DOHC vs. SOHC

Factual Questions
1

Recently, I purchased a 1999 Dodge Stratus. Under the hood is a nice little four cylinder engine. On top of the engine, it proudly proclaims “2.4 liter DOHC”. Now, I understand that 2.4 is the engine displacement and that generally bigger is more pwerful. I know that DOHC mean double over-head cam, and this is where my question lies.

Why is this important? Why should I be excited that I have this bit of technology? My other car ('98 Dodge Neon) says “2.0 liter SOHC”. It’s a smaller car, so has a smaller engine. But why the single over-head cam? Which is better and do I, the car-buying consumer, need to really care about this?

Thanks. I was just wondering. By the way, we really enjoy the Stratus. Nice car and not all that expensive.

2

It’s all about effeciency. Earlier engines had the cam shaft buried deep within the engine. Pushrods pushed up rocker arms, which thusly opened and closed valves. One day, somebody decided “Hey, since the valves are on top, why not put the cam shaft on top!” This is a Single Overhead Cam Engine. The singular camshaft has lobes which directly move the rocker arms, opening and closing the valves. The most effecient design is a Double overhead cam engine. This uses a separate camshaft for each side of the engine… Therefore, the camshaft’s lobes open and close the valves directly.
The reason to have overhead cams come down to effeciency. Opening and closing valves is necesary to the engine operating. The fewer mechanical steps between opening and closing the valves, the less energy expenditure and possibility of mechanical breakdown.

3

How does DOHC relate to Fuel Injected cars? Are the cams still there or does a computer (hooked to some king of actuators) take over all the functions of the cam shaft?

4

You still have cams and valves to let the intake air in and exhaust gases out.

Fuel injection justs puts the fuel in. Depending on the type, it can spray the fuel into the intake manifold, the runners leading to each intake valve, or into each cylinder.

5

To expand on Muffinmans’s excellent answer you need to visualize the whole arrangement of the cylinder head. The best arrangement for valves is a cross flow - the intake runner, intake valve, exhaust valve and exhaust manifold are all in a straight line across the head. That can be done with a single cam as on '90 BWM 325 but only by using rocker arms. A single cam head can be built without rocker arms as on a VW Rabbit, but not with the more efficient cross flow design.

6

Here’s another quick description of what happens in your engine:

Remember this phrase - “Suck, squeeze, bang, blow” (intake, compression, power, exhaust). That’s how I used to teach the 4-stroke cycle to student pilots in ground school.

Basically, this is what happens. There are two valves on top of the engine. One is an intake valve, and the other is an exhaust valve. There are a number of cylinders, with pistons moving up and down inside them. These pistons are connected to a crankshaft, which delivers the engine power to the rest of the drivetrain.

  1. “Suck”
    When the piston is at the top of the cylinder, the intake valve opens, and a mixture of air and fuel is injected into the cylinder (or sucked in) as the piston moves downwards.

  2. “Squeeze”
    Then the intake valve closes. The piston then begins to move back up, compressing the fuel/air.

  3. “Bang”
    When the piston reaches approximately the top of the cylinder, the spark plug fires and ignites the mixture. The expanding gases force the cylinder back down again. This is the power stroke, and creates the force that not only drives the vehicle, but forces other pistons up in the compression stroke. (The pistons are offset on the crankshaft, so that when one is in the power stroke, another is in the intake stroke and yet another is in the exhaust stroke, etc.)

  4. “Blow”
    Finally, the exhaust valve opens, and the piston goes up the cylinder, forcing the spent gases out. The intake valve then opens, and the whole process starts over.

You can see that there is some intricate timing involved here, and that’s where the camshaft, fuel injection, and spark plug timing comes in. The camshaft is connected to the crank, and will spin at the proper rate to ensure that the valves open and close when they should. Another timing system controls when the spark plugs fire, and yet another controls how the fuel is injected into the cylinders (or it may simply be drawn in by the vacuum created when the piston moves back down the cylinder). These timings are critical and all can be adjusted (for example, the intake valve typically will stay open slightly longer than when the piston hits dead bottom, because the inertia of the air will draw in a bit more fuel).

That’s your basic engine, and closely describes an older 4-stroke engine like you might find in a '67 chevy. Modern engines have all kinds of other details, like emissions controls, 4-valves per cylinder to improve air and fuel flow, etc. But the basic engine is still the same.

If you use an overhead cam you can get rid of pushrods and other mechanical componenents, which lowers the amount of intertia and friction inside the engine, making it more efficient and faster revving.

Modern engines are typically built with 4 cylinders in one straight line, and a single overhead cam works fine for those. But if you want more than four cylinders, you often want to place them in 2 rows of 3 or 4, because it makes the engine more compact and also cuts down on vibration. These rows are going to either be in a V shape (as in V-6 or V-8), or possibly horizontally opposed. In a V-style engine, you can get even greater efficiency by giving each row of cylinders its own camshaft. That’s your DOHC, and it further cuts down on internal complexity and friction.

7

Muffinman, thanks for the succinct explaination. I’m a bit confused by "both sides of the engine.’ Both of my cars’ engines have four-cylinder pistons arranged in a single row. What are the “both sides” of which you speak?

Jeff_42, both cars are fuel injected. That doesn’t relate to the camshaft(s). It’s the carburetor that’s missing. Instead, the fuel injector mixes the air with the fuel.

So, if I understand y’all correctly, my DOHC engine is using fewer parts to accomplish it’s 4-stroke mission. By fewer parts, we should hope for greater effieciency and fewer break-downs. Ain’t modern technology great?

8

Drum God, the two sides in question are probably the intake and exhaust sides of each cylinder. But of course you’ll need to decide based on context if they are speaking of the “sides” of a V arrangement, or the “sides” of each cylinder.

DOHC doesn’t necessarily mean less parts, just more reliable and less complicated mechanisms. Shorter linkages, lighter parts. It is going to depend on the specific engine, one may be best suited to a SOHC another to a DOHC. Its a balancing act between effeciency, cost, weight, and performance. You can’t really say which is better in a general sense, both have their advantages, and both are suited for different engines. I’d guess that majority of designs however use DOHC to optimize performance and spread the work load at a cost of more parts. A seperate camshaft is likely more relaible than rocker arms, but more expensive and heavier.

9

Just make sure you replace your timing belt at the recommended intervals!