RKTI 11.40 (Calculation of Injection Time ti from Relative Fuel Mass rk)

RKTI 11.40 Function Description

ti_w represents a physical value of injection time which is correct also during start conditions. During start the physical value of ti_b1, ti_b2 and ti_tvu_w has to be corrected by the user by a factor of 8, because start quantisation of ti_b1 is internally corrected by dividing by 8 to store large ti-values into a ‘word’ variable instead of a ‘long’ variable.

Please see the funktionsrahmen for the following diagrams:

1. Battery correction of injection time for injection valves, calculation frkte (fuel mass into injection time)

2. Calculation of ubatt correction of injector time for injectors

3. Correction for injected fuel mass if the reference pressure of the fuel rail pressure controller is not manifold pressure (i.e. with a returnless fuel rail).

4. Calculation of the injection time during start conditions

5. Calculation of the injection time after end of start conditions

This function calculates the effective injection time before fine tuning (tevfa_w, tevfa2_w) from the relative fuel mass (rk_w, rk2_w) and the factor frkte. With an ideal fuel supply system, tevfa_w + tvu_w, tevfa2_w + tvu_w should result in lambda of 1.0 in the combustion chamber, with pilot control to lambda = 1.0 and neutral values ​​of all mixture adaptations.

In practice, a deviation in lambda may occur due to injector nonlinearities or pulses in the fuel system. This deviation is corrected using the map FKKVS as a function of engine speed (nmot_w) and effective injection time (tevfa_w or tevfa2_w). The corrected effective injection time is te_w or te2_w. By adding the battery voltage correction for the injectors, the actuation time is calculated thus: ti_b1 = te_w + tvu_w. The function ACIFI controls the actuation times ti_b1 and ti_b2 for the associated injectors. In a single bank system (SY_stervk = false) the actuation times for bank 1 (ti_b1 or ti_b2) are forwarded to CIFI. In order to achieve the long injection times required during starting conditions, the quantization times ti_b1, ti_b2 are increased by a factor of 8 which thus expands the range to 1677.696 ms. The same applies for the additive quantity ti_tvu_w.

Therefore, a 16 bit value is required for the interface to the function ACIFI. This is important for runtime reasons for normal operation. During start conditions, VS100 measurements of the physically indicated injection time are multiplied by a factor of 8. The resolution during start conditions for ti_b1, ti_b2 and ti_tvu_w is 25.6 microseconds, whereas in normal operation it is 3.2 microseconds.

The RAM cells ti_w and ti_2_w show the physically correct injection time during both start conditions and also normal operation with a resolution of 16 microseconds. The resolutions are valid for a 20 MHz processor.

The minimum injection time TEMIN or TEMINVA is set when outputs B_va = true, B_temin = true or B_temin2 = true. This serves to lock out the lambda control. The threshold value TEMINVA is differentiated from TEMIN with a cold engine when the wall film degradation is not properly emulated by the thinning-delay because te_w limits TEMIN. At higher speeds it is possible that the available theoretical maximum injection time is not sufficient to obtain the required target torque. Therefore, an injection time timx_w that is larger than the maximum possible injection time timxth_w is deployed until the desired torque is withdrawn and timx_w is not larger than timxth_w. For this purpose, the control error dtimx_w is assigned to a PI controller. When the controller is active, the output controlled variable mitibgr_w represents the desired torque. When the controller is inactive, mitibgr_w receives the value 100%. The desired torque in %MDBGRG is obtained by initializing with mifab_w and mitibgr_w. In order to avoid jumps in the nominal torque, the integrator of the integral component is initialized with mifab_w.

The controller is activated as soon as timx_w exceeds the speed-dependent threshold timxth_w. The controller remains in operation until timx_w < timxth_w AND mitibgr_w > mifab_w. See Applications Information.

RKTI 11.40 Application Notes

Calculation of the constant KRKTE:

KRKTE = (rho_air x V_hcyl) / (100 x 14.7 x 1.67x10^–5 x 1.05 x Q_stat)

= (50.2624 x V_hcyl) / Q_stat

Where:

rho_air = air density (1.293 g/dm³ at 0°C and 1013 mbar)

V_hcyl = Volume of a cylinder hub in dm³

Q_stat = injector constant with n-heptane

1.05 = injector correction factor for petrol

14.7 = Stoichiometric air quantity at lambda = 1.0

1.67x10^–5 = conversion factor minutes to milliseconds.

Calculation of the correction for fuel supply systems where the reference pressure of the fuel pressure regulator is ambient pressure:

FRLFSDP = SQRT[pdr_evmes/(pdr_akt + (pu - ps))]

Where:

pdr_evmes = absolute pressure in the fuel system before the injectors at the injector constant (Qstat) generally 3 bar

pdr_akt = actual fuel system pressure

pu = ambient pressure

ps = intake manifold pressure

For systems that take their reference pressure from the intake manifold pu - ps = 0 is used in the calculation above.

It then applies to the entire relationship FRLFSDP = Ö(pdr_evmes/pdr_akt)

For a fuel pressure of 3 bar, the results for FRLFSDP (where dpus = pu - ps) are as follows:

• Boost pressure = 1800 mbar, ambient pressure = 600 mbar

For consistency reasons, 11 sampling points for vacuum and turbo are used with the turbo-values.

In the charge sampling and injection application in returnless fuel systems via the code word for the reference pressure for the fuel pressure regulator (CWPKAPP), the constant PSAPES (intake manifold pressure for injection application) is used as a substitute value where the modelled intake manifold pressure ps_w has not been applied. Thus the manifold pressure can be set directly with a VS100 processor. With the VS20 processor, the pressure PSAPES can be changed with an adjustment factor between 0 and 2 via the RAM cell vsfpses (pses_w = PSAPES x vsfpses).

The initial value for PSAPES is 1013 mbar. If this value (in conjunction with a factor of 2 from vsfpses) does not define the maximum manifold pressure for turbocharged engines with VS20, the one-off value of PSAPES must be increased with VS100.

Initialization:

Map size in program development nmot x tevfa_w = 10 x 10

FKKVS: Sample points

The characteristic field FKKVS corrects errors in the fuel system (pulses in returnless fuel systems)

The map size of FKKVS can be extended to about nmot x tevfa_w = 10 x 10 to 16 x 10.

This is especially important to simplify the application for proportional systems. The speed ​​sample points ​​should match the number and values of the map KFPRG in the function BGSRM.

TEMIN: 1 milliseconds

TEMINVA: 1 milliseconds so that overall, the same TEMIN is active

TEMINVA: 0 milliseconds so that it is inactive when the engine is cold and thinning delay B_va = true, te to TEMIN seated and so that the wall film is not broken down properly.

ti-resolution values ​​are valid for a 20 MHz processor frequency. Otherwise thery must be converted thus: 20 MHz / (current processor frequency [MHz]).

Start:

ti_b1, ti_b2 25.6 microseconds. Measurements from VS100 must be multiplied by a factor of 8.

ti_tvu_w 25.6 microseconds. Measurements from VS100 must be multiplied by a factor of 8.

ti_w, ti2_w 16 microseconds.

te_w, te2_w not available.

Normal:

ti_b1, ti_b2 3.2 microseconds.

ti_tvu_w 3.2 microseconds.

ti_w, ti2_w 16 microseconds.

te_w, te2_w 3.2 microseconds.

First inputs:

ZTSPEV = 240 seconds

TVTSPEV

DMIL

CWDMIL

Bit 0 true: controller activated

Bit 0 false: controller deactivated

Bit 1 true: inputs B_ba and B_bag both active

KMITIBGR = 15 %/ms*s

PVMITIBGR = 0.8 %/ms