Rpm to cylinder quantity correlation

[mjoekingz28]

I cannot quite put this together in my head, but I think that in a V8 each cylinder will fire but a few times per minute at 1000rpm- maybe 125 times? And a single cylinder thumper would obviously fire 1000 times a minute.


Is this correct or would each cylinder in a V8 fire 1000 times every rpm.







Oh no, new thought. Does a single cylinder even fire 1000 times per 1000rpm since there is the exhaust stroke? So maybe it fires 500 times per 1000 rpm.

 

[Shannow]

1 RPM is one revolution of the crankshaft per minute.

A two cycle engine fires once per revolution (per cylinder)

A four cycle engine fires once per two revolutions (per cylinder)

So at 1,000 RPM
* a 2 stroke does 1,000 firings
* a 4 stroke does 500 firings.

Multiply either by the number of cylinders for the number of firings per minute.

 

[Jetronic]

As shannow said.

So the V8 will fire each cilinder 500 times per minute, and the single cilinder (if it's a 4-stroke aswell) also.

 

[Astro14]

RPM refers to the number of crankshaft rotations per minute Each piston/rod combination is connected to the crankshaft, so each assembly goes up/down once on each rotation. So, for a single cylinder, that one assembly goes up/down 1,000 times at 1,000 RPM. On a V-8, each of the 8 rod/piston assemblies goes up/down 1,000 times at 1,000 RPM.

As was said, in a 4-stroke engine, that means 500 sparkplug firings for EACH cylinder at 1,000 RPM.

The number of up/down cycles per minute changes with RPM - but the net effect of more cylinders is that there are more power pulses, more firing events, during each rotation. So, on a single cylinder, there is a power pulse every 720 degrees of crankshaft rotation (for a 4-stroke). That's why they vibrate so much - big pulse every other rotation. On a 4 cylinder, then, you get a power pulse every 180 degrees (on average, crank design matters) - much smoother. Now, a V-8 gets a power pulse every 90 degrees (again, on average, the crank designs vary) so it is even smoother. And the V-12? Every 60 degrees of crank rotation sees a power pulse, so it's very, very smooth.

On that 4-stroke V-12 (the extreme case vs. the single cylinder), you're getting 500 firing events PER CYLINDER per minute at 1,000 RPM, but because there are 12 cylinders, you're getting 6,000 spark plug firings/power pulses per minute at 1,000.

Take a look at YouTube - when I was a kid, I built a model called the "Visible V-8" - and I'll bet that animations of what's going on inside an engine like a V-8 are on YouTube these days...

 

[Trav]

Originally Posted By: Astro14
when I was a kid, I built a model called the "Visible V-8" - and I'll bet that animations of what's going on inside an engine like a V-8 are on YouTube these days...


First engine I ever built.

 

[Alex_V]

On a 4-stroke engine each cylinder fires every other revolution, so at 2,500 RPM it has fired 1,250 times per minute. This is a universal principle and the only difference on multi-cylinder engines is that you have (however many) identical assemblies doing it simultaneously. So a V8, at 2,500 RPM, will have had 10,000 combustion cycles per minute by each cylinder firing 1,250 times.

2-stroke engines combine the power/intake and compression/exhaust strokes, and fire every revolution - making them less efficient but more powerful for a given displacement.

 

[CT8]

I built one as well.