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natural compressed gas 1
- Thread starter djdijohn
- Start date
The compression ratio data you mention is right, I've never looked at it up before. When we (Amoco in Farmington) had such horrible power on CNG, the Amoco Engineer responsible for the project did some research and said that the problem was low compression ratios and we pulled the conversion kits and sold the compressor. That is as far as I got.
The performance we saw was awful. Pulling a 6500 lbm trailer in a 1 ton truck up a reasonably shallow, but very long hill. On CNG the truck was in first gear and was running out of power at 5 mph. We flipped the switch to gasoline and by the top of the hill we were at the speed limit in 4th gear. CNG tank pressure was over 3000 psig. These were top of the line conversion kits and everyone had a great deal of interest in making it work (the pumper with the lowest gasoline to CNG ratio at the end of each month got a bonus).
Any idea of an alternative explanation for the poor performance?
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
The performance we saw was awful. Pulling a 6500 lbm trailer in a 1 ton truck up a reasonably shallow, but very long hill. On CNG the truck was in first gear and was running out of power at 5 mph. We flipped the switch to gasoline and by the top of the hill we were at the speed limit in 4th gear. CNG tank pressure was over 3000 psig. These were top of the line conversion kits and everyone had a great deal of interest in making it work (the pumper with the lowest gasoline to CNG ratio at the end of each month got a bonus).
Any idea of an alternative explanation for the poor performance?
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
ZDAS
Without further understanding the complete fuel system it's hard to speculate on the reason for the lack of power. If I had to guess the conversion was the old style with a mixer mounted to the throttlebody. I would guess if it a good conversion with a dual ECM and NG injectors you wouldn't have seen that drop off in power. Generally on NG spark maps are radically different from gasoline.
I've been involved in late model GM NG conversions where you couldn't detect the difference between Gasoline and NG without looking at the switch.
Without further understanding the complete fuel system it's hard to speculate on the reason for the lack of power. If I had to guess the conversion was the old style with a mixer mounted to the throttlebody. I would guess if it a good conversion with a dual ECM and NG injectors you wouldn't have seen that drop off in power. Generally on NG spark maps are radically different from gasoline.
I've been involved in late model GM NG conversions where you couldn't detect the difference between Gasoline and NG without looking at the switch.
So it is a timing issue instead of a fuel density issue?
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
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1
- #24
there is also the difference in stoich AFR - if you're trying to run near stoich w/gasoline at 14.7:1 and w/nat gas at 16.8:1, and you're putting the same amount of air in per cycle, then you'd have 12% less fuel mass into the engine per cycle w/cng.
...but you'd make it up on the LHV difference (47.141kJ/kg methane vs. 43.448kJ/kg gasoline)
...of course, the gaseous CNG will displace a heck of a lot more air in-cylinder than the liquid gasoline will, so you'll end up getting less air in. At STP, gasoline 719.7 kg/m3 and methane is 0.7 kg/m3... when the cylinder is full at stoich, gasoline takes up 0.0001/1 of the space, and cng takes 0.0929/1 of the space.
...If you have 1l/cyl displacement, operating at stoich and considering all of the above, you get about 3.07 kJ/cycle from your CNG and about 3.56 kJ/cycle from gasoline. That gives about 14% reduction in power.
if you compare the specs on the "regular" honda to the CNG version, you'll see that the power is down significantly, and compression ratio is up significantly... a 19% reduction in power. I would assume that timing is optimal in each case. Not sure where the rest of the lost power comes from.
DX Coupe
Engine Specifications
1.8L 140 hp I4
Horsepower 140 @ 6300 RPM
Torque (lb-ft) 128 @ 4300 RPM
Fuel Type/System Gasoline/MPFI
Valves/Valve Configuration 16/SOHC
Displacement (cc) 1799
Bore X Stroke (in.) 3.19 X 3.44
Compression Ratio 10.5:1
Turbocharger No
Supercharger No
GX sedan
1.8L 113 hp I4
Horsepower 113 @ 6300 RPM
Torque (lb-ft) 109 @ 4300 RPM
Fuel Type/System Compressed Natural Gas/MPFI
Valves/Valve Configuration 16/SOHC
Displacement (cc) 1799
Bore X Stroke (in.) 3.19 X 3.44
Compression Ratio 12.5:1
Turbocharger No
Supercharger No
...but you'd make it up on the LHV difference (47.141kJ/kg methane vs. 43.448kJ/kg gasoline)
...of course, the gaseous CNG will displace a heck of a lot more air in-cylinder than the liquid gasoline will, so you'll end up getting less air in. At STP, gasoline 719.7 kg/m3 and methane is 0.7 kg/m3... when the cylinder is full at stoich, gasoline takes up 0.0001/1 of the space, and cng takes 0.0929/1 of the space.
...If you have 1l/cyl displacement, operating at stoich and considering all of the above, you get about 3.07 kJ/cycle from your CNG and about 3.56 kJ/cycle from gasoline. That gives about 14% reduction in power.
if you compare the specs on the "regular" honda to the CNG version, you'll see that the power is down significantly, and compression ratio is up significantly... a 19% reduction in power. I would assume that timing is optimal in each case. Not sure where the rest of the lost power comes from.
DX Coupe
Engine Specifications
1.8L 140 hp I4
Horsepower 140 @ 6300 RPM
Torque (lb-ft) 128 @ 4300 RPM
Fuel Type/System Gasoline/MPFI
Valves/Valve Configuration 16/SOHC
Displacement (cc) 1799
Bore X Stroke (in.) 3.19 X 3.44
Compression Ratio 10.5:1
Turbocharger No
Supercharger No
GX sedan
1.8L 113 hp I4
Horsepower 113 @ 6300 RPM
Torque (lb-ft) 109 @ 4300 RPM
Fuel Type/System Compressed Natural Gas/MPFI
Valves/Valve Configuration 16/SOHC
Displacement (cc) 1799
Bore X Stroke (in.) 3.19 X 3.44
Compression Ratio 12.5:1
Turbocharger No
Supercharger No
Thank you for that. 14.8% lower torque at 4300 rpm explains a lot about the performance we saw.
Your explanation also helps me get my head around why the guys doing an LNG test the same time we were doing the CNG test had no noticeable power reduction.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
Your explanation also helps me get my head around why the guys doing an LNG test the same time we were doing the CNG test had no noticeable power reduction.
David Simpson, PE
MuleShoe Engineering
"Belief" is the acceptance of an hypotheses in the absence of data.
"Prejudice" is having an opinion not supported by the preponderance of the data.
"Knowledge" is only found through the accumulation and analysis of data.
Sorry i'm late jumping in. Well head natural gas is loaded with contaminates so with a good diesel filter it is great for homes or sheds. If you want to spend the money to clean and compress you should just continue to buy gas made from my crude. Oh and if you own the well do with it what you please with one exception, your state government most likely has a motor fuel tax so a chat with your local regulators could save you from getting taxed oh so kindly for every mile on your vehicle
Zadas04, Ivymike got most of the differences between gasoline and gasoline in a spark ignition engine. The other factor is related to the compression ratio and torque. Since methane has an octane rating of over 100, you could run a spark ignition engine upwards of 12.5 to 1 ration with methane, that alone would get back more than 15% more torque than a typical 9 to 1 comprssion.
Look up the HP difference on the plain ol 2.5 liter Subie and one that uses a turbo to artifically increase HP. Plain ol subie, 175 HP, simple turbo that is equal to a 11 to 1 compression ration, 250HP
Look up the HP difference on the plain ol 2.5 liter Subie and one that uses a turbo to artifically increase HP. Plain ol subie, 175 HP, simple turbo that is equal to a 11 to 1 compression ration, 250HP
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