Question about engine displacement

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thunderfog

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For those who know engine…engineering:

When it comes to cylinders and displacement, why are gas engines somewhat capped on cylinder size? For example it seems like the biggest I4 gas engine in a car is the Toyota 2.7L. What’s to stop a 3.7L I4? I figure more cylinders equates to a smoother running engine, but would there be ANY benefit to high displacement 4 cylinder as opposed to a smaller displacement V6? Does cylinder size reach a critical threshold at a certain ratio compared to overall displacement to where you get to being well past the point of any diminishing returns?

I ask this more from a marine perspective even though I’m asking about car engines. There is an outboard brand rated at 200hp that is nearly 2.9L, and another at 200hp that is a 3.4L V6. Both get rave reviews, and the I4 produces a lot of low end torque (and the V6 is all you would expect from a V6). It just got me thinking so I thought I would ping the engineers out there in BITOG world.
 
Mostly about vibration. Larger 4 cylinder engines need balance shafts.

I believe Porsche offered a 3L 4 in the 944 cars.

International Harvester had a 3.1 or 3.2L 4 cylinder back in the day. It was their 6.2L V8 cut in half IIRC.

Looking into this, even Pontiac had a 194 (3.2L) Inline 4 back in the early 1960s.

en.wikipedia.org

Pontiac Trophy 4 engine - Wikipedia

en.wikipedia.org en.wikipedia.org
 
Larger displacement engine require more reciprocating mass that make more vibration. To hit the performance needed more factors than displacement figure into it. Generally the larger the reciprocating mass the lower the top rpm is. Great in constant power applications but not in others.


Off the top of my head tractors in the early 70s were getting large displacements in gas engines. A Case 870 gas had a 4.9L four cylinder engine.
 
Cat made some large displacement inline 4 cylinder diesel engines that displaced 5.1L. I saw one of them torn down and the block must have weighed 500 pounds.
 
I believe one of my uncle's farm tractors had a large displacement four cylinder. It had a low RPM range so vibration wasn't an issue. Besides a tractor back then wasn't expected to be a smooth running machine. Only torque was important.
 
A General Electric 7FDL16 diesel engine is a V-16 with a 175.2L displacement. Similar engines are used in marine diesel applications. It's also got a massive turbo.

113674558.pCsSb9mc.jpg


Of course GE sold off its transportation division to Wabtec, but I guess that included the marine division.
 
thunderfog said:
For those who know engine…engineering:

When it comes to cylinders and displacement, why are gas engines somewhat capped on cylinder size? For example it seems like the biggest I4 gas engine in a car is the Toyota 2.7L. What’s to stop a 3.7L I4? I figure more cylinders equates to a smoother running engine, but would there be ANY benefit to high displacement 4 cylinder as opposed to a smaller displacement V6? Does cylinder size reach a critical threshold at a certain ratio compared to overall displacement to where you get to being well past the point of any diminishing returns?

I ask this more from a marine perspective even though I’m asking about car engines. There is an outboard brand rated at 200hp that is nearly 2.9L, and another at 200hp that is a 3.4L V6. Both get rave reviews, and the I4 produces a lot of low end torque (and the V6 is all you would expect from a V6). It just got me thinking so I thought I would ping the engineers out there in BITOG world.
Click to expand...

There have been many HUGE 4 cylinder engines. Usually though, an I6 is chosen (think large OTR trucks) because they are naturally balanced and so won't have the vibration problems.

A dear friend of the family had a 1920's vintage Minette launch, it was powered by a Van Blerck 4-cylinder:
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So the 4-pot with the same specs (5.5" bore, 6" stroke) was 570ci (9.34L). He had the "High Speed" version (Model E-4 Special):
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This is the actual engine:
 
How would 6 be balanced, if 4 would not? Not arguing, just asking...

OVERKILL said:
There have been many HUGE 4 cylinder engines. Usually though, an I6 is chosen (think large OTR trucks) because they are naturally balanced and so won't have the vibration problems.
Click to expand...
 
paulri said:
How would 6 be balanced, if 4 would not? Not arguing, just asking...
Click to expand...

This only applies to in-line 6 cylinder engines, which are naturally balanced, and by extension, V12's. That's why they were so popular in certain applications, particularly large displacement ones like OTR trucks. Each cylinder fires 120 degrees from the next resulting in two perfect rotations with the firing of all cylinders. This cancels out harmonics and is what makes them so smooth.

A 4-cylinder is not naturally balanced, neither are V8's, V6's...etc. But when you have more cylinders it makes that less apparent. That's why we have harmonic balancers, balance shafts...etc.
 
paulri said:
How would 6 be balanced, if 4 would not? Not arguing, just asking...
Click to expand...

I think a larger inline 4 needs a balance shaft, while 3 up, 3 down nature of an inline 6 is well balanced.

It's kind of technical and I don't fully understanding it, but here's the explanation.

automobile.fandom.com

Balance shaft

In piston engine engineering, a balance shaft is an eccentric weighted shaft which offsets vibrations in engine designs that are not inherently balanced (for example, most four-cylinder engines). They were first invented by British engineer Frederick Lanchester in 1904. Balance shafts are most...
automobile.fandom.com automobile.fandom.com
Balance shafts are most common in inline four cylinder engines which, due to the asymmetry of their design, have an inherent second order vibration (vibrating at twice the engine RPM) which cannot be eliminated no matter how well the internal components are balanced. Flat engines have their pistons horizontally opposed, so they are naturally balanced and do not incur the extra complexity, cost or power loss associated with balance shafts. This vibration is generated because the movement of the connecting rods in an inline engine is not symmetrical throughout the crankshaftrotation; thus during a given period of crankshaft rotation, the descending and ascending pistons are not always completely opposed in their acceleration, giving rise to a net vertical inertial force twice in each revolution whose intensity increases quadratically with RPM, no matter how closely the components are matched for weight.​
 
y_p_w said:
A General Electric 7FDL16 diesel engine is a V-16 with a 175.2L displacement. Similar engines are used in marine diesel applications. It's also got a massive turbo.

113674558.pCsSb9mc.jpg


Of course GE sold off its transportation division to Wabtec, but I guess that included the marine division.
Click to expand...
The 2-stroke EMDs are 645 & 710 CID per cylinder x 16. They run at 900 & 1050 rpm. The above puts out about 5,000 hp at the crank at 1050.
 
For automotive engines running at higher rpm, with one spark plug per cylinder as was the norm, a bore of about 4" or 100 mm is the limit for complete fuel combustion during the power stroke. With a few engineering tricks, some big-block V-8s went a little larger, but those engines were fuel hogs. Example: the Chevrolet 454 had a bore of 4.25". In street engines since World War II, few auto engines have gone much above 4".

The same principle applies to rotary engines. Mazda said in a patent a few years ago that it had determined the rotor width limit for complete combustion was no more than 70 mm. The rotor width corresponds to engine bore in a piston engine.

To be able to rev, larger engines have to limit stroke. They should be oversquare, meaning the bore is larger than the stroke. For low-speed torque in a lower-revving engine, a longer stroke is better. Most small four-cylinders are undersquare, meaning the stroke is larger than the bore.

This should show why modern Detroit V-8s have gone no larger than about 500 cubic inches.

For a given displacement, with the above limitations in mind, fewer cylinders are better because of friction loss through rings and gearings. Given a choice between a 5.0 liter straight-6 and a 5.0-liter V-8, the six will be a better choice for economy. When the bore exceeds the magic 4" limit, though, it's time to go to more cylinders.

Certain engine configurations cannot be completely balanced. These include any configuration with fewer than 6 cylinders. An inline-4, as mentioned, can have primary balance, but secondary imbalances cannot be eliminated, and the same is true for boxer-4s. An inline-6 is ideal, and for years Rolls-Royce used this type of engine. It isn't coincidence that the Chevrolet, Ford, and Chrysler I-6 engines are among those automakers' best and most durable.

A 90° V-8 has primary balance with only a minor secondary imbalance. A 60° or 120° V-12 is another ideal configuration. A V-6 has significant secondary imbalances. All of this is a gross simplification, so please don't shoot the messenger. The recent Ford 6.8-liter and Dodge 8.0-liter V-10s ideally should have been 72°, but were 90° to be able to use existing tooling, and tricks were required to balance these engines.

Huge slow-running diesels don't have to worry about a lot of these factors. A gigantic 2-stroke diesel propelling a ship is running at only a few hundred rpm at most.

Hope this helps the OP.
 
^^^

Awesome read, thanks for that, and for putting it in layman's terms that I can understand -- I'm now going down the bore and stroke rabbit hole. My next question would be where did the I6 go since it seemed to be ideal, but I understand there are other factors at play in engine manufacturing and design than durability (much to my dismay).
 
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