The Basics of the Four-Stroke Engine
There is an elegant genius to the way the four-stroke cycle powers our modern engines. Understanding how all the components work together will teach you the difference between crankshafts and camshafts, and can expand your appreciation of your car beyond the superficial appeal of a roaring exhaust or a bitchin' neon underglow kit.
Yes, my friend, under the hood is where it's at. Contained within the construction of iron, steel and aluminum is the core of why we love our cars so much. And it's all based on technology invented in the 1800s. We've been upgrading and improving on it since, but the core has always been based on the original four steps of internal combustion: intake, compression, power and exhaust.
At its most basic, the engine operates via a series of pistons which turn the crankshaft, which turns the wheels. Here's a simple look at the four-stroke process:
- Intake Stroke: The camshaft opens the intake valve, and the cylinder moves down to draw in fuel and air.
- Compression Stroke: The piston moves up, compressing the air/fuel mixture.
- Power Stroke: The spark plug ignites the air and fuel. The explosion forces the piston down, generating power to turn the crankshaft.
- Exhaust Stroke: The camshaft then opens the exhaust valve, and the piston moves up to force out the burned gas.
The cylinders go through each stroke sequentially and, in the case of a four-cylinder engine, one cylinder is always in a power stroke, assuring the crankshaft, and thus the camshaft, turns. For a V8 engine, two cylinders are in their power stroke at a time.
In a car with a carburetor, the air and fuel are mixed together in the carburetor and then fed into the cylinder. Today's cars have fuel injectors that force the air and fuel directly into the cylinder for better efficiency and performance.
The Basics of Camshafts and Crankshafts
So, what's a camshaft? What exactly does the crankshaft do? We're getting there. Two major pieces of a car's engine are the crankshaft and camshaft, and they work together to make sure the engine runs smoothly and properly. Understanding how the relationship works is a little like trying to figure out which came first, the chicken or the egg. The crankshaft is responsible for turning the camshaft via a timing belt (or chain), but the camshaft manipulates the intake and exhaust valves that force the pistons to move, which turns the crankshaft. The process is a perpetual loop that keeps the engine running.
How Crankshafts Work
The crankshaft, in addition to rotating the camshaft, is attached to the drivetrain, which makes the car move. The faster the crankshaft rotates, the more power that goes to the car's wheels. We're skipping a lot about the transmission, the clutch and axles and other aspects of what makes a car work, but for our camshaft vs. crankshaft discussion, this is all you need to know.
How a Camshaft Works
The role of the camshaft is to open the intake and exhaust valves at the correct time. The camshaft has egg-shaped "lobes" that lift the valves open as the shaft rotates. The valves are spring-loaded, so when the cam lobes rotate off, the valves automatically close. Expansion of compressed air forces the piston down and rotates the crankshaft.
Some engines use a single cam that operates both intake and exhaust valves, while others use two cams (one for intake and one for exhaust).
You may have seen the letters OHV, SOHC and DOHC and not known what they meant. Let's take a look at the three types of cams you'll see in most cars.
An overhead valve (OHV) setup uses a single camshaft with pushrods that manipulate rocker arms attached to the valves. As the camshaft rotates, the pushrods open the valves. This is the simplest of the three camshaft setups. The camshaft sits below the valves (hence the name), making this a very compact design compared to cams positioned above the heads. Many people prefer this cam setup because of the simpler design and ease of maintenance.
The single overhead cam (SOHC) uses a single camshaft above the cylinder heads to directly operate both intake and exhaust valves. You will find SOHCs with two valves (1 intake, 1 exhaust) per cylinder, three valves (2 intake, 1 exhaust) per cylinder or even four valves (2 intake, 2 exhaust) per cylinder.
Characteristics of OHV and SOHC:
- Fewer components means lighter weight and less production cost.
- Single-cam engines produce more torque at lower to middle RPM ranges.
- Better fuel efficiency
- The simpler design means fewer parts, and many believe this leads to improved reliability and easier Mustang maintenance.
- Single-cam engines tend to be narrower and shorter than DOHC and weigh less. This smaller size and reduced weight is important to people looking to mod their cars. It also allows the engine to be placed lower into the engine bay, lowering the center of gravity in relation to the car's overall center of gravity to improve stability and handling.
Similar to a single overhead cam, the dual overhead cam is positioned above the heads, but there are separate camshafts for the intake and exhaust valves. This allows for much more power generation at higher ranges.
Characteristics of DOHC:
- Dual overhead cam engines typically feature four valves per cylinder (2 intake, 2 exhaust).
- This setup provides better valve lift and better air flow through the combustion chamber, allowing the engine to produce more horsepower with smaller engine displacement.
- Produces higher top-end power output of torque and HP.
- The more complex design and added weight of additional components result in higher production cost.