Aluminium as a valve material?
The poppet valve is almost universally used in all forms of transport powered by an internal combustion engine. Motorsport has always had a buoyant two-stroke sector in the motorcycle arena, although this is on the wane now with Grand Prix motorcycle racing having bidden farewell to its last two-stroke class at the end of 2011. So, we have confined ourselves to a four-stroke future, and this generally means poppet valves. Brave attempts by forward-thinking engineers over the generations to try to replace the poppet valve have come to nothing in the mainstream automotive world, and the last serious attempt to dispense with it in Formula One was stymied by the introduction of a new regulation.
The question of whether, faced with a clean sheet of paper, engineers would invent something as complex and hard to control as a poppet valve for a high-speed engine is interesting, but not one that we will discuss at this juncture.
As engines have developed over the decades, so have their components, especially in recent times as speeds and output have risen. Steel valves were replaced by more corrosion-resistant austenitic grades. Higher strength superalloys have been adopted for the extremes of turbocharged and supercharged engines, and titanium alloys have remained a favourite of those chasing maximum performance from naturally aspirated engines, and even some companies using lightly boosted engines.
One material that is rarely mentioned in terms of poppet valves though is aluminium. The material offers some real advantages, especially its low mass, which means less closing force is needed, and lower closing forces require a lighter and shorter spring, and therefore a shorter valve. This means the height of an engine designed from the outset around such a valve could be shorter, with the attendant savings in casting mass.
There are a few such critical components in an engine where any mass saved can be multiplied by saving mass from other components as a direct consequence. However, any advantage in terms of valve control due to the lower density of an aluminium valve is offset by having a low modulus material.
You may ask, "Has anyone actually tried this?" The answer is yes, at least once, on a very advanced race engine. The material used was a metal matrix composite (MMC) aluminium material, based on a 2000 series alloy. The MMC in question was not specially formulated for high-temperature use and so was something of a compromise. Since then, however, there have been a number of material advances, both in aluminium MMC materials and in the availability of wrought alloys with improved high-temperature properties.
While these materials may not be suitable for turbocharged engines, there is some interest in their use in naturally aspirated applications, and at least one large valve company involved both in racing and mainstream automotive supply is producing aluminium poppet valves. It would be very interesting to see the results of engine testing with one of these new materials, or something formulated especially for the purpose.
Fig. 1 - Might we see aluminium poppet valves in race engines?
Written by Wayne Ward