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 Physics and the theoretical
 Horsepower vs CC
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JMalmsteen
Female New Member
13 Posts


Long Island/Lancaster, NY/PA
USA

Harley-Davidson

XL1200

Posted - 02/13/2014 :  6:25 PM                       Like
I am a physics teacher and this stuff is still making me crazy so I figured I would call upon the collective knowledge of the members of this site.

Apparently, the CCs of a motorcycle and the HP are very different things. For instance, I have a Suzuki S40 that is a 650 cc bike, but has around 30 hp. On the other hand, I have a Harley that is a 1200 cc bike and maybe it has 65 HP, I'll never know because Harley Davidson does not release HP numbers.

Now, there are sport bikes that are 600 cc and they can have 100 HP. A bike that I am looking at currently, the Yamaha FZ-09, is a triple with 847 cc with around 115 HP.

Why are ccs sometimes used to express the power of a motorcycle where other times it is useless because the true horsepower numbers are much more important, such as with a Honda CBR600rr vs. the 650 cc Suzuki Savage.

I would appreciate any insight into this mystery. Thanks, guys!

kacinpa
Male Advanced Member
802 Posts
[Mentor]


Lansdale, PA
USA

Triumph

Sprint GT

Posted - 02/13/2014 :  10:05 PM
Let me get this straight.

You teach PHYSICS?

The displacement of an engine is how much volume of air/fuel gets pumped through the cylinders. How much horsepower is dependent on the torque output and RPM range of the engine. This is determined not only by the displacement but by design elements of the engine, how the valve train moves the intake and exhaust, how many cylinders are used to create that displacement, what type of induction system, how the exhaust are designed, etc.

Horsepower is a measure of work:

Horsepower = Torque (lb/ft) * RPM / 5252

Also 1 Horsepower = 746 Watts

Generally with engines of a similar design, a larger displacement engine will have higher horsepower. A H-D V-twin 883 will have less HP than the 103ci engine. The Suzuki inline 4s go up in horsepower from the 650 to the 1,000 to the 1,300, but the 650 single in that S40 has way less power than the inline 4 650 in the GSXR.

This is why an S40 is a decent beginner bike, but a GSXR is a bad choice because of the high power output that comes on fast as the RPMs build.

Edited by - kacinpa on 02/13/2014 10:10 PM
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 02/14/2014 :  12:38 AM
Displacement is the size of the bucket. Power comes from how much you can get in the bucket and how many buckets(rpm) you carry in a minute.

How much you put in the bucket depends on valve size, how long the valves are open and how unrestricted the intake and exhaust allows the air to move. There are some other factors but those are the basics.
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JMalmsteen
Female New Member
13 Posts


Long Island/Lancaster, NY/PA
USA

Harley-Davidson

XL1200

Posted - 02/14/2014 :  4:44 AM
Thank you for the replies.

Yes, I teach math and physics in NY. I used to practice law and have an MBA in Finance and investments. That's my background.

For some reason, engineers are physics people are attracted to motorcycles.

I'm hoping to teach the MSF class in PA, and before doing something like that, I need to make sure that I can fix any gaps in my knowledge. I also want to be able to answer any questions that my physics kids have. For some reason, kids today don't seem to care how engines work or how their car works. Part of it is probably that cars are more complicated than they used to be compared to the '70's and even '80's cars before they computerized everything.

Edited by - JMalmsteen on 02/14/2014 5:06 AM
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scottrnelson
Advanced Member
6886 Posts
[Mentor]


Pleasanton, CA
USA

KTM

990 Adv, XR650L

Peer Review: 1

Posted - 02/14/2014 :  9:21 AM
For a better understanding of how much power you can get out an engine, read the writings of Kevin Cameron. He's a columnist for Cycle World magazine, but has also written several books. The best one would be Classic Motorcycle Race Engines: Expert Technical Analysis of the World's Great Power Units which discusses the tradeoffs of engine design for more than 50 engines with lots of details on how the various motorcycle companies squeezed more power out of their engines. The other great book on the subject is Sportbike Performance Handbook which gives more general details about how motorcycles work. You would think from the title that it's about how to make your bike faster, but it's really about how motorcycles work and how to make them work better.

Ultimately, the faster the engine goes, the more air/fuel mixture it can ignite in a given amount of time and the more power it can make. But there is a lot more to it than that, including how well the cylinder is filled and how well the mixture ignites once it's in there. The displacement directly affects overall power if everything else is equal, but bigger engines can't go as fast, so it isn't equal.

Both of the above books are definitely worth reading for anyone who wants to understand motorcycles better, especially for those interested in physics.
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JMalmsteen
Female New Member
13 Posts


Long Island/Lancaster, NY/PA
USA

Harley-Davidson

XL1200

Posted - 02/14/2014 :  11:02 AM
Great. I just ordered the second one :)
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/14/2014 :  11:10 AM   Join poster on Facebook as Friend  
These days not many manufacturers claim horsepower, probably out of fear that someone will dyno the bike and want their money back if it comes shy of the advertised power.

Some lawyers actually did that with small engine makers ie: lawn mower, snowblower etc. A class action case was won. Now all I see is cc size of those engines. Only problem with that is, I never knew what cc those things had, I only paid attention to relative HP. I have no idea if a 179cc is big enough, or 208cc or whatever.

But, the magazines routinely put their test bikes on a dyno, and publish what they got, on that day, on that bike, on that dyno.

I think the HP guys are the sporting guys, and the displacement guys are more likely the cruiser guys.

Both are probably a little silly about these things. The 600cc sportbike guy talks about 125 HP, but, it makes that at 12-13,000 rpm or higher. Not exactly an around town friendly power curve. The cruiser guy says, I have 1700cc (probably talk in inches though), and everyone is to assume that means BIG power and fast. The truth might be, it makes 70 HP, and the rev limiter is 5,500 rpm.

In the end, the power characteristics are designed into the engine for the typical use of the bike. The cruiser guy wanting stump pulling torque at a low rpm, and relaxed rpm at highway speed. The sportbike guy likes to rev it up, keep it in the powerband, squirting from corner to corner, but not likely to really utilize the maximum power, and rpm, in every gear, or everyday.

The standard bike, sport tourer guy likes the best of both worlds and has an engine that is designed to be a compromise of decent torque and decent horsepower. Typically a detuned sport bike engine.

In the end, it's really more about bragging rights, whether it's big cc or big power.
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Gryphon Rider
Male Junior Member
45 Posts


Calgary, Alberta
Canada

Honda

Valkyrie Tourer

Posted - 02/14/2014 :  2:05 PM
Let's start with torque. Torque is the twisting force ("moment", in physics terms) produced at the crankshaft. Air/fuel mixture is sucked into the engine with the intake valve open and as the piston moves away from the cylinder head. The intake valve closes and the piston moves up, compressing the mixture into a space recessed into the head, called the combustion chamber, above the cylinder. The ratio between the volume of the cylinder and combustion chamber together when the piston is down, and the volume of the combustion chamber alone when the piston is up, is the "compression ratio". The compressed mixture is ignited by the spark plug, and the pressure of the explosion pushes the cylinder back down. The exhaust valve opens, allowing the spent gas to be pushed out of the cylinder as the piston returns to the top. Once at the top, the exhaust valve closes and the intake valve opens and the cycle repeats its four strokes: 1. Intake - piston moves down 2. Compression - piston moves up, 3. Ignition (explosion) - piston driven down, 4. Exhaust - piston moves up. The two main things that influence the amount of torque the engine produces on its ignition stroke are the cylinder size and the compression ratio. A larger cylinder means more mixture, which produces a more powerful explosion. A higher compression ratio means that the combustion has higher thermal efficiency, meaning more of the fuel's energy is converted to mechanical energy and less is converted to heat. I'll let you do the research to understand how that works. Other secondary factors, such as valve and ignition timing, cylinder diameter to stroke proportions, fuel mixture homogeneity, and intake and exhaust gas flow all contribute to torque.

So, larger, higher compression, better designed engines produce more torque at a given RPM.

Now, we'll look at power. As stated by kapinka, power is directly proportional to engine speed, or more precisely, how much chemical energy in the gasoline is converted to mechanical energy in a given period of time. If two engines can produce the same amount of torque, the engine that is spinning twice as fast will produce twice the power, because it is converting twice the fuel to mechanical energy. Here's where it gets complicated in relating engine size to engine power. An engine with smaller pistons has less reciprocating mass and produces less energy for each ignition stroke, and thus puts less stress on all the moving parts related to the pistons. This enables the engine to spin faster before the stresses take a toll on the engine, thus producing more power per unit of engine size than a slower spinning engine. Thus a smaller but faster spinning engine can produce the same power as a larger but slower turning engine. This begs the question, from a performance perspective, why not only use small, light engines and spin the heck out of them to produce power? The answer relates to endurance and driveability. When we drive, we are not always using engine speeds where the most power is produced. In fact, we are almost always using low-to-mid engine speeds, where a larger engine's higher torque produces more power than a smaller engine's lower torque. Also, if our gear ratios are spaced out further, as we are accelerating, we need to use the mid engine speeds to get to the high engine speeds where more power is produced. A larger engine will seem to have consistent acceleration, while a smaller engine will have the acceleration be less at first, then build as the tachometer climbs higher.

One other consideration when looking at power vs. engine size in motorcycles, is that some motorcycle engines are two-stroke rather than four-stroke engines. Without explaining the "how", I will say that a two-stroke engine ignites the air/fuel mixture EVERY time the piston is moving down, not EVERY OTHER time, as a four-stroke engine does. This allows a two-stroke engine to make roughly twice the power as a four-stroke engine of the same size.

I'll add one more thing: The cruiser guy with a 1700cc engine that produces only 70HP needs to get his engine checked out; something's wrong!
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 02/14/2014 :  2:53 PM
Two stroke cycle engines don't meet EPA emissions requirements so they won't be found in late model street bikes or most other modern bikes for that matter. Two strokes have the intake, compression, combustion and exhaust of the four stroke cycle compressed into each up and down movement of the piston. Cylinder wall intake and exhaust ports are covered and uncovered by the piston and, possibly, reed or rotary valves in the intake ports. These limit the amount of air and fuel that can be stuffed into each cycle compared to cam operated valves on four strokes so "twice the power of four stroke" is not an actual figure.

Still, before EPA regulations arrived, two stroke engines ruled the race circuit due to significant, though not double, power advantages. They were also lighter.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/14/2014 :  3:17 PM   Join poster on Facebook as Friend  
Many 1700 cc ish engines from Harley, Yamaha, Kawasaki make high 60's HP. Many of the 1600cc versions make about 50 HP from the same manufacturers.

Brand new, no tune-up required yet.
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Gryphon Rider
Male Junior Member
45 Posts


Calgary, Alberta
Canada

Honda

Valkyrie Tourer

Posted - 02/15/2014 :  6:35 PM
quote:
Originally posted by rkfire

Many 1700 cc ish engines from Harley, Yamaha, Kawasaki make high 60's HP. Many of the 1600cc versions make about 50 HP from the same manufacturers.

Brand new, no tune-up required yet.


I'm sorry, I guess I made the assumption that because my 1520cc cruiser is rated at 95-100 hp at the crank, a 1700cc cruiser should make better than 70 hp. I hope they make up for it with locomotive-like torque.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/15/2014 :  8:33 PM   Join poster on Facebook as Friend  
Which is kind of what I was saying before, bigger displacement "must" mean more power right?

They know their customer though. So the big engine, tuned for big torque, that has to come into play right off the bat in the relatively small rpm range.

I've never ridden a big cruiser, but I'm sure it's satisfying to have a big thrust at low rpm from light to light.

In this discussion, that's the reason why a big inch motor makes little HP vs a small high revving engine with big HP, they are designing the characteristics for wants of the intended customer. Whether those wants are real or imagined...lol.

For the same reason, it's probably a pretty good first bike like the S40 Savage with a relatively low HP, but good off the line torque which may be forgiving in clutch and take off rpm errors.
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wbrownell9
Male Junior Member
58 Posts


New Castle, DE
USA

BMW

R1200RT

Posted - 02/16/2014 :  1:25 PM
Gryphon Rider gave a nice explanation of the sequence of events for a four-cycle engine but in the interest of accuracy I'd make one correction. The third event, the power stroke, begins with the rapid combustion of the fuel-air mixture, NOT an explosion. There is a difference between the two. Burning occurs because there's a flame front: in an engine it starts out as a tiny sphere at the spark plug gap which then expands as the mixture burns. In an explosion, the entire mixture goes off all at once, with no flame front.

If the mixture is truly exploding, you'll hear knocking/pinging/detonation (all different words for the same phenomenon) which is bad. Modern engines use knock sensors to adjust the timing and fuel delivery to minimize pinging, even when substandard gas is used. My bike's manufacturer recommends premium but the engine controls let it use regular without harm.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/16/2014 :  4:10 PM   Join poster on Facebook as Friend  
It may very well be possible that different fields of craft use a differnt definition than another field for the same word. It shouldn't be, if it's a scientific term, but I'm throwing out the possibility.

I come from a Fire and Explosion Invetigation field of reference.

The definition of Explosion is: "The sudden conversion of potential energy (chemical or mechanical) into kinetic energy with the production and release of gases under pressure, or the release of gas under pressure. These high pressure gases then do mechanical work such as moving, changing, or shattering nearby materials."

NFPA 921 Guide for Fire and Explosion Investigations. ( Sec.1-3 definitions)

With that definition in mind, it seems to describe what's happening inside an internal combustion engine. The difference between the engines explosion, and that of a typical "explosive" is the speed is whether the velocity of the reaction is slower than the speed of sound (deflagration) or faster than the speed of sound (detonation).

























t
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 02/16/2014 :  5:27 PM
Detonation is good in an explosive but bad in an IC engine. Engines develop power from combustion.

http://www.enginelogics.com/engine-detonation/
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/16/2014 :  10:28 PM   Join poster on Facebook as Friend  
See Pat, that guy in your link is using the term detonation in a totally foreign manner to me.

Engine designers talk about flame fronts etc, where they see the process in a slow motion manner. They do strive for a quick and complete combustion, for power, efficiency, and emissions. They'll set up the incoming air in a swirl, and try to introduce the fuel in a mist which is one of the biggest advantages with fuel injection.

I'd still call it an explosion, but deflagration not detonation.
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 02/17/2014 :  9:38 AM
Some people see jumping out of a perfectly good airplane in flight as recreation. It's all about perspective.
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/17/2014 :  2:16 PM   Join poster on Facebook as Friend  
quote:
Originally posted by greywolf

Detonation is good in an explosive but bad in an IC engine. Engines develop power from combustion.

http://www.enginelogics.com/engine-detonation/



That author is silent as to whether there is an explosion taking place in an engines combustion chamber, or not

He missed an oportunity to clarify his use of detonation, when he explains that the noise of detonation is caused by a force causing the structure of the engine to ring or resonate as if hit by a hammer. That wasn't calrifying to me.

If we take his definition of detonation as being unburned gas spontaneously combusts, ignited solely from the heat and pressure. Then, deisel engines, which are IC engines, operate just fine using "detonation".

I suspect the term detonation in the automotive world simply goes back to the origin of engines. Maybe like their use of the words engine and motor, even though they'd all agree the industry uses those terms interchangably, but shouldn't.
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gymnast
Moderator
4263 Posts
[Mentor]


Meridian, Idaho
USA

Harley-Davidson

Sportster Sport

Posted - 02/17/2014 :  2:41 PM
http://en.wikipedia.org/wiki/Engine_knocking
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rkfire
Advanced Member
1689 Posts


Stratford, CT
USA

Suzuki

Bandit

Posted - 02/18/2014 :  11:08 AM   Join poster on Facebook as Friend  
Gymnast, it's not what is engine knocking, it's the use of the word "detonation".

I began by trying to define the word explosion, then detonation as being faster than the speed of sound, and deflagration as slower than the speed of sound, and only after an assertion that there wasn't an explosion occuring in an engine.

Explosion seems to have been thought of as only a high explosion, from a high explosive, such as dynamite etc, and an engine having no explosion, but some sort of slow, ordinary combustion. The only difference between combustion and explosion of an ordinary flammable or combustiible material is, the speed, and whether it's occuring in an enclosed space or not.

So, take ordinary combustible materials, in a finely divided state, and put them in a confined space, and introduce ignition. Like the fuel air mixture in an engine, dust in a grain elevator, woodshop, sugar, or flour, etc can, and has caused an explosion. There's been some enormous explosions of this type, and often building have been leveled from the explosion.

Firefighters, oil tank scrappers, miners, utility workers in underground work, use a Combustible Gas Indicator. They are measuring for the presence of flammable gas, even if it is an oil, or gasolene tank. The indicator shows it as potentially UEL, LEL, which is abreviation for Upper EXPLOSIVE Limit, or Lower EXPLOSIVE Limit.

Another type of explosion is a BLEVE. That stands for Boiling Liquid Expanding Vapor EXPLOSION. Typically a flammable iquid container exposed to the heat from an uncontrolled fire. Pressure builds up beyond the containers ability, and there's a violent explosion and fire once the container has split open and releasing the gas. The flammable liquid is often propane, or GASOLENE. Tanks have been known to travel up to a mile or so from the blast.

Back to engines, isn't a backfire an explosion? They can occur in the intake or exhaust tracts of an engine, and can and have caused damage to those tracts. Carbs or air filter, mufflers damaged or split apart.

Why does an unmuffled engine exhaust make that loud booming noise? If the fuel air mixture was simply burning, wouldn't the exhaust note be like...pffft pfft, or woosh woosh. Or maybe it makes the noise it's making because there are explosions making quite the bam, bang, boom.

My point is that an explosion means more than the results of a high explosive. It's like the definition of fire. In my former business it was: Rapid oxidation with the evolution of light and heat. I'd imagine not many here would define it as a form of oxidation.
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greywolf
Male Moderator
1492 Posts
[Mentor]


Evanston, IL
USA

Suzuki

DL650AL2

Posted - 02/18/2014 :  1:27 PM
Explosive and internal combustion engine scientists and engineers use the same terms to mean different things. You'll just have to get used to it. Mixed usage is not uncommon in other fields either.
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