Number of Cylinders

[marcush.]

well ,I would look into the clearance Between Piston and head of the std
engine first .If you duplicate that clearance and assume the same stiffness of the crank things should be ok as long as your valvetrain gears ,belts or chain are properly dsigned to cope with the rpm.
Of course ,anything that is doifferent to the stock engine could be a course for engine failure .Common sense and testing will show..but there are virtually hundreds of reasons that may make the engine fail.....good luck!

[Reca]

Fergie_Racer :
Can you please post the bore ?
And, maybe I’m showing my ignorance but... what’s CFM ? cubic feet per minute ?
Thank you.

[Fergie_Racer]

Sorry I didn't give all the information before

Bore = 49mm
Stroke = 36.6mm
Total Volume = 550cc
Cyl Vol = 68.75cc
Air Restrictor = 20mm
Distance between bores = 60mm
Con-Rod length = 106mm
Compression ratio = 14.5
Peak Pressure = 38.9 bar

CFM is cubic feet per minute. I don't know what the SI equivilant is to that (CMM?).

I took all the dimmentions of the original engine, and tried to keep the exact dimentions of the combustion chamber untouched besides increasing the stroke by approx. 3mm. I read a few SAE papers today, and asked some professors for help, and started writing a matlab code to help calculate these vibrations.

[Reca]

So, 1 CFM = 4.72 10^-4 m3/s => 100 CFM = 4.72 10^-2 m3/s.
Meaning that, for a 4 stroke engine of 550 cc, this flow rate allows to reach (assuming volumetric efficiency = 1) : 4.72 10^ -2 [m3/s] / 550 10 ^ -4 [m3] * 120 = 10300 rpm.
But this leads me to a consideration. Assuming ideally the total area of the restrictor (diameter = 20 mm.) the velocity giving 100 CFM is 150 m/s while speed of sound is roughly 340 m/s (obviously it depends by air temp but that’s just for a ballpark evaluation). So you are actually using less than 50% of the geometrical area because of boundary layer thickness/separation. I don’t have direct experience of restrictors design but it looks a bit low, is that an usual result ? How it’s currently designed the restrictor you’re using ? Is the design imposed by SAE or rule only specifies the value for the minimum area of the intake ?

[Fergie_Racer]

I don't have much experience with the intake and restrictor design, but we use Fluent (a CFD program) to optimize our intake restrictor. Other teams have similar designs, and report pretty close to the same numbers as we are getting. The design is imposed by competition rules. All teams must pull their air through a 20mm restrictor for regular fuel (93-100 octane), or 18mm if they use ethinol. We are allowed to super/turbo charge our engines, but they much attached down wind of the restrictor. Our team hasn't dealt with turbo charging yet.

[AV]

Hello,

This forum is great ! I was wondering if any one could suggest a good book on Fundamentals of Engine Mechanics( which would include detailed analysis, math etc). I am a student at CU Boulder involved with FSAE team, I also did a couple of CFD analysis on the Intake system! using COSMOS and Fluent!. I would like to get enlightened on the mechanics aspect of Engine design, as I dont have much knowledge other than the basic Theory of machines knowledge, balancing etc!

Aravind

I don't have much experience with the intake and restrictor design, but we use Fluent (a CFD program) to optimize our intake restrictor. Other teams have similar designs, and report pretty close to the same numbers as we are getting. The design is imposed by competition rules. All teams must pull their air through a 20mm restrictor for regular fuel (93-100 octane), or 18mm if they use ethinol. We are allowed to super/turbo charge our engines, but they much attached down wind of the restrictor. Our team hasn't dealt with turbo charging yet.

[Reca]

Fergie_racer,

After a little search :
(displacement, restrictor diameter, peak power rpm => percentage of ideal area used at peak power assuming freestream temp = 15°C, hence speed of sound at the restriction is 372 m/s)

F3 engine : 2000 cc., 26 mm, 6000 rpm => 50%
Maserati Trofeo Light : 4244, 42.7, 7000 => 46%
Ferrari 360 GTC : 3586, 2x30.8, 8750 => 47 %
Ferrari 575 GTC : 5997, 2x32.1, 6300 => 52 %

The similarity of the results is quite good (*) so I assume that about 50% is a typical figure, 40% is currently your result so it’s just a bit lower than the previous data and, as already calculated, you should have peak revs at about 10000 rpm. As for turbo, as long as the restrictor area is the same theoretically it would be a very good idea.

(*) actually I’ve found also 6500 rpm for a F3 engine of few years ago having a 24 mm restrictor, that would mean 64%, but it was a non technical site with general info of presentation of F3 teams, probably not reliable, on the contrary the previous results are all from official data.

[Guest]

Sorry I didn't give all the information before

Bore = 49mm
Stroke = 36.6mm
Total Volume = 550cc
Cyl Vol = 68.75cc
Air Restrictor = 20mm
Distance between bores = 60mm
Con-Rod length = 106mm
Compression ratio = 14.5
Peak Pressure = 38.9 bar

CFM is cubic feet per minute. I don't know what the SI equivilant is to that (CMM?).

I took all the dimmentions of the original engine, and tried to keep the exact dimentions of the combustion chamber untouched besides increasing the stroke by approx. 3mm. I read a few SAE papers today, and asked some professors for help, and started writing a matlab code to help calculate these vibrations.

your rods are too long. you are detracting from the engines full capability due to rotational mass. 63mm rods would be perfect for that engine, it would give 1.75:1 ratio.

[Guest]

Hello,

This forum is great ! I was wondering if any one could suggest a good book on Fundamentals of Engine Mechanics( which would include detailed analysis, math etc). I am a student at CU Boulder involved with FSAE team, I also did a couple of CFD analysis on the Intake system! using COSMOS and Fluent!. I would like to get enlightened on the mechanics aspect of Engine design, as I dont have much knowledge other than the basic Theory of machines knowledge, balancing etc!

Aravind

You tried Amazon?

[gruntguru]

A few posts on this thread mention that longer stroke = more torque. As a general rule this is not true. For similar engines, peak torque (and MEP) is roughly proportional to displacement. For engines with the same displacement, those with more cylinders will be able to continue making useable torque up to higher revs and this means more power. Those with fewer cylinders will tend to develop peak torque at lower revs and are thus often referred to as "torquey". Long stroke vs short stroke is a similar story with long stroke engines being more "torquey" and short stroke engines making more power at higher revs.

[xpensive]

Welcome gg, it is most gratifying to notice a new author having his ducks in line on this subject. And if someone calls you a thread-archeologist now, just pretend it's raining.

[modbaraban]

Well it's common knowledge that archeology has its merits

Welcome to the forum, gruntguru.

[ced ampo]

Each design has its pros and cons. That's why FIA made a rule that only allows a single type of engine

[mike]

A few posts on this thread mention that longer stroke = more torque. As a general rule this is not true. For similar engines, peak torque (and MEP) is roughly proportional to displacement. For engines with the same displacement, those with more cylinders will be able to continue making useable torque up to higher revs and this means more power. Those with fewer cylinders will tend to develop peak torque at lower revs and are thus often referred to as "torquey". Long stroke vs short stroke is a similar story with long stroke engines being more "torquey" and short stroke engines making more power at higher revs.

generally p=cFx/t F corespond to the bore, x stroke and 1/t rpm
both bore and stroke has an affect on the torque of the engine

torque at low end does mean more torque consider this torque X rpm = power, graph torque(y) against rpm(x) and the area under the curve is the torque throughout the rpm range, so even if the peak torque is not as high longer stroke engines still has more torque as the curve is flatter

put it in real life context consider F20C and F22C1 from honda, do anyone actually think that the F20C has more torque?

[riff_raff]

I agree with Mike. The "torque" produced an engine is not really a function of the piston stroke. The torque value normally associated with engine performance is simply a measure of the mean torque value produced about the reaction point of the device used to provide a load on the engine crankshaft during the test. This device is normally the dynamometer brake, and that is why horsepower figures are usually quoted as "brake" horsepower.

Torque output varies widely during a single rotation of the crankshaft. At any given instant, torque can vary by up to 200%, depending upon the particular engine configuration. The instantaneous torque is a function of the cylinder pressures and the sum of the resulting forces at the crank axis due to the kinematics of the crank, pistons and conrods.

Maximum engine torque will normally occur at the point of max volumetric efficiency. And the max volumetric efficiency is helped by the inertia effects of the intake charge velocity. And of course, the intake charge velocity is a function of piston speed, which in turn is greater for long stroke engines than for short stroke engines at a given crank rotational speed. In short, long stroke engines only seem to be "torquier" due to the fact that their point of max VE occurs at a lower RPM.

I won't confuse you further by trying to explain horsepower.

Regards,
riff_raff