piston size 4

[jfoldbar]

hi all.
im new to this forum hope to learn from it.
i have a question i hope can be explained regarding engine piston size.

so i recently bought a new truck for my business. its an isuzu with a 5.2liter 4 cyl motor.
and i got thinking why are engine designers now making engines with larger fewer piston. 20 years ago a 5.2l motor would have been a v8. why is it now a 4cyl

thanks

 

[gruntguru]

BigClive (Chemical)28 May 19 12:41
The engine in question was on a DC-3/C-47 during the war. The 'plane had a non-operating starter motor on one engine and it was travelling around remote bases for a few weeks. The starting involved 4 or 5 tubes (presumably from aircraft or truck etc.) linked together with the end looped around the prop, stretched out by a group of blokes (4,5 20?). A restraining rope was tied from the prop to an immovable object (truck etc.?) The rope was cut, the engine spun over, starting and flinging off the tubes etc.
Great story BigClive - that would be a good one for Mythbusters to confirm.

If a truck was available, perhaps use it to replace the men and use a tree to replace the truck.

je suis charlie

 

[enginesrus]

Starting that way sounds plausible. There are a few tricks that can be tried in a pinch to start an engine that you don't have the strength to do by hand.

 

[Marquis]

"Do you see the contradiction? "Bigger" has a limit.

As SI cylinders get larger you reach a point where flame path gets too long and efficiency must suffer - unless you can provide multiple ignition points."

There's no contradiction - only a trade off.

And this is often somewhat overstated at that.

Long flame paths would manifest itself in the 10-90 % burn duration.

I have reams of data to show that this 'correlation' isn't a correlation at all but an unsubstantiated theory:

Example-
5.6 litre V8 with a 98 mm bore, single plug, pent roof, WOT peak power speed

10-90% burn= 24 degs

4 litre V8, 86 mm bore , single plug, pent roof chamber, WOT peak power speed

10-90 % burn = 25 degs

There is a massive bore difference there and there are other bigger fish to fry.

Another ignition point will surely help, however I think the benefits of this are shown more markedly in mitigation of knock limit (enabling higher compression ratio), faster burn at heavily throttled high engine speed conditions (0-5%s get very fast) and great combustion stability during cold start warm up retard mode.

Another example: The 5.7 litre Hemi showed an improvement of just over 1% BSFC when they fired it back to back in single spark vs dual spark mode. This is when the base engine design and spark plug position and in cylinder motion (which was known to be very poor in the early 'Eagle' Hemi design) are not optimized for single spark operation.

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Sideways To Victory!

 

[gruntguru]

Simple facts.

Diesel efficiency increases with cylinder dimensions in all practical sizes.

SI efficiency plateaus and falls when cylinder dimensions exceed some (much smaller) value.

Examples of different engines with "massive" (86mm vs 98mm) bore difference is light years from the point. Sure - that 98mm bore is still working fine (and the fact that SI engines are mass produced with even larger bore indicates that SI efficiency is still satisfactory in this size range). But what happens when you enlarge it to 198mm? No problem for a Diesel.

je suis charlie

 

[Lou Scannon]

To be sure, commercially successful SI engines exist with bores much larger than 200 mm. But their combustion systems bear no resemblance to SI automotive engines.

"Schiefgehen wird, was schiefgehen kann" - das Murphygesetz

 

[Marquis]

"Diesel efficiency increases with cylinder dimensions in all practical sizes.

SI efficiency plateaus and falls when cylinder dimensions exceed some (much smaller) value."
This is very vague.

Where and how does SI efficiency FALL or drop with increasing cylinder dimension?

Are you talking about Indicated Thermal Efficiency?

Not quite sure how cited example (with data) or 86 vs 98 mm is 'light years'from the point.

Are you referring to 10-90% burn slow down?
Completion or burn?
Making knock limit worse?

Please elaborate

The other point is that even a big bore diesel engine will have a slow down of combustion- and completion of burn aspects.
This can be managed by combustion chamber design, air motion control, better mixing etc, the same way that in a petrol engine these aspects can be managed too.

Blanket statements of 'Bigger cylinders make petrol engine efficiency fall and make diesel efficiency rise' need further examination.

Examples with data help also.

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Sideways To Victory!

 

[BrianPetersen]

The whole system needs to be taken into context.

A 100,000 horsepower engine for an oceangoing ship, with cylinders that you could walk into, might have a very high thermal efficiency when operating at rated load, but it will sink your jetski ... nevermind having a poor efficiency when operating a 100 hp load because of operating so far off of its design operating conditions. Operating at a small part load (due to an engine being oversized) is a very big efficiency-killer. YES the big engine has a higher thermal efficiency at its respective design operating condition, but that doesn't help if your load doesn't match that condition.

In the automotive context, the big-displacement engine choice is almost never going to be more economical than the smaller one on the grounds of "bigger cylinders", because almost all automotive and light truck engines are oversized for the part-load operation that constitutes almost all normal driving, and operating at a smaller fraction of part load is a bigger efficiency-killer than having smaller cylinders.

The exception seems to be the over-downsized turbo gasoline engines, especially if you are working them under load. The measures taken to protect themselves from blowing up (lower compression ratio than would otherwise be possible, delayed ignition timing to preclude detonation, enrichment to protect pistons and exhaust valves and turbochargers and catalysts from meltdown) are efficiency-killers.

 

[michaelwoodcoc]

If you read into the development of the honda insight 1st gen, for example, they decided to go with an inline 3 cylinder instead of a 4. They claimed it reduced internal engine friction, and reduced weight. You could probably do the math and see they they're right. Area around piston must be a source of friction. The amount of plain(plane)? bearings must be a source of friction, the number of valves, and so on and so forth.

You can now "downsize" a larger engine by "switching" from otto to atkins cycle, I think. The manufacturers use variable valve timing to keep the intakes open longer and push some air back out of the combustion chamber before compressing a smaller amount. This, however, doesn't reduce the drag from having potentially more cylinders, more valves, etc.

I think cylinder deactivation, as done in the past, is also a poor practice. We all know the notorious engine, was it the north star, which employed it to great failure.

 

[Marquis]

The limit to petrol down sizing seems to be about 300 ccs.

The benefits of the smaller size and 'de-throttling' for lower pumping are offset by the greater heat loss from the cylinder.

This is reflected in the BSFC.

From the example I outlined above: "From my database and experience on a petrol when the cylinder size reaches about 500 ccs, you get good best point BSFC (an outstanding number might be 235 g/kwh- for a PFI engine running a 10.5-10.75:1 CR). As you get bigger than this- the benefits level off and you don't get much better. A 300 cc cylinder of square bore and stroke would be lucky to attain a best point BSFC number of 270 g/kwh by comparison."

If you boost too hard you can run into the 'super knock phenomenon'- this becomes very apparent when you boost towards 25+ BMEP. Attention to exhaust valve open period (to minimize in cylinder residual- which can be a source of detonation)and oil control ring design (again- oil drop lets- a source of detonation).

In addition boosting harder often requires over fueling for component protection- which is why you often don't the see real world benefits to down size boosting as achieved in EPA and EUDC cycles.

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Sideways To Victory!

 

[gruntguru]

"Diesel efficiency increases with cylinder dimensions in all practical sizes.

SI efficiency plateaus and falls when cylinder dimensions exceed some (much smaller) value."
This is very vague. I only made a general claim.

Where and how does SI efficiency FALL or drop with increasing cylinder dimension? Somewhere between the 100mm bore you mentioned and the 1000mm bore of large diesels. Read a reference book like Taylor if you want more specific details. I have read it but don't recall the detail.

Are you talking about Indicated Thermal Efficiency? Both - ITE and BTE.

Not quite sure how cited example (with data) or 86 vs 98 mm is 'light years' from the point. Results from 14% difference in bore size on two different engines doesn't tell me much. 100mm bore (SI don't get a lot bigger without special ignition systems eg the gas engines I mentioned) is light years from big diesel engines.

Are you referring to 10-90% burn slow down? No
Completion or burn?
Making knock limit worse? Yes

Please elaborate

The other point is that even a big bore diesel engine will have a slow down of combustion- and completion of burn aspects. Really? Combustion rate and completion of burn are terms more relevant to SI engines.
This can be managed by combustion chamber design, air motion control, better mixing etc, the same way that in a petrol engine these aspects can be managed too. Pre-mixed combustion is a completely different kettle of fish. Flame travel distance and combustion duration (time not degrees) will kill you in the end. (detonation)

Blanket statements of 'Bigger cylinders make petrol engine efficiency fall and make diesel efficiency rise' need further examination. I have provided a reference. Enjoy your research.

Examples with data help also.

The internal combustion engine / by C.F.Taylor and E.S.Taylor
Taylor, Charles Fayette,

It is some years since I read it but there is an entire chapter devoted to cylinder sizing which formed the basis for my understanding of the subject.





je suis charlie

 

[Marquis]

I cite direct data based on engines I've developed and my experience. Yep, I go to the trouble. I wish I had the balls to cite data from a book I barely remember and cant prove when the burden of proof is on me. Magnificent!

Oh, and the reference is wrong, I checked.

Oh well



"Do not argue with a fool. He will drag you down to his level and beat you with experience." -

Mark Twain

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Sideways To Victory!

 

[RodRico]

A point of clarification...

Shouldn't "large bores have high heat loss," "long strokes have less heat loss," and "small cylinder displacements have higher heat loss" type rules of thumb be caveated with "all other factors being the same?" I often get surprised by how much effect the other factors can have.

I believe convection loss from the gasses to the cylinder walls is calculated by Q (Joules) = TimeStep (seconds) * Heat Transfer Coefficient (W/m2/K) * Area (m2) * (Gas Temp - Wall Temp)(K) where wall temp is typically around 366K (200F) to limit oil breakdown. In my model, I use Hohenberg's heat transfer coefficient (similar to Woschni's coefficient but shown by experiment to be more accurate for HCCI) shown below. Note the largest exponent is on mean piston speed which increases with stroke. Holding displacement, cylinder count, RPM, peak combustion temperature, wall temperature, and time step constant, my model indicates 63.5% efficiency with bore of 3.077 inches and stroke of 0.375 inches but 62.6% efficiency with a bore of 1.526 inches and a stroke of 1.526 inches. The largest difference between the two cases in the heat transfer calculation is mean piston speed (22 feet-per-second in the first case and 89 fps in the second). Equally surprising is the fact that efficiency for a given displacement does up when the number of cylinders is increased. With a 49.5 cc displacement in one cylinder, my model shows 63.5% efficiency but that figure goes up to 64.3% with four cylinders and 64.6% with six. The interaction between numerous variables appears to consistently break the "rules of thumb."

 

[gruntguru]

I cite direct data based on engines I've developed and my experience. Yep, I go to the trouble. I wish I had the balls to cite data from a book I barely remember and cant prove when the burden of proof is on me. Magnificent!

Oh, and the reference is wrong, I checked.

Oh well

- I made a very simple claim "Unlike diesels, SI engine efficiency drops above a certain cylinder size". I did not claim to know the specific limits, relevant data

- You refute that claim based on data from two engines - both less than 4" diameter bore. You have not disproved my claim so yes - the burden of proof remains on you.

- I provided a reference where you might find the information to broaden your knowledge.

Common sense plus a basic understanding of pre-mixed combustion.
As you increase the cylinder size of a SI engine the rpm will decrease -> convective heat transfer to the end-gas will increase. Relatively fixed flame speed means that time duration of the combustion process will increase -> radiant heat transfer to the end-gas will increase. Sound like a recipe for detonation? Might need to reduce the CR? Efficiency maybe suffers? You said as much yourself: "Bear in mind that the larger your bore - and cylinder size in general - limits you in the realistic attainable compression ratio you can achieve"

Here are some examples for you to think about.
The OP refers to a truck engine with displacement of 1.3 litres per cylinder. Do you know of any current on-road SI engines with cylinders this big?
The largest current SI cylinders I can find are on a Wartsila gas engine with 50cm bore. This engine is lean-burn and uses a sophisticated jet-ignition system to rapidly ignite the entire combustion chamber.
Current marine diesel cylinders are typically 100 cm bore and approximately 2,000 litres per cylinder

je suis charlie

 

[RodRico]

Ran across this today at

https://web.mit.edu/2.61/

http://www/Lecture notes/Lec. 15 Diesel engine characteristics.pdf

"DI" is direct injection, "IDI" is indirect injection (pre-chamber), and "SI" is spark ignition.

 

[Marquis]

RodRico, thanks for the plots, unfortunately it combines engine friction with thermal losses.

What would be more useful would be to show indicated thermal vs engine cylinder size that doesn't include the frictional aspect.


In all the Annand and Woschni models I've tuned to MEASURED single cylinder data, I've invariably had to tune it to the measured data. Usually separating open valve heat transfer to closed valve.

Starting with the equations/simulation- with no measured data is only a beginning and - once the engine was built and tested, invariably was some way off. The second iteration of the correlation of the model to measured data is where I found most of the REAL learning begins. This is invariably the downfall of every start up company I've consulted for.

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Sideways To Victory!

 

[RodRico]

Marquis,

The original poster asked about piston size, not thermal losses. A plot showing the clustering of displacement per cylinder provides valuable insight regarding design points that were selected for a variety of reasons including efficiency (heat loss, piston friction, pumping loss, etc.), emissions, manufacturing cost, design reuse across models, etc.

Yes, every model ends up getting tuned with measured data. That doesn't mean it can't provide insight into the interaction of variables during the design phase. Anyone who thinks a heat transfer or CFD model provides a final answer is naïve. The models are intended to get into the ball park and nothing more; a working model doesn't guarantee the engine will attain the indicated performance, but if the engine doesn't meet (or exceed) desired performance in the models, it most certainly won't meet them once built. In my current effort, the models exceed the targets that define a marketable product by 15% to 20%. It's going to take a lot of work and several iterations to find out what the final performance is.

Rod

 

[enginesrus]

For a spark ignition gasoline engine, in the old days 1930's to 1950's it was determined that around 6" diameter was the maximum size for the cylinder. And is the main reason that most all aircraft engines of the period have less than 6 inch diameter cylinders, for more power potential, the better thing to do was just add more cylinders.