditional Hall-Effect sensors mounted.

Next, the spark coil was added replace the magneto in order to fire the spark plug.  The initial test runs didn't have any special logic, the spark advance was adjusted manually from 15 to 31 degrees, just to see how the engine would respond.

Fuel Injection

The next step was pretty big.  It was going to involve removing the carburetor and adding a fuel injection system.  They don't typically make any parts for an engine this small, so a lot of time was spent trying to find parts that were small enough to retrofit onto the engine.

To supply fuel to the injector, an in-tank fuel pump was added (from a WRX).  It pressurizes the fuel line to 40psi, and the adjustable regulator maintains this pressure.  The original fuel injector was replaced with one from a Ford Festiva in order to reduce the flow rate (from 330cc to 120cc).

The throttle body is from an BMW R850 motorcycle which was restricted down to about 3/4" to better match the engine's intake.

Turbo Charger

Next was to add the turbo charger.  Actually, next was to find a turbo charger that was small enough for the engine.  The smallest available one I could find was a RHB31 (I'm told this is one from a Suzuki Swift 1.0l).  It would only be able to boost the engine between 2-4psi, but that would be enough for bench testing, so on it goes.

Since there engine does not have an oil pump, an evac pump is used to draw oil from the bottom of the engine through the turbo.  This keeps a vacuum in oil cavity preventing the oil from getting past the seals.  Normally an evac pump is used to pump excess oil away from the turbo when it's mounting point is low with respect to the oil pan.  This is typically not required if a turbo is mounted higher on the engine.

Drive by Wire Throttle

The last mechanical part that was added was a stepper motor mounted to the throttle body that controlled the throttle plate angle with a belt and pulley system.

Sensors

Along with the TDC and CAM position sensors, the engine is equipped with a MAP sensor, two temperature sensors (block temp, and intake air temp) and a TPS (throttle position sensor).  All of the sensor signals are routed back to the ECU board.  

ECU

The ECU is used to set the spark advance and fuel delivery based on the input sensor readings.  There are two main tables (Spark Advance and Fuel Delivery) that are user tunable using the PC software mentioned below.  The table values are set as a function of RPM and engine load, but correction factors for throttle input and temperature readings are also performed.

PC Software

In order to tune the engine, a PC application was developed to set the table values.  The application also reads back, reports and logs the sensor values to see how the engine is performing during the test run.  This allows you to see how the engine performs under different conditions and makes the tuning process much easier.

Completed Engine

A couple of pictures of the completed engine.

Quick Bench Test

June 13th, 2009