How Gasoline Direct Injection Technology Works

Gasoline Direct Injection, or GDI, is steadily making its way into racing from the top level down with added emphasis on fuel effciency. GDI technology can reduce the fuel consumption of race cars significantly and is therefore high on the hit list of engine developers wanting to improve their performance.

One key element of GDI technology is the extremely high pressure at which the fuel is fed to the injectors. This is often in the region of 150 bar, which is significantly higher than traditional port fuel injection. This extremely high pressure allows very precise injection of fuel, multiple times in one power stroke, which in turn makes for much a more effcient burn. Such high pressure means that high pressure fuel pumps must be used to feed the fuel rail. Normally there is low pressure fuel fed all the way up to the actual fuel rail and there sits a high pressure pump that feeds the rail. As the fuel pressure is integral to the calibration of the engine, making sure the high pressure fuel pump is correctly calibrated is a very important task.

The GDI pump unit consists of one or more cam lobes rotating at the same speed as the camshafts – once per engine cycle. The cam lobes are used to pressurise fuel and deliver it to the fuel rail in order to maintain the target fuel rail pressure. The pump contains two valves – an inlet valve and a control valve. The inlet valve controls low pressure fuel into the pump chamber, and also allows unrequired fuel back out from the chamber into the low pressure fuel circuit. The control valve is a “normally closed” configuration with the ECU controlling the opening time and duration.

The pump operates by generating pressure which is “normally” returned to the low pressure system unless the control valve is actuated by the ECU. This means the valve opens towards the end of the pump’s delivery stroke. For this reason, calibration of fuel mass flow is required so that the correct start angle can be determined as shown below.

 

Before any calibration is done it is important to configure the engine dataset or map for the type of pump in use. Parameters such as the number of pump lobes, the location of top dead centre of the lobes with respect to crank angle and the maximum start angle for the pump. It is important to verify the true location of the cam lobes of any pump as they may not be uniformly distributed. This also adds a complication when calibrating the flow rate of the engine as there can be varying pressure die to the nonuniform cam. In order to compensate for this it is useful to set an averaging function for the pump events, and this should then be turned off once the flow rate is calibrated. Additionally, if there is a variable valve timing system on the engine, there needs to be a compensation for any movement of the camshafts, as shown below.

Before starting the calibration of the pump the base angle map that allows the compensation for the effciency of the pump should be set to zero. This map is particularly useful at high engine speeds or after the engine volumetric effciency has peaked, but should also be the last thing to be properly calibrated.

The flow rate can now be calibrated in the pump flow rate map. This map references the current fuel rail pressure and the volume of fuel used during each cylinders injection sequence. This should be calibrated at steady state on a dyno, with the D and I terms of the PID controller for the demand valve control set to zero. Then set the target fuel rail pressure map to the minimum pressure in the Pump Flow Rate map.

Now run the engine at the speed where the volumetric effciency is maximum and adjust the engine load so the volume of fuel used is at the minimum value in the pump flow rate matrix. This value should also be the lowest value of the fuel rail pressure axis. A typical value for the pump flow rate angle in a large bore V8 engine is 90, although this base value will differ based on the expected fuel usage of the engine.

It is now necessary to tune the flow rate based on the P control term for the angle and the observed flow rate duration angle. Subtract the P term value from the flow rate angle until the P term is as near to zero as possible. An error of +/- 5 degrees is usually acceptable. Now return to the to the target fuel rail pressure map and set it to the next breakpoint in the pump flow rate map (60) and adjust the engine load to maintain the lowest volume of fuel used. Repeat the procedure for zeroing the P term and continue to work this way across the fuel rail pressure axis at the set volume of fuel used point. Then return the target fuel rail pressure map to the minimum value in the pump flow matrix and repeat the procedure for the next value of volume of fuel used. An experienced calibrator often extrapolates values to speed up the process. Once the calibration is complete it’s possible to work on the base angle map, which is used to compensate for any inconsistencies in the pump flow at high engine RPM. This is generally based on observing the pressure and smoothing out any inconsistencies. (Racecar Engineering)