Variable Valve Timing, often abbreviated to VVT, is a type of piston engine technology that deliberately delivers inconsistent timing of the intake and/or exhaust valves. The benefit of this is improved gas mileage and flexibility for an engine to deliver peak performance over a variety of driving conditions. For example, traditional piston engines often are required to sacrifice low-end torque for high-end power (or vice versa). A VVT engine more easily accommodates both of these preferred performance conditions.
Variable Valve Timing - How it Works
As with traditional piston engines, VVT engines use cams on a camshaft to drive the flow of air into the intake and exhaust valves. The timing of this valve lift directly affects how much air is taken in during each engine cycle. At times when the engine requires more air flow (for example high speeds or acceleration), a traditional piston engine often does not allow enough air to flow during each cycle, resulting in lower output performance. Conversely, a traditional piston engine that has been designed to feature longer exhaust and intake cycles will result in reduced fuel efficiency at slower speeds.
There are several proprietary VVT engine technologies that work slightly differently to prolong exhaust and intake cycles at high speeds and reduce cycles at slow speeds. The three major solutions to varying the valve timing of an engine are as follows:
- The actual timing of the intake or exhaust valves are slowed or sped up as needed
- Two sets of cam lobes are utilized and switched between as needed
- Timing and lift is continuously altered for maximum efficiency (called continuous variable valve timing)
History of Variable Valve Timing (VTT)
Variable valve timing was originally used in the 19th century to power steam engines. Fiat patented the first variable valve timing system for automotive use in the late 1960s. GM also patented an early system in 1975, but scrapped it due to lift problems. It was not until 1980 that a vehicle with VVT technology was made available for the North American market. This was achieved with the debut of the 1980 Alfa Romeo Spider. Throughout the 1980s and 1990s, many automakers followed suit by introducing VVT engines of their own. Notable landmarks include the Nissan NVCS (Nissan Valve-Timing Control System) in 1986 and the Honda VTEC system in 1989.
Due to increased pressure from the EPA and other government agencies, improved fuel efficiency is becoming a major concern for automakers. Today, many of them are turning to VVT technology to accommodate stricter rules and regulations regarding vehicle emissions and efficiency.
Proprietary Terms for VVT
While some automakers simply use "variable valve timing" or "VVT" to denote an engine with such technology, many manufacturers use proprietary technology and therefore affix a proprietary term to distinguish their engines from the competition. Following is a list of terms used by each automaker to denote an engine with VVT technology:
- Alfa Romeo - Twinspark technology
- Audi - VVT
- BMW - Valvetronic, VANOS and Double VANOS
- Ford - Variable Cam Timing
- GM - Double Continuous Variable Cam Phasing (DCVCP), Alloytec and Variable Valve Timing (VVT)
- Honda - VTEC, iVTEC and VTEC-E
- Hyundai - MPI CVVT
- Lexus - VVT-iE
- Mazda - S-VT
- Mitsubishi - MIVEC
- Nissan - N-VCT, VVL , CVTC and VVEL
- Porsche - VarioCam and VarioCam Plus
- Subaru - AVCS and AVLS • Toyota - VVT, VVT-i and VVTL-i
- Volkswagen - VVT
- Volvo - CVVT