I’m interested in finding out what the differences and advantages are of SOHC and DOHC engines, and if DOHC is inherantly connected to having four valves per cylinder. Also, what is the big advantage of having variable valve timing?
I’m not an automotive engineer, but maybe I can tide you over until one happens along.
DOHC is not always linked to four valves per cylinder. There was a version of the MGA called the Twin Cam, and that was in the late-50s. I’m sure (but now you’ve got me scratching my head a bit) that was only two valves per cylinder.
The correlation works pretty well the other way, though. I seem to remember reading that all four valve engines are DOHC (Or were until recently, it was an article about someone who invented a SOHC/four-valve design). I guess it was just too much of an engineering headache to drive four valves per cylinder off a single camshaft.
Variable valve timing might be kind of tricky to explain, but I imagine it’s a lot like advancing the ignition timing as the engine speeds up. If you figure out the lag when burning fuel or sucking air into the engine and work backwards, you can pick up a little bit of efficiency by anticipating it just that little bit.
But get with the times. I think most new Audis are five valves per cylinder.
The FIAT Spider 2000 has DOHC with 2 valves per cylinder.
And, Honda made several SOHC 4-valve engines IIRC. As did another couple people (Alfa?) So there are exceptions going both ways.
It was supposed to be cheaper to use one cam for four valves, but performance issues and wear issues changed that.
The new Turbo bug is 5-valve I think.
The way I understand it was: for a given multivalve engine, SOHC = less cost but more wear and less ability for high performance. DOHC = more cost, less wear, possibility to have strnage valve timings and cam lobe profiles.
Here’s the scoop on variable valve timing:
Your basic four-cycle engine does the following on each cycle:
[ul]
[li]Intake - A valve opens when the piston is on its way back down the cylinder. The vacuum created sucks in a mix of fuel and air. Then when the piston reaches the bottom, the valve closes, creating a sealed chamber[/li][li]Compression - the piston heads back up the cylinder, compressing the fuel/air mixture.[/li][li]Power - A spark plug ignites the compressed mixture. The expanding gases force the piston back down. Because the pistons are connected to a crankshaft, the shaft turns under force, transferring that power to the drivetrain. Also, the other pistons are connected to the crank, and some of this power is used to move the other pistons through other strokes.[/li][li]Exhaust - The exhaust valve opens, and the piston moves back up the cylinder, forcing the spent gases out.[/li][/ul]
Now, this description suggests that when the piston is at the top the intake valve opens and the exhaust valve closes, and when the piston is at the bottom the intake valve closes. But in reality, the valves are timed to lead and lag a little, because the gas has inertia. For example, when the piston is moving up in the exhaust stroke and hits the top, you want to leave the exhaust valve open just a little longer because the spent mixture will keep moving in that direction even though the piston is not moving up any more. On the intake stroke, you want the intake valve to stay open a little longer because a bit more fuel/air will continue into the chamber. This allows you to generate more power.
The amount of inertia in the mixture is dependent on the RPM of the engine. So the optimum timing for opening and closing valves is different for different engine speeds. Engines with fixed valve timing are set for an average value that delivers the best performance at an optimum RPM, and efficiency drops off at other speeds. But if you can vary the valve timing, you can maintain optimum performance through a wider range of RPM.
Overhead Camshafts are efficient because the cam is right over the valves, allowing lobes on the cam to move the valves directly or through a short connection. If the cam is on the bottom, you need pushrods connecting the camshaft to the valves. This adds mass to the drivetrain, which makes the the engine harder to rev and saps power.
On engines which have the pistons organized in a ‘V’ shape, you can put a cam over the top of each ‘V’, thus your DOHC. A single overhead cam is typically found in engines with an inline cylinder configuration, although there are exceptions to both.
Four valves per cylinder is another buzzword you might hear. This simply means that there are two intake and two exhaust valves on each cylinder. This allows the cylinder to breathe more freely, which improves power and efficiency.
If you hear someone say his engine is ‘ported and polished’, this means that someone took the time to smooth out the edges of the valve seats and polish the inside of them. This allows the air to move more smoothly without as much friction and turbulence, which improves performance. Factory cylinder heads are mass-produced, and often have imperfections and rough spots which trip up the air flow and decrease efficiency. Porting and Polishing corrects this. Also, when ‘porting’ the head, the size of the valve holes may be increased or re-shaped to improve air flow.
If you head someone say his engine is ‘balanced and blueprinted’, this means that a custom builder took the time to dynamically balance the major rotating components, which causes the engine to run smoother and waste less energy, thus delivering more power to the crankshaft. ‘Blueprinting’ essentially means returning a rebuilt engine to factory specifications, with close tolerances (perhaps better than the original factory engine).
There’s the skinny on your major performance factors for an engine.
>> allowing lobes on the cam
I believe the “lobes” are called “cams” and the “cam” is the “camshaft”. does that make sense?
One camshaft for the intake valves, and one for the exhausts.
OK, thanks. But why have multiple valves for the greater flow of air? Why not have just bigger openings? Is is because of the inertia of the bigger valves that would be necessary?
dhanson, that’s more in depth than I’d have gone. 
VTEC is usually used for greater top end power.
You have multiple because they have to be round. Now, you have a round cylinder, too. The easiest way to occupy the full space of a cylinder is either through one large opening, which is impossible in an engine, or through multiple smaller openings, which is what multiple valves are for.
Lobes are the lumpy things on the camshaft. Cam is just short for camshaft in every day speak, although, technically, there is a difference.
–Tim
Dave: It’s quite common for ‘performance’ engines to simply have larger valves, but you reach a limit - larger valves introduce problems in keeping them seated, more moving mass, etc. Two smaller valves positioned correctly can be more efficient.