Why can VTEC, a variable valve technology, make cars run faster?

The key to control engine ventilation is to control the intake and exhaust time. At high speed and low speed, the engine has different requirements for intake and exhaust time. With the increase of rotating speed, the intake and exhaust time becomes shorter and shorter, and the ventilation efficiency becomes worse and worse. In order to improve ventilation efficiency, it is necessary to adjust the intake and exhaust time. That is to say, the intake valve opens ahead of time and the exhaust valve closes later. If there is no variable valve, the designer should find a calibration suitable for the vehicle type. For example, ordinary cars are more suitable for low speed, and performance cars are more suitable for high speed.

If there is a variable valve, the engine can have better power in a wider speed range. In other words, both high speed and low speed can be considered. Neo with Nissan 2.0 engine? Compared with ordinary variable valves, the red line speed and maximum horsepower of VVL are increased by 25%. Fiat 1.8 variable valve engine can provide 90% of the maximum torque between 2000 and 6000 rpm. In some variable valve engines, the valve lifter also changes according to the speed. The higher the rotating speed, the greater the lift, which contributes to the efficiency of air intake and exhaust (when the rotating speed is low, the excessive lift will lead to the instability of performance fees and other problems).

Variable valves will affect the efficiency of waste recycling. Valve overlap can reduce fuel consumption during low-load cruising. Because a part of tail gas will participate in the calculation of combustion and performance cost again in the next cycle. This causes the engine to work at a sparse performance cost. VTEC system can be regarded as two sets of cams different from instant and elevator. The switching speed between two discontinuous cam states is 4500. A racing cam with similar design can add 30 horsepower to the 1.6 engine when the red line speed is close to 8000. But in order to maintain such a large horsepower, it is necessary to maintain the speed through frequent shifting. The low-speed cam only needs to adapt to the speed of 4500 (other engines may need to adapt to 6000), but the torque is not very favorable.

At low speed, the three rocker arms are independent. At medium and high speed, different cams are operated by the swing arm of the hydraulic lock to switch different durations and elevators. When the rotational speed is low, the air inlet and the air outlet are in a low rotational speed state; When the speed is moderate, the intake valve is in a high speed state and the exhaust valve is in a low speed state. Hydraulic control variable timing variable valve system is the simplest, cheapest, most used and least improved in performance. The structure is simple (only one hydraulic control structure), which can increase the torque in a larger speed range, the lift is unchanged, the valve opening and closing time is unchanged, and the high-speed horsepower is insufficient.