GB2497775A

GB2497775A - Reducing turbo lag in a compression ignition engine by combusting a secondary fuel in the exhaust system upstream of the turbine

Info

Publication number: GB2497775A
Application number: GB1121965.6A
Authority: GB
Inventors: Marcos Alonso
Original assignee: Nissan Motor Manufacturing UK Ltd
Current assignee: Nissan Motor Manufacturing UK Ltd
Priority date: 2011-12-21
Filing date: 2011-12-21
Publication date: 2013-06-26
Also published as: EP2607645A1; ES2560962T3; EP2607645B1; GB201121965D0

Abstract

A turbo lag reduction system 38 for a compression ignition engine includes a system 44 for delivering a secondary fuel 40 directly into the exhaust system 20 upstream of a turbocharger turbine 30. The secondary fuel is combusted in exhaust system 20 to accelerate the exhaust gases and to drive turbine 30. This spools up the turbine to deliver compressed intake air. The secondary fuel, e.g. urea, is different from the primary diesel fuel; and is combusted only during certain operating conditions, such as during engine idle, when the vehicle is about to accelerate. Injection of secondary fuel may be triggered by driver input; by selection of a gear and/or disengagement of the clutch; or where a traffic signal recognition camera is fitted to the vehicle, by detection of an amber or green light; or by any other selected method.

Description

Compression Ignition Enuine System and Method of Operating a Compression Ignition Engine System

Technical Field of the Invention

The present invention relates to turbocharged compression ignition engine systems and in particular to turbocharged compression ignition engine systems including arrangements for reducing turbo lag. The invention also relates to a method of operating turboeharged compression ignition engine systems to reduce turbo lag.

Background to the]nvention

Diesel engine systems incorporating a turbocharger to improve performance of the engine by increasing the mass of air entering the engine are well known. A turbocharger includes a turbine fluidly connected with an exhaust manifold of the engine so as to be driven by the exhaust gases. The turbine drives a compressor which compresses ambient air and directs it into an intake manifold of the engine.

A common problem with turbocharged compression ignition engine systems is turbo lag, sometimes a'so referred to as turbo delay. Turbo lag is the time delay between the accelerator being depressed and the exhaust gases accelerating the turbocharger to generate sufficient compressed air to increase engine power. A combination of inertia, friction, and compressor load are the primary contributors to turbo lag. Turbo lag can result in low torque at low engine speeds, the effect being especially noticeable when a vehicle starts to move under relatively high loads such as when towing.

A number of systems and methods have been proposed to reduce turbo lag or its effects. For example, it is known to inject compressed air from an auxiliary air tank into the engine intake manifold when the accelerator is depressed to provide a desired amount of air for the engine until the turbocharger is able to spin up to its full capacity and take over the air delivery demands. It is also known to use a high-speed electric motor to accelerate the turbocharger up to speed before a sufficient flow of exhaust gases is available. Whilst these arrangements are beneficial, there are drawbacks.

Using an electric motor to accelerate the turbine incrcascs turbine inertia; and gives rise to technical difficulties in terms of packaging and thermal management of the motor, which is located on or close to the exhaust system. Arrangements for injecting compressed air require additional equipment including a compressor and an auxiliary air tank. This type of arrangement is most suited for use in larger commercial vehicles having an air brake system where the compressed air for inj ecting into the intake manifold is supplied by the same compressor used to supply air for the braking system.

It is known from race and rally cars to retard engine ignition on upward gcarchanges, and to inject additional fuel into the exhaust manifold(s) -either directly, where the fuel is ignited by a spark plug and burned totally in the exhaust; or by overfuelling through the cylinders. In the latter technique, fuel is ignited in the combustion chamber by oae or more spark plugs, and bumed partly in the cylinder and partly in the exhaust system. This system will keep a turbocharger spinning at high engine speeds, but has a number of drawbacks; including high exhaust emissions due to the retarded ignition; explosion damage to the exhaust manifold and turbocharger, which are not designed as combustion chambers; and high noise levels -hence the informal nickname for this system of "bang bang". It is also suited more to petrol engines, than to diesels, as diesel iftel will not readily ignite without compression.

It is, however, known to reduce exhaust emission levels from diesel engines by injecting a mixture of urea and water into the exhaust system between a DPF (diesel particulate filter) and a catalytic converter. This system reduces nitrous oxide and carbon monoxide emissions by a chemical reaction with the urea. The urea is not burned, so there is no significant rise in pressure in the exhaust system. If a turbocharger is fitted to the engine, urea injection will not significantly affect its speed.

There is a requirement then for a turbocharged compression ignition engine system having an alternative arrangement for reducing turbo lag which overcomes, or at least mitigates, at least some of the problems ofthe prior art systems.

There is also a requirement for an alternative method of operating a turbocharged compression ignition engine system for reducing turbo lag which overcomes, or at least mitigates, at least some of the problems of the prior art methods.

Summary of the Invention

According to a first aspect of the invention, there is provided a compression ignition engine system comprising: an engine housing defining at least one cylinder, an air intake manifold and an exhaust system, each fluidly connected with the at least one cylinder, a turbocharger including a compressor in communication with the air intake manifold and a turbine in communication with the exhaust system, a primary fuel delivery system for delivering a primary fuel to a combustion chamber of the at least one cylinder, and a turbo lag reduction system including a control system and a secondary fue' delivery system for delivering a secondary fuel directly into the exhaust system upstream of the turbine under control of the control system.

The engine housing may also be known as an engine cylinder block.

The secondary ifiel delivery system may comprise at least one ifiel injector mounted to the exhaust system and configured to direct the secondary fuel directly into thc exhaust system. The engine housing may define a plurality of cylinders and the exhaust system may comprise an exhaust manifold having a plurality of branches fluidly connecting the cylinders with a combined flow path in the exhaust system. In 1 0 which case, the secondary fuel system may comprise at least one fuel injector for directing the secondary fuel directly into the combined flow path. Alternatively, the secondary fuel system may comprise at least one fuel injector for directing the secondary fuel directly into at least one branch of the manifold. The secondary fuel delivery system may comprise a plurality of fuel injectors for directing the secondary fuel into at least two branches of the exhaust manifold. In one embodiment, the secondary fuel delivery system comprises a plurality of fuel injectors configured to direct the secondary fuel into each branch of the manifold.

The secondary fuel delivery system may comprise a secondary fuel tank and a pump for pumping the secondary fuel from the secondary fuel tank to at least one fuel injector.

The control system may comprise a programmable controller; and may be configured such that, in use, secondary fuel is introduced into the exhaust system only under certain operating eoaditions. The controller may be programmed such that, in use, the secondary fuel is introduced into the exhaust system when the engine is idling. The control system may comprise at least one user input to enable a user to selectively activate and deactivate the turbo lag reduction system. The control system may be, or form part of an engine management control unit or engine management system.

The primary fuel may be diesel and thc sccondary fuel may be urea.

In accordance with a second aspect of the invention, there is provided a method of operating a turboeharged compression ignition engine system, wherein the turbocharged compression ignition engine system comprises an engine housing 1 0 defining at least one cylinder, an air intake manifold and an exhaust system, each fluidly connected with the at least one cylinder, a turbocharger including a compressor iii communication with the air intake manifold and a turbine in communication with the exhaust system, a primary fuel delivery system for delivering a primary fuel to a combustion chamber of the at least one cylinder; the method comprising using a secondary fuel delivery system to introduce a secondary fuel directly into the exhaust system upstream of the turbine under certain operating conditions such that the secondary fuel is combusted in the exhaust system, accelerating the exhaust gasses and driving the turbine.

The primary fuel may be diesel and the secondary the] may be urea.

The secondary fuel delivery system may comprise at least one fuel injector mounted to the exhaust system; and the method may comprise directing the secondary fuel directly into the exhaust system through the at least one fuel injector. The engine housing may define a plurality of cylinders and the exhaust system may comprise an exhaust manifold having a plurality of branches fluidly connecting the cylinders with a combined flow path in the exhaust system. In which case, the method may comprise using at least one fuel injector to direct the secondary fuel into the combined flow path. Alternatively, the method may comprise using at least one fuel injector to direct the secondary fuel into at least one branch of the manifold. The secondary fuel delivery system may comprise a plurality of fuel injectors; and the method may comprise using the injectors to direct fuel into at least two branches of the exhaust manifold. In one embodiment, the method comprises using the secondary fuel injectors to direct the secondary fuel into each of the branches of the manifold.

The secondary fuel may be combusted in the exhaust system substantially only when an exhaust valve of the at least one cylinder is open. In a multi-cylinder engine where the secondary fuel is injected into a common flow path in the exhaust, injection of the secondary fuel into the exhaust system may be timed so that the secondary fuel is combusted in the exhaust substantially only when at least one of the exhaust valves is open. In a multi-cylinder engine where the secondary fuel is injected into at least one branch of an exhaust manifold, injection of secondary fuel into any given branch of the exhaust manifold may be timed so that the secondary fuel is combusted in the branch substantially only when the exhaust valve of the cylinder with which the branch is fluidly connected is open.

In accordance with a third aspect of the invention, there is provided a vehicle having a turbocharged compression ignition engine system according to the first aspect of the invention and/or in which the method of the second aspect of the invention is carried out.

Detailed Description of the Invent ion

In order that the invention may be more clearly understood an embodiment thereof will now be described, by way of example only, with reference to Fig. 1, which is a schematic cross-sectional representation of part of a compression ignition engine system in accordance with an embodiment of the invention.

As illustrated in Fig. 1, a compression ignition engine system, indicated generally at 10, comprises an engine 11 having a housing 12 defining at least one cylinder 14 containing a piston 15. An air intakc manifold 16 is fluidly connected with an inlet port 18 of the cylinder and an exhaust system 20 is fluidly connected with an outlet port 22 of the cylinder. Inlet and exhaust valves 24, 26 for opening and closing the inlet and exhaust ports 18, 22 respectively are provided in the usual manner. Valves 24 and 26 are opened and closed by a conventional valve train (not shown).

A turbocharger 28 includes a turbine 30 connected to a compressor 32 by a drive shaft 34. The turbine 30 is fluidly connected with the exhaust system 20 so that it is driven by thc exhaust gases. The compressor 32 is drivcn by the turbine 30 through the shaft 34 and is fluidly connected with the intake manifold 16 so as to compress ambient air and to direct it through the intakc manifold to the cylindcr. An intercooler (not shown) may also be fitted in the engine intake tract.

Although not shown in the drawing, the compression ignition engine system also includes a primary fuel delivery system for delivering a primary fuel, typically diesel, to a combustion chamber of the at least one cylinder for combustion. The combustion chamber may be defined in the top of the piston or in the cylinder head or a combination of both as is well known in the art.

The primary fuel delivery system is conventional; and may typically comprise a fuel tank for the primary fuel, an injector for directing the primary fuel into the cylinder combustion chamber and at least one pump for pumping the fuel from the primary fuel tank to the injector. The dctails of the primary fuel dclivcry system are not essential to the present invention, which can be applied to compression ignition engine systems having any suitable fuel delivery arrangement for the primary diesel fuel; including direct and indirect injection arrangements.

As so far described, the compression ignition engine system 10 is conventional. In accordance with the present invention, the compression ignition engine system 10 incorporates a system 38 for reducing turbo lag by introducing a secondary fuel 40 into the exhaust system 20 upstream of the turbine 30 when the engine is idling, or when otherwise desired. The secondary fuel is combusted in a combustion area 42 in the exhaust system, also upstream of the turbine 20.

Combustion of the secondary fuel 40 in the exhaust system accelerates the exhaust gases which drive the turbine 30, and hence the compressor 32. When used whilst the engine 11 is idling, combustion of the secondary fuel in the exhaust results in the turbine being driven at a faster speed than would be the ease in a conventional engine system under the same operating conditions.

The turbo lag reduction system 38 includes a secondary fuel delivery system 44 having at least one fuel injector 46 mounted to the exhaust system 20 to direct the secondary fuel into the exhaust system upstream of the turbine 30, a secondary fuel tank 48 and a pump 50 for pumping the fuel from the tank to the injector. The turbo lag reduction system 38 includes a control system 52 for controlling operation of the secondary fuel delivery system 44. The control system may regulate operation of the pump 50 and/or the injector 46 as required. The control system 52 will typically comprise a programnable control unit and may be, or form part of an engine control unit and/or dngine managcment system that managcs the dngine system's overall operation. The control system 52 may also comprise user inputs that enable a driver to selectively activate and deactivate the turbo lag reduction system 38 and/or to change one or more of its operating parameters.

In a typical mode of operation, the lag reduction system 38 operates to deliver the secondary fuel 40 for combustion in the exhaust system when the engine is idling and the vehicle is stopped. Delivery of the secondary fuel is timed so that the secondary fuel 40 is combustcd wholly, or at least predominantly, whilst the exhaust valve 26 is open. This enables combustion of the secondary fuel to accelerate the flow of exhaust gasses 54 to the turbine 30. Injcction of the secondary fuel may commence just before the exhaust valve opens if desired; and the timing can be adjusted to obtain optimal performance for any given engine system and/or secondary fuel, and/or to take into account specific operating conditions.

Whilst the turbo lag reduction system 38 is operative and the secondary fuel is being combusted in the exhaust system, the turbocharger 28 provides compressed intake air 56. Consequently, the engine system is able to deliver an increased torque quickly in response to actuation of thc accelerator, with no delay; or only minimal delay. When the accelerator is depressed, torque output can be controlled by regulating the amount of primary fuel injected into the combustion chamber of the cylinder as there is already sufficient intake air available. Combustion of the secondary fuel 40 in the exhaust system 20 to reduce turbo lag is typically only required when the engine 11 is idling and during initial stages of acceleration. Once the flow of exhaust gases from the cylinder is sufficicnt to drive the turbine 30 at the required speed without further assistance, the control system 52 shuts down delivery of the secondary fuel 40 into the exhaust system 20.

The control system 52 monitors the operation of the engine 10 and the vehicle via sensors in a manner known in the art, and is programmed to activate the secondary fuel delivery system 44 when certain operating conditions are met. For example, the control system 52 may be programmed to activate the secondary fuel delivery system whenever the engine is idling and the vehicle is stopped. However, this may not make the most efficient use of the secondary fuel. Use of the lag reduction system is of particular advantage when the engine is put under a relatively heavy load, such as when towing or when the vehicle is heavily laden. The control system may have a user input to enable the driver to activate the turbo lag reduction systcm 38 when they feel it is required; and to deactivate the system at other times.

Ahernatively, the control system 52 may be configured to monitor engine performance, and to activate the secondary fuel delivery system 44 only when the engine is being placed under load conditions at or above a predetermined threshold level. In addition, or alternatively, the control system 52 may be configured to predict when a demand for increased torque is likely to arise, and to activate the secondary fuel delivery system 44 accordingly. In a manual transmission vehicle that is idling and stationary, the selection of a gear and/or depression of the clutch are often precursors to the engine being required to deliver torque as the driver subsequently pulls away. Thus, rather than delivering the secondary fuel 40 at all times whilst the engine is idling and the vehicle is stopped, the control system 52 could be configured to activate the secondary fuel delivery system when the engine is idling and the vehicle is stopped; but only also in rcsponse to a gear bcing scleeted and/or thc clutch pedal being depressed.

For a vchicle with automatic transmission, the driver will usually take their foot off the brake pedal before depressing the accelerator to pull away. In this situation, the control system 52 could be configured to activate the secondary fuel delivery system 44 when the driver releases the brake pedal whilst the engine is idling and the vehicle is stationary. Alternatively, the control system 52 may be configured to detect when the driver takes the automatic transmission out of a park mode into drive mode (forward or reverse) as an indication that a requirement for increased torque is likely to arise; and to then activate the secondaty fuel delivery system 44. In a further possible arrangement, thc control system 52 may be configured to activatc the secondary fuel dclivcry system 44 whcn the vchicle is stationary at a set of traffic signals in response to the signals turning from red to amber or green. The control system 52 in this case may comprise a traffic signal camera system which is configured to recognize a green or amber light. It will be appreciated that there are numerous ways in which the control system 52 can be configured to make efficient use of the secondary fuel, whilst reducing turbo lag to an acceptable level.

The secondary fuel 40 is a different fuel from the primary diesel fuel as the conditions in the exhaust system 20 are not suitable to cause diesel to eombust, there being insufficient pressure and temperature. Accordingly, an appropriate secondary fuel capable of being combusted in the exhaust system must be selected. Such a fuel should have a low autoignition point and be capable of burning at relatively low pressure. Its calorific value is not important, but it must not produce high levels of critical emissions, such as nitrous oxides or carbon monoxide. A high expansion rate from liquid to burnt gases is essential.

One suitable fuel is urea. Because the secondary fuel 40 is different from the primary fuel, the primary and secondary fuels must be kept separate from one another.

This requires the provision of a secoadary fuel tank to store the secondary fuel, a secondary thel pump and separate fuel lines. It will be appreciated however, that the primary and secondary fuel tanks could be combined in an integral dual tank assembly. Similarly, the primary and secondary fuel pumps could be combined in an integral dual pump unit with separate flow paths for the two fuels. A fuel gauge and an appropriate driver display for the secondary fuel will be provided to enable a driver to monitor levels of the secondary fuel and to re-fill as required.

The gauge for the secondary fuel could be included as a secondary function of the standard fuel gauge for the primary fuel. This function could be provided as an update of old technology, as the 1945 Rover P2 series car allowed the driver to check the engine oil level by pressing a button next to the fuel gauge, which disconnected the fuel level sender from the gauge and connected a corresponding oil level sender to the gauge instead. An appropriate update of this technology could be to site the fuel gauge changeover button on a dashboard-mounted touch screen, or on a steering column stalk as part of the vehicle trip computer control system. Either of these options would allow a standard combi-meter or instrument cluster to be used on vehicles with and without the turbo lag reduction system fitted. As a further alternative option, the level of secondary fuel could be displayed on the fuel gauge for a short period, for example five seconds, after the ignition is turned om the gauge then reverting to the conventional display of primary fuel level.

For simplicity, Figure 1 illustrates schematically an engine having a single cylinder. Many modern combustion ignition engines have multiple cylinders, with three, four and six cylinders or more being commonplace. For a multi-cylinder engine, the exhaust system 20 will typically comprise an exhaust manifold that defines a combined flow path at an outlet end and which has a plurality of branches at the inlet end for fluidly connecting the exhaust ports of the various cylinders to the combined flow path. Typically, the turbocharger turbine 30 is fluidly connected to the combined outlet flow path of the manifold; so that it is driven by the exhaust gasses flowing from each of the cylinders. In this type of arrangement, the secondary fuel delivery system 44 may use only a single fuel injector that is configured to deliver the secondary fuel 40 directly into the combined flow path, either iii the manifold itself or in an exhaust pipe between the manifold and the turbine 30.

However, in some applications and dependent on the speed of combustion of the fuel, there may be insufficient distance between the turbine and the point at which the manifold branches merge into the combined flow path to enable the fuel 40 to be injected directly into the combined flow path and combusted before reaching the turbine 30. In these situations, or where it is otherwise desirable, the secondary fuel 40 may be injected into one or more of the branches of the manifold. In some applications it may be sufficient to inject the secondary fuel 40 into only one, or some, of the branches. In other applications, the secondary fuel may be injected into all of the branches of the manifold. It will be appreciated that the secondary fuel delivery system 44 can be modified to incorporate an appropriate number of secondary fuel injectors mounted to the branches and connected with the secondary fuel pump as required. Where the secondary fuel 40 is injected into a branch of the manifold, combustion may take place wholly within the branch; or wholly within the combined flow path downstream from the branch; or partially within the branch, and partially within the combined flow stream.

As discussed above, the secondary fuel 40 is combusted whilst the exhaust valve 26 of the cylinder 14 is open in order to accelerate the flow of exhaust gases to the turbine. Where the engine has multiple cylinders 14 and the secondary fuel 40 is injected into a combined flow path in the exhaust system, injection of the secondary fuel is timed to corresponding with the opening of each of the exhaust valves so that the secondary fuel is combustcd substantially only when at least one of the exhaust valves is open. Where the engine has multiple cylinders 14 and the secondary fuel 40 is injected into one or more branches of an exhaust manifold, injection of the secondary fuel into any particular branch is timed to correspond with the opening of the exhaust valve 26 of the cylinder to which that branch is fluidly connected.

The above embodiment is described by way of example only. Many variations are possible without departing from the scope of thc invention.

Claims

<claim-text>CLAIMS1. A compression ignition engine system comprising: an engine housing defining at least one cylinder, an air intake manifold and an exhaust system, each fluidly connected with the at least one cylinder, a turbocharger including a compressor in communication with the air intake manifold and a turbine in communication with the exhaust system, a primary fuel delivery system for delivering a primary fuel to a combustion chamber of the at least one cylinder, and a turbo lag reduction system including a control system and a secondary fuel delivery system for delivering a secondary fuel directly into the exhaust system upstream from the turbine under control of the control system.</claim-text> <claim-text>2. A compression ignition engine system as claimed in claim 1, wherein the secondary fuel delivery system comprises at least one thel injector mounted to the exhaust system and configured to direct the secondary fuel directly into the exhaust system.</claim-text> <claim-text>3. A compression ignition engine system as claimed in claim 2, wherein the engine housing defines a plurality of cylinders and the exhaust system comprises an exhaust manifold having a plurality of branches fluidly connecting the cylinders with a combined flow path in the exhaust system, the secondary fuel system comprising at least one fliel injector for directing the secondary fuel directly into the combined flow path.</claim-text> <claim-text>4. A compression ignition engine system as claimed in claim 2, wherein the engine housing defines a plurality of cylinders and the exhaust system comprises an exhaust manifold having a plurality of branches fluidly connecting the cylinders with the combined flow path in the exhaust system, the secondary fuel system comprising at least one fuel injector for directing the secondary fuel directly into at least one branch of the manifold.</claim-text> <claim-text>5. A compression ignition engine system as claimed in claim 4, wherein the secondary fuel delivery system comprises a plurality of fuel injectors lbr directing the secondary fuel into at least two branchcs of the exhaust manilbid.</claim-text> <claim-text>6. A comprcssion ignition engine system as claimed in claim 4 or claim 5, wherein the secondary fuel delivery system comprises a plurality of fuel iijectors configured to direct the secondary fuel into each branch of the manifbld.</claim-text> <claim-text>7. A compression ignition engine system as claimed in any one of claims 2 to 6, wherein the secondary fuel delivery system comprises a secondary fuel tank and a pump fbr pumping the secondary fi.iel from the secondary fuel tank to at least one fuel injector.</claim-text> <claim-text>8. A compression ignition engine system as claimed in any one of the preceding claims, wherein the control system comprises a programmable controller and is configured such that, in use, secondary fuel is introduced into the exhaust system only under certain operating conditions.</claim-text> <claim-text>9. A compression ignition engine system as claimed in claim 8, wherein the controller is programmed such that, in use, the secondary fuel is introduced into the exhaust system when the engine is idling.</claim-text> <claim-text>10. A compression ignition engine system as claimed in any one of the preceding claims, wherein the control system comprises a user input to enable a user to selectively activate and deactivate the turbo lag reduction system.</claim-text> <claim-text>11. A compression ignition engine system as claimed in any one of the preceding c!aims, wherein the secondary the! is urea.</claim-text> <claim-text>12. A method of operating a turbocharged compression ignition engine system, wherein the turbo charged compression ignition engine system comprises an engine housing defining at least one cylinder, an air intake manithid and an exhaust system, each fluidly connected with the at least one cylinder, a turbocharger inc!uding a compressor in communication with the air intake manifold and a turbine in communication with the exhaust system, a primary thet delivery system thr delivering a primary fuel to a combustion chamber of the at least one ey!inder; the method comprising using a secondary fuel delivery system to introduce a secondary fuel direct!y into the exhaust system upstream from the turbine under certain operating conditions such that the secondary fuel is combusted in the exhaust system acee!erating the exhaust gasses and driving the turbine.</claim-text> <claim-text>13. A method as claimed in claim 12, wherein the secondary thetis urea.</claim-text> <claim-text>14. A method as claimed in claim 12 or claim 13, wherein the secondary the! delivery system comprises at least one fuel injector mounted to the exhaust system and the method comprises directing the secondary fuel directly into the exhaust system through the at least one fuel injector.</claim-text> <claim-text>15. A method as c!aimed in claim 14, wherein the engine housing defines a plurality of cylinders and the exhaust system comprises an exhaust maniflild having a plurality of branches fluidly connecting the cylinders with a combined flow path in the exhaust system, the method comprising using at least one el injector to direct the secondary friel into the combined flow path.</claim-text> <claim-text>16. A method as claimcd hi claim 14, whcrcin the engine housing dcfines a plurality of cylinders and the exhaust system comprises an exhaust manifold having a plurality of branches fluidly connecting the cylinders with a combined flow path in the exhaust systcm, the method comprising using at least onc find injector to direct the secondary fuel into at least one branch of the manifold.</claim-text> <claim-text>17. A method as claimed in claim 16, wherein the secondary firnl delivery system comprises a plurality of fuel injectors, the method comprising using the injectors to direct thel into at least two branches of the exhaust manithid.</claim-text> <claim-text>1.. A method as claimed in claim 17, wherein the method comprises using the secondary fuel injectors to direct thc secondary fuel into each of the branches of the manifold.</claim-text> <claim-text>19. A method as claimed in any one of claims 12 to 1, wherein, the sccondary fuel is combusted in the exhaust system substantially only when an exhaust valve of the at least one cylinder is open.</claim-text> <claim-text>20. A method as claimed in claim 19 when dependent on claim 15, wherein each cylinder has an exhaust valve and the injection of secondary fuel into the exhaust system is timed so that the secondary fuel is combustcd in the exhaust substantially only when at least one of the exhaust valves is open.</claim-text> <claim-text>21. A method as claimed in claim 19 when dependent on any one of claims 16 to 18, wherein each cylinder has an exhaust valve and the injection of secondary fuel into any given branch of the exhaust manifold is timed so that the secondary fuel is combusted in the branch substantially only when the exhaust valve of the cylinder with which the branch is fluidly connected is open.</claim-text> <claim-text>22. A vehicle comprising a compression ignition engine system as claimed in any one of claims 1 to 11 and/or in which the method of any one of claims 12 to 21 is carried out.</claim-text>