jim dozier
Lieutenant Commander
- Joined
- Jan 8, 2003
- Messages
- 1,970
Re: High compression heads
Just to second Merctech, gasoline internal combustion engines are designed around controlled ignitions of gas/air mixtures (and measurable flame front velocities). This applies controlled dynamic pressures on the pistons at timed intervals to maximize the downthrust on the piston/rod within the limits of the metallurgy of the engine and provide the torque on the crankshaft. Ignition timing is obviously critical to ignite the mixture at the appropriate time. Because the flame front and associated pressure wave travels at a measurable speed, ignition must occur before (in advance of) the optimun place in the crankshaft angle so that the the maximum pressure (which takes time to build) occurs where it will do the most good. Variations in mixtures (lean/rich) will affect flame front travel times, and compression ratios will affect ignitability of the mixture. Higher octane gas is used in higher compression engines to slow the ignitability and flame front down to appropriate levels. Pre-ignition is ignition that occurs prior to the electronic ignition. It can be due to too high compression igniting it (like a diesel), hot spots like carbon or sharp edges that retain prior combustion heat, or too low octane for the engine which is kind of the same as too high compression mentioned above. Pre-ignition (or too early ignition) is likely to cause detonation which is uncontrolled combustion (kind of like cancer is uncontrolled cell growth in an organism). Detonation results in cylinder pressures in excess of design limts and occurs at awkward crankshaft angles and can put a hole in a piston, bend a rod, hammer bearings, stretch cylinder bolts and blow the head off of engines (2 stroke or 4 stroke). Detonation is death to gasoline internal combustion engines. That is why the compression ratio discussion that started this post is important. Semantics is important for communicating our problems in a technical arena.
Just to second Merctech, gasoline internal combustion engines are designed around controlled ignitions of gas/air mixtures (and measurable flame front velocities). This applies controlled dynamic pressures on the pistons at timed intervals to maximize the downthrust on the piston/rod within the limits of the metallurgy of the engine and provide the torque on the crankshaft. Ignition timing is obviously critical to ignite the mixture at the appropriate time. Because the flame front and associated pressure wave travels at a measurable speed, ignition must occur before (in advance of) the optimun place in the crankshaft angle so that the the maximum pressure (which takes time to build) occurs where it will do the most good. Variations in mixtures (lean/rich) will affect flame front travel times, and compression ratios will affect ignitability of the mixture. Higher octane gas is used in higher compression engines to slow the ignitability and flame front down to appropriate levels. Pre-ignition is ignition that occurs prior to the electronic ignition. It can be due to too high compression igniting it (like a diesel), hot spots like carbon or sharp edges that retain prior combustion heat, or too low octane for the engine which is kind of the same as too high compression mentioned above. Pre-ignition (or too early ignition) is likely to cause detonation which is uncontrolled combustion (kind of like cancer is uncontrolled cell growth in an organism). Detonation results in cylinder pressures in excess of design limts and occurs at awkward crankshaft angles and can put a hole in a piston, bend a rod, hammer bearings, stretch cylinder bolts and blow the head off of engines (2 stroke or 4 stroke). Detonation is death to gasoline internal combustion engines. That is why the compression ratio discussion that started this post is important. Semantics is important for communicating our problems in a technical arena.