OHV, OHC, SOHC and DOHC (twin cam) engine design
OHC or SOHC engine continued
Acura 24-valve SOHC V6 engine. Photo:Acura
Honda successfully uses the SOHC design in its late V6 engines where 4 valves per cylinder are operated by a single camshaft. See the Acura TL 3.2L 24-valve SOHC V6 engine in the second photo (Acura is Honda's luxury vehicle division).
The main disadvantage of an OHC or SOHC engine is that it requires a timing belt or chain with related components, such as a tensioner and guides to run the camshaft(s). This means higher production cost and more complex design.
A timing belt needs to be replaced at regular intervals, typically every 60,000-120,000 miles. A timing chain can last longer, but it is sensitive to regular maintenance: if engine oil is not changed regularly, a timing chain stretches and wears out. Examples of cars with a SOHC engine include: 2001-2005 Honda Civic, 2007-2008 Honda Fit, 2003-2008 Subaru Forester, 2008-2012 Honda Accord V6.
DOHC or Twin Cam engine
4-cylinder 16-valve DOHC engine
GM 2.4L direct-injection DOHC Ecotec. Photo:GM
DOHC means Double OverHead Cam, or sometimes it could be called "Twin Cam" or "Double Cam." Most modern cars have DOHC engines.
A typical DOHC engine has two camshafts and 4 valves per cylinder. One camshaft operates intake valves that are installed on one side, while another camshaft controls exhaust valves on the opposite side.
With this design, camshafts can be installed further apart from each other. This allows the intake valves to be at a larger angle from the exhaust valves, which results in a more direct air flow through the engine with less obstruction. In other words, a DOHC engine can "breathe" better and thus produces more horsepower out of smaller engine volume.
Compare: The 3.5-liter V6 DOHC engine of the 2003 Nissan Pathfinder has 240 hp, similar to 245 hp of the 5.9-liter V8 OHV engine of the 2003 Dodge Durango.
Technologies like Direct Injection, Variable Valve Timing and Variable Valve Lift can be easily implemented in a DOHC engine, further improving fuel efficiency. For example, the 2013 Chevrolet Equinox SUV with a 2.4L 4-cylinder direct-injection Ecotec engine (in the photo) is rated at 32 mpg on the highway; something that not every family sedan can beat.
Downsides of the DOHC technology include a larger size and more complex design with additional timing belt or chain components. A timing belt needs to be replaced at recommended intervals, adding to maintenance costs. Examples of cars with a DOHC engine: 2008-2012 Honda Accord 4-cylinder, 2012-2014 Toyota Camry.
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