🎓 Evolution applied

Continuing from Part 7, let’s take a look at some basic examples where ECU’s and sensors have replaced mechanical processes and break them down.

Back to our trusty 4-stroke engine
Variant 1 – Petrol engine with carburettor & distributor. An accelerator cable is attached to the carburettor from the accelerator pedal.
Variant 2 – Petrol engine with fuel injection and coil pack. An accelerator cable is attached to the throttle body from the accelerator pedal.

Both variants have a cable connected to a pedal.

Variant 1 opens a butterfly in the carburettor which allows more air to flow drawing more fuel from the carburettor reservoir increasing the revs of the engine. The carburettor and pedal are spring loaded so when the pedal is released the butterfly returns to the closed position reducing the air flow and as a result less fuel.

Variant 2 opens a butterfly in the throttle body which is connected to a variable rotary sensor called a potentiometer. The potentiometer has 3 wires, a positive, a negative and a signal. The 3 wires run to the ECU. The positive is a constant voltage from the ECU, the negative is a constant ground, and the signal is variable. In the “idle” position the signal wire will send a very low voltage back to the ECU. As the potentiometer turns the voltage on the signal wire increases. This increase in voltage is interpreted by the ECU as more acceleration and as such tells the injectors to give more fuel. The butterfly and pedal are spring loaded so when the pedal is released the butterfly returns to the closed position sending a lower voltage to the ECU and as a result less fuel.

In this example the result of pressing the accelerator pedal resulted in more fuel and higher revs for both variants, the only difference is the delivery of the fuel. The ECU, injectors and throttle body have replaced the carburettor. Both variants have a cable & butterfly controlling air supply.

Sticking with our trusty 4-stroke
Variant 1 – Petrol engine with distributor and coil
Variant 2 – Petrol engine with coil pack and ECU

Both variants have spark plugs and HT leads.

Variant 1 has a mechanical distributor which is driven from the camshaft, a HT coil mounted somewhere in the engine bay and HT leads connecting the distributor cap to the coil and the spark plugs. The distributor is fitted and adjusted manually with reference to cylinder 1 TDC. As the distributer rotates it sends current through the HT leads to the plugs from the coil. The timing of this current is determined by its lobed shaft.

Variant 2 has a sensor mounted next to the crankshaft pulley, a coil pack mounted somewhere on the cylinder head and HT leads connecting the coil to spark plugs. It also has wires that connect it to the ECU. The crank shaft pulley is notched where all the notches are the same size except for one section where it looks like there are no notches. This section is the reference section of the pulley. The sensor reads the notches and sends this information to the ECU. The ECU then bases its calculations on the un-notched section of the pulley sending a signal to the coil pack telling it to send current through the HT leads to the plugs. The ECU can advance and retard the timing automatically as it sees fit.

Again the objective and final result is the same. The ECU, sensor and coil pack have replaced the distributor and HT coil. Both variants still have a HT coil of sort and HT leads that transport current to the plugs and a specific time during the 4-stroke cycle.

In the accelerator example above, we would not go fault tracing the distributor example if our problem was no acceleration. We would focus on what causes no acceleration right, the carburettor-cable setup or the throttle body-potentiometer-cable setup.

In the examples above it is clear to see where having a solid understanding of the fundamental basic operational requirements of a 4-stroke engine can be beneficial to fault tracing a fuel injected engine with an ECU. If we know what the result of a specific command should be, we can apply our knowledge of the basics to the fault tracing process. The most important thing to remember is understanding what does what, and what has replaced what, then tackle 1 section at a time.