by Rodger Caldwell

Created on: February 06, 2009   Last Updated: March 27, 2009

My best articles are not my highest rated ones, and I feel the reason for this is other Helium writers sometimes lack the database to appreciate my technical articles. Thus I am writing an elaboration of my motorcycle engine article, which also applies to car engines, fully explaining things.

Fuel economy standards have had engineers studying ways to preserve space and acceleration in cars that use less and less fuel for years, and some of their innovations are convoluted. Honda has camshafts with two different sets of cam lob profiles, one for maximum gas flow and cylinder filling at low engine speeds, one for high.

At low speeds the intake and exhaust valves should be completely closed, the intake valves at the end of the intake stroke, and the exhaust valves at the end of the exhaust stroke, to prevent mixture from escaping through the intake or exhaust valves once the piston begins its compression stroke, and to prevent mixture from being contaminated by exhaust gasses at the beginning of the intake stroke.

At high engine speeds the gasses are moving fast, and their momentum prevents them from going out a valve even if its partially open. This momentum also causes them to continue to enter the cylinder through the still open intake valve after the intake stroke is completed, and exhaust to continue to leave the cylinder after the exhaust stroke is completed through the still open exhaust valve. Also at low speeds the valves shouldn't be opened as far at any time as at high speeds, for at low speeds gas speed needs to be high for better scavenging of the exhaust gasses and better mixing of the fresh charge, and will be if the valves are open less than at high engine speeds.

At high engine speeds the momentum of the rapidly traveling gasses demands maximum valve opening for easiest gas flow. Honda valves have two sets of rocker arms. At low speeds the rocker arms operated by low speed cam lobs are engaged, at high speeds oil pressure forces a pin to disengage the low speed rocker arms, and engage the high speed ones. Four valve per cylinder combustion chambers also preserve maximum gas volocity at low engine speeds, and maximum gas flow at high speeds my varying valve area and flow capacity

One American car company devised a clever way of preserving good peak horsepower consistent with good low speed pulling power in its 3.8 liter V6, preserving simplicity and reliability. Buick's 3.8 liter V6 uses a relatively mild camshaft for good low speed torque and fuel economy, but still develops surprising peak horsepower at high speeds by employing short tube ram induction. At high speeds the intake gasses race through the tubes, and their momentum rams them into the engine, maximising cylinder filling and horsepower when the engine is turning fast. A relatively powerful, economical engine is thus achieved using push rods and one camshaft, allowing Buick to put money where it shows. Compare a Buick interior with a similarly priced car? Compare its reliability?

Buick has also developed continuously variable valve timing using oil pressure to advance or retard their camshafts on one of their engines. This varies how long the intake or exhaust valves remain open.

Various auto companies have since developed systems that continually vary both valve lift and duration, notably Infinity.

This article assumes the reader is acquainted with how an Otto engine basically operates. One can find out how by reading the other articles on this subject on Helium.

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