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Or more specifically, where the camshaft is and how it controls the intake and exhaust valves.
First up, let me dispell a myth. Pushrods (also called "overhead valve" - OHV) are not "old". And overhead cams are not "sophisticated". Sophisticated is a word that lazy auto writers use. Both pushrods and OHC engines have been around since the 20's.
Now back to the subject. In an OHV engine the camshaft that controls the valves is located inside the engine block. In a typical V6 or V8 engine, it's in the middle between the cylinder banks. There's only one (designs with 2 are on the drawing board at GM but not in production). It's driven by a short chain or belt. There are rods, the "pushrods" that follow the cams on the camshaft, and go up into the cylinder head, where they actuate the valves. There's a rocker-type mechanism on the valves, so the pushrod comes up, and this causes the valve to open down into the cylinder.
In an OHC engine, the camshaft is, as the name implies, "overhead". There's one or two (SOHC & DOHC) mounted in the cylinder head. Another myth: DOHC is "better" than SOHC. Not necessarily. Mercedes uses SOHC and their engines are great. Anyway, the cams can either directly actuate the valves, or in some cases there is a follower mechanism sortof like in an OHV engine. In the original Saturn 1.9l engine, the DOHC model had valves directly driven by the cams, the SOHC model used rockers.
Some advantages of an OHC/pushrod design:
More compact - the engine isn't as tall as an OHC engine, since the cam is hidden inside the block
cost - there's usually only the 1 camshaft, OHC engines require as few as one in a SOCH inline engine, or as many as 4 in a DOHC V-engine (don't get me started on VW's W8!)
Some advantages of OHC
multivalve - for the time being, most OHV engines only have 2 valves per cylinder. This makes them a little less efficient in terms of airflow, especially at higher rpms.
redline - an OHC engine can spin faster. The pushrods going up and down in an OHV engine can be problematic. Note how most OHV engines redline around 6,000, OHV can go much higher
Think about how the OHV 3.8l GM engine gets 210 hp. This is similar to what DOHC 3.0l engines can get. But that extra displacement helps produce a lot of torque at lower rpms.
The thing I'd send you away with from this little lesson is this: neither design is inherently better. Each has it's advantages. Some awesome engines are OHV (Corvette, GM's truck V8's, that 1,000 hp V16). The industry is mostly going to OHC though. It's probably because for a given engine displacement, you can get more output with a OHC engine. Fuel economy will suffer (check out the Nissan 3.5l equipped cars) but that's not really a concern for most right now. In the end it's all about how the vehicle performs - power, efficiency, cost. What's more important to you will lead you to determine what type of engine might be best for you.
jeff
posted by 209.244.8...
, Wed, 12 Mar 2003 14:33:09
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