huskar

Hello everyone, I need some math genius or somoene experienced with lpg fuel on megasquirt.I wanna run some low impedance lpg injectors on my MS3X. I bought them, but i dont know how to calculate req fuel for them. They are rated 100cc/ stroke at 255kpa and 24ms. That doesn't mean nothing for me and didn't help me with calculating req fuel for megasquirt. I run megasquirt on petrol for 2 years now ,but i want to run lpg now, as petrol prices goes sky high. I hope someone can help me,Thank you

Ted75zcar

iTrader: (2)

I will toss a couple of thoughts out there, this is at least a unique topic that carries some novelty.

Strictly speaking, there is nothing unique about megasquirt when it comes to req-fuel. This is a universal term/concept that applies to engine management. Thus, you should be able to get information that is specific from an alternative fuel forum.

The specifications you posted are without context from a traditional ICE (gasoline) perspective, so they can easily be misinterpreted. I would guess that you have a 100cc/min injector with a differential pressure of 2.55 bar, measured at 5000 rpm (4 stroke), but that is just a guess.

Stoich is ultimately used to derive req-fuel (for gasoline/ethanol blends), a quick googlefication indicates that lpg has a stoich of around 15.6 lbs/lb with an energy density similar to e85. This implies that you will need to deliver more fuel per stroke (higher load) to develop an equivalent hp to gasoline. It does also appear as if the octane rating is high, so theoretically I would assume this means you can run MBT timing safely. I did not research flame front speed, which is the primary characteristic for base timing numbers.

On the surface, I would think that 100cc is too small, assuming once again that the density of lpg is equal to or less than that of gasoline. Absolutely nothing to back this thought up, simply an uninformed hypothesis.

Here is a link that appears to offer some info, I did not review in detail.

Please post back your findings and progress. We rarely get new technical stuff around here these days.

jonboy

Depending on which injectors they are, you may not be able to run them directly from Megasquirt - most LPG injectors are peak/hold with a different peak and hold current compared to a petrol injector, so you may need an external driver board for them. The dead times are also huge compared to petrol injectors, and for most (but not all) you control the fuel delivery by drilling out the nozzles - the manufacturer will have details on which the nozzle size needs to be. In terms of timing - there's info out there as well - most advice is that timing needs to be more advanced at lower revs (as LPG burns more slowly), and less advance at higher - but I can't get round why less advance higher up...

If you have a look on MSextra there's a fair few people running MS and LPG (although mainly on big V8s). I've got a Land Rover running a 4.6 v8 with MS3 and LPG - but I've got an LPG ECU which drives the injectors directly based on injector pulses from MS, with a second fuel/spark table to tune the LPG.

huskar

As i said, there is no info what's the rpm when they measured flow, it's only said 100cc per STROKE, not per minute! When injector opens for 24ms and pressure is 2,55bar it will flow 100cc. So i'm trying to find, how many cc's is the flow for a minute and then calculate the req fuel. Jonboy is right, you can control fuel delivery by drilling nozzles, but the specs i have are for injectors without nozzles(so they work on full capacity). Dead time (opening) is 2ms and 0,8ms (closing). They are rated 60hp per cylinder, i'm currently using 214cc injectors as i'm in starting point of tuning the engine, my req fuel for petrol injectors is 15.

Ted75zcar

iTrader: (2)

Ahh, ok that is a lot more than 100cc/min. The math wasn't adding up.

24msec is 100% duty cycle at 5000 RPM.

Req fuel isn't that hard to derive. It is the theoretical duration of on time required for a given cylinder volume at 100% VE with 1 atm. Calculate cylinder swept volume, convert that to an air mass, You have stoich, determine the volume of fuel required to meet that mass ratio, calculate the amount of time your selected injector needs to be on to deliver that volume and there you are.

Lol, edit 3... 214cc gasoline is what about 40hp? Spot check (intuition) would be that req fuel should be about 67% with a 60hp injector.

Once again, totally off the cuff here

huskar

Thank you! My math and my english ,they are not really good, but i think i understand, and i'll try to calculate it.