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Varying timing for power and efficiency

 

One of the smartest pieces of auto engineering in the last couple of decades has been variable valve timing (VVT), a marvellous form of multi-tasking that you’d think must have been invented by a woman.

It makes engines smoother, delivers more power and at the same time, reduces emissions and improves fuel economy. Should you look for a car with variable valve timing? You betcha. Mind you, these days it’s on almost everything. Alfa-Romeo was first to use VVT in a production car, in 1980, but it took the Japanese, notably Honda and Nissan, to refine the system and make it widely available in the mass market. Internal combustion engines have intake and exhaust valves that open at the proper time to let in air and fuel and to let out exhaust gases. Before VVT, an engine used one or more camshafts to open and close the valves. The cam or cams were turned by a timing chain connected to the crankshaft. However, the dynamics of airflow through a combustion chamber change according to the engine’s revolutions per minute (rpm). This “one setting suits all” arrangement wasted fuel, reduced performance and did little for lowering exhaust emissions. Variable timing allowed engineers to tune valve operation for top performance and efficiency throughout the rev range, to achieve the big leap forward. Honda, for example, came up with two sets of camshaft profiles, one for low- and mid-range rpm and the other for high rpm. An electronic switch shifts between the two profiles at a specific rpm. Many VVT arrangements are similar, but each manufacturer will give all sorts of reasons about why its particular approach is best.


For example, Toyota went with a hydraulic instead of mechanical arrangement to change the cam phasing. VVT technology is always improving.  A goal is to make timing infinitely variable in the easiest and most reliable way possible. BMW's Valvetronic system is regarded as something of a landmark in the search for valve-timing Nirvana. Valvetronic steplessly alters the timing of each camshaft and the lift of the inlet valves. It was based on the company’s double VANOS system. Nissan has recently come up with technology that combines its Variable Valve Event and Lift (VVEL) and continuous valve timing control (C-VTC) to deliver what it says will be significantly enhanced performance. A clincher is that it can cut carbon-dioxide emissions by up to 10 per cent. Conventional engines control air intake using a throttle valve, VVEL-equipped engines do this directly and continuously at the intake valves. Diesel engines also benefit from VVT systems. However, their special requirements may result in solutions that differ significantly from those fitted to gasoline engines.

By the way, don’t confuse variable cylinder management for variable valve timing. That’s another technology entirely that shuts down cylinders when they’re not needed, like when cruising on the open road, primarily as a means of reducing fuel consumption. Although adding a layer of complexity to the motor, VVT mechanisms have generally had a good track record for reliable operation and are by now well understood by mechanics. There’s no reason to shun it when hunting for a used car.


What’s in an acronym?

The acronym inventers had a field day with the arrival of VVT.
It seems every manufacturer had to coin its own terminology. This is just a partial list.
BMW: Valvetronic, VANOS and Double VANOS
Honda: VTEC and i-VTEC
Hyundai: CVVT
Lexus: VVT-iE
Mitsubishi: MIVEC
Porsche: VarioCam and VarioCam Plus.
Subaru: AVCS and AVLS
Toyota: VVT, VVT-i and VVTL-i
Volvo: CVVT

What is air suspension?

An alternative to steel coil or leaf springs that uses bellows-like "bags" of pressurised air. It's versatile; ride quality can be adjusted from soft to sportingly hard and ground clearance adjusted easily.


 


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