CJRiojas

iTrader: (1)

I am trying to gather some understanding on how bore and stroke are chosen when designing an engine. For example; the hemi 5.7 has a bore/stroke of 3.92/3.58 whereas the LS1 is 3.9/3.622. What process do engine designers go through and what are the key factors that dictate what bore and stroke to use?

CJRiojas

iTrader: (1)

Nobody?! How does the different shape of the cylinder affect the air flow, fuel distribution and compression? Is there any real significant change?

wagonwes

A larger bore means you can run larger valves, letting more air in. A longer stroke makes more torque, but you cannot rev it as high because of a higher mean piston speed at the same RPM as a motor with a smaller stroke. There are many more variables to take into account though. In my opinion torque is more fun than horsepower, but if you are looking to build a drag only car, a large bore is the way to go.

All you ever want to know here: http://findarticles.com/p/articles/m.../ai_n15614621/

CJRiojas

iTrader: (1)

That was a fascinating article. However, the author failed to take into account that torque is Force over a distance. increasing the stroke from, say, 3.58 to 3.622 would increase torque considering an equal force, F, from:
F x 3.58 = T1 to F x 3.622 = T2
Where T2 is a higher torque. considering this, you can also factor in the friction coefficient, f. The friction coefficient wil be different for the 2 different stroke lengths so i will use f1 and f2:
(F - f1) x 3.58 = T3 or (F - f2) x 3.622 = T4
if f2 increases enough with the longer stroke, it can negate the increase in torque.
This is also not considering the cylinder compressing into the chamber where the friction coefficient is applied again.

Thanks for that article, it helped to start me thinking in the right direction. Anyone else have other factors that will be considered? I also saw the valve to sidewall clearance affected the velocity of the air flow. Can anyone key me in onto some formulas? I am an EE major and will not have the good fortune of taking a fluids course.

joecar

Some comments:

you have to use crank radius instead of stroke;
you also have to multiply by the sine of rod-crank angle;
you also have consider that F is dropping rapidly past 15°-20° ATDC.

[ the rod-crank angle becomes 90° when crank angle is approx 71°-75° ATDC; cylinder pressure has already dropped significantly by then ]


The larger stroke produces higher instantaneous piston velocity; frictional force is proportional to instantaneous velocity; the increased friction happens to balance out any torque increase.

So basically, an increase in stroke (keeping the bore constant) does increase torque, but the reason is that cylinder fill is now larger (more fuel/air mix to burn); this can also be achieved by increasing bore instead.

Long stroke has the disadvantage of higher piston speed and acceleration.


Take the advanced thermodynamics course.

The_Rizzle

iTrader: (4)

A longer stroke (vs bigger bore) seems to favor a lower rpm engine . Smaller bores are supposed to lower detonation resistance, but I'd have to find out where I read that. A larger bore also has more hidden end gasses in both the gasket groove, and b/w the crown and top ring. This would affect final emissions.
2 valve engines going for hp, its hard to get a large enough intake valve to flow w/o shrouding with the long stroke, smaller bore combo. 4 valve engines can get the flow w/ a smaller bore, so you'll see more of those with a smaller bore, longer stroke.

Overall block sizing (bore spacing) may also be a main criteria in the designing, which may overshadow any performance benefits of a bigger bore.

Heres one of the latest (and shorter) discussions at speedtalk. Tons of good info there. http://www.speedtalk.com/forum/viewt...bore+vs+stroke

Old SStroker

Reher makes many good points.

Remember that if you keep displacement constant and increase the stroke, the bore must become smaller. If we assume the cylinder pressure (lb/square inch) is the same, there is less Force on the rod/crank with the smaller bore. Force = pressure*area, so even though you have a longer (moment) arm with the longer stroke you have less force pushing on it. Ergo, no real torque advantage to a long arm with a fixed displacement. If you do the math the Force x distance is the same.

That is not always intuitive.

As far as the 3.58 vs 3.622 strokes, OEM designers often pick even mm (or inch) dimensions. 91 mm (3.583") or 92 mm (3.622"). They may then adjust the bore to get the displacement they want. Most modern engines are "hard metric" designs with nominal metric dimensions. If you are close to a mm dimension, why not? It makes it easier on the marketing folks.

The LS7 is an exception: 4.125" x 4.000" for 427.65 cubes. They could have done a 105 mm x 101 mm 426.79 cubes but us old V8 pushrod gearheads would not have liked that.


Jon

CJRiojas

iTrader: (1)

You are correct sir, i was not thinkin in terms of angular acceleration and velocity. With that point, why is it the sine? i though it would just be angular acceleration multiplied by the Force, etc? I wasnt very good at dynamics so excuse my questions if they are lower level engineering lol?

I wont be able to take that class either but it would be nice to. I really wanted to double major in EE/ME but after i took a dynamics course, it really discouraged me from doing that. if i can get passed that then i might do it in the future.



I havent finished reading the other link but thanks.

CJRiojas

iTrader: (1)

does that mean that engine designers typically begin with a set displacement in mind then choose the bore stroke to reach it?

Neither would I


Another point i forget. If the cylinder diameter is increased, the piston diameter is increased. The increase in diameter increases the overall surface area of the piston which also causes more friction. Just throwing that out there

Old SStroker

I think you meant more circumference, which means "longer" piston rings which create more drag/friction. The surface area of the piston head doesn't touch anything.

If you increase the bore (and piston circumference) by 5%, you increase displacement by 10.25%, but if you increase the stroke by 5% you only increase the displacement by 5%. Another reason bigger bores have less fricton than longer strokes.

FWIW, a longer stroke drags the rings not only farther on the bore, but also faster relative to a shorter stroke, both average and peak piston velocity. That causes more friction than a larger bore to achieve a given displacement.

Why OEM engines are designed to a given size is way more complex than just choosing a displacement. Similarly, race engines generally have a displacement limit and if the rules allow, different folks choose different bore/stroke combinations based on the physical restrictions imposed by things like bore spacing and available head designs.

Obviously not everyone thinks alike here.

For example, one of the very first competitive 5.7L LS engines was in the 2004 Cadillac CTS V racecar. It did not use the 3.90 bore 3.622 stroke of the production LS6 engine, but rather the 4.125 bore of the 7.0L LS7 with an 83 mm (3.268") stroke of the 4.8L. It used a version of the LS7 head a while before the LS7 engine was released in the Corvette.

In this case the displacement was fixed, but the bore/stroke was open...at least for a year or so until they were forced to use the LS6 architecture.

Jon

CJRiojas

iTrader: (1)

i wasnt talking about the piston head, i was talking about the piston to cylinder wall.

I guess it was a pretty complex question to ask. Wasnt trying to suggest one way was better than the other, just trying to gain some understanding behind it. Lots of really good info. Thanks for the insight.

joecar

Jon, I know you meant Force = pressure*area...

Old SStroker

Yep! lbs/in^4 doesn't make a lot of sense does it?

Thanks. I'll fix that.

Jon

Old SStroker

There is not all that much friction between the piston and the cylinder wall, especially if you fit the pistons correctly and use coated skirt pistons. The rings are the big piston-to-bore friction producer.

Interestingly, about the only part of the piston that actually touches the bore is the skirt area 90° from the pin axis. Enlarging the bore/piston diameter has very little effect on that area.

If making power is Job 1, which it is not necissarily the first priority for OEM designers, bigger bore/shorter stroke most often is the winner for all of the reasons outlined by others above.

Jon

c5blkvette

That is so... until the piston gets up to temperature. The pin bosses undergo the most thermal expansion, turning the piston from an oval shape at RT to round.

Old SStroker

Hmmm. The stock (cast) and aftermarket (forged) pistons I look at regularly have almost no skirt area at the pin ends, and I don't see used one with any scuff marks there. There is very little side thrust force (along the pin axis) to cause contact. If there were, there would be more than a 1/8" wide web which could contact the bore.

Does not thermal expansion equate to size (diameter) not mass? Coefficient of thermal expansion is inches(of expansion)/inch(of size)/degree temperature change. The diameter across the skirt expands the most. Actually it's more likely the head of the piston above the top ring that expands the most because it sees the most temperature change (delta T).

Jon

ZMX

Every engine you see is the culmination of design for the best COMPROMISE (every choice with an engine is a compromise)... the best compromise between cost, reliability, and effectiveness at its purpose - be that purpose broad (a SUV) or narrow (a race car).

Look at any engine getting massive HP/L naturally aspirated. They have good heads, small strokes, and large bores.

A large bore doesn't inhibit high RPM (ie pumping more air to consume more fuel and thus make more power) as much as a large stroke does. So, with a given displacement, large bore is better suited for power.

This is why we see the 200HP/L Yamaha R6 turning 16,500RPM. Or an F1 with 300HP/L turning to 20,000RPM. If grip weren't a factor, an F1 could accelerate from 0-60 in 1 second.

But, fuel tends to combust most efficiently in a "square" chamber. Equal bore and equal stroke. And, most people don't want to drive cars they have to rev to ten thousand RPM. So, most cars are somewhere around square for efficiency and for driver peace of mind. Plus, high revving requires a very expensive valvetrain.

It should be obvious to anyone paying attention that GM engines have relatively low HP/L. Their ideology tends toward large displacement and simple valvetrains. Is that any better or worse? Well, most import guys would tell you it's worse, but that's a load of crap. What matters is the complete package. And, there's a lot more to an engine than the bore and stroke. More importantly, there's a lot more to a vehicle than it's engine. No one puts a F1 engine in a truck, and no one puts a truck engine a F1.