Engine Displacement and Cylinders

[ivymike1031]

V12 engine is usually the practical limit in cylinder numbers.

There are, of course, exceptions. I don't think I've ever seen a modern engine with more than 24 cylinders. I did see a W-18 at the detroit auto show, which is about the most cylinders I've ever seen in a road vehicle.

[SaabJohan]

Basicly more cylinders -> lower effiency -> less power

this is because more cylinders means bigger surface area, and that means that more heat will go into the cylinderwalls and into the engines coolingsystem instead for going to the crankshaft.

But we can't make too big cylinders either because then the travelpath for the combustionflame will be too long so combustion will take too long time. It's wanted that the combustion goes fast, this means that small cylinders with small bore are good (and centrally placed sparkplugs), but the speed also depends of the turbulence created before ignition.

[Steel]

unless of course you have 2 plugs per cylinder, like my suzi bike engine.

[454Casull]

Is it possible to place two spark plugs into a car engine cylinder?

[ivymike1031]

Is it possible to use 2 plugs per cyl on a car engine? yes, Honda does that on the 2nd-generation IMA drivetrain. (Civic hybrid) It's also done on rotary engines, but that's not really the same thing...

[enginerd]

Mercedes also uses two plugs per cylinder. One fires before TDC and another fires shortly after TDC.

[Tom_S8]

Originally posted by enginerd
Mercedes also uses two plugs per cylinder. One fires before TDC and another fires shortly after TDC.

Alfa romeo also uses the same thing...

[TheMan5952]

So do the 4 cyl ford rangers, it helps reduce emissions.

[454Casull]

How does that work? Doesn't the resulting combustion push the piston backward (against its proper cycle)? I can understand if the plug fires at TDC and after, but not before and after.

[ivymike1031]

The spark plugs in every automobile that I can think of fire many degrees before TDC, even at idle. In general, the faster the engine is running, the more the timing is advanced.

The thing that you may be missing - it takes time for the charge to burn, and you want the peak pressure to happen at a useful point in the cycle (a little after TDC). You've got to give the flame a head start if you want that to happen. That's the big reason why spark-induced knock happens, and why faster-burning fuel helps to reduce knock. If you give the spark too much of a head start, you'll reach too high of a temp&pressure as you approach TDC, resulting in knock. If your fuel burns faster, you don't have to give the spark so much of a lead, and you can thus give yourself a bigger margin to avoid knock. If your mixture burned so fast that you could always ignite the mixture after TDC, you could run much higher compression ratios w/o fear of knock (you'd probably start to get a diesel-like pressure trace, but I'm not sure).

[SaabJohan]

The only time when the spark is fired very late is when you're starting your engine. If the fuel is ignited to early during startup the engine may go backward or create enough resistance so that startermotor will eventually break. And this is only done to engines with EMS.

Twin sparkplugs can be used on two ways, the can ignite both on the same time or not. Twin sparkplugs gives a few advantages under low and mid rpm and under part-load but not during high rpm and full-load.
Twin sparkplugs was used first/invented by Alfa Romeo.

[454Casull]

Ohhhhhhhhh!!!

/chalk one up for the dumbass

[TrvlynAlec]

this is probably going to sound really stupid, but what does TDC mean?

[ivymike1031]

top dead center: when the piston pin is farthest from the crankshaft axis of rotation.

[454Casull]

Twin sparkplugs gives a few advantages under low and mid rpm and under part-load but not during high rpm and full-load.

Why not?