Ignition System Theory Question
 

Just a general theory question.. is there more then 1 firing order pattern for spark plugs in a 4 cylinder engine?

http://www.howstuffworks.com/ignition-system.htm

It's showing the firing order as 1 3 2 4.. But by my figuring it should be 1 3 4 2??

Dawn

Most every inline four cylinder has a separate crank 'throw' for each cylinder so firing order can be in any order the manufacturer wants it to be. But due to balance issues, the 1,3,4,2 sequence is most popular. Also, on lot of newer cars with DIS ignition, the ignition is fired in cylinder pairs. Usually 1 and 4 are fired off the same coil while 3 and 2 are on another coil. Even if there is a single coil for each cylinder, it is likely that two of them are fired at the same time. It is setup so that one coil fires during the power stroke while the second fires during the exhaust stroke. Firing the plug on the exhaust stroke also is said to improve emissions.

What he said. :) AFAIK all Proteges fire 1-3-4-2.

The 1-3-2-4 firing order used to be much more popular before EFI became commonplace because it gives better mixture distribution at low rpm with a carburetor.

Most 4-cyl. carb manifolds are designed with a "tri-Y" runner cofiguration where the manifold starts with 2 runners at the carb- one to the front of the engine and one to the back- and each runner then splits into 2 more runners for each cylinder. On all carb engines, the air is mixed with fuel before it flows through the manifold (this is known as a "wet-flow" design) and the air must continue moving so that the fuel will remain mixed with it. However, the air will always stop when it encounters a closed intake valve.

What does this have to do with firing order? Think about the order that the intake valves open on the front 2 cylinders and the back 2 cylinders. On a 1-3-2-4 engine, it is front-back-front-back. However, on a 1-3-4-2 engine, it is front-back-back-front. Remember the tri-Y manifold? If the 1-3-4-2 firing order is used, the air in each runner will stall because the intake valve is closed for 2 consecutive strokes. This hurts low-rpm power, drivability and emissions because the fuel will not stay mixed. However, the 1-3-2-4 order will keep the air moving and improve performance because the air will only stall for 1 stroke at a time.

Modern EFI engines mix the fuel with the air only after it passes through the intake manifold (known as "dry-flow" design). Therefore, mixture distribution is not a problem and firing order can be whatever the engine designer wants. Ain't modern fuel management great? :)

OK, I know this has little to do with Proteges, but hey, you asked. :D

Wow thanks for all the info.. now I have more questions :)

You refer to the cylinder pairs as the back and front.. I thought they were in a line? is the 1/4 pair the front or the back?

Why would an engine designer want them to fire in an order that isn't most efficient.. unless the pairs are different? I thought 1/4 and 2/3 pairs were always together because it was symetrical.. lessens engine shaking?

Also.. how does firing in the exhaust cycle help emissions? it fires at the end of the exhaust cycle to burn any left over gas? Is it healthy to send burning stuff out through the valve(s)?

Dawn

Yes, I'm talking about an inline 4-cylinder. Typically, the "front" of an inline-4 is the the timing belt end, the "back" is the flywheel end, and the cylinders are numbered 1-2-3-4 from front to back. The front 2 cylinders I'm referring to are Nos. 1 and 2; the rear cylinders are Nos. 3 and 4.

Most modern inline-4 engines originally equipped with a single carburetor have a "tri-Y" intake manifold arrangement with one runner leading to cylinders 1 and 2, and the other runner leading to cylinders 3 and 4. This is what I wrote about in my last post.

This design was widely used in the old days because it is typically the easiest way to feed an inline-4 with one carb; tying cylinders 1 and 3 (or 2 and 4) together with a "tri-Y" in order to use a 1-3-4-2 firing order is very difficult with a wet-flow manifold because you have to twist the runners around each other, and fuel vapor does not like turning corners or traveling uphill. Other ways of feeding an inline-4 with one carb (such as a "log" style manifold) have serious and usually fatal fuel-flow and mixture distribution problems. Using individual carburetors for each cylinder also solves the problem, but this makes engine expensive, complicated and hard to tune.

And no, cylinders 1-4 and cylinders 2-3 do not necessarily have to be paired. Changing the firing order just requires a different crankshaft design. The 1-3-4-2 arrangement is inherently better balanced, but any firing order can be accomodated fairly easily on an inline-4 because of the small number of cylinders.

Basically, there are other things to consider when designing an engine other than balance. :D

Wow thanks so much for all your help.. I think I understand.. I can't wait to take apart an engine and see what all this stuff really looks like (covered in dirt and goo :) )..