What parameters make a long lasting engine?

[John Mcrain]

Not really. Because it usually represents a value at maximum load, thus not a normal condition. Mean piston speed would be way more informative.

Mean piston speed reffer to which RPM, at max torque,at max power or at redline?

 

[jack action]

For the purpose of this discussion, mean piston speed refers to the one measured at operating rpm.

 

[CosmologyHobbyist]

Thanks Ranger Mike for your detailed explanation based on real-world experience, I learned quite a bit.

 

[CosmologyHobbyist]

I know something that greatly impacts efficiency, is designing the engine to run at a single RPM. The design can be optimized around the speed of the combustion shockwave, which I believe to be around the speed of sound. This is practical for generators, ships, and locomotives using electric power transmission to wheels. Not so much for cars with mechanical transmission. I expect a single-RPM engine will tend to last longer than an engine designed to run smoothly over an RPM range.
Because of this, I have heard electric drive cars with IC generators, actually get better fuel mileage than mechanical drive cars.

 

[Baluncore]

Gasoline engines wear with a step at the top of the cylinder where the rings are forced outwards by peak cylinder pressure. The taper that forms high in the cylinder fatigues the rings more as they open and close, which also wears the rings and grooves. On over-revving, the step can break the rings.

Diesel engines inject fuel over a longer time, so cylinders wear more evenly with less step, making the rings and pistons last longer.

I know something that greatly impacts efficiency, is designing the engine to run at a single RPM.

If you restrict an engine to a single RPM, you can then also tune the input and exhaust. That makes possible a single speed, cleaner running, two-stroke engine, with an improved power to weight ratio. Impedance matching to a generator is done electrically.

How come all generator ICEs are not single speed, two-stroke, diesels ?

 

[CosmologyHobbyist]

How come all generator ICEs are not single speed, two-stroke, diesels ?

My question exactly. I assume because auto manufacturers are locked into existing manufacturing models, and cannot design up from a clean sheet of paper. A 15-hp diesel generator, plus a massive capacitor, would make a micro electric-drive car faster than a Ferrari.
Ultra-light, with 10-hp motor in each wheel, gives you 400-hp all-wheel-drive for acceleration until the motors need to cool off.
PS, there is an outfit that actually does this in California with a (BMW/MorrisMotors) Mini.

 

[John Mcrain]

For the purpose of this discussion, mean piston speed refers to the one measured at operating rpm.

How can calculate peak firing pressure from usual engine spec. parmeters,bore stroke,power ,torque etc..?
Is it possible?

 

[John Mcrain]

I know something that greatly impacts efficiency, is designing the engine to run at a single RPM. The design can be optimized around the speed of the combustion shockwave, which I believe to be around the speed of sound. This is practical for generators, ships, and locomotives using electric power transmission to wheels. Not so much for cars with mechanical transmission. I expect a single-RPM engine will tend to last longer than an engine designed to run smoothly over an RPM range.
Because of this, I have heard electric drive cars with IC generators, actually get better fuel mileage than mechanical drive cars.

What is single rpm engine?

 

[Baluncore]

What is single rpm engine?

Sorry; I knew what I meant, you are clearly not a mind reader.
I guess it could be one with a very short life, or a very slow engine.

I was actually referring to an engine designed to run at a fixed RPM of say 2000 RPM, with only a few percent variation either side.
The engine would be spun up to operating RPM by the generator operating as a starter motor. Once the lubrication pressure rises, the fuel injection is enabled, and it begins to drive the generator.

 

[jack action]

How can calculate peak firing pressure from usual engine spec. parmeters,bore stroke,power ,torque etc..?
Is it possible?

Not really. Only a full computer simulation - with knowledge of the intake & exhaust designs, etc. - would give you reliable numbers for the peak firing pressure within the cylinder.

But - with all that has been said in this thread - why do you want to know the peak firing pressure? If you want to design an engine for a longer life, mean piston speed should be your first criteria to set. Calculate it for different engines (diesel or gasoline, 2- or 4-stroke). You'll see that all the "working" engines (designed for long life) have lower mean piston speed (7-10 m/s) compared to typical vehicles (14-18 m/s), and high performance vehicles have the highest (20-30 m/s).

 

[John Mcrain]

Not really. Only a full computer simulation - with knowledge of the intake & exhaust designs, etc. - would give you reliable numbers for the peak firing pressure within the cylinder.

But - with all that has been said in this thread - why do you want to know the peak firing pressure? If you want to design an engine for a longer life, mean piston speed should be your first criteria to set. Calculate it for different engines (diesel or gasoline, 2- or 4-stroke). You'll see that all the "working" engines (designed for long life) have lower mean piston speed (7-10 m/s) compared to typical vehicles (14-18 m/s), and high performance vehicles have the highest (20-30 m/s).

Yes I calcualted for 15 different engines and and low value of HP/L and mean piston speed has long last diesel engines..

 

[RandyD123]

The

What is most important parameter when we must build long last internal combustion engine and why?

specific output : HP/Displacement ?
HP per each cylinder?
Torque per cylinder?
Displacement per cylinder?
HP / piston area ?
RPM / displacement of one cylinder?
Numbers of cylinder?
Low RPM as possible?
Low combustion pressure?
Long stroke ,narrow bore?
Piston average speed?

The answer is simple. Tight tolerances and proper lubrication.

 

[Stormer]

All of the above, and none of the above.
My simple answer is "maximum piston speed". Keep the piston speed low for the best lubrication and minimum fatigue of the piston rings.

How does low piston speed give best lubrication for piston rings? I would think that higher piston speed gives a stronger hydrodynamic force from the oil film and therefore keeps the piston rings out of contact with the cylinder wall. Unless the side forces from the connecting rod increase more than the hydrodynamic forces between the piston rings and the cylinder.

Have you tried researching your question at all (I mean other than just asking others for the information instead of looking for it yourself) ? We expect SOME effort on your part when you ask for help here.

OK, I guess we DON'T want him to do his own research before coming here.

Well in that case this forum can just be deleted. Because all questions can be answered by "google it" or "read up on it your self". But that is not a helpful "answer". In fact it is not a answer at all.

Pistons are not round but oval. This is because when at operating temperature the top of the piston where the piston rings are located is now Round due to thermal expansion. That time period from cold start to proper operating temperature is where all the wear happens.

Over time the piston will wear the cylinder into an oval shape. This is why you must re-bore cylinders in the engine blocks that have many miles on them.

Even if the pistons is oval when cold that does not mean that the piston rings are? The piston rings conform to the cylinder walls, not the piston shape right? So i don't understand how cylinders can wear to a oval shape unless the pistons them self are rubbing against the cylinder (and i guess the piston skirts do that to a certain extent but that is not because of the cold shape of the piston).

 

[Ranger Mike]

pistons do wear the cylinder very much in the direction of piston thrust. Usually 90 degrees from crankshaft center line. Piston skirt helps but over time they will wear.

 

[Stormer]

pistons do wear the cylinder very much in the direction of piston thrust. Usually 90 degrees from crankshaft center line. Piston skirt helps but over time they will wear.

But what has that have to do with the cold shape of the piston? Is not that because of the side forces form the crankshaft and connecting rods (and the piston skirts rubbing)?

 

[Ranger Mike]

the piston is oval in a cold state. When the piston iswarmed to operating temperature the shape is ROUND to provide maximum sealing of combution chamber and this is most effective shape to transfer the linear combustion power to the crankshaft and covert to rotary power.

 

[Stormer]

the piston is oval in a cold state. When the piston iswarmed to operating temperature the shape is ROUND to provide maximum sealing of combution chamber and this is most effective shape to transfer the linear combustion power to the crankshaft and covert to rotary power.

It's the piston rings that does the sealing, not the piston shape. And the piston rings float in the piston ring groves. So the rings take the cylinder shape, not the piston shape.

And my question is still how do you come to the conclusion that the shape of the piston when it is cold is what causes the oval wear to the cylinder?

 

[Ranger Mike]

i never said the oval shape of the piston causes oval wear to the cylinder.

 

[Stormer]

i never said the oval shape of the piston causes oval wear to the cylinder.

This is a quote of you and what i replied to:

Piston wear occurs most during start up. The engine is cold and oil has drained off the cylinder wall surface. When you cold start you do in fact have aluminum to iron rubbing. This causes wear on the es surfaces. Pistons are not round but oval. This is because when at operating temperature the top of the piston where the piston rings are located is now Round due to thermal expansion. That time period from cold start to proper operating temperature is where all the wear happens.

Over time the piston will wear the cylinder into an oval shape.

How can i interpret this in any other way than that you are implying that the oval shape of the pistons when they are cold is what causes the oval wear of the cylinder?
If not why are you even mentoning the pistons cold shape in a comment about cylinder wear?

 

[Ranger Mike]

you can interpert this any way you want. Not my problem! as i said, i never stated oval shape of the pistons when they are cold is the cause of oval wear. You mistakenly assumed this for some reason.
Piston thrust causes the cylinder wear during warm up.
It is coincidence that ovality is common wear pattern and is the cold state of the piston. I mention this because the orgional post asked for parameters to be considered for max life engine design. If you designed and engine with a round piston it would not last ten minutes.

 

[Stormer]

If you designed and engine with a round piston it would not last ten minutes.

No it would last a lot longer than 10 minutes...
In fact on a lot of homemade engines the pistons is turned on a simple manual lathe.
The only thing that means is that you need to have a larger tolerance between the piston size and the cylinder size to account for the uneven expansion of the piston. And that gives a larger unsupported portion of the piston rings because of the larger difference between the piston and the cylinder size. And On smaller engines this is less of an issue than on larger ones.

 

[Ranger Mike]

Go ahead and make um round. wish other racers had your ideas.

 

[Stormer]

Go ahead and make um round. wish other racers had your ideas.

Racers? Where did that suddenly come from?

Yes modern engines use oval pistons to account for the uneven expansion of the piston because of the uneven material thickness/geometry because of the design of a piston that is connected directly to a crankshaft with a connecting rod. That allows them to run tighter tolerances getting less piston slap and so on. Or on some big crosshead diesel ship engines the connecting rod to the piston runs in a straight line that allows you to pretty much eliminate the piston skirts and design the piston completely round making it expand evenly all the way around.

And no that does not mean that a round piston in a traditional engine could not run more than 10 minutes. As i said you just have to make the tolerances between the piston and the cylinder looser to account for the uneven expansion. Or you can even taper the skirt section of the piston to make it narrower.

The two are not mutually exclusive. Do you really think otto and diesel engines in the early 1900's had oval pistons?

 

[Ranger Mike]

glad you finally got it! good job!