What is a VVT engine

see also | Variable timing | Performance increase | Filling | Performance | Multi-valve | Supercharging | Comparison P / M |

 
Time and again, nature serves as a model for the development of innovative technologies. The comparison with the breathing technique of humans makes, for example, the functioning of VALVETRONIC clear:

When exerting a lot of effort, the person breathes deeply and long. If he needs less air, then he does not cut back the air supply by covering his nose and mouth a little, but rather he breathes shorter and shallower. In conventional gasoline engines, the throttle valve corresponds to the locked nose or mouth. VALVETRONIC with large valve lift (= deep, long breathing) or small valve lift (= shallow, short breathing) enables you to breathe like nature - without throttling and always efficiently.



Valvetronic

 


The innovative VALVETRONIC technology offers excellent cold start behavior and extremely smooth running due to its functionality. This is due to the principle of the fully variable valve train. In partial load operation, the VALVETRONIC works with comparatively small valve lifts of 0.5 to 2.0 millimeters. As a result, the fuel mixture only enters the combustion chamber through a narrow gap. Due to the high inflow speed, the mixture is ideally atomized even when the engine is cold. A mixture mist that is as fine as possible is also a prerequisite for a quick and even combustion process. Even smoother running is achieved at low loads, as the valves then only lift slightly.

 

BMW 316ti cylinder head

with Valvetronic

 

VALVETRONIC enables a fully variable valve lift of 0.0 to 9.7 millimeters. It is achieved by an intermediate lever that can be adjusted by an electric motor, which is placed between the camshaft and the rocker arm that actuates the valve. The VALVETRONIC not only works precisely, but also extremely quickly. The adjustment from minimum to maximum stroke can be done in just 300 milliseconds. In order to be able to use these setting options, very powerful controls are necessary. The VALVETRONIC therefore has its own 32-bit computer networked with the engine control.

 


Inlet valve with Valvetronic

The mixture supply is controlled via a variable valve lift without a throttle valve.

 

For the BMW driver, the new 316ti four-cylinder with VALVETRONIC means lower consumption, reduced exhaust emissions and, at the same time, better responsiveness and smoother running.

According to the EU, the 85 kW / 115 PS strong and 201 km / h fast 316ti compact consumes 6.9 liters of Super per 100 kilometers - 0.7 liters less than the previous model with 77 kW / 105 PS and well over a liter less than all of them Competitors in this class. The 316ti compact fulfills the EU4 pollutant standard.

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Valvetronic

Although all vehicle manufacturers like to speak of a "new engine" even with minor modifications, little has changed in the past decades with regard to the crucial components. So also with the mixture preparation. Nowadays, ignition is carried out using an electronic map and injection systems have replaced the carburettor, but the most important component still works as it did in the childhood days of engine construction: The throttle, installed somewhere in the intake tract, determines the volume of air or ignitable mixture that gets into the cylinder with its opening. The driver controls the position of the throttle valve with the accelerator pedal. What is unsatisfactory about this solution are the losses caused by undesired turbulence on the throttle valve.

 
fully variable valve lift

Valvetronic has now abolished the throttle valve. In future, their function will be taken over by the inlet valve (1). The inlet valve is not rigidly driven by the camshaft (2), but by means of an eccentric shaft (3) and intermediate lever (4). If the driver steps on the gas, an electric motor turns this eccentric shaft. The intermediate lever transmits the rotation of the cams, depending on the position of this shaft, to the valve.As a result, the stroke is variable. When the throttle is low, the valve has only a small stroke; when the throttle is full, the valve opens further.


 

 

What does the effort actually bring?

In engines with a throttle valve, when the intake valves open, the volume of the intake manifold between the valve and the throttle valve must be overcome before the air starts moving in the direction of the cylinders. The intake manifold volume thus forms a damper that reduces the response behavior. If it is possible to switch off the throttle valve and move the inlet valve to load control, then you can expect lower consumption, increasing power and a drastically better response of the engine to the command of the accelerator pedal. However, the Valvetronic motor is not dethrottled because the throttle valve is no longer used. There is still a throttle point - namely at the valve gap. However, dethrottling was made possible by the fact that, as a result of the better mixture preparation, a higher (internal) EGR rate can be used.

The reason for this is a significantly better atomization of the gasoline injected into the intake manifold at idle and in the lower partial load, because the intake mixture is accelerated enormously in the narrow valve gap and the resistance of the throttle valve and damping volume are eliminated. Eventually, the engine's cold start capability is dramatically improved. BMW also mentions a reduction in fuel consumption of 10% compared to the very good predecessor engine and an increase in torque in the lower speed range. This means that the consumption benefits mentioned for direct petrol injection can be achieved in a simpler way. BMW is by no means fundamentally against direct injection, but the previous systems are considered to be in great need of improvement because of the jagged piston crowns with the result of increased consumption at high load and speed and the increased tendency to form soot.

However, like most diesel engines, the BMW engine has a throttle valve, which is only used for correction and not for load control. This takes place exclusively via the variable lift of the inlet valves, with the timing of the inlet and outlet also being adjusted using the well-known Doppelvanos system by turning the camshafts. The lift of the exhaust valves, on the other hand, is not adjusted; the exhaust valves are operated as usual by the camshaft via roller rocker arms.

 

Super-class mechanics

Stroke between 0 and 10 mm


The new BMW four-cylinder with a displacement of 1.8 liters has four valves per cylinder, whereby, as usual, one camshaft operates the intake valve and the other operates the exhaust valve. Both rows of valves are operated by roller rocker arms. The exhaust camshaft lies directly above the rocker arms, while additional levers are arranged between the intake camshaft and the roller rocker arms. As usual with BMW, both camshafts are driven by chains; both are rotated by the actuators known as Vanos.

But these little intermediate levers on the inlet side have it all. They consist of a fine-grained special steel alloy and are manufactured with high precision using the investment casting process. Their contact surfaces with the adjustment shaft and with the roller rocker arms are ground to a thousandth of a millimeter in order to eliminate any deviation between the valve lifts. Above the intake camshaft is a second shaft mounted in the cylinder head, which is rotated by a VDO servomotor via a worm gear. It carries eccentrics, which are responsible for adjusting the valve lift and also prevent the intermediate levers from becoming independent. Because there is no “fixed” guide for the intermediate levers, they hang “loosely” between the adjustment shaft and the rocker arm. In the middle they have a role.

A hairpin spring presses the levers against both the camshaft and the adjustment shaft, while the hydraulic valve clearance compensation ensures that there are no rattling gaps at any point. On the underside, where they rest on the rocker arms, the contact surfaces are ground in a precisely determined curve shape that slides back and forth on the rollers with the cam movement. When idling, the sliding surface ensures a valve lift of only about 0.25 mm, while at full load the inlet valves open by 9.7 mm. With the same construction, however, one valve could also remain closed and only the second open. Or both valves remain closed if cylinder deactivation is planned. To do this, however, the eccentric shaft would have to be changed or divided into several individual sections.

The four-cylinder is designed so that it locks at 6500 revolutions per minute, while maximum output is achieved at a moderate 5500 revolutions per minute. The stability of the entire valve train is guaranteed up to around 7000 revolutions per minute.

VALVETRONIC and the Otto direct injection

Theoretically, there is hardly anything more natural for an engine developer than a direct-injection gasoline engine - with this concept, he can combine the performance potential of the gasoline engine with the unsurpassed consumption behavior of diesel. To date, however, this project has failed due to two factors: The availability of sulfur-free fuel required for a gasoline direct injection engine and the lack of durability of the indispensable and complex catalyst systems.

VALVETRONIC manages without the complex exhaust gas aftertreatment, which is essential for the gasoline direct injection engine., In contrast to the direct injection engine, a VALVETRONIC engine does not depend on sulfur-free fuel. The low consumption is achieved with all commercially available petrol fuels, so that the advantages of VALVETRONIC can be used without additional financial or time expenditure.

 

 

see also | Variable timing | Performance increase | Filling | Performance | Multi-valve | supercharging | Compare P / M |

Sources: BMW and various unknown sources


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Author: Johannes Wiesinger

edited: 02/19/2015
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