WO2000009871A1

WO2000009871A1 - Internal combustion engine with a piston and a cylinder and method for initiating combustion in such a cylinder

Info

Publication number: WO2000009871A1
Application number: PCT/SE1999/001363
Authority: WO
Inventors: Ulf Lundqvist; Bengt Johansson
Original assignee: Scania Cv Aktiebolag (Publ)
Priority date: 1998-08-13
Filing date: 1999-08-12
Publication date: 2000-02-24
Also published as: SE9802743L; SE9802743D0; SE513179C2

Abstract

In a combustion engine of piston-cylinder type, of the kind which uses preferably homogenous premixed fuel/air mixture and compression ignition, an ignition piston (3) is arranged in the engine's cylinder head (30) and a pushing device (4) is designed to impart to the ingnition piston (3) during the compression stroke of the working piston (2) a movement into the cylinder space (1) in order to control the ignition of fuel/air mixture present.

Description

INTERNAL COMBUSTION ENGINE WITH A PISTON AND A CYLINDER AND METHOD FOR INITIATING COMBUSTION IN SUCH A CYLINDER

FIELD OF THE INVENTION

The present invention relates to a combustion engine of piston-cylinder type in accordance with the preamble to patent claim 1 and a method according to the preamble to claim 13.

STATE OF THE ART

The problem of high NO_x emissions from conventional diesel engines arises from very high combustion temperatures in limited portions of the cylinder at the time of ignition, due to combustion taking place along a defined combustion front with low lambda numbers (air/fuel ratios). Combustion engines of the kind mentioned in the introduction are intended to prevent this emission problem and are known inter alia by the designation AT AC (Active Thermic Atmospheric Combustion) and may popularly be said to be a combination of a diesel engine and an Otto engine. In their case, a premixed fuel/air mixture is introduced into the cylinder and is ignited by compression when the working piston is in the vicinity of its top dead centre position in the ignition phase. AT AC engines afford considerable advantages, particularly in the form of small or no NO_x or CO discharges as well as high efficiency approaching that of diesel engines. AT AC engines overcome the problem by using a substantially leaner mixture (high lambda number) and lower combustion temperatures due to combustion being initiated substantially simultaneously within widespread regions of the combustion chamber. The overall result is a more even temperature distribution.

In combustion engines of the kind which employs combustion of a homogenous premixed fuel/air mixture it has been found possible to overcome the emission problem with regard to harmful pollutants. So-called homogenous combustion thus makes it possible for a premixed gas mass to be auto-ignited by compressive pressure. However, this type of engine suffers from a number of inherent disadvantages, which include a narrow load range. This means that acceptable operation depends on ignition taking place in a limited situation close to the top dead centre position (TDC) of the working piston. The commencement of combustion depends on a number of parameters, but primarily on the temperature in the combustion chamber and the lambda number. As the temperature itself depends to a large extent on the charging pressure and the load, the point in time at which the ignition of the gas mass commences will vary substantially. This means that the engine's efficiency will be poor and the thermal load on the engine will be large if ignition is too early, which tends to occur at high loads. If ignition is late, the efficiency will likewise be poor and the combustion incomplete, resulting in high HC (hydrocarbon) emissions, which tends to occur at low loads. A further problem is that the ignition commencement point is not stable. An example is the ignition commencement point being in the region around the TDC of the working piston. An infinitesimal change in the ignition commencement time towards earlier ignition relative to TDC causes an infinitesimal increase in the engine's thermal load, resulting in the next cycle in higher temperature etc., thereby driving the ignition commencement point still earlier relative to TDC in an escalating pattern with harmful consequences.

OBJECT AND MOST IMPORTANT CHARACTERISTICS OF THE INVENTION

One object of the present invention is to improve existing engines of the kind mentioned in the introduction so that a broader load range can be used in practice and the problem mentioned be prevented.

This object is achieved according to the invention by designing the engine according to the features in the characterising part of patent claim 1 and by a method for controlling an engine according to the characterising part of patent claim 13.

The result is the possibility of very accurate control of ignition commencement time to eliminate both the problem arising from early ignition and the problem arising from late or no ignition. It is in particular advantageous to take care to ensure that the working piston creates a compression ratio which is selected such as to prevent any possibility of auto-ignition. In an advantageous embodiment of the invention, the ignition piston has to be pushed very quickly into the cylinder, thereby causing further compression which is selected such as to ensure that ignition takes place. The resulting engine will be insensitive to operational malfunctions due to varying load, temperature changes etc. It will also be insensitive to different kinds of fuel. Higher efficiency may also be expected through a high compression ratio being achieved in practice.

The subclaims indicate further advantageous embodiments of the invention which result in easy and reliable operation regarding the influence of the ignition piston.

According to an advantageous embodiment, the engine control system is used for controlling the movement of the ignition piston. The control system, updated with operating data, can thus accurately control the compression in the cylinder so that compression resulting in auto-ignition in the prevailing situation (temperature, fuel/air quantity, engine load etc.) is reliably achieved in the correct phase situation of the cylinder.

Further advantageous embodiments are indicated by the other patent claims and by the following description.

BRIEF DESCRIPTION OF THE DRAWINGS

Further advantages are achieved by the features in the other claims and are indicated by the following detailed description of embodiments against the background of the attached drawings, in which:

Fig.l shows schematically part of a combustion engine with an arrangement according to the invention,

Fig.2 shows schematically a working diagram for an engine according to the invention, and

Fig.3 shows a modified embodiment of the invention.

DESCRIPTION OF EMBODIMENTS Fig.1 shows a first embodiment of an arrangement for control of ignition time in a multi- cylinder combustion engine of piston-cylinder type. The combustion engine is of the type which uses a premixed, preferably homogenous, fuel/air mixture which is ignited by compression. All the cylinders of the engine are of like design and Fig.l depicts one of them, whereby a cylinder 1 with a working piston 2 is provided, in the cylinder head of the engine, with a smaller piston (auxiliary piston) 3 which acts as an ignition piston and is therefore hereinafter called the ignition piston. The ignition piston 3 is urged into the cylinder 1 by a pushing device 4, in the form here of a preloaded spring device or, more closely defined, a coil spring acting as a compression spring. The spring device 4 abuts against a piston 5 which is connected to the ignition piston 3 via a pushrod 6 and which also has the function of a fluid piston and is for that purpose sealingly movable in a fluid cylinder 7. Both ends of the fluid cylinder are connected to a bypass line 8 which is controllable by a trigger valve 9. The trigger valve 9 is itself controlled advantageously by the engine's conventional control system in a manner which causes ignition to be initiated at the desired time.

In operation, the invention functions as follows: In the vicinity of the top dead centre position (TDC) of the working piston 2, the volume compressed by the working cylinder is selected, as previously indicated, such as to ensure that auto-ignition is not initiated. At a selected time relative to the position of the engine's crankshaft, the trigger valve 9 opens, whereupon the piston 5 is released in the cylinder and the spring device 4 with great force is quickly able to push the ignition piston 3 into the cylinder. This rapidly increases the compression in the cylinder 1 so as to ensure the reliable auto-ignition of the premixed fuel/air mixture introduced into the cylinder. The pressure created by the combustion pushes the ignition piston 3 back into its seat 10 against the counterpressure of the spring device 4, and the trigger valve 9 is caused to close so that the ignition piston 3 is locked in an upper position, after which the arrangement is ready to repeat the insertion when it is time for the next ignition to be initiated. Each commencement of combustion is also accompanied by a simultaneous increase in the pressure on the working piston 2, which performs its cycle in a conventional manner. It should be emphasised that the movement of the ignition piston 3 is almost instantaneous or takes place very rapidly and that the whole ignition piston insertion operation corresponds to approximately a couple of crankshaft degrees. This has to be compared with the working piston's movement in the vicinity of TDC, which, owing to the geometry, is halting and relatively slow. This is a further background to the instability as to when ignition is initiated with respect to crankshaft position.

The embodiment referred to with an urging spring device 4 means that the ignition piston 3 can be made self-setting and the system self-regulating. Thus in practical operation the ignition piston 3 will probably not be pushed to its position of maximum insertion into the cylinder 1 but rather, as soon as ignition has been initiated and the pressure of the combustion gases has built up in the cylinder, the ignition piston will be pushed back into its seat to a self-regulating extent.

Where applicable, the ignition piston 3 may, before starting, be pushed to an upper position by means of a fluid inflow line in the lower end of the fluid cylinder, but it is also possible to allow the ignition piston to be initially in its inserted position in the cylinder, since the pressure of the combustion gases resulting from ignition being initiated by the movement of the working piston will immediately push the ignition piston to a position in its seat. It is advantageous from the self-regulating point of view that the ratio between the spring constant of the spring device and the area of the ignition piston be approximately equal to Pmax.

It should be noted that Fig.l is greatly simplified and that parts such as pipes, lines and valves have been simplified or totally omitted for the sake of clarity.

The invention thus provides an engine with the possibility of a considerably broader operating load range and affords the advantages which are associated with engines having so-called homogenous combustion.

By way of example, it may be mentioned that various fuels are usable and that with gasoline, ethanol and methanol an initial compression of 10 to 14: 1 is achieved by means of the working piston (thereby ensuring that no auto-ignition occurs) followed, after insertion of the ignition piston, by a final compression of approximately 20: 1 , which thus ensures auto-ignition of these fuels. For other fuels such as diesel oil, other compression ratios may be appropriate, in particular initially 10 to 15:1 and finally 17 to 22:1. The ignition piston thus brings about an increase in the compression ratio by a number of units which may in certain cases be five units or more. It is also possible to increase the compression ratio by a smaller number of units, such as two units or, in certain cases, one unit. (An increase of one unit here means, for example, an increase in compression from 10: 1 to 11 : 1). It may in this context be mentioned that the aim is to bring about the movement of the auxiliary piston as quickly as possible in the period of time when the so- called pre-reaction takes place in the injected fuel mixture.

The material applicable for the ignition piston is conventional material for pistons in high- compression combustion engines, e.g. hard chrome plated steel.

Fig.2 shows a diagram of the prevailing pressure P in the cylinder as a function of the instantaneous volume V in the cylinder, i.e. in principle the crankshaft position. Curve 20 denotes the compression phase, 21 insertion of the ignition piston, 22 ignition, 23 expansion, 24 exhaust and 25 intake. The pressure rise is represented by curve 21.

The invention may be modified within the scope of the patent claims. Thus pushing devices other than springs, and trigger devices other than that depicted may be used. For example, mechanical, pneumatic or hydraulic pushing devices may be used.

In the embodiments described above, the ignition piston is inserted rapidly or even very rapidly, in order to directly initiate the ignition which follows directly or almost immediately after the ignition piston has been inserted. It is therefore the time when the ignition piston is inserted that controls the ignition time. In alternative embodiments of the invention, the ignition piston may be introduced significantly earlier and in a situation when the pressure in the cylinder is relatively low. In such embodiments the ignition piston may be introduced at any stage of the rising movement of the working piston during the engine's compression stroke. In such cases the ignition time depends on how far the ignition piston has been inserted. This allows the possibility of no great force being required for introducing the ignition piston, and of its introduction taking place relatively slowly. Such an embodiment does to some extent resemble engines with variable compression, but an essential difference is that according to the present invention it is the initiation of ignition that is primary and is the parameter which controls the insertion of the ignition piston. The ignition piston is thus used to control the ignition time in an engine with compression ignition.

In another modified embodiment of the invention, the ignition piston may be inserted to its inward position by means of a hydraulic system which acts upon the piston connected to the ignition piston. There is no inherent need to bring the piston quickly back to the inward position, provided that the need for the piston to be in the inward position before the next ignition is initiated is catered for, since there is a relatively long time before the next ignition is initiated. Nevertheless there are, from the combustion engineering point of view, advantages in being able to bring the piston back significantly more quickly. If it is brought back more quickly and yet in a controlled manner, it is possible to control both the pressure in the cylinder during combustion and the maximum pressure derivative during the cycle.

Fig.3 shows an embodiment whereby the valve according to the embodiment which refers to Fig.1 is replaced by a hydraulic system denoted generally by notation 11 and incorporating inter alia an oil sump, a hydraulic pump, valves and a control system. In this modified embodiment, this system 11 communicates via an upper line 12 and a lower line 13 respectively with the cylinder 7 above and below the piston 5. This system provides the possibility of deciding more freely how the piston 3 should be controlled and the points in time when the piston 3 should be inserted into, and brought back from, the engine cylinder 2. To this end, the system 11 receives signals representing various engine parameters for its control. In further modified embodiments, the hydraulic system may include an energy-recovering accumulator vessel which, analogously with the previously described spring 4, may store at least part of the pressure arising during combustion and use that energy in subsequent cycles for introducing the ignition piston 3 into the cylinder 1.

Claims

PATENT CLAIMS

1. Combustion engine of piston-cylinder type, of the kind which uses a premixed fuel/air mixture and compression ignition, characterised in that an ignition piston (3) is arranged in the engine's cylinder head (30) and that a pushing device (4) is designed to impart to the ignition piston (3) during the engine's compression stroke a movement into the cylinder space (1) in order to control the ignition of fuel/air mixture present in the cylinder.

2. Combustion engine according to claim 1, characterised in that the pushing device is a spring device (4).

3. Combustion according to claim 1 or 2, characterised in that the pushing device (4) is coupled to a fluid piston (5) which runs sealingly in a fluid cylinder (7), and there is between the ends of the fluid cylinder a controllable bypass line (8) which can be controlled in order to initiate the movement of the ignition piston (3).

4. Combustion engine according to claims 1, 2 and 3, characterised in that the ignition piston (3) is self-setting to its inserted position in the cylinder (1) through calibration of the spring constant of the spring device.

5. Combustion engine according to any one of claims 1-4, characterised in that the pushing device (4) is designed to impart to the ignition piston (3) a rapid movement into the cylinder space (1) in the region before the top dead centre position of the working piston (2) in order to initiate ignition of fuel/air mixture present in the cylinder.

6. Combustion engine according to any one of claims 1-4 and incorporating a control system for control of engine operation, characterised in that the control system is arranged to control by means of the pushing device (4) the movement and position of the ignition piston (3) in the cylinder space (1) during the operation of the engine.

7. Combustion engine according to claim 6, characterised in that the control system is arranged to adjust the position of the ignition piston (3) in the cylinder during the gas exchange phases to a position which depends on the operating conditions during the compression stroke with a controlled instantaneous compression ratio, thereby

5 resulting in controlled auto-ignition time.

8. Combustion engine according to claim 6 or 7, characterised in that the ignition piston (3) is arranged to be locked during at least part of the engine's expansion phase.

10

9. Combustion engine according to either of claims 6-7, characterised in that the ignition piston (3) is arranged to be movable during at least part of the engine's compression phase.

15 10. Combustion engine according to any one of claims 6-9, characterised in that the ignition piston (3) is arranged to be moved quickly outwards from the cylinder directly after ignition.

11. Combustion engine according to any one of claims 1-4 or 6-10, 0 characterised in that the ignition piston (3) is arranged to be driven by a hydraulic cylinder unit.

12. Combustion engine according to claim 11, characterised in that the hydraulic cylinder unit is connected to an energy-recovering accumulator vessel. 5

13. Method for control of ignition in a combustion engine of piston-cylinder type, of the kind which uses premixed fuel/air mixture and compression ignition, characterised in that a movement into the engine's cylinder space (1) is imparted to an ignition piston (3) in order to control the ignition of fuel/air mixture present in the 0 cylinder.

14. Method according to claim 13, characterised in that movement of the ignition piston (3) is triggered by controlled action of a fluid piston/cylinder arrangement (5,7,8,9) connected to it.

15. Method according to claim 13 or 14, characterised in that the ignition piston (3) is pushed back out of the cylinder by the action of the pressure created by combustion.

16. Method according to claim 13, 14 or 15, characterised in that the engine's working piston (2) compresses to a first compression ratio and that the ignition piston (3) compresses to a final compression ratio which is at least one unit higher but is with advantage more than two units higher or more than five units higher.

17. Method according to any of claims 13-16, characterised in that a rapid movement into the engine's cylinder space (1) is imparted to an ignition piston (3) in order to initiate ignition of fuel/air mixture present in the cylinder.