Degree of delivery

In an internal combustion engine with internal combustion, the degree of delivery describes the ratio of the fresh charge actually contained in the cylinder after completion of a charge cycle to the theoretically maximum possible charge . In business administration , the degree of delivery is an economic indicator .

Otto engine (external mixture formation)

Four-stroke cycle of an ideal-typical slow-running gasoline engine : 1: "Prime" ➝ 2: "compacting" ➝ 3: "Expand" ➝ 4: "pushing" real remains running fast on the cycle 1 inlet on the UT also opened to more time to allow the fresh charge to flow in while the piston is already starting to compress; the exhaust valve opens at the end of cycle 3 and between cycle 4 and 1 the opening times of the exhaust and intake valve overlap

In a conventional gasoline engine with external mixture formation , fresh gas ( mostly a gasoline-air mixture ) is sucked in. The pressure gradient between cylinder and intake manifold required for filling is either achieved solely by the intake stroke of the piston ( naturally aspirated engine ) or supported by a compressor ( supercharging ). If the gas exchange process were taking place infinitely slowly, the volume of the intake mixture would correspond exactly to the displacement , provided that the intake valves close precisely at bottom dead center (BDC) and the exhaust valves are closed during the entire process. ${\ displaystyle m_ {Z}}$

Because a gasoline engine runs at speeds of around 600 to 17,000 rpm, there is very little time for the inflow process , so that the incoming fresh charge firstly has to overcome a flow resistance and secondly it has to be accelerated and decelerated again. Due to pulsating pressure waves in the intake tract and cylinder, unfavorable currents and the valve overlap , part of the fresh charge that has already been introduced can flow away again. The effective volume of the fresh charge is usually smaller than the displacement, but can even be larger at certain speeds due to resonance effects ( resonance charging ).

The fresh charge is decisive for the amount of energy converted per work cycle and directly influences the torque and engine output via the mean effective pressure . An engine with a high delivery rate consequently has a high capacity-specific output .

The degree of delivery for a gasoline engine with external mixture formation is defined:

{\ displaystyle \ lambda _ {l} = {\ frac {m_ {Z}} {m_ {th}}} = {\ frac {m_ {air} + m_ {gasoline}} {m_ {th}}} = { \ frac {m_ {air} + m_ {gasoline}} {V_ {h} \ cdot \ rho _ {th}}}}

Diesel engine (internal mixture formation)

Four-stroke cycle for a diesel engine with direct injection (schematic)

In a diesel engine with internal mixture formation ( injection ) flows during the charge cycle only fresh air into the combustion chamber and diesel fuel is only injected during the compression stroke, so the volumetric efficiency that only the available combustion air refers. The degree of delivery for a diesel engine with internal mixture formation is defined:

{\ displaystyle \ lambda _ {l} = {\ frac {m_ {air}} {m_ {th}}}}

Otherwise, the same applies as for a gasoline engine , because the amount of fuel injected must be matched to the available fresh air charge in order to achieve the required combustion air ratio . While the gasoline engine is mainly controlled by throttling the delivery rate ( quantity control ), the diesel engine is also used ( quality control ) by changing the air ratio or stretching the fresh charge with inert exhaust gas ( exhaust gas recirculation ): this is necessary because the diesel cycle process requires a sufficiently high compression pressure even at part load to ensure ignition and even combustion of the mixture.

Two-stroke engine (flushing)

In two-stroke engines , the ratios of (despite its apparently simpler structure) gas exchange more complicated, since in the always incomplete rinsing a certain amount of exhaust gas remains which is involved in the combustion only as an inert ballast. In principle, the ratios for external mixture formation in classic gasoline engines and for internal mixture formation in two-stroke engines with direct injection such as the two-stroke diesel engine can be transferred .

Setting options

There are many ways to increase the delivery rate:

larger flow cross-section when changing gas (three-, four-, five-valve technology, slide control ).

The inlet valves usually close considerably after BDC, and the kinetic energy of the mixture means that further mixture flows in after BDC (reloading effect). With resonance charging , the cylinder charge can also be greater than the displacement in naturally aspirated engines.

Operation of a two-stroke engine with membrane inlet control and resonance exhaust (blue: air, green: gasoline-air mixture, gray: exhaust gases)

With a variable intake manifold , the resonance charging can be adapted to the speed so that the reloading effect can be used in a larger speed range.

The gas equation for air

{\ displaystyle m_ {air} = {\ frac {p \ cdot \ V} {R_ {L} \ cdot \ T}}}

leads to two further methods to increase the filling of the combustion chamber:

The supercharging of the engine (turbocharger, compressor) increases the pressure p of the fresh gas.
The charge air cooling lowers the temperature T of the fresh gas.

Both measures increase the value of the break and thus the air mass supplied to the engine.

Note: according to the definition, the theoretical air mass ( ) is calculated using the same values for density, pressure and temperature as the mass of the air actually supplied ( ). That means: charging and charge air cooling increase the cylinder charge, but not the degree of delivery . ${\ displaystyle m_ {th}}$ ${\ displaystyle m_ {air}}$

More terms

Air expenditure

The air consumption is the amount of fresh gas related to the theoretically possible (geometrical) fresh gas amount that goes through the combustion chamber during a work cycle. The fresh gas components that leave the combustion chamber immediately without participating in the combustion are also included. This can happen during the valve overlap (simultaneous opening of the inlet and outlet valve) or by the mixture flowing back into the intake pipe.

Degree of catch

The ratio between the fresh gas actually remaining in the combustion chamber and the amount of air required is called the degree of capture because the proportion of fresh gas has been "captured".

Rinsing degree

Charge change in the two-stroke engine (here with reverse scavenging )

The degree of flushing, on the other hand, indicates the ratio of fresh gas to the load (fresh gas and residual gas that could not escape). The idea that the residual gas (that is what is left over after combustion) escapes completely through the exhaust valve after combustion is an ideal that is not entirely correct. Hence the name of the degree of rinsing.

The degree of catch and the degree of flushing are important parameters, especially with two-stroke engines , because with this type of engine the outlet is almost always open when the inlet is also open. It can therefore hardly be avoided that fresh gas immediately escapes again through the outlet ( short-circuit purging ). The development goal is always to keep this flushing loss small.

Business administration

The volumetric efficiency ( English customer service level ) is in the operating economics of the quotient of the number of time and just order deliveries and the number of orders or orders .

{\ displaystyle {\ text {Degree of delivery}} = {\ frac {\ text {Deliveries}} {\ text {Number of orders}}}}

.

The degree of delivery improves when more deliveries are made for given orders and vice versa. However, a delivery rate of almost 100% requires high safety stocks , increases storage costs and thus the storage risk . On the customer side, a high degree of delivery is associated with a high level of readiness to deliver , good delivery service and thus a favorable supplier evaluation.

literature

Karl-Heinz Dietsche, Thomas Jäger, Robert Bosch GmbH: Automotive pocket book. 25th edition, Friedr. Vieweg & Sohn Verlag, Wiesbaden, 2003, ISBN 3-528-23876-3
Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1
Peter A. Wellers, Hermann Strobel, Erich Auch-Schwelk: Vehicle technology expertise . 5th edition, Holland + Josenhans Verlag, Stuttgart, 1997, ISBN 3-7782-3520-6

Individual evidence

↑ Michael Hompel / Volker Heidenblut, Taschenlexikon Logistik , 2011, p. 182

[1] Michael Hompel / Volker Heidenblut, Taschenlexikon Logistik , 2011, p. 182