EP2129894A2

EP2129894A2 - Combustion method for a reciprocating engine

Info

Publication number: EP2129894A2
Application number: EP08716564A
Authority: EP
Inventors: Walter Huebner; Amin Velji; Sebastian Hensel; Christof Schulz; Boris Kock; Norbert Peters; Olaf RÖHL; Konstantinos Boulouchos; Ulrich Spicher
Original assignee: Bayerische Motoren Werke AG
Current assignee: Bayerische Motoren Werke AG
Priority date: 2007-04-04
Filing date: 2008-03-15
Publication date: 2009-12-09
Also published as: DE102007016278A1; JP2013144983A; WO2008122343A3; US7874277B2; JP2010532441A; JP5747050B2; US20100083934A1; WO2008122343A2

Abstract

The invention relates to a combustion method, in particular for a four-stroke reciprocating engine comprising a combustion chamber, the volume of which can be modified by a reciprocating piston and fuel can be directly injected into the combustion chamber, at least one gas exchange inlet valve and a gas exchange outlet valve for a charge cycle being provided and the combustion chamber having a minimum volume at an upper charge-cycle dead-centre position (LOT) and at an upper ignition dead-centre position (ZOT). Said method comprises the following steps: introduction of an inlet gas into the combustion chamber (a) in an intake phase; introduction of a primary quantity of fuel (x) into the combustion chamber during the intake phase (a) and/or a compression phase (b); compression of the inlet gas and the fuel in the compression phase (b); ignition of a mixture of inlet gas and fuel formed in the combustion chamber (c); expansion and discharge of an exhaust gas formed by the combustion in an expansion phase (d), wherein a pilot quantity of fuel (e) is introduced into the combustion chamber before the primary quantity of fuel (x) is introduced; intermediate products of the pilot quantity of fuel are formed (f) and the primary quantity of fuel (x) is introduced into the combustion chamber during the compression phase (b) in such a way that the complete ignition of the mixture consisting of inlet gas and the intermediate products is suppressed and other intermediate products are formed until a controlled ignition of the mixture and other intermediate products takes place (c). The claimed combustion method fulfils the conflict of objectives of fuel consumption versus NOx emissions during lean combustion (spark-ignition and diesel) and thus future emission regulations, without fuel consumption losses.

Description

Combustion process for a reciprocating internal combustion engine

The invention relates to a combustion method, in particular for a 4-stroke reciprocating piston internal combustion engine having the features of the preamble of patent claim 1.

In direct-injection internal combustion engines with auto-ignition homogeneous lean fuel / air mixtures are often brought to auto-ignition, so that high efficiencies and improved exhaust emissions are achieved. In such so-called. HCCI or CAI internal combustion engines, also known as internal combustion engines with Raumzündverbrennung, a lean basic mixture of air, fuel and retained exhaust gas is usually formed in the lower part load and ignited itself. To higher loads occur by the auto-ignition steep pressure increases in the combustion chamber, which would lead to an impairment of the operation.

From the German patent DE 198 10 935 C2, for example, a method for operating a working according to the 4-stroke principle internal combustion engine is known in which a homogeneous, lean base mixture of air, fuel and retained exhaust gas is formed, which is burned after a compression ignition , This is to expand the engine operating range with compression ignition an activation phase interposed. During the compression of the retained exhaust gas, an activation fuel quantity is injected into the combustion chamber and distributed as homogeneously as possible with the remaining mixture components in the combustion chamber. As a result, thermal energy is supplied to the mixture through the compression so that a chemical reaction or ignition is initiated in the vicinity of the upper charge reversal dead point. By the timing and amount of the activation injection, the ignition timing of the fresh charge at the main combustion can be controlled.

In a dynamic operation of the internal combustion engine, d. H. With changing speed, it is very difficult with the generic combustion method to control the timing of the ignition timing exactly about the time and the amount of activation injection.

Object of the present invention is to show a combustion process, in particular for a 4-stroke reciprocating internal combustion engine, with a very precise control of the ignition timing is possible.

This object is achieved by the method step with the features in the characterizing part of patent claim 1. It has been demonstrated numerically that an uncontrolled chain reaction (self-ignition of the intermediate products of the fuel / air mixture) can be purposefully suppressed by the presence of fuel molecules, thus delaying the combustion and controlling and controlling its onset. By intermediates is meant a fuel having a higher burning rate than the starting fuel. This can be z. B. be a mixture of fuel, formaldehyde and hydrogen peroxide. One way of producing the intermediates is the so-called cold flame or cold combustion. This chemical process occurs in a temperature range between 700 K and 1000 K. According to the invention Now the combustion is delayed by continuous addition of fuel ("Nachfüttern with additional fuel ') until the desired ignition point is reached. Thereafter, the combustion can use controlled by spark ignition or auto-ignition, which then runs very fast and stable and the NOx level remains low due to the very fast and late reaction. Parallel to this, as a result of the high degree of ignitability of the mixture and / or the high flame propagation speed, the combustion can be very lean, which again considerably reduces the combustion temperatures. The fact that such a combustion process according to the invention leads to very low NOx emissions is demonstrated by results in lean-burn combustion with ignitable fuels, such as hydrogen or reformer gas. In the latter example, H ₂ - enriched gas is added to the gasoline. In principle, however, the Brennyerfahren invention can. can be used for almost every type of fuel such as gasoline, diesel, etc. For the production of intermediates, it is necessary to use a fuel which is characterized by a pronounced low-temperature kinetics. This is the case with higher hydrocarbons, which is why fuels such. As ethanol or natural gas are not suitable for the process according to the invention.

Preferably, the intermediates according to the claims 2 and 3 are formed during the upper charge alternation phase (LOT phase) by an undercut of the gas exchange valves, since then present in the compression phase, the intermediates very homogeneous in the combustion chamber.

According to the claims 4 and 5, ignition can be carried out either by a spark ignition, such as with a spark plug or by stopping the injection of additional fuel, followed by auto-ignition. A further emission reduction is possible according to claim 6, characterized in that the fresh gas is at least partially mixed with exhaust gas. This may be, for example, an external exhaust gas recirculation.

In addition to a stoichiometric operation of the internal combustion engine and a lean operation according to claim 7 is possible. For this purpose, the exhaust gas is formed in the combustion chamber at least partially with a high oxygen excess.

In the following the invention with reference to 3 figures for two modes is explained in more detail.

1 shows a cylinder internal pressure curve for a first operating mode of an inventively operated. Internal combustion engine.

2 shows a cylinder internal pressure curve for a second operating mode of an internal combustion engine operated according to the invention.

Fig. 3 shows numerically calculated temperature and lambda curves

(Air / fuel ratio) for an addition of fuel described in the invention ("further fuel refills").

Fig. 1 shows a cylinder internal pressure curve for an inventively operated 4-stroke reciprocating internal combustion engine in a first mode. Shown is schematically a cylinder internal pressure curve 1 via 720 ° crankshaft. An internal cylinder pressure of the engine is plotted across the Y axis and the 720 ° crankshaft is plotted across the X axis. The first pressure increase shown in dashed lines, formed by a gas exchange valve undercutting, corresponds to an upper charge cycle dead center (LOT) of a piston, the second pressure increase, viewed in the direction of the X-axis, corresponds to an upper ignition dead center (ZOT) of the piston.

In a further operating mode of the internal combustion engine and the first pressure increase can be omitted, for example, by a gas exchange valve overlap.

In a Gaswechselventilunterschneidung the Gaswechseleinlass- and Gaswechselauslassventile are closed at a position of the piston near the LOT, in a gas exchange valve overlap, the Gaswechseleinlass- and Gaswechselauslassventile are open at a position of the piston near the LOT.

The combustion method according to the invention is provided for the 4-stroke reciprocating internal combustion engine with a combustion chamber, the volume of which is variable by a liftable piston and wherein a fuel such. As gasoline, diesel, etc. directly into the combustion chamber einbringbar (injectible or einblasbar) is. Preferably, the fuel should contain carbon atoms or include hydrocarbons. The internal combustion engine has at least one Gaswechseleinlass- and a Gaswechselauslassventil for a gas exchange and the combustion chamber has the upper charge change dead center (LOT) and the upper Zündtotpunkt (ZOT) each on a minimum volume. The combustion process comprises the following process steps:

- introducing a fresh gas in an intake phase into the combustion chamber (a);

- introducing a main quantity of a fuel (x) into the combustion chamber during the intake phase (a) and / or a compression phase (b);

- Compressing the fresh gas and the fuel in the compression phase (b); - igniting a mixture formed in the combustion chamber of fresh gas and fuel (c);

Expanding and expelling an exhaust gas formed by a combustion in an expansion phase (d), wherein

a pilot quantity of fuel (e) is introduced into the combustion chamber before introduction of the main quantity of fuel (x);

- intermediates of the pilot quantity of the fuel (f) are formed (activation),

- The main amount of fuel (x) during the compression phase (b) is introduced into the combustion chamber such that a complete ignition of the mixture of fresh gas and the intermediates is suppressed and other intermediates that need not necessarily be identical to those from the activation phase, are formed until a controlled ignition of the mixture and the other intermediates takes place (c).

Intermediates is understood to mean a fuel which, for. B. has a higher burning speed than the starting fuel. This may be for example a mixture of fuel, partially oxidized hydrocarbons, formaldehyde and hydrogen peroxide. One way of producing the intermediates is the so-called cold flame or cold combustion. This chemical process occurs in a temperature range between 700 K and 1000 K.

The combustion chamber is understood here to mean the space which comprises the displacement volume and the compression volume, the volumes being formed by a cylinder inner side, a piston head and a cylinder.

FIG. 2 shows a cylinder internal pressure curve for a 4-stroke reciprocating internal combustion engine operated according to the invention in a second operating mode. The basic four-stroke operation of the reciprocating internal combustion engine speaks the one shown in Fig. 1. In contrast to the first operating mode, in the second operating mode the first fuel is introduced into the combustion chamber already in the LOT phase (h). Near the LOT, the temperatures of 700 K - 1000 K are also achieved in the combustion chamber in this second mode. In this second mode, the intermediates in the subsequent compression phase (b) are particularly homogeneous in the combustion chamber.

According to the invention, in this second operating mode before the ZOT further fuel is injected into the combustion chamber during the compression (x), so that a complete ignition of the mixture of fresh gas and the intermediates formed during the LOT phase (h) (activation ) is suppressed and further intermediates are formed by the majority of the fuel introduced into the combustion chamber, bjs a controlled (external or self) ignition des.Gemisches and the other intermediates takes place (c).

In addition to a 4-stroke reciprocating internal combustion engine, for example, a 6-stroke reciprocating internal combustion engine can be operated according to the invention.

FIG. 3 shows in a diagram simulation results on the influence of the continuous addition of fuel to the start of combustion. On the one hand, the temperature in Kelvin and on the other the air / fuel ratio is plotted along the Y axis, and the time in milliseconds via the X axis. During the first temperature increase to about 900 K, the intermediates are formed. These are prone to rapid second conversion (main combustion), but by further addition of fuel, the onset of combustion may be delayed. As long as fuel is added, the main conversion (temperature increase to over 2500 K) is suppressed. Preferably, the intermediates are formed according to the second combustion process during the charge cycle OT phase by an undercut of the gas exchange valves. Further, the main amount of the fuel is preferably injected or injected during the introduction of fresh gas into the combustion chamber. In a first variant, the ignition of the mixture in the combustion chamber can be effected by a spark ignition, for example with an ignition device, such as a spark plug. In a broad variant, the ignition of the mixture can be done by stopping the injection of the bulk of the fuel. In order to reduce the exhaust emissions even further, the fresh gas can advantageously be mixed at least partially with exhaust gas. Furthermore, the combustion method according to the invention allows lean engine operation, which means that the exhaust gas is formed in the combustion chamber at least partially with an excess of oxygen.

It is known that to increase the ignitability, a mixture is preconditioned (eg by pressure, temperature, mixture composition). This desirably increases the autoignition tendency and thus leads to corresponding chain reactions which initiate the combustion. However, this happens in the known combustion processes, such as HCCI or Schichtladebrennverfahren in an uncontrolled manner and thus there are limits in increasing the ignitability. In the meantime, it has been possible to prove numerically that this uncontrolled chain reaction (self-ignition) is deliberately suppressed by the presence of fuel molecules, thus delaying combustion and allowing its start to be controlled and controlled. The aim of the combustion process according to the invention is to delay the combustion by continuous addition of fuel ("refueling" of fuel) until the desired ignition point is reached, then the combustion can use controlled (by spark ignition or auto-ignition), which then runs very fast and the level of NOx remains low due to the very rapid and late reaction, and at the same time, due to the high level of ignitability of the mixture, the Run very lean combustion, which significantly reduces the combustion temperatures. The fact that such a combustion process leads to very low NOx emissions is demonstrated by results in lean-burn combustion with ignitable fuels such as hydrogen or reformer gas. The combustion process according to the invention resolves the conflict of objectives, fuel consumption versus NOx emissions during the lean-burn process (Otto and Diesel) and thus the fulfillment of future emission regulations without any loss of fuel consumption. Furthermore, it is a worldwide usable lean burn process. The inventive combustion process results in a significantly reduced effort and reduced costs for exhaust aftertreatment.

Claims

Combustion process for a reciprocating internal combustion engine Patent claims

1. combustion method, in particular jfür a four-stroke reciprocating internal combustion engine with a combustion chamber whose volume is variable by a liftable piston and wherein a fuel directly into the combustion chamber can be introduced, wherein at least one gas exchange inlet and a gas exchange outlet valve is provided for a change of charge and wherein the combustion chamber has a minimum volume at an upper charge cycle dead center (LOT) and an upper ignition dead center (ZOT), with the following process steps:

- introducing a fresh gas in an intake phase into the combustion chamber (a);

- Compressing the fresh gas and the fuel in the compression phase (b);

- igniting a mixture formed in the combustion chamber of fresh gas and fuel (c);

Expanding and expelling exhaust gas formed by combustion in an expansion phase (d); characterized by the following method steps:

- introducing a pilot amount of fuel (e) before introducing the main amount of fuel (x);

- forming intermediate products of the pilot amount of the fuel (f);

- Introduce the main amount of the fuel (x) during the compression phase (b) such that a complete ignition of the mixture of fresh gas and the intermediates is suppressed and further intermediates are formed until a controlled ignition of the mixture and the other intermediates takes place

(C).

2. combustion method according to claim 1, characterized in that after the expansion phase (h) in a LOT phase (h) a pressure and temperature increase in the combustion chamber is generated by an undercut of the gas exchange valves.

3. combustion method according to claim 2, characterized in that the pilot quantity of fuel (s) is injected before the LOT into the combustion chamber and the formation of the intermediates (f) in the LOT phase (h) takes place.

4. combustion method according to one of the claims 1 to 3, characterized in that the ignition of the mixture takes place by a spark ignition.

5. combustion method according to one of the claims 1 to 3, characterized in that the ignition of the mixture takes place by stopping the injection of the further fuel.

6. combustion method according to one of the claims 1 to 5, characterized in that the fresh gas is at least partially mixed with the exhaust gas.

7. combustion method according to one of the claims 1 to 6, characterized in that the exhaust gas is formed in the combustion chamber, at least partially with an excess of oxygen.