Interesting thing to think about

HokieRider

HokieRider

Member
I heard something the other day that i haven't been able to stop thinking about:

Modern MOTOGP bike engines are pushing close to, if not hitting, 18-20,000RPM. Because of their stroke lengths most of these engine's pistons with BREAK THE SOUND BARRIER somewhere between 23-25k.

Aviation has taught us that everything changes when you go faster than the speed of sound. Obviously things like vibration change dramatically, but also the characteristics of things like heat and moisture change.

Im not an engineer and I wasnt so great at physics, but something tells me that this is going to present a SERIOUS problem

Somehow i doubt my Buell 1200 pistons will ever threaten MACH 1...but wouldnt it be cool if they did
 
betarace

betarace

Member
Speed of sound is roughly 1275 feet per second. Most sport bikes incl your buell have piston speeds in the 4,000 to 5,000 range during normal operation. Balance is key.
 
HokieRider

HokieRider

Member
beta,

I think normal piston speeds are around 4000-5000 feet per minute

EX:
My buell has a stroke length of 3.8in
so 1 rotation covers 7.4 in

7.4x5000(rpm)=37000in
37000/12=3083ft/min
 
Meat

Meat

Member
Ruhe52;229118 wrote: Meat time to get your Nerd on. :popcorn:
Click to expand...
I saw this but have been burned too many times trying to explain any sort of physics/engineering/stress analysis to anyone on the forum.

People have preconceived notions about how physics works and there is nothing I can do to change that for most people.

It's a bummer because I love and live this kind of stuff but I have learned my lesson.

But the Mach numbers people have given are correct (760-795 mi/hr for almost all the conditions we will be in on the track). They depend on things like humidity, temp, pressure. Mach 1.0 at sea level is much faster (in ft/sec) than Mach 1.0 at 60,000 feet.
 
Meat

Meat

Member
And in the world of aeronautics, Mach 1.0 is considered transonic. Supersonic is usually ~Mach 1.2. But that is in aeronautical engineering, it could be different in other industries.
 
HokieRider

HokieRider

Member
Meat,
I can understand all that, the speed of sound changes depending on the relative conditions.

But are these engineers going to run into any BIG problems when pistons approach and possibly pass this speed?

I honestly have NO idea and would love to hear opinions.
 
Bubba Zanetti

Bubba Zanetti

Member
As a side note, this is one of the main reasons the two stroke purists got all pissed off at Honda for killing off the 2T’s. A four stroke is not cheaper to run if you hop it way up and spin it to death to make extra power. And that is one of the draw backs to a 4T, or any engine for that matter.

In the 60’s, when the East Germans figured out how to make two stroke race bikes scream and started winning championships. Honda did everything they could to make a 4T that could beat a 2T. Honda had a small 5 cylinder 125cc 4 stroke that could spin up to 20,000 rpms. The description of it running was ‘deafening’ and it still could not produce the power the 125 2T’s had. Of course all that has changed, but at a high price tag.

Anyway, I digress. Interesting question Hokie and I look forward to seeing what racing does with engine design.

BZ
 
S

sidepipe79

New Member
I am not sure we will ever see that problem. The guys in charge of MOTOGP are looking to cap the RPMs of future engines.
 
cooker1

cooker1

New Member
sidepipe79;229156 wrote: I am not sure we will ever see that problem. The guys in charge of MOTOGP are looking to cap the RPMs of future engines.
Click to expand...
Then it will turn into a 2 wheeled NASCRAP !!!!
 
Meat

Meat

Member
Hokie, this isn't actually my area of expertise but I would think one of the problems to work out is where the shock wave is as the piston becomes sonic, and how that affects the wave front of the burning gasses in the cylinder.

If the piston is just barely at Mach 1.0 the sonic fatigue probably wouldn't be much of an issue but I am not really sure.
 
Meat

Meat

Member
BTW: you can have discontinuous detonation/combustion if the shock caused by the sonic speed interrupts the wave front of the burning gasses in the cylinder, as I was guessing would be an issue.
 
rac_wv

rac_wv

New Member
Valve issues

I remember a while back an issue with F1. It was way over my head. Something along the lines of RPMs getting so high that the valves started becoming an issue. I think they got the pistons moving so fast that the valves didn't have enough time to close and were essentially locking open. I don't know if this was ever overcome, regulated out, or became a limitation.
 
You must log in or register to reply here.
Top