Bore x Stroke

[DannytheRadomski]

For higher RPMs is it better to have a wider bore and shorter stroke or vice-versa. Also how does the bore size and stroke length affect horse power and torque?

[Dash]

F = M * A

Think about it.

[Racer-X]

Think about the piston velocities and weights of the different combinations. Which one is going to have more inertia?

As far as which makes more torque vs. power. I have always been a believer in that displacement is displacement. If all other factors are equal it isn't going to matter so long as they breathe and rev the same. Now all other things are never equal, think about what I said above about piston velocities and how that is going to limit your revs. Then there is also the matter of how well it breathes. A bigger bore means bigger valves, which means more air without running crazy lift, which means more top end.

[PatClarke]

Danny, I think a quick Google search would find you more information on this subject than you are likely to read here.

Pat

[DannytheRadomski]

Just to clarify, I am a high school student who wants to get a jump start on knowing engines so I can join a FSAE team without a huge gap to make up. So I figured the people on here would know.
what does F=MxA stand for?

[acedeuce802]

A good bit of advice for before you start college is to learn to be more self sufficient when it comes to these types of questions. We don't currently follow through with this, but we used to have this rule. If someone asked a question, and the receiver of the question could find it on the first page of a google search, they were owed a beer (assuming they were 21). If you google "engine bore and stroke" and "f=m*a" you will get clear definitions to both of your questions. This forum should be more for the questions you have after you've spent 20 hours crunching numbers and researching your local library and numerous internet articles, and still can't grasp a concept.

[Jay Lawrence]

Danny,

Surely your highschool has a physics subject? If so, take the subject. In it you will learn some basics (like Force = Mass x Acceleration) that are a good engineering starting point. As for preparing to do FSAE engines, go out to a wrecker and buy an old/cheap (though preferably watercooled and definitely OHC) engine and investigate. This will teach you some mechanical skills as well as some engine 'theory vs. practice.'

As for your specific question: for a given capacity, a smaller bore will have to have a longer stroke ( = bigger crank = won't rev as high). Compare the bore/stroke of a typical diesel engine and it's max RPM with that of a typical petrol engine.

EDIT: Adam beat me to it. But yes, typically these sorts of forums are for when you've done a bit of research yourself

[Owen Thomas]

Good on you for learning on your own! High school students usually have... different priorities.

F = ma is the equation behind Newtons second law (Force = mass * acceleration). You will cover it in your high school physics course. That equation is affectionately referred to by people on this forum (and engineers in general) as the first step in solving any mechanics problem (ie anything involving forces, masses and accelerations(read:everything)).

Racer-X addressed your question quite well, I would suggest going through his post and wikipedia-ing the terms you do not yet understand. The answer will come to you with a bit of thought http://fsae.com/groupee_common/emoticons/icon_smile.gif

[DannytheRadomski]

Thank you everyone. Right now FSAE is a deciding factor in where I go to college.

My main thought that goes with this question is, since a wider bore and shallower stroke would mean less distance travelled by the piston, would you be able to achieve higher revs?

Thanks for the junkyard idea, I'll have to do that.

[Warpspeed]

Danny,
You can also build an engine that has a lot of bore (very large piston area) and a very small stroke, by having a greater number of smaller cylinders for a given total capacity.

[AxelRipper]

http://www.epi-eng.com/piston_...son_of_cup_to_f1.htm (http://www.epi-eng.com/piston_engine_technology/comparison_of_cup_to_f1.htm)

Enjoy.

[acedeuce802]

Also, since your new to this and seem to have an interest in engines: http://www.profblairandassocia...s/Back_to_basics.pdf (http://www.profblairandassociates.com/pdfs/Back_to_basics.pdf)

[Z]

Gordon Blair's "Back to Basics" article (linked above by Adam) sums up pretty much all you need to know about IC engines for FSAE...

Here is an even more basic summary of the "explosion" engine.
~~~o0o~~~

TORQUE.
For at least the last 50 years, any naturally-aspirated, normal sized engine (ie. not from toy-car or ship), running on reasonable 80+ octane fuel, generates a torque of about 100Nm per litre engine displacement. This is regardless of bore/stroke ratio.

With some good I/E pipe tuning, and higher compression ratio thanks to 95+octane fuel, you can add about 20% to that figure for a peak torque value. Supercharging simply multiplies the torque by the boost ratio.

So a NA 600cc FSAE engine has 60Nm torque, or maybe 72Nm peak torque with well tuned pipes. Supercharge to 2 bar absolute and it has 120 -140Nm. A 4 litre NA auto engine has 400 - 480Nm, and so on...

Incidentally, Blair (and most other engine engineers) uses BMEP for this measure of volume-specific torque. The conversion formula is;
Torque/litre (Nm/litre) = ~8 x BMEP (bar).
So Blair's figure of ~14 bar being typical of a good racing engine at peak power equates to 8 x 14 = 112 Nm/litre.
~~~o0o~~~

POWER.
Power is torque by rotational velocity. Since pi = ~3, in metric units Power (kW) = ~ Torque (Nm) x RPM/10,000.

So that 600cc FSAE engine (not considering its restrictor) revving to 12,000 rpm should put out about ~70Nm x 1.2 = ~84 kW maximum. If it can only rev to 8,000 rpm, then only ~70 x 0.8 = ~ 56 kW.

To convert to horsepower just add one-third. So 56 kW = 56 + ~19 = ~75hp. That figure is "at the crank", but roughly one-quarter is lost on the way to the ground. So 56 kW at the crank is (very) roughly 56 hp at the wheels (ie. as measured on a chassis dyno).
~~~o0o~~~

REVS.
So how fast can an engine go? For at least the last 50 years... any decent racing engine has its redline set at a "mean piston speed" of about 20m/s. MPS is directly related to the stroke and revs (see Blair article), so shorter stroke means higher peak revs.

For MPS = 20m/s:
100mm stroke => 6,000 rpm,
80mm => 7,500 rpm,
60mm => 10,000 rpm,
50mm => 12,000 rpm,
40mm => 15,000 rpm,
and so on.

Very well developed racing engines have maximum MPS up around 25m/s (see Jack Kane article linked in Axel's post). Pushing MPS beyond 30m/s is possible, say, with drag racing engines, but then their mean-time-before-failure (MTBF) is measured in seconds...
~~~o0o~~~

SUMMARY.
How much torque and power from, say, a NA 3 litre racing engine with 50mm stroke?
Max Torque = 3 x 120 = 360Nm.
Max Revs = 12,000 rpm (or 15,000 if big $$$ available).
Absolute Max Power <= 360 x 1.5 <= 540 kW <= 720 hp.

If they tell you any more, then tell'em they're dreaming! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z

[Jan_Dressler]

I agree on everything but the following:

Originally posted by Z:
So how fast can an engine go? For at least the last 50 years... any decent racing engine has its redline set at a "mean piston speed" of about 20m/s.
[...]
Very well developed racing engines have maximum MPS up around 25m/s
[...]
Max Revs = 12,000 rpm (or 15,000 if big $$$ available).
Max MPS of 25 m/s don't necessarily mean "big $$$ very well developed racing engines" (as in: "Formula 1-type budget") today... These values are reached bye some (albeit high performance) production engines.

Examples, MPS at redline:
Aprilia RSV4, 2010: 24,6 m/s
Honda CBR 1000RR, 2010: 24,6 m/s
Suzuki GSX-R 1000, 2010: 26,3 m/s
Car engines:
Honda F20C (the S2000 engine), 1999: 25,2 m/s
Honda K20A (various), 2001: up to 24,7 m/s
Honda K24A2 (various), 2004: up to 25,1 m/s
etc., these just came to my mind.

I agree on the "beyond 30 m/s" part, though. http://fsae.com/groupee_common/emoticons/icon_wink.gif

[Z]

Jan,

I originally wrote that part as:
"Very well developed racing engines (and anything built by Honda!) have maximum MPS up around 25m/s...".
I deleted the bit about Hondas because I have several of their industrial engines (water pumps, generators, etc.) and these run somewhat more slowly, so not quite true...

However, I note that four of your six examples are Hondas! http://fsae.com/groupee_common/emoticons/icon_biggrin.gif

(BTW, none of my Hondas has missed a beat in decades. I recently bought a cement mixer with a Chinese copy of a 6hp Honda. Worked ok for a while, but after six months in the sun all the rubber/plastic bits turned into either jelly, or chalk. http://fsae.com/groupee_common/emoticons/icon_frown.gif Should have paid the extra $100!!!)

Z

[DannytheRadomski]

I don't know if any of you have heard Jeremy Clarkson's quote about Honda, "Honda's plan was to make engines first and money hopefully." And that's why they make the best engines.

[Homemade WRX]

I do like Blair and am actually working with one of his associates currently.

Anyway, the rule of thumb calcs to see how they added up with my most recent Subaru engine project and it stacks up fairly well.

2.432L
3 bar boost (absolute)
875 Nm (roughly 650 lb-ft)
75mm stroke (101.6mm bore for the sake of the argument)
25 m/s @ 10,000 RPM (though it spins 10,200 for over rev)
Power 875 (roughly 1170 bhp)

Calculated BMEP this way, which I hadn't worked out before comes up as 45...
360 Nm/L / 8 = 45

Might start kicking around the numbers from my old senior project turbo 250 on E85/19mm

Originally posted by Z:
Gordon Blair's "Back to Basics" article (linked above by Adam) sums up pretty much all you need to know about IC engines for FSAE...

Here is an even more basic summary of the "explosion" engine.
~~~o0o~~~

TORQUE.
For at least the last 50 years, any naturally-aspirated, normal sized engine (ie. not from toy-car or ship), running on reasonable 80+ octane fuel, generates a torque of about 100Nm per litre engine displacement. This is regardless of bore/stroke ratio.

With some good I/E pipe tuning, and higher compression ratio thanks to 95+octane fuel, you can add about 20% to that figure for a peak torque value. Supercharging simply multiplies the torque by the boost ratio.

So a NA 600cc FSAE engine has 60Nm torque, or maybe 72Nm peak torque with well tuned pipes. Supercharge to 2 bar absolute and it has 120 -140Nm. A 4 litre NA auto engine has 400 - 480Nm, and so on...

Incidentally, Blair (and most other engine engineers) uses BMEP for this measure of volume-specific torque. The conversion formula is;
Torque/litre (Nm/litre) = ~8 x BMEP (bar).
So Blair's figure of ~14 bar being typical of a good racing engine at peak power equates to 8 x 14 = 112 Nm/litre.
~~~o0o~~~

POWER.
Power is torque by rotational velocity. Since pi = ~3, in metric units Power (kW) = ~ Torque (Nm) x RPM/10,000.

So that 600cc FSAE engine (not considering its restrictor) revving to 12,000 rpm should put out about ~70Nm x 1.2 = ~84 kW maximum. If it can only rev to 8,000 rpm, then only ~70 x 0.8 = ~ 56 kW.

To convert to horsepower just add one-third. So 56 kW = 56 + ~19 = ~75hp. That figure is "at the crank", but roughly one-quarter is lost on the way to the ground. So 56 kW at the crank is (very) roughly 56 hp at the wheels (ie. as measured on a chassis dyno).
~~~o0o~~~

REVS.
So how fast can an engine go? For at least the last 50 years... any decent racing engine has its redline set at a "mean piston speed" of about 20m/s. MPS is directly related to the stroke and revs (see Blair article), so shorter stroke means higher peak revs.

For MPS = 20m/s:
100mm stroke => 6,000 rpm,
80mm => 7,500 rpm,
60mm => 10,000 rpm,
50mm => 12,000 rpm,
40mm => 15,000 rpm,
and so on.

Very well developed racing engines have maximum MPS up around 25m/s (see Jack Kane article linked in Axel's post). Pushing MPS beyond 30m/s is possible, say, with drag racing engines, but then their mean-time-before-failure (MTBF) is measured in seconds...
~~~o0o~~~

SUMMARY.
How much torque and power from, say, a NA 3 litre racing engine with 50mm stroke?
Max Torque = 3 x 120 = 360Nm.
Max Revs = 12,000 rpm (or 15,000 if big $$$ available).
Absolute Max Power <= 360 x 1.5 <= 540 kW <= 720 hp.

If they tell you any more, then tell'em they're dreaming! http://fsae.com/groupee_common/emoticons/icon_smile.gif

Z