By Jim Kerr
New cars are very high tech machines. Everything from instrument clusters to automatic transmissions is electronically controlled, even on the lowest priced cars. There are many technologies that enhance our comfort or convenience, such as navigation systems or automatic climate controls, but when it comes to technologies, some provide more value to owners than others. One technology that offers better value to the consumer than perhaps any other is variable valve timing.
Variable valve timing has been around for many years. Honda was one of the first to make it common, with their VTEC engines, but most other manufacturers are now building engines with variable cam timing. It is one of those technologies I would look for if I were buying a new vehicle.
To better understand the benefits of variable valve timing, we should look at how it works. On a conventional engine, the camshaft is turned by a toothed belt or a roller chain driven by the crankshaft. The camshaft is a shaft with several eccentric lobes on it. As it rotates, the camshaft operates the valve train, opening the valves and letting them close. The shape of the eccentric determines when the valve opens and closes, and how high they lift. The longer a valve remains open, the more air and fuel can enter past the intake valve, or exhaust gases can exit past the exhaust valve.
Automotive engines operate over a wide rpm range and operate best with different valve timings at low speeds compared to high speeds. Variable valve timing offers the best timing at both low speed and high speed operation. At low speeds, the intake valve can be opened later and exhaust valve closed sooner. This smoothes out engine idle and produces strong low speed torque and throttle response. At higher engine speeds, this valve timing would restrict airflow and reduce high engine power. But by using variable valve timing to change the position of the camshaft in relation to the crankshaft, the intake valve can open sooner and the exhaust valve close later. This allows more airflow through the engine at high engine speeds to produce more power; however, this setting would produce poor low speed performance and a lumpy idle.
Hot Rodders often changed camshafts to change the valve timing but because these engines didn’t have variable valve timing, the camshaft design had to be selected that would only produce power in a narrow operating range. Fuel economy and low speed power was poor. Now, variable valve timing provides the best of both.
Mechanically, variable timing requires only a few components. Similar to previous engines, the camshaft is driven from the crankshaft by a chain, but instead of the cam sprocket being bolted directly to the camshaft, it is now able to rotate several degrees on a hub. The hub is bolted to the camshaft. In the most common design, there are chambers between the cam sprocket and hub with vanes connected to either the sprocket or the hub. Oil from the engine oiling system is fed into these chambers and the oil pressure moves the vanes, changing the position of the cam sprocket on the hub. An electric solenoid that is controlled by the engine computer controls the flow of oil to vary the cam sprocket position and vary the valve timing.
A simple change of computer programming can produce different engine operating characteristics. For passenger vehicles, the programming maximizes fuel economy and power. A great example of how effective this is can be found in the 2011 Mustang’s new 5.0 litre engine. This V8 engine has double overhead camshafts with variable cam timing on each of the four camshafts. The engine idles smoothly and quietly, is rated at 7.6 litres per 100 km fuel economy on the highway and produces an exciting 412 horsepower. During cruise operation, the computer can vary the valve timing to keep some of the exhaust gases in the engine cylinders. This lowers cylinder temperatures to reduce NOx emissions and does it without the necessity of an exhaust gas recirculation system.
There are many engines that have variable valve timing for only the intake valves or exhaust valves – not both. These engines are superior to engines without variable valve timing but the most efficient designs now have variable valve timing on both. Because this technology increases fuel economy and engine performance and lowers emissions, there are no real drawbacks. It’s definitely a technology that benefits consumers.