Is my math right? A 4 cycle engine at 2500 rpms fires each cylinder over 20 times a second?

[Kern Dog]

I was talking with my nephew today about how amazing that engines are and how it seems like everything has to go right for them to run as well as they do.
I told him that at freeway speeds when an engine is running at say....2500 rpms, each piston fires about 20 times a second. I based this on the cylinders firing on every other revolution given that this is a 4 cycle engine. That cuts the 2500 number to 1250. Then divide that by 60 to show what each second would see in terms of each cylinder firing.
I came up with 20.83.
This is something that I have never looked into this deeply before.
If this is true, if you shift at 6000 rpms, you're firing the pistons at 50 times in each second.
If this is true, that really is amazing.

 

[Jerry Hall]

First off a 4 cycle engine fires once every 4 revolutions. So 2500 divided by 4 divided by 60. 10.42 times a second if I'm thinking right. Still amazing.

 

[Kern Dog]

First off a 4 cycle engine fires once every 4 revolutions. So 2500 divided by 4 divided by 60. 10.42 times a second if I'm thinking right. Still amazing.

Are you sure about that? 4 cycles are Intake, compression, combustion and exhaust evacuation. That happens in 2 revolutions of the crank.

TDC to BDC for intake.
BDC to TDC for compression.
That is one revolution.
TDC to BDC for combustion.
BDC to TDC for exhaust evacuation.
Two revolutions.

I'm not arguing, I'm trying to wrap my head around this.

 

[Richard Cranium]

At 2,500 RPM, a spark plug in a four-stroke engine fires 1250 times per minute per cylinder. Since a four-stroke engine has two rotations of the crankshaft for each firing event, each cylinder fires once every two revolutions. Therefore, for a 2,500 RPM engine, each spark plug fires 2,500 / 2 = 1250 times per minute.

Divided by 60 = 20.83333

 

[Don Frelier]

We can do more math if you like.

1/20.833 = 0.048 seconds or 48 milliseconds between firing (2 rotations).

Now for argument sake say your timing is 36 degrees BTDC and you move it 2 degrees (either way)

So firstly 36 degrees is 36/720 degrees (2 rotations) = 1/20 of 48 millleseconds which is 2.4 milliseconds before the piston is all the way up.

If you move the timing say 2 degrees 2/720 = 1/360 of 48 milliseconds then you've adjusted the timing by 0.0001333 seconds or 133 microseconds.

So timing is everything as they say.

Now the acceleration and deceleration of the piston within the these timeframes is another whole discussion.
Especially for stroker motors.

And think fast.

 

[Jerry Hall]

Are you sure about that? 4 cycles are Intake, compression, combustion and exhaust evacuation. That happens in 2 revolutions of the crank.

TDC to BDC for intake.
BDC to TDC for compression.
That is one revolution.
TDC to BDC for combustion.
BDC to TDC for exhaust evacuation.
Two revolutions.

I'm not arguing, I'm trying to wrap my head around this.

Sorry, my thinking was screwed up. You are correct.

A 4-stroke, 4 cylinder engine fires twice per revolution. Just once. Each cylinder will move the crank 1/4 of a turn. this assumes a 4 stroke gas engine

Seems like as many of these things I have done, that I would have known. Getting old is clouding my thinking. Or, I didn't think long enough.

 

[toolmanmike]

@Kern Dog If you want to scramble your brain, check out these factoids about Drag cars. Top Fuel specifically.


* One dragster's 500-inch Hemi makes more horsepower then the first 8 rows at Daytona.

* Under full throttle, a dragster engine consumes 1 1/2 gallons of nitro per second, the same rate of fuel consumption as a fully loaded 747 but with 4 times the energy volume.

* The supercharger takes more power to drive than a stock hemi makes.

* Even with nearly 3000 CFM of air being rammed in by the supercharger on overdrive, the fuel mixture is compressed into nearly-solid form before ignition. Cylinders run on the verge of hydraulic lock.

* Dual magnetos apply 44 amps to each spark plug. This is the output of an arc welder in each cylinder.

* At stoichiometric (exact) 1.7:1 air/fuel mixture (for nitro), the flame front of nitromethane measures 7050 degrees F.

* Nitromethane burns yellow. The spectacular white flame seen above the stacks at night is raw burning hydrogen, dissociated from atmospheric water vapor by the searing exhaust gases.

* Spark plug electrodes are totally consumed during a pass. After 1/2 way, the engine is dieseling from compression-plus the glow of exhaust valves at 1400 degrees F. The engine can only be shut down by cutting off its fuel flow.

* If spark momentarily fails early in the run, unburned nitro builds up in those cylinders and then explodes with a force that can blow cylinder heads off the block in pieces or blow the block in half.

* Dragsters twist the crank (torsionally) so far (20 degrees in the big end of the track) that sometimes cam lobes are ground offset from front to rear to re-phase the valve timing somewhere closer to synchronization with the pistons.

* To exceed 300mph in 4.5 seconds dragsters must accelerate at an average of over 4G's. But in reaching 200 mph well before 1/2 track, launch acceleration is closer to 8G's.

* If all the equipment is paid off, the crew worked for free, and for once NOTHING BLOWS UP, each run costs $1000.00 per second.

* Dragsters reach over 300 miles per hour before you have read this sentence.

Did you know …

… that the nitromethane-powered engines of NHRA Top Fuel dragsters and Funny Cars produce approximately 7,000 horsepower, about 37 times that of the average street car?

… that one cylinder of the eight cylinders of a Top Fuel dragster or a Funny Car produces 750 horsepower, equaling the entire horsepower output of a NASCAR engine?

… that the gasoline-powered engines of NHRA Pro Stock cars produce about 1,200 horsepower, about eight times that of the average street car?

… that an NHRA Top Fuel dragster accelerates from 0 to 100 mph in less than .8-second, almost 11 seconds quicker than it takes a production Porsche 911 Turbo to reach the same speed?

… that an NHRA Top Fuel dragster leaves the starting line with a force nearly five times that of gravity, the same force of the space shuttle when it leaves the launching pad at Cape Canaveral?

… that an NHRA Funny Car is slowed by a reverse force more than seven times that of gravity when both parachutes deploy simultaneously?

… that NHRA Top Fuel dragsters and Funny Cars consume between four and five gallons of fuel during a quarter-mile run, which is equivalent to between 16 and 20 gallons per mile?

… that NHRA Top Fuel dragsters and Funny Cars use between 10 and 12 gallons of fuel for a complete pass, including the burnout, backup to the starting line, and quarter-mile run?

… that NHRA Top Fuel dragsters and Funny Cars travel the length of more than four football fields in less than five seconds?

… that NHRA Top Fuel dragsters can exceed 280 mph in just 660 feet?

… that from a standing start, NHRA Top Fuel dragsters accelerate faster than a jumbo jet, a fighter jet, and a Formula One race car?

… that a fuel pump for an NHRA Top Fuel dragster and Funny Car delivers 65 gallons of fuel per minute, equivalent to eight bathroom showers running at the same time?

… that the fuel-line pressure for NHRA Top Fuel dragsters and Funny Cars is between 400 and 500 pounds, about 20 times greater than the pressure on passenger-car fuel pumps?

… that depending on size and angle, the large rear wing on an NHRA Top Fuel dragster develops between 4,000 and 8,000 pounds of downforce?

… that the 17-inch rear tires used on NHRA Top Fuel dragsters and Funny Cars wear out after four to six runs, or about two miles? Some brands of passenger-car tires are guaranteed for 80,000 miles.

… that it takes just 15/100ths of a second for all 7,000 horsepower of an NHRA Top Fuel dragster engine to reach the rear wheels?

… that it's desirable for an NHRA Top Fuel dragster to race with its front wheels inches off the ground for about the first 200 feet of the run? This ensures proper weight transfer to the rear wheels, a crucial part of a good launch and quick run.

… that the nitromethane used to power the engines of NHRA Top Fuel dragsters and Funny Cars costs about $30 per gallon?

 

[Kern Dog]

Yeah, that is some amazing stuff!
Thanks for that.

 

[hanks73340]

I was never good at math but the above formulas that were explained was very interesting and eye opening. To bad this stuff isn't taught anymore in shop classes in the high schools but then again those were the first classes to be eliminated during cutbacks.

 

[BB BELLA]

what amazes me is a formula 1 engine operating a 19,000 rpm, those pistons are changing direction 316/sec.

 

[Bobzilla]

* Spark plug electrodes are totally consumed during a pass. After 1/2 way, the engine is dieseling from compression-plus the glow of exhaust valves at 1400 degrees F. The engine can only be shut down by cutting off its fuel flow.

This one is incorrect. If no "trouble" occurs on a clean pass, the spark plugs come out looking like they could go into a daily driver for another 100,000 miles. That list has been around for along time but I have not seen in a while. Still enjoy looking through it though.

 

[copper67sat]

… that the nitromethane-powered engines of NHRA Top Fuel dragsters and Funny Cars produce approximately 7,000 horsepower, about 37 times that of the average street car

… that NHRA Top Fuel dragsters can exceed 280 mph in just 660 feet?

Cool info. I find the science and engineering behind top fuel fascinating.

Not sure of your source, but I have seen/heard recently that they are now making 11,000 - 12,000 hp out of that same 500 cid and Brittany Force is going 300 mph in 1/8 mile. I was at the Gatornationals in 1992 when Bernstein broke 300 in the 1/4!

I've done the same calcs as Kern Dog and it's hard to comprehend how these mechanical devices operate at these speeds. I've worked on liquid rocket turbopumps that operate at over 30,000 RPM and there are some that go as high about 90,000! Incredible pieces of machinery.

 

[Kern Dog]

I have this thought that in the early days, guys would modify engines for more power until they blew up, then they would improve the parts, the results increased power and the cycle continued.
Look at the octane ratings of gasoline in the 50s. I thought I read where many engines were 6 to 1 compression or lower. You could run 65-70 octane and it wouldn’t knock.
I read about how unreliable 6 volt charging systems were with generators…. Cars with mechanical brakes, not hydraulic. It is amazing how far they have come. I haven’t seen a late model car with crank up windows, nobody settles for a simple AM radio either.

 

[BB BELLA]

they don't make them like they use to!
I remember my dad telling me about racing my uncle, one in a ford and the other in a plymouth! my dad saw the battery fly out from under the car when they took air over a hill crest landing! when they got to the finish line my uncle was bragging on his car when my dad bet him double or nothing his car wouldn't start after running it that hard!

 

[64BEL]

This demo shows the amount of fuel pushed through a nitro engine's cylinder during one run.

 

[WileE]