What is "shaving the head"?

[psr]

Assuming the valves don't hit the head & it becomes perfectly possible to increase the compression ratio from say from 11.0 to 13.0, wouldn't the torque improve? What happens to the engine life? I assume the life of the engine will not long last, is my assumption right? Is there a way to calculate by how much percent the life of the engine will come down?
There is a limit beyond which even air when compressed, will become hot.This is the principle of Diesel engine ignition.Petrol Vapor being a highly combustible Hydro Carbon will, at higher compressions self ignite without the spark which is called Pre-Ignition..which will lead to hole in piston.
Every engine is designed with parameters for specific design and altering it is asking for trouble.

There's also an interesting observation, not sure by how much, but atleast I was surprised. Here's the tech spec of Ducati GT1000 & Ducati 796 Monster. I know both of them are air cooled machines, but never expected their compression ratio in excess of 9.5 & I was little surprised as to understand how this is even possible. Even on single cylinder engines the compression ratio's are not more than 9.5 or 10, but these have in excess of 10. How is this even possible? So do these type of engine's also require premium or high octane fuel? Pls enlighten me. Tks.
The Ducati 796 has 11.1:1 CR.If you see the configuration it is a over square engine.The bike runs on higher Octane as otherwise it will have knocking problems.As a generalized thumb rule any engine with 9.5:1 or higher needs higher octane fuel, ie., 91 or above, to avoid detonation. With most modern engines there is a knock sensor which automatically Retards the engine Ignition Timing to avoid this,even with 87 rating fuel but at the cost of performance and mileage.

Edit - Oh!!! and one more important stuff - MILEAGE How does this factor affected when the compression ratio is increased. I assume again, it almost remains unchanged, but still would like to get this discussed. Tks.
A higher compression gives a higher power explosion , and hence improved mileage. Remember All ICE are Thermal engines

answers in bold.
Old Fox can give a better explanation than my crude one....

[aargee]

Tks PSR ji that I couldn't recall & relate the hole in piston & pre ignition stuff!!! Sry about that & Tks for explaining. And again, (assuming) one can overcome this piston hole with hardened or forged pistons isn't it? I know it required lots of engineering, but still asking for learning sake.

So is there a way by which we can determine the maximum limit to which the compression ratio can be increased? Also is the piston, cylinder & head are the only factors to determine the maximum possible compression ratio on an engine?

On the Ducati engines, I'm still not clear as how those air cooled engines are manufactured to with stand the compression ratio in excess of standard CR of 9.x on air cooled engine. Is it due to hardening of metals or Titanium?

Old Fox can give a better explanation

Yes, waiting for that one too

[jd666]

^^ there are a lot of other factors that go in there as well aargee. You can have a higher compression, but have a restricted inlet, which does not allow more air to come in, requiring more efficiency from the air fuel that has already entered. ( Higher compression, can let your reduce the amount of fuel for a given power output ).

Also higher the compression, more powerful the ignition. So more will be the stress on the piston, crank etc. The higher power will need a bigger flywheel or weight of crank to store/dampen the jerk, or more cylinders or something to smoothen out the power delivery of the engine.

So Basically, it can be minor engineering, or a massive overhaul of the whole engine design when playing around with compression ratos.

[Old Fox]

Tks PSR ji that I couldn't recall & relate the hole in piston & pre ignition stuff!!! Sry about that & Tks for explaining. And again, (assuming) one can overcome this piston hole with hardened or forged pistons isn't it? I know it required lots of engineering, but still asking for learning sake.

So is there a way by which we can determine the maximum limit to which the compression ratio can be increased? Also is the piston, cylinder & head are the only factors to determine the maximum possible compression ratio on an engine?

The engine's we are discussing here are primarily 'heat' engines. They convert 'heat' released by burning of fuel to produce mechanical work. And that's the defining factor be it efficiency, working limits, power output or reliability. Thermal limits are the defining element and way ahead of even the mechanical limits. Because metals and their mechanical properties are very intimately related to the temperatures they are operating in.

Switching from generalizations to specifics, the limiting factor for compression ratio is the fuel used as rightly pointed out by @psr. The 'spark-ignition' (or petrol/CNG/LPG) engine becomes a 'compression ignition' (Diesel/Heavy Oil) engine when on compression the temperature of intake air rises high enough to initiate spontaneous combustion of the fuel added to it. Petrol has a flash point way lower than Diesel and so a compression ratio of around 14:1 would cause spontaneous combustion in even the highest octane petrol.

An increase in compression ratio will definitely give a proportionate increase in torque PROVIDED we have fuels that don't self-ignite and the engine mechanicals have such reserve mechanical stress margins that it can handle the increased forces. It is not a coincidence that diesel engines are heavier, have stronger bottom ends and produce lots more torque at RPM's similar to petrol engines. The high compression ratios demand a heavier and stronger design and also produce better torque at lower RPMs.

Go on increasing only the compression ratio of a petrol engine and eventually you will have to feed it diesel to keep it working. Of course the spark plug would have become redundant by then . But yes there's a 'hole' here in the sense that Diesel fuel self- combusts only above 18:1. So anywhere between say 14:1 to 17:1, you have the engine but not the fuel to run it!

Increasing compression ratio is not just the only route to extracting more power from an engine. There's super-charging and turbo-charging as available, better and more flexible options.

On the Ducati engines, I'm still not clear as how those air cooled engines are manufactured to with stand the compression ratio in excess of standard CR of 9.x on air cooled engine. Is it due to hardening of metals or Titanium?

Air cooling is just another means of carrying away excess heat as is liquid cooling. A higher compression ratio will necessarily mean more waste heat when compared to a lower CR keeping other things constant. But then why should it be so difficult to remove that waste heat? An increased radiating surface in the form of deeper, larger and more wide-apart fins, better placement of the engine head wrt to the cooling airflow and an oil-cooler in alliance will get the job done pretty well. Its not so unbelievable a piece of engineering. Just see the 796's engine up close. See the huge size of the cooling fins, the placement of the engine exposing it efficiently to cooling air and the substantial oil-cooler. The engineering achievement of course is in the fact that usually larger fins mean stronger high-frequency vibrations (as those fins emulate the prongs of a tuning fork and respond in sympathy or 'resonate' in sync with the engine RPMs) but there are no such really bad buzzy vibes felt in the bike.

[psr]

Tks PSR ji that I couldn't recall & relate the hole in piston & pre ignition stuff!!! Sry about that & Tks for explaining. And again, (assuming) one can overcome this piston hole with hardened or forged pistons isn't it? I know it required lots of engineering, but still asking for learning sake.

If you have the patience then visit the Fiero/FX/F2 thread in Ownership expe., forum ...go to page 80 and post 796 onwards..read and you will see what high compression does...A friend Nanotechnology changed a Fiero 150cc bore and piston to RTR180 bore and piston since they are compatible,but didn't shape the head to the new dimension,and the rest is in the thread for you to read.