What does RPM gage measure?

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Originally Posted By: Vikas
OK, I am smirking here but a guy asked what is tachometer is measuring and you go on telling him about crankshaft! What an irony!! It is like when your 6 year old asks you where babies come from and you start explaining X and Y chromosome and cell membrane to her.


I have a big grin on my face too..... the "I have no idea what you guys are talking about so I'll just smile"
crazy.gif


One more question about automotive that I don't yet understand. What are people referring to when they talk about Liters? like 1.8L, 2.2L, 3.6L, 5.0L, etc. I know Liter is a measurement of volume of liquid (2L pop bottle). What in the engine holds 1.8L and of what is it holding?
 
Originally Posted By: Quattro Pete
Originally Posted By: Rick in PA
Got me to thinking about it. For extra credit: on the standard 4-cycle engine, the crankshaft makes two revolutions to move the displacement of the motor. For example, with a 2 liter motor the crankshaft rotates twice to move 2 liters of air,

Not sure I follow. If you look at the animated gif above, each piston travels all the way down and all the way back up during a single crankshaft revolution (360 degrees). Why are you saying it takes two revolutions? Or are you referring to a different engine design than what's depicted above?



Pretty sure Rick is talking about the fact that it takes two revolutions of the crankshaft to complete the 4-stroke cycle.
 
Originally Posted By: dparm
Originally Posted By: supton
VW intentionally has the speedometer read high;



Source? That's a strange claim.


Any VW forum; I'll look later if it matters. Long reported that VW reads 3% high, as, the word is, in Germany if someone gets a ticket and it turns out that the speedometer is at fault then the OEM is at fault and gets fined. So they read high intentionally to CYA, err, CTA. I've verified that SG reads mph slower than dash, and GPS likewise was the same. Odometer reads properly, as expected.
 
I believe Rick is not correct. A 4-stroke engine does not take two revolutions to move the displacement of the engine. By definition, an engine's displacement is (essentially) bore X stroke X number of cylinders. You take the square area defined the bore's circle and multiply that by the stroke to get the volume of that one cylinder. It takes exactly one engine revolution for the piston to travel down and travel up again, thus moving one displacement's volume of air.

Now, it does take two engine revolutions of a 4-stroke engine to complete the 4-stroke cycle, as Stelth notes above. But indeed takes one engine revolution to move one displacement's volume of air.
 
Originally Posted By: supton
Originally Posted By: dparm
Originally Posted By: supton
VW intentionally has the speedometer read high;



Source? That's a strange claim.


Any VW forum; I'll look later if it matters. Long reported that VW reads 3% high, as, the word is, in Germany if someone gets a ticket and it turns out that the speedometer is at fault then the OEM is at fault and gets fined. So they read high intentionally to CYA, err, CTA. I've verified that SG reads mph slower than dash, and GPS likewise was the same. Odometer reads properly, as expected.


Toyotas commonly read two mph high as well. The odometer is spot-on, but the speedometer is programmed to read a touch high. By law, a speedometer cannot read LESS than the actual speed, but it can read MORE. So many speedometers are programmed to read slightly higher, to allow for production variances, tire circumference differences, etc.

For what it's worth, I have noticed that Chrysler and Honda speedometers are be pretty accurate. Our Chrysler minivan and both of our current Honda vehicles are bang-on correct.
 
Originally Posted By: Eosyn
Originally Posted By: Vikas
OK, I am smirking here but a guy asked what is tachometer is measuring and you go on telling him about crankshaft! What an irony!! It is like when your 6 year old asks you where babies come from and you start explaining X and Y chromosome and cell membrane to her.


I have a big grin on my face too..... the "I have no idea what you guys are talking about so I'll just smile"
crazy.gif


One more question about automotive that I don't yet understand. What are people referring to when they talk about Liters? like 1.8L, 2.2L, 3.6L, 5.0L, etc. I know Liter is a measurement of volume of liquid (2L pop bottle). What in the engine holds 1.8L and of what is it holding?


Its displacing volume. Whether air or water or air/fuel mixture required for combustion.
The more volume the engine displaces the more potential power output the engine can produce.
 
There is an EU directive related to car speedometers that states:

Quote:

- The indicated speed must never be less than the actual speed, i.e. it should not be possible to inadvertently speed because of an incorrect speedometer reading.
- The indicated speed must not be more than 110 percent of the true speed plus 4 km/h at specified test speeds. For example, at 80 km/h, the indicated speed must be no more than 92 km/h.


So, this applies to all cars, not just VW. But it does not automatically mean that your VW speedo must read high. On my old Audi A4 it was actually spot on. But there is always a risk that some idiot will mount wrong size tires and make a speedo read low, so that's why they typically build out that buffer in the speedo.
 
Originally Posted By: Stelth
Originally Posted By: Quattro Pete
Originally Posted By: Rick in PA
Got me to thinking about it. For extra credit: on the standard 4-cycle engine, the crankshaft makes two revolutions to move the displacement of the motor. For example, with a 2 liter motor the crankshaft rotates twice to move 2 liters of air,

Not sure I follow. If you look at the animated gif above, each piston travels all the way down and all the way back up during a single crankshaft revolution (360 degrees). Why are you saying it takes two revolutions? Or are you referring to a different engine design than what's depicted above?



Pretty sure Rick is talking about the fact that it takes two revolutions of the crankshaft to complete the 4-stroke cycle.



Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.

A typical 4 gets two firings per 'rev'.

Some of you guys need to do some reading.
 
Originally Posted By: SteveSRT8
Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.


By 'full cycle', I think we mean the entire 4-stroke cycle. As in, intake, compression, power, exhaust. Every 4-stroke engine I'm aware of, regardless of the number of cylinders, needs two revolutions of the engine to complete the 4-stroke cycle for any given cylinder in that engine.
 
Thank you Pete and Clevy for helping me understand and the links. You two rock
thumbsup2.gif
thumbsup2.gif
 
Last edited:
Originally Posted By: SteveSRT8
Originally Posted By: Stelth
Originally Posted By: Quattro Pete
Originally Posted By: Rick in PA
Got me to thinking about it. For extra credit: on the standard 4-cycle engine, the crankshaft makes two revolutions to move the displacement of the motor. For example, with a 2 liter motor the crankshaft rotates twice to move 2 liters of air,

Not sure I follow. If you look at the animated gif above, each piston travels all the way down and all the way back up during a single crankshaft revolution (360 degrees). Why are you saying it takes two revolutions? Or are you referring to a different engine design than what's depicted above?



Pretty sure Rick is talking about the fact that it takes two revolutions of the crankshaft to complete the 4-stroke cycle.



Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.

A typical 4 gets two firings per 'rev'.

Some of you guys need to do some reading.


I need to do some reading? Let's see, in a 4-cycle engine, such as the V8 you reference, each cylinder has an intake and compression stroke, which require one revolution of the crankshaft, and then a power and exhaust stroke, which require another revolution of the crankshaft. 1+1=2, at least where I come from.
 
Originally Posted By: Hokiefyd
Originally Posted By: supton
Originally Posted By: dparm
Originally Posted By: supton
VW intentionally has the speedometer read high;



Source? That's a strange claim.


Any VW forum; I'll look later if it matters. Long reported that VW reads 3% high, as, the word is, in Germany if someone gets a ticket and it turns out that the speedometer is at fault then the OEM is at fault and gets fined. So they read high intentionally to CYA, err, CTA. I've verified that SG reads mph slower than dash, and GPS likewise was the same. Odometer reads properly, as expected.


Toyotas commonly read two mph high as well. The odometer is spot-on, but the speedometer is programmed to read a touch high. By law, a speedometer cannot read LESS than the actual speed, but it can read MORE. So many speedometers are programmed to read slightly higher, to allow for production variances, tire circumference differences, etc.

For what it's worth, I have noticed that Chrysler and Honda speedometers are be pretty accurate. Our Chrysler minivan and both of our current Honda vehicles are bang-on correct.


BMW speedometers read high as well.

My ES reads spot on, however my RX reads two mph high.
 
Originally Posted By: Stelth
Originally Posted By: stevesrt8


Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.

A typical 4 gets two firings per 'rev'.

Some of you guys need to do some reading.


I need to do some reading? Let's see, in a 4-cycle engine, such as the V8 you reference, each cylinder has an intake and compression stroke, which require one revolution of the crankshaft, and then a power and exhaust stroke, which require another revolution of the crankshaft. 1+1=2, at least where I come from.


I beg your pardon, I misunderstood the topic here completely.
 
Originally Posted By: SteveSRT8
Originally Posted By: Stelth
Originally Posted By: stevesrt8


Man, so far off it's not even funny!

What kind of engine? A regular 90 degree V8 needs one revolution to fire FOUR cylinders. One cylinder can indeed complete a full cycle in ONE revolution of the crank.

A typical 4 gets two firings per 'rev'.

Some of you guys need to do some reading.


I need to do some reading? Let's see, in a 4-cycle engine, such as the V8 you reference, each cylinder has an intake and compression stroke, which require one revolution of the crankshaft, and then a power and exhaust stroke, which require another revolution of the crankshaft. 1+1=2, at least where I come from.


I beg your pardon, I misunderstood the topic here completely.


No worries, Steve. I think you were responding thinking of the original topic (RPM's), and some of us had blasted off on a tangent (4-stroke cycle) that was only peripherally related. Isn't that what makes discussion boards fun?
thumbsup2.gif
 
Originally Posted By: Eosyn

Personally, in automatics, I'd rather have other gauges that will give information I'm more interested in, such as outside temp, avg MPGs, more tripometers (use these to keep track of my fluid changes). I just don't see the need for a tachometer in automatics (then again, I'm not a mechanic. perhaps they are used to troubleshoot problems)?



See, I think of ALL those other things you mentioned as useless fluff and I'd rather have the tach. To me, the key instruments for a vehicle are road speed (of course), engine temp, electrical voltage (or even better but harder to implement reliably: charge/discharge current), oil pressure, and engine RPM. Also useful is oil temp and transmission oil temp. The tachometer is really a diagnostic- even in a manual transmission, no one uses the tach to decide when to shift. You might glance at it briefly to see if its safe to DOWNshift, , but routine shifting by the tach is stupid... and takes your eyes off the road. Your ears are your upshift indicator except under the most extreme conditions, and then a big dragster-style shift light in the line of sight is a better instrument.

In automatics, the tach tells you if the transmission is slipping, what gear you're in climbing a hill (and whether you might ought to manually select a lower gear when towing), whether the torque convertor is locked etc. In a manual it tells you if the clutch is slipping, and as I mentioned its useful to gauge whether a downshift coming to a curve would over-rev the engine or not.
 
Originally Posted By: Clevy
Rpm measures Crank rotations per minute.

They are there just as another point of reference. The driver can see at what point to shift(manual)and whether there is any slip,which could indicate potential problems.
In the 70's and 80's they were in sportier type cars. Now they are there to make the driver think they are in a sporty type car.
Grabbing a signal out of the ECU to make a tach (AKA revolution per minute counter) needle on the dash move is cheap, but of little use. An oil pressure gauge or amp meter might just tell the owner something, a tach with an automatic transmission is just a decoration.
 
Originally Posted By: Stelth

I need to do some reading? Let's see, in a 4-cycle engine, such as the V8 you reference, each cylinder has an intake and compression stroke, which require one revolution of the crankshaft, and then a power and exhaust stroke, which require another revolution of the crankshaft. 1+1=2, at least where I come from.


Right

A full cycle consists of four segments each with 180* of crank rotation... Note valve timing is shifted somewhat by cam specs for performance but we are talking basics...

TDC = Top Dead Center -- BDC = Bottom Dead Center

#1 INTAKE, TDC to BDC, intake valve open, exhaust closed...

#2 COMPRESSION, BDC to TDC, both valves closed

#3 POWER(includes ignition), from TDC to BDC this is the 180* of rotation that really counts, again both valved closed

#4 EXHAUST, from BDC to TDC, exhaust valve opens, burned gases are pushed from cylinder...

The process repeats...

A V8 engine has two of these events completeing every 90* of crank rotation(a 4cyl only one)...

On start up power could theoretically begin after the first rotation(not likely in the real world, assumes cylinder at TDC, intake valve opening), then all four processes(two rotations) must be complete to maintain the cycle...
 
Originally Posted By: TFB1
Originally Posted By: Stelth

I need to do some reading? Let's see, in a 4-cycle engine, such as the V8 you reference, each cylinder has an intake and compression stroke, which require one revolution of the crankshaft, and then a power and exhaust stroke, which require another revolution of the crankshaft. 1+1=2, at least where I come from.


Right

A full cycle consists of four segments each with 180* of crank rotation... Note valve timing is shifted somewhat by cam specs for performance but we are talking basics...

TDC = Top Dead Center -- BDC = Bottom Dead Center

#1 INTAKE, TDC to BDC, intake valve open, exhaust closed...

#2 COMPRESSION, BDC to TDC, both valves closed

#3 POWER(includes ignition), from TDC to BDC this is the 180* of rotation that really counts, again both valved closed

#4 EXHAUST, from BDC to TDC, exhaust valve opens, burned gases are pushed from cylinder...

The process repeats...

A V8 engine has two of these events completeing every 90* of crank rotation(a 4cyl only one)...

On start up power could theoretically begin after the first rotation(not likely in the real world, assumes cylinder at TDC, intake valve opening), then all four processes(two rotations) must be complete to maintain the cycle...




I think the statement that caused confusion involved whether or not a 4-cycle engine "pumps" or "moves" its full displacement of air once per crank turn, or every two crank turns.

The answer is every TWO, disregarding volumetric efficiency and precise valve timing. For any given cylinder, every other turn of the crank occurs with the valves both CLOSED (compression/combustion "strokes") so no air is actually moved. On the other turn (exhaust/intake) the valves are open and (again disregarding timing, scavenging, miller cycle-like stuff, forced induction, and volumetric efficiency) the full displacement of air is moved.

So disregarding secondary effects and looking at it only as an air pump, a 440 cubic inch v8 (why not use the best and greatest as an example? ;-) ) moves 220 cubic inches of air from the intake manifold to the exhaust manifolds every turn of the crank.
 
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