Valvematic a new way to control engine speed

Hey there, time traveller!
This article was published 10/01/2014 (4673 days ago), so information in it may no longer be current.

Toyota has introduced a new variable-valve technology, and unlike most new technologies that first appear on higher-priced vehicles, Toyota is introducing this on their Corolla Eco models. They call this new feature Valvematic.

In almost all vehicles, engine speed is controlled by the throttle. Open the throttle to let more air in and the computer automatically increases fuel delivery as engine r.p.m. goes up. Some vehicles use mechanical linkages or cable to connect the gas pedal to the throttle plates, and on many newer vehicles, the throttle is electrically controlled by the engine computer as it receives information from the gas pedal. On Toyota’s Valvematic system, engine speed is controlled much differently. Airflow into the engine is controlled by varying the opening height of the intake valves.

The concept is not new. BMW has used it under the Valvetronic name and it works well, but typical of German ingenuity, there are many parts and it is a complex mechanical/electronic system. Toyota’s Valvematic takes a much simpler approach to varying valve lift.

Before looking at Valvetronic operation, there are a few things to understand. First, this system is different from Variable Cam Timing, which is used on many newer vehicles as well as still in use on the Corolla. Also, throttle plates can be installed on the engine, but are there only to provide a “safety” system. Almost all the time, the throttle plates can remain wide-open, which eliminates any air restrictions into the intake manifold. There is no vacuum in the intake manifold so pumping losses in the engine as air enters the cylinders are reduced, providing better efficiency and fuel economy. Finally, there some manufacturers who have used two-stage valve lift or opening (Honda for example), but the Toyota system is completely variable, all the way from 1 millimetre to 11 mm in lift height.

Mechanically, the Valvetronic system is a relatively easy addition to the existing engine design. The camshaft mounting plate that bolts to the top of the cylinder head is replaced with a Valvetronic plate that contains the camshafts and the Valvetronic hardware. There is no change in height of the engine. Other items such as valves, valve springs, rocker arms and even the camshafts remain essentially the same, with some differences to camshaft profile on the intake cam. What is added is a second shaft on the intake side of the cylinder head that holds oscillating arms and an electric motor on the end of the shaft to control the amount of valve lift.

In operation, the intake camshaft pushes on the oscillating arm, which in turn pushes on the rocker arm to open the valve. The key to the variable valve lift is the position of the oscillating arm. Every oscillating arm has a ramp-type profile that pushes on the rocker arm. As the oscillating arm is moved closer or further away from the rocker arm contact point, the ramp profile changes and so does the amount of lift.

To move the oscillating arm, the additional shaft has helical gears on it for each valve mechanism. The oscillating arms have a bearing pivot with an internal helical gear on it as well. The computer-controlled motor, which is mounted externally on the engine on the end of the additional shaft, moves a plunger internally in the shaft that causes the helical gears to rotate back and forth slightly. As the helical gears rotate, they cause the oscillating arm to change its fulcrum point where it pivots and this causes different parts of the ramp to contact the rocker arms, changing the lift.

It may sound complex, but in reality it is a relatively simple mechanical design that doesn’t require oil pressure to operate. The benefits are big, however. There is quicker engine response to driver demand and an increase in top-end power by at least 10 per cent due to the increased airflow into the engine. It is also efficient, improving fuel economy by five to 10 per cent depending on driving conditions. For now, the system is only on the Corolla but it wouldn’t surprise me to see it expand quickly to other models and perhaps even other manufacturers.

 

Jim Kerr is a mechanic, instructor of automotive technology, freelance journalist and member of the Automobile Journalists’ Association of Canada.

kerr.jim@sasktel.net