why not a larger inline 4 instead of v6?

[ElFenix]

Originally posted by: HokieESM

Originally posted by: ElFenix<brummm... no, torque is a direct result of the torque arm, which is longer in the 4 cylinder, ceteris paribus

Actually, this isn't true. It also depends on how many power strokes you're getting.... in addition to the overlap between the strokes. Note that a four-stroke Otto cycle demands that a four-cylinder engine only have one cylinder firing at any given time. A six can have more than this--albeit the cylinders are smaller. And there is NO reason that the strokes can't be identical..... the bore on the six could just be really small.

saying ceteris paribus gives both engines the same amount of squareness, valve design, metallurgy... it cancels out all factors except the number of cylinders and the volume of said cylinders. changing the squareness makes this whole thought excercise rather pointless.

 

[ElFenix]

Originally posted by: desertdweller

That is part of it, the longer stroke means more torque.

Now I'll ask you the question, why does a 4 cylinder have less torque than a v6 of equal
CID if a 4 cylinder has a longer stroke.

DD

show me two engines of very similar characteristics except the number of cylinders (and of course the volume of the cylinders). the rpm advantage and more-continuous force (less coasting) should be canceled out by the longer arm and larger explosion.

peak torque of course

 

[HokieESM]

ElFenix, then you've made it even easier. Lets look at a V8 and an I4:

At any given time, one cylinder is firing in the I4. TWO cylinders are firing in the V8. Even though the pistons are half the size, you still have two firing. Thus giving the exact SAME amount.

In real life, the friction on the V8 is typically lower because of less bearing area. Not to mention vibrational losses.

 

[ElFenix]

Originally posted by: HokieESM
ElFenix, then you've made it even easier. Lets look at a V8 and an I4:

At any given time, one cylinder is firing in the I4. TWO cylinders are firing in the V8. Even though the pistons are half the size, you still have two firing. Thus giving the exact SAME amount.

In real life, the friction on the V8 is typically lower because of less bearing area. Not to mention vibrational losses.

V8s cancel more vibrational forces than V6s do.

area doesn't have anything to do with friction.

 

[HokieESM]

also, its VERY difficult to get the combustion efficiency of a large cylinder equal to that of a smaller one.... which is the main reason you see small cylinders in formula one and CART. Don't you think they would have gone to inline-4s if they could make more power with the same displacement?

 

[WinkOsmosis]

Why would it matter how many strokes there are? Torque is the average of the force over however long it takes for all cylinders to fire... Engines have flywheels right? And in a 4 cylinder engine, one cylinder is always firing anyway. Look at a torque graph.. it's a smooth line, not a jagged WWWWWWWW, which it would be if the number of cylinders mattered.
My 2.4L I4 makes 155hp, which is coincidentally about the same hp/L and torque/L (or more) as V6s and V8s like Ford's 4.6L that makes 280hp or whatever.

 

[HokieESM]

Area has a GREAT deal to do with friction. I would love to see you get the same amount of friction from a 1" tire as a 20" wide one. The simple things they teach you in physics about a coefficient of friction times a normal force are very overly simplified.

 

[Bulk Beef]

It should also be pointed out that "better" does not neccessarily mean "higher peak torque" or "higher peak power". Ideally, you would have a torque curve as flat as possible (same torque across the rpm range), and a power curve that climbed in a straight line.

 

[ElFenix]

Originally posted by: HokieESM
also, its VERY difficult to get the combustion efficiency of a large cylinder equal to that of a smaller one.... which is the main reason you see small cylinders in formula one and CART. Don't you think they would have gone to inline-4s if they could make more power with the same displacement?

so worse control/harnessing of the explosion inherent to larger volumes? theres a real reason. good, we're getting somewhere.

 

[HokieESM]

Well ElFenix, neglecting friction, a V6 STILL has two cylinders firing (really more like 1.5 in terms of overlap)--so the equation still holds--the exact same amount of power. And a balanced V6 (with a sixty degree angle) is actually balanced BETTER than an inline-4.

 

[ElFenix]

Originally posted by: HokieESM
Area has a GREAT deal to do with friction. I would love to see you get the same amount of friction from a 1" tire as a 20" wide one. The simple things they teach you in physics about a coefficient of friction times a normal force are very overly simplified.

the 1" tire would rip itself apart and also you'd have a higher chance of a significant part of the tire losing grip by hitting a surface that it has a lower coefficient with.

 

[HokieESM]

Yes, much worse control. Read my LONG post earlier. Fuel-air ratios are hard to control in large cylinders. you have to give the air time to get in there in swirl--and it takes longer to fill a larger cylinder. Smaller cylinders can be filled more uniformly and combust more uniformly (hotspots don't help you in combustion).

 

[WinkOsmosis]

Originally posted by: HokieESM
Well ElFenix, neglecting friction, a V6 STILL has two cylinders firing (really more like 1.5 in terms of overlap)--so the equation still holds--the exact same amount of power. And a balanced V6 (with a sixty degree angle) is actually balanced BETTER than an inline-4.

How many cylinders are compressing?

 

[Zenmervolt]

Originally posted by: HokieESM
Yes, much worse control. Read my LONG post earlier. Fuel-air ratios are hard to control in large cylinders. you have to give the air time to get in there in swirl--and it takes longer to fill a larger cylinder. Smaller cylinders can be filled more uniformly and combust more uniformly (hotspots don't help you in combustion).

There are tricks for large cylinders, are there not? Things like exotic valve setups, multiple plug heads and gasoline direct injection?

ZV

 

[ElFenix]

Originally posted by: HokieESM
Well ElFenix, neglecting friction, a V6 STILL has two cylinders firing (really more like 1.5 in terms of overlap)--so the equation still holds--the exact same amount of power. And a balanced V6 (with a sixty degree angle) is actually balanced BETTER than an inline-4.

ok, so a properly made v6 is better balanced than an inline 4, another good answer. so now we have
1) v6s can be better balanced than an i4
2) explosion can't be controlled as well in larger displacement cylinders

 

[HokieESM]

no, the 1" tire wouldn't necessarily rip itself apart. Simplify it and don't think about tires... think about two bodies sliding on one another. Friction is caused by surface roughness of the two materials. It can be ROUGHLY approximated by the coefficient of friction times the normal--for VERY large areas. For finite areas, you have dynamic effects and lots of surface roughness effects (and edge effects, as well... but those are hard to quantify). Area very much does enter into the equation.... hence the wide tires on Indy cars. And believe me, if they could get the same cornering acceleration with narrower ones, they would love to use narrower ones--it decreases rolling AND air resistance.

 

[WinkOsmosis]

I still don't get that second order stuff.. I need diagrams, not equations!

 

[ElFenix]

Originally posted by: HokieESM
Read my LONG post earlier.

jesus how did i miss that? musta been too busy replying

thanks!

 

[HokieESM]

Originally posted by: Zenmervolt

Originally posted by: HokieESM
Yes, much worse control. Read my LONG post earlier. Fuel-air ratios are hard to control in large cylinders. you have to give the air time to get in there in swirl--and it takes longer to fill a larger cylinder. Smaller cylinders can be filled more uniformly and combust more uniformly (hotspots don't help you in combustion).

There are tricks for large cylinders, are there not? Things like exotic valve setups, multiple plug heads and gasoline direct injection?

ZV

There are SOME. But it doesn't make the problem easier.... you can also use those tricks on the smaller cylinders (which are by no means perfect). And its a volume problem--so increasing the bore by a factor of two makes your problem increase by a factor of 8 (its a volume issue). And it gets worse as RPMs rise--because the gas has to get in there faster.

 

[ElFenix]

Originally posted by: HokieESM And it gets worse as RPMs rise--because the gas has to get in there faster.

so does forced induction work better on the larger displacement cylinders?

 

[Zenmervolt]

Originally posted by: HokieESM

Originally posted by: Zenmervolt

Originally posted by: HokieESM
Yes, much worse control. Read my LONG post earlier. Fuel-air ratios are hard to control in large cylinders. you have to give the air time to get in there in swirl--and it takes longer to fill a larger cylinder. Smaller cylinders can be filled more uniformly and combust more uniformly (hotspots don't help you in combustion).

There are tricks for large cylinders, are there not? Things like exotic valve setups, multiple plug heads and gasoline direct injection?

ZV

There are SOME. But it doesn't make the problem easier.... you can also use those tricks on the smaller cylinders (which are by no means perfect). And its a volume problem--so increasing the bore by a factor of two makes your problem increase by a factor of 8 (its a volume issue). And it gets worse as RPMs rise--because the gas has to get in there faster.

Yeah, I know that it can't ever be truly overcome, I was just thinking about things that have been done to try to allieviate the problem.

ZV

 

[Bulk Beef]

The big problem as RPMs rise is that it has to burn faster, but the flame front can only travel so fast. RPMs get high enough, and you run out of time to burn your fuel/air.

 

[HokieESM]

Does it work BETTER? No. Basically, we're so far off of perfection on the air-fuel mixture distribution... that any "trick" for large cylinders would work just as well on small cylinders. Theoretically, yes, it would help more.

But the BIG thing is that large pistons increase reciprocating mass..... which severely limits RPM potential by increasing the operating stresses in the parts. And ALL engines (big or small) make their power up near their redline (albeit some redlines are lower than others).

 

[desertdweller]

Originally posted by: ElFenix

Originally posted by: desertdweller

That is part of it, the longer stroke means more torque.

Now I'll ask you the question, why does a 4 cylinder have less torque than a v6 of equal
CID if a 4 cylinder has a longer stroke.

DD

show me two engines of very similar characteristics except the number of cylinders (and of course the volume of the cylinders). the rpm advantage and more-continuous force (less coasting) should be canceled out by the longer arm and larger explosion.

peak torque of course

I looked for a short bit, couldn't find any examples.

By adding a longer stroke you have added to the weight of the rotating mass. With the more constant fireof a v6, you can
over come the force of the rotating mass much easier because there is less time between compression strokes to allow
the slowdown to occur. Also, a longer stroke produces more windage in the block further sapping power from the crank.

Then add in all the force against the crank from the wheels, the axles, the transmission and the weight of the vehicle,
and with all things being equal, you're allowing less time that the crank has no power to allow it to slowdown.

DD

 

[HokieESM]

Originally posted by: sward666
The big problem as RPMs rise is that it has to burn faster, but the flame front can only travel so fast. RPMs get high enough, and you run out of time to burn your fuel/air.

Yep. But you never get there... frequently, what you get is "knock"... which is combustion set off by the compression stroke and NOT the spark plug. So sometimes, you get an explosion while the cylinder is going up... which REDUCES power output. EDIT: my point being--flame fronts are REALLY fast. I've never seen/heard of this being the problem. Frequently, you see a hotspot in the engine producing knock (and power decline) first. And even then, most of the time, its occuring AFTER the peak power (if the engine is in good tune)... the combustion efficiency is already on the decline.

ever notice how engines reach their peak and die off? one would think that an engine would produce more and more power until it blew up..... but what happens is that the combustion efficiency starts to decline--and you're not getting as much power.