DE10141929A1 - Procedure for starting a gasoline engine - Google Patents

Abstract

The invention relates to a method for starting a gasoline engine, in particular a motor vehicle, the fuel-air mixture being greased in the starting phase to ensure that the gasoline engine starts up, fuel being deposited as a wall film in an intake manifold. Here, an intake manifold pressure is controlled during a start phase and a post-start phase in such a way that spontaneous evaporation of the wall film is avoided.

Description

The invention relates to a method for starting a gasoline engine, in particular one Motor vehicle, which to ensure a startup of the gasoline engine Fuel-air mixture is enriched in the start-up phase, with fuel as Deposits wall film in a suction pipe, according to the preamble of claim 1.

In gasoline engines, it can be too considerable in the post-start area after a cold start HC emissions come in the area of the limitation of a speed overshoot. Around the ramping up of the speed of a gasoline engine in cold start at various Ensuring fuel quality, manufacturing tolerances, etc. is a computationally high one The fuel-air mixture must be enriched. This strong enrichment builds there is a wall film of fuel on an intake manifold wall. After the successful Startup of the gasoline engine at the start leads to the so-called. Speed overshoot, which is necessary to safely achieve the desired idle speed. The Speed overshoots with a certain, adjustable time constant regulated until the stationary idling speed is reached. The speed is predominant regulated with the air path, this leads to a lower filling and the Intake manifold pressure drops quickly. This then leads to a high evaporation rate of the Wall film and a higher internal exhaust gas recirculation rate. The combustion is accordingly bad. The enrichment in the start has a decisive influence on the Wall film structure. The higher the enrichment, the more fuel is stored on the Intake manifold wall and evaporates during the transition in the post-start. The faster the Intake manifold pressure drops, the more fuel is assumed assuming a constant Wall film released at short notice. This leads to extremely rich engine operation and bad combustion. A low suction pressure causes a high internal Exhaust gas recirculation. This can, especially in conjunction with an extremely fat Engine operation, also lead to poor combustion.

The invention has for its object a method of the type mentioned above to improve that in the early post-start of an internal combustion engine Starting hydrocarbon emissions are significantly reduced.

This object is achieved by a method of the above. Kind of with the in Characteristics characterized claim 1 solved. Advantageous embodiments of the Invention are specified in the dependent claims.

For this purpose, it is provided according to the invention that an intake manifold pressure is controlled in such a way that that spontaneous evaporation of the wall film is avoided.

This has the advantage that there is an evaporation rate of the fuel wall film in the intake manifold reduced, so that there are significantly lower HC emissions.

For example, the ignition angle already becomes apparent during the startup of the gasoline engine adjusted so late that the speed of the gasoline engine is limited and the Speed gradient is limited.

An effective measure against over-greasing the engine is that in the Start phase and the post-start phase a throttle valve to a constant position is opened, the throttle valve being opened more than is necessary for the start. As a result, the intake manifold pressure drops only slowly due to the inflowing air.

In order to avoid overfatting of the mixture, a Injection amount of fuel considering a fuel wall film in the Intake manifold lowered so that a sum of injected fuel and fuel from the wall film corresponds to a desired amount of fuel in the cylinder.

In a preferred embodiment, each time the gasoline engine is started during the Start phase and the post-start phase, the speed curve monitored and recorded, the speed curve compared with a limit speed curve and the The speed is only reduced by means of the ignition angle if the Speed curve approximates to the limit speed curve by a predetermined value, the limit speed curve is determined such that below the Limit speed curve post-start emissions below a predetermined value result. The limit speed curve is determined, for example, by measuring a Test engine determined with approval fuel. This will make the motor Air ratio predictable. By countermeasuring with a Fast FID, Indication measuring technology or ion current probe becomes the limit of the still tolerable Overfat determined. Intake manifold pressure curves and speed curves are in Dependence on post-start emissions measured and recorded.

In an alternative, preferred embodiment, each time the gasoline engine is started during the start phase and the post-start phase, the speed curve is monitored and is recorded from the recorded speed curves and the Speed curve of the current start for the future of the current start calculates whether the gasoline engine is over-greased and the speed is reduced by means of the ignition angle only if there is a measure without this measure Overfatting of the gasoline engine would come. In the calculation, models for the motor enrichment and / or the intake manifold pressure drop included.

To ensure a reliable start, the speed is reduced by means of the ignition angle adjustment a minimum speed and the Ignition angle adjustment limited to a maximum value.

Further features, advantages and advantageous configurations of the invention result from the dependent claims, as well as from the following description of the Invention.

The invention is applicable and encompasses all gasoline engines Engine control measures that are carried out without hardware changes can. According to the invention, an intake manifold pressure curve is essentially controlled. The previous speed curve incl. The current speed gradient is used for this, to determine the intake manifold pressure curve in the future through a model and knowledge of the engine predict. This knowledge and the knowledge of how the intake manifold pressure curve must look so that no spontaneous evaporation of the fuel film on the Intake manifold wall is used to take countermeasures at an early stage. These measures preferably relate to ignition angle measures and possibly Secondary measures, such as lowering the injection quantity and actuation the throttle valve.

The intake manifold pressure can be calculated by balancing the inflowing and outflowing air mass flows and the air masses stored in the intake manifold. The air mass ≙ _{l, Br drawn in} by the engine is described by the following equation 1:


where n = engine speed, p _rg = residual gas _{partial pressure} , V _heff = effective displacement, R = gas constant and T _Br = combustion chamber _temperature .

The air mass flow ≙ _DK flowing in via the throttle valve is given in the following equation 2:


where φ _DK = flow coefficient of the throttle, ρ _{l, DK} = air density, p _a = ambient pressure and p _S = intake manifold pressure.

At constant intake manifold temperature T _S , the time derivative of the air mass ≙ _S stored in the intake manifold can be calculated according to the following equation 3:


where V _S = intake manifold volume and T _S = intake manifold temperature.

The conservation of mass on the intake manifold requires: ≙ _S = ≙ _DK - ≙ _{l, Br} .

This results in the intake manifold pressure gradient

With constant geometry and cold start, this can be simplified to Equation 4:


where C ₁ and C _{2 are} constants.

The intake manifold pressure gradient therefore results from two summands, both of depend on the geometry, the temperature and the intake manifold pressure. The first summand shows that the intake manifold pressure drops the slower the further the throttle valve is drawn up and the more air flows into the intake manifold. The second summand shows that the intake manifold pressure drops the faster, the higher the speed and the more higher is the air mass flow pumped out by the engine.

Parts of the injected fuel are deposited in the intake manifold and in the inlet channels as a wall film. The fuel mass stored in the wall film essentially depends on the geometry of the intake manifold, the engine air ratio and the intake manifold vacuum. If the operating point of the engine changes, fuel will either evaporate or accumulate according to the following equation 5:

Δm _{K, W} = F _TL. F _AW . (Α _{Tw, p} .Δp _S + α _{Tw λ} .Δλ) (5) FVV

where F _TL = intake air temperature factor, F _AW = wall area factor,
α _{Tw, p} = pressure coefficient (wall temperature), α _{Tw, λ} = pressure coefficient (air ratio),
Δp _S = intake manifold pressure change and Δλ = air ratio change.

There is also a mathematical relationship for the change over time, for which there is a fuel dependency:


where, for example:
τ = 1.72.p 2 | S - 4.58.p _S + 3.48
p _S = 0.4 bar: τ = 1.92 s
p _S = 1.0 bar: τ = 0.62 s

The quality of internal combustion is essentially determined by the engine air ratio according to equation 7:


where Kr stands for fuel, Br for combustion chamber and mot for engine. When the engine is operating at a steady point, the fuel mass stored in the wall film is constant and the mass of the fuel introduced into the combustion chamber corresponds to the mass m _{Br, EV} injected in the cycle. If the operating point changes, wall film effects lead to a deviation of the fuel mass introduced into the combustion chamber from the injected fuel mass. The following equation 8 represents the fuel mass introduced:

m _{Kr, Br} = m _{Kr, EV} + Δm _{Kr, Br, WF} (8)

where EV stands for inlet valve and WF stands for wall film. The expression Δm _{Kr, Br, WF} denotes the fuel mass supplied or removed to the wall film in the cycle.

The motor air ratio is calculated according to equation 9:

This clearly shows the drama in the post-start, where on the one hand that in the combustion chamber inflowing air through the empty suction pipe and it on the other hand, a sudden evaporation of the fuel according to equation 5 and 6 is coming. The may have an additional negative effect on the quality of combustion increased internal exhaust gas recirculation due to the low load. The process shown leads then overfat and engine fat. Incomplete results Combustion up to misfiring.

According to the invention, the second term of equation 4 is affected by the fact that the ignition angle is retarded during the run-up. This will the indicated torque and the increase in speed are reduced. Because in the In the initial phase, the intake manifold pressure is high, especially according to Equation 4 to a rapid drop in pressure. The invention by means of late adjustment of the An early reduction of the speed of the ignition angle is therefore an effective one Countermeasure.

Another effective measure against over-greasing the engine is not to close the throttle valve as far as usual when starting or to open it even after starting. This leads to an increase in the engine air mass in the combustion chamber ≙ _{L, Br} . The throttle valve is preferably kept constant during the start phase and the post-start phase, for example by setting the I component of an idle control to zero, especially for the start. When starting, the throttle valve is then opened further than would be necessary to start the engine. At the same time, the suction pressure does not drop as quickly because of the inflowing air. With the post-start idle control, smooth transitions are provided in order to avoid a sudden actuation of the throttle valve.

Another measure concerns the fuel mixture. Here, the injected fuel quantity m _{Kr, EV} is reduced, so that the sum in equation 8 remains constant despite evaporating from the wall film.

The intake manifold pressure can be determined using the formulas given. On The corresponding intake manifold model is already available in modern control units. The Two alternatives for a control strategy described below are presented Based on the engine air ratio or the intake manifold pressure drop.

In a first alternative, a test engine with a US or Measure the European approval fuel and so matched a wall film model as well the motor air ratio can be calculated. By countermeasuring with a Fast FID, Indiziermeßtechnik or ion current probes can still be a limit range determine tolerable overfat. On an experimental engine, intake manifold pressure and measure speed curves that lead to low or high post-start emissions to lead. A limit speed curve is determined that is safely tolerable Emissions. On each series engine, speed curves are in start and Post-start time recorded. If the speed curve approaches the limit curve, then the ignition angle is retarded.

In a second alternative, the speed at each start and at each series engine Post-start process tracked. On this basis and from the knowledge of the starts from the The past is calculated in the future of the current start. This calculation also includes models for engine enrichment or intake manifold pressure drop. If this calculation shows that over-greasing occurs without measures would, if necessary, be supported by critical values from a test engine or a comparable engine, the ignition angle is used to limit the speed. To the To ensure a reliable start, a minimum speed is specified and the Firing angle adjustment limited.

If it is not possible to prevent the engine from being over-greased, one of them is preferred development in secondary air concepts, for example, the Delay wall film evaporation. Such a delay significantly improves the Conditions for post-oxidation of unburned and partially oxidized components of fuel. A delay in evaporation is also influenced reached on the intake manifold pressure. However, avoiding evaporation no longer the aim is to open up more degrees of freedom. For example, one is longer During the overspeed overshoot, not to be viewed so critically, if it does leads to over-greasing of the engine, however, the post-oxidation of the unburned and partially oxidized components of the fuel by secondary air is particularly effective.

Claims (13)

1. A method for starting a gasoline engine, in particular a motor vehicle, the fuel-air mixture being greased in the starting phase to ensure that the gasoline engine starts up, fuel being deposited as a wall film in an intake manifold, characterized in that during a starting phase and a After the start-up phase, an intake manifold pressure is controlled in such a way that spontaneous evaporation of the wall film is avoided. 2. The method according to claim 1, characterized in that in the starting phase and in the post-start phase, a throttle valve is opened to a constant position, the throttle valve is opened more than is necessary for the start. 3. The method according to claim 1 or 2, characterized in that during the Post-start phase an injection quantity of fuel taking into account a Fuel wall film in the intake manifold is lowered so that a sum of injected fuel and fuel from the wall film of a desired The amount of fuel in the cylinder corresponds. 4. The method according to at least one of the preceding claims, characterized characterized in that during the startup of the gasoline engine The ignition angle is adjusted so late that the speed of the gasoline engine regulated and the speed gradient is limited. 5. The method according to claim 4, characterized in that each time the Otto engine during the start phase and the post-start phase the speed curve is monitored and recorded that the speed curve with a Limit speed curve is compared and that the limitation of the speed using the ignition angle is only carried out when the speed curve around approximates a predetermined value to the limit speed curve, the Limit speed curve is determined such that below the Limit speed curve post-start emissions below a predetermined Result in value. 6. The method according to claim 5, characterized in that the Limit speed curve by measuring a test engine with Approval fuel is determined. 7. The method according to claim 5 or 6, characterized in that for determining the limit speed curve with a Fast FID, indexing measurement technology or Ion current probe is measured against. 8. The method according to at least one of claims 5 to 7, characterized characterized in that to determine the limit speed curve Intake manifold pressure curves and speed curves depending on Post-start emissions are measured and recorded. 9. The method according to claim 4, characterized in that at each start of the Otto engine during the start phase and the post-start phase the speed curve monitored and recorded that from the recorded Speed curves and the speed curve or a speed gradient of the current starts for the future of the current start is calculated whether there is a Over-greasing of the gasoline engine comes, and that the limitation of the speed by means of the ignition angle is only carried out if it is without this measure would overfat the gasoline engine. 10. The method according to claim 9, characterized in that in the calculation Models for engine enrichment and / or intake manifold pressure drop included become. 11. The method according to claim 9 or 10, characterized in that from the recorded speed curves and the speed gradient of the current start an intake manifold pressure curve is calculated for the future of the current start. 12. The method according to at least one of claims 4 to 11, characterized characterized that the ignition angle adjustment to a maximum value is limited. 13. The method according to at least one of claims 4 to 12, characterized characterized in that for the regulation of the speed by means of Ignition angle adjustment a minimum speed is specified.