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hackish · 03-03-2010, 12:15 AM

Some years ago I had access to a device that measured the MON and the BTU's of heat in a given mass of fuel. We found that the MON and the energy content were strongly linked. Regular 87 octane gasoline had the highest BTU number and Sunoco 94 octane had the lowest. I have no idea what would have happened to the paper on that subject but I do remember the results.

I know fuel in North America is the average of RON and MON but here are a few factoids. First, a lot of premium gasolines use up to 5% ethanol to increase the octane rating. Ethanol definitely has a lower energy content than normal gasoline - look at an E85 engine! Most often the addition of anti-knock agents to the fuel serves to lower it's tendency to ignite and they don't tend to add anything good to the combustion event. Hotrodders will often come up with their own mix injecting water or methanol to increase the effective octane. Again, lowers the number of BTUs of energy in the fuel.

One last thing, a few years ago I was taking a long road trip with my 1992 honda civic. It was properly tuned on both 91 octane and 87 octane. In 2800km of driving I compared mileage on each tune and the 87 octane map won out by nearly 10%. Maybe if I get a new vehicle this year (non-turbo) I will spend some time and re-do the experiment. I'm also curious to see if cylinder balance has a significant impact on emissions and fuel consumption.

On the previous post about the accuracy of these little OBD2 scanners...
The ones that measure injector pulsewidth directly will also need to take into account the battery voltage as the injector opening time is affected by this. This is a tiny difference but over the million or so injections per tank of fuel it can add up to a significant amount. I know this because I did some sub-contracting for a carmaker where they had a fuel economy indicator on the dash - it was approximate at the best of times.

It also takes about 10 seconds to establish an OBD2 connection with an ECU so if you end up with your engine shutoff trick then you may actually be missing the startup and cranking enrichments that the ECU adds while starting the engine. On a cold engine they can be pretty significant - 200% the idle pulsewidth is pretty common. Many cars will also retard the ignition and add extra fuel for the first 15-20 seconds to improve the cat light-off for the cold start emissions some countries mandate.

On one vehicle I know of the reported fuel injector pulsewidth is simply wrong. I verified this with some pretty high tech instruments and I've also spent more than a year disassembling the factory code from inside the ECU.

For me, I usually track kilometres travelled with fuel pumped to fill the tank.

Sorry for making this so long. Didn't have time to shorten it.

03-03-2010, 12:15 AM	#192 (permalink)
hackish Calibration Engineer Join Date: Jan 2010 Location: Canada Posts: 86 Subie - '00 Subaru Impreza STi JDM 90 day: 22.49 mpg (US) Thanks: 1 Thanked 18 Times in 14 Posts	Some years ago I had access to a device that measured the MON and the BTU's of heat in a given mass of fuel. We found that the MON and the energy content were strongly linked. Regular 87 octane gasoline had the highest BTU number and Sunoco 94 octane had the lowest. I have no idea what would have happened to the paper on that subject but I do remember the results. I know fuel in North America is the average of RON and MON but here are a few factoids. First, a lot of premium gasolines use up to 5% ethanol to increase the octane rating. Ethanol definitely has a lower energy content than normal gasoline - look at an E85 engine! Most often the addition of anti-knock agents to the fuel serves to lower it's tendency to ignite and they don't tend to add anything good to the combustion event. Hotrodders will often come up with their own mix injecting water or methanol to increase the effective octane. Again, lowers the number of BTUs of energy in the fuel. One last thing, a few years ago I was taking a long road trip with my 1992 honda civic. It was properly tuned on both 91 octane and 87 octane. In 2800km of driving I compared mileage on each tune and the 87 octane map won out by nearly 10%. Maybe if I get a new vehicle this year (non-turbo) I will spend some time and re-do the experiment. I'm also curious to see if cylinder balance has a significant impact on emissions and fuel consumption. On the previous post about the accuracy of these little OBD2 scanners... The ones that measure injector pulsewidth directly will also need to take into account the battery voltage as the injector opening time is affected by this. This is a tiny difference but over the million or so injections per tank of fuel it can add up to a significant amount. I know this because I did some sub-contracting for a carmaker where they had a fuel economy indicator on the dash - it was approximate at the best of times. It also takes about 10 seconds to establish an OBD2 connection with an ECU so if you end up with your engine shutoff trick then you may actually be missing the startup and cranking enrichments that the ECU adds while starting the engine. On a cold engine they can be pretty significant - 200% the idle pulsewidth is pretty common. Many cars will also retard the ignition and add extra fuel for the first 15-20 seconds to improve the cat light-off for the cold start emissions some countries mandate. On one vehicle I know of the reported fuel injector pulsewidth is simply wrong. I verified this with some pretty high tech instruments and I've also spent more than a year disassembling the factory code from inside the ECU. For me, I usually track kilometres travelled with fuel pumped to fill the tank. Sorry for making this so long. Didn't have time to shorten it.