A look at Engine Load and BSFC numbers

[pgfpro]

This question comes up a lot, why does high engine load yield more efficient BSFC numbers?

Most people on this forum know that if they run around 80% engine load while accelerating their car to the desired speed they can improve their fuel mileage.

They also know if they get closer to 100% engine load the fuel mileage will suffer. This is due to the engineers building into the ecu a
"High Load Fuel Enrichment" map using a Threshold TPS or Load factor or sometimes both. Running at normal engine load your engine runs in
"closed loop" 14.7 stoich. When High Load Fuel Enrichment kicks in, your engine then switches to "open loop" mode, and reads off the fuel table and will be richer then 14.7 stoich. How much richer depends on the engine the load amount ,rpm, etc...

They're two main reason you have a "high load fuel enrichment".
High Load Fuel Enrichment is used for "Thermal Management purposes".
Thermal Management is basically adding more fuel to the engine for cooling reasons. The extra fuel pulls heat out of combustion chamber, cylinder, and piston top etc. At high engine load and low rpm you have a combustion event that is very slow so there is more time for the heat to transfer into the engine combustion chamber, engine block, piston etc.
The other main reason for running a high engine load fuel enrichment is at low rpm and high load there is more time during the combustion stroke for the engine to detonate. Example at 1200 rpm one engine cycle takes 100 milliseconds or (100ms) to complete. At 3000 rpm it only takes (40ms) for one engine cycle to complete. Higher rpm and a richer air to fuel ratio also promotes faster flame speed!!! This helps with detonation because the flame front travels faster across the combustion area before a hot spot can ignite before the spark plug ignites the air/fuel mixture. Higher rpm also promotes a better air fuel mixture.

So after reading all this it sounds like if we stay out of the "high load fuel enrichment" and run a higher rpm to promote better combustion cooling, faster flame speed, and better air/fuel mixture then higher rpm is the way to go???
Well nope!!!

The problems as most of you know is the intake pumping losses. Intake pumping losses are greater then all the above "high rpm" advantages.

Chart time:


In the above chart is some data from my logs of my car last year.

The engine is a 2.0L 85mm bore, 88mm stroke

The lowest line in the graph is at 3000 rpm

-9.36 inch/hg or 68 kpa (L) more pumping loss
3.94 lbs/min of air (N/A)
12.81 bsac (L) less power from air
25 engine cycles/sec (L) greater pumping losses, lower kpa at the intake valves
21 a/f ratio (L) raises pumping losses
15.7 whp (L) more power needed for higher mph
0.61 bsfc (L)
28.8 fps piston speed (W) great air/fuel mixture
35mpg @ 66mph (L)

The middle line in the graph is at 2350 rpm

-1.59 inch/hg or 95 kpa (W) less pumping losses
3.13 lbs/min air (N/A)
16.80 bsac (W) more power from air
19.5 cycles/sec (W) lower pumping losses, higher kpa at the intake valves
30 a/f ratio (W) lowers pumping losses by increasing higher load needed for the correct fuel amount for power output.
9.5 whp (W) lower power needed for lower mph
0.56 bsfc (W)
22.6 fps piston speed (W) fast enough to promote good air/fuel mixture
53mpg @ 53mph (W)

*Comparing between the two lines.
(W) win
(L) lose

Now there is the third line, the highest in the graph at 1200 rpm. FAIL

-1.02 inch/hg or 97 kpa (W) low pumping losses, great kpa at the intake valve
10 cycles/sec (L) very low air velocity killing air flow, and causing reversion
11.5 fps piston speed (L) major knock, from slow power stroke, and heat adsorbed into combustion chamber cylinder etc.

[Cobb]

Thanks for sharing. How did you make this graph? Can you run one with 91 or 93 octane to see how it effects it?

[pgfpro]

Thanks

The data came from ECM Tunning data logs from last year.
The 3000rpm log.


The 2350rpm log.


I then put the data into a EFI University engine simulator.
The graph that I'm showing is one I made by just putting that info into a spreadsheet for visualization purposes.
I like to visualize things, in this case it helps me put it into perspective of how much the engine is feeling the pumping losses per-second.

The 2350rpm sample shows the reason why I needed more fuel to over come pumping losses and the outcome of this is a poorer BSFC number.
25 intake strokes a second making a vacuum of -9.36 inch/hg the engine has to over come. Also a lower air pressure at the intake valves of only 68kpa.
The lower 68kpa air pressure is not as great at atomizing the fuel verses the 2350 rpm air pressure of 95kpa.

The 1200rpm graph showed me that there is a lot of time after the cylinder would fill and equalize cylinder pressure and start reversion.

Anyway I hope this will give some people on here some ideas, and if anything maybe we can have some great discussions.

To answer your question on octane? I'm was running 91 octane when these logs were made. I should of noted that? lol I'm sure if I were to run a higher octane it would result in a better MBT but this is the highest pump octane we have in our area.

[serialk11r]

I don't think it's really the enrichment's fault. You have to be giving the engine extremely high load to cause enrichment. If you look at any BSFC chart you'll notice that the efficiency gets really terrible at low rpm (like 1000), even at middle load. I think the reason for this is a combination of:
1. Slow intake air speed, poorer fuel atomization
2. Greater proportion of heat rejection into coolant, oil, pistons, etc.
3. Higher piston skirt friction, possibly bearing friction due to lower proportion of hydrodynamic lubrication

[pgfpro]

Originally Posted by serialk11r

I don't think it's really the enrichment's fault. You have to be giving the engine extremely high load to cause enrichment. If you look at any BSFC chart you'll notice that the efficiency gets really terrible at low rpm (like 1000), even at middle load. I think the reason for this is a combination of:
1. Slow intake air speed, poorer fuel atomization
2. Greater proportion of heat rejection into coolant, oil, pistons, etc.
3. Higher piston skirt friction, possibly bearing friction due to lower proportion of hydrodynamic lubrication

I agree with what your saying. Today,s cars have to be at almost WOT to go into open loop.

Its nice to see the effect's on running at low rpm, high load with a engine management system that you can control all the parameters. I have the ability to lock in the values and set a knock limit ignition-retard activation. This way I can get down to the wire on my tunes and push the envelope so to speak.

On my engine the payoff for no more great BSFC numbers is around 2100 rpm.
Anything lower the BSFC suffers dramatically (poor fuel atomization). Most of it comes from my intake port size (ports are very large and lose velocity at low rpm) and the other part has to do with running gasoline.

[Daox]

Another thing that I think is a big issue, and didn't see mentioned, is ignition timing. I think that after we hit the ~80% load, timing starts to be retarded to avoid knock. This reduces efficiency before the fuel enrichment happens as we're not able to take full advantage of properly timed ignition. I would imagine this leads to lower peak cylinder pressures, and thus lost power / efficiency.

[pgfpro]

Originally Posted by Daox

Another thing that I think is a big issue, and didn't see mentioned, is ignition timing. I think that after we hit the ~80% load, timing starts to be retarded to avoid knock. This reduces efficiency before the fuel enrichment happens as we're not able to take full advantage of properly timed ignition. I would imagine this leads to lower peak cylinder pressures, and thus lost power / efficiency.

Great point!!!

Here is my ignition timing table on my system. Most timing tables have the same type load verses timing values.


As you can see as load increases timing is retarded.

[Cobb]

Interesting comment about rpm and air flow. Many gen 2 insight owners find you can be fairly agressive up to 2500 rpms and still get pretty high mpg vs accelerating like a little old lady at 1500. Ive wondered about that.

Any idea how delaying closing of the intake valve to reduce pumping losses would effect this?

One of the main differences I notice with higher octane fuel is I can accelerate briskly at 4700 rpms on 93 vs 5600 on 87. Turning 900 rpms slower should account for some savings if you must make those type of moves for 10 seconds several times a tank for merging into traffic.

[Mista Bone]

pgfpro, are those timing tables factoring in base timing or not?

Like +16 BTDC to all the numbers?

[pgfpro]

Originally Posted by Mista Bone

pgfpro, are those timing tables factoring in base timing or not?

Like +16 BTDC to all the numbers?

They're factoring in base timing, so what ever cell is high-lighted that's what your true timing is on this system.

The factory timing map is very factory safe when it comes to the 4g63 turbo engine. Large gains can be made by just increasing timing numbers on the 4g63 engines.