2L 4Cyl. vs. 2L 8Cyl What's more fuel efficient?

[user removed]

tjts1, the 3 liter Porsche 4 cylinder had at least one balance shaft (twice engine RPM), maybe even two. Think the balance shafts were pioneered by Mitsubishi, but there could have been an earlier version.

The problem with 4 cylinders above 2.4 liter is they develop a harmonic porpoising motion from front to back when they go beyond 2.4 liters. Not sure of all the physics involved in that but I know it is generally understood among engine makers.

I know the Cummins and many other engines are larger displacement than 2.4 liters. I won't guess as to why they do not suffer from the same symptoms of front to rear engine rocking. It may be their mass, RPM, the fact they are diesel and not gas.
Maybe someone else has more info. I just feel like you can power virtually any vehicle up to two tons with 2.4 liters of 4 cyl. The wife's Sorento does it nicely at 3800 without passengers, with 2.4 liters and could provide much more power (which she never needs) with a turbo.

regards
Mech

[tjts1]

Yup, 4 cylinder is much smoother than 3 cyl in any displacement.

[serialk11r]

Old Mechanic, how about flat-4 engines? I know Subaru doesn't use balance shafts, and the EJ25s are pretty rev friendly I believe (although that may be because they have a somewhat short stroke).

[user removed]

Flat 4 engines are not subject to the porpoising issue. The problem is when the 1st and last cylinders (inline 4 cyl) fire at certain RPMs the forces trying to push the cylinder head in the opposite direction of the piston become harmonized and it amplifies the resonance when in harmony, with a fore and aft rocking motion.

This is especially prevalent in FWD cars that do not have a transmission bolted to the engine that is another 3 feet longer with a good distance between the engine and tranny mounts.

It did not seem to be a problem in the old days and many early 4 cylinder engines were a heck of a lot larger displacement than 2.4 liters.

Offenhauser - Wikipedia, the free encyclopedia

Big 4 cylinder high performance engine. Copy of the 1913 Peugeot Indy engine that was DOHC 4 valves per cylinder hemi (I think it was a Hemi).

regards
Mech

[baldlobo]

it's because the forces in a boxer are operating in opposition to each other; where as the inline 4 runs forward and back 1-4,2-3(flats go 1-2-3-4); along with it being a 360 crank change instead of 180 of the flat 4.

maybe the older engines inline 4's have a 90 degree offset instead of 360. along with thicker material and bigger crank/rod bearings lands. the 70's brought in the minimum is enough thought pattern

[user removed]

1917 Golden Submarine Replica - Autoweek

The Golden Submarine.

regards
Mech

[smokey442]

I've written a few reponses to this thought provoking topic but when I go to send it I get a window directing me to log on again. When I go back to what was originally written its gone. Ayone else experience this?

[drmiller100]

Originally Posted by Old Mechanic

tjts1, the 3 liter Porsche 4 cylinder had at least one balance shaft (twice engine RPM), maybe even two. Think the balance shafts were pioneered by Mitsubishi, but there could have been an earlier version.

The problem with 4 cylinders above 2.4 liter is they develop a harmonic porpoising motion from front to back when they go beyond 2.4 liters. Not sure of all the physics involved in that but I know it is generally understood among engine makers.

I
regards
Mech

On the link provided the Continue button goes into great depth about all this.

The link shows there are indeed two countershafts, and that they need to be rotating in opposite directions and talks about the harmonics which never do get solved.

The diesel solutions (cummins, perkins, VW, et al) all turn at pretty low speeds, and tend to be VERY heavy engines. less of an issue at lower rpms.

[tjts1]

The 3 cylinder vibrates a lot more than the 4 and it also requires a balance shaft. End of story.

[drmiller100]

Originally Posted by tjts1

The 3 cylinder vibrates a lot more than the 4 and it also requires a balance shaft. End of story.

I'm sorry, but this is not true.

I invite you to read the pages you posted. It does a REALLY good job of explaining it all - I never understood it until I read your article.

The 4 cylinder needs TWO counter rotating ballancer shafts, and it still has third order issues.

The 3 cylinder only needs one simple ballancer shaft, and it is intrinsicly more ballanced.