Variable Valve Timing

There are a couple of novel ways by which carmakers vary the valve timing. One­ system used on some Honda engines is called VTEC.

The variable cam system used on some Ferraris

 

VTEC (Variable Valve Timing and Lift Electronic Control) is an electronic and mechanical system in some Honda engines that allows the engine to have multiple camshafts. VTEC engines have an extra intake cam with its own rocker, which follows this cam. The profile on this cam keeps the intake valve open longer than the other cam profile. At low engine speeds, this rocker is not connected to any valves. At high engine speeds, a piston locks the extra rocker to the two rockers that control the two intake valves.
Some cars use a device that can advance the valve timing. This does not keep the valves open longer; instead, it opens them later and closes them later. This is done by rotating the camshaft ahead a few degrees. If the intake valves normally open at 10 degrees before top dead center (TDC) and close at 190 degrees after TDC, the total duration is 200 degrees. The opening and closing times can be shifted using a mechanism that rotates the cam ahead a little as it spins. So the valve might open at 10 degrees after TDC and close at 210 degrees after TDC. Closing the valve 20 degrees later is good, but it would be better to be able to increase the duration that the intake valve is open.

Ferrari has a really neat way of doing this. The camshafts on some Ferrari engines are cut with a three-dimensional profile that varies along the length of the cam lobe. At one end of the cam lobe is the least aggressive cam profile, and at the other end is the most aggressive. The shape of the cam smoothly blends these two profiles together. A mechanism can slide the whole camshaft laterally so that the valve engages different parts of the cam. The shaft still spins just like a regular camshaft -- but by gradually sliding the camshaft laterally as the engine speed and load increase, the valve timing can be optimized.
Several engine manufacturers are experimenting with systems that would allow infinite variability in valve timing. For example, imagine that each valve had a solenoid on it that could open and close the valve using computer control rather than relying on a camshaft. With this type of system, you would get maximum engine performance at every RPM. Something to look forward to in the future...
For more information on camshafts, valve timing and related topics, check out the links on the next page

Variable Valve Timing

 By Roy Berndt

With all the scientific technology computing power out there in the world today I have yet to hear an answer to the age old question: What came first, the chicken or the egg? Well the automotive industry has a similar dilemma that continues to plague its existence. Which is more desirable, large displacement and horsepower or smaller displacement and better fuel mileage?
What if you could have a large displacement, high-horsepower engine that you could call upon when you needed it and have a smaller displacement better fuel mileage engine available when you don’t.  For those of you old enough to remember, that would be a “Doublemint” commercial: double your pleasure and double your fun!
Well, that technology is here right now and growing in popularity and it has more names and acronyms than “Carter has pills” (another commercial, for those old enough).
The one that is probably best known is “Displacement on Demand” (DOD), but there is also “Active Fuel Management” (AFM) that GM has laid ownership to; “Multiple Displacement System” (MDS) from Chrysler; “Active Cylinder Control” (ACC) at Mercedes Benz; and “Variable Cylinder Control” (VCM) from Honda.
You may also hear it called cylinder deactivation. But no matter what you call it, the bottom line is that you have the ability to go down the road driving your gas guzzling road thumping V8 (or bigger) and have half or more of your cylinders go dead, start sipping fuel, run more efficiently and gain an increase of fuel economy up to as much as 20 percent. Imagine being on the highway with the cruise on and only 4 cylinders pushing you down the road (because you only need 30 percent of your available power). No one has to know, and you still retain your V8 dignity. It’s here today, it really works and it will be an even bigger part of future engine development of all sizes.  

HOW IT WORKS
Cylinder deactivation is used to reduce the fuel consumption and emissions of an internal combustion engine during light load operation (LLO).  Since we already know that the vehicle only needs 30 percent of its maximum engine power to keep moving (a body in motion tends to stay in motion; see Newton’s Laws), there are other issues that come into play during this reduced demand for power. 
The throttle valve is nearly closed, and the engine is literally starving for air, which is a very inefficient point of operation known as pumping loss. Larger displacement engines are throttled back so far during light load that the cylinder pressure at top dead center (TDC) can diminish as much as 50 percent. Low cylinder pressure means low fuel efficiency.
The use of cylinder deactivation at light load means there are fewer cylinders drawing air from  the intake manifold, which works to increase its fluid air pressure.  This reduces pumping losses and increases pressure in each operating cylinder.  Fuel consumption can be reduced by approximately 20 percent in highway conditions.
Cylinder deactivation is achieved by keeping the intake and exhaust valves closed for a particular cylinder, which creates an “air spring” in the combustion chamber. The trapped exhaust gases from the previous charge burn are compressed during the pistons up stroke and push down on the piston on the downward stroke. The compression and decompression of the trapped exhaust gases have an equalizing effect and overall there is virtually no extra load on the engine. 
The engine management systems also cut fuel delivery to the disabled cylinders.  The transition between normal engine operation and cylinder deactivation is kept smooth using changes in ignition timing, cam timing and throttle position thanks to electronic throttle control (ETC), commonly known as “drive by wire.” Two issues to overcome with all variable displacement systems are the unbalanced cooling and vibration tendencies of cylinder deactivation engines. This is done in different manners by different manufacturers and designs.

HISTORY
The oldest engine technological predecessor for the variable displacement engine is the hit and miss engine, developed in the late 19th century.  These single cylinder stationary engines had a centrifugal governor that cut the cylinder out of operation so long as the engine was operating above a set speed typically holding the exhaust valve open.

Cadillac L62 V8-6-4
The technology was first experimented on multiple cylinder engines during WWII, but was truly pioneered in 1981 on Cadillac’s ill-fated L62 V8-6-4 engine.  GM paved the way (and lost its way) with this innovative engine. Unfortunately, cylinder deactivation still carries a bit of a stigma among some older drivers with long memories that stemmed from this engine.  
Through the late ’70s Cadillac was under immense pressure to improve the fuel consumption of its V8 powered boulevard cruisers due to the introduction of the U.S. Corporate Average Fuel Economy (CAFÉ) regulations that forced the issue, so Cadillac developed a cylinder deactivation system in conjunction with the Eaton Corporation.
The existing 368 cid push rod V8 was used as the platform and an all-new valve control system allowed the sequential deactivation of two pairs of cylinders, creating an 8, 6 or 4 cylinder engine (see Figure 1). Interestingly, Cadillac chose to deactivate opposing pairs of cylinders rather than a bank of cylinders as is commonly done today.
Cadillac and Eaton developed a series of solenoids that were used to release the fulcrum on the intake and exhaust valves’ rocker arms.  The lifters and pushrods continued to operate as normal but the rocker arms sat motionless and the valves remained closed due to valve spring tension. When increased engine power was required, the solenoids returned the rocker fulcrums to their normal operating position and full valve operation resumed.
Coordinating the activation and deactivation of cylinders was an electronic control unit (ECU).  The ECU controlled the engine’s throttle body fuel injection system as well as the cylinder deactivation solenoids.
The engine was set to run on all 8 cylinders during starting, heavy acceleration and at all speeds up to 27 mph. At light to moderate engine load the system would deactivate pairs of cylinders as required.  It was stated that interstate driving only required 4 cylinders for the majority of the journey. It even had a dashboard display that showed the number of active cylinders and the system relied on the driver applying more throttle to maintain speed, a trait that many drivers never became comfortable with or accepted.
Despite being a technological marvel, the L62 V8-6-4 had numerous problems and Cadillac found itself faced with many legal battles.
The throttle body fuel injection (TBI) system (a carb with an injector) was a major cause of the problems. The TBI would continuously deliver fuel to all cylinders irrespective of the cylinder deactivation.  Fuel would build up in the intake ports of the deactivated cylinders until full engine operation was again required, at which point a fuel dump would occur in those cylinders and the engine would stumble momentarily.  
There just was not enough technology or electronics available at the time to resolve the issue, so the V8-6-4 died in 1982 after just one year. Now before you want to condemn Cadillac’s efforts, Google “Cadillac 16” and see what is happening in their latest and greatest efforts and you will be pleasantly surprised.