LPG Stoichiometric AFR = 15.8:1

[CNG]

AFR Lambda Table 19-02-20 01.png (153.06 KiB) Viewed 3061 times

It's really hard to fidn information on AFR/Lambda point for LPG and CNG.

Yes I can tell you LPG is 15.8 and CNG is 17.2, but the real-world numbers easily availaible for the others is absent even after an hour on the Google Poodle for LPG, let be CNG.

Fueling chart.jpg (130.42 KiB) Viewed 3061 times

Where is Lambda or AFR for peak power/ economy in LPG? And I'm hoping Simon has this... for CNG too?

[Pinger]

It is massively helpful when discussing AFRs to specify whether it is mass or volume that is being measured. Neither chart specifies this but at least I can guess the first one.
AFR for maximum power with gasoline (and other liquid SI fuels) is as much to do with the cooling effects of evaporating the fuel in the cylinder and obviously this is a phase change already completed with LPG and CNG. Less important with increasing octane as curbing detonation is the aim of said cooling.
Lean AFRs for economy are quite complicated and for 4T are more or less limited by combustion degradation or the slowness of lean combustion impinging on cycle efficiency. With 2T where there is a measurable cost for 'air work' lean burn is less effective as the increased pumping work providing the additional air negates any other benefits.

[LPGC]

Not much to add because Pinger nailed it.

Figures are mass.

Don't forget that a wideband lambda sensor is probably calibrated for petrol... If you're using a wide band sensor calibrated for petrol stoch will show as 14.7 regardless of what fuel you're using. I.e. If you're using such gauge to set up a CNG install and want stoch mixture you should still aim for 14.7 not 17.2, narrow and wide band lambda sensors key from amount of oxygen remaining in exhaust gasses. Some such sensors are calibrated to show mixture in terms of lambda ratio but if they're calibrated for petrol stoch ratio (1) will be 14.7:1 and rich ratio will be lower than 1 (e.g. 0.8 = 11.76:10)... But if you're running CNG a reading of 0.8 might be close to equivalent of 13.76:1

As Pinger says when it comes to which mixture will give best economy much will depend on specifics such as engine design and load but you won't be far from ideal with stoch.

Also as Pinger says... Some of the increase in power from running rich mixture on petrol comes from the cooling effect of liquid petrol evaporating to petrol vapour in the inlet manifold, almost gives a very slight supercharging effect by cooling air and increasing density. Also any petrol that doesn't evaporate won't be burned because the air can't get inside droplets of liquid to burn it, same reason we need a choke (or fuel injection strategy that gives rich mixture to a cold engine) on a petrol engine for cold starts, so running an engine on petrol at high rpm and high loads we might need a slightly rich mixture just to compensate for any portion of the petrol that doesn't evaporate (less time to evaporate at high rpm). With LPG or CNG it's all going to be vapour so we don't need rich mixture to compensate for the factors I've just mentioned. The engine might still make more power if we run a different to stoch mixture because by changing mixture we affect burn speed. We might want to run a slightly rich mixture at high loads anyway because it helps keep the valves cool. Liquid petrol is obviously much denser than petrol, LPG or CNG vapour so it takes up less volume in the inlet manifold and in cylinders during induction. If it takes up less volume in the manifold / inducting cylinders it leaves more volume for intake air... So richening mixture on petrol has less of a negative impact on VE volumetric efficiency (engine size and design versus amount of air per 4 stroke cycle it can suck in - VE changes with rpm) than richening mixture on LPG or CNG does. Put all this together and ideal mixture for maximum power on LPG or CNG is leaner than ideal mixture for maximum power on petrol, we still want a rich mixture for maximum power on LPG or CNG and we still want to protect the valves by running slightly rich mixture at high loads but it will make most power at a leaner mixture than on petrol and not least in part because if we go rich on LPG or CNG we start to impact on VE more than impact on VE from running rich mixture on petrol.

I started writing to put some figures/maths into this but it'd take too long and I need to get a move on with work lol.

[CNG]

tThank you both.

Unsure of the point of using AFR, yet everybody does, my WB gauge is set to display Lambda. Thus Lambda is 1.0 at stoich regardless of fuel.

Without the cooling effect, I did wonder how LPG/CNG running rich was a good thing. My whole reasoning came about when I understood that the AEB175 Leonardo richens mixture over 4000~rpm by default. Minded that my Land rover does not like much over 3500rpm I wondered what I was losing. For LPG/CNG running rich is as much a valve-cooling thing as increasing burn-speed.

[LPGC]

Running a bit rich does give you a bit more power on LPG, it also has the effect of further ensuring there is no spare oxygen to react with hot exhaust valves.