Peak pressure will always be ATDC unless you have a mechanical failure of a cylinder seal. This is because whatever pressure you record at __ degrees BTDC, the piston will still advance closer to the head as it reaches TDC, increasing this pressure. The only other way to have peak pressure BTDC would be if you rapidly cooled the cylinder after some point BTDC with say coolant, or a broken nozzle, or something similar. But for the sake of understanding, just be aware that peak cylinder pressure isn't going to continually occur BTDC on a functioning engine. The variance we are talking about will usually be the different between peaks that occur right at TDC, vs occuring at or beyond 5 to 10 or more degrees ATDC. And in terms of engine stress, the shape and onset of that pressure. Even the exact same maximum cylinder pressure value can be achieved in such a way with proper tuning that results in long component life when compared to having produced that same pressure, but in such a way that approaching TDC the pressure is reached VERY quickly, almost instantly as the fuel source detonates. The difference is akin to placing your shoe on the top of an empty coke can and pressing down slowly, and uniformly until you are applying 100lbs of force to that can, vs merely taking a 100lb plate and holding it in a fixture where it is barely touching the top of the can, and then releasing it onto the can. Even though the force will be 100lbs, the timeframe in which it was delivered makes all the difference. So even speaking in terms of maximum cylinder pressure isn't even addressing the route cause of engine failure, which is shock loading, and really anything that isn't smooth and uniform that results in forces acting within timeframes that exceed the mechanical strength of any components in such a way that the service life is drastically cut short, if not immediately in some cases.
That depends on the injection duration and engine rpm. Obviously longer durations and higher rpm will extend the crank angle over which the injection window takes place. If you say a timing advance of 15 degrees, injection delay of say, .5ms, and in injection duration of 2ms at 1656rpm (chosen for round numbers), you'd have an injection window that started at 10 degrees BTDC, lasting 20 degrees and ending at 10 degrees ATDC. That might represent a cruising situation. It's just an example, I didn't know if that's what you were looking for.
Say, ~5 degrees BTDC, with the spike hitting right at or slightly ATDC. That should be in the ballpark, although I haven't spent much time looking at part throttle cylinder pressures, so I can't say for sure.
This is what i was wondering. Because from what i read is propane accelerates the burn-but it burns longer down the stroke????? WTF. Anyways lets say it accelerates then burn and if what you said here is correct and the "pocket" of fuel begins to flash at 5* BTDC then if there is propane in there and it all begins to flash at the same time that may not be to big of a problem because its so close to TDC. Now lets say i am running a high performance engine with 25* timing and my fuel begins to flash at 15* BTDC I will lose efficiency and raise cyl pressures. Goes back to fuel efficiency being a good indicator of stress. If your doing the same work with less fuel it seems that there would be less stress on the motor.
I am confused on if propane accelerates the burn or slows it because I think that is key to the whole argument here. If it accelerates it like your saying and the burn begins at 5* BTDC and then it is done say 10* sooner than diesel alone would have. This would mean more internal stress because you made X power in less time spread. Ok that makes since.
Lets say it is in fact burning slower. This would be great because it spreads out the stress over more time and would increase engine life. But without any in cylinder data we cannot be sure about what is happening inside the cylinder.
I do know one FACT and that is it increases the efficiency beyond just the extra BTU’s. Some if not most of the increase beyond added BTUs may be due to an advanced timing effect. But-we advance timing all the time on our engines without causing any problems when done properly. I am saying that many propane failures could simply be due to basically over timing the motor. Take a stock engine for example. We safely increase timing by 5* and increase mileage by 10%. That’s great now say we inject propane into a stock motor with stock timing. It essentially has the same effect increasing timing by 5*. Now lets put them both together and the timing is raised by ten degrees and suddenly we have an over timed motor. Not to mention that the chip is dumping more fuel than stock which causes that “pool” of fuel to get bigger sooner. This could easily account for most engine failures because for some reason I only hear about failures on at high performance trucks which were running at minimum a chip (increased timing) and propane. I have not heard of any failures when running propane on a STOCK TIMED truck.
The company that makes the kit I run put propane injection on a stock engine and doubled the engine HP. I’m not saying they advocate that but it is possible. This engine should have blown up if propane causes that much extra stress. You were saying before that it causes as much stress as adding 3X the power on diesel only. So this engine made lets say 300hp stock 600hp on propane which is a 300hp increase which according to you was equivalent to a 900 hp increase which equals 1200hp from a 300 hp engine. That is pretty good for stock internals.
This is assuming the engine was properly timed to begin with. For instance, most people will pick up response and mileage by advancing their stock timing a bit, because that timing might not have been set specifically for maximum combustion efficiency by the OEM for various reasons, emissions often being the culprit here. In such a case (as if often found) a quicker burning fuel will be advantageous. And yes, state of tune and intended engine usage/rpm operating range directly effects whether or not cetane increases/decreases will be of benefit for the reasons you outlined above.
As above, in most cases, increasing timing over a stock calibration nets positive results, which is effectively what is occuring with the propane in many cases. Reading this paragraph I think by the end of it you have come to the same conclusion.
I was speaking in terms of WOT, where diesel fuel delivery would be constant, and the only variable would be with or without propane. In this case the addition of the propane decreased tailpipe smoke considerably, leading me to believe the burn must have occured quicker, started earlier or both.
