Hey, I decided to finally register here after a while just reading threads and posts that I found interested and perhaps here I can get and share more information related to what I think would be 'the key' for best fuel efficiency on engine modifications; lean burn.
Lean burn is an old invention, and what I've understood so far it could be possible to run with lambda = 1.40...1.60 if the combustion chamber, ignition and perhaps some amount of egr.. help enough with burning that lean mixture.
E85 is interesting atleast in my thoughts, and I've been running most of my vehicles with E85. Quite hard to cause engine knocking, works with ridiculous compression ratios and is cheaper than gasoline - but not enough to save any money if the fuel consumption is 25..30% more than with gasoline like it usually is.
I have been testing my current daily driver; BMW 523 E39 '99 w/ 5spd manual that I actually bought because it's easy and cheap to get ecu tuning without replacing it with aftermarket ecu (megasquirt or etc) or buying an emulator.
I have been driving around 6000km with only E85, and 2000..2500km of it with lean mixture while cruising at low load, so far no problems.
It's running on lambda=1.15 (+/- 0.05) and ignition timing at low loads is around 10 to 16 degrees more than factory tune.
2800rpm on 5th gear (100km/h gps speed) , low load = 45-48 degrees BTDC. Yes, needs a lot longer gear ratios.. way too much rpm for a 2.5 liter straight six engine.
Fuel consumption is currently about 10% more than with gasoline (95E10), which is quite good at this stage.
An interesting note:
- while cruising around 100km/h (gps speed) for more than 20km, the engine suddenly starts accelerating with same throttle amount, and I'm able to reduce the throttle so the current fuel consumption reads only 7-8 liters / 100km (true fuel consumption about 10% more than it reads) and then after a while it starts slowing down back to normal.
My thoughts are, it starts pre-igniting or it's getting fuel fumes from the tank, because of EVAP.
Spark plugs look good, no signs of knocking etc. I should actually be worried.
Pre-ignition may not be dangerous at part throttle, low load.
But I'm not the right person to tell that.. full throttle, yes - spark plugs gone.
There are some faults on the engine (and car) that should reduce fuel consumption more when fixed (hopefully soon), but after that I'm interested on going for more advanced modifications than mostly just ecu tuning.
I have a cylinder head from 2 liter variant, which has smaller valves, ports and combustion chambers.
I've already started modding it; welded combustion chambers and took the valve seats out. Bigger valve seats and valves coming, so it's possible to make the ports better.. intake port requires the most work since I'm trying to change it for 'high tumble flow'.
On factory lean burn engines, are there some clear differences in combustion chamber? Atleast Honda Insight has valve disable function(?) and intake ports made for swirl flow; which is quite odd for a 4 valve head (per cylinder) but understandable when it's disabling the other intake valve making it act more like a 2 valve head.
So in my case it's clear it's best going for tumble flow.
Compression ratio is another thing. Because I'm running the engine on E85, it's possible to benefit of quite extreme compression ratio.
I'm targeting to around 14 to 15:1 ratio, which should still work with the small cams.
Any thoughts/ideas?
If I find the previous threads related to lean burn engines with useful information, I'll post their links here.