CNG Advance curves vs LPG or 'The Liquid' with a classic

Natural Gas, CNG, LNG, NGV, CH4
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BritCNG
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Joined: Wed Jan 29, 2020 12:45 pm

CNG Advance curves vs LPG or 'The Liquid' with a classic

Post by BritCNG »

It's my understanding that LPG and CNG require more advance than 'The Liquid'
I run a single point 1960s Land rover with CNG. There's add-on TAPs (Timing Advance Processors) which give as much as 15 degs? You nail them into the low-tension side of a coil. Seems a lot to me? Is the advance curve parallel to petrol? I know that LPG isn't.

Only my 60s classic already runs 38 degrees advance at WOT. Add another 15, and we're at over 50 degs. Seems a lot to me? I've found this, are the relative curves shown typical?

I've asked this question on other forums, and never really found an answer.
Attachments
TAP Schematic.png
CNG vs Petrol.png

C3H8
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Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: CNG Advance curves vs LPG or 'The Liquid' with a classic

Post by C3H8 »

Sorry to take so long. It has been quite sometime since a question was posted on the CNG part of this forum. Tough to answer your question based on the numbers you stated. You say your Landrover is 1960's. On petrol I would expect to see a base timing in the 5 to 10 degree range, mechanical timing at 22 to 26 degrees at 3500 to 4000 RPM, and perhaps an additional 10 to 12 degrees of vacuum advance at low to mid range loads. Your stated 38 degrees at WOT on petrol sounds excessive considering the Octane rating of todays fuels. I would have expected 24 or 25 degrees because the vacuum would be at a minimum at WOT. All that being said NG has a much higher Octane rating then petrol. Petrol is typically at 87 ROH for regular and good quality NG is 125. The burn rate of NG is sufficiently slower then petrol which means you need a reasonable amount of advance to ensure the flame front ignites soon enough to provide maximum push just after the piston clears TDC. It is common for NG engines to be 20% to 25% ahead of a petrol curve and to also reach the intended curve quicker then petrol. The mechanical timing should top out at 2000 to 2500 RPM to achieve maximum power. Careful consideration has to be given to any modifications done to the engine such as increased compression ratios or cam changes and then adjust the timing accordingly. In theory you should hear any pre-ignition knocking however it is more difficult to hear on gaseous fuels. If you really are running at 38 degrees on petrol at WOT without any pre-ignition I would not be surprised to find you could run 53 on NG as long as that is at a higher RPM. If the timing is calibrated to come in at a lower RPM I believe that may be excessive.

C3H8
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Joined: Fri Sep 09, 2005 11:23 pm
Location: Winnipeg, Canada

Re: CNG Advance curves vs LPG or 'The Liquid' with a classic

Post by C3H8 »

I should have also mention that the curve you attached is typical, however you can see their example indicates what I was talking about. At WOT their curve shows about 12 or 14 degrees advance on petrol at 2500 RPM. The NG curve it topped out at approx. 38 degrees. In these cases the vacuum advance would not eve be engaged. I have seen many lower compression engines that are running 30 to 35 degrees on NG at RPMs as low as 1800 to 2500 RPM with no vacuum advance capability. Your stated 53 degrees would be high but it depends on the load factor. If that is the range under light load and your rover is running a lower compression it could be ok as the timing would drop off at WOT with minimum vacuum available. 1960's engines usually ran 9.5:1 - 10.5:1 ratios. Although high for the current octane of petrol it is well within a normal range for NG. NG engines can operate at even higher ratio's but then the advance would be lower.

storm
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Location: NSW, Australia

Re: CNG Advance curves vs LPG or 'The Liquid' with a classic

Post by storm »

C3H8 wrote:
Thu Mar 05, 2020 7:03 pm
1960's engines usually ran 9.5:1 - 10.5:1 ratios.
The 2.25 litre Petrol in the Series IIA Landrover was 7 or 8 to 1 and had been from the late 1950s in the Series I Landrover through to the Series III Landrover. Some design features were decades ahead of their time, features such as a roller cam and cam followers (brass rollers) hadn't even been considered by other manufacturers anywhere but Landrover used them. The big problem with LR engines was their small size and lack of compression. They could run for years, if the electrical system was maintained, but they were gutless. It wasn't until LR started using the Rover V8 and the Isuzu diesel that a LR could do more than pull a skin of a custard.
Fuel flow requirements calculations
Engine air flow requirement calculation: CFM = Cubic Inches x RPM x Volumetric Efficiency (VE) ÷ 3456

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