WO2019120802A1

WO2019120802A1 - Method of determining rail pressure in a common rail fuel system

Info

Publication number: WO2019120802A1
Application number: PCT/EP2018/081675
Authority: WO
Inventors: Andreas Nilsson
Original assignee: Delphi Technologies IP Ltd; Delphi France SAS
Current assignee: Phinia Delphi France SAS; Borgwarner US Technologies LLC
Priority date: 2017-12-20
Filing date: 2018-11-19
Publication date: 2019-06-27
Anticipated expiration: 2020-06-20
Also published as: GB201721449D0; GB2569579A

Abstract

A method of determining at least one operating parameter of a fuel injection system, said fuel injection system including a fuel rail adapted to supplied pressurized fuel to one or more fuel injectors, said fuel rail including a pressure sensor, comprising the steps of: a) obtaining a signal from the pressure sensor; b) analyzing the signal in the frequency domain; c) from the results analysis of step c) determining the at least one parameter.

Description

Method of Determining Rail Pressure in a Common Rail Fuel System

TECHNICAL FIELD

This invention is applicable to fuel injection systems which include a common rail (or accumulator volume) adapted to supply high pressure fuel to fuel injectors and which includes a (e.g. common rail) pressure sensor.

BACKGROUND OF THE INVENTION

An important parameter in the control of fuel injected engines is the fuel pressure in the common rail which is used to supply pressurized fuel to fuel injectors. This parameter is typically used in various engine control strategies.

In order to meet ever more stringent legislative emissions and diagnostics requirement as well as compensating for parts wearing over the lifetime of a combustion engine to ensure emissions and performance remains constant over life, more and more advanced control strategies are required.

At the same time, the computing power of the electronic controller(s) which control the fuel injection equipment, the air management, the exhaust after- treatment etc. has increased. With more advanced controllers, it is possible to include more advanced functionality that was previously possible, such as e.g. continuous rail pressure sampling and DSP (Digital Signal Processing) functions.

When fuel is injected by an injector into a combustion space, there is typically a pressure drop detectable in the common rail. Analysis of the pressure drop allows the quantity of fuel to be estimated. However, due to pressure ripples in the wave, it is difficult to get ascertain the exact pressure drop; furthermore a lot of filtering is required in this methodology. It Is an object of the invention to provide a method of determining rail pressure drop and/or fuel quantity injected which avoids the above mentioned problems.

SUMMARY OF THE INVENTION

In one aspect is provided a method of determining at least one operating parameter of a fuel injection system, said fuel injection system including a fuel rail adapted to supplied pressurized fuel to one or more fuel injectors, said fuel rail including a pressure sensor, comprising the steps of:

a) obtaining a signal from the pressure sensor;

b) analyzing the signal in the frequency domain;

c) from the results analysis of step c) determining the at least one parameter.

The parameter may be the pressure drop in the common rail or the amount of fuel injected, subsequent to one or more injections.

Step b) may comprise applying a Fast Fourier Transform to said signal to provide a FFT magnitude plot (signal) from said signal.

Step c) may comprise determining the magnitude of the main peak in the FFT magnitude plot and correlating this to fuel pressure drop or fuel quantity injected.

Step c) may comprise determining the magnitude of the lowest frequency peak of the FFT magnitude signal and correlating this to fuel pressure drop or fuel quantity injected.

Step c) may comprises analysing the FFT phase signal to determine the timing of injection.

Step c) may comprises analysing the FFT phase signal to determine the time separation between a plurality of injection pulses. BRIEF DESCRIPTION OF THE DRAWINGS

The present invention is now described by way of example with reference to the accompanying drawings in which:

Figure 1 shows plots of pressure drop against fuel quantity injected for different rail pressures;

Figure 2a and 2b shows plots of and injector waveform and corresponding pressure in the common rail measured by a pressure sensor;

Figure 3a and b shows the same plots as in figure 2a and b, and figure 3c shows the FFT of the rail pressure;

Figure 4a, 4b and 4c show corresponding plots of rail pressure against time, FFT amplitude against frequency and FFT amplitude against injected fuel quantity, for a range of different fuel injection quantities;

Figure 5 shows the plots of FFT amplitude against fuel quantity injected at different rail pressures of one crank cycle 720 degrees;

Figure 6a shows a plot of fuel injection quantity over time for two injections against sample number, figure 6b shows a plot corresponding to figure 6a showing total fuel injected against sample number, and figure 6c shows FFT amplitude over time for the injections;

Figure 7a shows the rail pressure over time, figure 7b shows corresponding the injector current drive-form showing, and fig 7c shows the total injected corresponding fuel in one example;

Figure 8 shows the amplitude of the FFT and the rail pressure drop corresponding to the test of figure 7a.

Figure 9a show the injection quantity (demand) for two injections, against sample number; figure 9b shows the corresponding separation demand, and figure 9c shows the FFT phase against sample number; and Figure 10 shows the shift in FFT phase of the rail pressure for the same frequency that was used in the case of figure 9a. DESCRIPTION OF THE PREFERRED EMBODIMENTS

The pressure drop in a common rail, caused by an injection, is proportional to the injected fuel. Figure 1 shows plots of pressure drop against fuel quantity injected for different rail pressures. The difference in gradient for different pressures is due to the bulk modulus of the fuel. Measuring the pressure drop in conventional methodology is tricky due to pressure waves inside the rail, especially after an injection.

Figure 2a and 2b shows pots of and injector waveform 1 and corresponding pressure 2 in the common rail measured by a pressure sensor respectively over time. As illustrated in figure 2b which shows rail pressure against time with respect to three injections designated with reference numeral 3, shown in figure 2a, the pressure before an injection can fluctuate by 10 bar and after an injection by over 20 bar. The pressure drop in the rail due to an injection increases by roughly 0.3 bar/mg at 1000 bar, so a 10 bar difference would lead to a 30 mg error in the estimation (the example in figure 2.2 is a 71 mg injection).

If one takes point samples before and after injection, the point samples might be on the peak or middle or bottom of some waves that are higher than cylinder frequency and one wouldn’t know about it.

If one take burst samples before and after then the burst can be averaged and will remove any waves that are shorter than the duration of the burst sample. However, if there is a wave that is longer than the burst sample but shorter than cylinder frequency then one would be unaware of this. A conclusion is that you have to have a longer burst sample - in fact you need to continuous sample in order to know what waves are there down to engine frequency in order to compensate for them. Anything less than continuous sampling will mean that there could be waves that you don’t know about. Although It is possible to filter out most of these fluctuations, this is tricky and it is impossible to filter all fluctuations. It will still be possible to do a good estimation of the injected fuel by average the pressure drop over a number of cycles. In one aspect the pressure in the common rail is analysed in the frequency domain and a method is to do the filtering by dividing the content of all pressure waves in the rail into frequency components, in order to focus on the relevant components.t are relevant to us.

In one method the fuel being injected into the cylinder is determined by analysing the rail pressure in the frequency domain, via e.g. FFT (Fast Fourier Transform), were amplitude and phase information provides data relating to injected fuel and separation between injection pulses. There is a low frequency component, which relates to the fluctuations in the rail, caused by injection (discharges the rail) and pumpings (charges the rail). This low frequency component can be converted into a fuel quantity, which can be compared to the demanded fuel and can compensate for changes in injected fuel due to e.g. coking and/or lacquering effect within the injector and mechanical wear. It can also be used for balancing the injectors. So in essence in one method, a FFT is applied to the pressure signal from a pressure sensor on the common rail, and the low frequency results used to determine rail pressure drop.

lniection quantity:

As mentioned an initial task is to detect the pressure drop in the rail due to an injection. If the pressure is measured before and after injection then fluctuations and waves means there will not be accurate measurement.

In the first step the pressure drop is determined by applying a FFT. Thus the pressure signal of the common rail pressure sensor is taken (over a respective time wind) and a FFT is applied to it which give a magnitude value for various frequencies, or frequency ranges (referred to typically as“bins”)

The pressure drop is cylinder synchronous so it appears mainly in the frequency bin that is at cylinder frequency. All the other waves that are not due to pressure drop are not at the cylinder frequency so they can either be ignored. Figure 3a and b shows the same plots as in figure 2a and b, and figure 3c shows the FFT 4 of the rail pressure; zooming in on the lowest frequency“peak” 5 , as this peak originates from wave caused by the pumping and injection events (injection discharges and pumping charges the rail).

Figure 4a 4b and 4c show corresponding plots of rail pressure against time, FFT amplitude against frequency and FFT amplitude against injected fuel quantity, for a range of different fuel injection quantities.

As can be seen in from the figures, the low frequency“peaks” on the FFT are proportional to the pressure drop/fuel being injected, as long as the pumping and injection not overlap. In case the pumping and injection do overlap, and if the pumps are equipped with control valves, the pumping can be disabled for the injector under test. In case of multiple injections, the low frequency peak in the FFT will correspond to the total fuel being injected.

So from the peak FFT magnitude, the pressure drop and or fuel quantity injected consequent to a fuel injection even 9comprising a single or multiple injection pulses) can be determined. The skilled person would be aware of methods to do this and how to correlate peak FFT magnitude to pressure drop and/or fuel quantity injected.

The FFT magnitude multiplied by the number of FFT samples used will give a voltage magnitude. The pressure drop can be determined form the voltage magnitude and depends on the calibration and range of a pressure sensor.

To convert pressure drop to fuel injected can be done in various ways including using prestored relationships such as MAPS or tables which correlated pressure drop to fuel quantity injected and may be dependent also on other parameters such as fuel rail pressure. Alternatively the quantity if fuel injected may be determined form pressure drop empirically by formula or numerical methods and would depend on factors such as also in addition the volume of the rail and the bulk modulus of the fuel.

Figure 5 shows the plots of FFT amplitude (e.g. at the first peak) against fuel quantity injected at different rail pressures (with respect to a FFT time window) of one crank cycle 720 degrees. There is useful information to be found in the amplitude and phase information to establish information regarding hydraulic separation between pulses.

Multiple Injections In some modem fuel injection strategies for singe injector, in one injection cycle there may be a plurality of injections e.g. pilot and main injection.

Figure 6a shows a plot of fuel injection quantity over time for two injections 11 and 12, against sample number (where there are a number of sample runs with two injections) figure 6b shows a plot corresponding to figure 6a showing total fuel injected against sample number, and figure 6c shows FFT amplitude over time for the injections. As can be seen the FFT amplitude equates to the sum of the pressure drops caused by the injections (total pressure drop is proportional to total injected fuel) when multiple injections are used.

Small quantity injections

Figure 7 shows a test of the minimum drive pulse of an injector. The top plot (fig 7a) shows the rail pressure over time, figure 7b shows the injector current drive- form showing, in this case, a burst of five pulses are scheduled (to amplify the pressure drop in the rail); the bottom plot (fig 7c) shows the total injected fuel. Figure 8 shows the amplitude of the FFT 13 and the rail pressure drop 14 corresponding to the test of figure 7; the FFT amplitude follows the measured rail pressure drop - it even improves the pressure drop detection slightly as it is not as sensitive to higher frequency waves in the rail. The pressure drop is measured by taking a burst of samples before and after the injections.

Start of injection

Figure 9 below shows the shift in FFT phase of the rail pressure for the same frequency that was used above to establish the relationship between injected fuel and FFT amplitude. As can be seen, the phase shifts with the separation (i.e. start of injection of pulse 2). Figure 9a show the injection quantity (demand) for a sample run with two injections; a pilot injection 16 which was a constant low level and a main injection 15 (where the quantity injected varied) against sample number; figure 9b shows the corresponding separation demand, and figure 9c shows the FFT phase against sample number. The sample number refers to the index of the particular sample run. So the figure shows the results from a number of sample runs each having two injections (e.g. a pilot and a main). The pilot quantity is the same for all the measurements in this example. The main injection is varied in eight steps and the hydraulic separation is swept for each pilot/main combination from 250 - 5000 ps. The sample number is just a particular measurements, so sample 1 a 2 mg pilot, 10 mg main and 250 ps separation, sample 2 mg pilot, 10 mg main and 500 ps separation, etc.

The FFT phase can be used to establish start of injection. It can also be used to establish if pulses have merged together or if the separation has changed. Figure 10 shows the shift in FFT phase of the rail pressure for the same frequency that was used above (in the case of figure 9) to establish the relationship between injected fuel and FFT amplitude. In this case, the separation between pulses has been fixed and the quantity (injection duration) varied to show the relationship between FFT phase and the injections. The figure shows the injected fuel quantity for the fuel injections, injection 1 and injection 2 designated with reference numerals 17,18, total injected fuel 19 and FFT phase 20. By establishing that the amplitude of the FFT of the frequency bin that is at cylinder frequency is proportional to the total injected fuel and that the phase of the frequency bin that is at cylinder frequency is (inversely) proportional to the separation between two pulses and that there is a phase change relationship to the first injection it is possible to: not only estimate total injected fuel, using the FFT amplitude but also estimate the start of injection for the pulses using the FFT phase. This could be done both as an average and cylinder by cylinder.

Claims

1. A method of determining at least one operating parameter of a fuel injection system, said fuel injection system including a fuel rail adapted to supplied pressurized fuel to one or more fuel injectors, said fuel rail including a pressure sensor, comprising the steps of:

a) obtaining a signal from the pressure sensor;

b) analyzing the signal in the frequency domain;

c) from the results analysis of step c) determining the at least one parameter; d) subsequently controlling an engine with said fuel injection system based on the parameter determined at step c).

2. A method as claimed in claim 1 where the parameter is the pressure drop in the common rail or the amount of fuel injected, subsequent to one or more injections.

3. A method as claimed in claims 1 or 2 wherein step b) comprises applying a Fast Fourier Transform to said signal to provide a FFT magnitude plot (signal) from said signal.

4. A method as claimed in claim 3 wherein step c) comprising determining the magnitude of the main peak in the FFT magnitude plot and correlating this to fuel pressure drop or fuel quantity injected.

5. A method as claimed in claim 3 or 4 wherein step c) comprises determining the magnitude of the lowest frequency peak of the FFT magnitude signal and correlating this to fuel pressure drop or fuel quantity injected.

6. A method as claimed in claims 3 wherein step c) comprises analysing the FFT phase signal to determine the timing of injection.

7. A method as claimed in claims 6 comprises analysing the FFT phase signal to determine the time separation between a plurality of injection pulses.