Re: Automotive: Please explain Variable Valve Timing in curent practice.

Greetings:

The design of internal combustion engines has evolved for more than a 
century and the engineer’s design trade-off has always included performance 
versus cost. In recent years environmental issues mandated by the government 
have also become a part of the trade-off which now includes performance 
versus cost versus emissions. In many ways variable valve timing (VVT) 
systems are following the same development path as automatic ignition timing 
systems pioneered, only decades later in time.  

Early in the development of automobile engines it was realized that the 
timing of the ignition had to be different for easily starting an engine and 
for the smooth running of an engine at speed. Initially a manual ignition 
timing adjustment lever was provided for the driver to start the engine and 
then to readjust the ignition timing when the engine came up to speed. This 
was followed in the 1930s by the development of automatic vacuum advance 
ignition timing controlled by the vacuum in the intake manifold of the 
engine.

Automotive engineers have to design internal combustion engine controls that 
are a compromise between smooth performance over the operating torque range 
of the engine and which also encompass the range of fabrication tolerances 
incurred in engine manufacturing.  Engineers realized that each engine's 
performance is a trade between manufacturing tolerances and cost and that a 
compromise had to be made in ignition timing to cover all of the engines 
being manufactured for a given model of automobile. In fact, because of 
differences in tolerance, each cylinder in an automobile engine has a 
different optimum firing angle for a given operating load and speed and an 
average value for all of the cylinders must be used for control in a given 
design.

With the advent of electronic ignitions and microcomputers in the 1970s and 
80s, a greater range of options in the control of ignition timing became 
available. However, the programmed look up table in each cars computer 
memory was still a compromise averaging the optimum firing angles of all of 
the cylinders in all of the engines being manufactured. With electronic 
control engineers dreamed that if suitable sensors 
could be invented, an optimum closed loop control system with feedback could 
be designed that might some day optimally control each cylinder 
individually.

During the 1990s we began to develop, mechanical, electronic and fiber optic 
cylinder pressure sensors that could be incorporated within a cylinder. 
Some designs inserted the sensor witin the sparkplug, eliminating the need 
for a costly special hole in each cylinder. If a low cost sensor could be 
designed then each cylinder could be individually operated optimally for all 
operating conditions. Because of sensor and control costs, individual 
control of each cylinders firing angle is still a laboratory project and not 
available in production cars. 

At the same time, to meet stringent emissions standards in the 1990s and 
beyond, engineers began to study the control of intake and exhaust valve 
timing. The first production systems have a controlled variable valve timing 
(VVT) actuator between the crank shaft and the valve cam shaft timing belts 
so that the opening and closing angles of all of the intake and exhaust 
valves can be controlled in parallel, in a manner similar to the vacuum 
controlled ignition timing developed in the 1930s only much more complex and 
costly. 

You can see a graphic, animated description of a typical variable valve 
timing (VVT) system now in production at the following web site:
 http://www.wabashtech.com/
vvt.htm

As with ignition timing, mechanical tolerances make each cylinder and it’s 
valves have different optimum setting for valve timing. Once again the best 
of all designs would be to individually control the valves for each 
cylinder. Also, the cylinder pressure sensor would be a good transducer to 
activate this control system if a robust, low cost design could be 
manufactured.

Today, automotive engineers are testing novel electromagnetic valve actuator 
(EVA) systems that provid benefits long sought by vehicle manufacturers to 
control engine intake and exhaust valves and meet market demands for greater 
engine performance and improved fuel economy. Preliminary test results show 
that the EVA timing system gives more than 10 percent increase in engine 
efficiency and a substantial improvement in power at low speeds.  Today a 
few EVA systems are now being sold in production automobiles (e.g. BMW VANOS 
system); however, they do not yet use individual cylinder pressure sensors.

The Cooperative Automotive Research for Advanced Technology (CARAT) Program, 
sponsored by the U.S. Department of Energy, provides funding for the design 
and development of advanced energy-efficient automotive components and 
systems. CARAT brings these innovative ideas and concepts to the attention 
of the automotive industry, and accelerates the introduction of these new 
energy-efficient technologies in advanced vehicle systems. 

You can read about the University of Illinois’ CARAT project on Variable-
Valve Timing Systems for Camless Engine operations on their web site at: 
 http://www.ipd.anl.gov/
carat/uiuc.htm

Best regards, Your Mad Scientist
Adrian Popa