Patent Description:
Conventional internal combustion engines operate based on the Otto and Diesel cycles. Such cycles are associated with a fundamental tension between increases in efficiency (and thus performance) and the generation of emissions of NOx, particulates and Carbon Dioxide. Modern day regulations on such emissions are growing increasingly strict as concerns over atmospheric pollution and global warming are rising. From a review of such engine cycles, it can be seen that increasing efficiency of a cycle leads to increased temperatures, which in turn lead to increased NOx formation and a material performance limitation on that efficiency. In order to mitigate NOx formation, it has been proposed that it is necessary to introduce extra plant complexity in the form of after treatment of the exhaust.

For both of the Otto and the Diesel cycles, the efficiency is predicated on the pressure at the end of compression. The Diesel cycle efficiency is also dependent on a rate of combustion, as the rpm and combustion rate influence a volume ratio between the start and end of combustion. Increasing the efficiency of modern engines is therefore also met with practical material limitations. This is because the peak temperatures and pressures associated with the engine may reach very high levels.

The formation of NOx compounds occurs in areas where the temperature of an air fuel mixture rises above <NUM>. For instance, this may occur for localised 'hot spots' or it may be on a larger scale, e.g. throughout the whole of an engine cylinder. NOx compounds are linked to human respiratory health issues and so production of such compounds and emission of these compounds into the atmosphere poses a significant health risk. Also, the formation of these compounds is endothermic so they are inherently of no use with regards to maximising conversion of chemical energy to work.

<CIT>, <CIT>and <CIT> disclose a split cycle internal combustion engine which uses a coolant injector for cryogenic fluids (fluids which have been condensed into its liquid phase via a refrigeration process). <CIT> discloses a split cycle engine where intake air from the compression cylinder is cooled before entering the combustion cylinder to keep the combustion temperature below <NUM> (<NUM>°F).

Aspects of the invention are as set out in the independent claims and optional features are set out in the dependent claims. Aspects of the invention may be provided in conjunction with each other and features of one aspect may be applied to other aspects.

Aspects of the disclosure will now be described, by way of example only, with reference to the drawings, in which:.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control a coolant system so that a peak temperature of combustion in a combustion cylinder is below a selected threshold. The controller may control a peak temperature of combustion to inhibit generation of NOx and particulates during combustion, this has a clear environmental benefit as these chemicals are known to be damaging to human health.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control opening and closing of an inlet valve for controlling the flow of working fluid into a combustion cylinder. The controller may control the inlet valve to open and close at selected times to control a peak temperature of combustion to inhibit generation of NOx and particulates during combustion, this has a clear environmental benefit as these chemicals are known to be damaging to human health.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control a reactivity adjuster to adjust the reactivity of fuel based on a received indication of operating conditions of the engine. The controller may control the reactivity adjuster to increase reactivity of the fuel when fuel reactivity is low. This may enable increased efficiency as combustion of fuel may be achieved for a greater proportion of the fuel.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control the timing of injection for a fuel injector for injecting fuel into a combustion cylinder. The controller may control timing of the injector to control a peak temperature of combustion in the combustion cylinder. This may enable the controller to inhibit generation of NOx and particulates during combustion, as lower peak temperatures could be achieved. This has a clear environmental benefit as these chemicals are known to be damaging to human health.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control a coolant system based on an estimate for the peak temperature of combustion, so that a peak temperature of combustion remains within a selected range. This may enable the controller to prevent the engine from operating at a sufficiently high temperature that NOx and particulates are released during combustion, and it may prevent the engine from operating at a sufficiently low temperature for engine performance to be compromised.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control a coolant system so that working fluid in a crossover passage will flow into a combustion cylinder at a speed greater than a speed threshold. This may enable greater mixing of the fuel with the working fluid prior to combustion. This may reduce the richness of the fuel, providing a leaner air-fuel mixture so that complete combustion of the fuel occurs and inhibits generation of particulates, such as soot. It may also reduce the presence of any 'hotspots' where combustion occurs at higher peak temperatures, which produce NOx or other undesirable pollutants.

In one example, a split cycle internal combustion engine is disclosed comprising a controller configured to control a cross-sectional area defined by an inlet valve to a combustion cylinder so that working fluid flows into the combustion cylinder at a speed greater than a speed threshold. This may enable greater mixing of the fuel with the working fluid prior to combustion. This may reduce the richness of the fuel and reduce the presence of any 'hotspots' where combustion produces NOx or particulates.

<FIG> shows a first example of a split cycle internal combustion engine <NUM> arranged to control a peak temperature of combustion so that it is below a selected threshold. The engine <NUM> is arranged to provide an indication of a peak temperature of combustion to a controller <NUM> which determines, based on this indication, a peak temperature of combustion. Based on the determined peak temperature of combustion, the controller <NUM> controls a coolant system to regulate a temperature of working fluid supplied to a combustion cylinder <NUM> of the engine <NUM>. In particular, the coolant system may be arranged to control this temperature so that working fluid in a crossover passage <NUM> between a compression cylinder <NUM> and a combustion cylinder <NUM> of the engine <NUM> is cool enough that when this working fluid is used in the combustion cylinder <NUM>, as part of the combustion process, a peak temperature of combustion does not exceed a selected threshold. The controller <NUM> may operate based on a feedback loop which controls the operation of the coolant system so that the temperature of the working fluid to be supplied to the combustion cylinder <NUM> may be controlled to be within a selected range. This may enable the peak temperature of combustion to be controlled so that, for example, generation of NOx compounds may be inhibited. The feedback loop may also be based on a cooling threshold, wherein in response to the controller determining that the peak temperature of combustion is below the cooling threshold the controller controls the coolant system to regulate the temperature of the working fluid so that the peak temperature of combustion exceeds the cooling threshold. This may enable the controller to control the engine to operate within a selected peak temperature range.

As illustrated, <FIG> shows a split cycle internal combustion engine <NUM> apparatus comprising a compression cylinder <NUM> and a combustion cylinder <NUM>. The compression cylinder <NUM> accommodates a compression piston <NUM>, which is connected via a connecting rod <NUM> to a respective crank on a portion of a crank shaft <NUM>. The combustion cylinder <NUM> accommodates a combustion piston <NUM>, which is coupled via a connecting rod <NUM> to a respective crank on a portion of the crank shaft <NUM>. The compression cylinder <NUM> is coupled to the combustion cylinder <NUM> via a crossover passage <NUM>. The crossover passage <NUM> may comprise a recuperator, which may be used for heat transfer. The compression cylinder <NUM> comprises an inlet port <NUM> for receiving fluid from outside the engine <NUM>, and an outlet port <NUM> coupled to the crossover passage <NUM>. The outlet port <NUM> comprises a valve, for example a non-return valve so that compressed fluid cannot flow back into the compression cylinder <NUM>. The combustion cylinder <NUM> comprises an inlet valve <NUM>, which is also coupled to the crossover passage <NUM>, and an exhaust valve <NUM> for passing exhaust from the combustion cylinder <NUM> to an exhaust. These couplings provide a fluid flow path between the compression cylinder <NUM> and the combustion cylinder <NUM> via the crossover passage <NUM>.

The engine <NUM> also comprises a coolant system. The coolant system is illustrated as comprising a liquid coolant reservoir <NUM> coupled to the compression cylinder <NUM> via a coolant injector <NUM>, which defines a liquid flow path. The coolant system may also comprise an injector for injecting coolant into the crossover passage <NUM>, although this is not illustrated in <FIG>. The coolant system may also comprise use of heat transfer via a recuperator. For example, this may comprise utilising heat in the exhaust from the combustion cylinder to heat the recuperator. It may comprise utilising the recuperator to transfer heat away from the split cycle internal combustion engine <NUM>. The engine <NUM> also comprises a fuel reservoir <NUM> coupled to the combustion cylinder <NUM> via a fuel injector <NUM> so that a fluid flow path is defined between the fuel reservoir <NUM> and the combustion cylinder <NUM>.

The engine <NUM> comprises a controller <NUM> and a plurality of sensors, which are illustrated as black dots coupled to the controller <NUM>. However, it is to be appreciated that the sensors illustrated are only exemplary and there could be a different number of sensors or they could be placed in different locations. For example, the inlet port <NUM> may also comprise a temperature sensor. The sensors could be coupled to the controller <NUM> through physical wires or could be connected wirelessly. In the example shown in <FIG> there is a compression sensor <NUM> within the compression cylinder <NUM>. The compression sensor <NUM> may for example be mounted proximate to the air inlet port <NUM> or proximate to the coolant injector <NUM>. The compression sensor <NUM> may comprise a temperature sensor. The example engine <NUM> shown in <FIG> also comprises a combustion sensor <NUM> within the combustion cylinder <NUM>. The compression sensor <NUM> may comprise a temperature sensor; it may comprise a pressure sensor. Also illustrate is a crossover sensor <NUM> within the crossover passage <NUM>. The crossover sensor <NUM> may comprise a temperature sensor; it may comprise a pressure sensor. Additionally, the engine <NUM> comprises a crank sensor <NUM> mounted to the crankshaft <NUM>. The crank sensor may provide an indication of torque demand from the engine. Also illustrated is an exhaust sensor <NUM> downstream of the exhaust valve <NUM> of the combustion cylinder <NUM>. The exhaust sensor <NUM> may comprise a temperature sensor; it may comprise a pressure sensor; it may comprise a lambda sensor configured to provide an indication of NOx concentration in the exhaust of the engine. In some examples, the liquid coolant reservoir <NUM> may also comprise a sensor, for example, for measuring a quantity, such as mass, of liquid contained in the reservoir <NUM>. The controller <NUM> is also coupled to the coolant injector <NUM>, and the fuel injector <NUM> and/or reservoir <NUM>.

The sensors are configured to send at least one signal to the controller <NUM> providing an indication of at least one parameter associated with the engine <NUM>. A parameter of the engine <NUM> may comprise a temperature of working fluid in the engine (in different locations, e.g. exhaust, compression cylinder <NUM>, crossover passage <NUM> etc.). It may comprise a pressure of working fluid in the engine; it may comprise a demand on the engine; it may comprise a value for NOx generation in the engine; it may comprise timings for the opening and closing of the inlet valve <NUM>; it may comprise timing for the injection of fuel into the combustion cylinder. A parameter of the engine <NUM> may comprise an indication of engine knocking, for example, this may be based on a received audio signal of the engine running. Engine knocking may occur when the fuel does not ignite at the correct time during the cycle of the piston, and may be detected based on listening to the noise of the engine, and thus an indication of engine knocking may be considered a parameter of the engine.

For example, in the example shown in <FIG>, the compression sensor <NUM> is configured to measure at least one parameter associated with the compression cylinder <NUM>. The combustion sensor <NUM> is configured to measure at least one parameter associated with the combustion cylinder <NUM>. The crossover sensor <NUM> is configured to measure at least one parameter associated with the crossover passage <NUM>. Additionally, the crank sensor <NUM> is configured to measure RPM for the engine <NUM>, and the exhaust sensor <NUM> is configured to measure at least one parameter of exhaust gas expelled through the exhaust valve <NUM> of combustion cylinder <NUM>. Such measurements of the at least one parameters provide an indication of a peak temperature of combustion in the combustion cylinder <NUM>. Each sensor may provide said indication of peak temperature to the controller <NUM> for the controller <NUM> to determine the peak temperature of combustion in the combustion cylinder <NUM>.

The engine <NUM> is arranged such that air is drawn into the compression cylinder <NUM> through the inlet port <NUM> of the compression cylinder <NUM>. The compression piston <NUM> is arranged to compress this air, and during the compression phase, liquid coolant may be added into the compression cylinder <NUM>. The crossover passage <NUM> is arranged to receive the working fluid via the outlet port <NUM> and pass it into the combustion cylinder <NUM> via the inlet valve <NUM>. The engine <NUM> is further arranged to add fuel from the fuel reservoir <NUM> to the working fluid in the combustion cylinder <NUM> via the fuel injector <NUM>, and combust the mixture of fuel and working fluid (for example via operation of an ignition source, not shown) to extract useful work via turning of the crankshaft <NUM>.

The fuel reservoir <NUM> is connected to the controller <NUM> so that the controller <NUM> controls the delivery of fuel into the combustion cylinder <NUM>. In some examples, the controller <NUM> is configured to determine the amount of fuel to be injected based on a received indication of at least one parameter of the engine <NUM>. For example the controller <NUM> may be configured to obtain the indication of the at least one parameter via a signal indicative of a peak temperature of combustion received from the exhaust sensor <NUM>, or a signal indicative of engine demand received from the crank sensor <NUM>.

In operation, the controller <NUM> is configured to receive an indication of a peak temperature of combustion. The signal is received from at least one of the sensors illustrated in <FIG>. For instance, the controller <NUM> may receive an indication of a temperature in the exhaust from the exhaust sensor <NUM>. In the event that the controller is receiving an indication from a sensor which does not directly measure the peak temperature of combustion, the controller determines an estimate for peak temperature of combustion in the combustion cylinder <NUM> based on the received indication. For example, the received indication of temperature in the exhaust may be used to infer the peak temperature of combustion in the combustion cylinder. In the event that the controller receives an indication from a sensor which directly measures a peak temperature of combustion, e.g. combustion sensor <NUM>, the controller may use the indication of peak temperature rather than separately determining the peak temperature.

The peak temperature of combustion typically occurs towards the end of the movement of the piston <NUM> from Top Dead Centre ('TDC') to Bottom Dead Centre ('BDC'). In the event that the controller <NUM> receives the indication from a sensor which cannot directly measure this peak temperature (e.g. which is not in the combustion cylinder <NUM>), the controller <NUM> is configured to determine an estimate the peak temperature based on the received indication. This may comprise use of a mathematical model which can estimate a peak temperature for combustion based on a value for a parameter of the engine (e.g. a temperature of the working fluid in the crossover passage). For example, such a model may comprise determining a value based on previous data for heat generation throughout the cycle of the engine and/or dissipation of heat and consequential cooling after combustion has occurred. The sensor may measure a parameter of the system and/or the working fluid (e.g. a temperature, a pressure) and this may be the indication provided to the controller <NUM>. Based on the indication, the controller <NUM> may use known thermodynamic relationships to determine an estimate for the peak temperature in the combustion cylinder <NUM>. For example, based on a received indication of pressure and temperature of working fluid, a density for the working fluid may be determined (e.g. based on the equation for state linking pressure, temperature and density).

In an example, the controller <NUM> may receive an indication of the peak temperature of combustion from a sensor measuring a parameter of the working fluid after combustion. For instance, the measurement may be made by the exhaust sensor <NUM>. The exhaust sensor <NUM> may be configured to measure the temperature of working fluid in the exhaust. Post-combustion temperature provides an indication of a peak temperature of combustion. An estimate of the peak temperature of combustion may be determined based on the post-combustion temperature using previous data, e.g. using a look-up table. It is to be appreciated that this may provide a good approximation to the peak temperature of the working fluid during combustion, as the time at which the working fluid flows through the exhaust valve <NUM> from the combustion cylinder <NUM> will be very shortly after the time at which the peak temperature of combustion was reached. The exhaust sensor <NUM> may therefore measure a post-combustion temperature, and based on this measurement, provide an indication of the peak temperature of combustion to the controller <NUM>. The controller <NUM> then determines, based on the post-combustion temperature, a peak combustion temperature. The peak combustion temperature is greater than the post-combustion temperature. The peak combustion temperature may be determined using a look-up table comprising a mapping between values for post-combustion temperatures and corresponding values for peak combustion temperatures.

In another example, the controller <NUM> may receive an indication of the peak temperature of combustion from a sensor measuring a parameter of the working fluid prior to combustion. For instance, the measurement may be made by a supply sensor, wherein a supply sensor may refer to any sensor which provides an indication of a parameter of the engine or working fluid prior to combustion, for example the indication may be from the compression sensor <NUM> or the crossover sensor <NUM>. The crossover sensor <NUM> may be configured to measure the temperature of the working fluid in the crossover passage <NUM> prior to it flowing into the combustion cylinder <NUM>. The crossover sensor <NUM> may therefore measure a pre-combustion temperature of the working fluid, and provide an indication of this to the controller <NUM>. The controller <NUM> then determines, based on the pre-combustion temperature an estimate for the peak temperature of combustion in the combustion cylinder <NUM>. The pre-combustion temperature is less than the peak combustion temperature. The controller <NUM> may determine an estimate for the peak combustion temperature using a look-up table comprising a mapping between values for pre-combustion temperatures and corresponding values for peak combustion temperatures. The values in the mapping may be determined using a mathematical model modelling the thermodynamics of the system to predict the temperatures. They may comprise values determined empirically.

It is to be appreciated that the look-up table used in either example may also comprise other parameters. The look-up table may therefore enable the controller <NUM> to determine an estimate of the peak temperature of combustion based on present conditions of the engine <NUM> and a temperature of the working fluid (e.g. the pre-combustion or post-combustion temperature). For example, one of the other parameters may comprise an indication of a demand on the engine <NUM>, which may be determined based on a signal received from the crank sensor <NUM>. One parameter may comprise a timer indicative of the duration of time for which the engine <NUM> has been running. This may provide an indication for the temperature of the engine, as during start-up of the engine operational temperatures will be lower whilst the engine heats up. The time the engine has been running may therefore provide an indication of a likely temperature of the engine itself. One parameter may comprise an indication of an overall temperature of the engine <NUM>. It is to be appreciated that the other parameters may comprise any suitable parameter which may influence the determination of the peak value of combustion in the combustion cylinder <NUM>. For example, during start-up of the engine <NUM>, the combustion cylinder <NUM> may be cooler than during normal operation, and so the increase in temperature of the working fluid between the pre-combustion temperature and the peak combustion temperature may be smaller than when the combustion cylinder <NUM> is hotter after extended use or in cases of high demand. Based on an indication of the temperature of the engine <NUM> (e.g. the combustion cylinder <NUM>), or for example a timer which indicates how long the engine <NUM> has been running, the mapping from the pre-combustion temperature to the peak combustion temperature may provide a more accurate estimation of the peak temperature of combustion in the combustion cylinder <NUM>.

The controller <NUM> is arranged to control the coolant system to cool the working fluid in response to determining that a temperature of the working fluid is greater than a selected threshold. During start-up of the engine <NUM>, the engine <NUM> will be operating at cooler temperatures and so the controller <NUM> may determine that the estimate of the peak temperature of combustion is well below the selected threshold. In which case, the controller <NUM> may control the coolant system so that little or no cooling occurs.

Once the engine <NUM> has progressed from the start-up conditions to a normal mode of operation, the controller <NUM> is configured to determine the peak temperature of combustion and control the coolant system to regulate the temperature of the working fluid. Controlling the coolant system is based on a feedback loop which comprises routinely monitoring the peak temperature of combustion and controlling cooling of the working fluid so that the peak temperature of combustion does not exceed a selected threshold. In response to determining that the peak temperature of combustion exceeds the selected threshold, the controller <NUM> is configured to operate the coolant system to increase cooling of the working fluid. In the example shown in <FIG>, this comprises controlling the coolant injector <NUM> to inject more coolant into the compression cylinder <NUM>. Although, it is to be appreciated that other ways of controlling the temperature of working fluid may be provided (e.g. by heat transfer using a recuperator). As the working fluid in the compression cylinder <NUM> is compressed, some of the increase in heat of the working fluid may be absorbed by the injected coolant. The coolant will absorb a certain portion of the heat to overcome its latent heat of vaporisation, which will act to inhibit the increase in temperature in the combustion cylinder <NUM>. Thus, by controlling the quantity of coolant injected into the combustion cylinder <NUM>, the controller <NUM> can control the heat of the working fluid. In particular, the controller <NUM> can influence the heat of the working fluid in the crossover passage <NUM> prior to the working fluid flowing into the combustion cylinder <NUM>.

The selected threshold comprises a criterion for the peak temperature of combustion. The controller may determine whether the criterion is satisfied or not based on a comparison comprising the estimated peak temperature of combustion and the criterion. The selected threshold may be a value for a maximum temperature, such that any peak temperature of combustion greater than this maximum temperature does not satisfy the criterion. The value for the selected threshold may be selected to inhibit the formation of NOx compounds. The controller may compare a value for the peak combustion temperature to the selected threshold, wherein the comparison is based on an average value for the peak temperature of combustion, i.e. a `global' value for the peak temperature for the entire cylinder. In other examples, the controller may compare a value for the peak combustion temperature to the selected threshold, wherein the comparison is based on a localised peak value for the peak temperature of combustion. The localised peak value may comprise a value for the highest peak temperature of combustion in any region of the combustion cylinder <NUM>. In some examples, the selected threshold may comprise an indication of both values. The selected threshold may require a temperature equal to or less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin; it may require a temperature of less than <NUM> Kelvin. It is to be appreciated that this value may be dependent on an equivalence ratio for the working fluid and fuel mixture and so may vary.

In response to determining that the peak temperature of combustion is greater than the selected threshold, the controller <NUM> controls the coolant system to regulate the temperature of the working fluid to be provided to the combustion cylinder <NUM>. As described above, the temperature is regulated using the coolant system. In one example, this may be by increasing the volume of coolant injected into the compression cylinder <NUM>, but additionally or alternatively it may be by controlling heat transfer away from a recuperator in the crossover passage. The controller <NUM> may be configured to determine the extent of the cooling based on the determined indication of the peak temperature of combustion. The coolant system may be operated in a continuous manner such that the volume of coolant injected is proportional to the amount of cooling required for the temperature of the working fluid to be cooled to less than the selected threshold. It may be operated in a discrete manner such that above a first selected threshold a first volume of coolant is injected, and above a second selected threshold a second volume of coolant is injected. There may be a plurality of such thresholds.

By controlling the coolant system to regulate the peak temperature of combustion in the combustion cylinder <NUM>, the controller <NUM> may therefore control the split cycle internal combustion engine <NUM> so that the combustion process is at lower temperatures to reduce production of NOx compounds.

It is to be appreciated that although the controller has been described as controlling the coolant system to inject more coolant, the same result could be achieved in other ways. For example, this may be achieved by injecting a different type of coolant, or coolant at a different temperature. Additionally, it is to be appreciated that the sensors are configured to provide the controller <NUM> with an indication of a peak temperature of combustion. However, this indication does not have to comprise a temperature, it could comprise a measurement of any suitable thermodynamic parameter from which the peak temperature of combustion could be determined. For example, using known thermodynamic relationships, a value for temperature may be determined based on a value for pressure.

In another aspect, the split cycle internal combustion engine <NUM> of <FIG> may operate using the timing of the inlet valve <NUM> to regulate the temperature of working fluid in the combustion cylinder <NUM>. The inlet valve <NUM> is operable to move from a closed state at a first position during the cycle of the piston to an open state at a second position during the cycle of the piston. When the inlet valve <NUM> is in the open state, working fluid in the crossover passage <NUM> may flow into the combustion cylinder <NUM>, and when the inlet valve <NUM> is in the closed state, the working fluid may not. In operation, the controller <NUM> may select the first and second position based on a selected threshold and/or a cooling threshold. These two positions may be selected so that they are separated by a selected time period; this time period may be constant and fixed and/or it may be variable. Combustion in the combustion cylinder <NUM> typically occurs at, or very close to the TDC position of the piston during the cycle. The first position is thus selected to be before TDC so that working fluid in the crossover passage <NUM> has time to flow into the combustion cylinder <NUM> before combustion occurs. The second position may be selected to be at or before TDC so that combustion provides a greater force on the piston. This is because, at combustion working fluid is expanded which causes the combustion piston <NUM> to move towards its BDC position. In the event that the inlet valve is still open during combustion, a portion of the working fluid may move back in to the crossover passage rather than provide a force on the combustion piston <NUM>. Thus, if the second position is selected so that the inlet valve is closed before expansion of the working fluid occurs then a greater force will be delivered to the combustion piston <NUM>.

As the first position is before TDC, there will be some compression of working fluid in the combustion cylinder <NUM> before combustion occurs. This will increase the temperature of this working fluid. The temperature of the working fluid prior to combustion will influence the peak temperature of combustion in the combustion cylinder <NUM>, and thus by controlling this compression-induced heat rise in the combustion cylinder <NUM>, the controller <NUM> can regulate the peak temperature of combustion in the combustion cylinder <NUM>. The amount of compression-induced heat rise in the combustion cylinder <NUM> will depend on the first position. The sooner after BDC the first position is, the greater the amount of heating of the working fluid. The controller <NUM> may therefore select the first position based on a determined amount of heating required. This may be determined based on the determined peak temperature of combustion in the combustion cylinder <NUM>, and thus a desired extra amount of heating for the working fluid to be at a selected temperature prior to combustion, such that the peak temperature of combustion is within a selected range.

For instance, in response to determination of an estimate of the peak temperature of combustion being greater than the selected threshold, the controller <NUM> selects the first position to be later during the cycle of the piston. In response to determining that the peak temperature of combustion is below a cooling threshold, the controller <NUM> selects the first position to be earlier during the cycle of the piston so that the working fluid may receive more heating. Likewise, the controller <NUM> may control the second position based on the peak temperature of combustion and the cooling and selected thresholds.

In another aspect, the split cycle internal combustion engine <NUM> of <FIG> may operate using the timing of the injection of fuel by the fuel injector <NUM> to regulate the temperature of working fluid in the combustion cylinder <NUM>. Injection of the fuel may occur at an injection position during the cycle of the piston. The injection may occur for a set time period; it may occur for a variable time period; the time period may be based on a volume of fuel to be injected. The controller <NUM> is configured to select the injection position based on the determined estimate for the peak temperature of combustion. For instance, in response to determining an estimate for the peak temperature of combustion which is greater than the selected threshold, the controller <NUM> may control the fuel injector <NUM> to inject fuel at a delayed injection position during the cycle of the piston. The delayed injection position may comprise a position during the cycle of the piston which occurs later than a present injection position. In response to determining an estimate for the peak temperature of combustion which is less than the cooling threshold, the controller <NUM> may control the fuel injector <NUM> to inject fuel at an earlier injection position during the cycle of the piston. The earlier injection position may comprise a position during the cycle of the piston which occurs before a present injection position.

Typically, combustion will occur at or very shortly after the TDC position of the combustion piston <NUM>. Controlling the combustion to occur at the TDC position may enable an expanding force to be applied on the combustion piston <NUM> for a greater length of time, whilst the combustion piston <NUM> returns to its BDC position. The volume in the combustion cylinder <NUM> defined by the location of the combustion piston <NUM> changes during the stroke of the piston, and will be at its lowest at the TDC position of the combustion piston <NUM>. Combustion at this TDC position may result in a greater expansion of the working fluid than combustion at a later position during the cycle of the piston. Combustion closer to TDC may also result in a greater change in temperature from the starting temperature than combustion later on after TDC. As a consequence, a peak temperature of combustion in the combustion cylinder <NUM> may be greater for an earlier starting combustion. Combustion will not occur without the fuel.

The controller <NUM> is configured to control the fuel injector <NUM> to inject fuel into the combustion cylinder <NUM> at an injection position during the cycle of the piston. The controller may delay injection of the fuel so that it is injected at a later position during the cycle of the piston (e.g. after TDC). Based on the determined estimate for the peak temperature of combustion in the combustion cylinder <NUM>, the controller may determine that the estimate for peak temperature is too high and may result in NOx generation. As a way of regulating the temperature in the combustion cylinder <NUM>, the controller may delay injection of the fuel so that combustion occurs at a later position during the cycle of the piston. The peak temperature of combustion may therefore decrease which may inhibit NOx generation.

In another aspect, the split cycle internal combustion engine <NUM> of <FIG> may operate using the controller <NUM> to control the coolant system to regulate the peak temperature of the working fluid supplied to the combustion cylinder <NUM> based on an estimate for the peak temperature of combustion in the combustion cylinder <NUM>. The controller <NUM> may use the estimate so that the peak temperature of combustion in the combustion cylinder <NUM> is within a selected range. In particular, during normal operation of the engine <NUM>, the controller may select the selected range so that the peak temperature of combustion in the combustion cylinder is not greater than the selected threshold and/or is not less than the cooling threshold. The selected range may be selected to be a range of values between the cooling threshold and the selected threshold. This may enable the controller <NUM> to control operation of the engine so that both efficiency and NOx generation satisfy selected criteria.

The controller <NUM> may determine the estimate for the peak temperature of combustion based on a received indication of a parameter of the engine. For example, the controller <NUM> may determine the estimate based on a received indication of a demand on the engine. In which case, the controller <NUM> may predict based on the indication of demand for the engine, and (e.g. an indication of a temperature of working fluid to be supplied to the combustion cylinder <NUM>), an estimate for the peak temperature of combustion that will be reached in the combustion cylinder <NUM>.

The prediction may be based on previous data associated with the engine <NUM>. For example, the controller <NUM> may access a look-up table comprising a mapping between a value, or values, for at least one engine parameter and a corresponding estimate for peak temperature. The controller <NUM> may comprise a machine learning element which comprises a model for predicting peak temperatures of combustion based on input data relating to the engine (e.g. parameters for the engine, or a log of measurements for the engine since it started running). This machine learning element may be 'trained' on data for which there is a known peak temperature of combustion associated with the input data. This may enable a prediction model of the machine learning element to learn and update based on training data so that the model may provide a more reliable and accurate system for predicting peak temperatures. Based on this estimate, the controller may control the coolant system so that a peak temperature of combustion in the combustion cylinder <NUM> is within the selected range.

The split cycle internal combustion engine <NUM> of <FIG> may regulate the temperature of the working fluid in accordance with examples described above. The temperature regulation may be based on a combination of above examples.

<FIG> shows a second example of a split cycle internal combustion engine <NUM> arranged to control a peak temperature of combustion so that it is below a selected threshold. The engine <NUM> of <FIG> is similar to the engine <NUM> of <FIG> and so components which perform substantially the same functions are associated with the same reference numerals and will not be described again.

The split cycle internal combustion engine <NUM> of <FIG> also comprises a reactivity adjuster <NUM>. The reactivity adjuster <NUM> is connected to the controller <NUM> so that the controller <NUM> may control operation of the reactivity adjuster <NUM>. The reactivity adjuster <NUM> is operable to adjust the reactivity of a fuel to be used during the combustion process. The reactivity adjuster <NUM> is illustrated as being operable to act on fuel (e.g. in the fuel reservoir <NUM>) to be injected into the combustion cylinder <NUM>. The reactivity adjuster <NUM> is also illustrated as being operable to act directly on fuel within the combustion cylinder <NUM>. The reactivity adjuster <NUM> is operable to increase the ability of a fuel to ignite. This may comprise at least one of: making the fuel more reactive and providing additional means for ignition of the fuel in the combustion cylinder <NUM>. The controller <NUM> may also control operation of the reactivity adjuster <NUM> in response to determining that the reactivity of the fuel is greater than an over-reactivity threshold. This may help reduce NOx formation as over-reactive fuel may produce a higher peak temperature of combustion.

In the example shown, the reactivity adjuster <NUM> comprises a system for directing electromagnetic radiation, e.g. laser or microwave radiation, at the fuel to provide an additional source of ignition for the fuel in the combustion cylinder <NUM>. This may provide a more targeted ignition mechanism and so may enable fuel to ignite in less favourable ignition conditions, such as when the combustion cylinder <NUM> is colder than an ignition threshold temperature. The controller <NUM> may be configured to control the reactivity adjuster <NUM> so that, in response to determining that a temperature in the combustion cylinder <NUM>, and/or a temperature of the working fluid, is less than the ignition threshold, the controller <NUM> controls the reactivity adjuster <NUM> to provide an additional source of fuel ignition. The reactivity adjuster <NUM> may comprise a system for selective energy transfer. The system for selective energy transfer may provide targeted radiation for certain compounds found within the fuel working fluid mixture to increase reaction rates. This may comprise targeted radiation for breaking up compounds which would produce improved combustion, e.g. breaking down CH<NUM> (methane) so that combustion may occur at a lower starting temperature, and thus a peak temperature of combustion may occur at a lower temperature, which in turn may inhibit NOx generation.

In some examples, the reactivity adjuster <NUM> may comprise a system for providing an oxidising agent or free radical to the fuel. This provision may be in the combustion cylinder <NUM>; it may be in the fuel reservoir <NUM> (for example, prior to injection of the fuel into the combustion cylinder <NUM>). The provision of an oxidising agent may enable a larger proportion of the fuel to ignite; it may increase the probability of initially igniting the fuel. For example, a suitable oxidising agent may comprise: oxygen or ozone. Although it is to be appreciated that any suitable oxidising agent may be added.

The controller <NUM> is configured to receive an indication of at least one of a pressure, a density and a temperature of the working fluid, and based on this to determine an ignition parameter of the working fluid. The determined ignition parameter may provide an indication of the ability of the fuel to ignite. For example, the ignition parameter may provide an indication of an expected proportion of the fuel which will ignite. The controller <NUM> is configured to determine the ignition parameter based on the received indication. For instance, this may comprise using a look-up table to identify, based on one or more values for thermodynamic properties of the working fluid, a value for the ignition parameter. These values may be determined theoretically and/or empirically. For example, the controller <NUM> may identify that the fuel is less likely to ignite when it is cold, and so, in response to receiving an indication that the temperature of the working fluid is cold, the ignition parameter may be determined to be a low value.

In response to determining that the ignition parameter is below an ignition threshold, the controller <NUM> is configured to operate the reactivity adjuster <NUM>. Operation of the reactivity adjuster <NUM> will contribute towards increasing the value for the ignition parameter, and thus to a probability that the fuel will ignite. The controller <NUM> may be configured to determine the extent of the operation of the reactivity adjuster <NUM> based on the determined ignition parameter. For instance, the extent of the operation of the reactivity adjuster <NUM> may be determined based on the size of the difference between the ignition parameter and the ignition threshold. There may be a plurality of ignition thresholds, and the controller <NUM> may determine the extent of the operation of the reactivity adjuster <NUM> based on which thresholds the ignition parameter satisfies. The reactivity adjuster <NUM> may provide benefits in particular during start-up of the engine <NUM>, when the ignition parameter may be below, even considerably below, the ignition threshold. For example, the temperature of the combustion cylinder <NUM> may be very low, and operation of the reactivity adjuster <NUM> may enable the fuel to ignite and thus enable combustion to occur at a much lower temperature.

<FIG> shows an exemplary temperature-entropy diagram for the operation of a split cycle internal combustion engine as illustrated in <FIG> or <FIG>. The dashed line shows the cycle for an engine with no cooling, and the solid line shows the cycle with cooling. This diagram is based on an approximation of the engine using a Nitrogen only cycle. Both cycles produce the same amount of heat output. In the cycle with coolant added, the bottom left point in the cycle has a lower value for both temperature and entropy when compared to the cycle with no cooling. This is due to an increase in mass and decrease in temperature as a consequence of the addition of coolant. Consequently, the top right point in the cycle with cooling is at a lower temperature and entropy to the cycle with no cooling. This point represents the peak temperature of combustion. The amount of cooling may therefore be controlled so that this peak temperature of combustion is below the selected threshold. This may inhibit generation of NOx, but this may avoid an associated decrease in efficiency of the engine, because the same amount of heat is released. This is because a ratio between an initial and final pressure in the combustion cylinder may be the same for both the cycle with cooling and the cycle with no cooling, and engine cycle efficiencies are determined based on such ratios. A slope of the line from the top left point to the top right point in each cycle represents an efficiency of the conversion from thermal energy into pressure. The flatter this slope is the more efficient the conversion is. As can be seen from <FIG>, the cycle with cooling may provide an increased efficiency for conversion from thermal energy into pressure as the slope is shallower.

<FIG> shows the exemplary temperature-entropy diagram for the operation of a split cycle internal combustion of <FIG> with lines of constant pressure added in. The lines of constant pressure illustrate that the ratio of final to initial pressures of combustion are the same for both cycles. As a result, the two cycles are operating at the same level of engine efficiency. However, as the temperature of the 'with cooling' cycle is controlled to be lower than that for the without cooling, a maximum temperature of combustion may be reduced. In turn, this may inhibit generation of NOx and/or particulates.

<FIG> shows a graph illustrating examples of different start temperatures for combustion mapping on to their respective final temperatures for combustion for N-Dodecane, Methane (CH<NUM>) and Iso-octane. The graph also provides an indication of values for the equivalence ratio for each of these fuels at the start temperature. The graph also illustrates the region of end temperatures above with NOx generation typically occurs; the line is illustrated at a final temperature of approximately <NUM> Kelvin. Typically, the value for temperature of NOx generation remains the same for the fuels discussed herein. For example, the selected threshold may be selected based on a typical value for the temperature at which NOx generation occurs. The graph also illustrates a region of start temperatures for which complete combustion of the fuel typically occurs. As illustrated, this region extends from a temperature of approximately <NUM> Kelvin to a temperature of approximately <NUM> Kelvin. For example, the cooling threshold may be selected based on the lower value of the range in which complete combustion occurs. This is because, for start temperatures of combustion below the lower value, the combustion may be inefficient as the fuel cannot completely ignite and combust.

The graph shows that for N-Dodecane as the fuel, and with a range of start temperatures from <NUM> Kelvin to <NUM> Kelvin, complete combustion may occur without entering into a final temperature of combustion in the NOx zone. The graph shows this occurring across the range of temperatures for equivalence ratios of <NUM> to <NUM>. For the equivalence ratios of <NUM> and <NUM>, the final temperature of combustion may not reach the NOx zone for lower starting temperatures within the range. However, for higher starting temperatures, the final temperature of combustion may reach the NOx zone. As one example, at an equivalence ratio of <NUM> and a starting temperature of approximately <NUM> Kelvin, the end temperature of combustion is approximately <NUM> Kelvin and not in the NOx zone. As another example, at an equivalence ratio of <NUM> and a starting temperature of <NUM> Kelvin, the end temperature of combustion is approximately <NUM> Kelvin, and in the NOx zone. This shows that by controlling the start temperature to be within a certain range, for a given equivalence ratio, the final temperature of combustion may avoid the NOx zone, and thus may inhibit the generation of NOx.

As illustrated in the graph, the equivalence ratio is the fuel-air equivalence ratio (φ). For Dodecane, at a fuel-air equivalence ratio of <NUM>, the air-fuel equivalence ratio (λ) is <NUM>. The fuel-air equivalence ratio may be selected based on a leanness threshold. A leanness ratio may be defined based on the fuel-air equivalence ratio. For example, the leanness threshold may be selected based on a fuel-air equivalence ratio of be <NUM>; it may be <NUM>; it may be <NUM>; it may be <NUM>; it may be <NUM>; it may be <NUM>. In the event that the leanness of the fuel and working fluid mixture is below a certain value, particulate generation may occur. The leanness threshold may be selected based on the certain value. Particulate generation may comprise generation of soot in the engine. The leanness of the mixture between the working fluid and the fuel may be controlled so that the mixture is lean enough to avoid generation of particulates. Typically, particulate generation occurs as a result of 'rich zones' of fuel, where the fuel does not mix with enough oxygen and so incomplete combustion occurs. Combustion may also be controlled to avoid generation of compounds HC and CO, the presence of which typically results in ineffective combustion. The equivalence ratio may be based on a local equivalence ratio; it may be based on an average equivalence ratio for the combustion cylinder; it may be based on both.

In another aspect, the engine <NUM> of either of <FIG> or <FIG> may operate so that the controller <NUM> controls at least one thermodynamic property of the working fluid in the crossover passage <NUM> so that the flow of that working fluid into the combustion cylinder <NUM> satisfies a selected criterion. In particular, the controller <NUM> is configured to control at least one of the pressure and the density of the working fluid in the crossover passage <NUM> so that the working fluid flows into the combustion cylinder <NUM> at a speed greater than a speed threshold. The speed threshold is selected so that the working fluid flowing into the combustion cylinder <NUM> enables a lean mixing of fuel with the working fluid in the combustion cylinder <NUM>. For instance, the working fluid may flow in through the inlet valve <NUM> at a speed so that it generates a large amount of turbulence and result in a fast flow of fluid past the fuel injector <NUM>. Upon injection of fuel into the combustion cylinder <NUM>, the fuel may be suitably dispersed as a result of the speed of the flow of the working fluid. This may reduce the number of 'pockets of fuel' which burn at a higher temperature than their surroundings, and thus it may reduce the amount of NOx and/or soot formation. This may also ensure that the fuel is completely reacted, leaving no pyrolysis products and so enabling a greater proportion of the fuel to be consumed to produce useful output.

In operation, the controller <NUM> is configured to receive an indication of at least one of a pressure and a density of the working fluid in the crossover passage <NUM>. The indication may be received from the crossover sensor <NUM>, which may be configured to measure a suitable thermodynamic parameter from which the pressure and/or density may be determined. The controller <NUM> may determine the pressure and/or density using a look-up table or a mathematical model which provides a mapping between the measured parameter and a corresponding value for pressure and/or density. The controller <NUM> is configured to compare this determined value to an input threshold. The input threshold may be a value for the measured parameter in the crossover passage which is expected to produce a flow of working fluid into the combustion cylinder at a speed greater than or equal to the speed threshold. The controller <NUM> is configured to control the pressure and/or density of the working fluid based on this comparison. The pressure and/or density of the working fluid are controlled so that the fluid flows into the combustion cylinder <NUM> at a speed greater than the speed threshold.

The speed threshold is selected to be a speed which results in a turbulent flow of fluid into, and within, the combustion cylinder <NUM> to provide a lean mixing of the fuel with the working fluid. A value for the speed threshold may be determined based on the pressure and/or density of the working fluid in the crossover passage <NUM>, as well as dimensions of the combustion cylinder <NUM> and the inlet valve <NUM>, which could be used to model the flow of the fluid into the combustion cylinder <NUM>. The value for the speed threshold is therefore selected so that flow of the working fluid in to the combustion cylinder <NUM> at the speed threshold results in a lean mixing of fuel. Lean mixing of the fuel is selected so that complete combustion may occur and particulate generation is inhibited. Optionally the leanness may be selected so that all of the fuel does not all ignite at once, but the fuel ignition is staggered over the duration of the combustion stroke, as this may provide a more consistent output of work by the engine <NUM>. For example, the speed threshold may be greater than <NUM> metres per second; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s; it may be <NUM>/s. However, it is to be appreciated that this value is dependent on different parameters of the engine and so may be considerably higher or lower. For example, pressure or density may influence the value for the speed threshold. Typically, at these speeds the flow of fluid into the combustion cylinder <NUM> will be choked flow, which results from a breaking of the sound barrier interfering with the flow of working fluid into the combustion cylinder <NUM>.

The input threshold is selected based on the speed threshold and/or a selected level of turbulence within the combustion cylinder <NUM>. For instance, the input threshold may be selected based on empirical data and/or a mathematical model which provides an indication of an associated level of turbulence within the combustion cylinder <NUM>. The input threshold may be selected so that working fluid with a pressure and/or density at the input threshold will flow into the combustion cylinder <NUM> at the speed threshold.

In response to determining that the pressure and/or density of the working fluid are greater than the input threshold, the controller <NUM> is configured to control the pressure and/or density of working fluid. The controller <NUM> may control the pressure and/or density of the working fluid using the coolant system. In response to determining that the pressure and/or density are below the input threshold, the controller <NUM> is configured to control operation of the coolant system. This may reduce the temperature of the working fluid, which may enable a greater pressure and/or density in the crossover passage <NUM>. For example, in an engine <NUM> at <NUM> and 7MPa, a reduction of the temperature to <NUM> would yield a <NUM>% increase in density. The extent of the operation of the coolant system may be determined based on an extent of the difference in value between the pressure and/or density of the working fluid and the input threshold. In the example of <FIG> and <FIG>, the coolant system comprises a coolant injector <NUM> for injecting coolant into the compression cylinder <NUM> of the engine <NUM>. Operation of this coolant system may comprise increasing the volume of coolant injected into the compression cylinder <NUM>. The controller <NUM> may also control the volume of coolant injected based on the determined pressure and/or density of the fluid.

The increased pressure in the crossover passage <NUM> will create an increased pressure differential between the crossover passage <NUM> and the combustion cylinder <NUM>, and so in response to the inlet valve <NUM> to the combustion cylinder <NUM> moving into the open state, the flow of working fluid into the combustion cylinder <NUM> will be at a greater speed. The increased density will provide an increase in the density of oxygen carrying gas in the combustion cylinder <NUM>. Another consequence of an increased density is that an initial temperature of the working fluid in the combustion cylinder <NUM> is lower, and thus the likelihood of NOx generation is reduced. This increased density provides an increased mass of gas, which results in increased pressure and less temperature rise upon combustion. Thus, this also reduces the peak temperature of combustion and so inhibits NOx production.

In another aspect, the split cycle internal combustion engine <NUM> may operate based on selected valve timings. The valve timings comprise a timing associated with the first position during the cycle of the piston at which the inlet valve <NUM> moves from the closed to the open state, and the second position during the cycle of the piston at which the inlet valve <NUM> moves from the open to closed state. The first and second position may be fixed, so that the inlet valve <NUM> moves at selected positions which are not controlled by the controller <NUM>. The controller <NUM> may therefore determine the input threshold based on the selected positions. This comprises determining, based on the conditions of the engine <NUM> at each position, a value for the input threshold so that working fluid in the crossover passage <NUM> at the input threshold may flow into the combustion cylinder <NUM> at a speed greater than the speed threshold after the inlet valve <NUM> has moved to the open state at the first position.

In another aspect, the split cycle internal combustion engine <NUM> may operate based on a control of movement of the inlet valve <NUM> for the combustion cylinder <NUM>. The inlet valve <NUM> may move from the closed state to the open state. Movement to the open state comprises movement of the valve so that a cross-sectional area is presented to the working fluid in the crossover passage for the fluid to flow through and into the combustion cylinder <NUM>. The inlet valve <NUM> may be configured to move between the closed state and a plurality of open states. The plurality of open states may comprise a series of discrete states in which a different cross-sectional area is defined; it may comprise a continuum of states in which the cross-sectional area differs continuously. The controller <NUM> is configured to control the movement of the inlet valve <NUM> so that a selected cross-sectional area is defined for the working fluid to flow through into the combustion cylinder <NUM>.

The controller <NUM> is configured to control the movement of the inlet valve so that the selected cross-sectional area is defined. The controller is configured to select the selected cross-sectional area so that the working fluid in the crossover passage <NUM> flows through the cross-sectional area and into the combustion cylinder <NUM> at a speed greater than a speed threshold. The controller <NUM> may determine the selected cross-sectional area based on a received indication of a parameter of the engine. For example, the controller <NUM> may be configured to use a mathematical model (e.g. based on Bernoulli flow) to determine an estimate for the speed of fluid flow into the combustion cylinder <NUM>. The controller <NUM> may determine, based on e.g. the mathematical model or a look-up table, the cross-sectional area needs to be limited to a selected cross-sectional area for the working fluid to flow into the combustion cylinder <NUM> at a speed below the speed threshold. The controller <NUM> may therefore control the inlet valve <NUM> to move to the open state, wherein movement to the open state comprises opening the valve, but not necessarily opening the valve to its fully-open state. Rather, the valve may be opened to a portion of its fully-open state, e.g. the inlet valve may be moved to a half-open state. The extent to which the inlet valve <NUM> is moved may be based on the received indication of pressure in the crossover passage <NUM>. For example, in the event that the pressure in the crossover passage <NUM> is very high, the controller <NUM> may control the inlet valve <NUM> to open to its fully-open state, as even with a much greater cross-sectional area, the working fluid may still flow into the combustion cylinder <NUM> at a speed greater than the speed threshold. In another example, the controller <NUM> may determine that the pressure in the crossover passage <NUM> is not very high and may thus control the inlet valve <NUM> to open to a just-open state in which the cross-sectional area defined is very small and thus results in a much faster flow of working fluid into the combustion cylinder <NUM>.

The controller <NUM> is configured to control the valve lift so that the working fluid flows into the combustion cylinder at a speed greater than a speed threshold. The speed that the working fluid flows into the combustion cylinder may be determined as a peak speed of flow, which typically will occur at, or very shortly after, opening of the inlet valve <NUM>. This speed may be determined based on a measurement from the exhaust sensor. For example, if the exhaust sensor determines that NOx and/or particulate generation is above a threshold level then the speed of flow is too low. By controlling the movement of the inlet valve <NUM> so that the speed of flow of the working fluid into the combustion cylinder <NUM> is greater than the speed threshold, the mixing of air and fuel in the combustion cylinder may provide a leanness ratio greater than a leanness threshold. The leanness threshold may inhibit generation of particulates as the fuel and working fluid is sufficiently mixed up that each unit of fuel is provided with sufficient oxygen for complete combustion to occur, and thus for particulate generation to be inhibited. Controlling the speed of flow to be greater than a speed threshold may also reduce the stress on the fuel injector <NUM>, because fewer requirements are placed on the fuel injector <NUM> with regards to mixing of the fuel and working fluid, which may prolong injector life. Additionally, running operating the inlet valve <NUM> to open at a low lift may reduce the time taken for the inlet valve <NUM> to move from its closed state to its open state as it has less far to move. This may speed up the process of getting working fluid into the combustion cylinder <NUM> from the crossover passage <NUM>. As a result, the inlet vale <NUM> may be opened later during the cycle of the piston.

The controller <NUM> may determine movement of the inlet valve <NUM> based on data regarding the design of the inlet valve <NUM>. For example, the dimensions of the valve may be considered, such as its shape, or surface friction levels. It is to be appreciated that the specifics of the fluid flow path from the crossover passage <NUM> into the combustion cylinder <NUM> (e.g. shape, length, diameter etc.) may influence the speed of flow. The controller <NUM> may access a look-up table which is specific to its inlet valve <NUM> when determining the cross-sectional area to be defined by the inlet valve <NUM> for fluid flow.

In another aspect, the split cycle internal combustion engine <NUM> may operate based on variable valve timings. This may comprise the controller <NUM> selecting the first and second position based on a determined value for the pressure and/or density of the working fluid, so that the working fluid flows into the combustion cylinder <NUM> at a speed greater than the speed threshold after the inlet valve <NUM> has moved to the open state at the selected first position.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. At step <NUM>, the method starts and proceeds to step <NUM> at which an indication of peak temperature is received. As discussed above, this indication may be received from one or more sensors, and may provide information about a parameter of the engine. At step <NUM>, based on the indication received at step <NUM>, a peak temperature of combustion in the combustion cylinder <NUM> is determined. The peak temperature may be determined as described above. At step <NUM>, the determined peak temperature is compared against the selected threshold. In response to determining that the peak temperature is less than the selected threshold, the method proceeds to step <NUM>, where the peak temperature is compared to a cooling threshold. At step <NUM>, if the determined peak temperature is greater than the cooling threshold, it is determined that the peak temperature of the engine is within a suitable range. The method then loops back to the start, where another indication of peak temperature is received. The looping may occur over a variable timescale, for example indications may be received at selected time periods; the indications may be received more frequently during start-up of the engine where values for parameters of the engine will vary more. In response to determining at step <NUM> that the determined peak temperature is greater than the selected threshold, or at step <NUM> that it is less than the cooling threshold, the method proceeds to step <NUM>. At step <NUM>, the coolant system is controlled to regulate temperature of the working fluid based on the determined peak temperature. The temperature may be regulated with the aim of moving the peak temperature of combustion to be within the suitable range. The method then loops back to step <NUM>. The frequency of received indications may be higher in response to the looping being from step <NUM> rather than step <NUM>.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. Steps <NUM> to <NUM> of the method correspond to steps <NUM> to <NUM> respectively of <FIG> described above and so are not described again. At step <NUM>, in response to the determined peak temperature being greater than the selected threshold, or the determined peak temperature being less than the cooling threshold, the first and second positions for the respective opening and closing of the inlet valve are selected to regulate the temperature of the working fluid. The temperature of the working fluid may be regulated with the aim of moving the peak temperature of combustion to be within a suitable range (e.g. between cooling threshold and selected threshold).

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. At step <NUM>, the method starts and proceeds to step <NUM>, where an indication of an engine parameter is received. At step <NUM>, an ignition parameter for the fuel is determined based on the indication received at step <NUM>. The ignition parameter may be determined as described above. At step <NUM>, the ignition parameter is compared to an ignition threshold. In response to determining that the ignition parameter is greater than the ignition threshold, the method proceeds to step <NUM> where the ignition parameter is compared to an over-reactive threshold. In response to the ignition parameter being less than the over-reactive threshold, the ignition parameter is considered to be within a suitable range and the method loops back to step <NUM>; this looping may be as described above. In response to the ignition parameter being either less than the ignition threshold or greater than the over-reactive threshold, the method proceeds to step <NUM> where the reactivity adjuster is operated, for example to adjust the working fluid so that the reactivity is within the suitable range for operation of the engine.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. Steps <NUM> to <NUM> of the method correspond to steps <NUM> to <NUM> respectively of <FIG> described above and so are not described again. At step <NUM>, in response to the determined peak temperature being greater than the selected threshold, or the determined peak temperature being less than the cooling threshold, an injection position for the injector is selected. The injection position is selected to regulate the temperature of the working fluid as described above.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. At step <NUM>, the method starts and proceeds to step <NUM> where an indication of peak temperature is received. An estimate for the peak temperature of combustion may be determined, as described above, based on the indication received at step <NUM>. At step <NUM>, the coolant system is controlled so that a peak temperature of combustion is within a selected range. This step may comprise, increasing and or decreasing cooling of the working fluid based on whether the peak temperature of combustion low or high with respect to the selected range.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. At step <NUM>, the method starts and proceeds to step <NUM> where an indication of an engine parameter is received. At step <NUM>, based on this indication of the engine parameter, a value for an engine parameter (in the example of <FIG>, a pressure and/or temperature) may be determined. This determination will be dependent on what is comprised within the indication. It may comprise use of a thermodynamic relation to process a value for one engine parameter to determine a value for another (a pressure or a temperature). At step <NUM>, the determined parameter (pressure and/or temperature) is compared to an input threshold. In response to the parameter being greater than the input threshold, the working fluid is considered to be suitable for use in the combustion cylinder, and the method loops back to <NUM>. In response to the parameter not being greater than the input threshold, the method proceeds to step <NUM> where the parameter (pressure/temperature) of the working fluid is controlled so that it may be in a suitable range for the working fluid to flow into the combustion cylinder <NUM> at a speed greater than the speed threshold. The method then loops back to step <NUM>.

A method of operation of a split cycle internal combustion engine, for example the split cycle internal combustion engine <NUM> of <FIG> and <FIG>, will now be described with reference to <FIG>. Steps <NUM> to <NUM> of the method correspond to steps <NUM> to <NUM> respectively of <FIG> described above and so are not described again. At step <NUM>, the movement of the inlet valve is controlled to define a cross-sectional area of the inlet valve opening through which working fluid flows from the crossover passage <NUM> into the combustion cylinder <NUM>. The cross-sectional area is selected, as described above, so that the working fluid flows into the combustion cylinder <NUM> at a speed greater than a speed threshold.

It is to be appreciated that whilst the description has been directed towards NOx, the term NOx may be considered to encompass any suitable Nitrogen Oxide compound, for example N<NUM>O, or any other combination of Nitrogen and Oxygen. It is not to be construed as limited directly to compounds containing a single Nitrogen atom.

It is to be appreciated that the cycle of the piston is cyclical and recurring and so reference to occurrence later in the cycle of the piston may refer to occurrence at a later time. Each cycle of the piston may be considered to commence with the combustion piston <NUM> at its bottom dead centre ('BDC') position. During the cycle of the piston, the combustion piston <NUM> then proceeds to move from its BDC position to its top dead centre (`TDC') position, before returning back to its BDC position. Thus, discussion of, for example the injector injecting fuel at an earlier/later position during the cycle of the piston or the inlet valve opening and closing at an earlier/later position during the cycle of the piston, is based on the cycle of the piston moving from BDC to BDC.

With reference to the drawings in general, it will be appreciated that schematic functional block diagrams are used to indicate functionality of systems and apparatus described herein. It will be appreciated however that the functionality need not be divided in this way, and should not be taken to imply any particular structure of hardware other than that described and claimed below. The function of one or more of the elements shown in the drawings may be further subdivided, and/or distributed throughout apparatus of the disclosure. In some embodiments the function of one or more elements shown in the drawings may be integrated into a single functional unit.

In some examples, one or more memory elements can store data and/or program instructions used to implement the operations described herein. Embodiments of the disclosure provide tangible, non-transitory storage media comprising program instructions operable to program a processor to perform any one or more of the methods described and/or claimed herein and/or to provide data processing apparatus as described and/or claimed herein.

The activities and apparatus outlined herein may be implemented with fixed logic such as assemblies of logic gates or programmable logic such as software and/or computer program instructions executed by a processor. Other kinds of programmable logic include programmable processors, programmable digital logic (e.g., a field programmable gate array (FPGA), an erasable programmable read only memory (EPROM), an electrically erasable programmable read only memory (EEPROM)), an application specific integrated circuit, ASIC, or any other kind of digital logic, software, code, electronic instructions, flash memory, optical disks, CD-ROMs, DVD ROMs, magnetic or optical cards, other types of machine-readable mediums suitable for storing electronic instructions, or any suitable combination thereof.

Claim 1:
A split cycle internal combustion engine (<NUM>) comprising:
a compression cylinder (<NUM>) accommodating a compression piston (<NUM>);
a combustion cylinder (<NUM>) accommodating a combustion piston (<NUM>);
a crossover passage (<NUM>) between the compression cylinder (<NUM>) and the combustion cylinder (<NUM>) arranged to provide working fluid to the combustion cylinder (<NUM>);
a controller (<NUM>) arranged to determine a peak temperature of combustion in the combustion cylinder (<NUM>) based on a received indication of a peak temperature of combustion in the combustion cylinder (<NUM>); and
a coolant system arranged to regulate a temperature of the working fluid supplied to the combustion cylinder (<NUM>);
wherein, in response to determining that the peak temperature of combustion exceeds a selected threshold, the controller (<NUM>) is configured to control the coolant system to regulate the temperature of the working fluid supplied to the combustion cylinder (<NUM>) so that a peak temperature of combustion in the combustion cylinder (<NUM>) is less than the selected threshold.