Controlling an Engine's Emission Levels
Combustion in existing engines involves flame fronts: When the air-fuel mixture in a gasoline or diesel engine ignites, a flame front swiftly expands throughout the combustion chamber. Unfortunately, this uneven combustion produces high levels of exhaust gases that must be purified. Bosch researchers intend to use the HCCI technology to replace this wasteful combustion method, at least in the partial engine load range.
The entire combustion process should take place as uniformly as possible throughout the combustion chamber. In future engines, fuel combustion will create no visible flame, and combustion will occur simultaneously throughout the entire combustion chamber. As a result, emissions of soot and nitrogen oxides will be reduced. The principle is analogous to the blue flame of a gas range, which burns with fewer emissions than does a smoky candlewick. Bosch researchers intend to approach this ideal state from two directions: From the diesel side and from the gasoline side.
In both engine designs, it’s important to optimize the system as a whole. To date, the operating states of a diesel or gasoline engine have been mapped in a large number of characteristic curve fields. If the driver steps on the gas, the engine management software scans these diagrams and tables, and adjusts the ignition, intake air charge and injection metering to meet the required operating state of the engine.
The HCCI method goes a step further: The engine regulates itself and its emissions by means of a pressure sensor in the combustion chamber. Such feedback control is required because homogeneous fuel combustion cannot simply be set to the right level, since HCCI combustion cycle differs statistically from the preceding one.
With a sensor in the combustion chamber, for instance on the glow plug or on the spark plug, the instantaneous operating conditions within the combustion chamber can be measured. The in-cylinder pressure signal has proven to be the best variable to be tracked for this purpose. The measured pressure in the combustion chamber is used to control the air management.
The proper mixture of fresh air and recirculated exhaust gas is of key importance. This can be achieved with external exhaust gas recirculation alone or in combination with internal exhaust gas recirculation by means of variable valve timing. This approach lowers the combustion temperature and thereby prevents nitrous oxide formation. But to optimize fuel consumption, final compression temperatures must be high. Feedback from the sensor ensures an optimal compromise for the driving situation at a given moment.
In addition to pressure sensing, temperature or knock sensors can also contribute to the overall timing of each cylinder cycle. In the gasoline engine in particular, it’s essential to prevent a drift toward uncontrolled knocking that might damage the engine.
In principle, the HCCI method can be used only at moderate engine speed in the partial-load range. This is due to the short time windows – between 50 and 120 milliseconds per cycle – available for achieving a homogenized mixture. As the engine speed increases, the time window shrinks until homogenization is no longer possible. Since exhaust emissions are regulated based on the extent of vehicle operation within representative driving cycles, the goal for HCCI is to cover as large of an area as possible in theses cycles.
If the driver operates outside this ideal cycle – for instance by fast acceleration in a low gear – the HCCI engine reverts to the conventional operating mode of the gasoline engine. This is why future engine designs will have to be able to work in both operating modes: HCCI at low and intermediate loads, and conventional combustion at full load.
Bosch researchers are striving to maximize the load range in which the HCCI method can be used. The development of this design is still a work in progress. It addresses charge composition and charge dynamics, injection timing and supercharging strategy, combustion chamber geometry and active combustion control. An example to control the charge dynamics is an electrohydraulic valve-train system (EHVS) which enables the operation of HCCI, as well as improves the performance of the conventional operation mode. Initial prototypes are already being tested, but there are still many technical hurdles to overcome before a market launch will be possible.
The large number of mutually interactive variables and sub-systems is a major engineering challenge, but it also provides the flexibility essential for converging to an optimal solution. After all, a properly matched combination of fuel, injection system, air management and exhaust treatment should always make it possible to create sufficiently stable operating conditions to meet prevailing regulatory or self-imposed emission and fuel economy targets. Bosch expects that appropriate engine design and engine management can already reduce raw emissions sufficiently to render additional exhaust treatment methods very economical, among other benefits.
Since particulate filters will, in the future, be standard equipment with diesel engines, the combustion method, for instance, can be adjusted to minimize nitrogen oxide emissions.
In both engine designs, it’s important to optimize the system as a whole. To date, the operating states of a diesel or gasoline engine have been mapped in a large number of characteristic curve fields. If the driver steps on the gas, the engine management software scans these diagrams and tables, and adjusts the ignition, intake air charge and injection metering to meet the required operating state of the engine.
The HCCI method goes a step further: The engine regulates itself and its emissions by means of a pressure sensor in the combustion chamber. Such feedback control is required because homogeneous fuel combustion cannot simply be set to the right level, since HCCI combustion cycle differs statistically from the preceding one.
With a sensor in the combustion chamber, for instance on the glow plug or on the spark plug, the instantaneous operating conditions within the combustion chamber can be measured. The in-cylinder pressure signal has proven to be the best variable to be tracked for this purpose. The measured pressure in the combustion chamber is used to control the air management.
The proper mixture of fresh air and recirculated exhaust gas is of key importance. This can be achieved with external exhaust gas recirculation alone or in combination with internal exhaust gas recirculation by means of variable valve timing. This approach lowers the combustion temperature and thereby prevents nitrous oxide formation. But to optimize fuel consumption, final compression temperatures must be high. Feedback from the sensor ensures an optimal compromise for the driving situation at a given moment.
In addition to pressure sensing, temperature or knock sensors can also contribute to the overall timing of each cylinder cycle. In the gasoline engine in particular, it’s essential to prevent a drift toward uncontrolled knocking that might damage the engine.
In principle, the HCCI method can be used only at moderate engine speed in the partial-load range. This is due to the short time windows – between 50 and 120 milliseconds per cycle – available for achieving a homogenized mixture. As the engine speed increases, the time window shrinks until homogenization is no longer possible. Since exhaust emissions are regulated based on the extent of vehicle operation within representative driving cycles, the goal for HCCI is to cover as large of an area as possible in theses cycles.
If the driver operates outside this ideal cycle – for instance by fast acceleration in a low gear – the HCCI engine reverts to the conventional operating mode of the gasoline engine. This is why future engine designs will have to be able to work in both operating modes: HCCI at low and intermediate loads, and conventional combustion at full load.
Bosch researchers are striving to maximize the load range in which the HCCI method can be used. The development of this design is still a work in progress. It addresses charge composition and charge dynamics, injection timing and supercharging strategy, combustion chamber geometry and active combustion control. An example to control the charge dynamics is an electrohydraulic valve-train system (EHVS) which enables the operation of HCCI, as well as improves the performance of the conventional operation mode. Initial prototypes are already being tested, but there are still many technical hurdles to overcome before a market launch will be possible.
The large number of mutually interactive variables and sub-systems is a major engineering challenge, but it also provides the flexibility essential for converging to an optimal solution. After all, a properly matched combination of fuel, injection system, air management and exhaust treatment should always make it possible to create sufficiently stable operating conditions to meet prevailing regulatory or self-imposed emission and fuel economy targets. Bosch expects that appropriate engine design and engine management can already reduce raw emissions sufficiently to render additional exhaust treatment methods very economical, among other benefits.
Since particulate filters will, in the future, be standard equipment with diesel engines, the combustion method, for instance, can be adjusted to minimize nitrogen oxide emissions.
