What is a 24-Valve DOHC Engine?
The valve train of a 24-cylinder double overhead cam (DOHC) engine is not the top of the line, but it's enough to give an average sports car the get-up-and-go that most consumers like to feel when they press the gas. The DOHC design allows for quieter, more efficient vehicles, making engines stronger without forcing them to be heavier.
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Identification
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A 24-valve double overhead cam engine is exactly what it sounds like: a combustion engine with a total of two dozen valves on its cylinders and two cam shafts positioned above the pistons and combustion chambers. The DOHC engine stands in contrast to overhead valve (OHV) designs that put the cam shafts below the combustion chambers, and single overhead cam engines where the valves are driven by only one camshaft.
Function
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The 24-valve DOHC engine design is used to increase the efficiency of the engine and allow for variable timing. An engine can only produce as much drive power as it can generate energy in its combustion chambers, and this is largely determined by the airflow in the chambers. The dual camshafts allows for twice as many valves per cylinder, maximizing the engine's output without increasing its overall volume.
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Features
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In some cases, the DOHC appellation does not refer to the absolute number of cam shafts in the engine. V8 engines, for example, have more than one bank of cylinder heads, but are still considered DOHC engines even though they have more than two camshafts in total, because they have two camshafts per cylinder head. It's not necessary to have two camshafts to mount multiple inlet or exhaust valves, and not all DOHC engines have multivalve cylinder heads, since the first dual cam engines had only two valves per cylinder. Almost all today, however have between three and five valves per cylinder.
Types
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24-valve DOHC engines generally come with 6 cylinders, each having four valves, or 8 cylinders, each with three. Many engine designs come in different versions, with the single cam varieties having exactly half as many total valves. One example is the Mitsubishi Cyclone V6, which has been used in many Dodge, Chrysler, and Mitsubishi vehicles from the late 1980s onward. The single cam version appeared in most of the models, but the DOHC version is what gave the 3000GT and Stealth their extra horsepower.
Considerations
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While double cam engines will tend to have higher overall power than single cam engines, it's the overall number of valves that tends to explain the difference. Engines with more valves have more torque at high RPMs and can therefore go faster before having to shift gears. More valves per cylinder is better than less, whether or not it's produced by double or single cam shaft designs.
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Resources
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