Simple 4-Stroke Question

[PaykanHunter]

OK Guys. There are no stupid questions yes? Just as I thought I knew everything about engines a friend of mine asked me a question that has puzzled me for few days.

 

Say you got a single cylinder 200cc engine. At 5000 RPM, which is, if my calculation is right, 83.3 Revolution per Second. If we want know how many times per second "intake" has occurred, then we have to divide that number by half (intake happens once per two revolution). Then 83.3/2= 41.65 times per seconds. If we multiple that to the cylinder capacity 200cc, that is 8.33 Liter of air per second.

 

Sucking 8.33L of air per second @ 5000RPM. Wouldn't that create a vortex? I mean that seems crazy. Please educate me here.

 

Cheers! 

 

 

[Mr_Bo11ox]

sounds about right to me, you have to factor in volumetric efficiency (you won't get 200cc of air on every intake stroke) so maybe knock 15-20% off but in principle that sounds about right

[PaykanHunter]

sounds about right to me, you have to factor in volumetric efficiency (you won't get 200cc of air on every intake stroke) so maybe knock 15-20% off but in principle that sounds about right

Thanks, but what is Volumetric efficiency, at higher revs, you get less capacity in the cylinder? 

[Junkman]

The calculation is correct and yes, it would create a vortex.

For this reason, there is an airstream calming plenum chamber ahead of the intake, which often contains a filterisation element, and is thus frequently but incorrectly called an air filter.

[Mr_Bo11ox]

yeah pretty much, the engine sucks in the air through the manifold and throttle and that, because its sucking through some level of restriction, and that airflow if having to stop and start moving with every cycle you don't get the full 200cc each time. What you do get is some difficult-to-calculate number that results from the level of restriction, the shapes the air has to flow round and the properties of the air

[RayMK]

Intake geometry, particularly on modern engines, is designed to increase 'swirl,' or in some cases to preferentially produce the best combustable mixture at the point of ignition.  There is so much going on so quickly that the Dyson effect does not have much time to form.  Large marine engines, because they have cylinders the size of dustbins (or larger) and run slowly may be a different matter  .

[Junkman]

Thanks, but what is Volumetric efficiency, at higher revs, you get less capacity in the cylinder?

 

Gas kinetics and compressible flow are extremely complex sciences and not easy to explain in a few sentences.

Each engine has its intake tract designed in a way to have optimal filling at a certain rpm. This gives the engine its "characteristic".

[PaykanHunter]

I understand that, but even at loss of 20% that is still a lot of air being sucked in the engine. Wouldn't that act like a vacuum cleaner? AND is this the real reason behind the fact that maximum Torque and maximum Horsepower don't happen to be at the same RPM? 

[Junkman]

It does. But the suction is pulsative, not continuous, so you have a resonance effect.

[PaykanHunter]

It does. But the suction is pulsative, not continuous, so you have a resonance effect.

 

So what you saying is that because the suction is not happening all the time, one every two revolution, then we can't simply compare the engine intake to a vacuum cleaner? 

[worldofceri]

yeah pretty much, the engine sucks in the air through the manifold and throttle and that, because its sucking through some level of restriction, and that airflow if having to stop and start moving with every cycle you don't get the full 200cc each time. What you do get is some difficult-to-calculate number that results from the level of restriction, the shapes the air has to flow round and the properties of the air

 

I sat through hours of thermodynamics lectures at uni and probably never really grasped what volumetric efficiency is.  Until now.  I wish Mr_Bo11ox were my uni professor and not some hard-to-understand Iraqi guy, I might be some incredibly brilliant engine design engineer by now.  Or something.

[Mr_Bo11ox]

Torque is just a straightforward force. Power is how often you can generate that force in a certain time window. One person might be able to lift a heavier absolute maximum weight onto a table than another. but the 'weaker' one might be able to lift his 10 times per minute as opposed to once a minute for the 'stronger' guy. IN this case the weaker guy is more powerful cos more weight is getting lifted onto tables in a certain time.

 

With a car engine there is a certain speed at which the cylinder fills up best with air and petrol, giving the most powerful explosion, and therefore thats the speed with the greatest torque (force). The engine might well generate a smaller force higher up the rev range, but be able to generate that force more frequently, and therefore is capable of doing more work to push the car along. Theres a point at which the force x the frequency maxes out, and thats your peak power.

[Dave_Q]

I understand that, but even at loss of 20% that is still a lot of air being sucked in the engine. Wouldn't that act like a vacuum cleaner? AND is this the real reason behind the fact that maximum Torque and maximum Horsepower don't happen to be at the same RPM? 

 

Torque is the actual turning force of the engine at any given moment.

 

Power (or work) is the rate at which it's produced.

 

Power is calculated from torque, the full derivation is here if you're interested.

 

http://www.epi-eng.com/piston_engine_technology/power_and_torque.htm

 

Power is a function of torque and revs therefore the maximum power is usually at the top of the rev range despite the torque having dropped off by then.

[dollywobbler]

I understand that, but even at loss of 20% that is still a lot of air being sucked in the engine. Wouldn't that act like a vacuum cleaner? 

 

Yes. To experience for yourself, find a car with a nice downdraught carburettor, rev the engine, then slam your hand down on the top of the carb. Do not wear latex gloves when you do this! You'll discover just how hard an engine sucks then. This is my side-of-the-road method of unblocking a carburettor jet.

[Christine]

/\  /\ Make sure your flies are done up , you dont want  to let your cock near the said downdraught carb.....

 

About 2 minutes in.........it sucks a lot of air ...  https://www.youtube.com/watch?v=8M1o2rpO_JY

[PaykanHunter]

Torque is just a straightforward force. Power is how often you can generate that force in a certain time window. One person might be able to lift a heavier absolute maximum weight onto a table than another. but the 'weaker' one might be able to lift his 10 times per minute as opposed to once a minute for the 'stronger' guy. IN this case the weaker guy is more powerful cos more weight is getting lifted onto tables in a certain time.

 

With a car engine there is a certain speed at which the cylinder fills up best with air and petrol, giving the most powerful explosion, and therefore thats the speed with the greatest torque (force). The engine might well generate a smaller force higher up the rev range, but be able to generate that force more frequently, and therefore is capable of doing more work to push the car along. Theres a point at which the force x the frequency maxes out, and thats your peak power.

 

For years I have asked this question and nobody could explain it like you did. THANKS MATE!...  

[Christine]

Does the "weaker" guy who is "stronger" , pull more birds ?  Darwins theory , an all that...

[Mr_Bo11ox]

He's not stronger, but he is more powerful

[PaykanHunter]

/\  /\ Make sure your flies are done up , you dont want  to let your cock near the said downdraught carb.....

 

About 2 minutes in.........it sucks a lot of air ...  https://www.youtube.com/watch?v=8M1o2rpO_JY

HOLY F***!!!!!!!!!!!!!................ That is one serious Vortex!.... 

[Junkman]

Yes. To experience for yourself, find a car with a nice downdraught carburettor, rev the engine, then slam your hand down on the top of the carb. Do not wear latex gloves when you do this! You'll discover just how hard an engine sucks then. This is my side-of-the-road method of unblocking a carburettor jet.

 

Depending on which car/engine you do this, it could also be a very efficient method of unblocking the blood vessels in your hand. Unblocking towards the environment I mean.

[The Moog]

For years I have asked this question and nobody could explain it like you did. THANKS MATE!...

We should launch an AS university.

 

Cav to do sales

DW Economics

Bollox hard shit to understand

Junkman modern history

 

 

Bit like unseen university I guess.... With more dried frogs

[dollywobbler]

Depending on which car/engine you do this, it could also be a very efficient method of unblocking the blood vessels in your hand. Unblocking towards the environment I mean.

That's a fair point. I have only done this with feeble 2cv engines. Probably best not to try this with a highly tuned big block. You'll lose your arm.

[Christine]

In poor countries ,  rebores are done by lobbing Vim or pumice powder into the vortex at 5000rpm...

 

 

If you see a car advertised in Greece with recon engine.. Avoid.

[Junkman]

That's a fair point. I have only done this with feeble 2cv engines. Probably best not to try this with a highly tuned big block. You'll lose your arm.

 

Worse. The resulting ragout would definitely clog up your carburetter jets.

[Christine]

A lot of people on this forum ought to be working for NASA ?

[Taff]

A lot of people on this forum ought to be working for NASA ?

 

NASA couldn't afford us

 

on a vaguely related note, earlier today I had a 14 year old ask me "Dad, what does a cam shaft do?"

[Junkman]

on a vaguely related note, earlier today I had a 14 year old ask me "Dad, what does a cam shaft do?"

 

The future is bright!

Buy that boy a beer.

[red5]

NASA couldn't afford us

 

on a vaguely related note, earlier today I had a 14 year old ask me "Dad, what does a cam shaft do?"

 

Good lad - he'll go far.

 

Strictly speaking nothing in itself - it's the cam lobes that open inlet and exhaust valves. A camshaft is literally a series of cam lobes mounted (machined) onto a common shaft.    ooer.