TO AN ENGINE

This invention relates to an engine system and particularly, but not exclusively, to an engine system for use with a dual fuel internal combustion engine, together with a method for controlling the supply of fuel to the engine.

Knock is a form of abnormal combustion which may occur in an internal combustion engine. The term knock is the name given to the noise which is transmitted through the engine structure when essentially spontaneous ignition of a portion of the fuel/air mixture takes place ahead of the normal combustion of this mixture.

This abnormal combustion process can cause a rapid release of much of the chemical energy in the fuel/air mixture, causing very high local pressures. The non-uniform nature of these local pressures causes pressure waves or shock waves of substantial amplitudes to propagate across the engine's combustion chamber, which may cause the chamber to resonate at its natural frequency.

The effect of knock on the engine depends upon its intensity and duration. Brief, low intensity knock may have no significant effect on the performance or durability of the engine. However continued and high intensity knock can lead to extensive engine damage. Such damage may include cylinder head gasket failure, piston or cylinder head erosion, or piston melting or holing. A knock sensor is a form of vibration sensor which is used to detect knock during the operation of an internal combustion engine.

Knock commonly occurs in spark ignition internal combustion engines in situations where, for example, the ignition timing is incorrect.

Knock does not normally occur in compression ignition internal combustion engines operating solely on diesel fuel.

A diesel fuelled compression ignition engine may be modified to run on a mixture of fuels whereby the first fuel is diesel fuel and the second fuel is often a gaseous fuel such as natural gas or propane. Such an engine is often termed a dual fuel engine. Such modifications are often made in the interests of economy since the cost of gaseous fuels, such as natural gas, is generally significantly less than that of diesel fuel. This can result in significant cost savings for vehicle operators whose vehicles cover high annual mileages.

In a dual fuel engine, when the engine is operating on a mixture of, for example diesel fuel and natural gas, the amount of the diesel fuel supplied to the engine is reduced to a level where it can still be ignited by the compression produced in the cylinders, with natural gas replacing the missing diesel fuel.

Subsequent combustion of the natural gas fuel is then initiated by the combustion of the reduced (or, pilot) quantity of diesel fuel injected into the engine.

In this situation, knock may occur as the natural gas fuel ignites if, for example, the natural gas / air mixture ahead of the normal combustion flame front spontaneously ignites.

Knock in a dual fuel compression ignition engine can be destructive to the combustion chamber components as described above.

Knock can be prevented in a dual fuel compression ignition engine by reducing the ratio of the first and second fuels supplied to the engine such that the knock levels remain below a safe limit. In one particular embodiment of the invention, in which the first fuel is diesel fuel and the second fuel is natural gas, knock may be prevented by reducing the ratio of natural gas to diesel fuel (substitution rate).

According to a first aspect of the invention there is provided an engine system comprising an engine adapted to operate in a first mode in which the engine is fuelled substantially entirely by a first fuel, and a second mode in which the engine is fuelled substantially entirely by a mixture of the first fuel and a second fuel, the engine comprising:

a first engine control unit (ECU) for controlling the amount of the first fuel supplied to the engine when the engine is operating in the first mode; a second ECU, operatively connected to the first ECU, and adapted to control the amount of the first and second fuels supplied to the engine when the engine is operating in the second mode; and

a knock sensor operatively connected to the second ECU, and adapted to sense knock in the engine,

whereby the second ECU is adapted to alter the ratio of the first and second fuels supplied to the engine when running in the second mode in response to the level of knock sensed by the knock sensor. By monitoring the level of knock sensed in the engine the system is able to continuously adjust the ratio of the first and second fuels supplied to the engine while ensuring the correct operation of the engine (i.e. by preventing the onset of knock).

The prevention of knock eliminates the associated mechanical damage which can occur to parts of the engine, which enhances the longevity and reliability of the engine.

Optionally, the engine comprises a plurality of cylinders and the knock sensor comprises a cylinder knock sensor at each cylinder. By sensing the level of knock at each individual cylinder the system is able to adjust the ratio of the first and second fuels supplied to the engine on a cylinder by cylinder basis.

This makes the system more effective at providing an optimum amount of the second fuel (i.e. optimising the substitution rate).

In an alternative embodiment, the knock sensor may comprise a cylinder knock sensor at each pair of adjacent cylinders. This can make the system simpler and cheaper to implement. In a further alternative embodiment, the knock sensor may comprise a single cylinder knock sensor on the engine block.

In embodiments of the invention, the or each knock sensor comprises a vibration sensor adapted to produce an analogue output signal.

When the knock sensor is a form of vibration sensor it will be able to produce an output whenever the engine is running. This output may be representative of the different events occurring during the engine cycle, such as induction, compression, ignition and exhaust.

During normal operation, an internal combustion engine generates horizontal and vertical shaking vibrations, fore and aft rocking moments, and various torsional vibrations, together with vibrations associated with the combustion process. These vibrations combine to produce a vibration signature for the engine.

As mentioned above, knock occurs when the fuel ignites irregularly and prematurely during the combustion process. This causes significant and erratic pressure differentials which produce a characteristic change to the engine's vibration signature.

Since the onset of knock generates unique vibration characteristics, the signal produced by the knock sensor at the onset of knock can be used to detect this event.

The or each knock sensor may be adapted to determine whether knock is above a predetermined threshold.

The second ECU may comprise a signal processing circuit adapted to receive the analogue output signal.

Optionally, the signal processing circuit comprises a low pass filter circuit and a knock signal acquisition circuit. The low pass filter is used to eliminate induced or pickup noise generated during the knock signal acquisition process.

Optionally, the knock signal acquisition circuit comprises a digital signal processing circuit integrated with a narrowband filter circuit.

The narrowband filter circuit is used to remove frequencies other than those of interest from the signal generated by the knock sensor.

The knock signal acquisition circuit may be tuned to the engine knock frequency, for a typical compression ignition internal combustion engine this can be between approximately 6 and 7 kHz. However, this engine knock frequency is a property of the configuration of the engine and, in other embodiments of the invention, it may be lower than 6 kHz or higher than 7 kHz.

In other embodiments, the signal processing circuit is adapted to produce a digital knock signal that is representative of the level of knock in the engine.

The generation of a digital knock signal which is representative of the level of knock in the engine is passed to the second ECU and is used to control the fuelling strategy of the first and second fuels.

According to a second embodiment of the invention there is provided a method for controlling the supply of fuel to an engine adapted to be fuelled by a first fuel in a first mode, and a mixture of a first and second fuel in a second mode, the method comprising:

sensing knock in the engine; and

altering the ratio of the first and second fuels supplied to the engine in the second mode in response to the level of knock sensed.

Optionally the engine comprises a first ECU operatively connected to a second ECU, and a knock sensor operatively connected to the second ECU, and the method comprises the additional initial steps of:

programming the engine to operate initially in the first mode in which the first ECU controls the amount of first fuel supplied to the engine; and

switching the mode of operation to the second mode in which the second ECU controls the amount of first and second fuels supplied to the engine.

In an embodiment of the invention, the first ECU is fitted to the engine by the Original Equipment Manufacturer (OEM) while the second ECU is an aftermarket addition.

In this embodiment, when the engine is running in the first mode, the first ECU will function as intended by the OEM since the engine will be fuelled entirely by the first fuel. This is because the second ECU does not influence the operation of the first ECU when the engine is running in the first mode.

However, when the engine is running in the second mode, the second ECU intercepts signals from first ECU which continues to operate as intended by the OEM, and determines the amounts of the first and second fuels which are to be supplied to the engine. According to a third embodiment of the invention there is provided a method for controlling the supply of fuel to an engine adapted to be fuelled by a first fuel in a first mode, and a mixture of a first and second fuel in a second mode, the engine comprising a first ECU operatively connected to a second ECU, and a knock sensor operatively connected to the second ECU, the method comprising the additional initial steps of:

programming the engine to operate initially in the first mode in which the first ECU controls the amount of first fuel supplied to the engine;

switching the mode of operation to the second mode in which the second ECU controls the amount of first and second fuels supplied to the engine;

sensing knock in the engine; and

altering the ratio of the first and second fuels supplied to the engine in the second mode in response to the level of knock sensed.

Optionally, the step of sensing knock in an engine comprises the steps of:

sensing a first knock signal at a first phase in the engine cycle; and sensing a second knock signal at a second phase in the engine cycle.

Fuel is injected by the fuel injection system into the engine cylinder toward the end of the compression stroke, just before the desired start of combustion. The liquid fuel, usually injected at high velocity as one or more jets through small orifices or nozzles in the injector tip, atomises into small drops and penetrates into the combustion chamber. The fuel vaporises and mixes with the high temperature and high pressure cylinder air. Since the air pressure and temperature are above the fuel's ignition point, spontaneous ignition of portions of the already mixed fuel and air occurs after a delay period of a few crank angle degrees.

A first knock signal acquired from the knock sensor at this point represents the initial combustion noise. This signal is used as a reference signal when determining whether knock has occurred later in the engine cycle since it is known that knock cannot have occurred at this point.

The cylinder pressure then increases as combustion of the fuel air mixture occurs. The consequent compression of the unburned portion of the fuel/air charge results in ignition of the fuel and air which has mixed to within combustible limits, which then burns rapidly. Injection continues until the desired amount of fuel has entered the cylinder. A second knock signal may then be acquired at this point in the engine cycle since it is known that knock may occur during this portion of the engine cycle.

The step of altering the ratio of the first and second fuels supplied to the engine in the second mode in response to the level of knock sensed, may comprise the steps of:

transmitting the first and second knock signals to a signal processing circuit; and

altering the ratio of the first and second fuels supplied to the engine in the second mode, in response to an output signal from the signal processing circuit.

The step of the first and second knock signals being transmitted to a signal processing circuit may comprise the steps of:

transmitting the first and second knock signals initially to a low pass filter; and then

to a knock signal acquisition circuit.

Optionally, the step of transmitting the first and second knock signals to a knock signal acquisition circuit comprises the steps of:

transmitting the first and second knock signals initially to a narrow band filter; and then

to a digital signal processing circuit.

By converting the analogue knock signals produced by the knock sensor into digital knock signals, these digital signals can be transmitted directly to the second ECU and used as part of the control strategy for altering the ratio of the first and second fuels supplied to the engine.

The step of altering the ratio of the first and second fuels supplied to the engine in the second mode, in response to an output signal from the signal processing circuit, may comprise the steps of:

comparing a ratio of the first knock signal to a corresponding second knock signal, to a predetermined value; and

altering the ratio of the first and second fuels supplied to the engine in response to the difference between the ratio of the first and second knock signals, and the predetermined value. The ratio of the first and second knock signals is then determined and compared to a predetermined value to determine whether knock has occurred in the engine and whether an adjustment is required to the ratio of the first and second fuels supplied to the engine.

The first knock signal effectively represents the level of background vibration during normal operation of the engine. By comparing the second knock signal to the first knock signal, the present invention effectively accounts for, and eliminates the effect of, this background vibration. This provides for a reliable and robust technique for determining whether knock occurs in the engine.

In embodiments of the invention, a high level of knock is defined by the second knock signal being greater than 1.2 times the first knock signal. In this situation, the amount of the second fuel supplied to the engine is reduced, and the amount of the first fuel increased accordingly, ensuring the total energy supplied remains the same.

In the case where the second fuel is a gaseous fuel, this reduction is achieved by reducing the gas substitution, typically in a 5% increment. Other ratios between the first and second knock signals may be used to effect a change in the amount of the second fuel supplied to the engine.

The step of sensing a first knock signal is carried out within a period commencing preferably between 1.0 and 3.0 ms, more preferably between 1.0 and 1.5 ms, after the commencement of a diesel injection signal; and

the step of sensing a second knock signal is carried out within a period following the end of the first knock signal sensing period;

each of the first knock signal sensing period and the second knock signal sensing period having a duration of between 2.5 ms and 3.5 ms.

The first knock signal is intended to be representative of the combustion noise of the engine, and is measured at a point in the engine cycle where diesel first flows from the injector and into the combustion chamber. There now follows a description of an embodiment of the invention, by way of non- limiting example, with reference being made to the accompanying drawings in which: Figure 1 is schematic representation of an engine system for a dual fuel engine according to a first embodiment of the invention; and

Figure 2 is a schematic representation of the engine system of Figure 1.

Referring to Figures 1 and 2, an engine system according to a first embodiment of the invention is designated generally by the reference numeral 10. The engine system 10 controls the supply of fuel to a diesel engine 20 that has been adapted to operate in a dual fuel mode in which it is fuelled by a combination of diesel fuel and natural gas.

The engine 20 is a six cylinder in-line diesel engine as used in a Mercedes Benz Axor commercial vehicle.

The supply of fuel to the engine 20 is controlled by a first ECU 30 and a second ECU 40. The first ECU 30 is the original equipment ECU supplied by the engine's manufacturer. The first ECU 30 transmits signals to the diesel injectors 50 and thus controls the supply of diesel fuel to the engine 20, when the engine 20 is operating in a diesel-only mode.

When the engine 20 is operating in a dual fuel mode, the second ECU 40 intercepts the signals from the first ECU 30, determines the mass of first fuel to be injected, and emulates an injector. It then produces a first modified signal and a second calculated signal of equivalent fuel energy to the original intercepted signal.

The first modified signal is transmitted to the diesel injectors 50 which supply a reduced quantity of diesel fuel to the engine 20. The second calculated signal is transmitted to the natural gas injectors 60 which supply a quantity of natural gas fuel to the engine 20

The engine system 10 further comprises two knock sensors 70 which are attached to the cylinder block of the engine 20. The knock sensors 70 are conventional automotive wideband knock sensors.

The first knock sensor 70 is positioned adjacent to the second cylinder in order to receive signals from the front three cylinders of the engine 20. The second knock sensor 70 is positioned adjacent to the fifth cylinder in order to receive signals from the rear three cylinders of the engine 20. In dual fuel mode, the engine 20 is fuelled by a mixture of diesel fuel and natural gas. The amount of natural gas supplied to the engine 20 is calculated by the second ECU 40 such that the combined calorific value of the diesel fuel and natural gas is equivalent to the calorific value of the diesel fuel supplied to the engine 20 when it is operating under the same speed and load conditions in a diesel only mode.

Turning now to Figure 2, the knock sensor 70 transmits a knock sensor signal to the knock sensor system 10 which is integrated in the second ECU 40. The engine system 10 includes a signal processing circuit 110 which further comprises a low pass filter 120 and a knock signal acquisition circuit 130. The knock signal acquisition circuit 130 includes a digital signal processing circuit 140 and a narrow band filter 150. The knock sensor signal is initially received by the low pass filter 120 which is configured to eliminate induced, or pickup, noise in the signal.

The knock sensor signal is then passed to the knock signal acquisition circuit 130 which is a tuned circuit being tuned to the engine's knock frequency. In the engine 20 of the present embodiment, this engine knock frequency is between approximately 6 and 7 kHz.

In the present embodiment, the knock signal acquisition circuit 130 comprises a Texas Instruments TPIC8101 integrated circuit (IC). This IC includes a digital signal processing circuit 140 and an integrated narrow band filter 150. The IC may be tuned to the engine's knock frequency by writing the appropriate 8 bit word to an external register.

The acquisition of the first knock signal and the second knock signal is made by the knock signal acquisition circuit 130 upon receipt of a logic control signal from a microcontroller 160 within the second ECU 40.

After acquisition of each of the first and second knock signals, the microcontroller 160 requests the amplitude of the corresponding signal from the knock signal acquisition circuit 130.

These signals are passed between the knock signal acquisition circuit 130 and the microcontroller 60 using a series communications interface. The process of acquiring the first and second knock signals is carried out at two specific corresponding points during each engine cycle. The first knock signal acquisition phase commences approximately 1.1 ms after the commencement of the diesel injection signal, at the maximum gas substitution level of 90%. This is the point at which diesel fuel starts flowing from the injector and into the combustion chamber. When the gas substitution level is less than 90%, the time, t _s , to the start of the first knock signal acquisition phase is defined as follows: t _s = 1.1 + (( 0.9 - g _s ) * 2 ) ms where: ts = delay between the commencement of the diesel injector signal and the start of the first knock signal acquisition phase; and

g _s = gas substitution level.

The duration of the first knock signal acquisition phase is approximately 3 ms.

At the end of the first knock signal acquisition phase, the amplitude of the first knock signal is requested by the microcontroller 160 from the knock signal acquisition circuit 130. The second knock signal acquisition phase follows immediately from the first knock signal acquisition phase and also has a duration of approximately 3 ms.

At the end of the second knock signal acquisition phase, the amplitude of the second knock signal is requested by the microcontroller 160 from the knock signal acquisition circuit 130.

No use is made of the signals from the knock sensors 70 during the remaining portion of each engine cycle. The amplitude of the second knock sensor signal is then compared to that of the first knock sensor signal and this ratio is then used to determine whether knock is present in the engine 20. A ratio of approximately 1.2 (i.e. the magnitude of the second knock sensor signal is approximately 1.2 times that of the first knock sensor signal) indicates that a high level of knock is present.

In these circumstances, the second ECU 40 instantly reduces the substitution rate of natural gas supplied to the engine 20, typically a reduction of 5% in the substitution rate of natural gas. A lower ratio will result in a correspondingly lower reduction in the substitution rate of natural gas supplied to the engine 20, for example a 1% reduction in the substitution rate of natural gas supplied.

If the ratio is only approximately 1.1 , the system makes no reduction to the substitution rate of natural gas supplied to the engine.

In situations where the ratio is lower than 1.1 , the second ECU 40 will increase the substitution rate of natural gas supplied to the engine 20 provided the following three criteria are satisfied:

(i) the amount of diesel fuel presently being supplied to the engine 20 exceeds a predetermined minimum value;

(ii) the amount of natural gas presently being supplied to the engine 20 is less than a predetermined maximum value, typically 90%.; and

(iii) the valve open time is limited.

The system treats the supply of both diesel fuel and natural gas to each separate cylinder in a discrete manner. Consequently, each cylinder may have a different amount of natural gas supplied to it. The engine system and method for controlling the supply of fuel to an engine have hereinabove been described in relation to their application to the engine from a Mercedes Benz Axor commercial vehicle.

However, it is to be understood that the engine system and method are equally applicable to compression ignition internal combustion engines of different displacement and/or cylinder configuration. By way of example, in an alternative embodiment, the engine system is used to control the supply of fuel to the diesel engine from a Mercedes Benz Actros commercial vehicle, which has a six cylinder vee configuration. In this alternative embodiment, the start of the first knock signal acquisition phase occurs at 1.1 ms after the commencement of the diesel injector signal.

The above described engine system and method have been described in relation to their application to an engine being configured to operate on either a first fuel or a mixture of a first fuel and a second fuel, with the first fuel being diesel fuel and the second fuel being natural gas. However, it is to be understood that the engine system and method may also be applied to engines in which the first and second fuels are other than diesel and natural gas respectively.