In fact it's all one big happy detonation.
Printable View
The original CR was chosen to be best when running the engines on boost all the time (1800+ rpm).
As most of our engines are no longer run this way due to expensive fuel etc installing NA pistons does make sense for boat that is run at displacement speeds all the time, as many of our members do.
There's no reason to change the injectors to match the increased compression unless you're looking for more power on top - it's not a gas engine. The increased dynamic compression will just make the engine run more efficiently when off boost.
And start easier with less smoke when cold.
Thank you luckydave and avenger that is the type of info I am looking for. I like your thoughts about how most are being run at a off boost rpm most of the time.
Every time one of us asks how our engine should be run, if it hurts it to be run at high speed, or how often it should be "blown out" it is useful to remember that they were designed for military use, principally to turn generators at 1800 rpm and 80% load.
The design intent was to optimize them for that load/speed. Running them any other way is essentially "misusing" them.
So, if you want to run an engine that was designed to run forever at 80% load, at 950 rpm and 20% load, you can expect under loading issues.
If you want to force the engine to do things it wasn't designed to do, raising the compression ratio might help.
[QUOTE=Avenger;376412
Detonation is a gas engine thing. Not really an issue with a diesel.
[/QUOTE]
Would you care to expand on this error or delete it.
What would you call that knocking sound coming from that diesel engine?
Rapid burning of fuel, not detonation. Diesel has effectively unlimited octane, detonation is just about impossible.
A engineer/writer in my world - the motorcycle industry - named Kevin Cameron published a concise writeup on detonation recently in Cycle World magazine. He's a much better writer than I am, so I'll just copy it here.
When a chemically-correct mixture of gasoline and air filling a sealed volume is ignited, the heat released by combustion raises its pressure to about 7 times its pre-combustion value. This pressure can drive the piston of an internal combustion engine to produce rotary power through a crank and connecting-rod linkage. By compressing the mixture before ignition, we can greatly raise the peak combustion pressure and so get more power from a given amount of fuel.
Igniting mixture at atmospheric pressure (14.7-psi at sea level) gives us 7 x 14.7 = 103-psi, but if we pre-compress the charge to 200-psi or a bit more before igniting it, we can get peak pressures in the range of 1000-1200-psi, greatly boosting torque and power. Because the amount of heat release is the same in both cases, all we have changed is how that energy is divided between work delivered to the piston and heat wasted out the exhaust. As we raise an engine’s compression ratio, causing it to generate higher peak combustion pressure, we find that exhaust temperature falls.
As we raise an engine’s compression ratio in trying to make it give higher torque, we reach a limit that is set by the robustness of the fuel molecules themselves. At some high compression ratio, as the mixture burns, the unburned mixture ahead of the advancing flame front is heated by both compression and radiation. If the last of the unburned mixture reaches a critical temperature level, molecular collisions become energetic enough to break off the weakest-bonded hydrogen atoms from the fuel. When these combine with oxygen atoms from the air in the mixture, highly reactive fragments called ‘radicals’ are formed. Most important among them are negatively-charged OH- radicals. As their population in the unburned mixture increases, a new and highly violent form of combustion becomes possible.
Normal combustion, which chemists call ‘deflagration’, is like a forest fire; the advancing flame heats the unburned material ahead of it, raising its temperature to its ignition point. In a perfectly still chemically correct mixture of gasoline and air, this flame velocity is quite low – of the order of one foot per second.
But when the unburned mixture has been heat-altered to generate a population of OH- radicals (call them “Oh-Aitch-minus”), its nature changes to that of a sensitive explosive. Deflagration is not an explosion – its speed is limited by the time taken by the flame front to heat what is in front of it to its ignition point. But in mixture containing a heat-generated population of radicals, a much faster form of reaction becomes possible -‘detonation’ (which in engines is also called ‘knock’). In detonating combustion, the chemical reaction proceeds, not by deflagration’s gradual heating to ignition, but by a wave of reaction pressure, propagating at the local speed of sound.
This forms a shock front, which is a zone just a few molecules thick, across which pressure suddenly rises to full reaction pressure. When this front hits piston or combustion chamber, it sounds like knocking rocks together under water. Flooring the throttle in a high gear at low rpm – lugging – creates ideal conditions for detonation – the irregular tinkling you hear.
It takes very little detonating charge to create big effects. Detonation happens in the last parts of the mixture to burn, out near the cylinder walls, because they have been heated and compressed the longest and the most. The shock wave from detonation entrains the layer of stagnant gas that usually adheres to piston crown and chamber surfaces. Because that ‘boundary layer’ normally has a powerful insulating effect, when detonation takes place there is a sudden increase in heat flow from combustion gas to piston and head (Riders on TZ Yamaha racers used to see a mysterious 5-deg C rise on the coolant temp gage).
Folks who hoped that using an exhaust gas temperature probe and gage would save them from detonation are disappointed, because when detonation begins, exhaust gas temperature falls. It makes sense. If shock-wave destruction of the boundary layer allows more heat to enter piston and chamber surfaces, less heat remains in the exhaust gas. This fact comes straight from the dyno, not from any theory!
Fuel’s resistance to breaking up at temperature this way is quantified as “Octane Number” (ON). The greater the ON, the higher the fuel’s resistance to detonation. In general, the more compact the molecule, the better it resists being knocked apart to form those awful OH- radicals that lead to detonation. And so we find that long straight-chain fuel molecules – such as those of n-heptane – are very easily knocked apart and knock like crazy, while more compact branched-chain forms – like triptane – hold onto their hydrogen atoms longer. Also in this compact category are the ring compounds, or “aromatics”, such as benzene, toluene, and xylene. When in 1977 the staged removal of the ON-booster tetraethyl lead from motor gasolines drove ON downward, fuel blenders tried to recover some of it by adding up to 40% aromatics to fuel. Lead removal was motivated by the fact that it poisoned emissions-reducing exhaust catalytic converters and that lead-bearing exhaust was thought to be reducing the intelligence of urban children.
Note that ON has zero connection to the energy content of fuels. Increased power can be had from fuels of higher ON only by operating at the higher compression ratio that such fuels can tolerate.
Operation on today’s no-lead motor gasolines would quickly destroy any of the WW II vintage aircraft piston engines if operated at high power. Yet modern motorcycle engines can safely use compression ratios as high as 12-to-one or a bit more because the engineered rapid combustion in such engines consumes the fuel-air mixture before sufficient charge heating to create detonation can take place.
Tetraethyl lead or TEL (which is desperately poisonous) was an almost magical anti-knock agent in gasoline. It operated by being a negative rate catalyst for the creation of OH- radicals. One gram per gallon had a big effect, but the second gram had much less, and so on. In wartime aviation gasoline, up to 6 grams of TEL per gallon were used. Its combustion product, lead oxide, sometimes produced cream-colored exhaust streaks down the fuselages of fighter aircraft in lean cruise (operation at combat power, which was over-rich for charge cooling, produced black exhaust streaks).
Many people even today use the terms 'detonation' and 'pre-ignition' as if they meant the same thing. They do not. Detonation occurs after the ignition spark, in the most compressed and heated part of the fuel-air charge, out near the cylinder wall, at the very end of otherwise normal combustion. This is why damage from light detonation takes the form of rough erosion of the piston's edges.
Pre-ignition is what it says it is – ignition of the fuel-air charge before the ignition spark. Because this forces the engine to compress a hot, burning charge (usually pre-ignition takes place near bottom center), the center of the piston crown heats rapidly because it is farthest from the cooler cylinder wall. This heating softens the piston, its crown sags and eventually is punched through by the pressure.
Detonation initially damages piston edges, but pre-ignition pushes the center of the piston into collapse.
Luckydave's learned colleague (and thank you for the excellent insight) notwithstanding, I understood the post I referenced to use the term detonation to refer to pre-ignition or "pinging" brought on by a fuel-air mixture igniting prior to the firing of the ignition system and it's intended timing of the combustion process, usually due to excessive cylinder pressure or hot spots in the combustion chamber creating an unintended ignition source. Since Diesels do not compress an air-fuel mixture, pre-ignition is an impossibility.
O god not this discussion again. Everyone need a lesson on stoichiometric combustion again?