What parameters make a long lasting engine?

[John Mcrain]

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?

 

[berkeman]

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?
Numbers of cylinder?
Low RPM as possible?
Low combustion pressure?
Long stroke ,narrow bore?
Piston average speed?

What is the context of your question? Is this for schoolwork?

 

[John Mcrain]

What is the context of your question? Is this for schoolwork?

No this is not schollwork,it is general discusion.

 

[phinds]

Well, then tell us what YOU think are the most important and why.

 

[berkeman]

No this is not schollwork,it is general discusion.

Okay. So what do you think are the most important things that lead to a longer life of internal combustion engines (ICEs)?

Oops, @phinds beat me to it, as usual...

 

[Baluncore]

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

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.

 

[John Mcrain]

Well, then tell us what YOU think are the most important and why.

I am not sure so I ask here in hope some mechanical engineer will make detail explanation..

But I know that all long last engines truck,bus etc has very low specific output, around 30-40HP/L
modern car diesel engine around 100HP/L, tanker only 4HP/L !
Tanker has 100 000HP and 25480 Liters !

 

[phinds]

I am not sure so I ask here in hope some mechanical engineer will make detail explanation..

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.

 

[jack action]

1. Low mean piston speed = better longevity.

The piston must accelerate and decelerate from zero, to its maximum speed, to zero again, all within one stroke. These accelerations create tremendous stresses on the engine parts and increase the friction losses in the bearings. The high speeds also increase the friction losses.

2. Next would be the heat dissipation characteristics. Overheating an engine is not good for longevity.

3. Finally, higher pressure on the piston will add to the stresses on the mechanical parts (which brings the same consequences as stated in point 1).

 

[phinds]

1. Low mean piston ...
2. Next ...
3. Finally ...

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

 

[Baluncore]

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

Understanding IC engines is a lifelong vocation. You cannot do it over the weekend.

A short answer reduces confusion and gets the reader headed in the right direction. A full explanation would be too long for this forum.

You can find the full explanation spread through a dozen different textbooks. But it is not until you get near the end that you will realize what is actually most important with regard to durability.

 

[John Mcrain]

3. Finally, higher pressure on the piston will add to the stresses on the mechanical parts (which brings the same consequences as stated in point 1).

I agree but brake mean effective pressure (BMFP)of tanker which last 50years is 35Bar and Ferrari high rpm engine has BMFP=12 Bar last only 100 000km.

So we can't strictly follow this numbers in sense of engine longivity because leads as to wrong conclusion that high cylinder pressure (high BMEP) means increeased engine life...

 

[DaveE]

You left out the most obvious parameter: Cost.

 

[Baluncore]

So we can't strictly follow this numbers in sense of engine longivity because leads as to wrong conclusion that high cylinder pressure (high BMEP) means increeased engine life...

That is why your question only has a general answer, which is not applicable to every specific engine configuration. Everything plays a part, including the driver and the service mechanic.

 

[Ranger Mike]

From a guy who grenade many many engines over the years racing, my main concern is heat and flex poor design where form does not follow function. More on this later.

Specially here, heat means excess heat, flex means dynamic flex ( non-controller harmonics).
Heat robs motor oil of lubrication properties over time. Heat destroys the valve spring life span. Then you have valve float then the piston smacks the valve, may break it off and engine eats it. Bam.

Heat can cook the motor oil and may even set it on fire. Heat can cause cracks I the block and cylinder head. Heat can warp cylinder heads and cause blown head gaskets. Heat can cause loss of coolant by changing liquid to steam and booom
Flex is where the load is more the original design limits. Flex of the crank shaft means poor lubrication, scored bearings eventual failure. Flex can mean crankshafts crack then break. Flex in valve train components mean erratic valve timing possible valve float, possible timing chain breakage. Flex here may be due to weak skinny push rods, weak timing chain /belts, accessories driven off cam timing irregularities ( distributor flex). Flex of cylinders in the engine case (block ) is horsepower robbing and long term liability.

This is one area severely overlooked when designing race engines.poor design where form does not follow function – Number one consideration is the oil system. Stock oil systems are great for grocery getters but terrible to race. Oil does two things, lubricate and cool. Too much oil pressure can be as bad as too little. Oil must be controlled at all levels of performance. Oil under pressure shoot up to the bottom of the piston and cools it. Too much and the piston rings are overcome and can not control it. We have smoke, fouled spark plugs and a mess. Not so obvious is the danger of too high oil pressure in the upper oil galleries. Typically the oil is meant to flow up top then make many right angle turns to various other engine components. When the speed gets too great, the oil ha a difficult time making the 90 degree turn, hence, poor oiling. One more danger is oil aeration. Passing many moving parts creates potential for introducing air bubbles into the oil and air don’t lube too good! In the old days valve lifters were in the oil gallery and after the oil passed all the way past 3 lifters the oil capability of number 4 was dramatically reduced. We found that we could supply oil into the oil galleries from both ends of the block and the oil would meet in the middle and then back fill all the 90 degree turns a lot more effectively. Proper oiling means controlling the oil to the valve train to keep the valve springs cool but not to over pump oil top side and overcome the valve seals. Oil should return the ethe oil pan as soon as possible. You don’t want a bunch of oil slinging around inside engine block as it robs power and goes where it should not. Oil sling can cause parasitic drag.
Flex – One of the most impressive engine blocks I ever saw was the 292 CID straight six truck block Ford made. It has 7 main bearing caps. I never heard of a crankshaft busting all the time this engine was raced. If you look at the small block Chevy V8 block the main bearing caps are bolter to the flat block surface. No lateral support against flex. This is one reason the y had to go to 4 bolt mains. The old Mopar (Chrysler) B block had enormous lateral support. Monster Hemi’s had the same deal and added a main cap bolt 90 degrees to the crank thru the engine block.
Mopar small blocks were famous for blown head gaskets. The cylinder head design was for 4 head bolts on each cylinder. The small block chevy had 5. One other flaw was the deck thickness the small block Mopar. This was the design engineers light weight thin wall casting block concept. They wanted a light weight V8 for the compact cars in the early 1960’s. But thin wall design meant ----- FLEX. We had to pour a concrete metallic mixture into the engine block at the water coolant passages until it was about an inch thick, This stiffened cylinders and tied them into each other to prevent flex. It still did not help the thin deck (thickness of iron material flat surface the cylinder head bolts to) but it helped longevity. Did not impact cooling either.

Any way that’s my take on things to consider.

 

[phinds]

Understanding IC engines is a lifelong vocation. You cannot do it over the weekend.

A short answer reduces confusion and gets the reader headed in the right direction. A full explanation would be too long for this forum.

OK, fair point.

 

[Lnewqban]

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

If you must build a long lasting reciprocating IC engine, study the characteristics of existing engines that were designed to achieve that: Diesel engines for big ships and stationary power generators.
Those are heavy (abundant iron-based metal) and robust (especially bearings and valves), which help with most of the phenomena that has been very well explained above.

Poor operation, frequent start-ups and neglected maintenance can destroy the best designed engine very briefly.

 

[jack action]

I agree but brake mean effective pressure (BMFP)of tanker which last 50years is 35Bar and Ferrari high rpm engine has BMFP=12 Bar last only 100 000km.

So we can't strictly follow this numbers in sense of engine longivity because leads as to wrong conclusion that high cylinder pressure (high BMEP) means increeased engine life...

I added this one at the last minute to make sure everything was covered (say using a supercharged engine fueled by nitromethane compared with a naturally-aspirated one fueled by gasoline), but the mean piston speed is really the most important one.

You can always make the engine beefier to withstand large pressures or add a more powerful cooling system to retrieve more heat, bu the accelerating parts is a catch 22: If you make them bigger, they are more resistant, but the inertia forces are also greater. Ultimately, the material strength becomes a limiting factor.

 

[Ranger Mike]

As usual Jack is correct..heartily agree, piston speed most important, if you want it to last.

 

[John Mcrain]

From a guy who grenade many many engines over the years racing, my main concern is heat and flex poor design where form does not follow function. More on this later.

Specially here, heat means excess heat, flex means dynamic flex ( non-controller harmonics).
Heat robs motor oil of lubrication properties over time. Heat destroys the valve spring life span. Then you have valve float then the piston smacks the valve, may break it off and engine eats it. Bam.

Heat can cook the motor oil and may even set it on fire. Heat can cause cracks I the block and cylinder head. Heat can warp cylinder heads and cause blown head gaskets. Heat can cause loss of coolant by changing liquid to steam and booom
Flex is where the load is more the original design limits. Flex of the crank shaft means poor lubrication, scored bearings eventual failure. Flex can mean crankshafts crack then break. Flex in valve train components mean erratic valve timing possible valve float, possible timing chain breakage. Flex here may be due to weak skinny push rods, weak timing chain /belts, accessories driven off cam timing irregularities ( distributor flex). Flex of cylinders in the engine case (block ) is horsepower robbing and long term liability.

This is one area severely overlooked when designing race engines.poor design where form does not follow function – Number one consideration is the oil system. Stock oil systems are great for grocery getters but terrible to race. Oil does two things, lubricate and cool. Too much oil pressure can be as bad as too little. Oil must be controlled at all levels of performance. Oil under pressure shoot up to the bottom of the piston and cools it. Too much and the piston rings are overcome and can not control it. We have smoke, fouled spark plugs and a mess. Not so obvious is the danger of too high oil pressure in the upper oil galleries. Typically the oil is meant to flow up top then make many right angle turns to various other engine components. When the speed gets too great, the oil ha a difficult time making the 90 degree turn, hence, poor oiling. One more danger is oil aeration. Passing many moving parts creates potential for introducing air bubbles into the oil and air don’t lube too good! In the old days valve lifters were in the oil gallery and after the oil passed all the way past 3 lifters the oil capability of number 4 was dramatically reduced. We found that we could supply oil into the oil galleries from both ends of the block and the oil would meet in the middle and then back fill all the 90 degree turns a lot more effectively. Proper oiling means controlling the oil to the valve train to keep the valve springs cool but not to over pump oil top side and overcome the valve seals. Oil should return the ethe oil pan as soon as possible. You don’t want a bunch of oil slinging around inside engine block as it robs power and goes where it should not. Oil sling can cause parasitic drag.
Flex – One of the most impressive engine blocks I ever saw was the 292 CID straight six truck block Ford made. It has 7 main bearing caps. I never heard of a crankshaft busting all the time this engine was raced. If you look at the small block Chevy V8 block the main bearing caps are bolter to the flat block surface. No lateral support against flex. This is one reason the y had to go to 4 bolt mains. The old Mopar (Chrysler) B block had enormous lateral support. Monster Hemi’s had the same deal and added a main cap bolt 90 degrees to the crank thru the engine block.
Mopar small blocks were famous for blown head gaskets. The cylinder head design was for 4 head bolts on each cylinder. The small block chevy had 5. One other flaw was the deck thickness the small block Mopar. This was the design engineers light weight thin wall casting block concept. They wanted a light weight V8 for the compact cars in the early 1960’s. But thin wall design meant ----- FLEX. We had to pour a concrete metallic mixture into the engine block at the water coolant passages until it was about an inch thick, This stiffened cylinders and tied them into each other to prevent flex. It still did not help the thin deck (thickness of iron material flat surface the cylinder head bolts to) but it helped longevity. Did not impact cooling either.

Any way that’s my take on things to consider.

Do you know what are engines with lonigest TBO(time between overahaul) and which common parameter of them reduced engine wear the most, low piston speed, low specific output(HP/L) or maybe high brake mean effective pressure(which don't make sense)?

I notice that this low rpm-long last engines has very high mean effective pressure,which increase heat and force on piston/ring/bearings..,isnt that weird ?

 

[John Mcrain]

You can always make the engine beefier to withstand large pressures or add a more powerful cooling system to retrieve more heat, bu the accelerating parts is a catch 22: If you make them bigger, they are more resistant, but the inertia forces are also greater. Ultimately, the material strength becomes a limiting factor.

If piston never really touch cylinder because of oil film(in engine without failure),why than piston speed increase wear?
Can you explain what happened in high piston speed engines to increase wear and reduced life time?

 

[Averagesupernova]

If piston never really touch cylinder because of oil film(in engine without failure),why than piston speed increase wear?
Can you explain what happened in high piston speed engines to increase wear and reduced life time?

I won't claim to be an expert about this but the fact that an oil film exists does not guarantee absolutely ZERO wear. The definition of the word lubricant should imply this.

 

[jack action]

Can you explain what happened in high piston speed engines to increase wear and reduced life time?

It's fatigue due to the constant pushing and pulling of the pistons (just the weight of them) and also causes the flex that @Ranger Mike was talking about in his post:

Fatigue

Fatigue is the main cause of broken connecting rods--especially in older engines. The constant compression during the power stroke and stretching during the exhaust stroke, over thousands of times a minute, eventually wears the metal out and it becomes brittle and finally breaks. If the oil is low or dirty it can speed up this process. Running the engine hot can also speed up the process. Sometimes a fairly new engine can have fatigued connecting rods if it is a rebuilt engine and the mechanic used cheap parts or the wrong parts for the engine.

Here's what a crankshaft looks like under stress (deformations are emphasized to better see them):

 

[John Mcrain]

It's fatigue due to the constant pushing and pulling of the pistons (just the weight of them) and also causes the flex that @Ranger Mike was talking about in his postere's what a crankshaft looks like under stress (deformations are emphasized to better see them):

View attachment 271435
View attachment 271436​

So key for durability is low piston speed and low specific power.

What is difference between mean effective pressure and brake mean effective pressure formula?Does formula include max torque or torque at peak power?
Which torque is better to choose when compare two engines?

 

[jack action]

What is difference between mean effective pressure and brake mean effective pressure formula?

MEP is the general formula where any value of torque can be used. BMEP is where the torque value comes specifically from a dynamometer reading.

Does formula include max torque or torque at peak power?
Which torque is better to choose when compare two engines?

You would usually plot BMEP against rpm, just like with torque or power. The rpm for the maximum value will necessarily coincide with the rpm of the maximum torque.

 

[Baluncore]

There are many ways of dying. To live a long life, you must avoid all of them for as long as possible. A durable engine will be designed, manufactured and maintained in a deliberate way, intended to avoid early failure. That is a multi-dimensional problem.

The operator controls when the engine will exceed the safe operating conditions that may reduce engine lifetime. Many drivers are hard on their vehicles, which leads to an early failure. A performance engine may need to be fitted with a RPM limiter, to protect it from it's operator. Keep the maximum engine RPM down.

A ships engine room is staffed by skilled engineers whose job it is to care for the engine. It is not unusual to see a spare connecting rod, protectively wrapped and bolted to an engine room bulkhead. It is continuous care that makes the difference.

We all know of engines with statistically poor reputations, engines that wear out quickly, or that fail catastrophically. Those engines were poorly designed, or not produced for the job they were later called on to perform.

 

[Stephen Tashi]

I wonder if "engine life" has technical definition. For example, if a engine blows a head gasket, does that count as end-of-life even if the engine can be repaired?

 

[Vanadium 50]

There are many ways of dying. To live a long life, you must avoid all of them for as long as possible. A durable engine will be designed, manufactured and maintained in a deliberate way, intended to avoid early failure. That is a multi-dimensional problem.

This. If you take the most likely cause of failure and make it much less likely, at some point the second most likely dominates. Continues improvement of the original element will have a smaller and smaller impact.

And it's probabilistic. Luck matters. It's also possible to do everything wrong and still have a not so bad outcome. Below is an engine that's gone 84000 miles on the original oil.

 

[Tom.G]

As a teenager I accidentally bought one that looked like that.

• Drain oil
• Refill with kerosene
• Run it for a short time AT IDLE
• Repeat a few times 'til the kerosene is somewhat cleaner
• Refill with oil
• Drive gently
• If it knocks
  • Stop immediately
  • Drain
  • Fill with the heaviest oil you can find (50W was heaviest available)
  • Sell immediately (you are about to lose a connecting rod)

Another car I had I bought from a wholesale consumer product salesman I worked with. Manual transmission, almost all short trip city driving, except he would shift according to the length of time he was in a gear, not vehicle or engine speed. Maintenance schedule was: If it won't run, get it to run, repeat.

It ran decent, until I took it on a 400 mile trip to a new job. On the Interstate Highway it suddenly started running very rough, noisy, and low power. Oh well, it still moved. When I got to surface streets it really strained to get moving after a stop.

Some investigation revealed the rear cylinder of the straight 6 was not working. Removing that spark plug made it drivable but VERY noisy. It was a rural area with a few scattered towns, so I probably didn't wake up more that 50 people or so!

The failure was insufficient oil to the rocker arms. The rocker arm shaft sheared off at the back end, therefore the #6 exhaust valve never opened.

A clean-up, replacing the rocker shaft and rocker arm allowed me to drive it about another 5 years, including 3/4 of the way across the country.

 

[John Mcrain]

MEP is the general formula where any value of torque can be used. BMEP is where the torque value comes specifically from a dynamometer reading.

You would usually plot BMEP against rpm, just like with torque or power. The rpm for the maximum value will necessarily coincide with the rpm of the maximum torque.

1. Is MEP value in engine specifications calculated for max torque or torque at peak power?
Here is example at link below ,where write mean effective pressure: 28.8Bar
https://www.mtu-solutions.com/cn/en/stories/marine/new-mtu-16-cylinder-series-8000-engine-for-marine.html

2. So MEP value has nothing to do with engine durability/life time(time between overahul),but can we say that engine with higher MEP value has higher efficiency?

 

[Ranger Mike]

A couple of observations.

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. It used to be that much wear happened when the engine was new and never fired up. Back then a lot of material was scrubbed off during “ break in”. period. Engineers developed “ plateau Honing’ that did not generate as much “Peaks” during the honing process in the cylinder and made break in a lot shorter until the piston rings were “ set”.
This same thing happens the main and connecting rod bearing at initial break in and cold starts. You have oil film but not a whole lot at cold starts and this is the metal to metal contact period. You could avoid this with a Pre-luber that is basically an aluminum cylinder with a “Piston plug inside”. Engine oil is plumbed to the cylinder and as the engine is running, oil pressure enters the cylinder and pushes the piston that compresses a mechanical coil spring. Before engine shut off, the drive operates a shut of valve that closes the engine supply. Now you have 2 quarts of engine oil trapped din this reservoir under 40 PSI. On cold start you release the valve and you per-lube the bearings with 40 PSI oil. And fire the engine.
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. The ovality only gets worse over time. You develop piston slap on bad cases of wear. Also the piston rings develop “ flutter” when severe ovality occurs. The rings will open and close at higher RPM (flutter). This introduces oil into the combustion chamber. Not good. Eventually the poor cast iron piston rings (Moly) will stop doing the job and you have major blow by or even ring breakage. Heat is one big contributor to ring failure. Heat is death to rings and valve springs.
Do not forget that automobile manufactures do not want you to have a life time engine. They would go out of business. So they use cast pistons (forged wear far longer but are more expensive). They use cheap piston rings. They know about pre-lube but this is expensive.
Piston speed is not about the piston. It is about the reciprocating mass connected to the crankshaft. Manufactures use cast pistons, cast rods and minimal life span connecting rod bolts. By article in Engine Builder Magazine Larry Carley Mar 15, 2017 - High engine speeds place enormous tensile loads on a rod. The inertia of the pistons reciprocating up and down multiplies the effective weight of a piston exponentially as RPM go up. The force generated by a piston hitting TDC at 1,000 RPM is 50 times its initial weight when the engine is not running. At 10,000 RPM, the effective weight of that same piston is 5,000 times greater! That’s a lot of force stretching the rods 166 times per second. Think about that for a second. It is a miracle we can get 150,000 miles about of production engines as we do. During catastrophic failure we usually have a lack of lubrication problem of stress failure. Very rarely will the piston fail. The con rod snaps at the small end near the wrist pin. The other common failure is the ovality of the connection rod bearing that happens over time because of this loading at top dead center 16 times a second at 1000 RPM. Very rarely will the connecting rod bolt break.

 

[John Mcrain]

. The force generated by a piston hitting TDC at 1,000 RPM is 50 times its initial weight when the engine is not running. At 10,000 RPM, the effective weight of that same piston is 5,000 times greater!

If piston weight is 200grams than effective weight at 10 000rpm is 1000 kg!

but that is way smaller than force which act on piston because of presssure,peak pressure in modern diesel engine is 205Bar(mercedes 2.0, 245HP, 500Nm, 4cylinder in line ),at bore 82mm piston area is 52cm2,that gives 205bar x52cm2= 10 660 kg! 10 tons at piston!this is tramendous force

 

[Vanadium 50]

Do not forget that automobile manufactures do not want you to have a life time engine. They would go out of business. So they use cast pistons (forged wear far longer but are more expensive). They use cheap piston rings. They know about pre-lube but this is expensive.

I don't think I agree with that. Back in the day, a car that went 100,000 miles was practically a miracle. Today 200,000 doesn't raise many eyebrows, and these were cars built a decade ago.

I think it is fair to say that the companies have no interest in developing an engine that last substantially longer than the rest of the car. Customers have no interest in buying such a vehicle either. What would be the point?

It's also worth thinking about even problem engines. Consider the Subaru Boxer and its head gasket issues. This is well-publicized and what people think of when they think of unreliable engines. These are engines that took ~100K miles before the problem showed up. In the 1970's this would be considered a great success. It took Subaru a decade to fix the problem because it took them maybe 8 years for the problem to show up.

 

[jack action]

1. Is MEP value in engine specifications calculated for max torque or torque at peak power?

You want to brag about max BMEP, so at max torque.

2. So MEP value has nothing to do with engine durability/life time(time between overahul),

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

but can we say that engine with higher MEP value has higher efficiency?

What a higher MEP says is: «You get more torque for the same displacement.» In other words, you can get the same torque with a smaller engine. Although MEP & efficiency can be related for comparable engines, MEP doesn't tell you about how much fuel you burn to get that energy, which is important to determine the efficiency.

If piston weight is 200grams than effective weight at 10 000rpm is 1000 kg!

Remember that this effective weight is proportional to the mean piston speed, not the rpm.

but that is way smaller than force which act on piston because of presssure,peak pressure in modern diesel engine is 205Bar(mercedes 2.0, 245HP, 500Nm, 4cylinder in line ),at bore 82mm piston area is 52cm2,that gives 205bar x52cm2= 10 660 kg! 10 tons at piston!this is tramendous force

Remember that this is at maximum load. An engine will not last long if used at maximum load all the time.

 

[cmb]

You left out the most obvious parameter: Cost.

Exactly.

But more than that.

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?

I think you are trying to ask what parameters have the least wear on the engine, but it is difficult to say.

Do you want to look at highly maintained engines, or zero maintenance engines? Can we add or change oil, is it a wet or dry sump, is it for steady speed operation or urban driving type?

But @DaveE has it in one word;

The longest lasting engines of all are those that COST the MOST to buy! If they cost a lot to buy one tends to maintain them very well, fix them when they go wrong and they keep going and going.

My grandfather's axe is the most reliable and longest lived axe of them all, if you see what I mean.

If you are talking about wear rate, then that is down to tribology, basically lowest friction losses and material compositions whose specifications far exceed those wearing loads. This is covered above; low mean effective crown pressures and temperatures, wide long block decks with multi-bolt bearing caps to stop flexing and plenty of space for cooling and avoid overcrowding the oil gunnels, etc. etc..

Basically, if you want 'an indestructible' reliable engine in your car, it'll be about 10 litres, 50bhp, 10mpg and weigh in at half a ton.

If you are talking about the durability of the first non-maintained part to fail (like a timing belt) then that is a very long discussion and has more to do with the engine's part's manufacturing processes than the engine and its design.

But remember the most cost effective reliable pen is a cheap Biro* because when it goes wrong you throw it and get another. It is a 'system-level' reliability rather than a 'parts-wise' reliability.

This draws a rather contradictory answer; the most reliable engine [thing] is one that is so valuable you maintain it so it never breaks down, but the best value-for-money reliability you can buy is something you can throw away.

*(I mention Biro because it is the commemoration day of Laszlo Biro's death today, FWIW.)