A traditional gasoline engine needs a mixture of air and fuel to create combustion inside the individual cylinder chambers (be they numbered 4, 6, 8, or 3 if you drive a DamnElantra®). If you simply let in a bunch of air and an Inland Empire Meth Lab amount of fuel, your car would first go Pah(!) then Kaboom. If you restrict the mixture of air and fuel too much, you will get no such Pah. This air-fuel mixture passes through the intake valve before being compressed and ignited, creating the car-go-now that we all seek. Post ignition, it’s shuffled out through the exhaust valve. Whence, four strokes: intake, compression, power and exhaust.
What determines when it is air-fuel feeding time for your hungry, hungry pistons and when it is time to let them expel their sweet, sweet exhaust gases? The Camshaft! Hop this here jumpsy-daisy to find out how.
First, we need to take a moment and explain a cam, singular. Essentially a circle with a bump on it, cams have been used by engineers for years to turn rotational motion into linear motion. Essentially, you have a circle with a bump on it with something the rests on said circle while it turns. When the bump comes along, whatever’s resting on the cam moves up with the bump. String a couple of cams on a stick and you’ve got yourself a camshaft. Link the rotation of the camshaft to the rotation of your engine and link the bumps to the valves of the engine, and you can see the basic principle at work. On to specifics…
The terminology’s a little odd, not unlike opposing sides of a political issue who both refer to themselves as “Pro” something. Nowadays, we’re basically talking about two main engine configurations: overhead valve (OHV) and overhead cam (OHC). (For you old-skool guys, we’ll get into flat heads another day). Jargon aside, an OHV engine has the camshaft in the bottom-end of the block, near the crankshaft, with pushrods traveling upwards to activate the valves. An OHC engine has the camshaft in the head, with the valves directly actuated by the camshaft and longish timing belt or chain stretching from the crankshaft up to the camshaft to drive it.
Dual overhead camshaft (DOHC) engines have one camshaft for the intake valves and one for the exhaust valves. DOHC setups make for more tunability when it comes to some of the more complex aspects of camshaft timing, as the open/close timing of the intake and exhaust valves can be set independent of each other.
Nowadays, the standard for a “modern” engine uses a DOHC configuration coupled with a “mutlivalve” configuration. As a minimum, you need one intake and one exhaust valve per cylinder. If you’re trying to flow more air/fuel in and more exhaust out more valves are better, so why not have two intake valves and two exhaust valves? Easy with a DOHC setup. Audi (among others) is known for running five valve-per-cylinder engines, e.g. the 40V V8 or the 20 V 4-banger with three intake valves and two exhaust.
The .gif above is a DOHC setup, while the one below is a SOHC setup.
This is what people are talking about when they compare a typically American “pushrod” V8 to a more modern OHC set up. In general, all of the features that allow greater flow help at higher engine speeds, but tend to hurt at lower RPMs. Hence, a DOHC Audi 4.2L v8 will make 400-something HP at 8000rpm while the OHV GM 6.0L LS2 V8 makes 400hp at 5500rpm and more torque across its whole rev-range than the DOHC unit(fainbois: the numbers are rough, for the purposes of illustration so back off). The point is: DOHC = more power from less displacement at higher RPM. OHV = lower HP/liter, but more low-end torque. (Again, gross generalizations for instructive purposes).
I mentioned above that the cam lobes are controlling the amount of air allowed to enter into this automotive chamber of secrets. They do this by lifting the intake valves for a certain amount of time per stroke, known as “duration”. The more air/fuel mix allowed into the combustion chamber, the bigger the PAH(!) at higher RPMs. This is why you often hear muscle car fans talking about “bigger cams”… they have increased the amount of valve lift and duration by swapping in an aftermarket piece. That lovely blub-blub-blub you hear from the ‘69 Camaro next to you at the stop light? That’s the product of a really hot cam setup that’s only happy above 4000rpm.
I know this explanation is not as cool as the Ghost-Riding Diff Explanation Team. If you are still confused, head on over to the HowStuffWorks link at the bottom. They do a much better job than I at explaining the cam, though I hope my rambling excuse to show off two cool .gifs helped you just a bit.
[Extra Source: Mad_Science – because my first shot at deciphering the mystery of the cam… wasa nata so gooda]
Don't Be Afraid to Ask Questions: Part 2 – The Camshaft
"OHC = lower HP/liter, but more low-end torque." Shouldn't that be "OHV = …"? Sorry to nitpick. I know what you mean, though.
As I understand it, in most OHV engines, you have a fixed lift and duration. Since the cam is rotating with engine speed, the lift (how far the valve opens) and duration (how long it is open) has to be tuned over a broad RPM range. What is a typical passenger car cam tuned for? I'm assuming between idle (~600 rpm) and peak HP and/or torque? A racer will put in a cam that is tuned higher in the rev range to take more advantage of the HP and torque curve, right? This, though, means a very rough idle unsuitable for a commuter.Loading…
The first time I held a cam in my hands and looked at all the lobes I was really impressed with the first guy who did all the math and machine work to figure out where the bumps went (8v 4-cylinder). Not long after that I cracked the bottom end and pulled out the crank.Loading…
Thanks for the great article…. I really do love you guys' (I'm counting Murilee as a guy) web page here.
Do you plan to continue on this topic, and educate the lesser of us on more details of the designs?? I hope so!
I'm going to have to see if i can find a moving .gif of an OHV, because i'd like to see one now with the pushrods in action. While I understand the concept, I'd like to see the comparision of the camshaft placement between the two designs.Loading…
Next up the magic geometry of the rotary engine?Loading…
I really hope this was Camshafts 101, and I look forward to the 200 and 300 level classes! You guys are awesome! thanks!Loading…
Social comments and analytics for this post…
This post was mentioned on Twitter by TheHooniverse: From the Hooniverse:: Don’t Be Afraid to Ask Questions: Part 2 – The Camshaft http://bit.ly/3M94az carchat…Loading…