GB2418898A

GB2418898A - Hybrid electric drive system optimising emissions from a vehicle

Info

Publication number: GB2418898A
Application number: GB0422366A
Authority: GB
Inventors: Stephen Latham
Original assignee: TRL Ltd
Current assignee: TRL Ltd
Priority date: 2004-10-08
Filing date: 2004-10-08
Publication date: 2006-04-12
Also published as: GB0422366D0

Abstract

An electric drive system (700) for a vehicle comprises a combustion engine (710) for providing a primary energy source for the vehicle; an energy storage unit (760) operably coupled to the combustion engine (710) for providing additional power for the vehicle; and an engine management system for monitoring an operational mode of the combustion engine (710). The engine management system is also operably coupled to the energy storage unit (760) and arranged to provide the additional power from the energy storage unit (760) for the vehicle intermittently, to substantially avoid emissions in a non-stoichiometric operational region of the combustion engine (710). In this manner, an electric drive system is described that optimises a combustion engine solely to produce emissions within acceptable limits.

Description

- 1- 2418898

ELECTRIC DRIVE SYSTEM AND METHOD FOR OPTIMISING EMISSIONS

FROM A VEHICLE

Field of the Invention

This invention relates generally to an improved electric drive system for optimising emissions from a vehicle. In particular, the present invention relates to supplementing a vehicle primary energy source with an auxiliary energy source.

Background of the Invention

Conventional vehicles have power systems that require a gasoline or diesel engine (the former using a spark emission mechanism), a clutch mechanism, and either a multi-speed standard or automatic transmission. The. .

general design of these vehicle power systems is typically the same, regardless of the intended use of the.

-....

vehicle. Modern spark ignition engines produce low A..

emission levels under 'most' operating conditions. The operation of the engines, however, can be very varied, .. ...

for example, high-speed Motorway (highway) driving.. ..

compared to urban stop-and-start driving.

However, very high Carbon Monoxide (CO) and Hydrocarbon (HC) emissions are still generated during short periods of high power demand, such as fast acceleration, hill climbing and high-speed motorway driving. Such modes of operation of the vehicle's engine are not currently used to determine whether the vehicle engine passes the EC emissions tests. High emissions may also occur during cold starting situations. - 2

For example, a Volkswagen stupor 1.4 litre car conforming to EURO 4 legislation was driven over a route in the city of Southampton, England, whilst emissions from the vehicle were continuously measured by on-board analytical equipment. Twenty-three separate tests were conducted over the same route by a variety of drivers. Analysis of the data collected showed that 86% of the total CO emissions and 31% of the total HC emissions were emitted during periods when the engine exceeded either 60% of maximum engine load or 4000rpm engine speed. However, and notably, the load and speed of the engine never exceeded these limits during most of these tests and the test that exceeded the limits most did so for less than 2.5% of driving time. Notably, the current European Commission emissions tests are performed when an engine of a vehicle is running in a low-to- medium load and speed .

normal operating mode, so fails to detect periods of extreme emissions. . . 20..

Relative emissions of HC and CO verses vehicle speed for a Euro 3 1 litre Vauxhall Corsa car for one of the tests. . are shown in FIG. 1 and FIG. 2 respectively. Of note is. ..

that vehicle emissions are substantially increased above a certain threshold speed, as the demand for power increases. Thus, stoichiometric conditions cannot be maintained. As known in the art, stoichiometric conditions are those conditions where the amount of fuel injected into the engine is just enough to react with all of the available oxygen in the intake air. It should be noted that speeds on motorways are rarely constant, and that accelerating at lower speeds than the threshold could still trigger intermittent high emission events.

FIG. 3 and FIG. 4 illustrate graphs of measured data for relative emissions of HC and CO verses time during the aforementioned emission trials. Of note, when considering FIG. 3 and FIG. 4, are the high emission (non-stoichiometric) periods, indicated as crosses on the sharp spikes 310, 410 respectively.

It is known that some manufacturers have built 'concept vehicles' using ultracapacitors/electric motors in conjunction with an internal combustion engine. Notably, these concept vehicles are only designed to either increase the overall power of the vehicle, or to downsize the engine. For example, Honda has developed a hybrid concept car, which uses an energy storage unit such as an ultracapacitor in conjunction with a motor/generator to supplement a downsized engine. Such a vehicle concept is. .

described at the following web-site: http://www.geocities. com/CapitolHill/3589/airp- . honda.html. . . . Other manufacturers have also produced hybrid concept.. e

vehicles using ultracapacitors. However, all of these. ..

hybrid concept vehicles employ batteries or fuel cells.

It is far from clear how such techniques can be re- produced easily and effectively in current vehicles, as they require a radical technological change. They also need complex engine management units, to control these extra components.

Referring now to FIG. 5, prior art US patent US 5,345,154 describes an engine system 500 comprising a combustion engine 510 that operates an alternator 520. The 4 - alternator 520 supplies electrical power to an electric continuously variable transmission for driving an ac motor 530 through a differential 540 to multiple wheels 550 (or for driving multiple independent ac wheel motors) in a vehicle. The combustion engine is configured to operate in a closed-loop (stoichiometric) power mode.

This allows the combustion engine 510 to produce the required power at a particular operating speed and throttle position. In particular, an energy storage unit, such as an ultracapacitor 560, is coupled to the ac motor 530 in order to provide electrical energy to the ac motor 530 to maximise fuel economy and 'minimise' engine emissions.

Thus, US 5,345,154 describes an engine system that aims to minimise the fuel consumption and engine carbon monoxide and hydrocarbon emissions. Furthermore, the. .

technique proposed in US 5,345,154 focuses on fuel consumption as well as maintaining stoichiometric operation of the engine (i.e. operate the engine in a closed-loop manner). This is more clearly shown in FIG. 6, which illustrates an example of an engine 'map'. FIG. .. .

6 shows a graph 600 of engine load (or torque) 610 versus engine speed 620 for a typical vehicle.

Notably, the technique proposed in US 5,345,154 will always employ the final drive power source in series.

This means that the ultracapacitor will be almost permanently in use. The primary power source is arranged to operate at 'X' 630 the point of maximum efficiency.

At all times when the primary power source is not operating at 'X', the ultracapacitor will be controlled in a manner to 'correct' this.

-

Furthermore, the engine system described in US 5,345,154 is an entirely electrical drive system, i.e. there is no mechanical drive between the engine and wheels. Thus, in order to utilise the concepts shown in US 5, 345,154, the design of current vehicles needs to be radically modified. The design would require the use of a relatively large alternator/generator to continuously supply power to the electric motor, which is the only direct source of power to drive the vehicle. Therefore, the total power available at the wheels in this 'series- type' system will be limited to that of the electric motor, rather than the combined engine and electric motor power in a parallel system. This necessitates using a much large electric motor and alternator, compromising space, weight and cost of the vehicle. :.

It is also noted that many vehicle manufacturers are downsizing IC engines to improve fuel economy. This may.

further restrict the low emission part of the emission. ë *

map and increase the disparity between actual and legislated emissions. ..

.. . . . A:.

Thus, there exists a need in the field of the present invention to provide for a simpler, cheaper system, for reducing emissions from a vehicle that can be realised in practical operating conditions, wherein the abovementioned disadvantages are substantially alleviated.

Statement of Invention 6 -

In accordance with a first aspect of the present invention there is provided an electric drive system for a vehicle, as claimed in Claim 1.

In accordance with a second aspect of the present invention there is provided an engine management system, as claimed in Claim 8.

In accordance with a third aspect of the present invention there is provided an energy storage unit, as claimed in Claim 9.

In accordance with a fourth aspect of the present invention there is provided a method of optimising vehicle emissions, as claimed in Claim 10.

In accordance with a fifth aspect of the present.. . invention there is provided an engine management system, : as claimed in Claim 16.. . 20.... . In accordance with a sixth aspect of the present invention there is provided an energy storage unit, as . claimed in Claim 17. . ..

Thus, in summary, the aforementioned problems with known techniques are resolved by use of an energy storage unit, such as an ultracapacitor, being configured to only deliver additional power to supplement a combustion engine's power in order to meet sudden power demands, such as rapid accelerations and hill climbing. In this manner, the combustion engine is always operating within a low emission (stoichiometric) mode. Thus, when considering FIG. 3 and FIG. 4, the high emission periods indicated as crosses on the sharp spikes would be effectively eliminated.

Thus, the inventor of the present invention has recognized and appreciated that the extra power required to support additional speed and load limits can be supplied from a different source than the engine, thereby substantially reducing overall engine carbon monoxide and hydrocarbon emissions.

Brief Description of the Drawing.

FIG. 1 and FIG. 2 illustrate graphs of measured data for relative emissions of HC and CO verses vehicle speed; FIG. 3 and FIG. 4 illustrate graphs of measured data for relative emissions of HO and CO verses time during .

emission trials; : FIG. 5 illustrates a known engine electrical drive. ; system, as proposed in US 5,345,154; and FIG. 6 illustrates a graph of engine load (power) versus. ..

engine speed for a typical vehicle.

Exemplary embodiments of the present invention will now be described, with reference to the accompanying drawings, in which: FIG. 7 illustrates an engine electrical drive system, in accordance with a preferred embodiment of the invention; 8 - FIG. 8 illustrates a graph of engine load (power) versus engine speed for an engine management system configured in accordance with the preferred embodiment of the present invention; and FIG. 9 is a flowchart illustrating a preferred method of optimising vehicle emissions using an auxiliary energy storage unit, in accordance with a preferred embodiment of the invention.

Description of Preferred embodiments

Referring now to FIG. 7, a direct drive engine system 700 is illustrated, in accordance with a preferred embodiment of the present invention. A direct drive engine system, in the context of the present invention, is one where there is a direct mechanical linkage between the. .

combustion engine and the wheels. The direct drive engine system 700 is configured with a combustion engine.

710, whereby the combustion engine (710) is mechanically.

-....

linked to the wheels of the vehicle. Combustion engine 710 supplies power both to an alternator 720 and the. . wheels 750, in parallel. Power is supplied to the wheels. ..

750 via a gearbox 740 as in most conventional designs.

However, according to the preferred embodiment of the present invention, the alternator 720 supplies current to an ultracapacitor 760, which is arranged to build up/store a sufficient 'additional' charge over time.

This additional charge is used to supply enough current to operate an electric motor 730 to drive the wheels 750 through the gearbox 740, in parallel with the engine. It is envisaged that the electric motor 730 is configured to drive the wheels 750 intermittently for short periods of 9 time, for example to address high-power demand. This reduces the power demand on the engine, thereby ensuring it always operates in a closedloop, low emissions mode, whilst ensuring the total torque available at the wheels is equivalent to that of the same size of engine operating in an open-loop mode.

The vehicle alternator 720 in the preferred embodiment of the present invention may be slightly larger than would normally be used to change the battery of a conventional vehicle. The slight increase in size provides the additional charge that the ultracapacitor 760 requires, to ensure that there is sufficient torque for all open- loop accelerations. However, the increase in size is likely to be minimal, since the electric motor 730 can potentially act in reverse, so as to recharge the ultracapacitor 760 by using regenerative braking. . .. I.

The vehicle's engine management system (not shown) is.

configured to restrict the fuelling rate to the engine, . so that the part of the engine map that normally requires open-loop conditions is never encountered. However, when.. e.

greater torque is required within this region, the engine. ..

management system discharges current from the ultracapacitor 760 to the electric motor 730 to supplement the engine power. In this manner, the combined power is transmitted to the wheels 750 via the gearbox 740. The rate of discharge and the driving force produced at the wheels is determined by the gear selected and position of the accelerator pedal, via the engine management system controls. -

A skilled artisan will appreciate that the engine management system would be configured to control numerous functions within the vehicle, in addition to the discharge of the ultracapacitor 760. For example, the engine management system would also be linked to the engine fuelling system, gear selection and other vehicle sensors, etc. In more general terms the engine management system is arranged to monitor the operational mode of the vehicle and utilise the additional power available from the energy storage unit as and when required, to operate just' within a closed-loop (stoichiometric) power mode, as illustrated in FIG. 8.

Referring now to FIG. 8, a graph 800 of engine load (power) 810 versus engine speed 820 for an engine . management system is illustrated, whereby the engine management system is configured in accordance with the.

preferred embodiment of the present invention. Notably, . the engine management system is configured to employ the additional (preferably ultracapacitor) power source only. . when the combustion engine 710 is operating in the. ..

vicinity of the stoichiometric/non-stoichiometric boundary 840.

Thus, the engine management system controls the emission performance of the vehicle via a pre-programmed map relating fuel injected or electric current discharged to the engine, speed and load demand. The load demand is indicated by the accelerator pedal (with automatic transmissions, the gear selection is also programmed).

The fuel injected is fine tuned to stoichiometric by use - 11 of an air fuel sensor, as with conventional engine designs, although this closedloop mode is always retained at normal engine operating temperatures.

In accordance with the preferred embodiment of the present invention, the engine management system preferably determines when the performance of the combustion engine 710 may be entering the non- stoichiometric region by using a pre-programmed engine map relating accelerator position and engine speed to engine load, with pre-defined threshold limits of engine load and engine speed. Thus, an engine map in this context may comprise a series of engine performance characteristics preferably with pre-defined or operational limits.

In this manner, it is envisaged that the: . stoichiometric/nonstoichiometric boundary 840 of FIG. 8 can be defined mathematically in terms of engine load 810.

and engine speed 820. Thus, the engine management system.

always ensures that the engine never enters an area of * the engine map which would normally require non-. . stoichiometric conditions...

Thus, as the engine management system is configured to monitor the engine load and engine speed, it is able to map an operational condition of the combustion engine directly onto graph 800 of FIG. 8. Furthermore, by employing a series of thresholds as determining factors, to identify when the combustion engine performance is approaching a non-stoichiometric mode of operation, the engine management system is able to initiate the - 12 additional power to be supplied by the energy storage unit 760 in good time.

When the engine load is above curve 840, the energy storage unit 760 supplies the extra power. This allows the engine to meet demands as high as that shown by curve 830, without the output and emissions of the combustion engine exceeding those of the stoichiometric region below curve 840.

The combination of elements shown in FIG. 7 can thus meet demand, up to that corresponding to an engine, i.e. one that would alone meet the demand shown by curve 830. So combustion engine 710 can be a 'down-sized' engine, without the risk of non-stoichiometric emission peaks at times of abnormally high engine demand. e'

It is also envisaged that the engine management system;.

may be arranged to prevent the combustion engine (710) operating in a nonstoichiometric region, if a power demand of the vehicle is identified as exceeding a level that would require the power from both the combustion -engine and the energy storage unit to enter such a non- Be. ..

stoichiometric region. Similarly, if the ultracapacity is identified as carrying no charge, the engine management system may be arranged to prevent the combustion engine (710) operating in a non-stoichiometric region based on the power from the combustion engine alone.

In an enhanced embodiment of the present invention, as mentioned earlier, it is envisaged that the engine - 13 management system also preferably identifies how much power to extract from the energy storage unit 760.

Since the engine and electric motor provides power in parallel to the gearbox and wheels, the size of the electric motor and ultracapacitor is reduced relative to a conventional series hybrid vehicle, such as that in FIG. 5.

Furthermore, it is envisaged that the electric motor will only be provided intermittently with the additional power, for example 1-2% of driving time.

Furthermore, these smaller components only provide enough additional drive to maintain the vehicle emissions within the limits of stoichiometric operation when the engine management system determines that the combustion engine .

is about to operate in a non- stoichiometric mode. . Advantageously, the proposed electric drive system could be more easily adapted to existing designs of vehicle. :e Referring now to FIG. 9, a flowchart 900 illustrates a.. A..

method of optimising vehicle emissions using an energy... .,

storage unit, in accordance with a preferred embodiment of the present invention. As a starting point, it is assumed that the combustion engine initially powers the vehicle in isolation, as shown in step 905.

The engine management system then monitors the operation of the combustion engine in step 910 and determines whether the non- stoichiometric threshold is being approached, as shown in step 915. If the nonstoichiometric threshold is not being approached in step - 14 915, the process loops between step 910 and step 915. If the non- stoichiometric threshold is being approached in step 915, the engine management system preferably identifies how much additional torque is required to supplement the combustion engine torque, in order to maintain its operation within stoichiometric limits, as shown in step 920.

The engine management system then extracts the identified additional power from the energy storage unit (preferably an ultracapacitor) to supplement the combustion engine torque to maintain its operation within stoichiometric limits, as shown in step 925. The process then loops back to step 910, to ensure that the operation of the combustion engine remains within the stoichiometric limits. .:e

It is noteworthy that the current trend towards .

downsizing engines has obvious benefits, since a smaller, . lighter and therefore more fuel-efficient power unit can.... . be used. However, downsizing the engines in such a .

manner clearly results in more frequent excursions into .

the high emission (non-stoichiometric) zone of FIG. 6. .. . This occurs since the maximum deliverable power in the stoichiometric range would be reduced but, the demand would be the same. Thus, the use of ultracapacitors in current concept vehicles is focused on supplementing the vehicle's performance, and often reducing fuel consumption, rather than optimising vehicle emissions, as proposed herein.

In an enhanced embodiment of the present invention, the energy storage unit may be used for cold-start operations. It is known that vehicle catalysts start to reduce emissions only once they have reached a predetermined temperature. Therefore, vehicle catalysts are designed with low thermal inertia, which allows them to rapidly heat up after starting. During this short period substantial emissions are produced, which (given that the fuel air mixture is generally non- stoichiometric) can amount to a significant proportion of the total emissions during a journey.

Furthermore, having low thermal inertia means catalysts also cool down quickly after the engine is switched off.

Therefore, in accordance with a preferred embodiment of the present invention, an electric preheating system for catalysts is beneficially employed. The energy storage unit, preferably an ultracapacitor, of the present.. . invention may be configured to provide such a convenient means of providing the energy to preheat catalysts from .

cold. This pre-heating might start, for example, as soon ... . as the driver unlocks the vehicle with a remote central ... . locking system.

::::. ..

It is envisaged that there are many further potential.. :.

applications for the energy storage unit described herein, such as defrosting windows during cold weather.

It is also envisaged that such a defrosting application may be performed in combination with catalyst heating.

Such an approach resolves the current problems with defrosting windows, because no convenient means has yet been devised that effectively clears car windows of ice without using a substantial amount of heat. - 16

The invention would reduce the emissions associated with an engine being run at a stand-still, purely to generate warm air to demist or de-ice screens before driving.

De-icing compounds are only temporarily effective during very cold weather, as people often continue driving after the ice has re-frozen on the window and visibility is still poor. Therefore, using the energy stored in ultracapacitors has significant safety benefits.

A yet further application of the energy storage unit hereinbefore described is to supply heat to raise the temperature of fabric and air in the vehicle interior.

One approach is envisaged as using a mobile phone signal to discharge the ultracapacitors remotely.

Alternatively, the discharge mechanism could be initiated by means of a timer. .

In this manner, it is envisaged that the energy storage unit could start to simultaneously heat the catalyst, . defrost the windows and heat the interior several minutes before the driver enters the vehicle. . The ultracapacitor could be potentially used for supplying electricity for any mobile application that requires a short surge of power.

It will be understood that the improved electric drive system for reducing emissions from a vehicle, as described above, aims to provide at least one or more of the following advantages: (i) A new application for ultracapacitors to reduce exhaust emissions from vehicles; - 17 (ii) An electric drive system that optimises a combustion engine solely to ensure vehicle emissions remain within acceptable limits; (iii) An electric drive system that avoids excessive vehicle emissions with an arrangement that can be easily and readily incorporated into present day vehicles; (iv) In addition to a combustion engine, the inventive concepts are equally applicable to supplementing a primary energy source for any other power unit for which sudden surges in demand cannot be met, without some disadvantageous operational or environmental aspect; (v) An electric drive system that offers the opportunity to allow a vehicle manufacturer to pass future vehicle emissions tests, irrespective of the Operating condition imposed on testing a performance of the combustion engine; and..

(vi) The potential to supply power to propel the.... . vehicle in situations were the engine is disabled. This.

could be either were the engine is automatically or deliberately switched off, for example in stationary ' traffic (thereby saving energy and starting emissions) or...

in the case were an engine has broken down and the vehicle is obstructing the traffic. Should the vehicle run out of the combustion fuel, sufficient drive might be available to move the vehicle to safety.

Thus, in summary, an electric drive system for a vehicle is described. The electric drive system comprises a combustion engine providing a primary energy source for the vehicle; an energy storage unit operably coupled to the combustion engine for providing additional power for - 18 the vehicle; and an engine management system for monitoring an operational mode of the combustion engine.

The engine management system is operably coupled to the energy storage unit and arranged to provide power from the energy storage unit for the vehicle intermittently, and to substantially avoid emissions, in a nonstoichiometric operational region (860) of the combustion engine.

A skilled artisan will appreciate that the electric motor may be configured to act directly on the wheels.

Therefore, in contrast to the known art described in US 5,345,154, the ultracapacitor and electric motor are configured to occasionally supplement the engine torque, rather than being used for most or all of the time as in more conventional hybrid designs. It is therefore feasible that standard alternator designs may be adequate for electric generation purposes in the preferred... . embodiment of the present invention. j.

Furthermore, a method of optimising vehicle emissions ' using an energy storage unit is described. The method...

comprises the step of monitoring an operation of a combustion engine of the vehicle by an engine management system and determining whether a nonstoichiometric threshold is being approached. The method further comprises the step of providing additional power from an energy storage unit by the engine management system to supplement combustion engine power intermittently, and to substantially avoid emissions in a nonstoichiometric operational region of the combustion engine. - 19

Furthermore, an engine management system and an energy storage unit have been described to perform the aforementioned method steps or operate within the aforementioned electric drive system.

Whilst the specific and preferred implementations of the embodiments of the present invention are described above, it is clear that one skilled in the art could readily apply variations and modifications that would still employ the aforementioned inventive concepts.

Thus, an electric drive system and a method of optimising vehicle emissions have been provided wherein the abovementioned disadvantages with prior art arrangements have been substantially alleviated. .

..-..

- . .. . . -

. - ë.. - 20

Claims

Claxm.

1. An electric drive system (700) for a vehicle comprising: a combustion engine (710) providing a primary energy source for the vehicle; an energy storage unit (760) operably coupled to the combustion engine (710) for providing additional power for the vehicle; and an engine management system operably coupled to the combustion engine (710) for monitoring an operational mode of the combustion engine (710), the electric drive system (700) characterized in that: the engine management system is also operably coupled to the energy storage unit (760) and arranged to provide additional power from the energy storage unit (760) for the vehicle intermittently and to substantially '.

avoid emissions in a non-stoichiometric operational..

region (860) of the combustion engine (710). .... . a....

2. An electric drive system (700) according to Claim - 1 further characterized in that the electric drive system .

(700) is a direct drive engine system, whereby the.. . combustion engine (710) is mechanically linked to the wheels of the vehicle.

3. An electric drive system (700) according to Claim 1 or Claim 2 further characterized in that the engine management system is arranged to identify whether the combustion engine (710) is operating within a stoichiometric region.

4. An electric drive system (700) according to any preceding Claim further characterized in that the engine management system introduces energy from the energy storage unit (760) when the combustion engine (710) is approaching operation in a non-stoichiometric mode.

5. An electric drive system (700) according to Claim 3 or Claim 4 further characterized in that the engine management system is arranged to identify whether the combustion engine (710) is operating within a stoichiometric region or approaching operation in a non- stoichiometric mode by comparing a monitored engine load and a monitored engine speed to an acceptable operating region.

6. An electric drive system (700) according to any of preceding Claims 3 to 5 further characterized in that the engine management system is arranged to prevent the combustion engine (710) operating in a nonstoichiometric.

region if a power demand exceeds a level that would . . require the power from the combustion engine and the energy storage unit to enter a non-stoichiometric region. 6. An electric drive system (700) according to any preceding Claim further characterized in that the energy storage unit (760) is an ultracapacitor 760.

7. An electric drive system (700) according to any preceding Claim further characterized in that the engine management system controls a rate at which additional power is taken from the energy storage unit (760) . 22

8. An engine management system adapted to operate in the electric drive system (700) according to any of preceding Claims 1 to 7.

9. An energy storage unit (760), for example an ultracapacitor, adapted to provide additional power to the electric drive system (700) according to any of preceding Claims 1 to 7.

10. A method of optimising vehicle emissions (900) using an energy storage unit, the method comprising the step of: an engine management system monitoring (910) an operation of a combustion engine of the vehicle; the method characterized by the steps of: determining (915) whether a non-stoichiometric threshold is being approached; and ' the engine management system providing (925) additional energy from an energy storage unit to supplement combustion engine power intermittently and to.

substantially avoid emissions in a non-stoichiometric operational region of the combustion engine.

11. A method of optimising vehicle emissions (900) according to Claim 10, wherein the method is employed in a direct drive engine system, whereby the combustion engine is linked mechanically to the wheels of the vehicle.

12. A method of optimising vehicle emissions (900) according to Claim 10 or Claim 11, wherein the method is further characterized by the step of: 23 identifying (920), in response to determining (915) that a non-stoichiometric threshold is being approached, how much additional power is required from an energy storage unit to supplement the combustion engine power to maintain operation of the combustion engine within stoichiometric limits.

13. A method of optimising vehicle emissions (900) according to Claim 12, wherein the method is further characterized in that the step of identifying (920) comprises comparing a monitored engine load and a monitored engine speed to an acceptable operating region.

14. A method of optimising vehicle emissions (900) according to any of preceding Claims 10 to 13, wherein the energy storage unit is an ultracapacitor.

15. A method of optimising vehicle emissions (900) ..

according to any of preceding Claims 10 to 14 further. . . characterized by the step of: the engine management system controlling a rate at which power is taken from the energy storage unit, to maintain a particular load or speed of the vehicle. .,.

16. An engine management system adapted to perform the method of optimising vehicle emissions (900) according to any of preceding Claims 10 to 15.

17. An energy storage unit (760), for example an ultracapacitor, adapted to perform the method of optimising vehicle emissions (900) according to any of preceding Claims 10 to 15. - 24

18. An electric drive system (700) substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 7 of the accompanying drawings.

19. A method of optimising vehicle emissions (900) substantially as hereinbefore described with reference to, and/or as illustrated by, FIG. 9 of the accompanying drawings. .. . ë -

.... -

- -- Abbe ease :- ..