Control strategy for plug-in hybrid electric vehicle

A method of controlling a plug-in hybrid electric vehicle including an electric propulsion system, an engine, and a catalytic converter associated with the engine, the method comprising: monitoring a state of charge of a battery of the vehicle when in a charge depletion mode; determining a rate of depletion of the state of charge; estimating from the rate of depletion a duration of a depletion period representing the time remaining until a minimum state of charge of the battery will be reached; determining a duration of a warming period of the catalytic converter; comparing the duration of the depletion period and the duration of the warming period; and activating the engine if the duration of the depletion period is less than or equal to the duration of the warming period.

RELATED APPLICATIONS

This application is a 35 U.S.C. § 371 national stage application of PCT Application No. PCT/EP2016/054032, filed on Feb. 25, 2016, which claims priority from Great Britain Patent Application No. 1503104.0, filed on Feb. 25, 2015, the contents of which are incorporated herein by reference in their entireties. The above-referenced PCT International Application was published in the English language as International Publication No. WO 2016/135269 A1 on Sep. 1, 2016.

TECHNICAL FIELD

The present disclosure relates to a control strategy for a plug-in hybrid electric vehicle. In particular, but not exclusively, the disclosure relates to a strategy for controlling warm-up of a catalytic converter of such a vehicle. Aspects of the disclosure relate to a method, to a control system, to a vehicle, to a processor, to a computer program product and to a computer readable medium.

BACKGROUND

Plug-in hybrid electric vehicles (PHEVs), similarly to conventional hybrid vehicles, include an electric propulsion system working alongside an internal combustion engine to provide motive power for the vehicle. As a result of the common features, PHEVs share many advantages with conventional hybrid vehicles in terms of driving efficiency.

As with conventional hybrids, in a plug-in hybrid electric vehicle the architecture can either take a parallel arrangement or a series arrangement. In a parallel arrangement, both the engine and the electric propulsion system can supply torque directly to a vehicle transmission. Typically the vehicle alternates between the two sources of torque, although at certain times the two sources are used in combination, for example during acceleration from low speed. In pure series arrangements, torque is supplied to the transmission by the electric propulsion system at all times, and the engine is used only as a generator to provide electrical power to the electric propulsion system. In other arrangements, such as ‘powersplit’ or ‘series-parallel’ configurations, the electric propulsion system and the engine can drive the vehicle wheels independently or in combination as required, with the engine acting as a generator when needed.

Relative to conventional hybrid vehicles, PHEVs offer the advantage that a battery of the vehicle can be charged from an external power source when the vehicle is not in use, in the same manner as an electric vehicle. As with electric vehicles, this allows a PHEV to operate in a purely electric mode over a significant distance, known as its ‘all-electric range’. In contrast, in a conventional hybrid vehicle the total electrical energy available for motive effort is much lower, since all electrical energy stored in the vehicle battery is recovered internally, for example through regenerative braking. Therefore, the distance that a conventional hybrid vehicle can cover in a purely electric mode is much more restricted than for a PHEV.

PHEVs are typically operated in two distinct modes: a charge depletion mode, in which battery charge is used at a relatively high rate; and a charge sustain mode, in which the vehicle is operated so as to maintain the battery charge within a defined tolerance band. These modes of operation are each described in more detail below.

The charge depletion mode corresponds to pure electric operation in which the internal combustion engine is inactive, and the vehicle is entirely driven by the electric propulsion system. Therefore, in this mode the level of charge stored in the vehicle battery (referred to hereafter as the ‘state of charge’, or ‘SoC’) is consumed relatively quickly. The rate at which charge is depleted varies according to the load that is applied to the electric propulsion system, which is primarily determined by the way in which the vehicle is driven. For example, aggressive driving characterised by hard acceleration depletes battery charge more rapidly, with more charge being expended per mile of travel, than more sedate driving. Therefore, aggressive driving reduces the all-electric range of the vehicle. Similarly, the load applied to the PHEV will also influence the all-electric range, for example if towing a load, or if driving on an inclined surface. A further consideration is that internal electrical loads, particularly air conditioning systems and entertainment systems, can have a significant effect on the all electrical range.

On starting the engine, the vehicle enters charge depletion mode provided that the initial SoC of the battery is sufficient. While the vehicle operates in charge depletion mode the SoC drops gradually until it reaches a minimum level below which the battery cannot support continued electric operation. At this point, the vehicle switches to the charge sustain mode. As the rate at which charge is depleted is dependent on the manner in which the vehicle is driven, the point at which the transition between modes occurs varies, and is not known in advance. For this reason, the charge sustain mode does not, for example, activate after a certain time period or a certain distance, but is instead triggered with reference to the ability of the battery to support driver demand, for example represented by a predetermined threshold level of charge in the vehicle battery.

Upon entering charge sustain mode the internal combustion engine is started, and the PHEV operates in generally the same manner as a conventional hybrid: for a parallel arrangement the engine becomes the primary source of motive power for the vehicle, and the electric propulsion system is used in parallel with the engine for optimal overall powertrain efficiency. For a series arrangement, the electric propulsion system continues to drive the transmission, but the engine charges the battery to compensate for subsequent electrical power demands.

In this mode the vehicle is controlled so as to maintain the SoC of the battery close to the threshold value that is used to trigger the charge sustain mode initially. This means that when the electric propulsion system is used, for example to aid in moving the vehicle away from a stationary position, the battery charge is replenished from internal sources such as regenerative braking or direct generation from the internal combustion engine.

It is noted that the vehicle continues in charge sustain mode until the battery is next charged from an external power source to raise its SoC above the minimum level. This is primarily because using electrical energy supplied from a grid is typically more economical and more energy efficient than using the engine to charge the electric machine; it would be relatively inefficient and costly to fully recharge the battery using the engine as a generator. Therefore, it is typically preferable to use the engine to maintain the SoC at a minimum level, within a certain tolerance band, and then recharge fully from an external source when the vehicle is not in use.

A problem arises in the PHEV arrangement in that when the engine is started during the transition between the charge depletion and charge sustain modes, components of an engine exhaust gas after-treatment system attached to the engine, such as catalytic converters, are at approximately ambient temperature. The skilled reader will appreciate that catalytic converters do not act to catalytically convert pollutants in exhaust gases until operating in excess of 550° C. It takes approximately 20 seconds for this temperature to be reached from an ambient of 20° C. if the engine is operating specifically for the purpose of heating exhaust gases.

Of particular concern are NOxemissions from the engine, and which the engine exhaust after-treatment system is arranged to manage. Many countries apply restrictions to vehicle NOxemissions, and if the vehicle operates with a catalytic converter below the optimal temperature for a prolonged period it is possible that the emissions may exceed defined limits in those countries.

In a conventional combustion engine powered vehicle, there is typically a period of engine idling following engine start before the vehicle moves away. The catalytic converter warms during this time such that it is effectively treats the exhaust gasses when load is subsequently applied to the engine. Warming of the exhaust gas after-treatment system may be optimised, for example, by using a high engine idling speed while a crankshaft of the engine is not coupled to a driveline of the vehicle, and/or by tuning the combustion to optimise generation of heat.

In contrast, in the PHEV arrangement, load could be applied to the engine almost immediately after engine start, and so the catalytic converter has not had time to warm. There is therefore a risk of exceeding emissions limits for a short period following transition from charge depletion mode to charge sustain mode.

It is against this background that the present invention has been devised.

SUMMARY OF THE INVENTION

According to an aspect of the invention, there is provided a method of controlling a plug-in hybrid electric vehicle including an electric propulsion system, an engine, and a catalytic converter associated with the engine, the vehicle being operable in a charge depletion mode and in a charge sustain mode. When the vehicle is operating in the charge depletion mode the method comprises monitoring a state of charge of a battery of the vehicle, determining a rate of depletion of the state of charge, estimating from the rate of depletion a duration of a depletion period representing the time remaining until a minimum state of charge of the battery will be reached. When the vehicle is operating in the charge depletion mode the method further comprises determining a duration of a warming period of the catalytic converter, comparing the duration of the depletion period and the duration of the warming period, and activating the engine if the duration of the depletion period is less than or equal to the duration of the warming period. The method further comprises switching the vehicle from operating in the charge depletion mode to the charge sustain mode when the battery of the vehicle reaches the minimum state of charge.

The method may comprise monitoring an ambient temperature, and determining the warming period on the basis of the ambient temperature and a predetermined engine speed at which the engine is to be driven during the warming period. In this embodiment, the predetermined engine speed may correspond to engine idling, or alternatively the predetermined engine speed may be an elevated speed relative to engine idling.

Determining the warming period may include determining a cooling rate of the catalytic converter.

The cooling rate may be determined based on a measure of mass air flow across the catalytic converter.

The method may comprise monitoring an ambient pressure, and determining the warming period on the basis of the ambient pressure.

Determining the warming period may include obtaining a predetermined look-up value of the warming period, to minimise the required processing power.

Determining the warming period may include determining the time elapsed since the catalytic converter was last operated.

Comparing the duration of the depletion period with the duration of the warming period may comprise determining a state of charge threshold and comparing the state of charge threshold with the state of charge of the battery to determine when the duration of the depletion period is less than or equal to the duration of the warming period. In this embodiment, the method may comprise assigning indices to ranges of states of charge, defining a threshold index containing the state of charge threshold, and comparing the state of charge of the battery with a boundary of the threshold index to determine when the duration of the depletion period is less than or equal to the duration of the warming period.

The method may comprise determining a first rate of depletion of the state of charge over a first time period, and determining a second rate of depletion of the state of charge over a second time period, wherein the first time period and the second time period overlap.

According to another aspect of the invention, there is provided a control system for a plug-in hybrid electric vehicle including an electric propulsion system, an engine, and a catalytic converter associated with the engine, the vehicle being operable in a charge depletion mode and in a charge sustain mode. The control system comprises sensing means arranged to monitor a state of charge of a battery of the vehicle when in the charge depletion mode, processing means arranged, when in the charge depletion mode, to determine a rate of depletion of the state of charge, to estimate from the rate of depletion a depletion period representing the time remaining until a minimum state of charge will be reached, and to determine a duration of a warming period of the catalytic converter, and output means arranged when in the charge depletion mode to activate the engine if the depletion period is less than or equal to the warming period, the output means further being arranged to switch the vehicle from operating in the charge depletion mode to the charge sustain mode when the battery of the vehicle reaches the minimum state of charge.

In other aspects the invention also extends to a computer program product executable on a processor so as to implement the method of the invention, to a non-transitory computer readable medium loaded with the computer program product of claim10, to a processor arranged to implement the method or the computer program product of the invention, and to a vehicle arranged to implement the method of the invention, or comprising the control system or the processor of the invention.

DETAILED DESCRIPTION

FIG. 1shows in schematic form a simplified architecture for a PHEV10. The arrangement depicted is of the ‘powersplit’ type that will be familiar to the skilled reader, although it should be appreciated that this architecture is referred to by way of example only; embodiments of the invention could find application in many alternative architectures, for example in pure ‘series’ or ‘parallel’ type arrangements.

As this architecture is well known in the art, it is described in broad terms only so as to provide context for the embodiments of the invention that are introduced subsequently. It should be understood that this architecture operates according to the principles of a PHEV outlined above.

The PHEV10includes an internal combustion engine12arranged to operate in parallel with an electric propulsion system in the form of a combined integrated motor generator (CIMG)14. The CIMG14is operable either as an electric motor or as a generator. Both the engine12and the CIMG14can supply torque to a transmission16, which via a driveline drives one or more axles18carrying a pair of wheels20.

The engine12is also operable to drive the CIMG14as a generator to produce electrical energy to be stored in a battery22of the vehicle. An inverter24disposed between the CIMG14and the battery22converts a raw electrical output of the CIMG14into a suitable battery input. The inverter24can also operate in reverse to allow the CIMG14to draw power from the battery22to produce torque. The battery22includes an input26configured for connection with an external electrical power source such as a grid supply, enabling full recharging of the battery22over an extended period when the PHEV10is not in use.

The transmission16includes first and second differential gears28,30which transfer torque between components of the system. The first differential gear28is coupled to an output shaft32of the engine12, an input/output shaft34of the CIMG14, and a driveshaft36associated with the transmission16. The first differential gear28is configured such that when the engine12outputs torque, both the driveshaft36and the CIMG14are driven. Conversely, when the engine12is inactive, the CIMG14can drive the driveshaft36. In this way, the transmission16facilitates an isolating arrangement in which the engine12and the CIMG14can each drive the transmission16independently of the other. Alternatively, the engine12and the CIMG14can drive the driveshaft36simultaneously. In an alternative embodiment, a clutch is provided between the engine12and the first differential gear to provide complete isolation of the engine12as required.

The driveshaft36is disposed orthogonally with respect to the axle18, and the second differential gear30is used to accommodate the change in angle. The second differential gear30also provides an appropriate rotational speed adjustment between the driveshaft36and the axle18.

An exhaust system is provided to direct exhaust gases away from combustion chambers of the engine12. The exhaust system includes an exhaust gas after-treatment system comprising a catalytic converter38that reduces pollutants such as NOxin the exhaust gases. As noted above, the catalytic converter38is only fully effective once it reaches its operating temperature. In turn, the catalytic converter38relies on passage of hot exhaust gases through it to raise its temperature. Therefore, if the catalytic converter38has not been operated recently and so is at ambient temperature, it remains at ambient temperature while the engine12is inactive. If the catalytic converter38is already warm following recent operation, it cools towards ambient temperature while the engine12is not running.

A warming period, defined as the time taken for the catalytic converter38to reach its operating temperature, is determined by a combination of its starting temperature, the rate of warming, and the target operating temperature. If the catalytic converter38has not been warmed recently, the starting temperature is governed primarily by the ambient temperature. Otherwise, the starting temperature is determined by the time elapsed since it was last operated, in combination with a cooling rate which is governed by the ambient temperature. It is also noted that the catalytic converter38cools more quickly when the vehicle is moving due to air flow across the converter. It is further noted that the starting temperature may be governed by an ambient pressure of the air.

The rate of warming varies according to the flow rate and heat flux of exhaust gases through the catalytic converter38, which is in turn determined by the engine speed and load, along with other engine settings such as ignition angle and injector timing. This warming is offset to some extent by air flow across the catalytic converter38as a result of vehicle movement, as noted above. Therefore, if the ambient temperature, the vehicle speed, and the engine speed and load are known, the duration of the warming period for a given target temperature can be estimated.

Turning now toFIG. 2, an example of a battery SoC profile40over a typical driving cycle is illustrated. The drive cycle includes an initial period of operation in charge depletion mode, giving rise to a first portion42of the graph in which the SoC falls generally linearly. This is followed by a period of operation in charge sustain mode, commencing just before the SoC reaches a minimum level of 20%, resulting in a second portion44of the graph in which the SoC value is maintained above this minimum level, oscillating slightly about a constant level of approximately 22.5%.

While the SoC profile40during charge depletion mode follows a generally linear trend, it is clear that the gradient of the line, corresponding to the depletion rate of the SoC, is variable.

As noted above, this is due to the fact that driving parameters such as acceleration demand impact the SoC depletion rate. As a consequence, the point in time at which the SoC will reach the minimum threshold value to trigger charge sustain mode is not known in advance.

However, advantageously, by measuring the gradient of the graph line over any given time period, an estimate can be made of the time at which the minimum SoC will be reached, referred to hereafter as the transition point. This estimate provides a helpful indication of when loading of the engine12will be required (i.e. when switching from the charge depletion mode to the charge sustain mode will be required), thereby enabling the engine12to be prepared for use in advance of this moment. The estimate could be refined by also including data from a GNSS system of the vehicle to enable enhanced prediction of future driver demand.

In particular, this means that the engine12can be started in anticipation of demand in order to allow time for the catalytic converter38to warm and become effective before the engine12provides drive to road wheels20. Therefore, the catalytic converter38can be made operational prior to the transition point, and so there is a reduced risk of exceeding emissions restrictions over the range of driving cycles.

In order to achieve this, the duration of the warming period must also be known. As mentioned above, this can be estimated from the ambient temperature, which can be obtained from on-board sensors included for other purposes, and the engine speed, which is predetermined. Therefore, the duration of the warming period can be estimated. Using this estimate in combination with the predicted transition point, the time at which the engine12should be started to allow sufficient time for the catalytic converter38to warm can be determined.

A subtle benefit of this approach is that it ensures that the engine12does not run for longer than needed to warm the catalytic converter38, therefore preventing it from intruding on the all-electric range. To illustrate this point, a more crude arrangement is considered in which a simple predetermined SoC threshold is used to trigger start of the engine12in advance of the transition point. In such an approach, the threshold is necessarily set to a level above a point corresponding to a maximum possible SoC depletion rate, to ensure that sufficient time is provided for the catalytic converter38to warm regardless of how the PHEV10is driven, or the loading applied to the engine12. This would result in the engine12running for longer than necessary for all but the most aggressive driving profiles. In contrast, the present invention ensures that smooth drivers enjoy an extended period of electric operation compared with more aggressive drivers, whilst ensuring that legislative emissions targets are met.

The implementation of the above prediction method is illustrated in the graph ofFIG. 2by a narrow cross-hatched band46at the intersection between the two portions of the graph, which represents the warming period of the catalytic converter38. As shown in the graph, the warming period begins in advance of the transition point; this is clear from the fact that the SoC continues to fall during the warming period, which is indicative of the PHEV10continuing to operate in charge depletion mode throughout this time. As a result, load is only applied to the engine12after completion of the warming period, thereby avoiding undesirable emissions of pollutants such as NOx.

It will be appreciated that the predicted transition point varies according to the changing gradient of the SoC over time. For this reason, it is envisaged that the gradient measurement will be regularly or continually updated, so as to provide as accurate a prediction of the transition point as possible at all times. For example,FIG. 2shows three separate measurements of the gradient of the line, from which three corresponding predictions of the transition point can be calculated. A method for calculating and using the predicted transition point is described in more detail later with reference toFIG. 4.

It is noted that in this example, each measurement is taken over a 90 second period so as to produce an average gradient value that is not sensitive to short-term fluctuations. It will be appreciated that the length of the period used to calculate the gradient could vary significantly, to ensure that fluctuations arising from transient influences such as waiting at a junction, or hard acceleration when overtaking, are filtered out.

Additionally, it should be noted that the gradient calculation is conducted with increasing frequency inFIG. 2, insofar as the time between the first calculation and the second calculation is larger than the time between the second and third calculations. This is because as the transition point approaches it becomes important to update the prediction more regularly. For improved accuracy, the measurement periods can be overlapped, such that the predicted transition time is updated every two seconds for example, without changing the duration of the period over which the gradient is measured. In this way a dynamic prediction output can be provided.

For convenience, indices may be used to represent bands of SoC values. Index values are included inFIG. 2on the vertical axis to the right of the graph, taking values from 0 to 6.

The index containing the predicted transition point can be identified, and an upper boundary of that index can be used as the trigger for engine start.

Since the prediction of the transition point is inherently imprecise, the use of indices simplifies the implementation of the prediction method and accommodates calculation tolerances. It is noted that the indices do not cover uniform bands of SoC values: for enhanced accuracy the bands become narrower in the area of interest in which engine start is likely to be triggered.

Moving on toFIG. 3, a range of operating parameters for the PHEV10are represented over a typical driving cycle. The graph ofFIG. 3represents a shorter time period than theFIG. 2graph, although the transition period remains approximately at the centre of the x-axis inFIG. 3. The y-axis scale for each line is not shown: the graph lines are presented in a manner that enables straightforward comparison with one another, with the general trends shown; the absolute magnitude of the y-values of each line is not required for this purpose.

FIG. 3plots seven variables, including: the speed of the CIMG48; the engine speed50; the SoC of the battery52; the catalyst temperature in the catalytic converter54; the vehicle speed56; the relative load on the engine58; and catalytic converter heating activity60.

The SoC52, which is plotted centrally in the graph, is indicative of the operating mode of the PHEV10, and so acts as a reference point by which other variables shown in the figure can be understood.

As inFIG. 2, inFIG. 3the SoC52has an initial period of falling gradually, corresponding to charge depletion mode operation. As would be expected, during this time the CIMG speed48is relatively high since it is providing motive power for the PHEV10, whereas the engine speed50is at a minimum indicating that it is inactive. As the engine12is inactive, the catalytic converter38is not heated. This is represented in the lowermost line60, which is a digital indication of whether the engine is acting purely to heat the catalytic converter38, as a minimum. The fact that the catalytic converter38is not heated during this period is also reflected in the plot of the catalyst temperature54which exhibits no change in this period.

About one third of the way through the illustrated time period, a rise in engine speed50indicates engine start. This commences warming of the catalytic converter38in readiness for subsequent loading of the engine12, as described above. This is reflected inFIG. 3in that the lowermost line60is at a maximum during this period, indicating that the engine is operating only to warm the catalytic converter38. In this embodiment, the engine12is allowed to idle during the warming period, which is reflected in the graph with the engine speed50at a constant, relatively low level. It is noted that the idling speed is taken into account in the estimation of the duration of the warming period.

There are several advantages to allowing the engine12to idle during the warming period. Firstly, minimal fuel is consumed, leading to low emissions. This helps to ensure that pollutants are not released in excessive quantities during this time. Secondly, no energy is wasted in overcoming a load if the engine12is idling, meaning that heat generated from fuel combustion is transferred to the catalytic converter38as efficiently as possible. A final consideration is that idling generates minimal noise and vibration, which is a benefit in terms of user perception.

However, in other embodiments the engine speed50could be raised above idling during the warming period in order to increase the flow rate of hot exhaust gases through the catalytic converter38and so warm it more rapidly. While this may reduce some of the benefits described above, this is balanced by a reduced warming period.

Once the engine12is running, the catalytic converter38begins to warm. This is represented in the lowermost line60as a maximum, providing a digital indication that the catalyst is being heated. This is reflected in the plot of catalyst temperature54, which rises steadily during this period.

Since the PHEV10continues to operate in charge depletion mode during the warming period, the SoC52continues to fall, and the CIMG speed48remains elevated.

The initial period, which includes the warming period, ends around half way through the illustrated period. This is followed by a period in which the SoC52remains at a substantially constant value, corresponding to operation in charge sustain mode. Accordingly, the engine speed50rises in this period as it takes over the burden of supplying motive power, apart from at times where the engine load58is zero.

The vehicle speed56throughout the drive cycle provides further context for the changes exhibited in the other variables.FIG. 3corresponds to a vehicle certification cycle derived from real-world drive cycles, and so includes periods of acceleration, deceleration, and steady speed. There is also a period in which the PHEV10is stationary. Engine load58is zero while the vehicle is decelerating or at rest.

In the initial period when the PHEV10operates in charge depletion mode, it is clear that the SoC52depletes at a generally constant rate, which corresponds to a generally constant vehicle speed56. If the vehicle were to accelerate or decelerate sharply, this would be reflected in the SoC trend. This illustrates how the driving profile influences the transition time, and therefore why it is necessary to continuously or periodically recalculate the gradient of the SoC line52to provide an updated prediction.

Once the PHEV10assumes charge sustain mode operation, the driver demand torque influences the interaction between the CIMG14and the engine12. For example, it is apparent that in some periods the CIMG14and the engine12operate together in parallel, for example as the vehicle accelerates from a stationary position; this is because the CIMG14is used to compensate for the relatively poor efficiency of the engine12in this scenario. Prior to this, the engine12stops as the PHEV10comes to rest, allowing the CIMG14to recover energy not wasted in friction during braking, and ensuring that fuel is not wasted while the PHEV10is stationary. This is consistent with a ‘start-stop’ control regime that will be familiar to the skilled reader.

The high CIMG speed48shown during vehicle deceleration is indicative of the CIMG14being driven as a generator by the wheels20during these times to recover electrical energy to ensure that the SoC is maintained above the minimum level.

The relative load58on the engine12adds a final layer of information to aid the reader's understanding. It is noted that no load58is applied to the engine12when it is inactive.

Following engine start, a minimal load58is applied to the engine12to induce idling while the catalyst is heated. Once the PHEV10enters charge sustain mode, the loading58on the engine12immediately increases as torque is arbitrated between the engine12and CIMG14to provide tractive effort to the wheels20. From that point onwards, the loading58varies according to driver demand, as reflected in the changing vehicle speed56.

FIG. 4illustrates a process62for predicting engine demand and for starting the engine12in advance for preparation of the catalytic converter38. In this embodiment the process62is performed by a dedicated controller, although in alternative embodiments a standard controller of the vehicle may be used to the same effect, for example a powertrain control module. To perform its function, the dedicated controller includes inputs that receive signals indicative of the vehicle parameters relevant to determining the transition point and the warming period, such as the engine speed, the battery SoC, etc.

The process62involves two concurrent sub-processes64,66, a first64to predict the time to the transition point, and a second66to estimate the warming period.

The first sub-process64begins with monitoring at step68the SoC of the battery22over a pre-defined time period, for example 30 seconds. The change in SoC during this time is determined, and from this a depletion rate is calculated at step70as the quotient of the change in SoC with the time elapsed. Then, a depletion period defined as the time until the minimum SoC is reached is determined at step72based on the calculated depletion rate.

Meanwhile, the second sub-process66begins with reading at step74from an internal memory associated with the controller a predetermined engine speed at which the engine12is to be driven during the warming period, and a target temperature for the catalytic converter38. The controller then detects at step76an ambient temperature using an input from an on-board temperature sensor. Then, an estimated duration of the warming period is determined at step78, using the initial and target temperatures to determine the temperature difference, and the engine speed to estimate a warming profile. A safety factor may be applied to the warming period to ensure that the catalytic converter38has sufficient time to warm.

The process62then continues by comparing at step80the outputs from the first and second sub-processes64,66, namely the depletion period and the warming period. If the duration of the depletion period is equal to or less than that of the warming period, the engine12is started at step82. If the depletion period is larger than the warming period, it is too early to start the engine12. Therefore, the process62returns to reiterate the first and second sub-processes64,66so as to obtain an updated depletion period, and also to update the warming period if there is a change in ambient temperature. As the predetermined engine speed during the warming period is fixed, when the second sub-process66re-iterates it starts from the step of monitoring the ambient temperature.

In an alternative embodiment, at the comparison step80the warming period and the depletion period are used to determine a threshold SoC that will be used to trigger engine start. In this embodiment, an error-factor can optionally be applied to the SoC threshold rather than to the warming period.

It will be appreciated by a person skilled in the art that the invention could be modified to take many alternative forms to that described herein, without departing from the scope of the appended claims.

For example, while the invention is described above in the context of a powersplit hybrid architecture, as noted previously embodiments of the invention could be used in a pure series or parallel hybrid architecture. In this case, the PHEV10switches between charge depletion mode and charge sustain mode in the same manner as in the parallel arrangement, with the difference being that the engine12is used solely to charge the battery22in charge sustain mode. The skilled reader will appreciate that the above described problems with emissions at engine start will be equally prevalent in a series arrangement, and so the same principle of starting the engine early in anticipation of demand applies.

In the above described examples a warming period is estimated based on vehicle parameters, however in another embodiment a predetermined warming period is used in order to simplify the process.

Further aspects of the present invention are set out in the following numbered paragraphs:1. A method of controlling a plug-in hybrid electric vehicle including an electric propulsion system, an engine, and a catalytic converter associated with the engine, the method comprising:monitoring a state of charge of a battery of the vehicle when in a charge depletion mode;determining a rate of depletion of the state of charge;estimating from the rate of depletion a duration of a depletion period representing the time remaining until a minimum state of charge of the battery will be reached;determining a duration of a warming period of the catalytic converter;comparing the duration of the depletion period and the duration of the warming period; andactivating the engine if the duration of the depletion period is less than or equal to the duration of the warming period.2. The method of paragraph 1, comprising monitoring an ambient temperature, and determining the warming period on the basis of the ambient temperature and a predetermined engine speed at which the engine is to be driven during the warming period.3. The method of paragraph 2, wherein the predetermined engine speed corresponds to engine idling.4. The method of paragraph 2, wherein the predetermined engine speed is an elevated speed relative to engine idling.5. The method of paragraph 1, wherein determining the warming period includes obtaining a predetermined look-up value of the warming period.6. The method of paragraph 1, wherein comparing the duration of the depletion period with the duration of the warming period comprises determining a state of charge threshold and comparing the state of charge threshold with the state of charge of the battery to determine when the duration of the depletion period is less than or equal to the duration of the warming period.7. The method of paragraph 6, comprising assigning indices to ranges of states of charge, defining a threshold index containing the state of charge threshold, and comparing the state of charge of the battery with a boundary of the threshold index to determine when the duration of the depletion period is less than or equal to the duration of the warming period.8. The method of paragraph 1, comprising determining a first rate of depletion of the state of charge over a first time period, and determining a second rate of depletion of the state of charge over a second time period, wherein the first time period and the second time period overlap.9. A control system for a plug-in hybrid electric vehicle including an electric propulsion system, an engine, and a catalytic converter associated with the engine, the control system comprising:a sensor arranged to monitor a state of charge of a battery of the vehicle when in a charge depletion mode;a processor arranged to determine a rate of depletion of the state of charge, to estimate from the rate of depletion a depletion period representing the time remaining until a minimum state of charge will be reached, and to determine a duration of a warming period of the catalytic converter; andan output arranged to activate the engine if the depletion period is less than or equal to the warming period.10. A computer program product executable on a processor so as to implement the method of paragraph 1.11. A non-transitory computer readable medium loaded with the computer program product of paragraph 10.12. A processor arranged to implement the method of paragraph 1, or the computer program product of paragraph 10.13. A vehicle arranged to implement the method of paragraph 1, or comprising the control system of paragraph 9 or the processor of paragraph 12.