DE102006041467A1 - Internal combustion engine e.g. Otto engine, for motor vehicle, has combustion chamber in which fuel/air mixture is chemically converted, where mixture temperature is increased by variables in valve train to initiate chamber`s self-ignition - Google Patents

Abstract

The engine has a combustion chamber and a fuel injection device, where a fuel/air mixture is chemically converted in the combustion chamber. The temperature of the fuel/air mixture is increased using variables in a valve train for initiating self-ignition associated with the homogeneous combustion in the combustion chamber. Controllers are provided for controlling combustion state and indicated mean pressure in the combustion chamber.

Description

The The invention relates to control concepts, especially in gasoline engines with homogeneous compression ignition Combustion.

The CO ₂ discussion makes the further reduction in fuel consumption the central research and development topic in the gasoline engine. Possible concepts for improving fuel consumption include direct injection, downsizing, the reduction of charge cycle losses, homogeneous self-ignition or even possible combinations of the above-mentioned methods. The advantages of homogeneous autoignition in gasoline engines lie in addition to the aforementioned improvement in fuel consumption in the simultaneous maintenance of the emission behavior of conventional gasoline engines.

In This patent application becomes regulatory concepts and potential for gasoline engines with homogeneous self-ignition discussed on the basis of a full-engine combustion process with auto-ignition. The control of the homogeneous with the proves to be problematic Combustion accompanying auto-ignition, since no direct parameter to initiate combustion, such as e.g. the one by the spark initiated ignition timing in conventional gasoline engines, or the start of injection in diesel engines. The control and regulation of heat release is one of the central development tasks here. Furthermore Consumption and emission statements are made for one driving cycle.

1. Introduction

The Reduction of the charge exchange losses as well as the improvement of the Substance properties by charge dilution also form in Otto motor Ignition method the lever for consumption reduction. This approach has been around for some time Conventional gasoline engines used, but usually fails the limited compatibility Flame front combustion at charge dilution, be it through air (lean) or by residual gas (external or internal EGR).

1 shows the indicated fuel consumption and the hydrocarbon and nitrogen oxide crude emissions of conventional Otto cycle combustion and room ignition combustion as a function of the residual gas mass.

While in conventional operation the Entdrosselung exhaust gas recirculation can be implemented only up to a certain running limit in an improvement in consumption, resulting for controlled auto-ignition far beyond this running limit still consumption improvements. The controlled auto-ignition can after 1 Also regarded as a consistent increase in the residual gas compatibility of the gasoline engine. Far beyond the permissible residual gas rates for stable engine operation lies the range in which self-ignition occurs due to the high temperatures of the residual gas-air mixture. This leads to a further possibility for consumption reduction. Due to the charge dilution by residual gas and since the reaction no longer concentrated in a flame front, but at the same time takes place in large areas of the combustion chamber, the temperatures remain below the critical NO _x formation temperature despite rapid implementation. Due to the high charge dilution, the NO x oxide emission is far below the ottomotor values and the hydrocarbon emissions are approximately at the same level.

2 shows the self-lighting in Otto engine combustion, which is characterized by a flame front propagation, and in homogeneous, controlled auto-ignition, in which a heat release takes place in the entire combustion chamber. The brightness is an indirect measure of the temperatures that occur.

Self-ignition with homogeneous combustion is characterized by a high heat release rate and short burning time compared to the gasoline engine (see 3 ). The temperature of the burned zone is well below the values of the ottomotrical values.

The rapid release of heat and the dependence on the compression end temperature also limits the use of controlled self-ignition. The controlled homogeneous self-ignition can be operated in a load and speed range, which is approximately the shift operation of today's direct injection gasoline engine ( 4 ).

The map area in the direction of low load is limited by delayed burns and possible dropouts. Towards high loads, the pressure increases increase and as the load continues to increase, knocking combustion occurs. At the same time, NO _x emissions are rising. A change of operating mode to a full load suitable combustion process is thus necessary for vehicle applications.

In order to be able to cover as large a map area as possible, moreover, a regulation of the start of combustion and the release of heat is absolutely necessary. This can be achieved via the mixture reactivity and the mixture temperature. The influence parameters on these two quantities are shown in Tab. Tab. 1: Classification of the control parameters for auto-ignition Mixture responsiveness mixture temperature - Fuel - charge temperature - Exhaust gas recirculation - Exhaust gas recirculation - Oxygen excess - Charging - Injection time - compression ratio - Variable valve train

The most sensible variant for achieving self-ignition is the increase of the mixture temperature with the help of variabilities in the valve train. Depending on the degree of variability (from fully variable with EMVS / EHVS to two-stage valve lift switching), different exhaust gas retention methods ( 5 ) [1].

Version 1 ( 5 , left picture) shows the sucking back of exhaust gas from the exhaust passage during the intake stroke. The inlet valve is opened accordingly late. Controlling the combustion over the injection instant when using direct injection is only possible to a limited extent because the effect is limited only to the change in the homogeneity of the cylinder charge (more homogeneous mixture with injection during the intake stroke and heterogeneous distribution with injection in the compression). However, the influence can be classified as relatively low and, in the case of poor homogenization, can also have negative effects on the emission behavior. Therefore, this method requires a cylinder-specific adjustment of the timing, so a fully variable valve train due to the requirement for individually adjustable residual gas levels.

Variant 2 ( 5 , middle image) is described in [2]. First, the exhaust gas is also pushed out of the cylinder conventionally. Then the fresh charge is drawn in during a short intake phase before the exhaust valve is opened a second time at the end of the intake stroke in order to suck the residual gas necessary for auto-ignition out of the exhaust. Dispensing with the exhaust of exhaust gas is a simple dethrottling strategy with "early intake closes." This offers advantages when changing the operating mode The same applies to the control of the autoignition timing as for variant 1. In addition to an intake-side valve lift switchover, the additional construction costs include a cylinder-specific change controllable element to be able to reopen the exhaust valve at the end of the intake stroke.

The third variant ( 5 , right picture) provides to retain the exhaust gas directly in the cylinder. For this purpose, the exhaust valve is closed well before reaching the gas exchange TDC and opened the inlet valve later. For this purpose, inlet and outlet side valve lift switching is necessary. The additional charge exchange or wall heat losses caused by the latter two methods of exhaust gas supply are comparable, but compared to a conventional throttle control but significantly lower ([2]).

Another possibility for influencing the mixture temperature is the application of a pre-injection with appropriate pre-reaction or pre-combustion of this amount of fuel. For this purpose, however, it is necessary that the combustion chamber is closed during the pre-combustion and also no fuel is discharged into the Auslasstrakt. This can be achieved by a corresponding valve undercutting with subsequent intermediate compression as in variant 3 (FIG. 5 , right picture). Advantages and disadvantages of the different variants are shown in Tab. 2.

Tab. 2: Advantages and disadvantages of various exhaust gas recirculation or retention concepts

2. The Raumzündverbrennung invention (RZV)

In order to achieve the auto-ignition of conventional gasoline fuels, temperatures of about 1000 K are required. In the usual compression ratios for gasoline engines (9 ... 13), the necessary temperature must therefore be additionally provided via exhaust gas from the previous cycle. The basis for the RZV investigations is a standard 4-cylinder direct-injection engine with wall-guided combustion process, which was modified according to the desired, with RZV representable map range and the technical feasibility in a vehicle-capable engine for a concept with valve undercut. In order to be able to display both full throttle, throttle-operated operation with "early intake closes" and RZV operation, inlet and outlet sides are provided with an over-cam lever with oil pressure actuated valve lift and an extended camshaft control range [3], because the camshaft phasing only for all four Cylinder-specific adjustment of the residual gas content as a control or control variable is eliminated, so that as a cylinder-specific adjustable parameter only the direct injection ( 6 ). By dividing the amount of fuel to an injection before the top dead center of the gas exchange and injection during the intake phase, the auto-ignition can be controlled. The combustion focus and the indicated mean pressure for each cylinder can be optimally adjusted if fuel is injected even before the gas exchange TDC. At überstöchiometri Shem engine operation contain the resulting combustion gas and thus also retained in the cylinder exhaust always residual air or oxygen.

This responds now in dependence from the pilot injection already in by the undercut resulting intermediate compression.

7 shows the calculated over the residual gas mass temperature at the time "inlet opens" for different pilot injection parts.This temperature difference represents the energy supplied in the valve Unterschneidungsphase.

The Temperature level reached in the intermediate compression exceeds the auto-ignition threshold (≈1000 K) clearly what is the basic requirement for implementation. episode the implementation is an increase the cylinder charge temperature by up to 130 K (about 10% pre-injection) at the moment "inlet opens" is the temperature reached by ZOT at the end of the compression fundamental importance for the triggering the combustion. Considered the expansion after gas exchange OT, the dilution of the Cylinder charge through the intake fresh air and the compression before ignition TDC, arises for the temperature at compression end (ZOT) a difference of over 60 K. between operation without and with 10% pre-injection. With a variable Pre-injection of 0 ... 10% so can temperature differences of 60 K at the end of compression. Thus, the suitable Distribution of injection quantity to pre- and main injection quantity for controlled control of auto-ignition.

3. Combustion controller

Why a simple combustion control with controlled auto-ignition is no longer sufficient, shows 8th , Although the consumption proves to be relatively robust to changes in the combustion situation, pollutant emissions are extremely sensitive. Especially the nitrogen oxide emission (which can not be reduced in lean operation) increases drastically when the combustion position is too early, because the maximum process temperature rises sharply.

• – λ-Regler
• – Regler für Verbrennungslage
• – Regler für den indizierten Mitteldruck

The controller concept for the RZV full engine is accordingly 9 at least composed of:

• - λ controller
• - Controller for combustion position
• - regulator for the indicated mean pressure

Control variable for the λ-controller forms the measured value of a λ-probe, manipulated variable is the Camshaft phase of the exhaust camshaft, i. the residual gas content in the cylinder. With unthrottled operation means less residual gas more fresh air in the cylinder causing a leaner air-fuel mixture. To the desired implementation of the pilot injection implementation in the intermediate compression to enable GOT is it necessary that there is enough unburned air (oxygen) located in the exhaust.

Of the 50% conversion point of the heating curve forms input for the combustion controller, which allows the determination and correction of the combustion position. Too late Location of the 50% turnover point if the pre-injection duration is increased, conversely, the pre-injection duration is reduced if the position is too early.

Of the Indicated mean pressure is the controlled variable for the load. Control output is the main injection duration. With low dispersion of the injector flow the individual injectors on the full engine and a corresponding injection model can be dispensed with this regulator. An estimate of the Indicated mean pressure with the help of one in compression and a pressure value sampled in the expansion is due to the depending on the implementation in the charge cycle OT variable charge exchange work difficult. For one precise Determination is an increased one Sampling rate and thus an increased Calculation required.

In addition to the standard on the engine available Sensors or actuators that provide the engine control unit with information or over Controlling the engine is a combustion chamber information from each Cylinder required. This can either be a cylinder pressure or be an ion current signal, depending on the downstream evaluation algorithm Degree crank angle resolution to sampling of a few values per cycle are possible. Basis for easy determination of the firing process or turnover points on a combustion engine form thermodynamic basic equations, as already described in [4].

10 shows how such a combustion control can be used for the equalization of indicated mean pressure and 50% conversion rate of all cylinders of a four-cylinder engine.

With the same feedforward control of all cylinders (same pre-injection quantity, same main injection quantity), significant differences in the indicated mean pressure of the individual cylinders can be seen; the 50% conversion point is too early for all cylinders. If the combustion control is switched on, a cylinder equalization is reached after a few cycles (depending on the controller rating). The fact that such a combustion control can respond to changes relatively quickly when correctly metered, shows 11 , Here, with the combustion control switched on, a load jump in the RZV mode is shown. Pre-controlled, the engine reacts from one cycle to the next and continues with the correct combustion position and medium pressure indicated for all cylinders.

4. Operating mode change

By the ones already mentioned Disadvantages of increase the burden of self-ignition Operation is a change of mode to conventional Ottobetrieb necessary. According to the definition of a two-stage variable valve gear inlet and outlet must change combustion mode within a combustion cycle (when switching the Auslassventilhubs), since the by the small exhaust valve lift achieved exhaust gas retention rates for ignited operation are too high. Switching the operating mode involves both Directions (petrol engine → RZV operation and RZV- → Otto Motor Operation) challenges.

tries on a single-cylinder unit have shown that a load-neutral Switching from Otto engine combustion to controlled self-ignition and conversely also with relatively small ones variabilities in the valve train load neutral can be displayed. Because the change between the combustion process is not over a continuous increase or reducing the residual gas content is possible (RZV must the autoignition temperature reach and ottomotorischer operation has only a limited Restgasverträglichkeit) must the actual combustion process change happen within a cycle. Parameters such as injection or ignition timing have to according to the different exhaust gas temperatures, air figures etc. over several cycles the stationary Values are adjusted.

12 shows the combustion process change from high pressure IC engine combustion (6 bar indicated mean pressure) to low load space ignition combustion (3 bar indicated mean pressure). Shown in the upper part of the figure are integral measures such as the indicated mean pressure per cycle and the maximum pressure increase, the exhaust gas temperature and the air ratio. The lower part of the figure shows the cylinder pressure curve, the valve lift curve and the intake manifold pressure curve for the combustion cycles shortly before and after the operating mode change. The problem when switching the combustion mode in the direction shown is the maximum pressure increase in the first self-ignited combustion cycles due to the higher temperature level of the exhaust and combustion chamber walls. If a high ottomotor load is driven before the auto-ignition operation, this effect is exacerbated. Therefore, before switching from high Otto load to low load with Raumzündverbrennung initially a lower ottomotor load should be set and then switched immediately to room ignition combustion. By additionally adjusting the exhaust gas retention rate to exhaust gas temperature (lower exhaust gas retention rates at the beginning of self-ignited operation) and delaying injection timing (worse mixture preparation), the maximum pressure rise in the first self-ignited combustion cycles can be captured at the steady state level.

In an engine control unit, this cyclical sequence of parameter settings must be stored in a sequence control in order to be able to ensure a mode change in every situation. For example, if the engine is operating in controlled idle mode and there is a requirement to switch to room ignition combustion (for example, when a limit speed or load is reached), the programmed sequencer will first go through before recording RZV operation controlled from maps. 13 shows such a process. Shown again are the cycle-resolved variables, indicated mean pressure and maximum pressure increase, as well as the air ratio and the engine speed. First, the engine is controlled from the designated maps in conventional operation. When the engine speed drops below 2000 rpm, in the example shown, room combustion combustion is switched over and a sequence control system is run through. Thereafter, controlled or regulated RZV operation can be recorded from maps.

5. Consumption and emission

in the stationary Motor operation is it possible the target variables for the dynamic Engine operation in the New European Driving cycle over the most common estimated points. This requires a vehicle with appropriate transmission and rear axle ratio be deposited. The basis is a Mercedes-Benz C-Class vehicle (Bj 2005) with rear axle ratio 3.07 and a 1.8 liter 4-cylinder engine. The area relevant to the NEDC is almost on the map field despite the small-volume engine limited at low speed and low load, resulting in a partial load firing process like the room ignition combustion accommodates. Are the operating points determined stationary at conditioned Motor driven, the result corresponds to a "NEDC hot", i.e. a driving cycle at operating warm Engine.

In this way, the overall system is considered, ie also benefits that bring the hardware requirements of the engine in the conventional Otto engine operation, have a share of the total consumption advantage achieved. Thus, in the end, only the fuel consumption improvement that is achieved in comparison with an Otto engine system with the same variability in the valve train stands for the overall combustion of the room ignition. The results are in 14 shown. The basis variant is based on a four-cylinder engine with port injection and a compression ratio of 9.3 without camshaft adjustment. Inlet and exhaust side continuous camshaft adjustment brings a consumption advantage of about 3%. The test engine, on the other hand, is equipped with a compression ratio of 11.5 and additional inlet and outlet valve lift switching. The consumption advantage caused by the higher compression ratio is about 2%. The use of the "early intake closes" throttling strategy (represented by the small intake valve lift and an early intake phase) results in a consumption advantage of about 3%, and if additional room combustion is used (in the drivable points), fuel consumption is improved by an additional 3%. Based on the basis, the improvement achieved by the combustion of room ignition is greater than 3%, but the overall concept must be observed: Consumption advantages resulting from individual technologies can not necessarily be added together the charged motor used.

A technology for reducing part-load consumption makes sense only if the resulting costs can be justified by the achieved improvement in consumption. This means that in contrast to the existing stratified direct injection method, the use of a NO _x storage catalytic converter must be avoided. Since the space ignition combustion must be operated stoichiometrically in order to achieve maximum efficiency, resulting in the combustion of nitrogen oxides in the conventional 3-way catalyst can not be reduced. The amount of nitrogen oxide emitted must therefore be at a low level. From the same point of view as the consumption potential, it is now also possible to make a statement about the increase in nitrogen oxide emissions downstream of the catalytic converter (see 15 ).

This is an estimation for a "cold test", since nitrogen oxides are emitted significantly only during the cold start phase in conventional operation If the engine has reached a certain temperature level, room ignition combustion can be activated in the drivable operating points 50% mark of the EU4 limit. the more emission caused by RZV is also (in contrast to the basic emission) age-independent. on the use of a NO _x storage can therefore be dispensed with.

6. noise

In addition to the control of combustion under controlled auto-ignition remains as a further challenge caused by a high maximum pressure increase during combustion combustion noise, which is depending on the operating point between the level of a gasoline engine and the level of a diesel engine (see 16 ).

It There are two main causes to avoid: On the one hand, cyclical variations the combustion situation, the engine noise appear uneven on the other hand, the actual pressure increase during the Combustion. The reduction of cyclical fluctuations can be achieved by means of the combustion control to a minimum. To reduce the maximum pressure increase during combustion, the burning speed must be lowered. This is done with the help of charge dilution reached. charge dilution turn, by increasing the compression ratio or by charging.

Just when the combustion noise can be represented with controlled self-ignition Operating area over be extended to the extent presented today.

For this demonstrate

1 the indicated fuel consumption (b) and the hydrocarbon (HC) and nitrogen oxide raw emissions (NOx) in conventional Otto engine combustion and room ignition combustion as a function of the residual gas mass (2000 rpm 3 bar imep),

2 self-lighting in conventional SI engine combustion characterized by flame front propagation and in homogeneous, controlled auto-ignition which releases heat throughout the combustion chamber; while the brightness is an indirect measure of the temperatures that occur

3 Heat release rates and temperatures (burned, unburned) with conventional Otto engine combustion and room ignition combustion (2000 rpm, 3 bar imep),

4 Limits for the use of controlled auto-ignition between knocking and incomplete combustion as a function of cylinder pressure and crank angle,

5 Different valve lift configuration for different exhaust gas retention or recirculation concepts:
Variant 1 (sucking back of residual gas from the outlet channel),
Variant 2 (dual sucking back of residual gas from the outlet channel by means of two exhaust valves),
Variant 3 (pure exhaust gas retention),

6 an injection strategy and an associated pressure curve in a controlled self-ignition combustion method according to the invention with negative valve overlap,

7 the calculated temperature change at the opening time of the inlet valve with a change in the pilot injection rate (1500 rpm, 4 bar imep),

8th Modification of indicated fuel consumption (isfc) and nitrogen oxide emissions at different location of the center of gravity of the combustion (MFB50) (2000 rpm, 4 bar imep),

9 a controller concept for an internal combustion engine with room ignition combustion with λ controller, controller for combustion position and regulator for the indicated mean pressure,

10 a diagram concerning the equality of all cylinders of a 4-cylinder controlled self-ignition engine in terms of 50% conversion (MFB50) and indicated mean pressure,

11 a diagram relating to the control of the cylinders of a 4-cylinder controlled self-ignition engine in terms of 50% conversion rate (MFB50) and indicated mean pressure at a load step,

12 a diagram of the combustion process change from SI combustion with high load (6 bar indicated mean pressure) to room ignition combustion (CAI) with low load (3 bar indicated mean pressure),

13 a high-speed (3 bar indexed mean-pressure) high-speed SI (SI) combustion-to-engine combustion combustion (CAI) low-speed chamber combustion index (3 bar mean-pressure index), where the air-fuel ratio λ and the maximum pressure increase change;

14 a schematic representation of the fuel consumption potential of a motor vehicle with 4-cylinder internal combustion engine with controlled auto-ignition,

15 an estimate of NOx emissions in a 4-cylinder controlled self-ignition engine; and

16 an estimate of the noise emissions of a 4-cylinder internal combustion engine with controlled auto-ignition in comparison to internal combustion engines, according to a conventional Otto or Die be operated selselverfahren (2000 rev / min, 2 bar imep).

7. Abbreviations, symbols and symbols indices

• EGR

  Exhaust gas recirculation

  CAI

  Controlled Auto Ignition, controlled auto-ignition

  CAD

  Degree crank angle (Crank angle degree)

  dQ / dΦ

  Per degree crank angle supplied energy

  EHVS

  Battery powered valve control

  EMVS

  Electromechanical / -magnetic valve control

  IV

  intake valve

  Λ

  Air-fuel ratio

  _Air

  air mass

  MFB50

  50% sales point of combustion

  m _fuel

  Fuel mass

  m _residual

  Residual gas mass

  _gas

  NEDC

  New European driving cycle

  OT

  Top Dead Center

  RZV

  homogeneous charge compression ignition

  SI

  spark ignition

  T _burnt

  Temperature in the burned

  T _unburnt

  Temperature in the unburned

  ZOT

  Top dead center (ignition)

  IMEP

  Indexed medium pressure

8. Literature

• [1] Wolters, P., Salber, W., Geiger, J., Duesmann, M. Controlled Auto Ignition Combustion Process with an Electromechanical Valve train SAE 2003-01-0032
• [2] Fraidl, G.K., Fuerhaupter, A., Piock, W.F., Csato, J. homogeneous self-ignition gasoline engine - challenges, Potentials and practical implementation Stuttgart Symposium 2005
• [3] Herrmann, H.-O., Herweg, R., Karl, G., Pfau, M., Stelter, M. The use of gasoline direct injection in gasoline engines with homogeneous compression ignition Combustion direct injection in gasoline engine V, Haus der Technik, eat 2005
• [4] Rassweiler, G.M., Withrow, L. Motion Pictures of Engine Flames Correlated with Pressure Cards SAE 800131
• [5] Willand, J., Nieberding, R.-G., Vent, G., Enderle, C. The Knocking Syndrome - Its Cure and its potential SAE 982483
• [6] Nieberding, R.-G. The compression ignition of lean mixtures as motor combustion process University of Siegen, dissertation 2002

Claims (2)

Internal combustion engine with a combustion chamber and a Fuel injection device in which a fuel / air mixture in the combustion chamber is chemically feasible. Method for operating an internal combustion engine, at injected in a process step fuel in a combustion chamber and chemically reacted with air.