On modern automobiles, the Electronic Control Unit (ECU
) of the car controls ignition timing
. One may begin to wonder, though, how does the ECU
know how to time ignition? It knows how because there is a Crankshaft Position Sensor
which tells the ECU
exactly where in the rotational cycle the crankshaft
is. The ECU
then takes into consideration several factors, and tells the fuel injector
when to shoot and the spark plug
when to fire.
However, that doesn't do any good if the camshaft is not rotating in the proper synchronization with the crankshaft, opening the valves when they should be open, allowing the ECU to properly ignite the fuel.
In the past, when overhead valve engines were the only kind of engines in existence, there was only one method of keeping the crank and cam properly aligned. These days, there are several methods used, although some have pretty much fallen by the wayside due to their disadvantages.
One thing to keep in mind is this: In a full rotation of the crankshaft, the combustion cycle is only half complete. This is because in a complete 4-stroke cycle, there are two crankshaft rotations - The first rotation pulls in the air, and then compresses it. The second allows expansion of the expanding gases, and expels it.
So, keep that fact in mind (2 crankshaft rotations for every 1 camshaft rotation) for the rest of this write-up.
The oldest system was very simplistic. There was a gear
attached to both the crankshaft and the camshaft. In order to rotate the camshaft a single time for the two rotations of the crankshaft, the camshaft
gear is twice as large as the crankshaft
This is a very efficient system, because you have two strong metal gears which are firmly linked and do not slip.
However, it's noisier than all get out. And since you have two metal gears that are very hard grinding together, it creates a lot of vibration. Add to that the loss of power due to the friction, and you have a simple system that isn't the least bit elegant. Quickly, a replacement for this was found.
Timing Chain and Sprockets
In this system, the timing gears are replaced by two smaller sprockets
, keeping, of course, the 1:2 size ratio between the crankshaft and camshaft sprockets. Connecting them is a simple timing chain
This set up has its own high and low points. Due to the fact there aren't two large gears touching, there is less friction than in a gear system, so less power is wasted trying to turn the camshaft. This results in more power being delivered to the transmission. Also, it's dozens of times more quiet. The action of moving the chain is almost smooth as silk, resulting in a more pleasurable driving experience.
Unfortunately, the chain needs constant lubrication. This requires that a timing cover be placed over the assembly so that the engine oil can reach it without leaking out. This further means that changing the sprockets and chains is a messy, difficult job.
It doesn't help that you're supposed to have this done every 75,000 miles or so, possibly even sooner if your car is older than 10 years. Consult your owner's manual for exact maintenance specifications.
Timing Belt and Sprockets
When manufacturers began to design overhead cam
engines, the camshaft became too far from the crankshaft to run them using a chain; chains have "play
", or slack
, and a chain that long would require too much slack and bind up frequently. They began using a new system, a timing belt
In this system, there are anywhere from 4 to 5 sprockets, and one belt. There's the small sprocket for the crankshaft. Near that is a double-sized one, which acts as a "redirect" towards the camshaft or camshafts (depending on whether the engine is SOHC or DOHC). You'll then have one more sprocket for each camshaft. Finally, you'll have a tension sprocket, which exists to help feed the belt back into the crank sprocket, was well as maintain the proper play of the belt.
This system is slightly better performing than timing sprockets and chains, because a plastic belt and plastic sprockets exhibit less mass than a metal chain and metal sprockets. Lower weight allows more power to go to the wheels.
Also, it requires no lubrication. This means it can exist outside of a sealed environment, which makes it dozens of times easier to change. Today's cars are not supposed to need a tune-up for 100,000 miles, and that includes the timing belt. Your mileage may vary, though, so once again, consult your owner's manual.