Throttle valve (motor)

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Throttle valve of a powerful Otto engine (red components; here 100% closed; without air filter )
Scheme of a throttle valve
Throttle valve, approx. 30% open

The throttle valve is the component of a gasoline engine with which the generated torque is adjusted by throttling the delivery rate ( quantity control ). It is located in the intake tract between the air filter and intake manifold or the fan-shaped intake manifold of the engine. In the case of carburetor engines , it is an integral part of the carburetor ; in the case of injection engines , it sits in a throttle valve housing. Exceptions include a. Racing engines and engines in sporty series vehicles (e.g. BMW M3 ), where each cylinder has its own throttle valve ("individual throttle").

background

Four-stroke cycle of an ideal, typically slow-running gasoline engine :
In stroke 1 , the piston
sucks mixture into the combustion chamber through the open inlet valve (left)

Otto engines are ideally operated with a constant combustion air ratio (or air number λ) of 1.0. In order to vary the torque (and power), the mixture quantity (quantity of fresh charge ) must therefore be metered with a throttle valve. Because the quantity of the fuel-air mixture is regulated, one speaks of quantitative mixture regulation or "quantity regulation" .

During the intake stroke, naturally aspirated engines generate a vacuum ( suction pressure ) through the piston moving downwards in the cylinder , which draws in the fresh charge. Carburetors or manifold injection add the dosed amount of fuel to the sucked in air. This initially does not result in a gaseous mixture, but rather a mixture of fresh air with as finely atomized fuel as possible , which only evaporates when the cylinder is heated and contributes slightly to its cooling.

construction

BMW M10 engine (BMW 2002tii) with a central throttle valve that branches off to four intake pipes

There are central injections in which the injection nozzle is located in front of the throttle valve, with multipoint injections there is usually one nozzle per cylinder in the intake manifold, i.e. after the throttle valve ( intake manifold injection ).

The throttle valve is usually a circular sheet metal which is rotatably mounted on a shaft and is arranged in a cylindrical tube and closes the tube in the closed state; Throttle valves deviating from the circular shape are rarer. There are also arrangements with several throttle valves. However, these do not supply a cylinder, but (like a flap) first of all the intake tract, from which the air is distributed to the individual cylinders. If the number of throttle valves and cylinders in an engine is the same (individual throttles), they are located directly on the intake ports and not in front of the manifold. Almost all sports engines are designed this way. Such a system can also be implemented as a register actuation in order to achieve a more comfortable engine run.

Dynamic control

Pressing the accelerator pedal opens the throttle valve. The flap does not necessarily open directly depending on the pedal position. The electronic transmission of the accelerator pedal position to the throttle valve means that a control unit can actively intervene in the opening of the valve. Both the speed of opening and the position of the flap are influenced as a function of the pedal position. This should improve the drivability of powerful engines or increase driving comfort. Assistance systems such as a cruise control system or slip control (traction control) are also possible.

Idle setting

If the throttle valve were to close the pipe cross-section completely while idling , the engine would die due to a lack of mixture for combustion. Over the years there have been various systems to provide the necessary amount of mixture for idling in the engine. The simplest method is a mechanical stop that leaves the flap a little open. In engines with carburettors that work with a constant cross-section, petrol is released into the throttle valve gap via an opening when idling. With the idle adjustment screw and the adjustable throttle valve stop , the amount and air ratio (composition) of the idle mixture can be adjusted.

On engines with manifold injection, the idle speed adjustment screw closes an additional hole that guides air past the throttle valve. The idling speed of the engine can be adjusted with the position of the screw. However, these systems required an additional device to increase the idle speed during a cold start. This was achieved, for example, with an additional air valve at the Bosch K-Jetronic . With the advent of fully electronic injection systems, the amount of air for idling was provided by a servomotor ( idling regulator or idling control valve ), which can actively raise and lower the idling speed at any time. Modern vehicles with electronic throttle valves often get by without an additional idle regulator, the throttle valve is also actively controlled by the control unit for idle regulation.

Regulation of the throttle valve position

If available, the position of the throttle valve is measured via the voltage drop on a potentiometer that is attached directly to its axis. With newer throttle valves, the setting angle is measured by two fixed Hall sensors and a rotatable magnet. The measured value is sent to the engine control unit , which uses the sensor data and the setpoint to regulate the position of the throttle valve. The control unit also calculates the correct fuel injection quantity from the position together with a few other values. The Bosch KE-Jetronic occupies a special position . This works - depending on the application - only with a full load switch or with an idle switch that transmits a signal to the control unit to influence the engine map.

Drive the throttle valve

In "classic" engines, the throttle valve is actuated directly by the accelerator pedal, for example via a cable or linkage; in modern, electronically controlled engines, this is done via an electric drive ( direct current motor with gearbox , rarely direct drive without gearbox or stepper motors ). This is called e-gas (electronic gas pedal ) or drive-by-wire .

Executions

In carburetors or engines with retrofitted gas systems based on the Venturi principle , the throttle valve is located at one point on the intake funnel behind the Venturi tube , where it merges into a cylindrical cross-section. In injection systems, it is located in the throttle valve housing.

Single throttle / single throttle valve injection

High-performance and racing engines often use one throttle valve per cylinder to improve responsiveness and gas flow at high speeds, while engines with a lower liter output get by with a total of one throttle valve. Engines with sharp camshafts also benefit from the decoupling of the oscillating air columns in the intake tract thanks to individual throttles. With carburetors, this has often been achieved through the use of double carburetors (e.g. 2 double carburetors on a 4 cylinder engine). In the case of injection systems, the same basic concept is achieved through corresponding intake ports with individual throttles, which often also accommodate the injection valve. This concept is e.g. B. used in the BMW M3 . In motorsport, this - like flat slide systems - is often used together with an airbox .

Flat slide

A special form for racing engines z. B. racing touring cars or formula racing cars (sometimes also with motorcycles) forms the use of flat slide valves in connection with injection systems. For each cylinder, a "slide" is pushed to the side like a guillotine , thus exposing the entire cross-section. At full load, unlike with the individual throttle, there is no throttle valve and throttle valve shaft in the air flow.

Valvetronic

The fully variable valve train Valvetronic developed by BMW controls the amount of mixture via the lift of the intake valves . In this way, the throttle losses of the throttle valve can be avoided and higher degrees of efficiency can be achieved. The lower throttle losses can be achieved through the better mixture preparation and the higher internal EGR rates that this makes possible, which generate a lower negative pressure in the cylinder. A throttle valve is still installed for emergency air operation. In normal operation it is open to the mechanical stop.

Diesel engines

In principle, conventional diesel engines do not require a throttle valve. In contrast to the Otto engine, the torque is set here by changing the air ratio λ. It is therefore not necessary to regulate the amount of mixture by throttling the intake air. A throttle valve can be used for purposes other than torque adjustment on a diesel engine, which is why some diesel engines still have throttle valves.

In order to prevent the diesel engine from running on or coasting down after the fuel supply has been switched off, a resistance can be generated by closing the exhaust or the intake pipe, which stops the engine immediately. In order to have a positive influence on exhaust gas formation or exhaust gas cleaning, it can make sense to limit the amount of fresh air drawn in, for which purpose a so-called swirl valve is used, which in principle works like a throttle valve. This is partially closed in partial load operation so that a higher exhaust gas recirculation rate can be used. Less oxygen during combustion lowers the combustion temperature, which results in significantly fewer nitrogen oxides. If the engine is enriched by increased fuel injection in addition to throttling, the fuel no longer burns completely; the hydrocarbons and carbon monoxide contained in the exhaust gas can be used for LNT catalyst regeneration .

Pre-chamber diesel engines from Daimler cars were a specialty. These had a throttle valve to enable vacuum-pneumatic readjustment of the centrifugal governor of the diesel injection pump in the partial load range.

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 .
  • Max Bohner, Richard Fischer, Rolf Gscheidle: Expertise in automotive technology. 27th edition, Verlag Europa-Lehrmittel, Haan-Gruiten, 2001, ISBN 3-8085-2067-1 .
  • Helmut Hütten: Fast engines. Dissected and coiffed. 9th edition, Motorbuch-Verlag, Stuttgart 1990, ISBN 3-87943-974-5 .