What Is Used to Regulate Valve & Ignition Timing in Internal Combustion Engines?

Getting an engine to run, much less run well, is no simple task. While any high-school auto-shop student may tell you how air and fuel gets into the cylinder, ignites and gets back out, the fact is that the techniques used today are the products of more than a century's worth of trial, error and refinement. Valvetrains and ignition systems are getting more complex by the day, edging ever closer to perfectly orchestrating the dance of physics that is an internal combustion engine.

Camshaft Basics

A camshaft is a shaft with "cam lobes" on it. A lobe essentially is a bump on one side of the shaft; as the shaft spins, that bump passes a given point once per revolution. If you place the camshaft directly on top of a valve, it'll push the valve open every time the lobe bump hits it. How far the valve opens -- the valve lift -- depends on how tall the lobe is; how long the valve stays open -- its duration -- depends on the width of the lobe. How fast the valve opens is determined by the angle of the lobe "ramp" from the "base circle" of the shaft to the tip of the lobe.

Overhead Cam vs. Pushrod Engines

You can divide four-stroke engine designs into one of two basic configurations: overhead-cam and cam-in-block, or "pushrod" designs. The OHC engine's configuration is very similar to that described above, where the cam lobes push down directly on top of the valve. In practice, OHC engines use a set of "cam follower" levers between the cam lobe and the valve; the cam pushes down -- or sometimes up -- on the follower, and the follower pushes open the valve. A cam-in-block engine is so called because the camshaft is in the engine block next to the cylinders. In this configuration, the cam lobes push up on cylindrical lifters, which in turn push up on pushrods. These pushrods push on the bottom of rocker arm levers, which push the valves down and open.

Cam Timing Compromises

Opening a valve just a little bit and quickly closing it helps boost low-rpm torque, fuel efficiency and idle quality by forcing air to speed up to get in and out of the cylinders. This approach ultimately limits airflow, and thus high-rpm horsepower. Opening the intake and exhaust valves at the same time -- increasing valve overlap -- increases high-rpm horsepower at the expense of fuel economy, low-rpm torque and emissions. So you either may use small cam lobes for torque, fuel economy, idle quality and emissions, or you may use large cam lobes for high-rpm horsepower.

Variable Valve Timing

Honda found a work-around for camshaft compromises when, in 1983, it introduced Variable Valve Timing and Engine Control -- colloquially known as VTEC. A VTEC system starts as an overhead-cam engine, but utilizes two cam lobes for each intake valve. The primary lobe is the smaller of the two; below about 4,000 rpm, the engine runs on the primary for superior driveability. At about 4,000 rpm, the secondary lobes follower, which up to this point has been sitting there doing nothing, locks onto the primary follower. Now the engine is running on the big "race" cam and making race-spec horsepower.

Controlling Overlap and Advance

An overhead cam design is dynamically superior to the pushrod design, which restricts the size and flow of the intake ports by forcing them to bend around the pushrods themselves. A dual overhead cam -- DOHC -- engine uses one cam to control the intake valves and another to control the exhaust valves, as opposed to a single overhead cam -- SOHC -- engine that uses one cam to open both the intake and exhaust valves. The DOHC design itself isn't inherently superior to a SOHC, but for one fact: It allows a computer to mechanically rotate the intake cam relative to the exhaust cam, thus increasing or decreasing valve overlap. This provides another opportunity to tailor the valves' opening and closing events to suit engine rpm. Many manufacturers use some variant of the cam phasing system with or without a VTEC equivalent, but combining camshaft phase control with variable timing brings the valvetrain about as close to perfect as we've seen yet.

Ignition Controls

Compared to valve control, ignition control is child's play. On most distributor-driven systems, the ignition advance -- when the spark plug triggers either before or after the piston reaches the top of its travel -- is determined by the position of the distributor rotor relative to the distributor shaft. At a certain rpm, a set of spring-loaded counterweights on the shaft move outward, engaging a mechanism that rotates the rotor that triggers the spark plugs. Vacuum advance distributors contain a second mechanism to control rotor position, but this one runs according to engine vacuum -- an indicator of engine load and rpm. Modern systems do away with such gimmickry by using a computer that triggers the ignition coils and calculates the appropriate advance given rpm, engine load, air temperature, air/fuel ratio and barometric pressure.

References

• Engine Management: Advanced Tuning; Greg Banish
• Valvetrains for Internal Combustion Engines; Ina Schaeffler
• Team Del Sol: What Is VTEC?