Information Needed for Tuning Direct Injection

Tuning a D.I. vehicle is a complex task that necessitates precise and detailed information about the vehicle’s components and system behavior. This data is referred to as "characterisation". Before you start, here’s what you need to know:

Initial assessment.

Before putting a Direct Injection vehicle on the dyno, it's crucial to conduct a viability assessment. This will help you determine whether the time and resources required for tuning are justified by the potential benefits. Tuning a D.I. engine involves intricate characterisations , which if you need to gather yourself, can be time-consuming—often taking a full week or longer.

Key information and characterisations you'll need for D.I. tuning

1. Injector type: solenoid or piezo?

Identify the type of injectors in your vehicle—Solenoid or Piezo. The distinction is critical as most aftermarket ECUs cannot control Piezo injectors. Typically, Solenoid injectors feature a large spherical metal bulge, unlike the smaller, more intricate Piezo injectors.

2. Injector current profile.

Determining the injector current profile requires reverse engineering using an oscilloscope with a current probe. You will need to capture and analyse the waveform generated by the injector to establish the peak current, hold current, and response times.

This is done by disconnecting the injector harness and connecting the oscilloscope in series with the injector while the engine runs, to capture the entire current waveform. The waveform will show the initial peak current (which opens the injector), the hold current (which keeps the injector open) and the response times.

3. Injector flow characterisation.

This process necessitates a flow bench specifically designed for direct injectors, allowing you to measure flow rate and injector dead times, at various different operating pressures.

4. Fuel pump type.

Know your fuel pump type—whether it’s normally open, normally closed, or uses Pulse Width Modulation (P.W.M.). Each type interacts differently with the ECU and influences how fuel pressure is managed in the system.

A normally open fuel pump means that the circuit is open, i.e. not allowing current to flow, when the fuel pump is not activated. When the fuel pump needs to be activated, the circuit is closed, allowing current to flow and activating the pump.

This is a simple on/off control strategy. A normally closed fuel pump is the opposite, with the circuit being closed until the fuel pump needs to be activated.

PWM control allows for variable speed control of the fuel pump by rapidly turning it on and off at varying duty cycles. PWM allows for more precise control of the fuel pump compared to simple on/off control. A Google search may be able to determine your fuel pump type.

One trick to test if your pump is normally open or closed, is to unplug your fuel pump while the engine is running, to see if the fuel rail pressure increases or decreases, if it’s normally open it will decrease, if it’s normally close it will increase.

5. Camshaft profile for the fuel pump.

To establish the camshaft profile, measure the lift and duration of each cam lobe associated with the high-pressure pump. This data is crucial for optimizing injection timing. To find this information yourself, you will require a dial gauge and a degree wheel. You will only need to establish the profile once per engine type, i.e. every factory BMW B58 engine has the same Cam Shaft Profile.

6. Pump dead time, and injector timing.

Information on pump dead time and injector timing can usually be obtained from the factory ECU. If unavailable, these must be derived through reverse engineering. Injector timing will be dialed in as part of the tuning process, but knowing the factory map for this in advance can speed up the process.

High-pressure fuel pump control

Once the initial setup and characterisations are complete, you’ll need to tune the high pressure fuel pump control related systems, to ensure stable fuel pressure across all operating conditions. This is complex due to the many variables affecting control, camshaft position, pump dead time, battery voltage. Fuel pressure isn’t constant in DI engines, and needs to be able to change very quickly with load. This step could also take several days to a week, depending on the quality of your work in the initial set up.

Normal fuel tuning

After achieving consistent fuel pressure control, you can proceed to ‘normal fuel tuning’. Port injected engines generally have fixed fuel pressure (or a very small range of pressure), and fixed injector timing that has little consequence even if not fixed. In other words, you only need to optimize the amount of fuel injected under different RPM & load conditions to get a port injected engine running reasonably well.

With DI, fuel pressure will vary by a factor of about 25 from cranking to WOT, and this means a small error at low pressure suddenly becomes a big error when you increase the pressure 25 times. Injector timing is critical as you only have a short window of time to inject into and which varies with RPM/Load.

You will need to optimize the fuel injection in the same way as port at all RPM and Load conditions, and you also have target pressure and injector timing adding a big influence which needs to be optimised at the same time. This could triple the tuning time required for general fuel tuning. Cold and hot start tuning is also more difficult on DI engines and will require more time.

Support from Link Engine Management

At Link Engine Management, we aim to facilitate your tuning process. We offer pre-characterised Injector Current Profiles and Cam Shaft Profiles for some popular engines, saving you the effort of reverse engineering these components yourself, which can significantly cut down on your preparation time. To see if your engine, or components of your DI system are characterised, we have an up-to-date list here.

Why choose link? Utilising data from Link not only saves time but also ensures you're working with tested information. This allows you to identify potential issues early in your tuning project, and concentrate on the tuning itself.

With all the necessary information in hand, you are set to begin tuning your direct injection vehicle on the dyno. While similar to tuning traditional port injection with modeled fuel, it includes additional considerations like injection timing and fuel rail pressure management.

For more details, and resources, please visit the support section of our website, join our forums, or reach out to our 24/6 Tech Support team. Happy tuning!