Medium pressure

This article deals with the mean effective pressure of reciprocating engines; for the medical term, see mean arterial pressure .

Schematic pv diagram of the idealized gasoline process.
The mean pressure corresponds to the average height of the blue area.

The mean effective pressure is a calculation variable to assess the efficiency and the gas exchange of reciprocating piston engines regardless of the engine displacement or size. It is the quotient of the mechanical work performed by the engine during one work cycle (in Newton meters, N · m) and its displacement (in cubic meters, m ³ ). The meter in the newton meter and one meter in the cubic meter are shortened, so that at the end there is an indication in newtons per square meter (N · m ⁻² = Pa ), i.e. a pressure. This pressure is usually given in bar (1 bar = 100,000 N · m ⁻² ). The mean effective pressure cannot be measured like the compression pressure, for example , but can only be determined by calculation. In simplified terms, the accelerator pedal in the vehicle can be viewed as an actuator for the medium pressure. ${\displaystyle p_{\text{m}}}$ ${\displaystyle V_{\text{h}}}$

There are two types of medium pressure:

• The effective mean effective pressure can be calculated from the output torque or as a quotient from the work done on the crankshaft during a complete work cycle and the displacement : ${\displaystyle p_{me}}$ ${\displaystyle M}$ ${\displaystyle W_{\text{e}}}$ ${\displaystyle V_{\text{h}}}$

${\displaystyle p_{\text{me}}={\frac {2\cdot \pi \cdot M}{V_{\text{h}}\cdot i}}={\frac {W_{\text{e}}}{V_{\text{h}}}}}$

( is the number of work cycles per revolution: 0.5 for four-stroke engines and 1 for two-stroke engines . In the case of rotary engines, due to the four- stroke working cycle, the working volume is also calculated, which is calculated from the chamber volume as follows:, where the number If the mean effective pressure of the rotary engine is to be calculated from the chamber volume instead of the working volume , this should be assumed.) ${\displaystyle i}$ ${\displaystyle i=0,5}$ ${\displaystyle V_{h}}$ ${\displaystyle V_{k}}$ ${\displaystyle V_{h}=2\cdot V_{k}\cdot n}$ ${\displaystyle n}$ ${\displaystyle V_{k}}$ ${\displaystyle V_{h}}$ ${\displaystyle i=n}$

• The indicated mean effective pressure corresponds to the indicated work (see also volume work ) on the piston. ${\displaystyle p_{\text{mi}}}$

${\displaystyle p_{\text{mi}}={\frac {W_{\text{i}}}{V_{\text{h}}}}}$

In slow-running engines ( steam engines ), a writing measuring device ( indicator ) could be connected to the working cylinder and the content of the positive area circumscribed by the pressure curve could be planimized (measured) in its diagram in order to determine the size of the indicated work. Nowadays the indicated mean pressure is determined with electronic measuring devices .

The quotient of the two aforementioned mean pressures is called the mechanical efficiency and is a dimensionless measure for all mechanical losses:

${\displaystyle \eta _{\text{mech}}={\frac {p_{\text{me}}}{p_{\text{mi}}}}}$

Air ratio and mean pressure

Medium pressure and specific consumption
diagram for the gasoline engine
mixture, bottom left "lean", top right "rich"

Gasoline engine

Otto engines are operated at an air ratio of around . This enables exhaust gas cleaning with the three-way catalytic converter. A lambda probe in front of the catalytic converter measures the oxygen content in the exhaust gas and forwards signals to the control unit of the mixture regulator. The task of the mixture regulator is to keep the air ratio close to through the injection quantity . The most efficient operation occurs with a slightly lean mixture of . The highest torque (medium pressure) is achieved with a rich mixture of approx . This is also where the highest ignition speed and reaction speed of the gasoline-air mixture are set. Regular combustion is no longer guaranteed beyond the ignition limits ( for gasoline engines) (combustion misfires). ${\displaystyle \lambda =1}$ ${\displaystyle \lambda =1}$ ${\displaystyle \lambda \approx 1{,}1}$ ${\displaystyle \lambda \approx 0{,}85}$ ${\displaystyle 0{,}6<\lambda <1{,}6}$

Diesel engine

Diesel engines, on the other hand, work with a lean mixture of up to . The highest torque (medium pressure) is normally not achieved with a rich mixture of approx. Because of the soot limit. At part load, the air ratio is greater than and reaches values ​​of up to about 5 when idling. ${\displaystyle \lambda =1{,}4}$ ${\displaystyle \lambda =1}$ ${\displaystyle \lambda >1{,}4}$

Typical values ​​for medium pressure

Typical values ​​for medium pressure

Motor type				${\displaystyle p_{\text{m}}\,[\mathrm {bar} ]}$	comment
Series engines	Diesel engine	Charged	Two-stroke marine diesel engine	20th	z. B. MAN 5G45ME
			High-performance car engine	31.5	z. B. BMW B57
			2 liter turbo diesel	20.5	z. B. VW EA189
		Free sucking	1.9 liter naturally aspirated diesel	8.5	z. B. VW EA188
	Gasoline engine	Charged	High-performance car engine	29	z. B. Mercedes-Benz M 133
		Free sucking	Car engine, 4  valves per cylinder	11.5	z. B. Mazda 2-liter MZR
			Car engine, 2 valves per cylinder	10.5	z. B. BMW M20 B20
			Motorcycle engine, 4 valves per cylinder	13.5	z. B. Kawasaki Ninja ZX 600 G
Racing engines	Gasoline engine	Charged	Formula 1 engine, 1980s	57	Honda F1 (1987), maximum mean pressure
		Free sucking	Formula 1 engine 3.0 l ten-cylinder	15th

If the effective mean effective pressure is plotted against the specific fuel consumption as a function of the air ratio of the engine, the fish-hook curve is obtained for gasoline engines .

See also

• Compression pressure

Individual evidence

1. ↑ Technikhomepage.de - The thermodynamics of the internal combustion engine
2. ↑ Honda Power Curves. (PDF; 999 kB) GrandPrixEngines.co.uk, accessed on December 2, 2015 (English). 

literature

• Richard van Basshuysen, Fred Schäfer: Handbook Internal Combustion Engine Basics, Components, Systems, Perspectives. 3rd edition, Friedrich Vieweg & Sohn Verlag / GWV Fachverlage GmbH, Wiesbaden, 2005, ISBN 3-528-23933-6

• Wolfgang Beitz, K.-H. Küttner (Hrsg.): Taschenbuch für den Maschinenbau / Dubbel 15th edition, Springer Verlag, Berlin, Heidelberg, New York, 1983, ISBN 3-540-12418-7