why not a larger inline 4 instead of v6?

[Eli]

Originally posted by: 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.

Yep.. Listen to this man.

The flame front isn't the slowest reacting thing in the typical engine, your valve springs are. Your valves will float before your pistons are moving faster than the flame front.

You also have to take into account that max RPM is determined by a lot of things, one of which is stroke... The longer the stroke, the more torque you're going to get.. but the lower your max RPMs.

Another thing that effects power and RPM is the camshaft grind. You can change the characteristics of any engine dramatically by just changing the cam profile.

Remember that torque and horsepower are basically the same thing. Horsepower is just a measure of torque at certain rpms. (HP = Torque * RPM / 5252).

It is torque that moves you forward.

 

[Bulk Beef]

The flame front isn't the slowest reacting thing in the typical engine, your valve springs are.

I was speaking specifically of the limitations of combustion at high rpm, but yes, valves will float (and other bits may break) before you get there. And actually, in F1 in the early 90s maybe, a few manufacturers moved to bore/stroke ratios approaching 1.8:1, and they did start running into that problem.

 

[DivideBYZero]

Originally posted by: SuperSix
There's also engine compartment space/design, V6's are somewhat shorter.

Only useful for a logitudal installation. Thats why for Transverse mounting VW invented the VR6. A six in the space of a four.

 

[HokieESM]

Originally posted by: sward666

The flame front isn't the slowest reacting thing in the typical engine, your valve springs are.

I was speaking specifically of the limitations of combustion at high rpm, but yes, valves will float (and other bits may break) before you get there. And actually, in F1 in the early 90s maybe, a few manufacturers moved to bore/stroke ratios approaching 1.8:1, and they did start running into that problem.

If I recall correctly, the actually problem in the 92 season (when Ferrari was having this problem) was that there was a hot-spot under one of the intake valves. So they got premature detonation and then re-detonation when the plug fired. The also had a problem that the "overlap" in the valves caused a "pocket" of fuel-air mixture to go uncombusted behind the exhaust valve. So the flame front "sort of" didn't have enough time--but it was more a function of cylinder head design than the engine moving faster than flame speed.

Flame speeds, just for your info, are VERY fast. Several times the speed of sound. For a "perfect" combustion chamber, flame speed wouldn't be an issue.... although it does become an issue if there are too many obstructions--like big exhaust valves. Combustion chamber design is a VERY complicated matter--I went to grad school with a guy who now works for IndyCar doing this exact same thing.

Oh, and for those of you not in the know--Formula 1 cars frequently turn 17,000 rpm or so (and they have to resort to pneumatically actuated valves to prevent valve float)... so no worries of this flame-speed really being an issue in your 7000 (or even 9000) rpm engine in a street car.

 

[CraigRT]

I think the Ford Ranger has a 3L inline 4
someone told me that. but I don't follow Ford too much so I really have no idea.

 

[HokieESM]

There ARE some large I4s out there.... frequently, they end up being half of V8s (ever notice how GMs 3800 V6 really is a V8 with two cylinders lopped off)?

The Altima has a 2.5L four (and so does the Sentra SE-R). And the Honda CR-V has a 2.4 or 2.5L, I believe.

 

[Dulanic]

Originally posted by: Yield
I think the Ford Ranger has a 3L inline 4
someone told me that. but I don't follow Ford too much so I really have no idea.

For 2002 The Edge Model is 3L but its a 6 not a 4. The XL Model is a 4 and is 2.3L

Going further back like 2000... the 2WD XLs were 2.5L 4s. The 4WD XLs were 3.0L 6s.

The older 2.5L 4s sucked tho... I had to drive a ranger for my last job... 135 HP in a pickup just doesnt cut it. With all the crap I had in there for work it would slow down going up hills with the A/C on. The newer 2.3L are a bit better with around ~150 HP.

 

[NikPreviousAcct]

An inline engine also provides you with a smoother ride since your car doesn't shake side to side. It's much harder to feel up and down vibrations than it is side to side.

 

[JOSEPHLB]

since the last couple of posts mentioned something about a Ford Ranger.... I'd thought I'd throw this in here as well:

"Before we get into the stages of modifications, here's a little background on the four cylinders offered in the Ranger over the years.

The carbureted 2.0 was offered as the base engine in the Ranger from its introduction in 1983 until 1990. It is a debored version of the 2.3, so anything said about or offered for the carbureted 2.3 should apply.

The carbureted 2.3 was offered as a "step up" from the 2.0 from 1983 until 1985 when it was no longer available.

The fuel injected 2.3 came on the scene in 1985 and received the dual plug head and DIS in 1989. This iteration remained constant until 1997, and is probably in more Rangers than any other engine.

In 1998 Ford stroked the 2.3 to 2.5 liters, but didn't change much else. Anything applying to the injected 2.3 applies to the 2.5 as well."


History Of The 4-Banger!

 

[HokieESM]

Originally posted by: ffmcobalt
An inline engine also provides you with a smoother ride since your car doesn't shake side to side. It's much harder to feel up and down vibrations than it is side to side.

Well, the pistons move up and down, but the crankshaft rotates--and its far from symmetrical. So the "vibration" is usually circular--so you have some in every direction (otherwise, why would you have three or four motor mounts--usually on all sides). Compounded by the fact that most inline-fours have rather large counterweights on the crank--which makes vibration worse. And none of this really correlates to the "ride" of the car--that's the suspension. And even an engine that vibrates reasonably poorly can be fixed by good motor mount placement (particularly fluid mounts): check out a Toyota Camry--the I4 version. Very quiet and smooth--you could double crank one if you weren't paying attention.

But you have somewhat of a point. Inline-6s (which are much more inherently balanced than an I4) have small counterweights and don't vibrate nearly as much. Mostly, the vibration issue comes down to engine balance. Which is an odd thing. Theoretically, you could make an I-360 (with 1 degree crank angle) and it would be VERY smooth. But we have to reach a practical state--I think I6s don't have an imbalance until the 6th order.

Also, I should point out (after talking to some friends/coworkers who do kinematics for a living) that you SELDOM lose enough energy through vibration (in an IC engine) for it to be much of an issue. And the vibration issue can EASILY be solved by good motor mounts. But they do point out that the NOISE produced by a smaller cylinder is easier to "damp" out than a large cylinder one--hence the "piston slap" one hears on large cylinder engines at low RPM (ever hear a non-turboed diesel in a semi?). Not to mention, with multiple power strokes occuring, its not a discrete phenomena--so it "smooths" the noise band (listen to the "purr" of a V12 at a car show).

 

[CraigRT]

Originally posted by: Dulanic

Originally posted by: Yield
I think the Ford Ranger has a 3L inline 4
someone told me that. but I don't follow Ford too much so I really have no idea.

For 2002 The Edge Model is 3L but its a 6 not a 4. The XL Model is a 4 and is 2.3L

Going further back like 2000... the 2WD XLs were 2.5L 4s. The 4WD XLs were 3.0L 6s.

The older 2.5L 4s sucked tho... I had to drive a ranger for my last job... 135 HP in a pickup just doesnt cut it. With all the crap I had in there for work it would slow down going up hills with the A/C on. The newer 2.3L are a bit better with around ~150 HP.

yeah, 130 horse pickup is pretty weak.