Honda Power Unit Hardware & Software

[gruntguru]

So we are now agreed that 14 degrees ATDC is were PCP/PPP happens. that point produces maximum MBT, it also produces the best power stroke duration

No. Incorrect.
.
.

. for all that to happen ignition point is set to happen at a certain point BTDC and thereafter it is constantly on the move according to engine needs. the ignition point is set at maximum MBT and retarded at knock limit.

Yes. Correct.

[PlatinumZealot]

Detonation not withstanding... where ignition timing has to be dialed back to be closer to TDC (retarded).

[saviour stivala]

Detonation not withstanding... where ignition timing has to be dialed back to be closer to TDC (retarded).

In an ICE when the fire is lit it is a burn not an explosion (that would be like detonation). As RPM increases there is less time for maximum cylinder pressure to be reached at the right place in piston travel (crank degrees). So the fire is lighted sooner to get max pressure at the right place/time.
Ignition is set at maximum MBT and retarded at knock limit. Retarding ignition retard MBT at crank degrees, further After top dead center and not nearer to top dead center. Moving MBT nearer to top dead center will accelerate detonation/knock.

[Bandit1216]

About knock: would knock even be a main concern? Would underneath picture apply to the nowadays F1 engines?

[gruntguru]

Detonation not withstanding... where ignition timing has to be dialed back to be closer to TDC (retarded).

In an ICE when the fire is lit it is a burn not an explosion (that would be like detonation). As RPM increases there is less time for maximum cylinder pressure to be reached at the right place in piston travel (crank degrees). So the fire is lighted sooner to get max pressure at the right place/time.
Ignition is set at maximum MBT and retarded at knock limit. Retarding ignition retard MBT at crank degrees, further After top dead center and not nearer to top dead center. Moving MBT nearer to top dead center will accelerate detonation/knock.

PZ is right. Ignition timing is always BTDC so retarding is moving the ignition point closer to TDC.

[saviour stivala]

Detonation not withstanding... where ignition timing has to be dialed back to be closer to TDC (retarded).

In an ICE when the fire is lit it is a burn not an explosion (that would be like detonation). As RPM increases there is less time for maximum cylinder pressure to be reached at the right place in piston travel (crank degrees). So the fire is lighted sooner to get max pressure at the right place/time.
Ignition is set at maximum MBT and retarded at knock limit. Retarding ignition retard MBT at crank degrees, further After top dead center and not nearer to top dead center. Moving MBT nearer to top dead center will accelerate detonation/knock.

PZ is right. Ignition timing is always BTDC so retarding is moving the ignition point closer to TDC.

Grandguru. ha ha ha, I got it now, very clever of you, very silly of me. "ignition timing is always BTDC so retarding is moving the ignition point closer to TDC" Yes that is correct.

[godlameroso]

Detonation not withstanding... where ignition timing has to be dialed back to be closer to TDC (retarded).

In an ICE when the fire is lit it is a burn not an explosion (that would be like detonation). As RPM increases there is less time for maximum cylinder pressure to be reached at the right place in piston travel (crank degrees). So the fire is lighted sooner to get max pressure at the right place/time.
Ignition is set at maximum MBT and retarded at knock limit. Retarding ignition retard MBT at crank degrees, further After top dead center and not nearer to top dead center. Moving MBT nearer to top dead center will accelerate detonation/knock.

PZ is right. Ignition timing is always BTDC so retarding is moving the ignition point closer to TDC.

Hate to say it but that's called advance. Advance means ignition point is sooner, retarding means it happens later. If your ignition timing is set to 26° BTDC, and you move ignition point to 30° BTDC that's advancing timing, it occurs earlier than baseline. If you move the ignition point to 14° relative to baseline(assuming similar burn duration etc) that is known as retared timing. You are timing PPP further down the piston stroke.

Now, combustion speed is partially dependant on engine speed, so yes the ideal ignition point will move through the rev range and load applied because knocking is also a big limit which will alter ideal PPP(a compromise for reliability may retard PPP 2 or 3 degrees). However, PPP will not be far removed from the ideal ~14 degrees ATDC.

[henry]

I believe the current discussion revolves around 2 different ignition technologies. Conventional with a centrally placed spark to initiate combustion and TJI with combustion initiated by a set of radial spokes of hot combustion products.

In my mind the key difference, in relation to the discussion, is the different distances the flame front(s) have to travel to reach all of the cylinder contents. For the centre spark that distance is ~40mm. For the TJI it depends on the number of spokes, for 4 it’s ~31mm 6 it’s ~21mm and for 8 it’s down to ~16mm. I assume that the difference in flame travel distance serves as a reasonable proxy for the time to reach peak pressure.

If the flame speed is 60m/s, I’ve read this is in the ballpark for highly turbulent combustion, the time taken for the conventional is around 0.7msec and for the TJI about half that.

At 12000 rpm the crank is rotating at 72°/msec so there is likely to be a 25° difference in pressure rise time.

So irrespective of where the peak pressure point lies it seems apparent that more of the combustion pressure rise from TJI will fall on the helpful rather hindering side of TDC.

[gruntguru]

Good to think of the problem in those terms Henry. You haven't considered the fact that conventional spark radiates from a single point and the volume traversed begins slowly but accelerates as the area of the "flame front wall" grows. With multiple nuclei the TJI system consumes the charge very rapidly from the start.

[godlameroso]

I believe the current discussion revolves around 2 different ignition technologies. Conventional with a centrally placed spark to initiate combustion and TJI with combustion initiated by a set of radial spokes of hot combustion products.

In my mind the key difference, in relation to the discussion, is the different distances the flame front(s) have to travel to reach all of the cylinder contents. For the centre spark that distance is ~40mm. For the TJI it depends on the number of spokes, for 4 it’s ~31mm 6 it’s ~21mm and for 8 it’s down to ~16mm. I assume that the difference in flame travel distance serves as a reasonable proxy for the time to reach peak pressure.

If the flame speed is 60m/s, I’ve read this is in the ballpark for highly turbulent combustion, the time taken for the conventional is around 0.7msec and for the TJI about half that.

At 12000 rpm the crank is rotating at 72°/msec so there is likely to be a 25° difference in pressure rise time.

So irrespective of where the peak pressure point lies it seems apparent that more of the combustion pressure rise from TJI will fall on the helpful rather hindering side of TDC.

Flame speed varies considerably based on fuel temperature, engine speed, compression ratio, equivalence ratio, CC geometry, port geometry(whether it promotes tumble/swirl or some combination), and other things. Pressure rise from combustion is one thing, mechanical pressure rise is another, to get the most out of the engine you must find the best compromise between the two. Graphs 2, 3, and 4 are for TJI.

[gruntguru]

Peak pressure at 10* ATDC (2nd image).

[tok-tokkie]

X scale is ms not °

[henry]

@gruntguru, Yes the multiple flame fronts of TJI do acccess the combustion volume much more quickly than the conventional spark. I was going to try to calculate that but the TJI gets much too complex because there are multiple flame fronts. Also I imagine the actual combustion chamber is nothing like the flat disc in my assumptions, which would only be about 3mm thick.

@godlamerosa. Thanks for the graphs. I’ll need to look at them a few times to get my head round them. One thing that leaps out is the peak jet speed which is much higher than I imagined, I’d assumed around 200 m/sec.

[Sieper]

I am an absolute noob (on this front, as well as many others ) but before I saw the TJI "pre chamber" spark plugs (a mild let down) I envisioned a "pre chamber" as a small room above the cylinder with several thin hollow channels running from that room and exiting into the main cylinder side walls from all sides, introducing the sparks from all sides at the same time.

[saviour stivala]

@gruntguru, Yes the multiple flame fronts of TJI do acccess the combustion volume much more quickly than the conventional spark. I was going to try to calculate that but the TJI gets much too complex because there are multiple flame fronts. Also I imagine the actual combustion chamber is nothing like the flat disc in my assumptions, which would only be about 3mm thick.

@godlamerosa. Thanks for the graphs. I’ll need to look at them a few times to get my head round them. One thing that leaps out is the peak jet speed which is much higher than I imagined, I’d assumed around 200 m/sec.

"with a 3mm disc", i am assuming you mean on top the piston, the big part of the combustion sapce is in the pistons 4 valve clearance indentations, the pistons gets much closer then that at TDC. the NA engines were struggling to get past a comprssion ratio of 13 because of said valve indantations in the pistons running a valve lift well past 17mm and marks of valves hitting pistions was common.