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I do realize that it stands for revolutions per minute, but what are the revolving pieces that are measured? Assuming it is the crankshaft, is RPM calculated by the crank shaft sensor, or by some other means? Why do some old, carbureted cars not have a tachometer--is it for the lack of the ECU and the sensor?

Here's a scenario:
A 1,000cc 4-stroke engine with 4 cylinders is running at 1k RPM. How many times does each cylinder fire per second? How can this be calculated?

Also, does the piston provide force to the crank shaft with each stroke, even during the exhaust stroke? Does the engine create equal torque with each stroke of the piston, or do each of the four strokes apply different amounts of power? If so, why don't the RPMs fluctuate with each different stroke of a cylinder?

  • Plenty of carbureted cars have tachometers (RPM meter), and plenty of fuel-injected cars don't have carbureters--so I'm not sure fuel-delivery is related to ability to measure RPM. – Trevor D Mar 4 '16 at 21:28
  • @ᴘᴀᴜʟsᴛᴇʀ2 You're right, but I did it anyway.. lol Please review and let me know if I managed to pooch anything in my train of thought on this.. – cdunn Mar 4 '16 at 21:34
  • The difference would be cycle vs revolution – Mike Oct 19 '17 at 14:41
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Ok, let's start from the same picture so we're on the same page:

Simplified Engine layout

These engines are four stroke engines, which means the piston goes up and down a total of 4 times, twice up, and twice down for each cycle.

So to answer your questions:

Of course I do realize it stands for revolutions per minute, but revolutions of what?

As you stated later, it's revolutions of the crank shaft.

I assume the crank shaft, how is it calculated, is it by the crank shaft sensor? Why do some old cars that runs with a carburetor don't have the RPM meter is it for the lack of the ECU and the sensor?

As for how it's measured, there are both mechanical and electrical ways to measure it. Older cars didn't have a tachometer just because they were not popular, and to limit the cost. Many older cars did have tachometers though, just mechanically driven. I'm not 100% sure where the pickoff was for this, but if I had to bet it was just a gear off the crankshaft that gets reduced to a needle movement on the dash. In the same way that the speedometer was usually a gear in the transmission that did the same thing just for wheel speed.

As for the electrical methods, yes, it's just a sensor talking to the ECU which sends the right PWM (Pulse Width Modulation) signal to a gauge in the dash.

Also assuming a 1000 cc big 4 stroke engine with 4 cylinders running at 1k RPM how many times are each cylinder firing per second or how can that be calculated, also does the piston "revolve" the crank shaft with each stroke even at the exhaust stroke with the same torque or is it different at the firing stroke, if so how is the RPM not constantly going up and down with each different stroke

To lay this out, lets look at how many times the shaft rotates with each stroke. From the diagram you can see that a full up and down path of the piston makes for one revolution of the crankshaft. That means in a four stroke engine the shaft rotates twice (two revolutions) for every four stroke cycle, which means one power stoke for every two revolutions of the crank. So, if the crank is turning 1000 times per minute, that would be 500 power strokes (firing of the spark plug) on each cylinder in that one minute. Since there are 4 cylinders, there would be 2000 sparks in that one minute across the whole engine.

And no, the piston does not produce the same torque on every stroke. In fact it only produces torque on the power stroke. The other three it's just along for the ride.

As for why RPM does not vary constantly, the plugs don't all fire at once. The power strokes are spread out so that during the rotation of the crank the power is divided amongst the 4 cylinders so one of them is almost always producing power and turning the crank. In addition, the flywheel has a lot of mass, and tends to smooth out the roughness of each individual cylinder firing during it's power stroke. Otherwise it would be a much rougher cycle.

I think that covers it all, if anyone finds something I missed, or if I managed to get something wrong, please yell and I'll just edit it so we have this right.

I hope that helps!

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    Older vehicles which had a mechanical tac would usually get that off of a gear on the distributor shaft. Also, the RPM can vary all the time, but usually only on a very small scale. The engine will keep it near a target RPM, but it may only vary by ~1-2%. I know this is being anal about it, but like to be clear. The tac will only show what I consider to be an "average" of the RPM. Considering how most tacs have the scale crammed into such a small area, it only makes sense they won't register completely accurate. Great answer BTW!!! +1 – Pᴀᴜʟsᴛᴇʀ2 Mar 4 '16 at 21:45
  • In some carbureted vehicles, the tach runs off the coil. That's how its hooked up in my 70 pickup. – rpmerf Mar 4 '16 at 21:46
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    Never saw this before. Nice answer. – DucatiKiller Mar 18 '16 at 0:06
  • I put a dash from a petrol audi 80 into my audi 80 that had a 1.6L diesel engine and to make the tacho work used a magnet glued to the crank pulley with a hall effect transducer and some electronics to correct the pulses to make the tacho correct as the tacho was originally driven by the spark pulses... – Solar Mike Mar 15 '17 at 22:01
  • Mechanical tachs are often driven off the end of the cam shaft. Cam shafts generally turn at half engine speed which makes it a little easier to convert the cable spinning to a gauge reading. – Tim Nevins Oct 19 '17 at 15:10
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The piston goes up and down TWICE for every revolution of the Crankshaft. One compression, one exhaust stroke. If the crank is rotating at 1000rpm... every piston will go up and down 2000 times. The piston goes from top dead center to the bottom of the stroke and back up in 1/2 a revolution.... a 4 stroke refers to the piston going up down up down, (up is one stroke, down is one stroke... the piston travels the entire length of the stroke 4 times every revolution.) Every 1/4 turn of the crank, the top of the piston will go all the way up or all the way down. 4 strokes is one cycle..... one cycle is one revolution of the crank. Torque is not measured at the crank or during a a compression stroke.... which is the proper term... not a power stroke. The spark plug fires just before the piston reaches top dead center (10 degrees BTDC before top dead center).... since the explosion is while the fuel air mixture is being compressed.... (ignition timing) it sends the piston back down with force so that it can make it back up to exhaust the exploded and come back up to do it over again.... every cylinder thousands of times per minute.... the more fuel that is let in, the faster the crank will be sent around.... Torque is usually measured at the wheels on a Dyno.... low torque will not spin the dyno as fast..... The wheels spin in relation to torque.... high torque... the wheels spin faster than the car can move.... burning rubber.... low torque.... you cant break the tires loose.... except if you have less resistance.... like gravel, wet pavement or dirt. Hope that helps.....

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    This is incorrect - the piston goes up and down once per crankshaft revolution - twice per camshaft revolution. The four-stroke cycle takes two crankshaft revolutions. – Nick C Apr 27 '17 at 8:29
  • nick is correct 1 crankshaft rotation = 2 strokes. 1st the {Intake Stroke} piston moves DOWN from TDC to BDC end of intake stroke. 2nd {Compression Stroke} piston moves UP from BDC to TDC end of compression stroke. {3rd Power Stroke} spark ignites mixture the piston is forced DOWN from TDC to BDC this is the end of power stroke. 4th {Exhaust Stroke} piston moves UP from BDC to TDC this is the end of exhaust stroke thus completing the 4th stroke of the 4 stroke engine so you see crankshaft rotates 2 times to complete 4 strokes. – gvett Apr 25 '18 at 4:22

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