Charge exchange
The exchange of the working medium in intermittently working displacement machines is called charge exchange . The most important representatives are reciprocating internal combustion engines in which the working area burned exhaust gas from combustion enabled fresh gas (the fresh charge is exchanged). It is an essential element in the work process of engines with different combustion processes, i.e. gasoline engines and diesel engines , both for two-stroke and four-stroke reciprocating piston and rotary piston engines such as the Wankel engine .
Basics
Particularly at high engine speeds, poor gas exchange with reduced fresh charge and increased exhaust gas retention affects both engine performance and the quality of combustion. Only a gas exchange that is optimally matched to preferred speed ranges ensures efficient operation with maximum performance and low pollutant emissions. The essential parameter is primarily the degree of delivery or air consumption as a measure of the effectiveness of the gas exchange. When two-stroke engine also the volume-based are Spülgrad and the trapping efficiency important.
Depending on the speed significant influence for the optimization of the charge cycle, the valve timing , the number of valves per cylinder and the opening cross-section and diameter results from valve lift, as well as the dynamic pressure ratios in both the intake system and in the exhaust tract , wherein optionally a charge incl. intercooling be taken into account. Optimizing the gas exchange therefore requires careful coordination of all elements, including the intake tract and exhaust system . In doing so, dynamic resonance effects are also specifically exploited at preferred speeds ( resonance charging ), which is particularly useful for engines that predominantly run at a fixed speed ( power units & CHP units or also for lawnmowers, etc.).
Gas exchange in a four-stroke reciprocating engine
In the four-stroke engine , the change in volume of the working space is used alternately for work and gas exchange: The reciprocating piston performs the gas exchange in two of the four cycles: pushing out and sucking in :
| 1. | Suction | Charge cycle | |
| 2. | Work cycle | Compress | |
| 3. | Work cycle | Expand | |
| 4th | Push out | Charge cycle |
The gas exchange in four-stroke engines is usually controlled by inlet and outlet valves on the cylinder head , which seal the combustion chamber and release fresh gas and exhaust gas separately, so that when the piston moves upward, exhaust gas is pushed into the outlet channels and when the piston moves downward, the fresh mixture is sucked in through the inlet channels . Until shortly after the end of the Second World War, motors with slide control were also built. In modern engines, globe valves are used which meet requirements more easily and better at lower costs. The valves are operated by a camshaft via tappets and rocker arms or rocker arms.
Gas exchange losses and timing
The course of the gas pressure in the working space (cylinder) can be shown in a p-α diagram or in a pV diagram , where α denotes the crank angle and V the displacement :
The gas exchange is comparable to a pumping process, which consumes a certain part of the engine power. In relation to the schematic representation of a four-stroke engine process, gas exchange losses can be divided up as follows:
| No. | from | to | designation | comment |
|---|---|---|---|---|
| 1 | Aö | UT | Loss of expansion work | Working gas could theoretically still do expansion work; however, the outlet is opened before BDC to give the exhaust gas more time, which can reduce the expulsion work. |
| 2 | UT | OT | Loss due to expulsion work | The working gas escapes through the outlet valve with throttling losses and against the flow resistance of the exhaust system. |
| 3 | OT | UT | Loss due to suction work | Fresh mixture (or air) is drawn in through the inlet valve. Losses occur at the throttle point of the valve and through the negative pressure in the intake manifold. This is particularly high in gasoline engines with load control by means of a throttle valve at part load. |
Efforts to make engines more efficient include, in addition to many other measures such as a reduction in friction losses, also a reduction in gas exchange losses, whereby load control with a throttle valve in particular must be avoided so that suction does not have to take place against negative pressure in the intake manifold: A variable valve control minimizes losses by instead controls the filling level by adjusting the closing of the inlet valve ( Miller engine ).
Influence of the valve timing
With conventional valve control with fixed control times, the design has the following influence on the performance characteristics:
- Aö (outlet opens): Early (late) AÖ causes high (low) losses in expansion work, but reduces (increases) the expansion work.
- It (inlet closes) influences the filling and thus the torque characteristics of an engine much more strongly than the other timing: early ES is favorable for high torque in the lower speed range, but requires weak filling at the rated speed; late ES (sports engine) results in high rated power with loss of filling at low speeds.
- Eö and As (area of valve overlap ): If there is a large valve overlap, part of the fresh gas that is already flowing in at the same time can still escape unused when the exhaust gas is drawn in (purging loss similar to that of two-stroke engines), which in engines with a mixture intake worsens the effective efficiency and λ l <λ a . On the other hand, the high level of residual gas emissions and the extended intake increase the delivery rate and thus the performance, which is mainly used in sports engines.
In modern engines without variable valve control, the fixed timing is approximately at the following values ( ° KW means degrees crank angle ; timing in brackets are extreme interpretations):
| Gasoline engine | Diesel engine | |
|---|---|---|
| Aö [° KW v. UT] | (70) 50-40 | 50-40 |
| As [° KW after OT] | 4 - 30 (40) | 5 - 30 |
| Eö [° KW v. OT] | (40) 30 - (5) 10 | 25 - 0 |
| It [° KW after subtitle] | 40 - 60 (80) | 30 - 40 (70) |
Gas exchange in a rotary engine
The rotary engine also works according to the four-stroke process, but uses a slot control (similar to two-stroke engines ) instead of valves , whereby the piston takes over the control of the gas exchange by releasing or closing openings in the casing or side window. The timing is determined by the geometry of the slots. Here, too, both the intake tract and the exhaust system can be tuned to resonance for specific speeds.
Gas exchange in a two-stroke engine
In reciprocating piston engines that work according to the two-stroke process, the charge is changed between the work strokes by flushing out the exhaust gases with a fresh charge. They have control slots that the piston releases or closes so that, unlike the four-stroke engine, they do not have a valve train. To do this, you need a pump for the fresh gas. In small two-stroke petrol engines, this is usually the underside of the piston in the crankcase. The air supply to the crankcase is opened via a diaphragm valve or the lower edge of the piston, while the piston moving upwards creates a negative pressure which sucks in the fuel-air mixture. In the working stroke, the piston compresses the mixture in the crankcase. Shortly before bottom dead center, the overflow duct leading from the crankcase to the working chamber opens, through which the fresh gas flows into the working chamber and pushes the exhaust gas out.
Large two-stroke diesel engines have external scavenging fans combined with turbochargers. The exhaust valves controlled by a camshaft close before the intake so that they can be charged. The inlet via slots is controlled by the piston.
The two-stroke engine basically performs the gas exchange at bottom dead center, with fresh gas pushing the exhaust gas out of the cylinder. However, this dynamically extremely complex purging process does not work optimally and usually only achieves incomplete replacement in a compromise with the lowest possible loss of unburned fresh gas flowing through (see also: degree of capture ). In principle, three theoretical borderline cases can be considered:
- Displacement scavenging : Fresh gas and exhaust gas are ideally separated by a front and do not mix: The scavenging works optimally as fresh gas pushes the exhaust gas out of the cylinder as completely as possible.
- Dilution flushing: incoming fresh gas mixes continuously into the cylinder contents, the excess of which flows off via the exhaust gas path, whereby the fresh gas proportion increases steadily in the course.
- Short-circuit purging (undesirable): Fresh gas escapes directly to the outlet without contributing to purging the cylinder charge.
An effective charge exchange = flushing process should avoid the escape of fresh charge directly into the outlet as far as possible without too much exhaust gas remaining in the combustion chamber: An excessive exhaust gas proportion can impair the engine running if the mixture does not burn through properly, so that the performance drops and the emissions Incompletely burned hydrocarbons increases sharply.
Rinsing method
A number of flushing methods have been developed for the two-stroke engine . In most of them, the gas exchange is controlled by slots. This means that there are openings in the cylinder running surface that the piston passes over and thus closes or opens. Not only the upper edge of the piston, but also the lower edge, as well as pockets or openings in the piston skirt, can serve as a control element.
- In reverse flushing, there is an outlet on one side of the cylinder between two inlet slots. The gas flows from the inlet slots through the entire working space and is deflected by the opposite cylinder wall towards the outlet.
- Cross-flow flushing : inlet and outlet are opposite. The fresh gas flow isdirected upwardsthrough an inletslot openingtangentially into the cylinder or a nose on the top of the piston. This type of purging can also be connected to a slide valve in the outlet channel in order to achieve asymmetrical control times, but the effort is very high.
- Longitudinal flushing ( direct current flushing ): In this case, there is usually a lift valve in the cylinder head that is responsible for the outlet, while the inlet is controlled via slots through the piston. In this way it can be achieved that the outlet opens earlier and closes earlier than the inlet, which makes charging possible. However, the technical and thus financial outlay in production is significantly higher than with a slot control. The process is used for large diesel engines in ships.
- Longitudinal flushing with opposed piston : two pistons run in one cylinder. While the fresh gas is supplied in a slot-controlled manner in one piston, it also escapes through slots in the second. The disadvantage is the high manufacturing cost and the large temperature difference between the two pistons. An asymmetrical control diagram is also possible here.
- Longitudinal purging with double pistons : two cylinders directly next to or behind one another share a common combustion chamber. The pistons sit on a split or forked connecting rod. One piston controls the inlet slots in one cylinder, the other controls the outlet slots accordingly.
Longitudinal flushing is the most effective variant of all processes.
Avoidance of flushing losses
In principle, scavenging losses are not critical in engines with injection into the cylinder ( internal mixture formation ), since only air is used for scavenging and so no fuel is lost.
For two-stroke engines that are charged with an external compressor, external mixture formation by means of intake manifold injection or the injection of fuel gas can also be modulated in such a way that purging takes place first with clean air and the fuel is only supplied shortly before the end. This allows a generous flushing of the exhaust gases with flushing losses only from fuel-free air.
This principle is also used for two-stroke engine with a simple carburetor implemented by the piston in the milled pockets transfer ports in the upper dead center connected to clean air ducts, so that the overflow channel is present a clean air column. If the piston opens the overflow duct at bottom dead center, the exhaust gas is first flushed out of the working space with the clean air contained therein. Only then does the excessively rich mixture flow in, which can hardly be lost.
Resonance charging
In order to optimize the degree of delivery or air consumption, the gas exchange can be significantly supported by resonance charging with a system of ducts and resonance chambers that is matched to the speed, both in the intake tract and on the exhaust side. This works for a single cylinder or for the engine as a whole, whereby the periodic suction and exhaust cycles of the individual cylinders are superimposed with their correspondingly increased frequency.
Resonance charging is particularly suitable for engines that predominantly run at a fixed speed ( generator sets & CHP units or also for lawnmowers, etc.), but it also optimizes vehicle engines for preferred speed ranges. In engines with turbocharging , resonance charging is often used at low speeds in order to compensate for the torque weakness of the turbocharger .
Suction resonance
The gas exchange is significantly influenced by the intake tract: When the intake valves are opened, the negative pressure runs through the intake manifold as a wave front at the speed of sound and is reflected at its open end in reverse as an overpressure wave that runs back towards the cylinder, where it causes an additional charge or just before the intake -Conclusion can prevent the fresh charge already introduced from flowing back. The coordination of the intake tract over the length of the intake manifold determines the running time of the pressure wave and thus the effectiveness depending on the speed. The first systems of the so-called oscillating tube charging offered an optimal gas exchange only in a narrow speed range, but the first variable variable intake manifold systems with two and later three different intake manifold lengths appeared early on. Some engine manufacturers are now using infinitely variable intake manifolds, depending on the speed.
In the case of six and twelve-cylinder engines, the combination of resonance and oscillating tube charging is ideal. The resonance effects are effective at low engine speeds, while the gas oscillations are effective in the upper engine speed range due to the short intake manifolds. In this case, the suction pipes of six cylinders are connected via a collecting container with a flap in the middle. Two further resonance tubes lead from the container into a resonance collector. The flap is closed in the lower speed range. Three cylinders therefore suck out of a collecting container and, via a resonance pipe, from the common resonance collecting container; comparable to a long suction pipe. In the power setting for higher speeds (with normal gasoline engines from approx. 4000 rpm) the flap is opened and all six cylinders are supplied from a tank via short oscillating tubes.
Exhaust gas resonance
Similar to the intake tract, the pressure waves generated by the periodic exhaust can also be used on the exhaust side with a resonance exhaust , which particularly supports the gas exchange for two-stroke engines. By means of a suitable geometry of the exhaust system, optimized to the nominal speed, it can be achieved that the fresh gas mixture that has initially escaped into the exhaust system is pushed back into the combustion chamber by a reflected pressure wave.
The two-stroke engine is - like, to a lesser extent, the four-stroke engine and, in general and to varying degrees, other heat engines - a resonance system, the performance of which, however, unlike the four-stroke engine, depends very much on the vibration properties of the gases used ( inertia ).
literature
- Hans-Hermann Braess, Ulrich Seiffert: Vieweg manual automotive technology. 2nd edition, Friedrich Vieweg & Sohn Verlagsgesellschaft mbH, Braunschweig / Wiesbaden, 2001, ISBN 3-528-13114-4 .
- Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3 .
- Pischinger, Franz: Internal combustion engines, lecture reprint. Chair of Applied Thermodynamics, 1987, self-published.