FIELD OF INVENTION:

The invention relates to fuel injection systems in general and controlling the fuel injection timings in particular.

BACKGROUND OF THE INVENTION:

Different injection patterns within one injection cycle are known in the prior art. To reduce emissions, Diesel Particulate Filters (DPF) are used in the exhaust path. These filters accumulate carbon particles which can be oxidised at a certain temperature level. To achieve this temperature first measure is to reduce combustion efficiency, e.g. by injecting fuel after the main injection (Ml) with a so called early - post injection (Pol). The combustion of this fuel will not generate considerable torque to the crankshaft but releases heat. If this is not sufficient, it is also known to inject late Pol, which will not burn in the cylinder any more. The fuel will evaporate and the hydrocarbons will be oxidised in the diesel oxidation catalyst (DOC) by again releasing heat, this time close to the DPF or even in catalytic section of the filter. The heat released by post injections is used to burn out the soot in the DPF. An engine control module controls the timings of the injections.

The US 20050224044 discloses one such method to control the timings of the injections.

ADVANTAGES OF THE INVENTION:

In the prior arts, the post injection may happen in multiple steps by splitting the quantity of fuel into many parts and injecting them as post injections (POIs). The number of Pols not only affect the charge loss from the power supply module of an engine control unit (ECU), but also the time to recharge the required capacitors in the power supply module will be reduced with increased band of crank angle utilized for injections.

The invention proposes a method to optimize the timing of the post injections so that there is sufficient time for the charging of the required capacitors in the power supply module.

This invention shall enable a better charged condition of the booster capacitor and in ideal case enable to remove the dedicated recharge coil in the Engine Control Unit (ECU). Instead of this coil the coil in the injector solenoid can be used while the injector is not injecting.

BRIEF DESCRIPTION OF THE DRAWINGS:

Figure 1: Shows the schematic of the invention

DESCRIPTION OF THE INVENTION:

Shown in fig. 1 is a schematic of the invention. Here only the components which are relevant to describe the invention are shown. Other components are the standard ones used in a vehicle, hence not shown.

Shown in fig. 1 is an engine 10 controlled by an ECU 12. The engine has input port 13, an exhaust port 14, and an injector 16. There is only one cylinder shown as part of the engine but in reality, there may be plurality of cylinders. The injector 16 is controlled by the ECU 12 through the power supply module 18. A pressure sensor 19 continuously provides the pressure information to the ECU 12.

As mentioned earlier, it is common practice to inject fuel in parts in one injection cycle. One injection cycle may comprise pilot injection, main injection and post injection (Pol).

In the schematic, the air is drawn through the intake port 13 and compressed by the piston 20 in the cylinder. As the piston 20 approaches top dead center of the cylinder, the ECU 12 actuates the fuel injector 16 to initiate the pilot injection and then the main fuel injection.

Once the main injection event is completed, the ECU 12 stops the injection. After a predefined time, the ECU 12 initiates the post injection. The post injection may be split into multiple injections. The post.injection does not add torque to the engine but helps in increasing the temperature.

Figure 2 shows the timing diagrams of the injections, the X axis representing the time and the Y axis symbolically representing the injections. The peak 200 represents the pilot injection, peak 202 representing main injection and the peaks 204 representing the post injections. In some engines there may not be any pilot injection but start with the main injection.

The invention proposes a method to start the post injections as early as possible after the main injection, but without causing combustion in the cylinder.

The pressure sensor 19 in the cylinder is monitored continuously by the ECU 12. Initially, once the main injections are completed, the post injections are started at an approximated timing reference ''t' from the end of main injection. The reference timing 't' is such that the fuel injected in the Pol does not ignite and also provides maximum charging period for the power supply. For this the Pol has to start at the earliest after the main injection.

The invention describes how to use the signal of the combustion pressure to determine the earliest possible time 'V to start the Pol.

The engine position management function in the ECU 12 gives the actual engine position at any time. As the cylinder pressure is known, using the gas equation the temperature of the gas inside the cylinder is computed by the ECU 12, taking into account heat transfer at wall and piston blow by.

Based on the computed temperature inside the cylinder, the ECU 12 can precisely determine when the temperature is low enough to avoid ignition of fuel which will be injected as a post injection (Pol). Based on the temperature computed, the ECU 12 starts a sequence of split Pols.

When fuel does not ignite, the pressure will further drop because piston is still running down, heat is lost to the wall and energy is consumed for the evaporation of the fuel.

If with the starting time of V, the pressure in the cylinder does not show any positive change in gradient, the starting of POIs is advanced by a known value for the next injection cycle.

The advancing of the starting of Pols for subsequent injection cycles is done till a time when the pressure shows a positive change in gradient. At this point the starting of POI is retarded by a pre-defined value. This is shown in fig 3. In fig 3, the X axis represents the time and the Y axis represents the pressure.

Shown in fig. 3a is the pressure curve when the fuel from the post injections is not combusting. Hence the pressure keeps dropping.

Shown in fig. 3b is the pressure curve when the fuel from the post injections is combusting. At the combustion point the pressure has a positive change in gradient, as shown by 300.

Thus the starting time of the post injection is optimized in a closed loop operation as mentioned below:

As long as the pressure drop is observed, as in fig. 3a, it is possible to further advance the late Pol timing gradually. The moment the pressure has a significant, unexpected change in the pressure profile, as shown in fig. 3b, the timing will be again retarded by a small offset.

As the pressure is monitored for every post injection, the ECU 12 can determine whether the pressure in the cylinder is decreasing or is there any positive change in the gradient positive gradient in the pressure. The positive change in the gradient in the pressure indicates that the fuel injected in the post injections is combusting. If the pressure is continuously decreasing, it indicates that the fuel is not combusting.

When the ECU determines that there is no positive gradient in the pressure of the cylinder, then the timing reference V is advanced to start the Pol early, the post injection is shifted towards the left on the X axis.

When the ECU determines that there is positive change in the gradient in the pressure of the cylinder, then the timing reference 't' is retarded to start the Pol late for the subsequent injection cycles.

The invention proposes a method to determine the optimized timings for post injections in a closed loop.

WE CLAIM:

1. A method to determine earliest timing for post injection in an internal combustion engine, the said method comprising the steps:

-determining the end of main injection

-computing cylinder heat by monitoring cylinder pressure of the internal combustion engine

-determining an approximate timing for a post injection in dependence of the cylinder heat and starting the post injection

-advancing and/or retarding the timing of the post injection for subsequent injection cycles in dependence of the pressure variations in the cylinder such that the post injections will not cause positive variation in the cylinder pressure.

2. A method according to claim 1 wherein injection cycle comprises at least one of a main injection an at least one of post injection.

3. A method according to claim 1 wherein the said post injection is split into a plurality of injections

4. An electronic control unit - ECU (35) to optimize the starting time of post injections in a cylinder, the said ECU adapted to read cylinder pressure from a pressure sensor and compute the temperature in the said cylinder; the said ECU adapted to compute a starting time for post injections for the present injection cycle in dependence of the computed temperature; the said ECU further adapted to advance or retard the starting time of the post injections for the subsequent injection cycles to avoid any positive variation in the pressure in the cylinder.