AUTONEWS

This Is How Variable Valve Timing Allows a Modern Car to Save Fuel

You could say that modern cars are under great pressure to save money. I'm not just referring to costs, as some users complain, but especially in terms of fuel consumption and emissions due to the increasingly restrictive European Union pollution regulations. Meanwhile, we as drivers expect good performance from the latest engines. Could that be a contradiction? Perhaps not, thanks to the focus of this article, variable valve timing.

The goal of this technology is for the valves in an engine's cylinder head (those that allow gases to enter or exit) to no longer open or close at the same time as they did years ago. Thanks to variable valve timing, the opening times can now be adapted to the driving situation at any given time. Because the demand for fuel is not the same when driving in a city as on a fast road like a highway. Thus, as our colleagues at Auto Motor und Sport tell us, what changes most frequently are the control timings and valve overlap. The former defines when the camshafts open or close the valves in relation to the crankshaft, while the latter is the time the intake and exhaust valves are open at the same time. Both influence the filling of the combustion chamber and, therefore, the resulting engine torque, which ultimately translates into power.

The end result is that the engine consumes less fuel with the same or greater power, something that is closely related to emissions. This is especially noticeable in hybrid or mild hybrid engines, where the best combination of work performed by the thermal and electrical components is sought.

How Variable Valve Timing Works...As part of the work performed by a combustion engine, the proper air-fuel mixture determines performance and efficiency. And for this to be optimal, the cylinder head valves must be precisely controlled. In older engines, this always happens within the same "cycle," regardless of how hard you press the accelerator. It's certainly simpler, but completely outdated.

Keep in mind that depending on the accelerator pedal's demands, the engine may need air sooner or later. For example, under high load, it must flow quickly to the cylinders, so variable valve timing activates the longest valve overlap phase. On the other hand, with more relaxed driving, the overlap is significantly less.

Returning to those older engines, since their valves always work the same in both situations, unnecessary losses occur, either due to improper filling of the combustion chamber or resistance of a throttle valve.

Advanced systems can do even more. Even within this world of valve timing, there are even more advanced systems, which, as you'll understand, are those marketed by premium brands. These can control not only intake timing but also exhaust timing, further enhancing the positive effects.

They do this with systems such as valve lift control. This requires the inclusion of movable camshafts with different cams within the engine or special solutions such as BMW's Valvetronic system. In this system, the valves are continuously controlled to save energy and improve engine response time. Audi's AVS is similar, which in this case performs this operation through a movable sleeve that activates two cam contours with different strokes.

To this, we must add the cylinder deactivation common in some engines offered by Volkswagen Group brands. When driving at a constant speed with little accelerator pedal input, some cylinders are shut down individually. This can happen between 1,300 and 3,200 rpm or at low torque, completely automatically and without the driver noticing. Ford has also included it in its EcoBoost engines, in this case by "releasing" one of the three cylinders that make up the engine block.

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