Rub20B

Nice work..

Since a short time I am calibrating Otto engines at work (before I was only doin diesel) and what we use to determine knock on engine dyno is just a copper tube mounted to stock knock sensor location. the tube is hung on rubber strap to isolate any unwanted interference with the ground or dyno wall.

As some people already said, before knock is seen on the raw pressure trace (viewing full range) its definately there and audible.

Our indication system calculates a knock amplitude, this is the amplitude of the raw pressure signal as compared to the filtered signal. I do not know the filter setting for both signals. Rule of thumb is that we say the combustion is knocking when the knock amplitude is more than rpm/1000, so lets say 5 bar at 5000 rpm. Knock window is usally -10 to 60°.

What I have found out out on NA engines is that knock is really timing dependant and much less lambda dependant. on turbo engines usually enrichment is only there to stay below maximum allowed EGT, going from lambda .8 to .9 usually changes very little in knock but a lot on temp before turbine.

MBT is usally so that max pressure occurs around 13° ATDC, at higher load you are depending on the fuel either knock or maximum cylinder pressure limited.

With 95 RON and 9.5 compression we see at 20 bar MEP max pressure occuring at 20-35 deg depending on rpm. with a higher octance fuel like natural gas one can run always lambda 1 at 20-22 bar (havent tested more) and still be only maximum cylinder pressure limited. downside of natural gas is that it unlike gasoline only burns till lambda .92 and does provide close to no drop in EGT nor gain in torque when going richer than lambda 1.