Compression ratio and engine life

[ptuomov]

If I take a given engine and increase compression the temperature and pressure go up so do the loads and thermal stresses. It's extremely difficult to quantify what effect this have on life You won't find the answer here though as with everything it depends and why OEM spend millions trying to find out

There are a couple dozen basic design constraints in an engine. The peak cylinder pressure goes up, so if the head rigidity or cylinder tower strength are the binding constraints then maybe compression increase is a problem. Most times it’s not. I believe that heat problems on the exhaust side will be reduced by higher compression ratio. And the knock limits counter that, by forcing later ignition timing and thereby higher exhaust heat loads.

[MadBill]

Higher CR = lower exhaust valve and system temperatures...

[modok]

In the OLD engines with flexy crank problems, sometimes higher CR makes it worse, then again sometimes it makes it better. Apparently....interrupting the harmonics???

[Geoff2]

I would be more worried about the rod big end going out of round at high rpm & pinching the bearing as the rod tries to arrest the piston at TDC. I think this load is more damaging to parts & more likely to cause failure than an increase in CR.

[kimosabi]

Not to diss your question, but diesel engines go hundreds of thousands of miles on same set of bearings. Cylinder preparation, proper seating of the rings, ring tension, oil control, fuel control, and proper servicing all come into play on mileage limitations. Fuel injection has really limited the fuel wash down of cylinder walls and associated ring wear and contamination of oil. Joe-71

Diesels in general run at half the rpm, also in general they have stronger blocks and diesel as a fuel also acts alot like a lube, unlike gasoline that desolves and dries up things. Diesel in the engine oil is not as critical as fuel from blowby. On our biggest pilot boat we have two 16litre/980cuin 1000hp Scania's. Full speed is 2300rpm, cruise speed is 2000rpm. Our two 480hp 5.8litre/357cuin Yanmars are rated 3300rpm max.

If I turned them up to 5000rpm I don't think either would live really long.

[SupStk]

As compression ratio raises the power increase gets smaller and risks get bigger. For the stuff I do 15:1 is pretty much my limit.

[PackardV8]

If I turned them up to 5000rpm I don't think either would live really long.

But that has nothing to do with the compression pressures. Industrial engines often have long strokes, heavy pistons and valve gear, plus the inherent limits to diesel ignition process. It took Audi and Peugeot years of testing to learn how to build the high RPM diesels they raced at LeMans.

[kimosabi]

If I turned them up to 5000rpm I don't think either would live really long.

But that has nothing to do with the compression pressures. Industrial engines often have long strokes, heavy pistons and valve gear, plus the inherent limits to diesel ignition process. It took Audi and Peugeot years of testing to learn how to build the high RPM diesels they raced at LeMans.

It's all related to one another. High compression pressures demand sturdier components and block i.e pistons etc etc. As far as I know the TDI Lemans engines max out at 5000rpm. They are super efficient common rail engines now though with fuel pressures up tp 2800bar and don't run on normal diesel fuel. FWIW our Yanmar 6LY3-ETP engines are almost square with 106mm bore/110mm stroke.

[ptuomov]

For gasoline engines, air flow and component strength limit rpm. For diesel engines, I think it’s the combustion process. I’d guess (but don’t know) that a high rpm diesel needs a small bore and some fancy high pressure fuel injection system to get the diesel fuel into the piston top and burning quickly enough.

[The Badger]

All interesting inputs that I'll have to take a minute to think about, but while gm is warranting there engines I guess the real big sign that it's ok would be Mazda and there sky active engines.

[Tom Walker]

Increasing the compression ratio, within reasonable limits, increases the thermal efficiency of the engine. With increased efficiency, you will get more work and less emissions from the same amount of fuel burn, which is what the OEM manufacturers are after with their increasing compression ratios. Nox formation can increase with the elevated temperatures from higher compression, but most emissions will improve. As stated by someone in a previous post, exhaust temps should be lower with a higher compression which should make life easier for the exhaust valves and cooling system, a cleaner burn should help with keeping the oil in better shape longer. So, some of the effects of a higher compression could help the longevity of the engine. I believe the highest compression ratio that the fuel octane limits will permit provide the best mileage, best power, best part throttle response and lowest emissions. Most of this pertains to engines in everyday type of use and naturally aspirated, racing engines and forced induction applications require a little more thought as to what is best. For the most part, a higher compression may put a little more stress on some parts,(bearings, pistons, head gaskets) but make life easier for other parts,(exhaust valves, cooling system). Higher compression by itself, should not cause a shorter life expectancy for the engine unless detonation is happening. With in reasonable limits, higher compression should lead to higher efficiency, which brought about the Diesel engine with its very high compression ratios and incredible efficiency.

[MadBill]

Maybe this is the solution, CR that's instantly/infinitely variable between 8 & 14:1... http://www.enginelabs.com/news/infiniti ... ut-in-2019

[peejay]

All interesting inputs that I'll have to take a minute to think about, but while gm is warranting there engines I guess the real big sign that it's ok would be Mazda and there sky active engines.

VW has been doing 11:1 with boost, 13:1 naturally aspirated, for about two product cycles now.

The engines aren't the part that is commonly failing on the cars. When the engines do fail, it is generally oiling-related because VWAG propagates the myth that oil doesn't need to be changed.

One thing that should be noted is that combustion chamber shape is highly important, as is piston crown shape, and a lot of effort is spent on this. Also to note is that bore size is generally 80-83mm and no larger.

[Alaskaracer]

Higher compression means higher cylinder pressures, both because of the higher compression directly and also because of faster burn around TDC.

Where did I say always?
[/quote]

Here, it is implied....

[ptuomov]

Higher compression means higher cylinder pressures, both because of the higher compression directly and also because of faster burn around TDC.

Where did I say always?

Here, it is implied....
[/quote]

Fair enough. Here’s what I think. If you increase compression, and hold everything else constant in the engine, the peak cylinder pressure always goes up. If you change other things in the engine at the same time, the peak cylinder pressure usually but not always goes up. Do you agree with that?